High FOXA1 levels induce ER transcriptional reprogramming, a pro-metastatic secretome, and metastasis in endocrine-resistant breast cancer

Aberrant activation of the forkhead protein FOXA1 is observed in advanced hormone-related cancers. However, the key mediators of high FOXA1 signaling remain elusive. We demonstrate that ectopic high FOXA1 (H-FOXA1) expression promotes estrogen receptor-positive (ER+) breast cancer (BC) metastasis in a xenograft mouse model. Mechanistically, H-FOXA1 reprograms ER-chromatin binding to elicit a core gene signature (CGS) enriched in ER+ endocrine-resistant (EndoR) cells. We identify Secretome14, a CGS subset encoding ER-dependent cancer secretory proteins, as a strong predictor for poor outcomes of ER+ BC. It is elevated in ER+ metastases vs. primary tumors, irrespective of ESR1 mutations. Genomic ER binding near Secretome14 genes is also increased in mutant ER-expressing or mitogen-treated ER+ BC cells and in ER+ metastatic vs. primary tumors, suggesting a convergent pathway including high growth factor receptor signaling in activating pro-metastatic secretome genes. Our findings uncover H-FOXA1-induced ER reprogramming that drives EndoR and metastasis partly via an H-FOXA1/ER-dependent secretome.

A Multiparameter Molecular Classifier to Predict Response to Neoadjuvant Lapatinib plus Trastuzumab without Chemotherapy in HER2+ Breast Cancer

PURPOSE

Clinical trials reported 25% to 30% pathologic complete response (pCR) rates in HER2+ patients with breast cancer treated with anti-HER2 therapies without chemotherapy. We hypothesize that a multiparameter classifier can identify patients with HER2-"addicted" tumors who may benefit from a chemotherapy-sparing strategy.

EXPERIMENTAL DESIGN

Baseline HER2+ breast cancer specimens from the TBCRC023 and PAMELA trials, which included neoadjuvant treatment with lapatinib and trastuzumab, were used. In the case of estrogen receptor-positive (ER+) tumors, endocrine therapy was also administered. HER2 protein and gene amplification (ratio), HER2-enriched (HER2-E), and PIK3CA mutation status were assessed by dual gene protein assay (GPA), research-based PAM50, and targeted DNA-sequencing. GPA cutoffs and classifier of response were constructed in TBCRC023 using a decision tree algorithm, then validated in PAMELA.

RESULTS

In TBCRC023, 72 breast cancer specimens had GPA, PAM50, and sequencing data, of which 15 had pCR. Recursive partitioning identified cutoffs of HER2 ratio ≥ 4.6 and %3+ IHC staining ≥ 97.5%. With PAM50 and sequencing data, the model added HER2-E and PIK3CA wild-type (WT). For clinical implementation, the classifier was locked as HER2 ratio ≥ 4.5, %3+ IHC staining ≥ 90%, and PIK3CA-WT and HER2-E, yielding 55% and 94% positive (PPV) and negative (NPV) predictive values, respectively. Independent validation using 44 PAMELA cases with all three biomarkers yielded 47% PPV and 82% NPV. Importantly, our classifier's high NPV signifies its strength in accurately identifying patients who may not be good candidates for treatment deescalation.

CONCLUSIONS

Our multiparameter classifier differentially identifies patients who may benefit from HER2-targeted therapy alone from those who need chemotherapy and predicts pCR to anti-HER2 therapy alone comparable with chemotherapy plus dual anti-HER2 therapy in unselected patients.

Abstract PD10-03: PD10-03 Super-enhancer-oriented integrative bioinformatics identifies aberrant KLF4 signaling in endocrine-resistant breast cancer (BC)

Background: Recent studies suggest that enhancer reprogramming underlies heterogeneity and disease progression in estrogen receptor-positive (ER+) BC. Cell-type/state specific transcription is governed by high-order assemblies of master transcription factors (TFs) and epigenetically defined regulatory regions including super-enhancers (SEs). We previously showed that aberrant activation of the pioneer TF FOXA1 promotes enhancer and transcriptional reprogramming in endocrine-resistant BC, involving the ER and the AP-1 FRA1 and c-JUN TFs. As SEs maintain a robust cell-type/state specific core transcriptional regulatory circuitry (CRC) in developmental and tumorigenic processes, we sought to identify key additional TFs in SE/FOXA1-driven CRCs in endocrine resistance, which could serve as attractive therapeutic targets. Methods: TF binding motif at the shared SEs (mapped by H3K27ac ChIP-seq) between MCF7-parental (P) cells with ectopic FOXA1 overexpression (OE) and the endogenous FOXA1-amplifed tamoxifen-resistant (TamR) cells was analyzed by HOMER. ER-bound SEs distinguishing TamR vs. P cells were defined by integrating the SEs with our prior ER ChIP-seq data (PMID 28507152). KLF4 motif within these ER-bound SEs was scanned using FIMO and linked to nearby genes by intersection with the previously defined promoter-tethered regions (PTRs) (PMID 24141950). Differential gene expression in MCF7-TamR cells upon KLF4 knockdown (KD) by 3 unique siRNAs was analyzed using limma from edgeR. The biological and clinical significance of the KLF4-dependent genes was analyzed using Gene Ontology and survival modeling with METABRIC and the ER+ metastatic BC cohort (SABCS19-GS2-02). Cell migration was assessed by the wound-healing assay. Results: We identified KLF4 among the top enriched TF binding motifs at the shared SEs in FOXA1-overexpressing MCF7-P cells and the FOXA1-amplified TamR cells. Analysis of our prior RNA-seq data of MCF7-P and TamR cells upon OE or siRNA KD of FOXA1, FRA1, or c-JUN (PMIDs 27791031, 31826955, 32424275, SABCS21-PD1-05) revealed KLF4, the Yamanaka factor for induced pluripotent stem cells, as a common target activated by the FOXA1/FRA1/c-JUN axis. We next identified 44 genes commonly down-regulated upon KLF4 KD in the MCF7-TamR cells. This KLF4-dependent 44-gene set was enriched in biological processes of embryonic development and tumor progression, preferentially dependent on ER in MCF7-TamR vs. P cells, highly elevated in ER+ metastases vs. primary tumors, and associated with poor outcome in ER+ BC treated with endocrine therapy. KLF4 KD, using the 2 siRNAs that generated similar pathway perturbations in MCF7-TamR cells, reduced TamR cell migration. Notably, among the genes co-dependent on KLF4 and ER in TamR cells, PYGB was the only gene with a PTR residing in an ER-bound SE established in TamR but not P cells. Glycogen phosphorylase B, encoded by PYGB, is the rate-limiting enzyme in glycogen degradation and plays a role in the progression of various tumors. Expression of KLF4, FOXA1, and FRA1 are commonly activated during the differentiation of human embryonic stem cells into foregut endoderm and in the inner core of fibroblasts of first-trimester human placenta villi, suggesting a unique role of KLF4 in mediating lineage-specific CRC, possibly by engaging PYGB and the glycogen metabolic pathway in advanced ER+ disease. Conclusions: Using SE-oriented integrative bioinformatics, we identified KLF4 as a potential novel target in the FOXA1/AP-1 transcriptional axis. As KLF4 binding motif resides in the unique ER-bound SEs of TamR cells, KLF4 likely forms an auto-regulated loop amplifying CRC in transcriptional reprogramming, among which the PYGB/glycogen metabolic pathway merits further investigation in endocrine-resistant ER+ disease.

Citation Format: Chia Chia Liu, Lanfang Qin, Shanunak Sathe, Sarmistha Nanda, Jamunarani Veeraraghavan, Ofir Cohen, Nikhil Wagle, Mothaffar Rimawi, C. Kent Osborne, Xiaoyong Fu, Rachel Schiff. PD10-03 Super-enhancer-oriented integrative bioinformatics identifies aberrant KLF4 signaling in endocrine-resistant breast cancer (BC) [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr PD10-03.

Abstract P1-13-17: Hyperactivation of the EGFR pathway is associated with resistance to tucatinib in HER2-positive breast cancer models

Background: The HER2-specific tyrosine kinase inhibitor (TKI) tucatinib (Tuca) recently approved for advanced HER2+ breast cancer is making a move towards the early setting. Given its growing use, resistance is inevitable, as observed in the HER2CLIMB study, where only one patient with brain metastasis remained progression free after 2 years on Tuca. Driven by the prevailing lack of knowledge about the mechanisms of resistance, in this study, we sought to define these mechanisms and identify treatment strategies to overcome them. We previously reported (SABCS 2021) that our BT474 TucaR models acquired EGFR amplification and showed elevated levels of phosphorylated (p) and total (t) EGFR, pHER2, pHER3, and downstream pAKT and pS6. Since the HER pathway is activated by ligands, here we aim to assess if hyperactivation of EGFR via high levels of its ligands is an alternative mechanism of Tuca resistance. Materials and Methods: Our recently developed HER2+ BT474 (ATCC and AZ) cell models with acquired resistance to Tuca (TucaR) developed through long-term exposure to gradually increasing doses of Tuca and their naïve parental (P) were used. Genomic (DNA-seq), transcriptomic (RNA-seq), and proteomic (western blot) characterization were performed. Changes in cell growth and migration were assessed by methylene blue and Incucyte wound healing assays, respectively. Results: RNA-seq analysis demonstrated that the levels of TGFα was significantly higher in our BT474 TucaR models compared to P cells. Our results now demonstrate that exogenous supplementation of EGF to BT474-P cells rescues the Tuca-mediated inhibition of pEGFR, pHER2, and the downstream pAKT, pERK, and pS6 levels. Exogenous EGF was also found to reduce the levels of apoptosis, as assessed by cleaved PARP, mitigating the Tuca-induced cell death. Exogenous EGF or TGFα rendered naïve BT474 and SKBR3 cells resistant to Tuca while neratinib, a pan-HER TKI, effectively inhibited this ligand-driven cell growth. We previously showed that the HER signaling reactivation observed in our EGFR-amplified TucaR cells was inhibited by the EGFR-specific TKI gefitinib (Gef) (SABCS 2021) and that the TucaR cells displayed enhanced migratory capabilities (AACR 2022). Here, we demonstrate that in addition to curbing the growth of TucaR cells, Gef, either alone or together with Tuca, also markedly reverts the migration of the TucaR cells. Knockdown (KD) of EGFR but not HER2 selectively and substantially inhibited the migration of the TucaR cells. KD of EGFR also had a marked cell killing effect on only the TucaR cells, whereas HER2 KD inhibited the growth of P but not TucaR cells. Our findings are consistent with the notion that while the P cells are functionally dependent on HER2, in TucaR cells the survival dependence could be rewired to rely primarily on the hyperactive EGFR signaling. Genomic analysis further revealed that in addition to EGFR amplification, the AZ TucaR cells also acquired a gain of YES1, a src family receptor tyrosine kinase implicated in cancer cell growth, invasion, and metastasis. Functional studies using 2 siRNAs, however, showed that YES1 KD had no effect on the growth of TucaR cells, and the migration of both TucaR and P cells was equally affected by YES1 KD, precluding the potential role of YES1 in driving the resistant and enhanced migratory phenotypes. Conclusions: Hyperactivation of the EGFR pathway via amplification of EGFR or increased expression of its ligands confers resistance to Tuca, which may be overcome using dual/pan-HER TKIs or the combination of potent EGFR and HER2 inhibitors. Given the rapidly evolving treatment landscape of HER2+ breast cancer and biomarkers of resistance, our novel findings have potentially crucial therapeutic implications and suggest that rationally sequencing the currently available TKIs may be clinically important.

Citation Format: Fu-Tien Liao, Tia Gordon, Chia Chia Liu, Pier Selenica, Yingjie Zhu, Juber Patel, Sarmistha Nanda, Lanfang Qin, Xiaoyong Fu, Andrea Gazzo, Antonio Marra, Juan Blanco-Heredia, Britta Weigelt, Jorge Reis-Filho, C. Kent Osborne, Mothaffar Rimawi, Rachel Schiff, Jamunarani Veeraraghavan. Hyperactivation of the EGFR pathway is associated with resistance to tucatinib in HER2-positive breast cancer models [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr P1-13-17.

Abstract LB517A: The role of EGFR in resistance to tucatinib and its therapeutic implications

Tucatinib (Tuc) was recently approved for metastatic disease and is moving towards the early setting in HER2+ breast cancer (BC). Given the increasing clinical use of Tuc, resistance will likely soon emerge as a challenge. Here, we explore the yet unknown mechanisms of resistance to Tuc and identify treatment strategies to overcome it. Our recently developed models of BT474 (AZ and ATCC) with acquired resistance to Tuc (TucR) and their sensitive parental (P) were used. DNA-seq, RNA-seq, and RPPA/western blot were performed. Knockdown studies were performed using EGFR siRNA. Drug efficacy studies involved cell growth assays by imaging-based or methylene blue assays. We recently reported (SABCS 2021) that our BT474 TucR models acquired EGFR amplification. The TucR cells displayed elevated levels of phosphorylated (p) and total (t) EGFR, pHER2, pHER3, and downstream pAKT and pS6, which were substantially suppressed by the EGFR-specific tyrosine kinase inhibitor (TKI) gefitinib (Gef) or even further when combined with Tuc. Our new results demonstrate that EGFR knockdown selectively inhibits the growth and pHER2 levels in TucR vs P cells, supporting our hypothesis that heterodimerization of amplified EGFR with HER2 leads to higher pHER2 levels in TucR cells. We have recently also shown that TucR models were hypersensitive to Gef and this inhibition was further enhanced with Gef+Tuc, implying their survival dependence on EGFR. Here, we demonstrate that the TucR cells made resistant to 200nM Tuc maintain their resistant growth and elevated EGFR-dependent signaling even when exposed to 500nM, and can begrown as xenografts in the presence of clinically relevant dose of Tuc, emphasizing their true resistance via amplified EGFR. Importantly, both TucR models vs P cells were cross-resistant to trastuzumab but maintain partial sensitivity to TDM1. While the EGFR-specific antibody cetuximab (Cet) was partially effective as a single agent only in the ATCC model, it potently inhibited growth and induced cell killing in combination with Tuc in both models. A significantly greater inhibition in cell growth and survival was also observed when trastuzumab or TDM1 was combined with either Gef or Cet. Taken together, our results suggest that the activation of HER2 and the resistant growth and survival in the TucR models is completely dependent on the amplified EGFR, which we are currently further corroborating by additional mechanistic and xenograft studies. Whilst we have previously reported that resistance to lapatinib and neratinib confer cross-resistance to Tuc, our recent findings show that resistance to Tuc may be overcome using dual/pan-HER TKIs or the combination of potent EGFR and HER2 inhibitors. Overall, our novel findings hold crucial implications in light of the current treatment landscape of HER2+ BC and biomarkers of resistance, and places a particular emphasis on considerations to sequence currently available TKIs.

Citation Format: Jamunarani Veeraraghavan, Fu-Tien Liao, Tia Gordon, Pier Selenica, Sarmistha Nanda, Lanfang Qin, Yingjie Zhu, Juber A. Patel, Andrea Gazzo, Fabio Stossi, Michael A. Mancini, Carolina Gutierrez, Britta Weigelt, Jorge S. Reis-Filho, C. Kent Osborne, Mothaffar F. Rimawi, Rachel Schiff. The role of EGFR in resistance to tucatinib and its therapeutic implications [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr LB517A.

Effect of mevalonate pathway inhibitors on outcomes of patients (pts) with HER2-positive early breast cancer (BC) in the ALTTO trial

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Background: Our preclinical findings suggest a role for the mevalonate pathway (MVA) in treatment resistance in HER2+ BC by providing alternative growth and survival signaling to bypass potent HER2 blockade, which could be overcome by the MVA inhibitors statins and nitrogen-containing bisphosphonates (NBs). Here we explored the effect of MVA inhibitors’ use on pts’ outcomes in the ALTTO trial (BIG2-06; NCT00490139). Methods: In the ALTTO trial, 8381 pts with HER2+ BC were randomized to 1 year of adjuvant lapatinib (L), trastuzumab (T), L+T, or T→L. All pts with documented treatment start with statins or NBs < 1 year after randomization were considered as MVA inhibitors users. Survival curves, with a median follow-up of 6.9 years, for disease-free survival (DFS), distant relapse-free interval (DRFI), BC-specific survival (BCSS), and overall survival (OS) according to MVA inhibitors use were estimated by the Kaplan Meier method and Log-rank test. All multivariate survival analyses employed a Cox proportional hazards regression model, adjusting for tumor size, nodal status, hormonal receptor (HoR), menopausal status, BMI, timing of chemo, and randomization arm. We considered interactions terms in Cox’s model between MVA inhibitors use and randomization arm, hormonal status, and BMI group. Results: Among the 8381 pts included in this study, 493 and 299 were statins or NBs users, respectively. Table 1 summarizes the significant differences in pts’ characteristics according to MVA inhibitors use ( P <.005). In multivariate survival analyses, only NBs use was associated independently with better BCSS (HR, 0.44; 95% CI, 0.23 - 0.84; P = 0.014). Statin use was not independently associated with prognosis but only in interaction with pts characteristics: worse DFS, BCSS and OS in pts treated with L+T, worse DRFI and OS in pts treated with HoR+ BC (respective interaction P-values <0.05 in the Cox’s model). Conclusions: NBs independently predicted improved BC-specific outcome in pts with HER2+ BC treated with adjuvant anti-HER2 therapy. Statin use was associated with an inferior outcome in pts with HoR+ disease and/or those treated with L+T. Whether this inferior association in statin users may reflect the underlying predisposition factors that can weaken the efficacy of anti-HER2 treatments and whether this effect was observed only in the L+T arm due to the more potent inhibition of the HER2 signaling pathway remain open questions. Further clinical investigations on the impact of MVA inhibitors on the outcome of pts with HER2+ BC are warranted. [Table: see text]

Abstract PD8-06: Acquired resistance to tucatinib is associated with EGFR amplification in HER2+ breast cancer (BC) models and can be overcome by a more complete blockade of HER receptor layer

Background: With recent approval of the irreversible pan-HER tyrosine kinase inhibitor (TKI) neratinib (N) and the HER2-specific TKI tucatinib (T) in the advanced setting and their edging towards the early setting in HER2+ BC, resistance will likely emerge as a challenge, as illustrated by the HER2CLIMB study, where only one patient with brain metastasis remained progression free after 2 years on T. We set out to define the mechanisms of resistance to T and treatment strategies to overcome it. Materials and Methods: Our previously characterized HER2+ BT474 models with acquired resistance to lapatinib (L; LapR) or N (NrbR) (SABCS20-PD3-09), and our recently developed models of BT474 and SKBR3 with acquired resistance to T (TucaR) developed through long-term exposure to increasing doses of T (up to 200nM) and their naïve parental (P) were used. Genomic (DNA-seq), transcriptomic (RNA-seq), and proteomic (RPPA, western blot) characterization were performed. Drug efficacy studies involved cell growth assays by the imaging-based IncuCyte system. Results: We recently reported that while LapR is associated with acquisition of HER2 L755S mutation, which partially reactivates the HER pathway, NrbR is associated with the additional co-acquisition of a pathogenic PIK3CA mutation. Preliminary analysis of 2 BT474 TucaR models (ATCC and AZ) showed highly elevated levels of phosphorylated (p) and total (t) EGFR, suggesting EGFR signaling activation. Levels of pHER2, pHER3, and downstream pAKT and pS6 were also markedly higher in the TucaR models compared to P or short-term T. The TucaR but not LapR or NrbR models exhibited EGFR amplification, explaining the higher EGFR levels and signaling. Further, the elevated pEGFR, pHER2, pHER3, pAKT, and pS6 levels in TucaR models were substantially suppressed by the EGFR-specific TKI gefitinib (G) (50, 500nM) or even further when combined with T (500 nM G+200nM T). These results suggest that the higher pHER2 levels in TucaR models is probably due to heterodimerization of the amplified EGFR with HER2 and subsequent HER2 phosphorylation. In contrast to the P cells where the apoptotic marker cleaved (c)-PARP was not induced with G alone (50, 500nM), but with T (200nM) or 500nM G+T, in the TucaR model, 500nM G alone was enough to induce c-PARP, which was further enhanced when combined with T, implying the survival dependence of TucaR cells on EGFR signaling. The TucaR models were hypersensitive to G compared to P cells, and this growth inhibition was further enhanced with G+T. Whilst we previously reported that the LapR and NrbR cells were cross-resistant to T, the TucaR cells remained highly sensitive to the pan-HER TKIs N, poziotinib, and pyrotinib. Finally, in a second HER2+ model SKBR3, at 200nM TucaR, we observed elevated pEGFR, pHER2, pHER3, and pAKT levels, the underlying mechanism of which is under investigation by genomic and molecular analysis. In-depth characterization of our TucaR models to determine the differential gene expression and signatures is ongoing to gain additional mechanistic insights. Conclusions: Our findings suggest that whilst complete blockade of the HER layer using N is evaded by acquisition of HER and PIK3CA mutations, resistance to the HER2 TKI T is associated with EGFR amplification, a finding that underscores the HER signaling pathway redundancy and cross-talk between HER receptors to compensate for partial blockade of the pathway. Further, while resistance to L and N confers cross-resistance to T, resistance to T may be overcome using pan-HER TKIs or the combination of potent EGFR and HER2 inhibitors. Together, our findings hold crucial implications in light of the current treatment landscape of HER2+ BC, with a particular emphasis on the considerations to strategize the treatment sequence of currently available TKIs.

Citation Format: Jamunarani Veeraraghavan, Sreyashree Bose, Ragini Mistry, Pier Selenica, Sarmistha Nanda, Lanfang Qin, Andrea Gazzo, Yingjie Zhu, Michael A Mancini, Fabio Stossi, Britta Weigelt, Jorge S Reis-Filho, C. Kent Osborne, Mothaffar F Rimawi, Rachel Schiff. Acquired resistance to tucatinib is associated with EGFR amplification in HER2+ breast cancer (BC) models and can be overcome by a more complete blockade of HER receptor layer [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr PD8-06.

Abstract P4-01-01: Resistance to next generation tyrosine kinase inhibitors (TKIs) in HER2-positive breast cancer (BC): Role of HER and PIK3CA mutations and development of new treatment strategies and study models

Background: We recently reported that acquired resistance to the dual HER1/2 TKI lapatinib (Lap) was mediated by HER2 L755S, while resistance to the pan-HER TKI neratinib (Nrb) was associated with co-acquisition of an additional pathogenic PIK3CA mutation. Though the role of HER2 mutations is gaining attention in HER2-positive (+) BC, less is known about their role and clinical implications in next generation TKI resistance, particularly when co-occurring with PIK3CA mutations in HER2+ BC. Investigating optimal treatment combinations and the development of new clinically relevant 3D models are warranted.Materials and Methods: HER2+ BT474 parental (P) cells and models with acquired resistance to Lap (LapR) and Nrb (NrbR) (SABCS20-PD3-09) were used. Xenografts established in mice using P, LapR, and NrbR cells and 3D organoids derived from these xenografts using the Hans Clevers (HC, PMID 29224780) or Mark Burkard (MB, PMID 31175091) method were characterized by qRT-PCR and western blot. Drug efficacy was assessed by growth changes in 2D and 3D models using the IncuCyte system or by microscopy-based analysis. Results: We previously showed that Lap and Nrb resistance confers cross-resistance to tucatinib (Tuca) and trastuzumab, and that targeting the HER and downstream PI3K pathway, especially using small molecule agents that are key for treatment of CNS lesions, is effective only in combination with Nrb or poziotinib (Pozio), but not Tuca. Our new studies revealed that the MEK inhibitor (i) AZD6244 (selumetinib; Sel), mTORi everolimus (Eve), and selective estrogen receptor degrader fulvestrant (Ful) were not effective as single agents in inhibiting the growth of either LapR or NrbR models. Whilst the LapR cells were highly sensitive to the irreversible HER1/2 TKI afatinib (Afa) and the irreversible dual/pan-HER TKI pyrotinib (Pyro) as single agents, the NrbR models were cross-resistant to both TKIs, highlighting the importance of the co-occurring PIK3CA mutation in resistance. Interestingly, Afa and Pyro were only partly effective when combined with Eve+Ful, Sel+Eve, or Sel+the PIK3CAi alpelisib in inhibiting NrbR growth. Consistent with our previously reported findings for Nrb and Pozio, Pyro was highly effective with TDM1. As opposed to the P xenografts, the LapR and NrbR tumors grew in the presence of the respective TKI, confirming their resistant phenotype in vivo. P and resistant xenograft-derived organoids (XDOs) were successfully established using the HC but not MB method, but the HC-derived XDOs were subsequently grown in MB condition and used for molecular and functional studies. Preliminary characterization showed that the LapR tumors and XDOs harbor HER2 L755S, whereas the NrbR tumors and XDOs also have a concomitant PIK3CA E542V mutation, findings that are in line with our 2D results, suggesting that the xenografts and XDOs retain and recapitulate the molecular profile of their 2D or tumor counterparts. Early drug efficacy studies indicate that, akin to the 2D models, the LapR XDOs are highly sensitive to Nrb, whereas both the LapR and NrbR XDOs exhibit cross-resistance to Tuca but remain sensitive to Pozio.Conclusions: Our data suggest that the potency of next generation irreversible HER TKIs in HER2+ BC may be challenged by the emergence of mutations in HER2, together with other co-occurring downstream mutations, such as PIK3CA. Our findings present a clear roadmap for the development of combinatorial therapies that should be individualized for patients with HER2+ BC. Our newly developed XDO strategy may offer a new platform to confirm and prioritize optimal drug combinations to overcome this resistance and may facilitate the near future development of patient-derived organoids for precision medicine of resistant HER2+ BC.

Citation Format: Sreyashree Bose, Ragini Mistry, Chia Chia Liu, Sarmistha Nanda, Lanfang Qin, Pier Selenica, Andrea Gazzo, Yingjie Zhu, Michael A. Mancini, Fabio Stossi, Irmina Diala, Lisa D. Eli, Britta Weigelt, Jorge S. Reis-Filho, Mothaffar F. Rimawi, C. Kent Osborne, Rachel Schiff, Jamunarani Veeraraghavan. Resistance to next generation tyrosine kinase inhibitors (TKIs) in HER2-positive breast cancer (BC): Role of HER and PIK3CA mutations and development of new treatment strategies and study models [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr P4-01-01.

Activation of the IFN Signaling Pathway is Associated with Resistance to CDK4/6 Inhibitors and Immune Checkpoint Activation in ER-Positive Breast Cancer

PURPOSE

Cyclin-dependent kinase 4 (CDK4) and CDK6 inhibitors (CDK4/6i) are highly effective against estrogen receptor-positive (ER⁺)/HER2^- breast cancer; however, intrinsic and acquired resistance is common. Elucidating the molecular features of sensitivity and resistance to CDK4/6i may lead to identification of predictive biomarkers and novel therapeutic targets, paving the way toward improving patient outcomes.

EXPERIMENTAL DESIGN

Parental breast cancer cells and their endocrine-resistant derivatives (EndoR) were used. Derivatives with acquired resistance to palbociclib (PalboR) were generated from parental and estrogen deprivation-resistant MCF7 and T47D cells. Transcriptomic and proteomic analyses were performed in palbociclib-sensitive and PalboR lines. Gene expression data from CDK4/6i neoadjuvant trials and publicly available datasets were interrogated for correlations of gene signatures and patient outcomes.

RESULTS

Parental and EndoR breast cancer lines showed varying degrees of sensitivity to palbociclib. Transcriptomic analysis of these cell lines identified an association between high IFN signaling and reduced CDK4/6i sensitivity; thus an "IFN-related palbociclib-resistance Signature" (IRPS) was derived. In two neoadjuvant trials of CDK4/6i plus endocrine therapy, IRPS and other IFN-related signatures were highly enriched in patients with tumors exhibiting intrinsic resistance to CDK4/6i. PalboR derivatives displayed dramatic activation of IFN/STAT1 signaling compared with their short-term treated or untreated counterparts. In primary ER⁺/HER2^- tumors, the IRPS score was significantly higher in lumB than lumA subtype and correlated with increased gene expression of immune checkpoints, endocrine resistance, and poor prognosis.

CONCLUSIONS

Aberrant IFN signaling is associated with intrinsic resistance to CDK4/6i. Experimentally, acquired resistance to palbociclib is associated with activation of the IFN pathway, warranting additional studies to clarify its involvement in resistance to CDK4/6i.

Abstract 1077: Acquired neratinib resistance is associated with acquisition of HER2 and PIK3CA mutations and can be overcome using potent drug combinations in HER2-positive breast cancer models

The role of HER2 and PIK3CA mutations in anti-HER2 resistance is gaining more importance in HER2-positive (+) breast cancer. We recently reported that acquired resistance to lapatinib (Lap)-containing regimens is mediated by HER2 L755S, which could be overcome using the irreversible pan-HER tyrosine kinase inhibitor (TKI) neratinib (Nrb). However, less is known about the role of L755S in resistance to next-generation TKIs, particularly when co-occurring with PIK3CA mutations. HER2+ BT474 cell models with primary or sequential acquired resistance (R) to Lap (LapR) or Nrb (NrbR) and their parental (P) counterparts were profiled for alterations in signaling and gene expression by RPPA, western blot, and RNA-seq. For drug efficacy studies, change in cell growth was assessed using imaging-based high-throughput system. Proteomic profiling revealed partial restoration of HER2 phosphorylation and downstream signaling in the LapR and NrbR derivatives. RNA-seq analysis showed that the LapR and NrbR models, but not P cells, harbor HER2 L755S mutation. Importantly, the NrbR but not LapR cells also co-acquire a PIK3CA pathogenic mutation. GSEA analysis of RNA-seq data showed significant downregulation of G2/M checkpoint in the R derivatives compared to P cells, suggesting genetic instability. In line with the presence of HER2 and PIK3CA activating mutations and HER pathway reactivation in the R models, GSEA revealed an enrichment of mTORC1 and KRAS signaling in the R cells. Furthermore, enrichment of epithelial mesenchymal transition signature and downregulation of apical surface genes was observed in the R models compared to P cells, suggestive of their aggressive phenotype. Interestingly, the LapR cells remained sensitive to Nrb, though a higher dose (IC50: ~50nM) was required compared to P cells (IC50: ~2nM). The LapR and NrbR cells were cross-resistant to the HER2-selective TKI tucatinib, and trastuzumab. We recently showed that the NrbR cells were either partially or completely sensitive to poziotinib or TDM1, respectively, suggesting their therapeutic promise against HER2- and PIK3CA-mutant tumors. Of note, our studies using small molecule agents targeting HER and its downstream pathway to facilitate treatment of CNS lesions suggest that AKT or mutant PIK3CA inhibitors are effective only when combined with either neratinib or poziotinib, but not tucatinib, findings which we are currently expanding to xenograft-derived organoids. Overall, our findings suggest a complex disease evolution upon resistance to neratinib but indicate their potentially continued efficacy in overcoming resistance through drug combinations. Ongoing integrative omics analysis to determine the genomic and mutational complexity and landscape will uncover additional mechanistic insights and guide the discovery of other actionable targets.

Citation Format: Jamunarani Veeraraghavan, Ragini Mistry, Sarmistha Nanda, Sreyashree Bose, Chia Chia Liu, Vidyalakshmi Sethunath, Martin J. Shea, Tamika Mitchell, Meenakshi Anurag, Michael A. Mancini, Irmina Diala, Alshad S. Lalani, Fabio Stossi, C. Kent Osborne, Mothaffar F. Rimawi, Rachel Schiff. Acquired neratinib resistance is associated with acquisition of HER2 and PIK3CA mutations and can be overcome using potent drug combinations in HER2-positive breast cancer models [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 1077.

Neratinib plus trastuzumab is superior to pertuzumab plus trastuzumab in HER2-positive breast cancer xenograft models

Lapatinib (L) plus trastuzumab (T), with endocrine therapy for estrogen receptor (ER)+ tumors, but without chemotherapy, yielded meaningful response in HER2+ breast cancer (BC) neoadjuvant trials. The irreversible/pan-HER inhibitor neratinib (N) has proven more potent than L. However, the efficacy of N+T in comparison to pertuzumab (P) + T or L + T (without chemotherapy) remains less studied. To address this, mice bearing HER2+ BT474-AZ (ER+) cell and BCM-3963 patient-derived BC xenografts were randomized to vehicle, N, T, P, N+T, or P+T, with simultaneous estrogen deprivation for BT474-AZ. Time to tumor regression/progression and incidence/time to complete response (CR) were determined. Changes in key HER pathway and proliferative markers were assessed by immunohistochemistry and western blot of short-term-treated tumors. In the BT474-AZ model, while all N, P, T, N + T, and P + T treated tumors regressed, N + T-treated tumors regressed faster than P, T, and P + T. Further, N + T was superior to N and T alone in accelerating CR. In the BCM-3963 model, which was refractory to T, P, and P + T, while N and N + T yielded 100% CR, N + T accelerated the CR compared to N. Ki67, phosphorylated (p) AKT, pS6, and pERK levels were largely inhibited by N and N + T, but not by T, P, or P + T. Phosphorylated HER receptor levels were also markedly inhibited by N and N + T, but not by P + T or L + T. Our findings establish the efficacy of combining N with T and support clinical testing to investigate the efficacy of N + T with or without chemotherapy in the neoadjuvant setting for HER2+ BC.

Abstract PS5-29: Insights into the molecular underpinnings of the mevalonate pathway-YAP/TAZ-driven anti-HER2 therapy resistance in HER2+ breast cancer (BC)

Background. We recently reported that the biosynthetic mevalonate (MVA) pathway that produces cholesterol and isoprenoid intermediates, regulates the YAP/TAZ (Y/T) transcriptional co-activators to promote resistance to treatment regimens that effectively inhibit HER2 signaling in HER2+ BC. We further showed that the mTORC1 complex and survivin protein, which contribute to HER2-elicited oncogenic signaling in HER2-driven BC cells, are alternatively activated by the MVA pathway-Y/T axis in anti-HER2 therapy resistant cell models. Of note, other recent reports also showed that increased Y/T activity enables resistant cells to proliferate when other oncogenic pathways (e.g., RAS, EGFR, and FGFR) were effectively inhibited. Here, we sought to further determine the molecular underpinnings of MVA-Y/T axis-driven anti-HER2 therapy resistance to discover novel therapeutic targets and predictive biomarkers for HER2+ BC.Methods. SKBR3 HER2+ BC parental (P) cells and their lapatinib plus trastuzumab (LT) resistant (LTR) derivatives with sustained HER2 inhibition were treated with the MVA pathway inhibitor simvastatin (Sim), with or without the MVA metabolite to rescue Sim’s inhibitory effects. P and LTR cells were also transfected with control or combined Y/T siRNAs. The transcriptomes of all treatment groups were assessed by RNA-seq. Integrative bioinformatics analyses were used to identify differentially expressed (DE) genes and gene sets with functional annotations in LTR vs. P cells upon different interventions.Results. We found that cell cycle and cell proliferation processes were among the top common DE molecular signatures preferentially downregulated (DN) in LTR vs. P cells by both Sim and Y/T knockdown (KD). The top common genes preferentially DN in LTR vs. P cells include the cell cycle regulatory genes CDCA3 and ERCC6L, and the nucleotide metabolism genes TYMS and RRM2. Interestingly, 20% of the genes preferentially DN in LTR vs. P cells by Sim or Y/T KD were predicted to be direct Y/T transcriptional targets based on previously reported ChIP-seq data (PMID: 26258633). Of the Y/T-dependent genes, a significant enrichment of Sim-repressed genes was observed in both P and LTR cells (P = 1.2e-115 and P = 5.5e-138, respectively). The proportion of these enriched genes was higher in LTR vs. P cells (61% vs. 29%). Of note, we found that the global inhibited genes in LTR cells upon Sim or Y/T KD were significantly enriched for the genes DN by short-term LT treatment in P cells (P &lt; 2.2e-16). Likewise, of the genes nominated as putative molecular players in the MVA pathway-Y/T-mediated resistance, BIRC5 (survivin), CDC6, KIF2C, RRM2, and TYMS were recently also reported to be DN in HER2+ tumors treated with neo-adjuvant LT in the PAMELA trial (NCT01973660), a finding that is in line with what we observed in our P cells treated with short-term LT. Conclusions. Upon acquisition of resistance to sustained HER2 inhibition, the MVA pathway-Y/T axis takes over the regulation of pro-proliferative transcriptional programs that are generally downstream of HER2 signaling in treatment-naïve HER2+ BC. The MVA pathway-Y/T axis leads to Y/T-driven transcriptional reprogramming, an emerging mechanism of therapy resistance to anti-HER2 and other targeted therapies that warrants further investigation. The identification of multiple cell cycle related processes as putative targets of the MVA pathway-Y/T axis presents additional targetable vulnerabilities and implies that inhibitors of Y/T and cell cycle checkpoints may help circumvent anti-HER2 resistance in the clinical setting.

Citation Format: Vidyalakshmi Sethunath, Xiaoyong Fu, Pamela L Luna, Sarmistha Nanda, Martin Shea, Jamunarani Veeraraghavan, Carmine De Angelis, Huizhong Hu, Chad Shaw, Mothaffar Rimawi, C. Kent Osborne, Rachel Schiff. Insights into the molecular underpinnings of the mevalonate pathway-YAP/TAZ-driven anti-HER2 therapy resistance in HER2+ breast cancer (BC) [abstract]. In: Proceedings of the 2020 San Antonio Breast Cancer Virtual Symposium; 2020 Dec 8-11; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2021;81(4 Suppl):Abstract nr PS5-29.

Abstract PD3-09: HER2 L755S mutation is acquired upon resistance to lapatinib and neratinib and confers cross-resistance to tucatinib and trastuzumab in HER2-positive breast cancer cell models

Background: The role of HER2 mutations in anti-HER2 resistance is gaining more importance in HER2-positive (+) breast cancer (BC). The common HER2 L755S mutation is further enriched in metastatic lesions compared to primary tumors. Despite their mounting significance, effective therapies for HER2-amplified/mutant tumors are lacking. We recently reported that acquired resistance to lapatinib (Lap)-containing regimens is mediated by HER2 L755S, which could be overcome by the irreversible pan-HER tyrosine kinase inhibitor (TKI) neratinib (Nrb). However, less is known about the role of L755S in resistance to new generation TKIs, and the clinically implementable therapeutic strategies to overcome it. Materials and Methods: Our recently developed HER2+ BT474 cell models with acquired resistance to Lap (LapR) or Nrb (NrbR), developed through long-term exposure to increasing doses of the respective drug, and their naïve parental (P) counterparts were used. The resistant derivatives and their cognate P cells were subjected to proteomic (Reverse phase protein array (RPPA) and western blot) and transcriptomic (RNA-seq) characterization. For drug efficacy studies, change in cell growth was assessed using the in situ imaging-based high-throughput IncuCyte system. Results: Proteomic profiling of the resistant models and their P equivalents revealed partial restoration of HER2 phosphorylation and downstream signaling in the LapR and NrbR derivatives. Consistent with activated mTOR signaling observed in the resistant cells, we detected reduced levels of phospho (p)-RAPTOR S792, which is otherwise essential to inhibit the mTOR complex 1 (mTORC1). In addition, p-P38MAPK T180/Y182 levels were reduced. RNA-seq analysis revealed the presence of HER2 L755S mutation in the LapR and NrbR derivatives, but not in P cells, suggesting that the HER signaling reactivation could be attributed to acquisition of HER2 L755S. Interestingly, the NrbR cells co-harbor other pathogenic mutations in key BC related genes, the therapeutic and functional significance of which is being investigated. Importantly, the NrbR derivatives were cross-resistant to Lap and the monoclonal antibody trastuzumab (T). Next, we determined the efficacy of Nrb and the HER2-selective TKI tucatinib (Tuca), both recently approved for metastatic HER2+ BC, either alone or in combination with T. Nrb effectively inhibited the growth of LapR cells, although a higher dose (IC50: ~50nM) was required to inhibit the growth compared to that needed for naïve P cells (~IC50: ~2nM). When combined with T, Nrb was effective in inhibiting the LapR cell growth, though the inhibitory effect may very well be driven entirely by Nrb. On the other hand, the resistant derivatives were cross-resistant to Tuca, both as a single-agent and in combination with T. We then evaluated the efficacy of the antibody drug conjugate TDM1 and the irreversible pan-HER TKI poziotinib. In contrast to the high sensitivity of P cells to both these agents, a spectrum of effect was observed in the NrbR derivatives, with responses ranging from partial growth inhibition by poziotinib to complete response with TDM1, suggesting their therapeutic potential against tumors harboring HER2 mutations. Conclusions: Our findings suggest that HER2 mutations, particularly HER2 L755S, that emerge under the pressure of potent HER2-targeted therapy may confer cross-resistance to other single agent or combination HER2-targeted therapy. This holds important therapeutic implications in light of current treatment landscape. An in-depth molecular characterization of our resistant models to determine the differential gene expression and mutational profile is ongoing to gain additional mechanistic insights and to guide discovery of other actionable targets.

Citation Format: Jamunarani Veeraraghavan, Ragini Mistry, Sarmistha Nanda, Vidyalakshmi Sethunath, Martin Shea, Tamika Mitchell, Meenakshi Anurag, Michael A. Mancini, Fabio Stossi, C. Kent Osborne, Mothaffar F. Rimawi, Rachel Schiff. HER2 L755S mutation is acquired upon resistance to lapatinib and neratinib and confers cross-resistance to tucatinib and trastuzumab in HER2-positive breast cancer cell models [abstract]. In: Proceedings of the 2020 San Antonio Breast Cancer Virtual Symposium; 2020 Dec 8-11; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2021;81(4 Suppl):Abstract nr PD3-09.

Abstract SP139: HER2 targeted therapy: Determinants of response and mechanisms of resistance

HER2-positive (+) breast cancer (BC), accounting for 15-20% of all BCs, is characterized by overexpression, mostly via gene amplification, of HER2. HER2 is a key member of the HER family of 4 tyrosine kinase receptors. Multiple clinically available HER2-targeted therapies, including monoclonal antibodies, tyrosine kinase inhibitors (TKIs), and antibody-drug conjugates have revolutionized the outcome of patients with HER2+ BC. Despite these effective therapies, intrinsic and acquired resistance still occurs, posing a major challenge in the clinical management of this disease. A better understanding of the determinants of response and mechanisms of resistance may help develop personalized treatment approaches and new strategies to overcome resistance. Tumors that are truly addicted to HER2, clinically reflected especially under chemotherapy-sparing HER2-targeted therapy regimens, are associated with high and homogeneous levels of HER2 gene amplification, protein, and activity. Even in these HER2-addicted tumors, the efficacy of anti-HER2 therapy can be jeopardized by deregulations in the downstream PI3K pathway (e.g., PIK3CA mutations), which can lead to constitutive activation of the PI3K/AKT pathway and resistance. Given the functional redundancy of signaling from multiple HER receptor dimers and compensatory signaling within the pathway, dual anti-HER2 therapy has proven superior to single agents in achieving a more comprehensive blockade of the entire HER receptor layer and in anti-tumor efficacy. Further, in the HER2+ tumors that co-express ER, an unblocked, re-expressed and/or reactivated ER signaling can provide alternative proliferative and survival signals to evade sustained HER2 blockade, thus underscoring the need for concurrent blockade of HER2 and ER signaling. Nevertheless, effective inhibition of HER2 might prove challenging in some cases due to molecular masking of the HER receptors (e.g., mucins) or due to the de novo presence or acquisition of genetic, epigenetic or post-translational alterations in HER2 itself, including activating HER2 mutations (e.g., L755S), and p95HER2. We recently reported that acquired resistance to HER2-targeted therapy, especially TKIs, is mediated by the common HER2 L755S mutation, the clinical importance of which is underscored by the observation that this and other HER mutations are further enriched in the metastatic lesions compared to primary HER2+ tumors. On the other hand, when HER2 does remain effectively inhibited under potent HER2-targeted therapy, resistance can arise due to the upregulation of alternative escape pathways that transmit proliferative stimuli. These include activation of other receptor tyrosine kinases (e.g., AXL, FGFR), other downstream/intracellular signaling (e.g., SRC, YES1), or metabolic pathways (e.g., FASN and mevalonate pathways). Our recent data suggest that the mevalonate pathway offers an escape mechanism by providing alternative signaling through the YAP/TAZ-mTORC1-survivin axis to activate a transcriptional program that promotes resistant cell proliferation and survival, which can be overcome using inhibitors of mevalonate pathway (e.g., statins). Importantly, activation of the key cell cycle regulator cyclin D1/CDK4 complex has been shown to mediate resistance to HER2-targeted therapy and that CDK4/6 inhibitors, at least partly by also inhibiting mTORC1 activity, can overcome this resistance. Finally, the role of tumor microenvironment, including host immune components (e.g., TILs) and extracellular matrix components signaling via integrins, have been shown to play a role in modulating tumor response to treatment and in resistance. Together, these findings suggest new strategies to enhance sensitivity and overcome resistance to HER2-targeted therapy, some of which are already under clinical development.

Citation Format: R Schiff, J Veeraraghavan, C De Angelis, C Osborne, MF Rimawi. HER2 targeted therapy: Determinants of response and mechanisms of resistance [abstract]. In: Proceedings of the 2020 San Antonio Breast Cancer Virtual Symposium; 2020 Dec 8-11; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2021;81(4 Suppl):Abstract nr SP139.

Therapeutic Targeting of Nemo-like Kinase in Primary and Acquired Endocrine-resistant Breast Cancer

PURPOSE

Endocrine resistance remains a major clinical challenge in estrogen receptor (ER)-positive breast cancer. Despite the encouraging results from clinical trials for the drugs targeting known survival signaling, relapse is still inevitable. There is an unmet need to discover new drug targets in the unknown escape pathways. Here, we report Nemo-like kinase (NLK) as a new actionable kinase target that endows previously uncharacterized survival signaling in endocrine-resistant breast cancer.

EXPERIMENTAL DESIGN

The effects of NLK inhibition on the viability of endocrine-resistant breast cancer cell lines were examined by MTS assay. The effect of VX-702 on NLK activity was verified by kinase assay. The modulation of ER and its coactivator, SRC-3, by NLK was examined by immunoprecipitation, kinase assay, luciferase assay, and RNA sequencing. The therapeutic effects of VX-702 and everolimus were tested on cell line- and patient-derived xenograft (PDX) tumor models.

RESULTS

NLK overexpression endows reduced endocrine responsiveness and is associated with worse outcome of patients treated with tamoxifen. Mechanistically, NLK may function, at least in part, via enhancing the phosphorylation of ERα and its key coactivator, SRC-3, to modulate ERα transcriptional activity. Through interrogation of a kinase profiling database, we uncovered and verified a highly selective dual p38/NLK inhibitor, VX-702. Coadministration of VX-702 with the mTOR inhibitor, everolimus, demonstrated a significant therapeutic effect in cell line-derived xenograft and PDX tumor models of acquired or de novo endocrine resistance.

CONCLUSIONS

Together, this study reveals the potential of therapeutic modulation of NLK for the management of the endocrine-resistant breast cancers with active NLK signaling.

A Novel Neoplastic Fusion Transcript, RAD51AP1-DYRK4, Confers Sensitivity to the MEK Inhibitor Trametinib in Aggressive Breast Cancers

PURPOSE

Luminal B breast tumors are more aggressive estrogen receptor-positive (ER+) breast cancers characterized by aggressive clinical behavior and a high risk of metastatic dissemination. The underlying pathologic molecular events remain poorly understood with a paucity of actionable genetic drivers, which hinders the development of new treatment strategies.

EXPERIMENTAL DESIGN

We performed large-scale RNA sequencing analysis to identify chimerical transcripts preferentially expressed in luminal B breast cancer. The lead candidate was validated by reverse transcription PCR in breast cancer tissues. The effects of inducible ectopic expression or genetic silencing were assessed by phenotypic assays such as MTS, transwell, and transendothelial migration assays, and by clonogenic assays to assess MEK inhibitor sensitivity. Subcellular fractionation, Western blots, and immunoprecipitation were performed to characterize the protein products and elucidate the engaged mechanisms.

RESULTS

Here we report a novel tumor-specific chimeric transcript RAD51AP1-DYRK4 preferentially expressed in luminal B tumors. Analysis of 200 ER+ breast tumors detected RAD51AP1-DYRK4 overexpression in 19 tumors (9.5%), which is markedly enriched in the luminal B tumors (17.5%). Ectopic expression of RAD51AP1-DYRK4, but not wild-type RAD51AP1, leads to marked activation of MEK/ERK signaling, and endows increased cell motility and transendothelial migration. More importantly, RAD51AP1-DYRK4 appears to endow increased sensitivity to the MEK inhibitor trametinib through attenuating compensatory activation of HER2/PI3K/AKT under MEK inhibition.

CONCLUSIONS

This discovery sheds light on a new area of molecular pathobiology of luminal B tumors and implies potential new therapeutic opportunities for more aggressive breast tumors overexpressing this fusion.

Abstract 1911: HER2 L755S mutation is associated with acquired resistance to lapatinib and neratinib, and confers cross-resistance to tucatinib in HER2-positive breast cancer models

Despite the availability of potent HER-targeted agents, de novo and acquired resistance is common and continues to pose a major challenge, especially in the advanced setting. Amassing evidence point to the importance of HER2 mutations, including the most common HER2 L755S mutation, in mediating anti-HER2 resistance. The HER2 L755S mutation, in particular, is observed to be enriched in metastatic lesions compared to primary breast tumors. The need for effective therapy to treat tumors harboring HER2 mutations prevails. We have previously reported that acquired resistance to lapatinib (L)-containing treatments, mediated by HER2 L755S, could be overcome by the recently FDA-approved irreversible pan-HER tyrosine kinase inhibitor (TKI) neratinib (N). While N has shown great promise in patients with HER2-mutant metastatic breast cancer, its efficacy is somewhat limited. More recently, tucatinib, a HER2-selective TKI, has been shown to be effective in HER2-positive (+) brain metastases. Its potency in the context of HER2 mutations, however, has not yet been fully studied. In this study, we used the HER2+ BT474-L resistant (LR) cells, harboring endogenous HER2 L755S mutation, and parental (P) cells to first determine whether tucatinib may be effective in overcoming resistance mediated by HER2 mutations. Our results showed that while N effectively inhibited the growth of LR cells, although at a dose higher than that needed to inhibit the growth of naïve P cells, tucatinib failed to inhibit the growth of LR cells. Our results suggest that HER2 L755S mutation may confer cross-resistance to tucatinib. To further study mechanisms of resistance to 2nd generation anti-HER2 agents, we recently developed cell models with acquired resistance to N, through long-term exposure of the BT474-P and LR cells to increasing doses of N. These cells were profiled by reverse phase protein array (RPPA) and western blot analysis, which revealed restoration of HER2 phosphorylation in the NR derivatives, despite being cultured in the continuous presence of N. Interestingly, RNA-seq analysis revealed the presence of HER2 L755S mutation in all the NR derivatives, but not in the P cells, suggesting that the reactivated HER2 signaling observed in NR cells could be attributed to the emergence/acquisition of HER2 L755S mutation. Furthermore, while the P cells were highly sensitive to tucatinib, L, and the monoclonal antibody trastuzumab (T), the NR derivatives were totally resistant to these agents, suggesting that N resistance may also confer cross-resistance to tucatinib, L, and T. Additional molecular characterization to examine differential gene expression and mutational profile of the resistant derivatives, as well as testing of novel anti-HER2 regimens and drug combinations targeting downstream mediators to overcome resistance, both in vitro and in vivo, is ongoing.

Citation Format: Jamunarani Veeraraghavan, Ragini Mistry, Sarmistha Nanda, Vidyalakshmi Sethunath, Martin Shea, Tamika Mitchell, Meenakshi Anurag, Michael A. Mancini, Fabio Stossi, C. Kent Osborne, Mothaffar F. Rimawi, Rachel Schiff. HER2 L755S mutation is associated with acquired resistance to lapatinib and neratinib, and confers cross-resistance to tucatinib in HER2-positive breast cancer models [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 1911.

Therapeutic role of recurrent ESR1-CCDC170 gene fusions in breast cancer endocrine resistance

Background

Endocrine therapy is the most common treatment for estrogen receptor (ER)-positive breast cancer, but its effectiveness is limited by high rates of primary and acquired resistance. There are likely many genetic causes, and recent studies suggest the important role of ESR1 mutations and fusions in endocrine resistance. Previously, we reported a recurrent ESR1 fusion called ESR1-CCDC170 in 6–8% of the luminal B breast cancers that has a worse clinical outcome after endocrine therapy. Despite being the most frequent ESR1 fusion, its functional role in endocrine resistance has not been studied in vivo, and the engaged mechanism and therapeutic relevance remain uncharacterized.

Methods

The endocrine sensitivities of HCC1428 or T47D breast cancer cells following genetic perturbations of ESR1-CCDC170 were assessed using clonogenic assays and/or xenograft mouse models. The underlying mechanisms were investigated by reverse phase protein array, western blotting, immunoprecipitation, and bimolecular fluorescence complementation assays. The sensitivity of ESR1-CCDC170 expressing breast cancer cells to concomitant treatments of tamoxifen and HER/SRC inhibitors was assessed by clonogenic assays.

Results

Our results suggested that different ESR1-CCDC170 fusions endow different levels of reduced endocrine sensitivity in vivo, resulting in significant survival disadvantages. Further investigation revealed a novel mechanism that ESR1-CCDC170 binds to HER2/HER3/SRC and activates SRC/PI3K/AKT signaling. Silencing of ESR1-CCDC170 in the fusion-positive cell line, HCC1428, downregulates HER2/HER3, represses pSRC/pAKT, and improves endocrine sensitivity. More important, breast cancer cells expressing ectopic or endogenous ESR1-CCDC170 are highly sensitive to treatment regimens combining endocrine agents with the HER2 inhibitor lapatinib and/or the SRC inhibitor dasatinib.

Conclusion

ESR1-CCDC170 may endow breast cancer cell survival under endocrine therapy via maintaining/activating HER2/HER3/SRC/AKT signaling which implies a potential therapeutic strategy for managing these fusion positive tumors.

A multiparameter classifier to predict response to lapatinib plus trastuzumab (LT) without chemotherapy in HER2+ breast cancer (BC)

1011

Background: Several trials have shown 25-30% pathologic complete response (pCR) rates in patients with HER2+ BC treated with LT therapy (+ endocrine therapy if ER+), but no chemotherapy (CTX). We hypothesize that a multiparameter classifier, comprised of HER2 gene and protein levels, intratumor heterogeneity (ITH), HER2-enriched (E) subtype, and PIK3CA mutation status can identify patients whose tumors are “addicted” to HER2 signaling and are likely to achieve pCR from a CTX-sparing de-escalation strategy. Methods: Baseline specimens from 2 trials (TBCRC023 [NCT00999804] and PAMELA [NCT01973660]) of neoadjuvant CTX-sparing LT (+ endocrine therapy if ER+) in HER2+ BC were used. HER2 protein and ITH (scored for % 3+ by IHC), and gene amplification (HER2:CEP17 ratio and copy number (CN) by CISH) were measured on the same slide by the dual gene protein assay (GPA). HER2-E and PIK3CA mutation status were assessed by research-based PAM50 and MSK-IMPACT platforms, respectively. A decision tree algorithm was used to determine the GPA cutoffs and to construct the classifier of response (by pCR) in TBCRC023, which was then validated in PAMELA. Results: Of the evaluable patients from TBCRC023 (N = 130) and PAMELA (N = 151), GPA data were available for 121 and 94 cases, respectively. Both cohorts exhibited similar distributions for HER2 ratio, CN, and % 3+, and a strong correlation between HER2 ratio and CN (R > 0.92). In TBCRC023, 73 cases had data from GPA, PAM50, and IMPACT, of which 15 had pCR. Recursive partitioning identified cutoffs of HER2 ratio > 4.6 and % 3+ > 97.5% in both the GPA data cohort (N = 121) and complete data cohort (N = 73). With PAM50 and IMPACT data, the model added HER2-E and PIK3CA wild-type (wt). For practical reasons, the classifier was locked as HER2 ratio ≥ 4.5 AND % 3+ ≥ 90% AND PIK3CA-wt AND HER2-E, which yielded a PPV of 55% and NPV of 94%. Validation in PAMELA using 45 cases with data for all 3 assays yielded PPV of 44% and NPV of 82%. 29 TBCRC023 cases without IMPACT data could be predicted to be non-pCR, of which 26 were correct (NPV = 89%). In PAMELA, 66 additional cases could be predicted to be non-pCR, of which 54 were correct (NPV = 81%). Conclusions: We have constructed a multiparameter classifier that can predict pCR with targeted therapy alone that compare to pCR rates of CTX + dual anti-HER2 in unselected patients. Prospective validation in a clinical trial is warranted. [Table: see text]

Towards personalized treatment for early stage HER2-positive breast cancer

Advances in HER2-targeted therapies have improved the survival of patients with HER2-positive breast cancer. The standard-of-care treatment for localized disease has been chemotherapy and 1 year of adjuvant HER2-targeted therapy, typically with the anti-HER2 antibody trastuzumab. Despite the effectiveness of this treatment, disease relapse occurs in a subset of patients; thus, focus has been placed on escalating treatment by either combining different HER2-targeted agents or extending the duration of HER2-targeted therapy. Indeed, dual HER2-targeted therapies and extended-duration anti-HER2 therapy, as well as adjuvant therapy with the anti-HER2 antibody-drug conjugate T-DM1, have all been approved for clinical use. Emerging evidence suggests, however, that some patients do not derive sufficient benefit from these additional therapies to offset the associated toxicities and/or costs. Similarly, the universal use of chemotherapy might not benefit all patients, and treatment de-escalation through omission of chemotherapy has shown promise in clinical trials and is currently being explored further. The future of precision medicine should therefore involve tailoring of therapy based on the genetics and biology of each tumour and the clinical characteristics of each patient. Predictive biomarkers that enable the identification of patients who will benefit from either escalated or de-escalated treatment will be crucial to this approach. In this Review, we summarize the available HER2-targeted agents and associated mechanisms of resistance, and describe the current therapeutic landscape of early stage HER2-positive breast cancer, focusing on strategies for treatment escalation or de-escalation.

Abstract PD7-01: Identification of a high FOXA1-induced pro-metastatic enhancer signature in endocrine-resistant and metastatic breast cancer

Background: Forkhead box A1 (FOXA1) is a pioneer transcription factor (TF) for chromatin binding and function of other lineage-specific TFs essential for the normal development of endoderm-derived organs. Aberrant FOXA1 signaling, due to genetic amplification or mutations and/or overexpression, has been frequently detected in metastatic tumors of the breast, prostate, pancreas, bladder and thyroid, suggesting a general role and mechanism of FOXA1-driven tumorigenesis and disease progression. We recently reported that high levels of FOXA1 (H-FOXA1) promote endocrine-resistant (EndoR) and metastatic phenotypes in estrogen receptor (ER)+ breast cancer (BC) cells. Here we sought to uncover the role and the mechanisms by which H-FOXA1 promotes EndoR metastatic BC.

Methods: Genomic sequencing data from an ER+/HER2- metastatic BC cohort (n=781, MSK-IMPACT; cBioportal) were used to compare mutations and copy number alterations of FOXA1 and ESR1. Genome-wide FOXA1-chromatin binding (cistrome) and distribution of the enhancer marks histone H3 lysine 27 acetylation (H3K27ac) and lysine 4 mono-methylation (H3K4me1) were analyzed by ChIP-seq in MCF7 cell model with inducible H-FOXA1. FOXA1 cistrome, H3K27ac distribution, and transcriptome of a FOXA1-overexpressing pancreatic ductal adenocarcinoma cell model (PDA-hT2) were obtained from NCBI GEO (GSE99311). The core regulatory circuitry (CRC) Mapper was used to identify auto-regulatory loop of TFs induced by H-FOXA1. Gene Ontology was used for gene set functional annotation. FOXA1-associated enhancers of ER+ metastatic vs. primary tumors were analyzed using the H3K27ac epigenome data (European Nucleotide Archive, PRJEB22757).

Results: The FOXA1 and ESR1 genetic amplification and mutations displayed a largely mutually exclusive pattern in ER+/HER2- metastatic BC, suggesting a role of hyperactive FOXA1 signaling in promoting EndoR and metastatic BC distinct from that of the ESR1 mutations. FOXA1 overexpression in BC cells resulted in increased FOXA1 DNA binding and the establishment of more regions with gained H3K27ac and/or H3K4me1, suggesting a more accessible and active chromatin state. H-FOXA1-induced upregulated genes were enriched for the gained H3K27ac or H3K4me1, especially for the enhancers with both marks. An enhancer signature with gained and overlapped H3K27ac and H3K4me1 predicts genes enriched for proliferation, anti-apoptosis and developmental signaling. Upregulated genes induced by H-FOXA1 with gained enhancers were further enriched for pro-metastatic processes, sharing the same characteristics of cellular morphogenesis during embryonic development. Similar results were obtained using integrated data from the PDA-hT2 cell model, sharing enriched pro-metastatic genes predicted by the H-FOXA1-induced enhancer signature. A CRC auto-regulatory TF loop, comprising components of the AP-1 and SMAD families, was predicted to amplify the impact of this enhancer signature on activation of the pro-metastatic transcriptional programs. In line with our preclinical findings, epigenetic changes of active enhancers in ER+ metastatic vs. primary BC were associated with the H-FOXA1-induced enhancer signature.

Conclusions: Our study suggests that in ER+ metastatic BC, genetic alterations of FOXA1 leading to hyperactive FOXA1 signaling involves epigenetic evolution to promote a pro-metastatic enhancer signature. This genome-wide H-FOXA1-induced enhancer signature supports the role of H-FOXA1 in unleashing oncogenic activities of lineage-specific TFs in many types of metastatic tumors. Developing therapeutics targeting FOXA1 itself or key components of the H-FOXA1-induced CRC is warranted to treat or prevent EndoR and metastatic BC effectively via targeting the entire aberrant transcriptional programs.

Citation Format: Xiaoyong Fu, Resel Pereira, Carmine De Angelis, Sarmistha Nanda, Lanfang Qin, Jamunarani Veeraraghavan, Pier Selenica, Britta Weigelt, Jorge S Reis-Filho, Agostina Nardone, Rinath Jeselsohn, Myles Brown, Mothaffar F Rimawi, C Kent Osborne, Rachel Schiff. Identification of a high FOXA1-induced pro-metastatic enhancer signature in endocrine-resistant and metastatic breast cancer [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr PD7-01.

Abstract P6-04-02: Integrative cistromic/transcriptomic profiling identifies a high FOXA1/ER-activated pro-metastatic secretome in endocrine-resistant breast cancer

Background: The estrogen receptor (ER) plays an evolving role in conferring endocrine resistance (EndoR) via altering chromatin binding and transcriptional reprogramming in ER+ metastatic breast cancer (BC). We have recently reported that high levels of FOXA1 (H-FOXA1), a pioneer factor for ER-chromatin binding and function, promote EndoR and metastatic phenotypes in ER+ BC cells. Here we further investigated the interplay between FOXA1 and ER by cistromic and transcriptomic profiling of ER+ BC cells expressing H-FOXA1 and a consequential pro-metastatic secretome associated with poor outcome of ER+ BC.

Methods: FOXA1 and ER cistromes were analyzed by ChIP-seq in MCF7 cells expressing ectopic H-FOXA1 or treated with estrogen or the growth factor EGF. Transcriptomes of ER+ BC cells (MCF7, ZR75-1, and T47D) expressing ectopic H-FOXA1 and our established EndoR cell models (MCF7, ZR75-1, T47D, and 600MPE) were analyzed by RNA-seq. A core gene signature (CGS) and secretome gene sets induced by H-FOXA1 were identified by integrative analyses of cistromic/transcriptomic data and public secretory protein databases. Multivariant proportional hazards modeling and Kaplan-Meier log-rank test were performed to assess prognostic significance of the secretome gene sets using the METABRIC and KMplotter datasets. A transcriptomic dataset of ER+ primary and metastatic BC (The Metastatic Breast Cancer Project) was used to determine secretome expression changes in metastatic vs. primary tumors and the correlation with FOXA1 levels. A tamoxifen-resistant ER+ metastatic mouse model we previously established was used to determine pro-metastatic secretome gene expression.

Results: Ectopic H-FOXA1 expanded and reprogramed the FOXA1 cistrome in ER+ BC cells and induced a CGS shared by multiple H-FOXA1-expressing cancer cell models, including prostate and pancreatic cancer cells. H-FOXA1 redirected an estrogen-independent ER cistrome in ER+ BC cells, but had no effect on the FOXA1 cistrome under short-term treatment with estrogen or EGF. Upregulated CGS was enriched for secretome-encoding genes with variance in ER dependency across multiple EndoR cell models. The H-FOXA1/ER-activated (n=10), but not the ER-repressed or -independent secretome, predicted poor disease-free survival in ER+ BC treated with endocrine therapy (METABRIC, n=1,103). Multivariate analyses further identified each of four secretome genes (ST6GALNAC2, SERPINI1, S100P, and CD55) significantly contributing to overall survival of ER+ BC. The prognostic significance of this secretome signature was also observed in relapse-free survival and distant metastasis-free survival of ER+, but not ER- BC, using the KMplotter dataset. The expression levels of the identified secretome were significantly increased in ER+ metastatic (n=147) vs. primary (n=48) tumors (including 31 pairs) and positively correlated with FOXA1 mRNA levels in metastases. Notably, there was no association between the H-FOXA1/ER-regulated pro-metastatic secretome and the ESR1 mutations. Finally, H-FOXA1-induced EndoR metastatic xenograft tumors were associated with elevated secretome gene expression.

Conclusions: We identified an H-FOXA1/ER-activated pro-metastatic secretome via integrative analyses of the FOXA1/ER cistromes and transcriptomes of our preclinical models expressing H-FOXA1. This secretome signature predicts poor outcome of ER+ BC treated with endocrine therapy and has no correlation with the ESR1 mutations in ER+ metastatic BC. The increased expression of these secretome genes in ER+ metastatic vs. primary tumors suggests continuing interplay of FOXA1 and ER in promoting pro-metastatic transcriptional programs during ER+ disease progression, which warrants further investigation.

Citation Format: Xiaoyong Fu, Resel Pereira, Carmine De Angelis, Sarmistha Nanda, Lanfang Qin, Martin J Shea, Tamika Mitchell, Jamunarani Veeraraghavan, Vidyalakshmi Sethunath, Carolina Gutierrez, Balázs Győrffy, Ofir Cohen, Nikhil Wagle, Agostina Nardone, Rinath Jeselsohn, Myles Brown, Mothaffar F Rimawi, C Kent Osborne, Rachel Schiff. Integrative cistromic/transcriptomic profiling identifies a high FOXA1/ER-activated pro-metastatic secretome in endocrine-resistant breast cancer [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr P6-04-02.

Abstract GS2-01: High levels of interferon-response gene signatures are associated with de novo and acquired resistance to CDK4/6 inhibitors in ER+ breast cancer

Background: The CDK4/6 inhibitors (i) palbociclib (Palbo), ribociclib, and abemaciclib remarkably improved the outcome of patients with metastatic ER+/HER2- breast cancer (BC) and are now under clinical investigation in early BC. Despite high efficacy, de novo and acquired resistance to CDK4/6i is common. Elucidating the molecular basis for sensitivity and resistance to CDK4/6i is crucial to identify predictive biomarkers and novel therapeutic targets to improve patient outcome. Materials and Methods: MCF7, T47D and ZR75-1 parental (P) BC cells and their derivatives made resistant to tamoxifen, estrogen deprivation (EDR), or fulvestrant were used. The P and EDR models of MCF7 and T47D cells were chronically exposed to increasing concentrations of Palbo to generate derivatives with acquired resistance to Palbo (PalboR). The transcriptomic profiles of P, endocrine-resistant (EndoR) and PalboR models were determined by RNA-seq. IC50s were determined by exposing MCF7, T47D, and ZR75-1 P and EndoR lines (n=12) to increasing concentrations of Palbo. Cell growth was assessed by methylene blue staining, and changes in the mRNA and protein levels of key cell cycle molecules were assessed by RT-PCR and Western blot, respectively. Gene expression data from the Cancer Dependency Map (DepMap), baseline tumors from the NeoPalAna (NCT01723774) and neoMONARCH (NCT02441946) neoadjuvant trials, as well as the TCGA and METABRIC datasets were interrogated for correlations of gene signatures and patient outcome (by KMPlot). Results: Palbo treatment resulted in a dose-dependent inhibition of the growth of P and EndoR BC cell lines, with varying degree of sensitivity among the models. GSEA analysis comparing the least sensitive (IC50&gt;350nM) vs. the most sensitive (IC50&lt;100) cell lines identified the ‘interferon gamma response’ (IFNg) and ‘interferon alpha response’ (IFNa) as the top-ranked hallmark enriched signatures. Likewise, DepMap analysis of ER+/HER2- BC cell lines (n=11) revealed that cells with low CDK4 dependency scores displayed high IFN-signaling. We derived a 35-gene signature (termed ‘IFN-Related Palbociclib-Resistance Signature’, IRPS) comprised of genes belonging to the INFg and INFa gene sets that positively correlated with the Palbo IC50 values of our collection of P and EndoR lines. To extend these findings to primary ER+ BC, we interrogated transcriptomic data from the NeoPalAna and neoMONARCH trials that evaluated neoadjuvant CDK4/6i with endocrine therapy. In both trials, the IFNg, IFNa, and IRPS gene signatures were highly enriched in patients with tumors exhibiting intrinsic resistance to CDK4/6i. We next investigated the underlying molecular changes and their association with IFN-signaling in our acquired resistant PalboR cell lines. Compared to the untreated cells, the PalboR models commonly displayed alterations in several components of the cyclin D-CDK4/6-Rb axis, including elevated expression of cyclin-D1, -E1, and CDK6, and reduced levels of Rb. Notably, the PalboR derivatives commonly displayed a dramatic activation of IFN/STAT1-signaling compared to their short-term treated or untreated counterparts. In primary ER+/HER2- tumors, the IRPS score was significantly higher in lumB vs. lumA subtype and correlated with increased gene expression of immune checkpoints (PD-1, PD-L1, CTLA-4), endocrine-resistance, and poor prognosis. Conclusion: Aberrant IFN-signaling predicts resistance to CDK4/6i in both ER+/HER2- BC cell lines and in primary BCs from neoadjuvant clinical trials. Experimentally, acquired resistance to Palbo is associated with activation of the IFN-pathway suggesting its involvement in resistance to CDK4/6i. Future studies are warranted to provide mechanistic insights into the association of IFN-signaling with response to CDK4/6i.

Citation Format: Carmine De Angelis, Xiaoyong Fu, Maria Letizia Cataldo, Agostina Nardone, Valerie M. Jansen, Jamunarani Veeraraghavan, Sarmistha Nanda, Lanfang Qin, Vidyalakshmi Sethunath, Resel Pereira, Matteo Benelli, Ilenia Migliaccio, Luca Malorni, Joshua Donaldson, Pier Selenica, David N. Brown, Britta Weigelt, Jorge S. Reis-Filho, Ben H. Park, Sara A. Hurvitz, Dennis J. Slamon, Mothaffar F. Rimawi, Rinath Jeselsohn, Kent Osborne, Rachel Schiff. High levels of interferon-response gene signatures are associated with de novo and acquired resistance to CDK4/6 inhibitors in ER+ breast cancer [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr GS2-01.

HER2-enriched subtype and pathological complete response in HER2-positive breast cancer: A systematic review and meta-analysis

BACKGROUND

HER2-positive (HER2+) breast cancer (BC) comprises all the four PAM50 molecular subtypes. Among these, the HER2-Enriched (HER2-E) appear to be associated with higher pathological complete response (pCR) rates following anti-HER2-based regimens. Here, we present a meta-analysis to validate the association of the HER2-E subtype with pCR following anti-HER2-based neoadjuvant treatments with or without chemotherapy (CT).

METHODS

A systematic literature search was performed in February 2019. The primary objective was to compare the association between HER2-E subtype (versus others) and pCR. Selected secondary objectives were to compare the association between 1) HER2-E subtype and pCR in CT-free studies, 2) HER2-E subtype within hormone receptor (HR)-negative and HR+ disease and 3) HR-negative disease (versus HR+) and pCR in all patients and within HER2-E subtype. A random-effect model was applied. The Higgins' I² was used to quantify heterogeneity.

RESULTS

Sixteen studies were included, 5 of which tested CT-free regimens. HER2-E subtype was significantly associated with pCR in all patients (odds ratio [OR] = 3.50, p < 0.001, I² = 33%), in HR+ (OR = 3.61, p < 0.001, I² = 1%) and HR-negative tumors (OR = 2.28, p = 0.01, I² = 47%). In CT-free studies, HER2-E subtype was associated with pCR in all patients (OR = 5.52, p < 0.001, I² = 0%) and in HR + disease (OR = 4.08, p = 0.001, I² = 0%). HR-negative status was significantly associated with pCR compared to HR + status in all patients (OR = 2.41, p < 0.001, I² = 30%) and within the HER2-E subtype (OR = 1.76, p < 0.001, I² = 0%).

CONCLUSIONS

The HER2-E biomarker identifies patients with a higher likelihood of achieving a pCR following neoadjuvant anti-HER2-based therapy beyond HR status and CT use. Future trial designs to escalate or de-escalate systemic therapy in HER2+ disease should consider this genomic biomarker.

HER2-Enriched Subtype and ERBB2 Expression in HER2-Positive Breast Cancer Treated with Dual HER2 Blockade

BACKGROUND

Identification of HER2-positive breast cancers with high anti-HER2 sensitivity could help de-escalate chemotherapy. Here, we tested a clinically applicable RNA-based assay that combines ERBB2 and the HER2-enriched (HER2-E) intrinsic subtype in HER2-positive disease treated with dual HER2-blockade without chemotherapy.

METHODS

A research-based PAM50 assay was applied in 422 HER2-positive tumors from five II-III clinical trials (SOLTI-PAMELA, TBCRC023, TBCRC006, PER-ELISA, EGF104090). In SOLTI-PAMELA, TBCRC023, TBCRC006, and PER-ELISA, all patients had early disease and were treated with neoadjuvant lapatinib or pertuzumab plus trastuzumab for 12-24 weeks. Primary outcome was pathological complete response (pCR). In EGF104900, 296 women with advanced disease were randomized to receive either lapatinib alone or lapatinib plus trastuzumab. Progression-free survival (PFS), overall response rate (ORR), and overall survival (OS) were evaluated.

RESULTS

A total of 305 patients with early and 117 patients with advanced HER2-positive disease were analyzed. In early disease, HER2-E represented 83.8% and 44.7% of ERBB2-high and ERBB2-low tumors, respectively. Following lapatinib and trastuzumab, the HER2-E and ERBB2 (HER2-E/ERBB2)-high group showed a higher pCR rate compared to the rest (44.5%, 95% confidence interval [CI] = 35.4% to 53.9% vs 11.6%, 95% CI = 6.9% to 18.0%; adjusted odds ratio [OR] = 6.05, 95% CI = 3.10 to 11.80, P < .001). Similar findings were observed with neoadjuvant trastuzumab and pertuzumab (pCR rate of 66.7% in HER2-E/ERBB2-high, 95% CI = 22.3% to 95.7% vs 14.7% in others, 95% CI = 4.9% to 31.1%; adjusted OR = 11.60, 95% CI = 1.66 to 81.10, P = .01). In the advanced setting, the HER2-E/ERBB2-high group was independently associated with longer PFS (hazard ratio [HR] = 0.52, 95% CI = 0.35 to 0.79, P < .001); higher ORR (16.3%, 95% CI = 8.9% to 26.2% vs 3.7%, 95% CI = 0.8% to 10.3%, P = .02); and longer OS (HR = 0.66, 95% CI = 0.44 to 0.97, P = .01).

CONCLUSIONS

Combining HER2-E subtype and ERBB2 mRNA into a single assay identifies tumors with high responsiveness to HER2-targeted therapy. This biomarker could help de-escalate chemotherapy in approximately 40% of patients with HER2-positive breast cancer.

TBCRC023: A Randomized Phase II Neoadjuvant Trial of Lapatinib Plus Trastuzumab Without Chemotherapy for 12 versus 24 Weeks in Patients with HER2-Positive Breast Cancer

PURPOSE

Prior neoadjuvant trials with 12 weeks of dual anti-HER2 therapy without chemotherapy demonstrated a meaningful pathologic complete response (pCR) in patients with HER2-positive breast cancer. In this trial, we sought to determine whether longer treatment would increase the rate of pCR.

PATIENTS AND METHODS

TBCRC023 (NCT00999804) is a randomized phase II trial combining a Simon phase II design in the experimental arm with a pick-the-winner design, not powered for direct comparison. Women with HER2-positive breast tumors measuring ≥2 cm (median = 5 cm) were randomized in a 1:2 ratio to 12 versus 24 weeks of lapatinib and trastuzumab. Letrozole (along with ovarian suppression if premenopausal) was administered in patients whose tumors were also estrogen receptor (ER) positive. All evaluable patients were assessed for in-breast pCR.

RESULTS

Ninety-seven patients were enrolled (33 in 12-week arm and 64 in 24-week arm), of whom 94 were evaluable. Median age was 51 years, and 55% were postmenopausal. Median tumor size was 5 cm, and 65% were ER-positive. The rate of pCR in the 24-week arm was 28% and numerically superior to the 12-week arm (12%). This was driven by increased pCR in the ER-positive subgroup (33% vs. 9%). Study treatment was well tolerated, with grade 1-2 diarrhea and acneiform rash being the most common toxicities.

CONCLUSIONS

Treatment with dual anti-HER2 therapy for 24 weeks led to a numeric increase in pCR rate in women with HER2-positive breast cancer, without using chemotherapy. If validated, this approach may help identify patients who may benefit from deescalation of therapy.

Evaluation of the Predictive Role of Tumor Immune Infiltrate in Patients with HER2-Positive Breast Cancer Treated with Neoadjuvant Anti-HER2 Therapy without Chemotherapy

PURPOSE

Tumor-infiltrating lymphocytes (TIL) are associated with benefit to trastuzumab and chemotherapy in patients with early-stage HER2⁺ breast cancer. The predictive value of TILs, TIL subsets, and other immune cells in patients receiving chemotherapy-sparing lapatinib plus trastuzumab treatment is unclear.Experimental Design: Hematoxylin and eosin-stained slides (n = 59) were used to score stromal (s-)TILs from pretreatment biopsies of patients enrolled in the neoadjuvant TBCRC006 trial of 12-week lapatinib plus trastuzumab therapy (plus endocrine therapy for ER⁺ tumors). A 60% threshold was used to define lymphocyte-predominant breast cancer (LPBC). Multiplexed immunofluorescence (m-IF) staining (CD4, CD8, CD20, CD68, and FoxP3) and multispectral imaging were performed to characterize immune infiltrates in single formalin-fixed paraffin-embedded slides (n = 33).

RESULTS

The pathologic complete response (pCR) rate was numerically higher in patients with LPBC compared with patients with non-LPBC (50% vs. 19%, P = 0.057). Unsupervised hierarchical clustering of the five immune markers identified two patient clusters with different responses to lapatinib plus trastuzumab treatment (pCR = 7% vs. 50%, for cluster 1 vs. 2 respectively; P = 0.01). In multivariable analysis, cluster 2, characterized by high CD4⁺, CD8⁺, CD20⁺ s-TILs, and high CD20⁺ intratumoral TILs, was independently associated with a higher pCR rate (P = 0.03). Analysis of single immune subpopulations revealed a significant association of pCR with higher baseline infiltration by s-CD4, intratumoral (i-) CD4, and i-CD20⁺ TILs.

CONCLUSIONS

LPBC was marginally associated with higher pCR rate than non-LPBC in patients with lapatinib plus trastuzumab treated HER2⁺ breast cancer. Quantitative assessment of the immune infiltrate by m-IF is feasible and may help correlate individual immune cell subpopulations and immune cell profiles with treatment response.

Targeting the Mevalonate Pathway to Overcome Acquired Anti-HER2 Treatment Resistance in Breast Cancer

Despite effective strategies, resistance in HER2⁺ breast cancer remains a challenge. While the mevalonate pathway (MVA) is suggested to promote cell growth and survival, including in HER2⁺ models, its potential role in resistance to HER2-targeted therapy is unknown. Parental HER2⁺ breast cancer cells and their lapatinib-resistant and lapatinib + trastuzumab-resistant derivatives were used for this study. MVA activity was found to be increased in lapatinib-resistant and lapatinib + trastuzumab-resistant cells. Specific blockade of this pathway with lipophilic but not hydrophilic statins and with the N-bisphosphonate zoledronic acid led to apoptosis and substantial growth inhibition of R cells. Inhibition was rescued by mevalonate or the intermediate metabolites farnesyl pyrophosphate or geranylgeranyl pyrophosphate, but not cholesterol. Activated Yes-associated protein (YAP)/transcriptional coactivator with PDZ-binding motif (TAZ) and mTORC1 signaling, and their downstream target gene product Survivin, were inhibited by MVA blockade, especially in the lapatinib-resistant/lapatinib + trastuzumab-resistant models. Overexpression of constitutively active YAP rescued Survivin and phosphorylated-S6 levels, despite blockade of the MVA. These results suggest that the MVA provides alternative signaling leading to cell survival and resistance by activating YAP/TAZ-mTORC1-Survivin signaling when HER2 is blocked, suggesting novel therapeutic targets. MVA inhibitors including lipophilic statins and N-bisphosphonates may circumvent resistance to anti-HER2 therapy warranting further clinical investigation. IMPLICATIONS: The MVA was found to constitute an escape mechanism of survival and growth in HER2⁺ breast cancer models resistant to anti-HER2 therapies. MVA inhibitors such as simvastatin and zoledronic acid are potential therapeutic agents to resensitize the tumors that depend on the MVA to progress on anti-HER2 therapies.

Abstract 4474: Novel neoplastic RAD51AP1-DYRK4 fusion transcript in aggressive luminal breast cancers

Background: Estrogen receptor positive (ER+) breast cancer is known as luminal breast cancer, which can be classified into A and B intrinsic subtypes. While the luminal A tumors have a favorable outcome following endocrine therapy, the luminal B tumors are characterized by higher proliferation index, aggressive clinical behavior, early relapse following endocrine therapy, and high risk of metastatic dissemination. Clinically the treatment options are limited, and it is even difficult to clearly define these deadly tumors. The underlying pathological molecular events remain poorly understood, and recent genome sequencing studies have revealed a paucity of actionable oncogenic drivers, which hinders the development of new treatment strategies.

Experimental design and methods: Large-scale analyses of breast cancer RNAseq data from The Cancer Genome Atlas (TCGA) were performed to identify the driver gene fusions. ER+ breast tumor tissues were screened by RT-PCR. To test the function of RAD51AP1-DYRK4 transcripts in breast cancer, we engineered the fusion cDNA containing the E9-E2 chimeric ORF together with endogenous5’ translation start sequences into a doxycycline-inducible lentiviral vector, which was then transduced into T47D luminal breast cancer cells. The overexpression (OE) and endogenous fusion knockdown (KD) models were then subjected to diverse functional assays including MTS, Clonogenic, soft agar colony formation, migration and invasion.

Results: In this study, a large-scale analysis of breast cancer transcriptome revealed a tumor-specific RAD51AP1-DYRK4 fusion transcript preferentially overexpressed in luminal B tumors. Molecular analysis of 200 ER-positive breast cancer tissues detected strong RAD51AP1-DYRK4 expression in 19 tumors (9.5%), which is markedly enriched in the luminal B subtype (17.5%). The fusion encodes c-terminal truncated RAD51AP1 protein fused to an out frame peptide from DYRK4, which leads to the loss of the RAD51 interacting domain. Ectopic expression of RAD51AP1-DYRK4 but not wild-type RAD51AP1 significantly increased invasiveness of luminal breast cancer cells. Further, we have identified the endogenous RAD51AP1-DYRK4 protein in fusion-overexpressing cells, silencing of which leads to decreased cell viability.

Conclusions: In summary, this study identifies the first tumor-specific transcription-induced chimera that is preferentially overexpressed in the luminal B breast cancer, and the underlying mechanism. The results suggest that RAD51AP1-DYRK4 transcript may drive the more aggressive form of luminal breast cancers.

Citation Format: Chia Chia Liu, Jamunarani Veeraraghavan, Ying Tan, Jin-Ah Kim, Xian Wang, Rachel Schiff, Xiao-Song Wang. Novel neoplastic RAD51AP1-DYRK4 fusion transcript in aggressive luminal breast cancers [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 4474.

Abstract 4827: The therapeutic superiority of neratinib in combination with trastuzumab compared to pertuzumab plus trastuzumab in HER2-positive in vivo breast cancer models

Neoadjuvant clinical trials in HER2+ breast cancer showed that lapatinib (L) plus trastuzumab (T), combined with endocrine therapy for ER+ tumors, achieved meaningful complete pathologic response rates without chemotherapy. The irreversible pan-HER kinase inhibitor neratinib (N) has shown greater potency compared to L in the preclinical setting. However, the efficacy of N in combination with T (N+T) and how it compares to pertuzumab (P) +T (without chemotherapy) has not been well studied. We hypothesize that dual HER2 inhibition using N+T will be highly efficacious due to more complete blockade of the HER pathway, with comparable or better potency than P+T. Here, we evaluate the therapeutic efficacy and molecular mechanisms of N, P, and T, either alone or in combination, in cell- and patient-derived xenograft (PDX) models. Immunodeficient mice bearing BT474-AZ cell (ER+/HER2+), and BCM-3963 PDX tumors (ER-/HER2+, wild-type PIK3CA) were randomized to vehicle, N, T, P, N+T, or P+T, with simultaneous estrogen deprivation in BT474-AZ xenograft model. Study endpoints included: (i) treatment outcome - time to tumor regression (TTR) and progression (TTP) (tumor halving/doubling over baseline, respectively), and rate and time to complete response (CR and TCR, respectively); and (ii) biomarker analysis - immunohistochemistry (IHC) and western blot (WB) analysis of tumors harvested 2-4 days post-treatment to assess key biomarkers. In the BT474-AZ model, while tumor regression was observed in 100% of N, P, T, N+T, and P+T treated mice, the tumors treated with N+T regressed faster compared to P (p&lt;0.001), T (p=0.004), and P+T (p=0.044). Further, N+T was superior to N (p=0.018), and T (p=0.007) alone in achieving accelerated CR. In the BCM-3963 model, which was refractory to T, P, or T+P, while CR was achieved in 100% of N and N+T treated mice, the combination of N+T accelerated the attainment of CR compared to N alone (p=0.026). IHC analysis of short-term treated tumors showed that Ki67, pAKT, and pMAPK levels were significantly inhibited by N and N+T, but not by T, P, or P+T. Compared to P+T, N and N+T more potently inhibited Ki67, suggesting the superiority of N-containing regimens in suppressing tumor cell proliferation. Likewise, WB analysis showed that N and N+T markedly inhibited pHER2 (Y1248), pEGFR (Y1068), pAKT (S473), pERK, and pS6 levels, compared to P+T, suggesting a more potent blockade of the HER pathway by N-containing regimens, especially after short-term treatment. In the BT474-AZ model, short-term N+T treatment yielded greater inhibition of pHER2 (Y1248) and survivin levels, compared to N alone. These preclinical findings establish the efficacy of combining N with T for HER2+ breast cancer and support further clinical testing to investigate the efficacy of N+T without chemotherapy in the neoadjuvant setting for patients with HER2+ breast cancer.

Citation Format: Jamunarani Veeraraghavan, Vidyalakshmi Sethunath, Martin J. Shea, Tamika Mitchell, Resel Pereira, Lanfang Qin, Sarmistha Nanda, Carmine De Angelis, Kristina Goutsouliak, Irmina Diala, Alshad S. Lalani, Sepideh Mehravaran, Susan G. Hilsenbeck, Chandandeep Nagi, Carolina Gutierrez, Mothaffar F. Rimawi, C. Kent Osborne, Rachel Schiff. The therapeutic superiority of neratinib in combination with trastuzumab compared to pertuzumab plus trastuzumab in HER2-positive in vivo breast cancer models [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 4827.

Abstract 376: Therapeutic role of ESR1-CCDC170 gene fusion in breast cancer endocrine resistance

Background: Gene fusions resulting from genomic rearrangements are important drivers for cancer initiation and progression. Estrogen receptor-positive (ER+) breast cancer is the most common type of breast cancer, and luminal B confers a more aggressive phenotype and higher risk of early relapse and prone to resistance to endocrine therapy. Identify new druggable genetic driver will be critical to improve clinical outcome of aggressive luminal B tumors. Our lab has identified a recurrent rearrangement between the estrogen receptor gene ESR1 and its neighbor gene CCDC170, in 6-8% of luminal B tumors. The exon 2 of ESR1 fused with the exon 6, 7, 8 or 10 of CCDC170, which enables the expression of different-sized N-terminally truncated CCDC170 (ΔCCDC170) under ESR1 promoter. Ectopic expression of ΔCCDC170 led to malignant transformation phenotypes in ER+ breast cancer cells, such as increase the cell migration, invasion and anchor-independent cell growth, as well as enhance xenograft tumor growth in mice.

Methods: The effect of ESR1-CCDC170 on endocrine resistance was examined by cologenic assay in vitro, and by xenograft mouse model in vivo. Bioinformatics approaches, Reverse Phase Protein Array (RPPA) analysis and Bimolecular Fluorescence Complementation assay were used to elucidate the potential molecular mechanism.

Results: This study aimed to determine the role of ESR1-CCDC170 in breast cancer endocrine resistance and elucidate the potential mechanisms as well as the therapeutic effect thereof. Our results show that ectopic expression of fusion variants in ER+ T47D cell line sustains the cell proliferation ability under estrogen deprivation (ED), and decreases the sensitivity to 4-OH Tamoxifen treatment. While silencing of ESR1-CCDC170 in fusion positive HCC1428 cell line increases the sensitivity to 4-OHTamoxifen and Fulvestrant treatment. Results from T47D xenograft tumor models show that ESR1-CCDC170 fusion induces endocrine resistance in vivo. Further mechanism studies show that ESR1-CCDC170 possesses a potential ATP binding pocket, has the ability to form homodimers.

Conclusion: These results demonstrate the important role of ESR1-CCDC170 fusions in breast cancer endocrine resistance, and suggest that ESR1-CCDC170 fusion could be a potential therapeutic target for the treatment of breast cancer.

Citation Format: Li Li, Jamunarani Veeraraghavan, Yiheng Hu, Xian Wang, Ying Tan, Rachel Schiff, Xiaosong Wang. Therapeutic role of ESR1-CCDC170 gene fusion in breast cancer endocrine resistance [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 376.

Abstract 4757: Targeting the mevalonate pathway in HER2+breast cancer to overcome resistance and enhance anti-HER2 therapy efficacy

Background: Despite the advent of HER2-targeted therapies, including the monoclonal antibody trastuzumab (T) and the HER1/2 inhibitor lapatinib (L), for HER2+ breast cancer (BC), resistance still poses a major challenge. L and L+T resistant (R) HER2+ cells with inhibited HER2 signaling showed upregulation of RNA levels of the mevalonate pathway (MVA) enzymes (which were inhibited by short term treatment of parental (P) cells with L or L+T) and increased sensitivity to MVA inhibition by statins (e.g. simvastatin (Sim)), suggesting MVA’s role as an escape mechanism of resistance. Here we investigated the therapeutic potential of another MVA inhibitor Zoledronate (ZA) and the role of mTOR and YAP/TAZ (Y/T) in mediating this resistance. Lastly, we tested if co-blockade of the MVA or its downstream effectors further sensitizes HER2+ models to anti-HER2 therapies.

Methods: SKBR3 and AU565 P cells and their LR and LTR derivatives were used. The effects of MVA perturbations on cell growth and HER2 or MVA signaling were studied by methylene blue staining and Western blot (WB), respectively. Y/T activity was tested by a luciferase reporter assay and functionally validated by siRNA knockdown and dominant-active (DomA) YAP overexpression. YAP target gene expression was assessed by RT PCR. SCID-Beige mice bearing HER2+ BCM-3963 PDX tumors were treated with vehicle, L, Sim, or L+Sim and monitored for time to complete response (CR).

Results: ZA, like Sim, showed a selective inhibition of cell growth and mTOR signaling in R vs. P cells, which was rescued only by the downstream metabolite geranyl geranyl pyrophosphate (GGPP), but not by the upstream metabolite mevalonate, indicating the on-target effect of ZA. Increased Y/T activity in R models was confirmed, and both Sim and ZA inhibited TAZ levels and induced phospho-YAP levels, which were rescued by the corresponding downstream metabolites. Y/T knockdown inhibited growth and mTOR signaling in R vs. P cells, and DomA YAP negated the mTOR inhibition by Sim. Sim and ZA also significantly decreased levels of the Y/T target gene survivin in R vs. P cells, and the expression was rescued by the downstream metabolites. Inhibition of MVA by Sim or ZA or its downstream signaling effectors, Y/T (by siRNA) and mTOR (by everolimus), enhanced the L sensitivity in P cells. Conversely, DomA YAP reduced the sensitivity of P cells to L. In the presence of Sim or ZA, L treatment more strongly inhibited levels of phospho-S6, a downstream target of mTORC1, compared to L alone. Preliminary in vivo data showed that treatment with L+Sim vs. L alone shortened the median time to CR and numerically increased CR rates.

Conclusions: The MVA pathway mediates anti-HER2 therapy resistance via Y/T, survivin, and mTOR, in some cell models and this resistance can be overcome by Sim and ZA. The potential of MVA pathway inhibition to enhance anti-HER2 therapy efficacy warrants further clinical studies.

Citation Format: Vidyalakshmi Sethunath, Huizhong Hu, Carmine DeAngelis, Jamunarani Veeraraghavan, Lanfang Qin, Martin Shea, Tamika Mitchell, Sarmistha Nanda, Resel Pereira, Susan G. Hilsenbeck, Mothaffar F. Rimawi, Kent C. Osborne, Rachel Schiff. Targeting the mevalonate pathway in HER2+breast cancer to overcome resistance and enhance anti-HER2 therapy efficacy [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 4757.

A combinatorial biomarker predicts pathologic complete response to neoadjuvant lapatinib and trastuzumab without chemotherapy in patients with HER2+ breast cancer

BACKGROUND

HER2-positive (+) breast cancers, defined by HER2 overexpression and/or amplification, are often addicted to HER2 to maintain their malignant phenotype. Yet, some HER2+ tumors do not benefit from anti-HER2 therapy. We hypothesize that HER2 amplification levels and PI3K pathway activation are key determinants of response to HER2-targeted treatments without chemotherapy.

PATIENTS AND METHODS

Baseline HER2+ tumors from patients treated with neoadjuvant lapatinib plus trastuzumab [with endocrine therapy for estrogen receptor (ER)+ tumors] in TBCRC006 (NCT00548184) were evaluated in a central laboratory for HER2 amplification by fluorescence in situ hybridization (FISH) (n = 56). HER2 copy number (CN) and FISH ratios, and PI3K pathway status, defined by PIK3CA mutations or PTEN levels by immunohistochemistry were available for 41 tumors. Results were correlated with pathologic complete response (pCR; no residual invasive tumor in breast).

RESULTS

Thirteen of the 56 patients (23%) achieved pCR. None of the 11 patients with HER2 ratio <4 and/or CN <10 achieved pCR, whereas 13/45 patients (29%) with HER2 ratio ≥4 and/or CN ≥10 attained pCR (P = 0.0513). Of the 18 patients with tumors expressing high PTEN or wild-type (WT) PIK3CA (intact PI3K pathway), 7 (39%) achieved pCR, compared with 1/23 (4%) with PI3K pathway alterations (P = 0.0133). Seven of the 16 patients (44%) with HER2 ratio ≥4 and intact PI3K pathway achieved pCR, whereas only 1/25 (4%) patients not meeting these criteria achieved pCR (P = 0.0031).

CONCLUSIONS

Our findings suggest that there is a clinical subtype in breast cancer with high HER2 amplification and intact PI3K pathway that is especially sensitive to HER2-targeted therapies without chemotherapy. A combination of HER2 FISH ratio and PI3K pathway status warrants validation to identify patients who may be treated with HER2-targeted therapy without chemotherapy.

Abstract P6-17-12: Neratinib in combination with trastuzumab is superior to each alone and to pertuzumab plus trastuzumab in HER2-positive in vivo breast cancer models

Background: Lapatinib (L) plus trastuzumab (T) alone or with endocrine therapy for HER2+/ER+ tumors but without chemotherapy, yielded complete tumor eradication in xenograft models. In neoadjuvant trials (NCT00548184, 00999804, 01973660), a substantial number of patients achieved pathologic complete response with this same strategy. The irreversible pan-HER inhibitor neratinib (N) has been recently approved by the FDA for early stage HER2+ breast cancer and has shown greater potency compared to L in the preclinical setting. However, the therapeutic efficacy of N in combination with T (N+T) and how it compares to pertuzumab (P) +T (without chemotherapy) has not been well studied.

We hypothesize that dual HER2 inhibition using N+T will be highly efficacious and more effective than P+T due to more complete blockade of the HER pathway. Here, we evaluate the therapeutic efficacy of N, P, and T, either alone or in combination, with a primary focus on comparing N+T vs. P+T in established cell line- and patient-derived xenograft (PDX) models.

Methods: Athymic nude and SCID/Beige mice bearing BT474-AZ cell line (ER+/HER2+), and BCM-3963 PDX tumors (ER-/HER2+, wild-type PIK3CA), respectively were randomized to vehicle, N (20mg/kg, 5 days/week), T (10mg/kg, twice a week), P (6mg/kg, once a week), N+T, or P+T, with simultaneous estrogen (E2) deprivation (ED) in BT474-AZ model. Treatment response was assessed by biweekly tumor measurements. Study endpoints included time to tumor regression (TTR) and progression (TTP) (tumor halving/doubling over baseline, respectively), and the rate and time of complete response (CR and TCR, respectively). Results were analyzed using survival analysis (Kaplan-Meier estimates) and generalized Wilcoxon tests.

Results: In the BT474-AZ model, mice treated with E2+vehicle and ED+vehicle showed steady tumor growth, with a median TTP of 8 and 25 days, respectively. While tumor regression was observed in 100% of mice treated with N, P, T, N+T, and P+T, tumors treated with N+T regressed faster compared to P (p&lt;0.001), T (p=0.004), and P+T (p=0.044). Further, N+T was superior to N (p=0.018) and T (p=0.007) alone in achieving accelerated CR. In the BCM-3963 model, tumors treated with vehicle, T, P, and P+T continued to grow with a median TTP of 11, 16, 19, and 17 days, respectively. In contrast, CR was achieved in 100% of N and N+T treated mice. Importantly, combining N with T accelerated the attainment of CR compared to N alone (p=0.026). Molecular and pathologic analysis of short-term treated tumors in both models to evaluate alterations in HER signaling, cell proliferation, and apoptosis is ongoing.

Model/TreatmentN of miceMedian TTP (Days)Median TTR (Days)Median TCR (Days)CR (%)BT474-AZ     E2+Vehicle98--0ED+Vehicle1025--0ED+N13-214100ED+T12-519100ED+P12-185492ED+N+T13-210100ED+P+T14-414100BCM-3963     Vehicle1511--0N15-417100T1416--0P1319--0N+T19-614100P+T1617--0

Conclusions: Our findings establish the preclinical efficacy of combining N with T for HER2+ breast cancer and warrant further clinical testing to investigate the efficacy of N+T without chemotherapy in the neoadjuvant setting for patients with HER2+ breast cancer.

Citation Format: Veeraraghavan J, Sethunath V, Qin L, Shea MJ, Mitchell T, De Angelis C, Nanda S, Diala I, Lalani AS, Hilsenbeck SG, Rimawi MF, Osborne CK, Schiff R. Neratinib in combination with trastuzumab is superior to each alone and to pertuzumab plus trastuzumab in HER2-positive in vivo breast cancer models [abstract]. In: Proceedings of the 2018 San Antonio Breast Cancer Symposium; 2018 Dec 4-8; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2019;79(4 Suppl):Abstract nr P6-17-12.

The oral selective oestrogen receptor degrader (SERD) AZD9496 is comparable to fulvestrant in antagonising ER and circumventing endocrine resistance

Background

The oestrogen receptor (ER) is an important therapeutic target in ER-positive (ER+) breast cancer. The selective ER degrader (SERD), fulvestrant, is effective in patients with metastatic breast cancer, but its intramuscular route of administration and low bioavailability are major clinical limitations.

Methods

Here, we studied the pharmacology of a new oral SERD, AZD9496, in a panel of in vitro and in vivo endocrine-sensitive and -resistant breast cancer models.

Results

In endocrine-sensitive models, AZD9496 inhibited cell growth and blocked ER activity in the presence or absence of oestrogen. In vivo, in the presence of oestrogen, short-term AZD9496 treatment, like fulvestrant, resulted in tumour growth inhibition and reduced expression of ER-dependent genes. AZD9496 inhibited cell growth in oestrogen deprivation-resistant and tamoxifen-resistant cell lines and xenograft models that retain ER expression. AZD9496 effectively reduced ER levels and ER-induced transcription. Expression analysis of short-term treated tumours showed that AZD9496 potently inhibited classic oestrogen-induced gene transcription, while simultaneously increasing expression of genes negatively regulated by ER, including genes potentially involved in escape pathways of endocrine resistance.

Conclusions

These data suggest that AZD9496 is a potent anti-oestrogen that antagonises and degrades ER with anti-tumour activity in both endocrine-sensitive and endocrine-resistant models.

Abstract SY01-01: Endocrine resistance in metastatic breast cancer: Mechanisms and new therapeutic strategies

Multiple lines of preclinical and clinical evidence suggest a continued, albeit altered, role of estrogen receptor (ER) in endocrine resistance, a major hurdle in the clinical management of ER+ breast cancer. In the recent years, several studies that explored the biology of ER in recurrent metastatic disease in patients and in endocrine-resistant preclinical models have uncovered various mechanisms underlying altered ER activity and endocrine resistance. Some of these mechanisms are observed to emerge under the pressure of chronic endocrine therapy, through selective clonal expansion, resulting in acquired resistance. Recurrent gain-of-function mutations in the ligand-binding and other domains of the ER encoding gene ESR1 have been identified in 20-50% of the patients with endocrine-resistant metastatic breast cancers. Tumor biopsies have long been useful in identifying various mutations, but circulating tumor DNA in patients with advanced disease is now also emerging as a clinically valuable resource enabling the noninvasive detection of ESR1 mutations. Most of the ESR1 mutations confer constitutive ligand-independent ER activity, causing resistance to estrogen deprivation (i.e., aromatase inhibitors) and reduced sensitivity to the selective ER modulator (SERM) tamoxifen and the selective ER downregulator (SERD) fulvestrant. Less frequent ESR1-related genomic alterations, including gene amplifications, rearrangements, and gain-of-function gene fusions identified mostly in patients with recurrent metastatic disease, represent additional mechanisms of acquired endocrine resistance. While the therapeutic value of newly developed orally bioavailable SERMs/SERDs such as GDC-0810 and GDC-0927 for patients with tumors harboring ESR1 mutations is yet to be clinically proven, we anticipate that ongoing studies will address this open question. Additionally, the recent clinically potent CDK4/6 inhibitor palbociclib, in combination with fulvestrant, has shown promising therapeutic efficacy against tumors harboring ESR1 mutations. Acquired amplification of the aromatase-encoding gene CYP19A1, recently reported in ~20% of aromatase inhibitor-treated relapsed patients, has also been linked to endocrine resistance due to increased aromatase activity and intratumoral estrogen levels.

Besides genomic alterations in ER, crosstalk between growth factor receptor (GFR) signaling pathways and ER can also mediate resistance by altering the activity of ER and its co-regulators, and as a result, the ER-dependent transcriptional program. In particular, the bidirectional crosstalk between ER and the HER family and other GFRs, including in ER+/HER2- tumors that harbor HER2 activating mutations and FGFR1 amplification in 10-15% of patients with ER+ breast cancer, has been implicated in endocrine resistance. Combined treatment with endocrine therapy plus inhibitors against pertinent GFRs or their downstream signaling components to circumvent resistance is currently under preclinical and clinical evaluation. Other factors that contribute to endocrine resistance are changes in ER-interacting/co-regulator proteins such as the ER co-activator SRC3, transcription factor complex AP-1, and the pioneer factor FOXA1. Most recently, a new smal- molecule inhibitor targeting SRC3 has been shown to inhibit the levels and/or transcriptional activity of the SRC family of coactivators and the growth of endocrine-naïve and resistant tumors in vivo.

Clinical findings and preclinical data from in vivo mouse xenograft models have shown that the endocrine-resistant tumors harbor increased AP-1 activity, which mediates a shift in the ER transcriptional program and promotes endocrine resistance and metastatic capabilities. Numerous preclinical findings have long suggested the crucial role of the ER pioneer factor FOXA1 in cancer development and progression. In breast cancer, we first reported that high levels of FOXA1, by gene amplification and/or overexpression, promote acquired endocrine resistance across multiple endocrine-resistant cell and xenograft mouse models. In the clinical setting, recent sequencing studies have reported FOXA1 genomic aberrations, including gene amplification, recurrent mutations, and overexpression via activating mutations in the promoter region, in about 10% of ER+ primary tumors, and enrichment in metastatic tumors. Further, in the metastatic setting, FOXA1 genetic aberrations and ESR1 mutations associated with aromatase inhibitor resistance appear to be mutually exclusive, suggesting the involvement of different mechanisms in high FOXA1-induced endocrine resistance and metastasis. Our new studies in acquired endocrine-resistant preclinical models suggest that FOXA1 gene amplification and/or overexpression results in a global enhancer and transcriptional reprogramming that activates key downstream transcription factors, including the hypoxia-induced factor HIF2α, and the prometastatic secretome. Our findings thus support the notion that, at least in a subset of patients with ER+ breast cancer, high levels or activity of FOXA1 are drivers of endocrine resistance and metastasis, presenting new therapeutic targets and novel therapeutic approaches along its reprogrammed transcriptional axis at both the regulatory and downstream effector levels.

These findings underscore the need for a more careful assessment of the genomic, epigenomic, and signaling landscape of recurrent lesions in order to devise more tailored therapies to effectively circumvent and/or prevent endocrine resistance and metastasis. In addition, a more complete suppression of the levels and activity of ER, its co-activators, and alternative survival pathways may prove to be a better therapeutic strategy in effectively combating endocrine resistance. Effective therapeutic strategies may include novel epigenomic approaches to inhibit gene expression and activity of ER and its co-activators, more potent SERDs, small-molecule inhibitors of SRC3 and other ER co-activators, signal transduction inhibitors that modulate levels and activity of ER and its co-activators, a new HIF2α small-molecule inhibitor currently under clinical evaluation in renal cell carcinoma, and development of drugs that specifically target the pioneer factor FOXA1.

Citation Format: Rachel Schiff, Jamunarani Veeraraghavan, Xiaoyong Fu. Endocrine resistance in metastatic breast cancer: Mechanisms and new therapeutic strategies [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr SY01-01.

Abstract P3-06-03: Dual p38/NLK kinase inhibitor as potential novel therapeutic agent for tamoxifen-resistant luminal breast cancer

Background: Tamoxifen is the most commonly used endocrine agent for estrogen receptor (ER) positive breast cancer (also known as luminal breast cancer). However, approximately half of the patients develop resistance after initial response to tamoxifen. To date, no effective targeted therapy exists to overcome it. We previously identified the role of nemo-like kinase (NLK), a serine-threonine kinase that functions in stress response and neurite outgrowth, in breast endocrine resistance. In addition, activation of p38 MAPK has been reported to modulate ER signaling and promote endocrine resistance. We identified a highly selective dual p38 and NLK kinase inhibitor (PNKI) through analysis of public kinase profiling datasets, and evaluated its therapeutic effect in endocrine-resistant breast cancers using in vitro and in preclinical mouse models. Experimental design and methods: To determine the in vitro therapeutic window of PNKI, we treated an acquired tamoxifen-resistant cell line (MCF7-TamR) and a benign breast epithelial cell line (MCF10A) with gradually increasing doses of PNKI. To determine the effect of PNKI on tamoxifen-resistant breast cancer cells, we treated primary tamoxifen-resistant breast cancer cell line BT483, and MDAMB415, together with acquired tamoxifen-resistant line MCF7 TamR, T47D TamR, and ZR-75-B TamR, with 0.5 uM PNKI in the presence of different doses of Tamoxifen. To evaluate the therapeutic effect of PNKI in a T47D-derived xenograft tumor model with acquired tamoxifen resistance, we administered PNKI alone or in combination with Fulvestrant, the second-line endocrine therapy agent, or with Everolimus, the mTOR inhibitor that could improve patient outcomes in several clinical trials. Mice bearing xenografts were randomized into six treatment groups (Vehicle, PNKI, Fulvestrant, Fulvestrant+PNKI, Everolimus, Everolimus+PNKI). Tumor growth was tracked closely. The tumors harvested two weeks after treatments started were profiled with Reverse Phase Protein Array (RPPA) to assess the early signaling changes after treatments. The therapeutic effect of PNKI were also evaluated in a patient-derived xenograft (PDX) model of de novo endocrine resistant breast cancer. Mice bearing the PDX tumors were randomized to four treatment groups (Vehicle, PNKI, Everolimus, Everolimus+PNKI) and tumor growth curve was measured timely. Results: PNKI showed an in vitro therapeutic window at 0.1-1μM for MCF7-TamR cells. Breast cancer cell lines with either de novo or acquired Tamoxifen resistance became more sensitive to tamoxifen when treated with 0.5uM PNKI. The concomitant treatment of PNKI and Everolimus results in significant decreased tumor burden and prolonged progression free survival in the both T47D-TamR xenograft tumors and re-transplanted de novo endocrine-resistant PDX tumors compared to other treatments. RPPA data of T47D-TamR tumors harvested following 2-week treatments revealed that several key survival signaling in breast cancer are repressed only when PNKI are combined with Everolimus. Conclusion: The dual p38 and NLK inhibitor (PNKI) exhibited potential therapeutic value as adjuvant agent to the mTOR inhibitor everolimus for acquired or de novo tamoxifen-resistant luminal breast cancers.

Citation Format: Wang X, Cao X, Veeraraghavan J, Qin L, Kim J-A, Tan Y, Hilsenbeck SG, Schiff R, Wang X. Dual p38/NLK kinase inhibitor as potential novel therapeutic agent for tamoxifen-resistant luminal breast cancer [abstract]. In: Proceedings of the 2017 San Antonio Breast Cancer Symposium; 2017 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2018;78(4 Suppl):Abstract nr P3-06-03.

Abstract P4-04-03: Hyperactive FOXA1 activates super-enhancer-engaged HIF2α/EPAS1 to promote endocrine-resistant metastatic ER-positive breast cancer

Background: We have recently reported that acquired endocrine resistance (Endo-R) in multiple ER+ breast cancer (BC) Endo-R cell models is driven by high levels of FOXA1 (High-FOXA1), via gene amplification and/or overexpression (OE), leading to coordinated reprogramming of the FOXA1 genomic binding (cistrome) and transcriptome. Forced FOXA1 OE in parental (P) cells induced similar transcriptional reprogramming leading to Endo-R and metastasis. Recent clinical data showing enrichment of FOXA1 amplification in ER+ metastases further support the clinical importance of our findings. However, the molecular components and the mechanism of High-FOXA1-induced transcriptional reprogramming in Endo-R and metastasis are unknown.

Methods: High-FOXA1-containing MCF7 tamoxifen-resistant (TamR) and P/FOXA1-OE cells were used in this study. An integrative multi-OMICS approach was employed to analyze transcriptome (RNA-seq), FOXA1 cistrome, and histone H3K27 acetylation (ac) (ChIP-seq). Intersection of High-FOXA1-induced transcriptome and distinct FOXA1 cistrome-predicted genes defined a High-FOXA1 core gene signature (CGS). Gene Set Enrichment Analysis (GSEA) and Gene Ontology (GO) were used for functional annotation. Cell growth and migration/invasion were measured by a bright-field automated cell counter and Transwell insert system. Altered gene expression was measured by RT-qPCR. High-FOXA1 signaling inhibition included gene knockdown (siRNA) or pharmacologic blockade (the EPAS1 inhibitor PT2385). The predictive role of EPAS1 and the associated gene signature were analyzed using publicly available BC datasets.

Results: FOXA1 OE reprogrammed the FOXA1 cistrome in P cells to resemble that of the TamR cells. The FOXA1 cistrome was significantly correlated with the deposition of H3K27ac in TamR vs. P cells (P&lt;2.2e-16). Similarly, the differentially expressed genes in TamR vs. P cells were enriched for FOXA1 binding at enhancers demarcated by H3K27ac (P=8e-125). The FOXA1-CGS was linked to multiple metastasis-related GO terms including “hypoxia response”, enriched for the cancer secretome gene set (P=4.1e-16), and highly represented in the Endo-R transcriptome across our multiple cell models (MCF7, 600MPE, and CAMA1) (P&lt;0.01). Integrative analysis of H3K27ac-defined super-enhancers (SEs) and altered cistrome/transcriptome upon High-FOXA1 nominated EPAS1, a hypoxia-inducible transcription factor (TF), as a top candidate of SE-activated TFs amplifying High-FOXA1 signaling. EPAS1 blockade markedly repressed the secretome genes (e.g., IL8 and S100P) and cell migration and invasion in TamR cells. Primary ER+ tumors (TCGA) with high EPAS1 are enriched for a cancer secretome gene set (P=3e-4). High EPAS1 predicts poor distant metastasis-free survival in ER+ BC treated with endocrine therapy (P=.034).

Conclusions: High-FOXA1 induces transcriptional reprogramming by coordinating histone enhancer marks to activate EPAS1 via an SE mechanism, which in turn mediates transcriptional reprogramming, partly via inducing a pro-metastatic secretome, to promote Endo-R and metastasis. Targeting the High-FOXA1/EPAS1 axis to block transcriptional reprogramming may offer a new therapeutic strategy to prevent and treat Endo-R metastatic ER+ BC.

Citation Format: Fu X, Pereira R, De Angelis C, Veeraraghavan J, Shea MJ, Nanda S, Feng Q, Jeselsohn R, O'Malley BW, Brown M, Osborne CK, Schiff R. Hyperactive FOXA1 activates super-enhancer-engaged HIF2α/EPAS1 to promote endocrine-resistant metastatic ER-positive breast cancer [abstract]. In: Proceedings of the 2017 San Antonio Breast Cancer Symposium; 2017 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2018;78(4 Suppl):Abstract nr P4-04-03.

Abstract P4-03-04: Targeting the mevalonate pathway in HER2-positive breast cancer to overcome resistance to anti-HER2 therapy

Background: Despite the advent of HER2-targeted therapies for HER2+ breast cancer (BC), including the monoclonal antibody trastuzumab (T) either alone or in combinations, resistance still poses a major clinical challenge. Using our broad panel of HER2+ cell lines made resistant (R) to T alone (TR), and to lapatinib plus T (LTR), we observed that in resistant models where HER2 remains inhibited, especially the LTR derivative, the mevalonate (MVA) pathway is activated and provides an alternative proliferative signal, including the activation of mTOR, to drive resistance. While these resistant cell models were hypersensitive to the widely-used cholesterol-lowering statins, the role of other MVA pathway inhibitors such as zoledronic acid (ZA), which is in clinical use to treat bone metastasis, in overcoming resistance to HER2-targeted therapy has not been explored. Based on recent reports and our preliminary data using reverse phase protein array (RPPA) analysis, the YAP/TAZ transcription factor (TF) emerged as a potential mediator of MVA pathway signaling to mTOR. Here, we investigated the therapeutic efficacy of additional MVA pathway inhibitors and the role of YAP/TAZ in mediating resistance to HER2-targeted therapy.

Methods: HER2+ SKBR3 and AU565 BC cells and their LTR derivatives were used. Changes in cell growth upon genetic and pharmacologic inhibition of the MVA pathway were quantified by methylene blue staining. Luciferase reporter assays and western blots (WB) measured changes in total and phosphorylated (S127 and S381/inactive) YAP protein levels to examine activity of the YAP/TAZ TF complex. To validate the function of YAP/TAZ in resistance, we performed YAP/TAZ knockdown (siRNA), overexpression of dominant-active YAP constructs (S381A, S381/127A), and qRT-PCR assessment of YAP/TAZ target gene expression.

Results: ZA, like simvastatin (Sim), selectively inhibited the growth of resistant cells in a dose-dependent manner. This inhibition was rescued by geranyl geranyl pyrophosphate (GGPP), a downstream metabolite, but not by MVA, an upstream metabolite, indicating the on-target effect of ZA. Further, ZA and Sim combination showed a synergistic growth-inhibitory effect in R but not in parental (P) cells. YAP/TAZ luciferase reporter assays and phosphorylated YAP and total TAZ levels by WB, confirmed the increased activity of YAP/TAZ in R models, which was selectively inhibited by Sim or ZA and was rescued by the corresponding downstream metabolites. YAP/TAZ knockdown selectively inhibited resistant cell growth and mTOR signaling in R vs. P cells, and dominant-active YAP/TAZ rescued the mTOR inhibition by Sim. YAP/TAZ inhibition by siRNA or by Sim significantly decreased the expression of YAP/TAZ target gene survivin in R vs. P cells, and the Sim inhibition was rescued by MVA.

Conclusions: The MVA pathway plays a critical role in mediating resistance to anti-HER2 therapy, which was overcome by Sim and ZA either alone or in combination. Given the synergistic effect of Sim and ZA, their combination may offer a therapeutic strategy to overcome HER2-targeted therapy resistance. Our results also reveal the role of YAP/TAZ in MVA pathway-mediated HER2-targeted therapy resistance, which could suggest new biomarkers and therapeutic targets.

Citation Format: Sethunath V, Hu H, De Angelis C, Veeraraghavan J, Qin L, Rimawi MF, Osborne KC, Schiff R. Targeting the mevalonate pathway in HER2-positive breast cancer to overcome resistance to anti-HER2 therapy [abstract]. In: Proceedings of the 2017 San Antonio Breast Cancer Symposium; 2017 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2018;78(4 Suppl):Abstract nr P4-03-04.

Abstract P4-03-05: AP-1 as a potential mediator of resistance to the cyclin-dependent kinase (CDK) 4/6-inhibitor palbociclib in ER-positive endocrine-resistant breast cancer

Background: The CDK4/6-inhibitor palbociclib (Palbo) in combination with endocrine therapy (ET) substantially improves progression-free survival compared to ET alone. However, almost all initial responders eventually develop resistance and relapse. Delineating the early adaptive signaling and the mechanisms underlying resistance to CDK4/6 inhibition is therefore crucial to identify new biomarkers and therapeutic targets to enhance the efficacy of Palbo and improve patient outcome.

Materials and Methods: MCF7 parental (P) cells and derivative lines made resistant (R) to tamoxifen (TamR), estrogen deprivation (EDR), or fulvestrant (FulR) were used. The MCF7P line and its endocrine-R (EndoR) derivatives were exposed to increasing concentrations of Palbo to generate acquired Palbo-R (PDR) models. The proteomic/signaling profiles of P and EndoR cells upon short-term Palbo treatment and as PDR develops were determined using reverse-phase protein arrays (RPPA). Transcriptional activity of the activator protein-1 (AP-1) transcription factor (TF) was measured by luciferase reporter assay. Global AP-1 blockade was achieved using a pINDUCER system to express doxycycline (Dox)-inducible dominant-negative (DN) c-Jun that lacks the transcriptional activation domain. Cell growth and colony formation were assessed using methylene blue staining and clonogenic assays, respectively. Levels of phosphorylated (p)-RB and CDK2 were assessed by Western Blot.

Results: In P and all EndoR cell models, Palbo inhibited cell growth and colony formation in a dose-dependent manner, though the inhibitory effect was greater in the EndoR cells compared to P cells [IC50 value of P cells &gt;3 times that of EndoR lines (p&lt;0.001); clonogenic % inhibition at 100nM = 54 in P and &gt;85 in EndoR lines (p&lt;0.001)]. Across the P and all EndoR models, short-term Palbo treatment resulted in increased levels of several membrane and intracellular signaling molecules, TFs, and enzymes. Among these, the AP-1 TF components c-Jun and p-c-Jun showed the highest increase across all models, with the utmost change observed in the TamR model (Fold-change = 4.4, 4.0 for total and p-c-Jun, respectively). Since we also observed that acquired resistance to Palbo in the TamR model was associated with higher AP-1 transcriptional activity and increased total and p-c-Fos levels, we assessed the efficacy of combining Palbo with AP-1 blockade in the TamR model. In two independent TamR clones ectopically expressing inducible DN-c-Jun, AP-1 blockade (+Dox) in combination with Palbo was highly effective in inhibiting cell growth and reducing p-RB and CDK2 levels compared to single agent treatments. In addition, in both the TamR/DN-c-Jun-expressing clones, the combination of Palbo, AP-1 blockade, and fulvestrant resulted in cell death and a significantly greater cell growth inhibition compared to any dual or mono treatments.

Conclusion: Our results suggest activation of AP-1 as a potential mechanism of resistance to Palbo in ER+ EndoR models. Transcriptomic profiling of the Palbo-sensitive and R cells, currently underway, will provide an in-depth understanding of the role of AP-1 as well as other key targets and associated molecular mechanisms in Palbo resistance.

Citation Format: De Angelis C, Nardone A, Cataldo ML, Veeraraghavan J, Fu X, Giuliano M, Malorni L, Jeselsohn R, Osborne KC, Schiff R. AP-1 as a potential mediator of resistance to the cyclin-dependent kinase (CDK) 4/6-inhibitor palbociclib in ER-positive endocrine-resistant breast cancer [abstract]. In: Proceedings of the 2017 San Antonio Breast Cancer Symposium; 2017 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2018;78(4 Suppl):Abstract nr P4-03-05.

HER2 Reactivation through Acquisition of the HER2 L755S Mutation as a Mechanism of Acquired Resistance to HER2-targeted Therapy in HER2+ Breast Cancer

Purpose: Resistance to anti-HER2 therapies in HER2+ breast cancer can occur through activation of alternative survival pathways or reactivation of the HER signaling network. Here we employed BT474 parental and treatment-resistant cell line models to investigate a mechanism by which HER2+ breast cancer can reactivate the HER network under potent HER2-targeted therapies.Experimental Design: Resistant derivatives to lapatinib (L), trastuzumab (T), or the combination (LR/TR/LTR) were developed independently from two independent estrogen receptor ER+/HER2+ BT474 cell lines (AZ/ATCC). Two derivatives resistant to the lapatinib-containing regimens (BT474/AZ-LR and BT474/ATCC-LTR lines) that showed HER2 reactivation at the time of resistance were subjected to massive parallel sequencing and compared with parental lines. Ectopic expression and mutant-specific siRNA interference were applied to analyze the mutation functionally. In vitro and in vivo experiments were performed to test alternative therapies for mutant HER2 inhibition.Results: Genomic analyses revealed that the HER2L755S mutation was the only common somatic mutation gained in the BT474/AZ-LR and BT474/ATCC-LTR lines. Ectopic expression of HER2L755S induced acquired lapatinib resistance in the BT474/AZ, SK-BR-3, and AU565 parental cell lines. HER2L755S-specific siRNA knockdown reversed the resistance in BT474/AZ-LR and BT474/ATCC-LTR lines. The HER1/2-irreversible inhibitors afatinib and neratinib substantially inhibited both resistant cell growth and the HER2 and downstream AKT/MAPK signaling driven by HER2L755S in vitro and in vivoConclusions: HER2 reactivation through acquisition of the HER2L755S mutation was identified as a mechanism of acquired resistance to lapatinib-containing HER2-targeted therapy in preclinical HER2-amplified breast cancer models, which can be overcome by irreversible HER1/2 inhibitors. Clin Cancer Res; 23(17); 5123-34. ©2017 AACR.

De-escalation of treatment in HER2-positive breast cancer: Determinants of response and mechanisms of resistance

Overexpression and/or gene amplification of HER2, a crucial member of the HER family of four receptors, occur in about 15-20% of breast cancers and define an aggressive subtype of the disease. Activated HER homo and heterodimers govern a complex and redundant downstream signaling network that regulates cell survival and metastasis. Despite treatment with effective HER2-targeted therapies, many HER2-positive tumors fail to respond, or initially respond but eventually develop resistance. One of the upfront reasons for this treatment failure is failure to accurately select the tumors that are truly dependent on HER2 for survival and so would benefit the most from HER2-targeted therapy. In these truly HER2-addicted tumors (i.e. physiologically dependent), resistance could be the result of an incomplete inhibition of signaling at the HER receptor layer. In this regard, preclinical and clinical studies have documented the superiority of combination anti-HER2 therapy over single agent therapy to achieve a more comprehensive inhibition of the various HER receptor dimers. HER2 can be further activated or reactivated by mutations or other alterations in HER2 itself, or in other HER family members. Even when a complete and sustained HER inhibition is achieved, resistance to anti-HER therapy can arise by other somewhat dominant mechanisms, including preexisting or emerging alternative signaling pathways such as the estrogen receptor, deregulated downstream signaling components, especially of the PI3K pathway, and the tumor immune microenvironment. Most of the clinical trials that have investigated the efficacy of anti-HER2 therapies took place in the background of aggressive chemotherapy regimens, thus confounding the identification of key factors of resistance to the anti-HER2 treatments. Recent studies, however, have suggested that some HER2-amplified tumors may benefit from anti-HER2 therapy combined with only a single chemotherapy agent or in the absence of any chemotherapy. This de-escalation approach, a promising therapeutic strategy, is currently being explored in the clinic. In this review, we summarize the major molecular determinants that play a crucial role in influencing tumor response and resistance to HER2-targeted therapy, and discuss the growing need for patient stratification in order to facilitate the development of de-escalation strategies using HER2-targeted therapy alone with no chemotherapy.

Abstract 5741: Evaluating the role of recurrent ESR1-CCDC170 fusion in breast cancer endocrine resistance

Background: Recurrent gene fusions resulting from chromosome rearrangements are central to the formation of cancer. Previously, our lab identified recurrent rearrangements between the estrogen receptor α (ERα) gene ESR1 and its neighbor gene CCDC170, in ~6-8% of luminal B tumors, a more aggressive form of ER+ breast cancer that has worse clinical outcome after endocrine therapy. ESR1-CCDC170 rearrangements enable the expression of different-sized N-terminally truncated CCDC170 (ΔCCDC170) under ESR1 promoter, such as the E2-E7 or E2-E10 variants in which exon 2 of ESR1 fused with exon 7 or exon 10 of CCDC170. Consistent with the behavior of luminal B tumors, ectopic ΔCCDC170 expression in ER+ breast cancer cells transformed cells to a more aggressive form and led to reduced endocrine sensitivity in vitro, as well as enhanced xenograft growth in vivo. In the present study, we examined the role of ESR1-CCDC170 in breast cancer endocrine resistance in vivo and further elucidated the potential engaged mechanisms of its actions.

Results: Our in vivo endocrine sensitivity study showed that, while tamoxifen (Tam) treatment rendered tumor regression in the vector-expressing T47D xenograft tumors, it made the E2-E7 overexpressing T47D xenografts static and less likely to regress, whereas the E2-E10 overexpressing xenografts, though grew initially upon Tam treatment, became cytostatic and showed significant reduction in tumor regression. Kaplan-Meier analysis revealed a significantly worse progression-free survival for E2-E7 (p&lt;0.01) and E2-E10 (p&lt;0.001) overexpressing tumors treated with Tam compared to the control tumors. These data suggest that ESR1-CCDC170 variants differ in level of reduced responsivity to Tam, and may render the T47D xenografts less sensitive or resistant to Tam in vivo. Further studies suggest that ΔCCDC170 protein preferentially localizes to cytoplasm (different from wild-type CCDC170 protein enriched in nucleus), physically interacts with ERα and HER2, and forms homodimers. RPPA analysis showed that silencing of ESR1-CCDC170 repressed ERα and BCL2 protein levels, as well as total/phospho-HER2 levels, consistent with the findings of ΔCCDC170 interactions with ERα and HER2. Importantly, analysis of ΔCCDC170 protein sequence revealed a potential high-affinity ATP-binding pocket at its C-terminus, suggesting that ΔCCDC170 may be directly druggable.

Conclusion: These data suggest a potential role of ESR1-CCDC170 in mediating breast cancer endocrine responsivity, which may possibly through cytoplasmic mislocalization of ΔCCDC170 and interactions with ERα and HER2. Further studies will affirm and elucidate the role of overexpressed or endogenous ESR1-CCDC170 in breast cancer endocrine resistance, and pinpoint the precise mechanisms of its oncogenic function, in order to develop a novel target therapy against it for the fusion-carrying patient community around the world.

Citation Format: Yiheng Hu, Jamunarani Veeraraghavan, Xian Wang, Ying Tan, Jin-Ah Kim, Rachel Schiff, Xiao-Song Wang. Evaluating the role of recurrent ESR1-CCDC170 fusion in breast cancer endocrine resistance [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 5741. doi:10.1158/1538-7445.AM2017-5741

Comprehensive functional analysis of the tousled-like kinase 2 frequently amplified in aggressive luminal breast cancers

More aggressive and therapy-resistant oestrogen receptor (ER)-positive breast cancers remain a great clinical challenge. Here our integrative genomic analysis identifies tousled-like kinase 2 (TLK2) as a candidate kinase target frequently amplified in ∼10.5% of ER-positive breast tumours. The resulting overexpression of TLK2 is more significant in aggressive and advanced tumours, and correlates with worse clinical outcome regardless of endocrine therapy. Ectopic expression of TLK2 leads to enhanced aggressiveness in breast cancer cells, which may involve the EGFR/SRC/FAK signalling. Conversely, TLK2 inhibition selectively inhibits the growth of TLK2-high breast cancer cells, downregulates ERα, BCL2 and SKP2, impairs G1/S cell cycle progression, induces apoptosis and significantly improves progression-free survival in vivo. We identify two potential TLK2 inhibitors that could serve as backbones for future drug development. Together, amplification of the cell cycle kinase TLK2 presents an attractive genomic target for aggressive ER-positive breast cancers.

Luminal B oestrogen receptor positive breast cancers are generally aggressive tumors with poor outcomes. Here, the authors show that the kinase TLK2 is amplified and overexpressed in these tumors and correlates with reduced survival, TLK2 inhibition induces apoptosis in vitro and improves survival in mice.

Amplification of TLK2 Induces Genomic Instability via Impairing the G2-M Checkpoint

Managing aggressive breast cancers with enhanced chromosomal instability (CIN) is a significant challenge in clinics. Previously, we described that a cell cycle-associated kinase called Tousled-like kinase 2 (TLK2) is frequently deregulated by genomic amplifications in aggressive estrogen receptor-positive (ER⁺) breast cancers. In this study, it was discovered that TLK2 amplification and overexpression mechanistically impair Chk1/2-induced DNA damage checkpoint signaling, leading to a G₂-M checkpoint defect, delayed DNA repair process, and increased CIN. In addition, TLK2 overexpression modestly sensitizes breast cancer cells to DNA-damaging agents, such as irradiation or doxorubicin. To our knowledge, this is the first report linking TLK2 function to CIN, in contrast to the function of its paralog TLK1 as a guardian of genome stability. This finding yields new insight into the deregulated DNA damage pathway and increased genomic instability in aggressive ER⁺ breast cancers.

IMPLICATIONS

Targeting TLK2 presents an attractive therapeutic strategy for the TLK2-amplified breast cancers that possess enhanced genomic instability and aggressiveness. Mol Cancer Res; 14(10); 920-7. ©2016 AACR.

Recurrent and pathological gene fusions in breast cancer: current advances in genomic discovery and clinical implications

Gene fusions have long been considered principally as the oncogenic events of hematologic malignancies, but have recently gained wide attention in solid tumors due to several milestone discoveries and the advancement of deep sequencing technologies. With the progress in deep sequencing studies of breast cancer transcriptomes and genomes, the discovery of recurrent and pathological gene fusions in breast cancer is on the focus. Recently, driven by new deep sequencing studies, several recurrent or pathological gene fusions have been identified in breast cancer, including ESR1-CCDC170, SEC16A-NOTCH1, SEC22B-NOTCH2, and ESR1-YAP1 etc. More important, most of these gene fusions are preferentially identified in the more aggressive breast cancers, such as luminal B, basal-like, or endocrine-resistant breast cancer, suggesting recurrent gene fusions as additional key driver events in these tumors other than the known drivers such as the estrogen receptor. In this paper, we have comprehensively summarized the newly identified recurrent or pathological gene fusion events in breast cancer, reviewed the contributions of new genomic and deep sequencing technologies to new fusion discovery and the integrative bioinformatics tools to analyze these data, highlighted the biological relevance and clinical implications of these fusion discoveries, and discussed future directions of gene fusion research in breast cancer.

Abstract PD2-05: Evaluating the role of recurrent ESR1-CCDC170 in breast cancer endocrine resistance

Background

Recurrent gene fusions resulting from chromosome translocations are critical genetic aberrations causing cancer. In our previous study, we identified recurrent rearrangements between ESR1 and its neighbor, CCDC170, in 6-8% of luminal B tumors. Luminal B subtype is a more aggressive ER+ breast cancer, with a higher risk of early relapse after endocrine therapy. These rearrangements enable the expression of N-terminally truncated CCDC170 (ΔCCDC170) under ESR1 promoter. Consistent with the behavior of luminal B tumors, ectopic ΔCCDC170 expression in ER+ breast cancer cells, led to markedly increased cell motility, invasion, anchorage-independent growth, and reduced endocrine sensitivity in vitro, as well as enhanced xenograft growth in vivo. In the present study, we studied the role of ESR1-CCDC170 in breast cancer endocrine resistance in vivo and explored the potential mechanism.

Methods

To study endocrine resistance in vivo, we transplanted T47D cells stably overexpressing (OE) control (empty) construct or 2 ΔCCDC170 fusion variants (E2-E7 and E2-E10) bilaterally to 4-6 week old female athymic nude mice (supplemented with 17β-estradiol pellets). The tumor growth was monitored biweekly and tumor volume was measured by the formula 1/2(length × width2). When the tumors reach 150–200 mm3, mice were randomly allocated to vehicle or tamoxifen (tam) treatment groups. For ERE luciferase assay, cells were co-transfected with ERE luciferase reporter (ERE-TK-Luc) and pCMV β-galactosidase. The luciferase levels were measured and normalized to β-gal activity. For immunoblot analysis, T47D OE cells were estrogen-deprived, serum-starved, and treated with vehicle, estrogen (E2) or tam. Reverse Phase Protein Array (RPPA) analysis was performed using ∼200 validated antibodies against an array of key signaling molecules in cancer.

Results

Our in vivo endocrine sensitivity study showed that, while T47D vector control tumors mostly regressed after tam treatment, the regression of E2-E7 tumors was significantly slower. Moreover, E2-E10 tumors continued to grow despite tam treatment. These observations suggest that ΔCCDC170 may render the T47D xenografts less sensitive to tam in vivo. Kaplan–Meier analysis revealed a significantly worse progression-free survival (defined by tumor doubling time) for E2-E7 (p&lt;0.01) and E2-E10 (p&lt;0.001) tumors treated with tam compared to control tumors. ΔCCDC170 expression in T47D cells enhanced the ER transcriptional activity in the presence of E2 but not tam, suggesting that the fusion-mediated endocrine-sensitivity changes is unlikely due to restoration of classic ER activity. Immunoblot analysis of T47D OE cells revealed hyperactive growth factor signaling even after serum withdrawal, which was not significantly affected by tam treatment. Preliminary RPPA analysis revealed upregulation of key signaling molecules in T47D cells expressing ΔCCDC170, such as Her3, AMPK, Akt, Erk, c-Myc, and Src-3.

Conclusion

These data suggest a potential role of ESR1-CCDC170 in mediating breast cancer endocrine resistance, presumably due to hyperactive growth factor signaling endowed by this fusion. Further studies are required to elucidate the role of endogenous ESR1-CCDC170 in breast cancer endocrine resistance, and discover the precise engaged mechanisms.

Citation Format: Hu Y, Veeraraghavan J, Wang X, Tan Y, Kim J, Schiff R, Wang X-S. Evaluating the role of recurrent ESR1-CCDC170 in breast cancer endocrine resistance. [abstract]. In: Proceedings of the Thirty-Eighth Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2015 Dec 8-12; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2016;76(4 Suppl):Abstract nr PD2-05.