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.