Modeling the novel SERD elacestrant in cultured fulvestrant-refractory HR-positive breast circulating tumor cells

PURPOSE

Metastatic hormone receptor-positive (HR+) breast cancer initially responds to serial courses of endocrine therapy, but ultimately becomes refractory. Elacestrant, a new generation FDA-approved oral selective estrogen receptor degrader (SERD) and antagonist, has demonstrated efficacy in a subset of women with advanced HR+breast cancer, but there are few patient-derived models to characterize its effect in advanced cancers with diverse treatment histories and acquired mutations.

METHODS

We analyzed clinical outcomes with elacestrant, compared with endocrine therapy, among women who had previously been treated with a fulvestrant-containing regimen from the recent phase 3 EMERALD Study. We further modeled sensitivity to elacestrant, compared with the currently approved SERD, fulvestrant in patient-derived xenograft (PDX) models and cultured circulating tumor cells (CTCs).

RESULTS

Analysis of the subset of breast cancer patients enrolled in the EMERALD study who had previously received a fulvestrant-containing regimen indicates that they had better progression-free survival with elacestrant than with standard-of-care endocrine therapy, a finding that was independent estrogen receptor (ESR1) gene mutations. We modeled elacestrant responsiveness using patient-derived xenograft (PDX) models and in ex vivo cultured CTCs derived from patients with HR+breast cancer extensively treated with multiple endocrine therapies, including fulvestrant. Both CTCs and PDX models are refractory to fulvestrant but sensitive to elacestrant, independent of mutations in ESR1 and Phosphatidylinositol-4,5-Bisphosphate 3-Kinase Catalytic Subunit Alpha (PIK3CA) genes.

CONCLUSION

Elacestrant retains efficacy in breast cancer cells that have acquired resistance to currently available ER targeting therapies. Elacestrant may be an option for patients with HR+/HER2- breast cancer whose disease progressed on fulvestrant in the metastatic setting.

TRANSLATIONAL RELEVANCE

Serial endocrine therapy is the mainstay of management for metastatic HR+breast cancer, but acquisition of drug resistance highlights the need for better therapies. Elacestrant is a recently FDA-approved novel oral selective estrogen receptor degrader (SERD), with demonstrated efficacy in the EMERALD phase 3 clinical trial of refractory HR+breast cancer. Subgroup analysis of the EMERALD clinical trial identifies clinical benefit with elacestrant in patients who had received prior fulvestrant independent of the mutational status of the ESR1 gene, supporting its potential utility in treating refractory HR+breast cancer. Here, we use pre-clinical models, including ex vivo cultures of circulating tumor cells and patient-derived xenografts, to demonstrate the efficacy of elacestrant in breast cancer cells with acquired resistance to fulvestrant.

Phase 1/2 dose escalation study of NUV-422, a potent inhibitor of cyclin-dependent kinases 2, 4, and 6, in recurrent or refractory (r/r) high-grade gliomas (HGG) and solid tumors

TPS3173

Background: CDKs that govern the G1-S transition of the cell cycle (CDK2, CDK4 and CDK6) are deregulated in many cancers. CDK2 expression, in particular, is associated with worse overall survival in glioblastoma (GB), disease-free recurrence in prostate cancer, and resistance to approved CDK4/6 inhibitors (CDK4/6i) in breast cancer. These provide rationale for inhibition of CDK2/4/6 as a potential novel therapeutic strategy in these cancers. NUV-422 is a potent (low nM) small molecule CDK2/4/6i with limited activity against CDK1, a target potentially associated with toxicities in other CDKi. Preclinical studies have shown that NUV-422 has favorable blood-brain barrier penetration, inhibited growth of multiple glioma cell lines in vitro, and exhibited antitumor activity in GB xenograft models. NUV-422 also exhibited antitumor activity in multiple patient-derived xenograft (PDX) models of HR+ HER2- mBC resistant to CDK4/6i, and PDX models of prostate cancer resistant to anti-androgens. Methods: NUV-422-02 (NCT04541225) is a phase 1/2, first in human, open label, multicenter study to evaluate single-agent NUV-422 (given orally) in patients with advanced solid tumors (r/r HGG, r/r HR+ HER2- mBC, or r/r mCRPC). The Ph 1 dose escalation will use a 3+3 design to evaluate safety, tolerability, and PK of NUV-422 and establish the recommended phase 2 dose (RP2D). Ph 1 also includes a randomized surgical substudy to characterize PK and pharmacodynamics (PD) of preoperative NUV-422 in resected GB tumor tissue. After the RP2D is identified, parallel enrollment into phase 2 expansion cohorts will begin. Cohort 1 will evaluate isocitrate dehydrogenase wild type (IDH-WT) GB. Cohort 2 will evaluate HR+ HER2- mBC (without active brain metastases); Cohort 3 will evaluate mCRPC; and Cohort 4 will evaluate HR+ HER2- mBC with active brain metastases. The Ph 2 primary endpoint is objective response rate. Secondary efficacy endpoints include clinical benefit rate, duration of response, progression-free survival, and overall survival. Response assessments will be based on response criteria appropriate to the tumor type (HGG [RANO]; HR+HER2- mBC [RECIST 1.1 or RANO-Brain Metastases]; mCRPC [RECIST 1.1, PCWG3, prostate-specific antigen decrease]). Blood, urine, or tumor tissue will be obtained to assess safety, PK, PD, and for additional exploratory analyses to characterize NUV-422 mechanism of action. Enrollment was initiated in December 2020, and dose escalation is ongoing. The NUV-422 program will also be expanded in 2022 to include additional studies in mBC and mCRPC in combination with standard of care treatments. Clinical trial information: NCT04541225.

Abstract OT1-04-04: EMERALD: A randomized, open-label, phase 3 trial to evaluate the efficacy and safety of elacestrant (RAD1901), a novel oral selective estrogen receptor degrader (SERD), vs investigator’s choice of endocrine therapy for ER+/HER2- advanced breast cancer following CDK4/6 inhibitor therapy

Background: Estrogen receptor-positive (ER+) breast cancer (BC) comprises ~70% of all BC and advanced/metastatic ER+ disease (mBC) remains a major clinical challenge. The addition of CDK4/6i to endocrine therapy (ET) has improved progression-free survival (PFS); however, novel treatments are needed after progression. Putative mechanisms of endocrine resistance, such as ESR1 mutations (mESR1), also indicate the need for additional therapies. Elacestrant, an oral SERD, demonstrated anti-tumor activity in preclinical models of ER+ BC, including models resistant to CDK4/6 inhibitors (CDK4/6i) and models with mESR1. An interim evaluation of a phase 1 trial (NCT02338349) of elacestrant in heavily pretreated patients (pts) with mBC, demonstrated an overall response rate (ORR) of 27% with a PFS of 5.4 mo (Bardia, SABCS, 2017). Responses were seen in pts with prior CDK4/6i and with wild-type (WT) or mESR1.

Methods: This is a multicenter, international, randomized, open-label, active-controlled phase 3 trial for post-menopausal women or men with mBC. Pts must have received 1-2 prior lines of ET, ≤1 line of chemotherapy for mBC, and have documented progression on a CDK4/6i. Pts with measurable disease (RECIST v1.1) or bone-only disease are eligible. Pts are randomized 1:1 to elacestrant (400 mg orally daily) or investigator’s choice of fulvestrant or an aromatase inhibitor. Stratification factors include ESR1 mutation status (detected by ctDNA), prior fulvestrant treatment and presence of visceral disease. The co-primary endpoints are PFS by blinded independent review committee (IRC) in pts with mESR1 and in all pts (mESR1 or mESR1 not detected). Secondary endpoints include: overall survival; PFS by investigator review; ORR, duration of response, and clinical benefit rate; safety; pharmacokinetics; and quality of life. Approximately 466 pts will be enrolled to detect 340 PFS events in all pts (power ≥90%, hazard ratio (HR) = 0.667) and 160 PFS events in the mESR1 subset (power ≥80%, HR = 0.610), overall α level at 2-sided 5% using the Hochberg procedure. The EMERALD study is open for enrollment. NCT03778931

Citation Format: Philippe Aftimos, Aditya Bardia, Virginia G Kaklamani, Janice Lu, Teeru Bihani, JungAh Jung, Alfred T Anderson-Villaluz, Maureen G Conlan, Javier Cortes. EMERALD: A randomized, open-label, phase 3 trial to evaluate the efficacy and safety of elacestrant (RAD1901), a novel oral selective estrogen receptor degrader (SERD), vs investigator’s choice of endocrine therapy for ER+/HER2- advanced breast cancer following CDK4/6 inhibitor therapy [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 OT1-04-04.

EMERALD: Phase III trial of elacestrant (RAD1901) vs endocrine therapy for previously treated ER+ advanced breast cancer

Elacestrant is a novel, nonsteroidal, orally bioavailable selective estrogen receptor degrader (SERD) that has demonstrated activity in patients with estrogen receptor (ER)-positive/HER2-negative breast cancer previously treated with endocrine therapies including fulvestrant and/or CDK 4/6 inhibitor therapy, and in those with ESR1 mutations (ESR1-mut) known to confer endocrine resistance. Herein, we describe the design and methodology of EMERALD, an international, multicenter, randomized, open-label, active-controlled, Phase III clinical study comparing the efficacy and safety of elacestrant to standard-of-care endocrine monotherapy treatment (fulvestrant or an aromatase inhibitor, per investigator's choice) in patients with ER-positive/HER2-negative advanced breast cancer. Primary end points are progression-free survival in ESR1-mut patients and in all patients (NCT03778931; EudraCT 2018-002990-24).

EMERALD: A randomized, open label, phase III trial to evaluate the efficacy and safety of elacestrant (RAD1901) versus investigator’s choice (IC) of endocrine therapy (ET) for ER+/HER2- advanced breast cancer (BC) following CDK4/6 inhibitor (CDK4/6i) therapy

TPS1104

Background: Estrogen receptor-positive (ER+) BC comprises ~70% of all BC and advanced/metastatic ER+ disease (mBC) remains a major clinical challenge. The addition of CDK4/6i to ET has improved progression-free survival (PFS); however, novel treatments are needed after progression. Putative mechanisms of endocrine resistance, such as ESR1 mutations (mESR1), also indicate the need for additional therapies. Elacestrant, an oral selective estrogen receptor degrader (SERD), demonstrated anti-tumor activity in preclinical models of ER+ BC, including models resistant to CDK4/6i and models with mESR1. An interim evaluation of a phase 1 trial ( NCT02338349 ) of elacestrant in heavily pretreated patients (pts) with mBC, demonstrated an overall response rate (ORR) of 27% with a PFS of 5.4 mo (Bardia, SABCS, 2017). Responses were seen in pts with prior CDK4/6i and with wild-type (WT) or mESR1. Methods: This is a multicenter, international, randomized, open-label, active-controlled phase 3 trial for post-menopausal women or men with mBC. Pts must have received 1-2 prior lines of ET, ≤1 line of chemotherapy for mBC, and have documented progression on a CDK4/6i. Pts with measurable disease (RECIST v1.1) or bone-only disease are eligible. Pts are randomized 1:1 to elacestrant (400 mg orally daily) or IC of fulvestrant or an aromatase inhibitor. Stratification factors include ESR1 mutation status (detected by ctDNA), prior fulvestrant treatment and presence of visceral disease. The primary endpoints are PFS by blinded independent review committee (IRC) in pts with mESR1 and in all pts (WT or mESR1). Secondary endpoints include: overall survival; PFS by IRC in WT, PFS by investigator review; ORR, duration of response, and clinical benefit rate; safety; pharmacokinetics; and quality of life. Approximately 466 pts will be enrolled to detect 340 PFS events in all pts (power ≥90%, hazard ratio (HR) = 0.667) and 160 PFS events in the mESR1 subset (power ≥80%, HR = 0.610), overall α level at 2-sided 5% using the Hochberg procedure. The EMERALD study is open for enrollment. Clinical trial information: NCT03778931.

Abstract P4-13-03: Elacestrant (RAD1901) demonstrates anti-tumor activity in models resistant to CDK4/6 inhibitors

Background: Approximately 75% of all breast cancers diagnosed are estrogen receptor-positive (ER+) and currently approved endocrine therapies rely heavily on blocking of the ER signaling pathway. In recent years, the combination of an endocrine agent with other targeted agents have been evaluated to address endocrine resistance and improve progression-free survival (PFS). Recently, it was demonstrated that the addition of a cyclin-dependent kinase 4/6 (CDK4/6) inhibitor to an endocrine agent roughly doubles PFS, leading to the approval and use of certain CDK4/6 inhibitors in combination with either aromatase inhibitors in the first-line metastatic setting or in combination with the selective estrogen receptor degrader (SERD), fulvestrant, in the second-line metastatic setting. While combining CDK4/6 inhibitors and endocrine therapy can result in significantly increased PFS, patients eventually progressed on these combinations, and to date, there is no cure for patients with advanced metastatic ER+ breast cancer. Given the increased use of CDK4/6 inhibitors in the ER+ breast cancer treatment paradigm, it will be important to understand how treatment resistance to CDK4/6 inhibitors manifests in order to optimize therapeutic strategies to target this patient population. We have previously described elacestrant (RAD1901), a novel and orally bioavailable SERD, as an inhibitor of ER+ breast cancer growth in in vitro models and in in vivo patient-derived xenograft (PDX) models. Importantly, elacestrant inhibited the growth of PDX models that were derived from heavily pretreated patients, models harboring mutations in ESR1, and models insensitive to standard of care endocrine therapies. Given these results, we hypothesized that elacestrant would have anti-tumor activity in a CDK4/6 inhibitor-resistant setting. Herein, we describe elacestrant activity in multiple in vitro and in vivo models of CDK4/6 inhibitor resistance in both wild-type and mutant ESR1 backgrounds.

Methods: In vitro models of estrogen-independent ER+ breast cancer, harboring either wild-type or mutant ER, were exposed to increasing concentrations of approved CDK4/6 inhibitors: palbociclib, ribociclib, or abemaciclib. ER expression/signaling, changes in cell cycle mediators, and the effects of elacestrant and other SERDs were examined in these representative models.

Results: Despite prolonged exposure to CDK4/6 inhibitors, the resistant cell lines retained ER, ER signaling, and importantly, ER-driven proliferation. Elacestrant induced dose-dependent growth inhibition in CDK4/6 inhibitor-resistant cells, and this effect was independent of the CDK4/6 inhibitor used to generate resistance. Elacestrant also demonstrated in vivo tumor growth inhibition of CDK4/6 inhibitor-resistant ER+ PDX models.

Conclusions: Our preclinical data demonstrate that elacestrant is a SERD that can inhibit tumor growth in a CDK4/6 inhibitor-resistant setting and provides rationale for examining elacestrant in patients that have progressed on a combination of endocrine therapy with a CDK4/6 inhibitor.

Citation Format: Patel H, Tao N, Arlt H, Bihani T. Elacestrant (RAD1901) demonstrates anti-tumor activity in models resistant to CDK4/6 inhibitors [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 P4-13-03.

Abstract P6-20-08: Anti-tumor activity of elacestrant (RAD1901) in models harboring ESR1 mutations resistant to standard of care therapies

Background: Estrogen receptor positive (ER+) breast cancers make up approximately 75% of all breast cancers diagnosed and ER, a protein encoded by the ESR1 gene, plays a major role in the initiation, growth and survival of these cancers. Current targeted therapies inhibit the ER pathway by either blocking the synthesis of the natural ligand of ER, estradiol, (aromatase inhibitors (AI)), or by antagonizing and/or degrading the receptor (selective estrogen receptor modulators (SERMs) and selective estrogen receptor degraders (SERDs)). AIs are used in both the adjuvant and metastatic setting and recent clinical reports have shown that 20-50% of patients that had received AI therapy had detectable mutations in the ER ligand binding domain (ER-LBD). Two frequently found point mutations in the ER-LBD, Y537S and D538G, have been shown to result in estradiol-independence and constitutive activation of ER, consistent with their ability to cause resistance to AIs. While the selection of ESR1 mutations post-AI has been demonstrated clinically, the clinical response of ESR1 mutant tumors to fulvestrant, an approved SERD, is not fully understood. Preclinical studies have suggested that ESR1 mutations can cause decreased binding and a corresponding decrease in potency of ER antagonists, including fulvestrant (SERD) and tamoxifen (SERM). Conversely, clinical data from the SoFEA, PALOMA-3, and FERGI trials suggested the presence of ESR1 mutations did not alter fulvestrant activity. The limited clinical data that exists, however, is based on retrospective study designs with relatively small data sets, making it difficult to accurately predict fulvestrant activity against specific mutations and the activity of fulvestrant against tumors that harbor multiple mutations. In fact, recent additional data from the PALOMA-3 trial suggests that the Y537S mutation, specifically, was selected out in clinical samples from patients treated with fulvestrant, more closely matching preclinical results. This suggests there may be certain contexts of ESR1 mutations where fulvestrant may have limited activity. It will be important to further understand the consequence of specific mutations and to utilize therapies that have activity against all ESR1 mutations. We have previously described elacestrant (RAD1901), a novel orally bioavailable SERD, that exhibited activity in multiple ER+ breast cancer models. Interestingly, elacestrant exhibited similar effects to fulvestrant in in vitro ESR1 mutant models, however, in some in vivo PDX models harboring the Y537S mutation elacestrant inhibited growth, while fulvestrant had limited activity. Here, we describe a more complete in vivo dataset describing elacestrant activity versus fulvestrant in multiple patient-derived xenograft (PDX) models harboring ESR1 mutations.

Methods: Multiple PDX models harboring natural mutations in ESR1 or genetically-engineered CRISPR models were used to assess the anti-tumor efficacy and the pharmacokinetic/pharmacodynamic properties of elacestrant and fulvestrant.

Results: Elacestrant significantly inhibited the growth of xenograft models harboring ESR1 mutations, including those harboring Y537S or D538G mutations and models that were insensitive to fulvestrant and tamoxifen.

Citation Format: Patel H, Tao N, Arlt H, Bihani T. Anti-tumor activity of elacestrant (RAD1901) in models harboring ESR1 mutations resistant to standard of care therapies [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 P6-20-08.

Abstract P4-04-17: Elacestrant (RAD1901) demonstrates anti-tumor activity in a fulvestrant-resistant PDX model

Background: Breast cancer is subdivided into categories based on tumor receptor status, with estrogen-receptor positive (ER+) breast cancer making up approximately two-thirds of all breast cancers diagnosed. In advanced or metastatic disease, dependence on ER signaling is often retained even after multiple rounds of endocrine therapy, supporting the use of ER targeting agents for several lines of treatment prior to chemotherapy. Selective estrogen receptor degraders (SERDs) have gained recent attention as ER antagonists given their ability to target and degrade ER. Fulvestrant, the only approved SERD on the market, is currently used as a second-line therapy in the metastatic setting, however, the intramuscular route of administration and pharmacokinetic properties of fulvestrant have fueled the development of more potent and orally bioavailable SERDs. We have previously described elacestrant (RAD1901), a novel and orally bioavailable SERD, as an inhibitor of ER+ breast cancer growth in preclinical patient-derived xenograft (PDX) models, including those that are insensitive to fulvestrant. Here, we describe elacestrant activity in an ER+ PDX model that had been treated with multiple rounds of fulvestrant treatment.

Methods: An ER+/PR+ PDX model, MAXF-713, derived from a treatment-naïve patient, was passaged multiple times in the presence of fulvestrant over the course of a year. After each round of fulvestrant treatment, elacestrant or fulvestrant were evaluated for anti-tumor activity. Pharmacodynamic endpoints including changes in ER and ER target gene expression were also evaluated.

Results: Despite repeated exposure to fulvestrant, the PDX model retained sensitivity to elacestrant. In fact, regardless of the number of prior rounds of fulvestrant treatment, elacestrant produced consistent tumor growth inhibition within every passage, while the effects produced by fulvestrant exhibited a high degree of variability.

Conclusions: Our data demonstrate that elacestrant is a SERD that can inhibit tumor growth in a fulvestrant-resistant preclinical setting and provides rationale for examining elacestrant in the clinical setting in patients that have progressed on fulvestrant treatment.

Citation Format: Arlt H, Garner F, Bihani T. Elacestrant (RAD1901) demonstrates anti-tumor activity in a fulvestrant-resistant PDX model [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 P4-04-17.

Abstract P4-04-14: Anti-tumor activity of elacestrant (RAD1901) in combination with alpelisib (BYL-719) in patient-derived xenograft models of ER+ breast cancer

Estrogen-receptor positive (ER+) breast cancers make up approximately 70% of all breast cancers diagnosed. While patients with ER+ breast cancer have a better prognosis than other subtypes of breast cancer, the majority of those with advanced metastatic disease will eventually relapse. This has been attributed, in part, to mutations in the ER gene that result in constitutive activation of ER and contribute to aromatase inhibitor treatment resistance. As a strategy to deliver a more durable response in this setting, the use of selective estrogen receptor degraders (SERDs) that target and inhibit both wild-type and mutant ER has gained widespread attention. Indeed, fulvestrant, the only approved SERD on the market, is currently used as a second-line therapy in the metastatic setting, however, the intramuscular route of administration and pharmacokinetic properties of fulvestrant have fueled the development of orally bioavailable SERDs. We have previously described elacestrant (RAD1901), a novel and orally bioavailable selective estrogen receptor degrader (SERD) as an inhibitor of ER+ breast cancer growth in preclinical models, including those that harbor ER mutations and those that are insensitive to fulvestrant. In addition to ER mutations, the activation of parallel oncogenic pathways can also drive endocrine resistance, with the PI3K/Akt/mTOR pathway chief among those driving growth and treatment resistance to endocrine therapy. Consistent with these above-mentioned findings, recent clinical strategies to treat advanced ER+ disease have involved combining SERDs with PI3K inhibitors. Alpelisib (BYL-719) is a PI3K-alpha specific inhibitor that is being developed in combination with endocrine agents for the treatment of ER+ breast cancer. Here, we examined the effect of elacestrant in combination with alpelisib, in two ER+ breast cancer PDX models (one harboring wild-type ER and one harboring a Y537S mutation in the ER gene). The combination of elacestrant (10mg/kg) and alpelisib (35mg/kg) was well tolerated and resulted in significant tumor growth inhibition in both PDX models. Interestingly, in the mutant ER PDX model, the combination resulted in significantly greater growth inhibition relative to either compound alone. These data suggest the dual inhibition of the ER and PI3K signaling pathways with elacestrant and alpelisib produces significant anti-tumor activity in clinically-relevant PDX models, including those harboring ER mutations.

Citation Format: Sankaran B, Garner F, Hattersley G, Purandare D, Bihani T. Anti-tumor activity of elacestrant (RAD1901) in combination with alpelisib (BYL-719) in patient-derived xenograft models of ER+ 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 P4-04-14.

Abstract P4-04-09: New oral SERD elacestrant (RAD1901) shows efficacy in breast cancer models harbouring ESR1 mutations and enhances the antiproliferative activity of mTORC1 and CDK4/6 inhibitors

Background: Targeting estrogen receptor (ER) signalling is the main therapeutic option for ER+ breast cancer (BC). However, over 30% of patients relapse with endocrine resistance emphasising the need for improved therapeutic strategies. The prevalence of ESR1 mutations in relapsed tumours highlights the sustained reliance on ER signalling, rationalising continued targeting of ER. Unlike other endocrine therapies such as aromatase inhibitors (AI) and tamoxifen, selective ER degraders (SERDs) are competitive ER antagonists, that induce a conformational change in ER resulting in ubiquitination and degradation via the proteasomal pathway. Fulvestrant has shown clinical utility in advanced BC but is limited by its pharmaceutical properties highlighting the need for SERDs with enhanced bioavailability and pharmacokinetic properties. Here, we show that elacestrant (RAD1901) an orally bioavailable SERD, has antitumor activity in endocrine sensitive and resistant models of ER+ BC. Furthermore, elacestrant enhances the efficacy of mTORC1 inhibitor, everolimus and CDK4/6 inhibition, in model systems.

Methods: Several ER+ BC lines adapted to long-term E deprivation (LTED) and harbouring wild-type or a naturally occurring ESR1 mutation, were treated with elacestrant or fulvestrant +/- estradiol (E2). Effects on cell proliferation, cell signalling, cell cycle, transcription, ER protein stability and ER genomic binding were assessed. Efficacy in combination with everolimus, palbociclib and abemaciclib was also evaluated.

Results: Cell proliferation assays in 2D and spheroids in the presence of 0.01nM E2 showed a concentration-dependent decrease in proliferation in response to elacestrant and fulvestrant. GI50 values for elacestrant in general were 10-fold higher than fulvestrant but equated to doses that are clinically achievable for the drug. Most importantly, elacestrant suppressed proliferation of two LTED models harbouring ESR1 mutations, MCF7 LTEDY537C (GI50 5nM) and SUM44-LTEDY537S (GI50 100nM). GI50 values of elacestrant and fulvestrant showed similar reduction of ER, progesterone receptor (PGR) and cyclinD1 together with decreased phosphorylation of retinoblastoma (RB), concordant with cell cycle arrest. Chromatin immunoprecipitation (ChIP) for ER in response to elacestrant or fulvestrant showed a reduction in recruitment of ER to TFF1, GREB1 and PGR promoters and concomitant reduction in mRNA expression of these genes in the presence of E2. Elacestrant-mediated ER depletion was dependent on the 26S proteasome, as addition of the proteasome inhibitor MG132, fully blocked elacestrant depletion of ER, similar to the effect of MG132 treatment in preventing fulvestrant-mediated ER turnover. The combination of elacestrant with CDK4/6 inhibitors, palbociclib or abemaciclib, demonstrated additivity compared with monotherapy. In addition, elacestrant inhibited growth of palbociclib-resistant MCF7 LTED cells.

Conclusion: These findings highlight the potential utility of elacestrant as a 1st or 2nd line drug in the treatment of ER+ BC. The results support the testing of elacestrant versus fulvestrant after relapse on an AI, either alone or in combination with a CDK4/6 inhibitor.

Citation Format: Martin L-A, Pancholi S, Simigdala N, Nikitorowicz-Buniak J, Ribas R, Garner F, Bihani T, Dowsett M. New oral SERD elacestrant (RAD1901) shows efficacy in breast cancer models harbouring ESR1 mutations and enhances the antiproliferative activity of mTORC1 and CDK4/6 inhibitors [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 P4-04-09.

Effective combination therapies in preclinical endocrine resistant breast cancer models harboring ER mutations

Although endocrine therapy is successfully used to treat patients with estrogen receptor (ER) positive breast cancer, a substantial proportion of this population will relapse. Several mechanisms of acquired resistance have been described including activation of the mTOR pathway, increased activity of CDK4 and activating mutations in ER. Using a patient derived xenograft model harboring a common activating ER ligand binding domain mutation (D538G), we evaluated several combinatorial strategies using the selective estrogen receptor degrader (SERD) fulvestrant in combination with chromatin modifying agents, and CDK4/6 and mTOR inhibitors. In this model, fulvestrant binds WT and MT ER, reduces ER protein levels, and downregulated ER target gene expression. Addition of JQ1 or vorinostat to fulvestrant resulted in tumor regression (41% and 22% regression, respectively) though no efficacy was seen when either agent was given alone. Interestingly, although the CDK4/6 inhibitor palbociclib and mTOR inhibitor everolimus were efficacious as monotherapies, long-term delayed tumor growth was only observed when co-administered with fulvestrant. This observation was consistent with a greater inhibition of compensatory signaling when palbociclib and everolimus were co-dosed with fulvestrant. The addition of fulvestrant to JQ1, vorinostat, everolimus and palbociclib also significantly reduced lung metastatic burden as compared to monotherapy. The combination potential of fulvestrant with palbociclib or everolimus were confirmed in an MCF7 CRISPR model harboring the Y537S ER activating mutation. Taken together, these data suggest that fulvestrant may have an important role in the treatment of ER positive breast cancer with acquired ER mutations.

Abstract 4160: Impact of oral selective estrogen receptor degrader RAD1901 in preclinical models of endocrine sensitive/resistant breast cancer

Aromatase inhibitors (AI) are the mainstay for treatment of postmenopausal estrogen receptor positive (ER+) primary breast cancer (BC). However, many patients relapse whilst retaining a functional ER. The selective ER degrader (SERD) fulvestrant has been evaluated as a potential 2nd or 3rd line therapy for patients who relapse on AI treatment. Although exhibiting promising potential, its low bioavailability has limited its use to combination therapy in metastatic patients. Here, we assessed the efficacy of the orally available SERD, RAD1901, in a panel of BC cells with varying genetic backgrounds modelling patients both sensitive and resistant to AI therapy (LTED). Fulvestrant was used in parallel to allow relative responses to be compared. Cell proliferation assays in 2D and spheroids the presence of 0.01nM 17β-estradiol showed a concentration-dependent decrease in proliferation in response to RAD1901 and fulvestrant. GI50 values for RAD1901 in general were 10-fold higher than fulvestrant but equated to doses that are clinically achievable for RAD1901. Most importantly, RAD1901 effectively suppressed proliferation of two LTED models harbouring naturally occurring ESR1 mutations, MCF7 LTEDY537C (GI50 5nM) and SUM44-LTEDY537S (GI50 100nM). GI50 values of RAD1901 and fulvestrant showed similar reduction of ER, progesterone receptor (PGR) and cyclin D1 together with decreased phosphorylation of retinoblastoma (RB), concordant with cell cycle arrest. Furthermore, chromatin immunoprecipitation (ChIP) for ER in response to RAD1901 or fulvestrant showed a 70% reduction in recruitment of ER to TFF1, GREB1 and PGR promoters and concomitant reduction in mRNA expression of these genes. In MCF7-Arom cells, combination of letrozole with RAD1901 or fulvestrant showed enhanced antiproliferative effect compared to letrozole alone. The addition of RAD1901 to CDK4/6 inhibitor palbociclib or abemaciclib demonstrated additivity compared with monotherapy. In addition, RAD1901 effectively inhibited growth of palbociclib-resistant MCF7 LTED cells.These preclinical findings highlight the potential utility of RAD1901 as a potent drug in the treatment of ER+ BC. Combined with its bioavailability profile, RAD1901 warrants clinical testing versus fulvestrant after relapse on an AI, either alone or in combination with a CDK4/6 inhibitor.

Citation Format: Sunil Pancholi, Joanna Nikitorowicz-Buniak, Ricardo Ribas, Nikiana Simigdala, Fiona Garner, Teeru Bihani, Stephen R. Johnston, Mitch Dowsett, Lesley-Ann Martin. Impact of oral selective estrogen receptor degrader RAD1901 in preclinical models of endocrine sensitive/resistant breast cancer [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 4160. doi:10.1158/1538-7445.AM2017-4160

Elacestrant (RAD1901), a Selective Estrogen Receptor Degrader (SERD), Has Antitumor Activity in Multiple ER+ Breast Cancer Patient-derived Xenograft Models

Purpose: Estrogen receptor-positive (ER⁺) breast cancers are typically treated with endocrine agents, and dependence on the ER pathway is often retained even after multiple rounds of antiestrogen therapy. Selective estrogen receptor degraders (SERD) are being developed as a strategy to more effectively target ER and exploit ER dependence in these cancers, which includes inhibiting both wild-type and mutant forms of ER. The purpose of this study was to evaluate the efficacy of a novel orally bioavailable SERD, elacestrant (RAD1901), in preclinical models of ER⁺ breast cancer.Experimental Design: Elacestrant was evaluated as a single agent and in combination with palbociclib or everolimus in multiple ER⁺ breast cancer models, including several patient-derived xenograft models.Results: Elacestrant induces the degradation of ER, inhibits ER-mediated signaling and growth of ER⁺ breast cancer cell lines in vitro and in vivo, and significantly inhibits tumor growth of multiple PDX models. Furthermore, we demonstrate that elacestrant in combination with palbociclib or everolimus can lead to greater efficacy in certain contexts. Finally, elacestrant exhibits significant antitumor activity both as a single agent and in combination with palbociclib in two patient-derived breast cancer xenograft models harboring ESR1 mutations.Conclusions: These data underscore the potential clinical utility of elacestrant as a single agent and as a combination therapy, for both early- and late-stage ER⁺ disease. Clin Cancer Res; 23(16); 4793-804. ©2017 AACR.

Differential regulation of mTOR signaling determines sensitivity to AKT inhibition in diffuse large B cell lymphoma

Agents that target components of the PI3K/AKT/mTOR pathway are under investigation for the treatment of diffuse large B cell lymphoma (DLBCL). Given the highly heterogeneous nature of DLBCL, it is not clear whether all subtypes of DLBCL will be susceptible to PI3K pathway inhibition, or which kinase within this pathway is the most favorable target. Pharmacological profiling of a panel of DLBCL cell lines revealed a subset of DLBCL that was resistant to AKT inhibition. Strikingly, sensitivity to AKT inhibitors correlated with the ability of these inhibitors to block phosphorylation of S6K1 and ribosomal protein S6. Cell lines resistant to AKT inhibition activated S6K1 independent of AKT either through upregulation of PIM2 or through activation by B cell receptor (BCR) signaling components. Finally, combined inhibition of AKT and BTK, PIM2, or S6K1 proved to be an effective strategy to overcome resistance to AKT inhibition in DLBCL.

Abstract 1814: RAD1901, an orally available SERD, as an effective combination partner in ER+ breast cancer

Breast cancer is subdivided into categories based on a patient's estrogen receptor (ER), progesterone receptor (PR) or Her2 expression status. Estrogen receptor positive (ER+) breast cancer makes up approximately 70% of all breast cancers diagnosed and given the dependence on ER signaling in this disease segment, most treatment modalities focus on inhibiting some aspect of this pathway. Indeed, preventing estrogen synthesis (e.g. with aromatase inhibitors) and modulating ER pathway activity (e.g. with tamoxifen) continue to be mainstays in the standard of care for ER+ breast cancer patients. While patients typically respond well to these agents, a majority of patients will relapse, emphasizing the need to understand the specific mechanisms that can contribute to clinical resistance. One such mechanism is the activation of compensatory or concurrent signaling pathways that can stimulate growth (eg. Her2, CDK4/6) and/or confer survival (eg. PI3K, AKT, mTOR). The recent approval of palbociclib (CDK4/6i) or everolimus (mTORi) in combination with an aromatase inhibitor for the treatment of advanced ER+ disease demonstrates the effectiveness of combining anti-hormonals with targeted agents. While these combinations have superior efficacy compared to the use of aromatase inhibitors alone, alterations in ER itself, such as amplification and mutations, have been described as a second mechanism that can drive resistance to aromatase inhibitors in patients. These findings highlight the need for a selective estrogen receptor downregulator (SERD) that can degrade both wild-type and aberrant forms of ER in order to more effectively treat this patient population. We have recently demonstrated that treatment with RAD1901, an orally bioavailable SERD, results in consistent and robust tumor growth inhibition of patient derived xenograft (PDx) models, regardless of ER status. We hypothesized that combining RAD1901 with agents that inhibit compensatory signaling pathways would mitigate both mechanisms of resistance in ER+ breast cancer patients, leading to greater efficacy. We performed an efficacy screen with RAD1901 in patient derived xenograft (PDx) models with varied genetic backgrounds, allowing us to mimic clinical phenotypes and to better represent the heterogeneity within the ER+ breast cancer patient population. These models harbored a wide range of ER expression levels, in addition to other genetic alterations, which accurately represented the patients’ diverse treatment history profiles. Based on results from the screen, models were selected for additional studies to determine which targeted agent, if any, can be combined with RAD1901 to achieve maximal efficacy. By doing this, we were able to correlate response with genetic background in clinically relevant models, potentially allowing us to predict treatment strategies that have a higher likelihood of success for specific patients.

Citation Format: Teeru Bihani, Jeffrey L. Brown, Dinesh M. Purandare, Gary Hattersley, Fiona Garner. RAD1901, an orally available SERD, as an effective combination partner in ER+ breast cancer. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 1814.

Abstract PR10: RAD1901, an orally available selective estrogen receptor downregulator, has potent anti-tumor activity in in vitro and in vivo models of ER+ breast cancer

Breast cancer is the most frequent type of cancer diagnosed in women, with over 200,000 new cases diagnosed in the US each year. Treatment strategies are typically based on the tumors receptor status; that is, whether a tumor expresses estrogen receptor (ER), progesterone receptor (PR), or Her2. Estrogen receptor positive (ER+) breast cancers comprise approximately two-thirds of all breast cancers. For decades the approach to treat ER+ disease revolved around altering the ligand interactions with the receptor; either by preventing estrogen binding (e.g., tamoxifen) or preventing estrogen biosynthesis (aromatase inhibitors). While patients typically respond well to these agents, estrogen-independent ER activity and recurrent ER mutations are increasingly being reported as contributing factors to endocrine resistance and continue to be a clinical hurdle. Given this growing unmet medical need, selective estrogen receptor downregulators, or SERDs, have gained widespread attention as new therapeutic treatment strategies for ER + disease. Indeed, fulvestrant has been shown to downregulate ER and cause tumor growth inhibition in many ER+ breast cancer models. However, in the clinic fulvestrant appears to be limited by PK exposure properties and this, combined with its intramuscular route of administration, underscores the need for novel orally available SERDs. Here, we describe RAD1901, an orally administered SERD that binds ER and targets it for degradation in a dose-dependent manner. Biochemical affinity binding studies and cocrystallization experiments revealed insights into RAD1901 complexes with both wild-type and mutant forms of ER. In addition, RAD1901 treatment resulted in decreased cell proliferation in in vitro breast cancer cell lines and had profound single agent tumor growth inhibition in in vivo xenograft models. Consistent with these findings, RAD1901 treatment resulted in decreased expression levels of ER target genes. Interestingly, the extent of tumor growth inhibition induced by RAD1901 in vivo was dependent on ER expression levels, demonstrating the specificity of RAD1901 and predicting its activity in ER-driven cancers. Importantly, RAD1901 was also able to induce significant tumor growth inhibition in clinically relevant and representative patient-derived xenograft models, at a level similar to or greater than fulvestrant. In conclusion, our preclinical data demonstrate that RAD1901 is an orally available SERD, with potent single agent antitumor activity. RAD1901 is currently under clinical investigation in post-menopausal women with advanced ER+ disease.

Citation Format: Teeru Bihani, Jeffrey Brown, Gary Hattersley, Fiona Garner. RAD1901, an orally available selective estrogen receptor downregulator, has potent anti-tumor activity in in vitro and in vivo models of ER+ breast cancer. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2015 Nov 5-9; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2015;14(12 Suppl 2):Abstract nr PR10.

AZD2014, an Inhibitor of mTORC1 and mTORC2, Is Highly Effective in ER+ Breast Cancer When Administered Using Intermittent or Continuous Schedules

mTOR is an atypical serine threonine kinase involved in regulating major cellular functions, such as nutrients sensing, growth, and proliferation. mTOR is part of the multiprotein complexes mTORC1 and mTORC2, which have been shown to play critical yet functionally distinct roles in the regulation of cellular processes. Current clinical mTOR inhibitors only inhibit the mTORC1 complex and are derivatives of the macrolide rapamycin (rapalogs). Encouraging effects have been observed with rapalogs in estrogen receptor-positive (ER(+)) breast cancer patients in combination with endocrine therapy, such as aromatase inhibitors. AZD2014 is a small-molecule ATP competitive inhibitor of mTOR that inhibits both mTORC1 and mTORC2 complexes and has a greater inhibitory function against mTORC1 than the clinically approved rapalogs. Here, we demonstrate that AZD2014 has broad antiproliferative effects across multiple cell lines, including ER(+) breast models with acquired resistance to hormonal therapy and cell lines with acquired resistance to rapalogs. In vivo, AZD2014 induces dose-dependent tumor growth inhibition in several xenograft and primary explant models. The antitumor activity of AZD2014 is associated with modulation of both mTORC1 and mTORC2 substrates, consistent with its mechanism of action. In combination with fulvestrant, AZD2014 induces tumor regressions when dosed continuously or using intermittent dosing schedules. The ability to dose AZD2014 intermittently, together with its ability to block signaling from both mTORC1 and mTORC2 complexes, makes this compound an ideal candidate for combining with endocrine therapies in the clinic. AZD2014 is currently in phase II clinical trials.

Synergistic induction of apoptosis by combination of BTK and dual mTORC1/2 inhibitors in diffuse large B cell lymphoma

Diffuse large B cell lymphoma is generally treated by chemotherapy and there is an unmet medical need for novel targeted therapies or combination therapies. Using in vitro screening, we have identified the combination of ibrutinib, an inhibitor of the tyrosine kinase BTK, and AZD2014, an mTOR catalytic inhibitor, as being highly synergistic in killing ABC-subtype DLBCL cell lines. Simultaneous inhibition of BTK and mTOR causes apoptosis both in vitro and in vivo and results in tumor regression in a xenograft model. We identify two parallel mechanisms that underlie apoptosis in this setting: cooperative inhibition of cap-dependent translation, and the inhibition of an NF-κB/IL10/STAT3 autocrine loop. Combined disruption of these pathways is required for apoptosis. These data represent a rational basis for the dual inhibition of BTK and mTOR as a potential treatment for ABC-subtype DLBCL.

Cyclin D1 activity regulates autophagy and senescence in the mammary epithelium

Overexpression of cyclin D1 is believed to endow mammary epithelial cells (MEC) with a proliferative advantage by virtue of its contribution to pRB inactivation. Accordingly, abrogation of the kinase-dependent function of cyclin D1 is sufficient to render mice resistant to breast cancer initiated by ErbB2. Here, we report that mouse cyclin D1(KE/KE) MECs (deficient in cyclin D1 activity) upregulate an autophagy-like process but fail to implement ErbB2-induced senescence in vivo. In addition, immortalized cyclin D1(KE/KE) MECs retain high rates of autophagy and reduced ErbB2-mediated transformation in vitro. However, highlighting its dual role during tumorigenesis, downregulation of autophagy led to an increase in senescence in cyclin D1(KE/KE) MECs. Autophagy upregulation was also confirmed in human mammary epithelial cells (HMEC) subjected to genetic and pharmacologic inhibition of cyclin D1 activity and, similar to our murine system, simultaneous inhibition of Cdk4/6 and autophagy in HMECs enhanced the senescence response. Collectively, our findings suggest a previously unrecognized function of cyclin D1 in suppressing autophagy in the mammary epithelium.

Mitosis hit with an ATM transaction fee: aurora B-mediated activation of ATM during mitosis

In this issue of Molecular Cell, Yang et al. (2011) demonstrate that Aurora B phosphorylates ATM, leading to its mitotic activation and ability to phosphorylate Bub1 and regulate the spindle checkpoint, thus maintaining genomic integrity.

Dissecting the senescence-like program in tumor cells activated by Ras signaling

Activated Ras signaling can induce a permanent growth arrest in osteosarcoma cells. Here, we report that a senescence-like growth inhibition is also achieved in human carcinoma cells upon the transduction of H-Ras(V12). Ras-induced tumor senescence can be recapitulated by the transduction of activated, but not wild-type, MEK. The ability for H-Ras(V12) to suppress tumor cell growth is drastically compromised in cells that harbor endogenous activating ras mutations. Notably, growth inhibition of tumor cells containing ras mutations can be achieved through the introduction of activated MEK. Tumor senescence induced by Ras signaling can occur in the absence of p16 or Rb and is not interrupted by the inactivation of Rb, p107, or p130 via short hairpin RNA or the transduction with HPV16 E7. In contrast, inactivation of p21 via short hairpin RNA disrupts Ras-induced tumor senescence. In summary, this study uncovers a senescence-like program activated by Ras signaling to inhibit cancer cell growth. This program appears to be intact in cancer cells that do not harbor ras mutations. Moreover, cancer cells that carry ras mutations remain susceptible to tumor senescence induced by activated MEK. These novel findings can potentially lead to the development of innovative cancer intervention.

Enhanced selenium effect on growth arrest by BiP/GRP78 knockdown in p53-null human prostate cancer cells

Redox modification of thiol/disulfide interchange in proteins by selenium could lead to protein unfolding. When this occurs in the endoplasmic reticulum (ER), a process known as unfolded protein response (UPR) is orchestrated for survival through activation of PERK-eIF2alpha (PERK: double-stranded RNA-activated protein kinase-like ER kinase; eIF2alpha: eucaryotic initiation factor 2alpha), ATFalpha (ATFalpha: activating transcription factor 6) and inositol requiring 1 (IRE1)-x-box-binding protein 1 (XBP1) signalings. All three UPR transducer pathways were upregulated very rapidly when PC-3 cells were exposed to selenium. These changes were accompanied by increased expression of UPR target genes, including immunoglobulin heavy chain-binding protein/glucose-regulated protein, 78 kDa and CCAAT/enhancer binding protein-homologous protein/growth arrest- and DNA damage-inducible gene (CHOP/GADD153). Induction of BiP/GRP78, an ER-resident chaperone, is part of the damage control mechanism, while CHOP/GADD153 is a transcription factor associated with growth arrest and apoptosis in the event of prolonged ER stress. Knocking down BiP/GRP78 induction by small interference RNA produced a differential response of the three transducers to selenium, suggesting that the signaling intensity of each transducer could be fine-tuned depending on BiP/GRP78 availability. In the presence of selenium, CHOP/GADD153 expression was raised even higher by BiP/GRP78 knockdown. Under this condition, the selenium effect on wild-type p53-activated fragment p21 (p21(WAF)), cyclin-dependent kinase (CDK)1 and CDK2 was also magnified in a manner consistent with enhanced cell growth arrest. Additional experiments with CHOP/GADD153 siRNA knockdown strongly suggested that CHOP/GADD153 may play a positive role in upregulating the expression of p21(WAF) in a p53-independent manner (PC-3 cells are p53 null). Collectively, the above findings support the idea that UPR could be an important mechanism in mediating the anticancer activity of selenium.

Differential oncogenic Ras signaling and senescence in tumor cells

Several studies have shown that forced expression of oncogenic H-ras can induce a senescence-like permanent growth arrest in normal cells. Here we report that expression of oncogenic H-ras in human osteosarcoma U2OS cells also resulted in a senescence-like flat and enlarged cell morphology and permanent growth arrest. In contrast to normal human fibroblasts, U2OS cells were arrested independently of the p16 and ARF tumor suppressors. Treatment with a MEK inhibitor or a p38MAPK inhibitor interrupted oncogenic H-ras-induced growth arrest in U2OS cells, suggesting that activation of MAPK pathways is important. To further determine whether this process is unique to oncogenic H-ras signaling, we examined the effect of oncogenic K-ras on normal cells and human osteosarcoma cells. Similar to oncogenic H-ras, oncogenic K-ras also induced senescence in normal fibroblasts, while transforming immortalized mouse fibroblasts. However, in contrast to oncogenic H-ras, oncogenic K-ras failed to induce a permanent growth arrest in osteosarcoma U2OS cells. Additionally, cells transduced with oncogenic K-ras exhibited distinguishable cellular changes compared to those transduced with oncogenic H-ras. In summary, we report for the first time that oncogenic H-ras signaling can trigger a senescence-like growth arrest in tumor cells, independent of the p16 and ARF tumor suppressors. This result suggests that tumor cells may harbor a senescence-like program that can be activated by ras signaling. Moreover, our study uncovered a cell type-dependent differential response to oncogenic K-ras, as compared to oncogenic H-ras.