Weiyi Toy

ESR1 ligand-binding domain mutations in hormone-resistant breast cancer

Seventy percent of breast cancers express estrogen receptor (ER), and most of these are sensitive to ER inhibition. However, many such tumors for unknown reasons become refractory to inhibition of estrogen action in the metastatic setting. We conducted a comprehensive genetic analysis of two independent cohorts of metastatic ER-positive breast tumors and identified mutations in ESR1 affecting the ligand-binding domain (LBD) in 14 of 80 cases. These included highly recurrent mutations encoding p.Tyr537Ser, p.Tyr537Asn and p.Asp538Gly alterations. Molecular dynamics simulations suggest that the structures of the Tyr537Ser and Asp538Gly mutants involve hydrogen bonding of the mutant amino acids with Asp351, thus favoring the agonist conformation of the receptor. Consistent with this model, mutant receptors drive ER-dependent transcription and proliferation in the absence of hormone and reduce the efficacy of ER antagonists. These data implicate LBD-mutant forms of ER in mediating clinical resistance to hormonal therapy and suggest that more potent ER antagonists may be of substantial therapeutic benefit.

Rapid induction of apoptosis by PI3K inhibitors is dependent upon their transient inhibition of RAS-ERK signaling

The effects of selective phosphoinositide 3-kinase (PI3K) and AKT inhibitors were compared in human tumor cell lines in which the pathway is dysregulated. Both caused inhibition of AKT, relief of feedback inhibition of receptor tyrosine kinases, and growth arrest. However, only the PI3K inhibitors caused rapid induction of cell death. In seeking a mechanism for this phenomenon, we found that PI3K inhibition, but not AKT inhibition, causes rapid inhibition of wild-type RAS and of RAF-MEK-ERK signaling. Inhibition of RAS-ERK signaling is transient, rebounding a few hours after drug addition, and is required for rapid induction of apoptosis. Combined MEK and AKT inhibition also promotes cell death, and in murine models of HER2(+) cancer, either pulsatile PI3K inhibition or combined MEK and AKT inhibition causes tumor regression. We conclude that PI3K is upstream of RAS and AKT and that pulsatile inhibition of both pathways is sufficient for effective antitumor activity.

Estrogen receptor alpha somatic mutations Y537S and D538G confer breast cancer endocrine resistance by stabilizing the activating function-2 binding conformation

Somatic mutations in the estrogen receptor alpha (ERα) gene (ESR1), especially Y537S and D538G, have been linked to acquired resistance to endocrine therapies. Cell-based studies demonstrated that these mutants confer ERα constitutive activity and antiestrogen resistance and suggest that ligand-binding domain dysfunction leads to endocrine therapy resistance. Here, we integrate biophysical and structural biology data to reveal how these mutations lead to a constitutively active and antiestrogen-resistant ERα. We show that these mutant ERs recruit coactivator in the absence of hormone while their affinities for estrogen agonist (estradiol) and antagonist (4-hydroxytamoxifen) are reduced. Further, they confer antiestrogen resistance by altering the conformational dynamics of the loop connecting Helix 11 and Helix 12 in the ligand-binding domain of ERα, which leads to a stabilized agonist state and an altered antagonist state that resists inhibition.

Activating ESR1 Mutations Differentially Impact the Efficacy of ER Antagonists.

Recent studies have identified somatic ESR1 mutations in patients with metastatic breast cancer and found some of them to promote estrogen-independent activation of the receptor. The degree to which all recurrent mutants can drive estrogen-independent activities and reduced sensitivity to ER antagonists like fulvestrant is not established. In this report, we characterize the spectrum of ESR1 mutations from more than 900 patients. ESR1 mutations were detected in 10%, with D538G being the most frequent (36%), followed by Y537S (14%). Several novel, activating mutations were also detected (e.g., L469V, V422del, and Y537D). Although many mutations lead to constitutive activity and reduced sensitivity to ER antagonists, only select mutants such as Y537S caused a magnitude of change associated with fulvestrant resistance in vivo Correspondingly, tumors driven by Y537S, but not D5358G, E380Q, or S463P, were less effectively inhibited by fulvestrant than more potent and bioavailable antagonists, including AZD9496. These data point to a need for antagonists with optimal pharmacokinetic properties to realize clinical efficacy against certain ESR1 mutants.Significance: A diversity of activating ESR1 mutations exist, only some of which confer resistance to existing ER antagonists that might be overcome by next-generation inhibitors such as AZD9496. Cancer Discov; 7(3); 277-87. ©2016 AACR.This article is highlighted in the In This Issue feature, p. 235.

Genomic profiling of ER+ breast cancers after short-term estrogen suppression reveals alterations associated with endocrine resistance

Genomic profiling of ER+/HER2− breast tumors after short-term estrogen deprivation revealed alterations associated with intrinsic resistance and provided mechanistic insights. A patient look at cancer resistance A variety of drugs that inhibit estrogen signaling are in use for breast cancer, but patients often develop resistance to these treatments. To understand how this resistance develops, Giltnane et al. evaluated 143 patients who were receiving the aromatase inhibitor letrozole to block estrogen signaling before undergoing surgery for breast cancer. By performing genomic analysis on these patients’ tumors, the authors were able to identify not only changes in gene expression and estrogen receptor gene fusions that correlated with resistance to therapy but also potential leads for future treatments that could help overcome this resistance. Inhibition of proliferation in estrogen receptor–positive (ER+) breast cancers after short-term antiestrogen therapy correlates with long-term patient outcome. We profiled 155 ER+/human epidermal growth factor receptor 2–negative (HER2−) early breast cancers from 143 patients treated with the aromatase inhibitor letrozole for 10 to 21 days before surgery. Twenty-one percent of tumors remained highly proliferative, suggesting that these tumors harbor alterations associated with intrinsic endocrine therapy resistance. Whole-exome sequencing revealed a correlation between 8p11-12 and 11q13 gene amplifications, including FGFR1 and CCND1, respectively, and high Ki67. We corroborated these findings in a separate cohort of serial pretreatment, postneoadjuvant chemotherapy, and recurrent ER+ tumors. Combined inhibition of FGFR1 and CDK4/6 reversed antiestrogen resistance in ER+ FGFR1/CCND1 coamplified CAMA1 breast cancer cells. RNA sequencing of letrozole-treated tumors revealed the existence of intrachromosomal ESR1 fusion transcripts and increased expression of gene signatures indicative of enhanced E2F-mediated transcription and cell cycle processes in cancers with high Ki67. These data suggest that short-term preoperative estrogen deprivation followed by genomic profiling can be used to identify druggable alterations that may cause intrinsic endocrine therapy resistance.

The SERM/SERD Bazedoxifene Disrupts ESR1 Helix 12 to Overcome Acquired Hormone Resistance in Breast Cancer Cells

Acquired resistance to endocrine therapy remains a significant clinical burden for breast cancer patients. Somatic mutations in the ESR1 (estrogen receptor alpha (ERα) gene ligand-binding domain (LBD) represent a recognized mechanism of acquired resistance. Antiestrogens with improved efficacy versus tamoxifen might overcome the resistant phenotype in ER+ breast cancers. Bazedoxifene (BZA) is a potent antiestrogen that is clinically approved for use in hormone replacement therapies. We find BZA possesses improved inhibitory potency against the Y537S and D538G ERα mutants compared to tamoxifen and has additional inhibitory activity in combination with the CDK4/6 inhibitor palbociclib. In addition, comprehensive biophysical and structural biology studies show that BZA’s selective estrogen receptor degrading (SERD) properties that override the stabilizing effects of the Y537S and D538G ERα mutations. Significance Bazedoxifene (BZA) is a potent orally available antiestrogen that is clinically approved for use in hormone replacement therapy (DUAVEE). We explore the efficacy of BZA to inhibit activating somatic mutants of ERα that can arise in metastatic breast cancers after prolonged exposure to aromatase inhibitors or tamoxifen therapy. Breast cancer cell line, biophysical, and structural data show that BZA disrupts helix 12 of the ERα ligand binding domain to achieve improved potency against Y537S and D538G somatic mutants compared to 4-hydroxytamoxifen.

Immunohistochemical analysis of estrogen receptor in breast cancer with ESR1 mutations detected by hybrid capture-based next-generation sequencing

Estrogen receptor-α (ER-α), encoded by ESR1, is detected by immunohistochemistry in approximately 70% of invasive breast cancers and serves as a strong predictive biomarker. ESR1-activating mutations in the ligand-binding domain have been reported in up to 35–40% of ER-positive metastatic breast cancers and are associated with endocrine therapy resistance and disease progression. At present, it is unclear whether ESR1 mutations alter the immunohistochemical detection of ER performed in routine clinical practice. In this study, ESR1 mutations in breast cancer were identified utilizing Memorial Sloan Kettering-Integrated Mutation Profiling of Actionable Cancer Targets (MSK-IMPACT), a Food and Drug Administration-approved hybridization capture-based next-generation sequencing assay. Five hundred and eighty-six breast cancers from patients with locally advanced or metastatic disease were analyzed using MSK-IMPACT in the study period. ESR1 somatic alterations were identified in 67 breast cancer samples from 66 patients. Immunohistochemical analysis of ER, progesterone receptor, and human epidermal growth factor receptor 2 was performed on the primary and treated breast cancers from these patients at the time of diagnosis. Twenty unique ESR1 mutations were identified involving the ligand-binding domain, all in breast cancer samples from patients previously treated with endocrine therapy. The most frequent mutations were D538G (n = 22), Y537S (n = 7), and E380Q (n = 7). All breast cancer samples with an ESR1 mutation were ER-positive by immunohistochemistry. Review of the ER immunohistochemistry in the paired untreated primary tumor and treated tumor from 34 patients showed no detectable change in the ER-positive immunohistochemical status (median percentage of invasive tumor cells with nuclear staining: untreated primary tumor 90%, treated tumor 95%). We conclude that ESR1 mutations do not appreciably diminish ER-positive staining by immunohistochemistry. In addition to standard biomarker testing by immunohistochemistry, the assessment of ESR1 mutations by molecular testing can help guide the clinical management of patients with ER-positive breast cancer in the setting of endocrine resistance and progression of disease.

New Class of Selective Estrogen Receptor Degraders (SERDs): Expanding the Toolbox of PROTAC Degrons

An effective endocrine therapy for breast cancer is to selectively and effectively degrade the estrogen receptor (ER). Up until now, there have been largely only two molecular scaffolds capable of doing this. In this study, we have developed new classes of scaffolds that possess selective estrogen receptor degrader (SERD) and ER antagonistic properties. These novel SERDs potently inhibit MCF-7 breast cancer cell proliferation and the expression of ER target genes, and their efficacy is comparable to Fulvestrant. Unlike Fulvestrant, the modular protein-targeted chimera (PROTAC)-type design of these novel SERDs should allow easy diversification into a library of analogs to further fine-tune their pharmacokinetic properties including oral availability. This work also expands the pool of currently available PROTAC-type scaffolds that could be beneficial for targeted degradation of various other therapeutically important proteins.

Abstract 4854: Bazedoxifene inhibits ESR1 somatic mutants with improved potency compared to tamoxifene and raloxifene

Proceedings: AACR 107th Annual Meeting 2016; April 16-20, 2016; New Orleans, LA Despite continued administration of antiestrogen therapies, approximately 50% of all estrogen receptor alpha (ERalpha) positive breast cancers will present new metastatic lesions. The acquisition of secondary hormone-resistant metastatic breast cancers represents a significant clinical barrier towards life-long disease free survival for the patient. Somatic mutations to the ERalpha gene (ESR1) Y537S and D538G represent a novel mechanism of acquired antiestrogen resistance because they confer hormone-free transcriptional activity and reduced selective estrogen receptor modulator (SERM) and selective estrogen receptor degrader (SERD) potency. Fulvestrant, a SERD, was the only molecule that could completely ablate mutant ERalpha activity. Unfortunately, fulvestrant possesses poor pharmacologic profiles that limit its therapeutic utility. Bazedoxifene (BZA) is a potent mixed SERM/SERD and has improved pharmacokinetics and oral bioavailability compared to fulvestrant. We show that BZA inhibits Y537S and D538G ESR1 somatic mutation transcriptional activity with a greater potency than the SERMs 4-hydroxytamoxifen (TOT) and raloxifene (RAL). Further investigations into the biophysical and structural basis for BZA action suggest that BZA increases the conformational dynamics of helix 12, a key molecular switch that governs ERalpha action resulting in SERD-like properties and improved potency against the somatic mutations compared to TOT and RAL. Citation Format: Sean W. Fanning, Venkat Dharmarajan, Christopher G. Mayne, Weiyi Toy, Kathryn E. Carlson, Teresa A. Martin, Jason Nowak, Jerome Nwachukwu, David J. Hosfield, Emad Tajkhorshid, Sarat Chandarlapaty, Patrick Griffin, Yang Shen, John A. Katzenellenbogen, Geoffrey L. Greene. Bazedoxifene inhibits ESR1 somatic mutants with improved potency compared to tamoxifene and raloxifene. [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 4854.

Abstract 863: Differential activity and SERD sensitivity of clinical ESR1 mutations

Background: Mutations in the ligand binding domain of ESR1 have been identified as recurrent alterations in ER+ metastatic breast cancer patients previously treated with aromatase inhibitors. Selective estrogen receptor degraders (SERDs) such as fulvestrant have been speculated to be a rational therapeutic approach to inhibiting these mutants by promoting receptor inhibition and degradation. Whether such drugs can potently and durably inhibit the distinct ESR1 LBD mutants both in vitro and in vivo has not been well characterized. In this study, we investigated the activities of different SERDs against a set of ESR1 LBD mutants identified in the clinic. Methods: The diversity of ESR1 LBD mutations was analyzed by next generation sequencing of metastatic breast tumors from over 900 patients treated at MSKCC. Laboratory models of these different mutants were generated to interrogate their activities and drug sensitivity. We utilized in vitro reporters of ER driven transcription, proliferation assays, and both cell line derived and patient derived models to characterize the different mutants. Results: In addition to previously characterized mutations, D538G, Y537S/N/C, L536R, and E380Q, we identified a number of novel ESR1 LBD mutations in this series such as S432L, L469V, and Y537D among others. We found that while most ESR1 LBD mutations promoted estrogen independent ER function, some did not. We also found that different activating ER mutants had significant differences in the degree to which they induced estrogen independent signaling. Consistent with these findings, we found that while most SERDs could antagonize all of the mutant receptors, there were significant differences in potency. Whereas certain ESR1 mutants such as E380Q were inhibited at similar concentrations as wild type receptors, other mutants such as Y537S required higher concentrations. These differences appeared to impact the in vivo efficacy of the FDA approved SERD, fulvestrant, which is known to have poor pharmacokinetic properties. However, a SERD with high bioavailability and potency, AZD9496, was found to be sufficient to fully inhibit mutant driven tumor growth from WT, D538G and Y537S expressing tumors in vivo. Conclusions: Altogether, the data suggest that clinical ESR1 LBD mutations have distinct effects in activating the receptor and differentially impact the efficacy of ER antagonists. In order to be broadly effective against different ESR1 LBD mutants, SERDs may require highly optimized pharmacokinetic properties. Citation Format: Weiyi Toy, Hazel Weir, Pedram Razavi, Michael Berger, Wai Lin Wong, Elisa De Stanchina, Joœe Baselga, Sarat Chandarlapaty. Differential activity and SERD sensitivity of clinical ESR1 mutations. [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 863.