Leofal Soto

PURPOSE: Estrogen receptor (ER) alpha signaling is a known driver of ER-positive (ER+)/human epidermal growth factor receptor 2 negative (HER2-) breast cancer. Combining endocrine therapy (ET) such as fulvestrant with CDK4/6, mTOR, or PI3K inhibitors has become a central strategy in the treatment of ER+ advanced breast cancer. However, suboptimal ER inhibition and resistance resulting from the ESR1 mutation dictates that new therapies are needed. EXPERIMENTAL DESIGN: A medicinal chemistry campaign identified vepdegestrant (ARV-471), a selective, orally bioavailable, and potent small molecule PROteolysis-TArgeting Chimera (PROTAC) degrader of ER. We used biochemical and intracellular target engagement assays to demonstrate the mechanism of action of vepdegestrant, and ESR1 wild-type (WT) and mutant ER+ preclinical breast cancer models to demonstrate ER degradation-mediated tumor growth inhibition (TGI). RESULTS: Vepdegestrant induced ≥90% degradation of wild-type and mutant ER, inhibited ER-dependent breast cancer cell line proliferation in vitro, and achieved substantial TGI (87%-123%) in MCF7 orthotopic xenograft models, better than those of the ET agent fulvestrant (31%-80% TGI). In the hormone independent (HI) mutant ER Y537S patient-derived xenograft (PDX) breast cancer model ST941/HI, vepdegestrant achieved tumor regression and was similarly efficacious in the ST941/HI/PBR palbociclib-resistant model (102% TGI). Vepdegestrant-induced robust tumor regressions in combination with each of the CDK4/6 inhibitors palbociclib, abemaciclib, and ribociclib; the mTOR inhibitor everolimus; and the PI3K inhibitors alpelisib and inavolisib. CONCLUSIONS: Vepdegestrant achieved greater ER degradation in vivo compared with fulvestrant, which correlated with improved TGI, suggesting vepdegestrant could be a more effective backbone ET for patients with ER+/HER2- breast cancer.

The mammalian SWItch/Sucrose Non-Fermentable (SWI/SNF) helicase SMARCA4 is frequently mutated in cancer and inactivation results in a cellular dependence on its paralog, SMARCA2, thus making SMARCA2 an attractive synthetic lethal target. However, published data indicates that achieving a high degree of selective SMARCA2 inhibition is likely essential to afford an acceptable therapeutic index, and realizing this objective is challenging due to the homology with the SMARCA4 paralog. Herein we report the discovery of a potent and selective SMARCA2 proteolysis-targeting chimera molecule (PROTAC), A947. Selective SMARCA2 degradation is achieved in the absence of selective SMARCA2/4 PROTAC binding and translates to potent in vitro growth inhibition and in vivo efficacy in SMARCA4 mutant models, compared to wild type models. Global ubiquitin mapping and proteome profiling reveal no unexpected off-target degradation related to A947 treatment. Our study thus highlights the ability to transform a non-selective SMARCA2/4-binding ligand into a selective and efficacious in vivo SMARCA2-targeting PROTAC, and thereby provides a potential new therapeutic opportunity for patients whose tumors contain SMARCA4 mutations.

The identification of VHL-binding proteolysis targeting chimeras (PROTACs) that potently degrade the BRM protein (also known as SMARCA2) in SW1573 cell-based experiments is described. These molecules exhibit between 10- and 100-fold degradation selectivity for BRM over the closely related paralog protein BRG1 (SMARCA4). They also selectively impair the proliferation of the H1944 “BRG1-mutant” NSCLC cell line, which lacks functional BRG1 protein and is thus highly dependent on BRM for growth, relative to the wild-type Calu6 line. In vivo experiments performed with a subset of compounds identified PROTACs that potently and selectively degraded BRM in the Calu6 and/or the HCC2302 BRG1 mutant NSCLC xenograft models and also afforded antitumor efficacy in the latter system. Subsequent PK/PD analysis established a need to achieve strong BRM degradation (>95%) in order to trigger meaningful antitumor activity in vivo. Intratumor quantitation of mRNA associated with two genes whose transcription was controlled by BRM (PLAU and KRT80) also supported this conclusion.

Abstract The BCL6 transcription repressor (B-cell lymphoma 6, BCL6) protein has been shown to be a key molecular driver of diffuse large B-cell lymphoma (DLBCL). Somatic mutations of the BCL6 gene include gross- or cryptic-chromosome translocations and point mutations that have been shown to result in the deregulation of BCL6 expression. These BCL6 abnormalities also contribute to a subgroup of high-risk (HR) aggressive double- and triple-hit (DH/TH) lymphomas with very poor outcomes. We have developed highly specific, potent and orally bioavailable BCL6 PROteolysis TArgeting Chimera (PROTAC TM) degraders that demonstrate potent in-vitro and in-vivo efficacy in multiple pre-clinical DLBCL models. Ten of 12 germinal center B-cell (GCB) and two of four activated B-cell (ABC) DLBCL cell lines show significant growth inhibition in-vitro with BCL6 PROTAC TM treatment, demonstrating a critical dependence on BCL6. This array of sensitivity across genetically variable cell lines suggests that BCL6-dependence is not just associated with BCL6-mutated DLBCLs. A more advanced BCL6 PROTAC TM, ARVN-71228, achieves >95% BCL6 D max in-vitro at a DC 50 of <1 nM in the OCI-Ly1 model following 24 hr treatment, degrading BCL6 equally well in the nuclear, chromatin-bound and cytosolic cell fractions. BCL6 degradation is associated with dose-dependent G1 cell cycle arrest and elevated apoptosis that increases over time (24 vs 72 hours). In head-to-head BCL6 degradation and growth inhibition studies using OCI-Ly1, the ARVN-71228 BCL6 PROTAC TM demonstrates superior activity compared to recently published BCL6-targeted degraders/inhibitors and heterobifunctional molecules. Importantly, medicinal chemistry efforts have resulted in the successful development of orally bioavailable BCL6 PROTAC TM degraders for in-vivo dosing. Time-course studies show >95% BCL6 loss within four hours which is maintained at 8-, 16- and 24-hours. Genes repressed by BCL6 such as BLIMP1 and PTPN6 are derepressed and show increased protein levels 24 hours post-dose. ARVN-71228 achieves regressions in the GCB OCI-Ly1 CDX model. Future studies plan to look at rational drug combinations with BCL6 PROTAC TM degraders to find collaborative or synergistic pathways to target, especially in the HR-DLBCL subtypes where there is a high unmet medical need. Disclosures Gough: Arvinas: Current Employment, Current equity holder in publicly-traded company, Divested equity in a private or publicly-traded company in the past 24 months. Sherman: Arvinas: Current Employment, Current equity holder in publicly-traded company, Divested equity in a private or publicly-traded company in the past 24 months. DeCarr: Arvinas: Current Employment, Current equity holder in publicly-traded company, Divested equity in a private or publicly-traded company in the past 24 months. Eaton: Arvinas: Current Employment, Current equity holder in publicly-traded company. Milanovic: Arvinas: Current Employment, Current equity holder in publicly-traded company. Bookbinder: Arvinas: Current Employment, Current equity holder in publicly-traded company. Pizzano: Arvinas: Current Employment, Current equity holder in publicly-traded company. Altieri: Arvinas: Current Employment, Current equity holder in publicly-traded company. Corradi: Arvinas: Current Employment, Current equity holder in publicly-traded company, Divested equity in a private or publicly-traded company in the past 24 months. Xiao: Arvinas: Current Employment, Current equity holder in publicly-traded company. Gallego: Arvinas: Current Employment, Current equity holder in publicly-traded company, Divested equity in a private or publicly-traded company in the past 24 months. Soto: Arvinas: Current Employment, Current equity holder in publicly-traded company. Lingamaneni: Arvinas: Current Employment, Current equity holder in publicly-traded company. Chen: Arvinas: Current Employment, Current equity holder in publicly-traded company, Divested equity in a private or publicly-traded company in the past 24 months. Zhang: Arvinas: Current Employment, Current equity holder in publicly-traded company, Divested equity in a private or publicly-traded company in the past 24 months. Wang: Arvinas: Current Employment, Current equity holder in publicly-traded company. Dong: Arvinas: Current Employment, Current equity holder in publicly-traded company. Chirnomas: Arvinas: Current Employment, Current equity holder in publicly-traded company. Berlin: Arvinas: Current Employment, Current equity holder in publicly-traded company. Hornberger: Arvinas: Current Employment, Current equity holder in publicly-traded company. Snyder: Arvinas: Current Employment, Current equity holder in publicly-traded company, Divested equity in a private or publicly-traded company in the past 24 months. Taylor: Arvinas: Current Employment, Current equity holder in publicly-traded company, Divested equity in a private or publicly-traded company in the past 24 months.