Mark Bookbinder

259 Background: The Androgen Receptor (AR) remains the principal driver of castration-resistant prostate cancer during the transition from a localized to metastatic disease. Most patients initially respond to inhibitors of the AR pathway, but the response is often relatively short-lived. The majority of patients progressing on enzalutamide or abiraterone exhibit genetic alterations in the AR locus, either in the form of amplifications or point mutations in the AR gene. Given these mechanisms of resistance, our goal is to eliminate the AR protein using the PROteolysis TArgeting Chimera (PROTAC) technology. Methods: Here we report an orally bioavailable small molecule AR PROTAC degrader, ARV-110, that promotes ubiquitination and degradation of AR. This molecule has been characterized in in vitro degradation and functional assays, and DMPK, toxicology and preclinical efficacy studies. Results: ARV-110 robustly degrades AR in all cell lines tested, with an observed half-maximal degradation concentration (DC 50 ) of ~1 nM. ARV-110 treatment leads to highly selective AR degradation, as demonstrated by proteomic studies. In VCaP cells, PROTAC-mediated AR degradation suppresses the expression of the AR-target gene PSA, inhibits AR-dependent cell proliferation, and induces apoptosis at low nanomolar concentrations. Further, ARV-110 degrades clinically relevant mutant AR proteins and retains activity in a high androgen environment. In mouse xenograft studies, greater than 90% AR degradation is observed at a 1 mg/kg PO QD dose. Significant inhibition of tumor growth and AR signaling has been achieved in LNCaP, VCaP and prostate cancer patient derived xenograft (PDX) models. Notably, ARV-110 demonstrates in vivo efficacy and reduction of AR-target gene expression in a long term, castrate, enzalutamide-resistant VCaP tumor model. Conclusions: In summary, we report preclinical data on an orally bioavailable AR PROTAC degrader, ARV-110, that demonstrates efficacy in multiple prostate cancer models. ARV-110 has completed IND-enabling studies and FIH studies are planned for 1Q2019.

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.

Abstract ARV-471, an estrogen receptor (ER) alpha PROTAC, is a hetero-bifunctional molecule that facilitates the interactions between estrogen receptor alpha and an intracellular E3 ligase complex, leading to the ubiquitylation and subsequent degradation of estrogen receptors via the proteasome. ARV-471 robustly degrades ER in ER-positive breast cancer cell lines with a half-maximal degradation concentration (DC50) of ˜ 2 nM. PROTAC-mediated ER degradation decreases the expression of classically-regulated ER-target genes (PR, GREB1, TFF) and inhibits cell proliferation of ER-dependent cell lines (MCF7, T47D). Additionally, ARV-471 degrades clinically-relevant ESR1 variants (Y537S and D538G) and inhibits growth of cell lines expressing those variants. In an immature rat uterotrophic model, ARV-471 degrades rat uterine ER and demonstrates no agonist activity. Daily, oral-administration of single agent ARV-471 (3, 10, and 30 mpk) leads to significant tumor volume regressions of estradiol-dependent MCF7 xenografts and concomitant tumor ER protein reductions of >90% at study termination. Moreover, when a CDK4/6 inhibitor is combined with ARV-471 in the MCF7 model, even more pronounced tumor growth inhibition is observed (˜130% TGI), accompanied by significant reductions in ER protein levels. In an ESR1 Y537S, hormone-independent patient-derived xenograft model, ARV-471 at 10 mpk completely inhibited growth and also reduced mutant ER protein levels. Taken together, the preclinical data of ARV-471 supports its continued development as a best-in-class oral ER PROTAC-degrader. Citation Format: Flanagan JJ, Qian Y, Gough SM, Andreoli M, Bookbinder M, Cadelina G, Bradley J, Rousseau E, Willard R, Pizzano J, Crews CM, Crew AP, Taylor I, Houston J. ARV-471, an oral estrogen receptor PROTAC degrader for breast cancer [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 P5-04-18.

Abstract ARV-471, an estrogen receptor (ER) alpha PROTAC® protein degrader, is a hetero-bifunctional molecule that facilitates the interactions between estrogen receptor alpha and an intracellular E3 ligase complex, leading to the ubiquitylation and subsequent degradation of estrogen receptors via the proteasome. ARV-471 robustly degrades ER in ER-positive breast cancer cell lines with a half-maximal degradation concentration (DC50) of ~ 1 nM. PROTAC® mediated ER degradation decreases the expression of classically regulated ER-target genes and inhibits cell proliferation of ER-dependent cell lines (MCF7, T47D). Additionally, ARV-471 degrades clinically relevant ESR1 variants (Y537S and D538G) and inhibits growth of cell lines expressing those variants. In an immature rat uterotrophic model, ARV-471 degrades rat uterine ER and demonstrates no agonist activity. Daily, oral-administration of single agent ARV-471 (3, 10, and 30 mpk) leads to significant anti-tumor activity of estradiol-dependent MCF7 xenografts and concomitant tumor ER protein reductions of >90% at study termination. Moreover, when a CDK4/6 inhibitor is combined with ARV-471 in the MCF7 model, even more pronounced tumor growth inhibition is observed (131% TGI), accompanied by significant reductions in ER protein levels. In an ESR1 Y537S, hormone-independent patient-derived xenograft model, ARV-471 at 10 mpk completely inhibited growth and also significantly reduced mutant ER protein levels. Taken together, the preclinical data of ARV-471 supports its continued development as a best-in-class oral ER PROTAC® protein degrader. These preclinical data supported the clinical development of ARV-471 for the treatment of patients with breast cancer. The discovery, chemical structure and initial clinical data of ARV-471 will be presented. Citation Format: Lawrence B. Snyder, John J. Flanagan, Yimin Qian, Sheryl M. Gough, Monica Andreoli, Mark Bookbinder, Gregory Cadelina, John Bradley, Emma Rousseau, Julian Chandler, Ryan Willard, Jennifer Pizzano, Craig M. Crews, Andrew P. Crew, John Houston, Marcia Dougan Moore, Ron Peck, Ian Taylor. The discovery of ARV-471, an orally bioavailable estrogen receptor degrading PROTAC for the treatment of patients with breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 44.

Abstract The Androgen Receptor (AR) remains the principal driver of castration-resistant prostate cancer during the transition from a localized to metastatic disease. Most patients initially respond to inhibitors of the AR pathway, but the response is often short-lived. The majority of patients progressing on enzalutamide or abiraterone exhibit genetic alterations in the AR locus, either in the form of amplifications or point mutations in the AR gene. Given these mechanisms of resistance, our goal is to eliminate the AR protein using the PROteolysis TArgeting Chimera (PROTAC) technology. Here we report an orally bioavailable small molecule ARV-110 that leads to ubiquitination and degradation of AR. ARV-110 completely degrades AR in all cell lines tested, with an observed 50% degradation concentration (DC50) < 1 nM. PROTAC-mediated AR degradation suppresses the expression of the AR-target genes PSA and FKBP5, inhibits AR-dependent cell proliferation, and induces potent apoptosis in VCaP cells. ARV-110 degrades clinically relevant mutant AR proteins and retains activity in a high androgen environment. In mouse xenograft studies, greater than 90% AR degradation is observed at a 1 mg/kg PO QD dose. Significant inhibition of tumor growth and AR signaling can be achieved in both an intact and castrate setting. Further, ARV-110 demonstrates in vivo efficacy and reduction of oncogenic Erg protein in a long term, castrate, enzalutamide-resistant VCaP tumor model. DMPK and exploratory toxicology studies show robust oral, dose proportional drug exposure in rodent and non-rodent species. In summary, we report preclinical data on ARV-110, an orally bioavailable androgen receptor PROTAC degrader that demonstrates efficacy in enzalutamide-resistant prostate cancer. Citation Format: Taavi Neklesa, Lawrence B. Snyder, Ryan R. Willard, Nicholas Vitale, Kanak Raina, Jennifer Pizzano, Deborah Gordon, Mark Bookbinder, Jennifer Macaluso, Hanqing Dong, Zheng Liu, Caterina Ferraro, Gan Wang, Jing Wang, Craig M. Crews, John Houston, Andrew P. Crew, Ian Taylor. ARV-110: An androgen receptor PROTAC degrader for prostate cancer [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 5236.