Abstract 3075: Enhanced Efficacy of ARV-471, a Novel PROTAC® Estrogen Receptor Degrader, in Combination with Targeted Agents in Estrogen Receptor-Positive (ER+) Breast Cancer Models

ARV-471 is a selective, orally bioavailable PROteolysis-TArgeting Chimera (PROTAC®) small molecule that induces wild-type and mutant estrogen receptor (ER) alpha degradation via the ubiquitin-proteasome system. ARV-471 demonstrates superior ER degradation and antitumor activity compared to fulvestrant in endocrine sensitive and resistant xenograft models and has shown significant ER degradation and promising clinical benefit in late-line ER+ breast cancer patients. Dual pathway inhibition combining ER targeting agents with CDK4/6 or PIK3CA/mTOR pathway inhibitors is now a central strategy for treatment of advanced ER+ breast cancer. However, resistance to aromatase inhibitors resulting from ESR1 gene mutations, suboptimal ER degradation and the intramuscular route of administration of fulvestrant underscore a need for superior orally bioavailable endocrine therapy backbones for these combinations. Here, we assessed the effects of ARV-471 in combination with CDK4/6 or PIK3CA/mTOR pathway inhibitors in preclinical models of ER+ breast cancer. In vitro studies revealed evidence of synergistic interactions between ARV-471 and the CDK4/6 inhibitors abemaciclib and ribociclib, the mTOR inhibitor everolimus, or the PIK3CA inhibitors alpelisib and inavolisib in ER+ breast cancer cells. Evidence of synergistic effects between ARV-471 and everolimus was also observed in ER+ breast cancer cells expressing ER Y537S or D538G mutations. ARV-471 in combination with CDK4/6, PIK3CA or mTOR inhibitors led to tumor regressions in MCF7 xenografts when compared to single agents alone. ARV-471 also displayed greater anti-tumor activity in combination with abemaciclib, ribociclib or inavolisib than that observed with fulvestrant in combination with these agents. Taken together, these data suggest the potential utility of ARV-471 as a combination partner for clinically relevant targeted agents for treatment of early and late-stage ER+ disease. Citation Format: Jessica Teh, Elizabeth Bortolon, Jennifer Pizzano, Melissa Pannone, Sean Landrette, Richard Gedrich, Ian Taylor. Enhanced efficacy of ARV-471, a novel PROTAC® estrogen receptor degrader, in combination with targeted agents in estrogen receptor-positive (ER+) breast cancer models [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 3075.