ARV-471, an estrogen receptor (ER) PROTAC degrader, combined with palbociclib in advanced ER+/human epidermal growth factor receptor 2–negative (HER2-) breast cancer: Phase 1b cohort (part C) of a phase 1/2 study.

Erika P. Hamilton Sarah Cannon Research Institute/Tennessee Oncology, Nashville, TN info_outline Erika P. Hamilton, Anne F. Schott, Rita Nanda, Haolan Lu, Chi Fung Keung, Richard Gedrich, Janaki Parameswaran, Hyo S. Han, Sara A. Hurvitz

Authors Erika P. Hamilton Sarah Cannon Research Institute/Tennessee Oncology, Nashville, TN info_outline Erika P. Hamilton, Anne F. Schott, Rita Nanda, Haolan Lu, Chi Fung Keung, Richard Gedrich, Janaki Parameswaran, Hyo S. Han, Sara A. Hurvitz Organizations Sarah Cannon Research Institute/Tennessee Oncology, Nashville, TN, University of Michigan Rogel Cancer Center, Ann Arbor, MI, University of Chicago Medical Center, Chicago, IL, Arvinas, Inc., New Haven, CT, Moffitt Cancer Center, Tampa, FL, UCLA Jonsson Comprehensive Cancer Center, Los Angeles, CA Abstract Disclosures Research Funding Pharmaceutical/Biotech Company Background: ARV-471 is a novel, potent, orally bioavailable PROteolysis TArgeting Chimera (PROTAC) protein degrader that selectively targets the ER. In xenograft models, ARV-471 demonstrated substantially greater ER degradation and antitumor activity compared with the selective ER degrader fulvestrant. In the phase 1 dose escalation portion (Part A) of the first-in-human phase 1/2 study, ARV-471 monotherapy was well tolerated and showed antitumor activity in patients with ER+/HER2- locally advanced or metastatic breast cancer who had previously received endocrine therapy and a cyclin-dependent kinase (CDK)4/6 inhibitor; the clinical benefit rate (rate of confirmed complete or partial response or stable disease ≥24 weeks) was 40% (95% CI: 26%–56%) in 47 evaluable patients. The phase 2 VERITAC expansion cohort (Part B) is further evaluating ARV-471 monotherapy in this patient population. Palbociclib, a CDK4/6 inhibitor, plus fulvestrant is a standard treatment option for patients with ER+/HER2- breast cancer who have had disease progression on endocrine therapy. ARV-471 plus palbociclib resulted in substantially greater tumor growth inhibition in xenograft models compared with palbociclib plus fulvestrant, supporting further investigation of the ARV-471 plus palbociclib combination in patients with ER+ breast cancer. Here we describe Part C of the phase 1/2 study, which evaluates the safety and clinical activity of ARV-471 plus palbociclib in patients with breast cancer who previously received endocrine therapy. Methods: Eligible patients (aged ≥18 years) have histologically or cytologically confirmed ER+/HER2- advanced breast cancer and have received ≥1 prior endocrine therapy and ≤2 prior chemotherapy regimens for advanced disease; prior CDK4/6 inhibitor therapy is permitted. Patients with known symptomatic brain metastases requiring steroids are excluded. ARV-471 and palbociclib will be administered orally once daily in 28-day cycles; ARV-471 will be given continuously and palbociclib for 21 days followed by 7 days off treatment. Primary objectives are to evaluate the safety and tolerability of ARV-471 plus palbociclib and select the recommended phase 2 dose and schedule of the combination (based on the incidence of dose-limiting toxicities during the first cycle and the frequency and severity of adverse events and laboratory abnormalities). Secondary objectives are to assess preliminary antitumor activity of ARV-471 plus palbociclib (based on overall response rate per Response Evaluation Criteria in Solid Tumors v1.1, clinical benefit rate, progression-free survival, and duration of response) and pharmacokinetic parameters. Clinical trial information: NCT04072952.