Abstract PS12-06: Phase II randomized trial of fulvestrant with or without enzalutamide in ER+/Her2- primary breast cancer (BC)

Background: Almost all ER+ breast cancers express androgen receptor (AR), but its function is uncertain. AR expression is associated with more indolent tumors, however high AR expression relative to ER is associated with endocrine resistance, and in the absence of estradiol or if ER function is blocked, preclinical data would suggest that AR can take over to signal cell survival and proliferation. This non-blinded randomized phase II trial of neoadjuvant fulvestrant with or without enzalutamide was performed in women with T2 or greater ER+/Her2- primary breast cancer. Methods: Eligible patients were women with ECOG 0-2, ER+/Her2- primary breast cancer cT2 or greater. A total of 4 months of therapy was given (fulvestrant 500 mg IM weeks 1, 3, 5, 9, and 13). Patients were randomized to receive enzalutamide 160 mg po daily on a continual basis for 16 weeks. Stratification factors were clinical node status (N0 vs N1/2) and T-stage (T2 vs T3/4). Surgery was planned for week 17. Fresh tumor biopsies were required at study entry and at ~4 weeks on therapy. Tissue, both fresh frozen and FFPE, was also obtained at time of surgery. The PEPI score at time of surgery was the primary endpoint for efficacy. The statistical design were parallel phase II trials with the experimental arm designed as a Simon 2-stage. The expectation for the control arm was a PEPI score of 0 in 16% and 32% for the combination arm. Additional enrollment of 12 patients would ensue if 4 or more achieved a PEPI score of 0 within the first 22 patients enrolled onto the combination arm. 27 patients would be enrolled into the fulvestrant alone arm. Results: The Simon 2-stage criteria were met and the trial continues to accrue up to 11 more evaluable patients. Thus far, 58 patients were consented of whom 50 were treated. Median age was 61.5 years (45-83); PS 0 (0-2). Among the 46 patients with information on AEs, there are 25 patients with Grade 2 AEs, including 5 with fatigue, 3 with headache, and 2 with hot flashes. There are 5 patients with grade 3 AEs including abdominal pain, gallbladder obstruction, ALT elevation, hyperglycemia, and hypertension. We also have 2 patients with grade 4 AEs , chest pain-cardiac and cardiac disorder-other. There is no grade 5 event. Paired samples at baseline and at 4 weeks are collected so far from 49 patients. 42 patients have completed surgery, while two patients have not undergone surgery. Conclusions: The combination of fulvestrant plus enzalutamide had manageable side effects. The trial is now in its extended stage since PEPI score = 0 was achieved in at least 4 of the first 22 patients on the combination arm. Extensive molecular studies of paired fresh biopsies from pretreatment and at 4 weeks are underway. These analyses and correlations with clinical outcome will be described. Citation Format: Anthony D Elias, Monica Fournier, Gregory A Vidal, Sharon Sams, Nicole Spoelsta, Peter Kabos, Jennifer R Diamond, Elena Shagisultanova, Anosheh Afghahi, Jose Mayordomo, Julia Wulfkuhle, Emanuel Petricoin, Lisa Carter, Kathryn Zolman, Stephanie Armstead, Alyse Winchester, Tessa McSpadden, Gloria Crawford, Virginia Borges, Dexiang Gao, Jennifer Richer. Phase II randomized trial of fulvestrant with or without enzalutamide in ER+/Her2- primary breast cancer (BC) [abstract]. In: Proceedings of the 2020 San Antonio Breast Cancer Virtual Symposium; 2020 Dec 8-11; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2021;81(4 Suppl):Abstract nr PS12-06.