Abstract CT144: Randomized phase II trial of preoperative fulvestrant with or without enzalutamide for ER+/Her2- breast cancer

Background: Most estrogen receptor alpha positive (ER+) breast cancers (BCs) express androgen receptor (AR) protein, but its function is unclear. High AR relative to ER is associated with endocrine resistance. This randomized phase II trial of neoadjuvant fulvestrant (F) with or without the anti-androgen enzalutamide (E) was designed to determine if the addition of E to F in women with ≥T2 ER+/Her2- primary BC would increase the % patients (pts) with limited residual disease at time of surgery as measured by modified preoperative endocrine predictive index (PEPI). Methods: 4 months of therapy was given prior to surgery (F 500 mg IM weeks 1, 3, 5, 9, and 13) and pts were randomized to receive E 160 mg po daily for 16 weeks, stratified by node status and T-stage. Tumor biopsies were required at study entry and after 4 weeks on therapy (Wk5). PEPI score at time of surgery was the primary endpoint for efficacy. The minimax two-stage design had 80% power with type I error rate of 0.08. Reverse phase phosphoprotein array (RPPA), IHC for ER/PR/AR/GR/Ki67, and Polaris multiplex immunofluorescence (IF) panel for myeloid lineage immune cells were performed. Average signal levels were compared across arms, PEPI score (0, >0), PEPI by arm, and Ki67 (<10%, ≥10%). Comparisons at both baseline and with treatment were considered. Significance was assessed with empirical Bayes moderated t-statistics. Since exploratory, p-values were not adjusted for multiple testing. Results: 69 pts consented; 59 evaluable. 95% completed surgery. PEPI=0 observed in 10 (17%). Of 33 pts on FE arm, 21 had T3/T4 tumors, 91% ER+/PR+, median AR expression 80%, Ki67 15%. Of 26 patients on F arm, 19% had T3/T4 tumors, 96% ER+/PR+, median AR expression 85%, Ki67 10%. PEPI=0 was achieved more frequently on the FE arm (8, 24%) than the F arm (2, 8%) (p=0.16). Toxicity was as expected with endocrine therapy. IHC of baseline vs Wk5 biopsies showed decreased ER, PR, and Ki67 levels by Wk5 that remained decreased at time of surgery. AR and GR were significantly decreased only in the FE arm at the time of surgery. RPPA revealed that baseline EGFR (Y1604 and Y992) was higher in tumors with PEPI=0 tumors, whereas the average baseline mTOR activation was higher among pts with PEPI>0. Significant changes detected by RPPA upon treatment included a significant decrease in AR. Myeloid-derived suppressor cells (MDSC) were significantly reduced by treatment only in the FE arm. Conclusions: The combination FE had manageable side effects. PEPI=0 was achieved more frequently on the FE arm (8/33) than the F arm (2/26). Activated EGFR was higher pretreatment in tumors achieving PEPI=0. mTOR pathway was elevated pretreatment in PEPI>0 tumors (also observed with resistance to FE in ER+/HER2- metastatic disease (SABCS 2021). When comparing pretreatment tumor to Wk5, total AR by RPPA was the most differentially decreased protein in PEPI=0 tumors. AR signaling is known to support immunosuppressive cells and we observed a marked decrease in MDSCs with treatment only on the FE arm. Citation Format: Anthony D. Elias, Alyse Staley, Monica Fornier, Gregory A. Vidal, Sharon Sams, Nicole Spoelstra, Peter Kabos, Jennifer R. Diamond, Elena Shagisultanova, Rosa I. Gallagher, Julia Wulfkuhle, Emanuel Petricoin, Kathryn Zolman, Stephanie Biller, Vida Alami, Tessa McSpadden, Virginia Borges, Lyndsey S. Crump, Dexiang Gao, Jennifer K. Richer. Randomized phase II trial of preoperative fulvestrant with or without enzalutamide for ER+/Her2- breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 2 (Clinical Trials and Late-Breaking Research); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(8_Suppl):Abstract nr CT144.