Abstract P1-17-01: Response of persistent metastatic ER+/Her2- breast cancer treated with fulvestrant plus enzalutamide

Abstract

Abstract Background: The clinical implications of the androgen receptor (AR), particularly in the context of aromatase inhibitor (AI) refractory metastatic breast cancer (MBC), are unclear. While AR is associated with more indolent primary tumors, in the absence of low estradiol/blocked ER, AR can exert a pro-survival signal. Thus, in a phase II trial of Fulvestrant (Fulv) plus Enzalutamide (Enza) in ER+/Her2- MBC (NCT02953860) we analyzed serial biopsies pre- and post-treatment. Methods: Eligible patients were women with ECOG 0-2, ER+/Her2- MBC. Fulv at 500 mg IM days 1, 15, 29 and every 4 weeks thereafter and Enzaat 160 mg po daily on a continual basis were administered. Biopsies were required at study entry and at ~4 weeks on therapy. The clinical benefit rate at 24 weeks (CBR24) was the primary endpoint for efficacy. We performed mutational analysis using a modified Archer VariantPlex Solid Tumor Assay to detect mutations in ESR1 exon 8 and 67 other gene hotspots. We examined estrogen, progesterone, androgen and glucocorticoid receptor protein by IHC and multiplex for immune cells and PD-L1. Frozen cores were utilized to perform reverse phase protein array (RPPA) based protein pathway activation analysis of over 150 proteins from LCM-enriched tumor in baseline and post-treatment metastatic biopsies. Comparisons of Responders (progression free survival (PFS) equal to or longer than 24 weeks) and Non-Responders were performed using moderated t-tests on log2 transformed data. Results: 32 patients were eligible, median age was 61 years (46-87), and 90.6% had visceral disease with an average of 4 prior non-hormonal therapies and 3 prior hormonal agents (including 37.5% with prior Fulv). PFS >24 weeks was observed in ~22% of patients treated with Fulv plus Enza, including 42% of those who had prior Fulv. When stratified by both AR and ER protein levels, median time to progression was 59 days (95% CI: 55 to Inf) when both targets were high (greater than or equal to 10%), but only 14 days (95% CI: 13 to Inf) when both were less than 10%. Metastases with ESR1 mutations in the ligand binding domain had significantly higher levels of ER and PR protein than those with wild type ESR1 (p<0.05), while AR did not significantly differ. ER significantly decreased following 5 weeks post Fulv plus Enza in a paired t-test (p<0.003). ESR1 mutation positive metastases had significantly more T helper cells, T regulatory cells and macrophages than those with wild type ESR1. In contrast, those with TP53 or PIK3CA mutations had higher CD8+ T cells, but also increased T regulatory cells compared to those WT for these genes. PD-L1 increased with treatment in all patients by paired t test (p<0.03). RPPA analysis indicated that activation of mTOR pathway proteins was associated with non-response to Fulv plus Enza and patients with PIK3CA and or PTEN mutated disease had a shorter progression free survival time following treatment, with the hazard of disease progression for participants with PIK3CA or PTEN mutated disease being 2.27 times (95% CI: 0.94 to 5.46) than without these mutations (p=0.068). Conclusions: PFS >24 weeks was observed in 22% of patients treated with Fulv plus Enza, including 42% who had prior Fulv treatment, suggesting contribution of the anti-androgen. Response was significantly better when metastases were >10% for ER and AR. Poor response to Fulv plus Enza was significantly associated with mTOR pathway activation and patients with PIK3CA and or PTEN mutated metastases had a significantly shorter PFS. Mutation status also affected hormone receptor expression and immune infiltrates. The increase in PD-L1 protein following treatment with Fulv plus Enza warrants further pre-clinical investigation into whether the addition of anti-androgen can enhance efficacy of checkpoint inhibitor therapy in ER+ metastatic disease resistant to standard endocrine therapy approaches. Citation Format: Jennifer Richer, Nicole Spoelstra, Alyse Winchester, Julia Wulfkuhlue, Rosa Gallagher, Sharon Sams, Gregory Vidal, Peter Kabos, Jennifer Diamond, Elena Shagisultanova, Anosheh Afghahi, Jose Mayordomo, Tessa McSpadded, Gloria Crawford, Virginia Borges, Dexiang Gao, Emanuel Petricoin, Anthony Elias. Response of persistent metastatic ER+/Her2- breast cancer treated with fulvestrant plus enzalutamide [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr P1-17-01.