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, high AR relative to ER in primary breast cancer is associated with endocrine resistance, and in the absence of estradiol or low or blocked ER, AR can exert a pro-survival signal. In a phase II trial of fulvestrant plus enzalutamide in ER+/Her2- MBC we analyzed serial biopsies pre- and post-treatment

Methods: Eligible patients were women with ECOG 0-2, ER+/Her2- MBC. Fulvestrant 500 mg IM days 1, 15, 29 and every 4 weeks thereafter and Enzalutamide at 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 to detect mutations including ESR1 exon 8 mutations.We examined estrogen, progesterone, androgen and glucocorticoid receptors, multiplex analysis of immune cells and PD-L1, and performed reverse phase protein array (RPPA) based protein pathway activation analysis of over 150 proteins/phosphoprotein drug targets from LCM-enriched tumor epithelium in baseline and post-treatment metastatic biopsies. Comparisons of PFS Responders (PFS longer than or equal to 24 weeks) and PFS Non-Responders (PFS shorter than or equal to 5 weeks) were performed using moderated t-tests on log2 transformed data.

Results: 32 were eligible and median age was 61 years (46-87); PS 1 (0-1); a median of 2 prior chemotherapy and 2 prior hormonal therapies for metastatic disease (including 7 with prior Fulvestrant) and 90% had visceral disease. ESR1 mutant metastases had higher levels of ER and PR than those with wild type ESR1 (p<0.05). In a paired t test, ER and Ki67 decreased (p<0.05) with treatment. RPPA analysis of the baseline biopsy sample indicated activation of mTOR pathway proteins associated with non-response, while phosphorylated RB, EGFR and IRS1 were associated with response (PFS greater than 24 weeks). ESR1 mutation positive metastases had significantly more T helper cells, T regulatory cells and macrophages than those with wild type ESR1.PD-L1 increased with treatment in all patients by paired t test (p<0.03).

Conclusions: Clinical benefit lasting 6-12 months was observed in 23% of patients. Our studies show important differences in hormone receptor expression and immune infiltrates in ESR1 mutated disease. RPPA based pathway activation mapping showed that mTOR pathway activation was associated with shorter PFS (p<0.05). Since PD-L1 expression significantly increased with treatment, we conclude that mTOR and/or PD-L1 directed therapies could be useful with Fulvestrant and Enzalutamide in patients with MBC refractory to current standard of care treatments

Citation Format: Jennifer K. Richer, Nicole S. Spoelstra, Alyse Winchester, Julia Wulfkuhle, Sharon B. Sams, Gregory Vidal, Peter Kabos, Jennifer Diamond, Elena Shagisultanova, Anosheh Afghahi, Jose Mayordomo, Tessa McSpadden, Gloria Crawford, Virginia Borges, Dexiang Gao, Emanuel Petricoin, Anthony D. Elias. Laboratory analyses of metastatic ER+/Her2- breast cancer treated with fulvestrant plus enzalutamide [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 2867.