Differential Odds of Response in ILC Versus IDC Correlate with Changes in the TIME in a Phase II Trial of Pre-Operative Fulvestrant with or Without Enzalutamide

Background: The majority of ER+ breast cancers (BC) express androgen receptors (AR). In a randomized phase II trial for women with ER+/HER2- primary BC T2 or greater, neoadjuvant fulvestrant (Fulv) alone or with enzalutamide (Combo) was given for 4 months prior to surgery. A total of 59 patients were evaluable: 33 on Combo and 26 on Fulv. The addition of AR blockade to Fulv reduced residual tumor at time of surgery as measured by modified preoperative endocrine predictive index (PEPI) score. Fresh tumor biopsies were required at study entry (baseline), after 4 weeks on therapy (W5), and at surgery. The Combo arm achieved PEPI=0 more frequently (24%: 8/33) than Fulv (8%: 2/26). Interestingly, the odds of response were 4.6-fold (95% CI: 0.9-22) higher for patients with invasive lobular cancer (IDC) versus invasive ductal (IDC). Results: When examining all tumors, gene expression analyses showed significantly decreased estrogen response and cell division gene sets in tumors in both arms; however, only Combo treated tumors exhibited significant enrichment of immune activation genes sets, including interferon gamma, complement, inflammation, antigen processing, and B and T cell activation. AR protein was significantly reduced by time of surgery in the Combo arm only (P<0.05) as measured by immunohistochemistry. AR was also significantly lower at time of surgery in the PEPI=0 as compared to PEPI >0 (P<0.05) and in the Ki67 responders versus non-responders (p<0.02). Because of the 4.6-fold higher odds of PEPI=0 response in ILCs versus IDC, we examined phosphoproteins that changed with treatment in these two histologic groups by reverse phase phosphoprotein assay (RPPA) from frozen tumor sections. Both AR and phosphoS650 AR showed significantly more decrease from BL to W5 in ILC versus IDC. Cyclin D1, S6RP, HIF-1 alpha, ATP citrate lyase, were significantly lower in ILC than IDC, while CHK1 and ALK were higher. As would be expected with the higher odds of response in ILC, cell cycle proteins decreased significantly more with treatment in this histologic subtype, as did growth/survival and metabolism proteins. Polaris Multiplex immunofluorescence, used to study the tumor immune microenvironment (TIME), revealed that the number of tertiary lymphoid structures (TLS) per area surrounding resected tumor at time of surgery was higher in the Combo arm and was significantly higher near tumors that achieved PEPI=0 (P<0.03). Additionally, the average number of TLS/mm2 was higher in the ILC versus IDC (P < 0.059). T regulatory cells were reduced in the Combo arm (p<0.0002) and in Ki67 responsive tumors (p<0.004); however, T regs were not significantly different in ILC versus IDC. Tumor-associated macrophages decreased by time of surgery only in the Combo arm (p<0.0001), only in tumors that achieved PEPI=0 (p<0.0005) and trended towards decreased numbers in ILC vs IDC. Conclusions: AR inhibition in combination with a SERD activates the immune system in ER+ BC, particularly in ILC. Citation Format: Jennifer Richer, Anthony D. Elias, Alyse W. Staley, Monica Fornier, Gregory A. Vidal, Vida Alami, Sharon Sams, Nicole S. Spoelstra, Andrew Goodspeed, Peter Kabos, Jennifer R. Diamond, Elena Shagisultanova, Rosa I Gallagher, Julia Wulfkuhle, Emanuel Petricoin, Kathryn Zolman, Tessa McSpadden, Christian Rickert, Kimberly R. Jordon, Jill E. Slansky, Virginia F. Borges, Dexiang Gao. Differential odds of response in ILC versus IDC correlate with changes in the TIME in a phase II trial of pre-operative fulvestrant with or without enzalutamide [abstract]. In: Proceedings of the San Antonio Breast Cancer Symposium 2024; 2024 Dec 10-13; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2025;31(12 Suppl):Abstract nr PS18-07.