Abstract 2175: Combined anti-CSF-1R and anti-TIM-3 overcome immune-mediated mechanisms of PARP inhibitor (PARPi) resistance in BRCA1-associated triple negative breast cancer

Abstract Background: Poly (ADP-Ribose) polymerase inhibitors (PARPi) have improved outcomes for BRCA-associated triple negative breast cancer (TNBC); however, resistance develops, resulting in lack of durable responses. Our previous work demonstrated that PARPi activates the cGAS/STING pathway inducing recruitment of anti-tumor CD8+ T-cells, which are required for tumor clearance. These studies led to clinical trials testing PARPi plus immune checkpoint blockade (ICB). However, clinical trials demonstrated that PARPi + ICB is not superior to PARPi monotherapy, indicating T-cells in the tumor microenvironment (TME) are inhibited. We previously showed that PARPi induce CSF-1R+ suppressive tumor associated macrophages (TAMs) that restrict anti-tumor immune responses, contributing to PARPi resistance. Removing TAMs with anti-CSF-1R therapy significantly enhanced overall survival (OS) when combined with PARPi in naïve tumors. Here, we test if targeting CSF-1R can overcome PARPi resistance and if ICB can further enhance PARPi + anti-CSF-1R therapy in both naïve and PARPi-resistant tumors. Methods: Mice bearing naïve or PARPi resistant BRCA1-deficient TNBC (K14-Cre;Brca1f/f;Trp53f/f) tumors were treated with PARPi ± anti-CSF-1R ± ICB and followed for tumor size and OS. Flow cytometry was employed to determine alterations in TME composition. Results: In PARPi-resistant tumors, TAMs expressed higher levels of CSF-1R and PD-L1 and lower levels of antigen presentation machinery compared to PARPi-naïve tumors, indicating increased immunosuppression. Additionally, in PARPi-resistant tumors, CD4+ and CD8+ T-cells expressed higher levels of PD-1 compared to naïve tumors, indicating increased exhaustion. PARPi + anti-CSF-1R therapy significantly restored PARPi efficacy and increased overall survival in PARPi-resistant tumors. The combination of PARPi + anti-CSF-1R + anti-TIM-3 significantly increased OS in PARPi-resistant tumors, whereas combination of PARPi + anti-CSF-1R + anti-CTLA-4 did not improve therapy efficacy. While the combination of PARPi + anti-CSF-1R with anti-PD-1 improved therapy in PARPi-naïve tumors, it did not improve responses in PARPi-resistant tumors. PARPi + CSF-1R + anti-TIM-3 significantly increased CD8+ T-cell infiltration and granzyme B production indicating an increased anti-tumor immune response. Additionally, anti-TIM-3 increased TRAIL production by TAMs indicating a more pro-inflammatory phenotype. Conclusion: We demonstrate that PARPi-resistant tumors have increased T-cell exhaustion and infiltration of immunosuppressive TAMs. Importantly, our data show that targeting TAMs can overcome acquired PARPi resistance, which select ICB can further enhance defining a novel strategy addressing a critical unmet medical need for PARPi-treated patients with BRCA-associated breast cancer. Citation Format: Adam Nelson, Anita K. Mehta, Madeline G. Townsend, Daniel E. Michaud, Madisson Oliwa, Kelly F. Zheng, Carlos Wanderley, Patrice A. Lee, Nicholas A. Saccomano, Filipa Lynce, Nabihah Tayob, Geoffrey I. Shapiro, Jennifer L. Guerriero. Combined anti-CSF-1R and anti-TIM-3 overcome immune-mediated mechanisms of PARP inhibitor (PARPi) resistance in BRCA1-associated triple negative breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 2175.