Background Poly (ADP-ribose) polymerase inhibitors (PARPi) have improved outcomes of BRCA-associated breast cancer (BC); however, treatment responses are often not durable. Our preclinical studies demonstrated PARPi activates the cGAS/STING pathway inducing recruitment of anti-tumor CD8+ T-cells, required for tumor clearance. These studies led to clinical trials testing PARPi plus ICB. However, single-arm and randomized trials of PARPi + ICB have not yet demonstrated efficacy superior to PARPi monotherapy, underscoring the need to characterize the tumor microenvironment (TME) during PARPi therapy to identify optimal strategies to enhance T-cell activation. We recently showed that PARPi induce CSF-1R+ suppressive tumor associated macrophages (TAMs) that restrict anti-tumor immune responses, contributing to PARPi resistance. Removing TAMs with CSF-1R-blockade significantly enhanced overall survival (OS) when combined with PARPi compared to PARPi monotherapy in preclinical models, which was CD8-T-cell-dependent. Here, we investigate if combined CSF-1R blockade and PARPi can be further enhanced by ICB.
Methods Mice bearing Brca1-deficient TNBC (K14-Cre;Brca1f/f;Trp53f/f ) tumors were treated with PARPi (talazoparib) ± small molecule inhibitor of CSF-1R (ARRAY-382; CSF-1Ri) ± anti-PD-1 and followed for survival. Flow cytometry and Nanostring gene expression analysis were employed to elucidate changes in the TME following short-term treatment.
Results PARPi + CSF-1Ri-treated mice cleared 7/10 tumors by day 98, compared to 2/8 mice treated with PARPi monotherapy. PARPi + CSF-1Ri-treated mice had 100% protection upon tumor rechallenge, whereas PARPi monotherapy provided 50% protection. The addition of anti-PD-1 to PARPi did not enhance OS compared to PARPi monotherapy. Triple combination of anti-PD-1 + PARPi + CSF-1Ri demonstrated similar clearance of tumor by day 98 (7/10) compared to PARPi + CSF-1Ri. Triple combination led to superior long-term tumor clearance; whereat day 301, 5/10 mice were tumor-free compared to 2/10 treated with PARPi + CSF-1Ri. Flow cytometry revealed an increasing trend of infiltrating Granzyme B+CD8+T-cells in triple therapy-treated tumors, compared to PARPi + CSF-1Ri-treated tumors. Triple therapy-treated tumors had increased expression of genes associated with anti-tumor immune response (CD80, CD86, PD-L1, IL-1β, IFNβ, IFNγ), antigen presentation, and T-cell chemotaxis (CXCR3 ligands) compared to PARPi + CSF-1Ri.
Conclusions These data confirm targeting immunosuppressive TAMs induces favorable anti-tumor responses that enhance PARPi therapy, which is associated with immunologic memory and can be further enhanced by ICB, with improved long-term durability of tumor clearance and further favorable modulation of the TME. Trials combining CSF-1R-blockade and PARPi are under development for BRCA-associated BC; our results suggest addition of ICB may further improve efficacy.
Acknowledgements We would like to thank J.Jonkers for providing reagents for animal experiments. We are grateful for the expertise and help from the following core facilities: The Dana-Farber Animal Research Facility, the Dana Farber Flow Cytometry Core and the Brigham and Women’s Center for Advanced Molecular Diagnostics Research Core Lab. This work was supported by Susan G. Komen Foundation (CCR18547597), NCI Cancer Systems Biology Center of Excellence Grant (U54-CA225088), Terri Brodeur Breast Cancer Foundation, The Harvard Ludwig Center, NIH DF/HCC. SPORE in Breast Cancer (P50 CA168504), NIH NCI R01/R37 CA 269499 and The Concern Foundation.
Ethics Approval All murine experiments were approved by BWH Center for Comparative Medicine and Institutional Animal Care & Use Committee (Protocol #2020N000142)
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