Abstract 6521: Macrophage-targeting immunotherapy enhances chemotherapy response in primary and metastatic triple-negative breast cancer models

Abstract Triple-negative breast cancer (TNBC) is a highly aggressive breast cancer subtype that lacks targeted treatment options. While immune checkpoint blockade leads to significant benefits in the treatment of TNBC when in combination with chemotherapy, recurrence, metastasis, and chemoresistance still remain major challenges. The purpose of this study was to assess the ability of a colony-stimulating factor 1 receptor inhibitor (CSF-1Ri) to shift the phenotype of the tumor microenvironment (TME) in murine models of TNBC, and the potential benefit of such in enhancing standard of care chemotherapy. We focused on targeting tumor-associated macrophages (TAMs) as they are the most abundant immune infiltrate in the TNBC TME and are associated with worsening overall survival (OS) and metastasis free survival (MFS). Targeting TAMs, specifically M2-like TAMs, has been shown to inhibit tumor progression and improve OS in the clinic. We selected pexidartinib (PLX) as the CSF-1Ri of choice, given its translational potential (FDA approved) and gemcitabine (GEM) as chemotherapy, given its ability to target myeloid derived suppressor cells and M2-like TAMs. For primary TNBC we employed 4T1 mammary carcinoma cells transfected with luciferase reporter gene in BALB/c mice as an immunocompetent model. Mice were injected with 100, 000 cells in the 4th mammary fat pad, and tumors established over the course of fifteen days. Mice were then exposed to daily administrations of PLX (8 mg/kg), and weekly administrations of GEM (60 mg/kg). Tumor volumes were recorded every other day. The TME was analyzed with flow cytometry (FC), immunohistochemistry (IHC), and multiplex immunofluorescence (mIF). For the metastatic model, mice were inoculated with 100, 000 4T1-luc cells in the flank. On day six, mice were exposed to treatments as outlined above. Mice underwent survival surgery to remove the primary tumor on day thirteen and were then treated with adjuvant therapy. Metastasis and OS were tracked. FC analysis indicated only combination therapy (11. 8% ± 2. 1) significantly decreased total TAMs compared to vehicle (22% ± 2) in the primary tumor. Combination therapy also showed the greatest depletion of M2-like TAMs (2. 2% ± 0. 6), with a significant decrease compared to both vehicle (15. 6% ± 0. 4) and GEM (6. 6% ± 0. 9). This TME modulation was confirmed using IHC and mIF. After four cycles of treatment, combination therapy (232. 9 mm3 ± 130. 8) had a significant effect on tumor burden compared to GEM alone (369. 7 mm3 ± 165. 8), indicating a correlation between tumor phenotype and therapeutic benefit. Furthermore, combination therapy led to the greatest prolongation of MFS and OS in the resection model, indicating that combination therapy has efficacy in the neoadjuvant and adjuvant setting. In conclusion, targeting TAMs in TNBC primes the tumor for combination with chemotherapy in TNBC. Citation Format: Matthew Emilio Fernandez, Yasir Alshehry, Laura Graham, Jennifer E. Koblinski, Harry D. Bear, Doug H. Sweet, Sandro R. da Rocha. Macrophage-targeting immunotherapy enhances chemotherapy response in primary and metastatic triple-negative breast cancer models abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts) ; 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84 (6Suppl): Abstract nr 6521.