Targeting Interactions Between Tumor-Associated Macrophages and Tumor Cells to Enhance Chemotherapy Response in Murine Models of Triple-Negative Breast Cancer

Among women in the United States, breast cancer (BC) is the most diagnosed malignancy, and the second leading cause of cancer-related deaths. Approximately 15-25% of all breast cancer cases are classified as triple-negative breast cancer (TNBC). TNBC is the BC subtype with the worst prognosis, which is driven by high rates of metastasis and recurrence and lacks targeted treatment options. As a result, there is an urgent unmet clinical need for the development of more effective pharmacological interventions for TNBC. In this work, we seek to address this need by targeting tumor-associated macrophages (TAMs), as they are the most abundant immune infiltrate in the TNBC tumor microenvironment (TME), and high TAM abundance is associated with worse patient outcomes in TNBC. This association between abundance and patient outcomes is also present when observing alternatively activated, anti-inflammatory, M2-like TAMs. We sought to target the interactions between TNBC cells and TAMs by inhibiting the CSF1/CSF1R axis, as well as the CD47/SIRPa axis. To inhibit CSF1R, we selected pexidartinib (PLX) given its translational potential as the only FDA approved CSF1R inhibitor. We sought to block the CD47/SIRPa axis using an anti-CD47 mAB because TNBC cells express high baseline levels of CD47, and this expression is further increased upon exposure to many chemotherapies, including gemcitabine (GEM). Increased levels of CD47 is associated with evasion of phagocytosis, increased cancer stemness, and lower metastasis free survival (MFS) and overall survival (OS). To test the efficacy of treatment, we utilized a highly sophisticated survival surgery murine model, thus allowing us to recapitulate the clinical setting by administering (neo)adjuvant therapy and track the development of metastases and record both MFS and OS. The combination therapy PLX+GEM significantly depleted total and M2-like TAMs compared to vehicle and GEM monotherapy, while shifting the M1/M2 ratio towards an M1-like dominant phenotype. PLX+GEM also greatly impacted tumor-infiltrating lymphocyte (TIL) populations by significantly increasing B cells, CD4+, and CD8+ T cells. These drastic changes in the TNBC TME resulted in significant prolongations in MFS and OS compared to all monotherapies. Similarly, the combination of anti-CD47+GEM significantly depleted total and M2-like TAMs compared to vehicle and GEM monotherapy. However, while anti-CD47+GEM did not drastically affect TIL populations, it did lead to significant depletions in myeloid-derived suppressor cell (MDSC) populations, and significant increases in dendritic cell (DC) populations. These shifts in myeloid cell populations resulted in combination therapy significantly increasing MFS and OS to all monotherapy approaches. When all three monotherapies are combined into a triple combination (PLX+anti-CD47+GEM), the result was the greatest depletion of total TAMs and the greatest magnitude of M2-like TAM reduction. Triple therapy also retained the myeloid cell effects demonstrated by anti-CD47+GEM, with it significantly reducing MDSCs while leading to the greatest infiltration of DCs. However, while triple therapy had significantly lower TIL abundance compared to PLX+GEM, it still demonstrated the ability to partially overcome anti-CD47 mediated TIL reductions. As a result of the aforementioned TME remodeling, triple therapy resulted in the greatest prolongation of MFS and OS. These results suggest that PLX+GEM, anti-CD47+GEM, and triple therapy are all promising therapies that warrant further investigation and optimization.