Abstract PD2-05: CSF-1R antibody targeting therapy with combined metronomic chemotherapy enhances a B and T cell response for the treatment of metastatic triple negative breast cancer

Abstract Background: Increased macrophage infiltration is associated with the poorest outcome following neo-adjuvant chemotherapy in patients with triple negative breast cancer (TNBC). We have extensively characterized three different preclinical syngeneic claudin-low Trp53-/- tumor models (T11, T12 and 2151R), that have high Tumor Associated Macrophage (TAM) infiltration. These “claudin low” models closely phenocopy the EMT/TAM subtype observed in patients. Treatment using conventional chemotherapy, Cyclophosphamide (CTX) in combination with Pexidartinib (PXB, PLX3397) an anti CSF-1R small molecule inhibitor yielded a durable response in two of the models, T12 and 2151R (Singh et al. Cancer Res. 2022). However, due to potential liver toxicity and dual targeting of c-kit and FLT-3, PXB is not moving forward into the clinic for TNBC. Accordingly, we asked if SNDX-ms6352 a specific, high affinity monoclonal antibody targeting CSF-1R in combination with CTX might provide an alternative approach for treating primary TNBC as well as established lung metastases. Methods: Tumor chunks were transplanted into the mammary fat pad to generate primary tumors. Single cells from primary T12 mammary tumors were introduced via tail vein (TV) injection to obtain lung metastases. After fifteen days for primary tumors and twelve days following TV injection, respectively, the mice were randomized into treatment groups and administered four weekly treatments of IgG, CTX, SNDX-ms6352 or CTX+SNDX-ms6352. Primary tumors, lungs and mammary glands were harvested and fixed overnight in 4% paraformaldehyde then placed in 70% EtOH for paraffin embedding and stained for H&E, immunohistochemistry (IHC), immunofluorescence (IF) and imaging mass cytometry (IMC). Results: Following, four weekly treatments of CTX+SNDX-ms6453 we observed a significantly decreased primary tumor volume as compared to IgG, CTX and SNDX-ms6352 alone. Mice that fully responded to the treatment were re-challenged with tumor cells in the contralateral mammary gland to determine if there was a long-term immune memory response. We observed a fifty percent tumor re-challenge rejection in both T12 and 2151R. Increased expression of CD20 B and CD8 T cells was observed within the mammary gland of the complete responders, this was accompanied by macrophage depletion in the SNDX-ms6352 and combination treated mice. SNDX-ms6352 induced macrophage apoptosis as evidenced by the activation of cleaved caspase 3. For lung metastases TV injected mice were administered BrdU to identify micro-metastatic lesions. Following four weekly combination treatments of CTX+/-SNDX-ms6352 we observed a decreased lung metastatic burden and increased overall survival as compared to IgG, CTX and SNDX-ms6352 alone. Interestingly we also observed increased CD8 T-cell infiltration, but only in the combination treated mice following 28 and 56 days post treatment. Conclusion: These results suggest that targeting macrophages enhances the immunostimulatory effect of low dose cyclophosphamide in treating not only primary tumors, but also established lung metastasis via the activation of the tumor immune microenvironment. Supported by CA148761 and T32 CA203690 grants. Citation Format: Diego Pedroza, Weiguo Wu, Paul Porter, Xiang Zhang, Jeffrey Rosen. CSF-1R antibody targeting therapy with combined metronomic chemotherapy enhances a B and T cell response for the treatment of metastatic triple negative breast cancer. [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr PD2-05.