Abstract 2157: Rational combination of chemotherapeutic agents with immune checkpoint inhibitors in orthotopic metastatic breast cancer models

Abstract Breast cancer is broadly diagnosed clinically and deadly among women. Due to heterogeneous nature of cancer and resistance, standalone treatments often fail. Consequently, combination therapy is essential as it targets multiple pathways and enhances efficacy and reduce resistance. Clinically relevant orthotopic mammary fat pads (MFP) implanted models are used for evaluating combination therapies. In the present study, we have evaluated efficacy of anti-PD1, anti-CTLA-4 antibody and Doxorubicin (DOX) as standalone and combination regimen in orthotopic breast cancer models [4T1, EMT6]. 0.5x10^6 cells were implanted orthotopically into fourth mammary fat pad. Mice were randomized based on tumor volume (∼100mm3) and treatment was initiated with anti-PD1 and anti-CTLA-4 at 10mg/kg, i.p; Q4D x 4 doses and DOX 4mg/kg, i.v; Q4Dx 4 doses as standalone or in combination. Tumor volume, changes in body weight, clinical signs, mortality, distant organ metastasis were monitored upto 4 weeks. At the end, lungs were harvested weighed, perfused with Indian ink, and fixed in 10% neutral buffered formalin for histopathology evaluation. In 4T1 tumor model, treatment with standalone anti-PD1, anti-CTLA-4, and DOX resulted in non-significant tumor growth inhibition (TGI). Whereas combination of anti-PD1 and anti-CTLA-4 with Doxorubicin demonstrated moderate TGI and marginally reduced distant organ metastasis compared to the standalone regimen. In EMT6 tumor model, standalone IO agents anti-PD1, anti-CTLA-4, and Doxorubicin treatment exhibited moderate TGI. However, combination of anti-PD1 and anti-CTLA-4 with DOX achieved synergistic significant tumor growth inhibition which was evident from absence of distant organ metastasis and resulted in prolonged survival. Overall, outcome of this study suggests combination therapy with conventional chemoagents, and immune checkpoint blockade can prevent progression to metastatic disease and may limit to overcome resistance. Such combination regimen represents a multimodal translational approach to enhance chemotherapeutic efficacy for the treatment of metastatic breast cancer. Citation Format: Balaji Ramachandran, Satheeshkumar Rajendiran, Rajendran Venkidusamy, Kowshik Karmakar, Raghu Patil, Krishnappa Haladasappa. Rational combination of chemotherapeutic agents with immune checkpoint inhibitors in orthotopic metastatic breast cancer models [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 2157.