Synchronously implanted cold tumor abrogates anti-PD-1 response of hot tumor in a syngeneic mouse model

Background and Objective: Synchronous cancer describes a new or second primary cancer that develops at the same time as the first primary cancer, either in the same organ or in a different organ with the same molecular basis. Synchronous cancer is not considered metastatic relapse. The prevalence of synchronous cancer varies depending on the cancer type, but overall, it is a rare event occurring in 4.5% to 11.7% of patients with multiple primary malignancies. The impact of a second primary tumor on the progression and treatment response of the first tumor remains poorly understood. This study investigates the interaction between two distinct primary tumors- MC38 colorectal cancer and LL/2 lung cancer- in a synchronous syngeneic mouse model. Specifically, we evaluated how the presence of LL/2 tumor influences the efficacy of anti-PD-1 immunotherapy against MC38 tumor. Changes in immune cell population and cytokine profiles in circulation and within the MC38 tumor microenvironment were also examined. These findings shed light on how inter-tumoral communication affects therapeutic efficacy in the presence of synchronous malignancies. Method and Result: C57BL/6 mice were inoculated subcutaneously with 5 x 10 5 MC38 cells on the right flank. Mice in group 1 and 3 received an additional MC38 inoculation on the left flank, while group 2 and 4 received 5 x 10 5 LL/2 cells at the left flank. When MC38 tumor reached approximately 100 mm 3 , mice were treated with 10 mg/kg of anti-PD-1 antibody intraperitoneally every 4 days. Tumor growth, body weights, immune cell populations, and cytokine levels in blood and tumor tissues were assessed after 10 days of treatment. The body weights of the mice remained no significant differences between groups. Interestingly, although the tumor volumes in group 2 were only slightly higher than in group 2, the tumor volumes in group 4 were significantly higher than in group 3, indicating the presence of LL/2 tumor greatly impaired the efficacy of anti-PD-1 immunotherapy against MC38 tumor. FACS analysis and cytokine analysis confirmed the changes of immune profiling that led to the insensitive of MC38 tumor to the immunotherapy. Conclusion: The presence of a second primary cancer can profoundly affect the immune checkpoint inhibitor therapeutic response of the first primary tumor through inter-tumoral immune modulation. These findings emphasize the importance of evaluating tumor-tumor communication when designing treatment strategies for patients with synchronous cancer. WuXi AppTec Biology-IVPU offers bilateral tumors models and comprehensive in vivo capability for preclinical oncology drug efficacy studies.