Abstract 6155: The combination of cisplatin and temozolomide triggers immune surveillance through antigen-dependent and independent mechanisms

The combination of chemotherapy and immune checkpoint inhibitors (ICIs) has shown dramatic activity in a variety of malignancies. However, the addition of ICIs to standard of care (SOC) chemotherapy is not clinically effective in mismatch repair proficient (MMRp) colorectal cancer (CRC). We have previously reported that priming treatment with temozolomide (TMZ) induces hypermutability and immunotherapy clinical benefit in the molecular subset of CRCs characterized by inactivation of O6-methylguanine methyltransferase (MGMT). We reasoned that the combination of TMZ with cisplatin (CDDP), another active alkylating agent, might similarly increase mutational burden and induce cancer immunogenicity in the larger group of MMRp / MGMT wild-type tumors, which account for more than 80% of CRC cases. We investigated the impact of cytotoxic agents on the immunogenomic features of CRC, focusing on TMZ, CDDP, and their combination, in comparison to the SOC treatments (5-fluorouracil, irinotecan and oxaliplatin). Whole exome sequencing (WES) and RNA sequencing were performed to identify mutational signatures and for neoantigen prediction. We assessed treatment-induced antigenicity and immunoediting by injecting CRC cells that had been treated in vitro with chemotherapy into syngeneic immunodeficient and -competent mice. In parallel, we evaluated the activity of chemotherapy and immunotherapy performed in vivo in order to capture the effect of chemotherapy in the tumor immune microenvironment. Immune infiltration was assessed using a dedicated flow cytometry panel. We found that combinatorial treatment with CDDP and TMZ induces clonal and subclonal hypermutability resulting in an increase in predicted neoantigens. This combination specifically alters the immune fitness of the tumors and ultimately leads to immunoediting of chemotherapy-induced neoantigens and CD8+ T cell-mediated tumor rejection. Treatment-induced hypermutation and immune surveillance were also confirmed in an immune-refractory breast cancer model. Complete rejection of CDDP- and TMZ-treated tumors is associated with persistent immune surveillance that extends to the untreated counterpart. The same effect is not observed when cells are primed with the clinically approved combination of 5-flourouracil, oxaliplatin and irinotecan (FOLFOXIRI). In vivo chemotherapy potentiates immune effectors, allowing for durable responses and complete rejections when anti-PD1 is combined with CDDP and TMZ. These results suggest that rational combinations of alkylating agents can promote immune surveillance by enhancing immunologically relevant hypermutability and antigenicity and by remodeling the immune microenvironment. This can be exploited to promote cancer immunogenicity in otherwise immune-refractory CRC. Citation Format: Pietro Paolo Vitiello, Rosaria Chila, Vito Amodio, Paolo Battuello, Vittorio Battaglieri, Sharon Scardellato, Gaia Grasso, Federica Pisati, Claudio Tripodo, Achille Anselmo, Francesca Clemente, Federica Di Nicolantonio, Giovanni Crisafulli, Benoit Rousseau, Luis A. Diaz, Giovanni Germano, Alberto Bardelli. The combination of cisplatin and temozolomide triggers immune surveillance through antigen-dependent and independent mechanisms [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 6155.