Abstract 700: Impact of heterogeneity for mismatch repair activity on colon tumor development and therapeutic response

Abstract Background: Colorectal cancers (CRC) that are deficient for DNA mismatch repair (dMMR) proteins have high levels of genetic instability and consequently high mutational burden, creating numerous neoantigens that can elicit an immune response. Therefore, dMMR CRC are the best candidates for immune checkpoint inhibition (ICI) compared to proficient DNA mismatch repair (pMMR) CRC. Early clinical trials revealed that only 30-55% metastatic dMMR CRC responded to ICI. Recent studies utilizing allograft models have demonstrated that tumor immune landscape can be primed with radiopharmaceutical therapy (RPT). Although early results have been quite promising, allograft models often fail to accurately predict efficacy for many reasons, but most often are due to a lack of heterogeneity. Methods: To better understand how heterogenous MMR expression impacts tumor development and response to ICI alone or in combination with RPT, we developed a new mouse model that develops three tumor types: (1) homotypic dMMR tumors that express green fluorescent protein, (2) homotypic pMMR tumors that express red fluorescent protein, and (3) heterotypic tumors with a mixture of dMMR and pMMR cells. Surveillance bright field and fluorescent colonoscopies allow us to assess treatment response by measuring tumor size and proportion of dMMR and pMMR tumor cells in real-time. These outcomes are verified through ex vivo imaging and histological analysis. Results: Tumor response to anti-PD-L1 ICI was dependent on MMR status. Homotypic dMMR tumors exhibited the strongest response: significantly smaller in treated mice than controls, exhibited areas of vascular congestion and increased CD8+ cytotoxic T-cells infiltration. Response of heterotypic MMR tumors varied depending on percentage of dMMR cells; tumors with a high percentage of dMMR cells remained small, whereas tumors with a low percentage of dMMR had an outgrowth of pMMR cells. Treated and untreated homotypic pMMR tumors were indistinguishable. To enhance the response of all three tumor types, mice were treated with dual ICI treatment, anti-PD-L1 and anti-CTLA-4, or RPT. Animals administered the tri-therapy (dual ICI + RPT) survived significantly longer with fewer tumors than animals treated with either dual ICI or RPT alone. Conclusions: Our innovative mouse model allows for the highly detailed characterization of tumor response to therapies. Simplistic allograft models may not adequately account for the dynamic effects that neighboring cell death may have on radio-resistant cells, on tumor vascular perfusion and oxygenation, or on tumor immune infiltration and adaptive immune recognition of remaining cells. We begin to fill these critical gaps in knowledge and therefore the results from our studies will help guide the translation and integrated use of ICIs and RPTs in clinic. Citation Format: Santina M. Snow, Dawn Albrecht, Paul A. Clark, Caroline P. Kerr, Joseph J. Grudzinski, Justin J. Jeffery, Hansel Comas Rojas, Reinier Hernandez, Kristina A. Matkowskyj, Jamey P. Weichert, Zachary S. Morris, Richard B. Halberg. Impact of heterogeneity for mismatch repair activity on colon tumor development and therapeutic response abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts) ; 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84 (6Suppl): Abstract nr 700.