Abstract 6343: Frameshift neoantigen-based vaccine testing in an intra-cecal implantation mouse model of Lynch syndrome

Lynch syndrome (LS), also known as hereditary non-polyposis colorectal cancer (HNPCC), is caused by monoallelic germline mutations in mismatch repair (MMR) genes (e.g., MLH1, MSH2, MSH6, and PMS2), which predispose individuals to an increased risk of developing various cancers (mainly colorectal cancer and endometrial cancer). The hallmark of LS-associated tumors is high microsatellite instability (MSI-H) characterized by increased mutations at coding mononucleotide repeats (cMNR), which usually lead to frameshift mutations (FSMs) in coding exons. These recurrent FSMs are thought to play a central role in the increased cancer risk. Vaccines targeting neoantigens produced by FSMs can elicit immune responses, which may prevent or suppress LS tumor formation and progression. Indeed, frameshift neoantigen-based vaccines have been developed and are being tested in LS or MSI-H patients in clinical trials (e.g., NOUS-209). To accelerate vaccine development and optimization, we have developed and characterized an intra-cecal tumor implantation model using tumor fragments derived from a LS mouse model (Msh2LoxP/LoxP;Villin-Cre; VCMsh2). The model was examined for its suitability for preclinical vaccine evaluation in prevention (vaccination followed by fragment implantation) and treatment (fragment implantation followed by vaccination) settings. Primary cecal tumors were adenocarcinoma mixed with mucinous feature. In contrast to subcutaneous tumor implant models that never develop metastasis, metastatic lesions were detected in the lung, liver, and lymph nodes within two months post intra-cecal tumor implantation. In both settings, vaccinated animals had intra-cecal tumors that were smaller in size and a lower rate of metastasis and lower number of metastatic nodules as compared to unvaccinated controls (2 vs. 13 nodules in preventive setting and 1 vs. 4 nodules in therapeutic setting, respectively). FSM analysis by targeted next generation sequencing of a panel of 50 genes revealed that some loci had lower mutation rates and variant allele frequencies, but not at all vaccine targets, in tumors from vaccinated groups compared to controls, indicating that tumor cells harboring certain FSMs targeted by the vaccine were not eliminated. Vaccine target composition and formulation need to be further optimized. Interestingly, plasma cell free DNA (cfDNA) showed lower FSM rates in vaccinated groups, which correlated with a lower metastatic rate. This is consistent with data from VCMsh2 model vaccinated with frameshift peptide vaccines. Additional studies are needed to confirm whether this is neoantigen vaccine specific. In summary, this intra-cecal implantation LS model appears suitable for preclinical neoantigen-based vaccine testing and may help elucidate aspects of tumor cell/DNA shedding and distant metastasis. Citation Format: Yurong Song, Brandon Somerville, Shaneen S. Baxter, Jason D. Marshall, Lei Wei, Chelsea Sanders, Qiang Hu, Song Liu, Alan Hutson, Baktiar Karim, Simone Difilippantonio, Ligia Pinto, Shizuko Sei, Robert H. Shoemaker. Frameshift neoantigen-based vaccine testing in an intra-cecal implantation mouse model of Lynch syndrome [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 6343.