Abstract LB197: A pooled mutant KRAS peptide vaccine activates polyfunctional T cell responses in patients with resected pancreatic cancer

Abstract Background: Oncogenic mutations in KRAS are expressed in up to 90% of pancreatic ductal adenocarcinomas (PDAC). Vaccination against mutant KRAS (mKRAS) is thus a promising approach as an off-the-shelf immunotherapeutic treatment for PDAC. We developed a mKRAS peptide vaccine targeting 6 common KRAS mutations (G12V, G12A, G12C, G12R, G12D, or G13D (NCT04117087). We evaluated the mKRAS specific T cell response induced by vaccination in combination with immune checkpoint inhibitors (ICIs) in patients with resected PDAC. Materials and Methods: This is an ongoing pilot study of a pooled mKRAS long peptide vaccine in combination with ICIs in patients with resected PDAC and mutations in one of 6 KRAS mutations in our vaccine. Patients with no evidence of disease on imaging within 6 months of completion of adjuvant chemotherapy were vaccinated with the mKRAS vaccine (0.3mg/peptide and 0.5mg poly-ICLC (Hiltonol: Oncovir)) weekly for 4 doses followed by booster vaccines every 8 weeks. Patients also received ipilimumab (1mg/kg, every 6 weeks for 2 doses) and nivolumab (3mg/kg, every 3 weeks in the priming phase) followed by nivolumab (480mg, flat dose in boost phase). To evaluate the expansion of mKRAS-specific T cells in the periphery, pre- and post-vaccination peripheral blood mononuclear cells (PBMCs) were restimulated with control or individual mKRAS peptides and assessed for IFNγ release by ELISPOT. To further phenotype the responding CD4 and CD8 T cell compartments, peptide-restimulated PBMCs were evaluated for T cell activation, proliferation, memory and exhaustion marker expression by CyTOF. Paired single-cell RNA and TCR sequencing are being performed to isolate mKRAS-specific TCRs and their corresponding transcriptional profiles. These TCRs are being functionally validated in vitro using CRISPR-Cas12a-based genome knock-in of human T cells. Results: At the time of data cut off (1/12/2023) 8/11 patients enrolled had a positive mKRAS-specific T cell response in post-vaccine sample timepoints defined by a >5 fold increase in IFNγ producing T cells after peptide restimulation. CyTOF analysis of peptide-restimulated PBMCs demonstrated expansion of polyfunctional (IFNγ+IL2+TNFα+) mKRAS-specific CD4 and CD8 T cells with both central and effector memory phenotypes after vaccination. mKRAS-specific CD4 T cells were induced at a greater proportion. Our single-cell analysis has identified and validated a novel CD4+ TCR that recognizes KRAS G12V in the context of HLA-DRB1*07:01. Conclusions: This study thus far indicates the induction of de novo, high quality mKRAS-specific T cells in the periphery post-vaccine. Ongoing studies will define TCR diversity and clonality of mKRAS-specific T cells to each mKRAS epitope. Overall, our data will be used to identify peripheral T cell-based biomarkers that may be able to predict response to mKRAS-targeted immunotherapy. Citation Format: Amanda L. Huff, Saurav D. Haldar, Emily Davis-Marcisak, Thatcher Heumann, Gabriella Longway, Alexei Hernandez, Maximillian F. Konig, Brian Mog, Ludmila Danilova, Luciane Kagohara, Julie M. Nauroth, Amy M. Thomas, Elana J. Fertig, Won Jin Ho, Elizabeth M. Jaffee, Nilo Azad, Neeha Zaidi. A pooled mutant KRAS peptide vaccine activates polyfunctional T cell responses in patients with resected pancreatic cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 2 (Clinical Trials and Late-Breaking Research); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(8_Suppl):Abstract nr LB197.