Abstract 4111: Multi-peptide cancer vaccines targeting KRAS induce significant anti-tumor efficacy
Authors: Li Chi Han Samson , Toh Melvin 
Published: March 2024

Abstract
Background and objective: Kirsten rat sarcoma viral oncogene homologue (KRAS) is frequently mutated in human cancers. KRAS mutants offer the potential for the development of cancer-specific immunotherapy, as epitopes harbouring mutations are strong neoantigens to which the immune system generates cancer-specific anti-tumor effects. However, previous clinical trials of KRAS-targeted vaccines failed to generate satisfactory anti-cancer efficacy. Evidence suggested that CD4 T cell activity plays a critical role in augmenting the cytotoxic effect of CD8 T cells. Here, we design long peptides based on the MHC II hotspots on the entire KRAS protein that activate both CD4 and CD8 T cells to kill KRAS-driven cancer cells. Long peptides also cover a wider range of HLA alleles to increase their utility in a broader spectrum of patients. Our objective is to create a multi-peptide vaccine simultaneously targeting the mutant and wild-type (WT) regions of human KRAS protein.
Methods: By selecting MHC II hotspots via multiple epitope prediction algorithms, we designed and synthesized 8 long peptides with lengths between 25 to 30mers. One of them harboured a G12D mutation, while the others covered the WT regions of KRAS. In addition, the corresponding keyhole limpet hemocyanin (KLH) conjugated peptides were also generated. We first demonstrated their immunogenicity in mouse in vivo models. Different combinations of peptides adjuvanted with either CpG oligodeoxynucleotide + aluminum (CpG+alum) or complete/incomplete Freud’s adjuvant (CFA/IFA) were immunized into Balb/c mice before determining the immune response by mouse interferon γ (mIFNγ) ELISPOT assay. Subsequently, an in vivo prophylactic anti-tumor study was conducted by immunizing this multi-peptide vaccine in Balb/c mice and observing the growth inhibition of the CT26-derived tumors.
Results: T cell responses were observed in two peptide mixes with or without G12D mutation, suggesting that both mutant-harbouring peptide and WT peptides were immunogenic. We showed that KLH-conjugated peptides and CpG+alum elicited stronger immune responses than the naked peptides and CFA/IFA respectively. Furthermore, peptide mapping showed that the G12D mutant-harbouring peptide and two other WT peptides may contribute to the overall T cell response in the Balb/c mouse. In the prophylactic anti-tumor study (n = 10), immunization of KRAS naked peptide and KLH-conjugated peptide vaccines significantly inhibited tumor growth by 51.2% (****p &lt; 0.0001) and 44.7% (***p &lt; 0.001) respectively, compared to control. Subsequent mIFNγ ELISPOT assay showed that the tumor inhibitory effect was attributed to the activation of antigen-specific T cell response of the vaccines.
Conclusion: Our multi-peptide vaccines targeting both the mutant and WT regions of KRAS could generate KRAS-specific T cell responses and elicit significant anti-tumor effect in mouse models.
Citation Format: Chi Han Samson Li, Melvin Toh. Multi-peptide cancer vaccines targeting KRAS induce significant anti-tumor efficacy [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(6_Suppl):Abstract nr 4111.