Optimization of an active mutant KRAS peptide vaccine in colorectal cancer

PROJECT SUMMARY. Colorectal cancer (CRC) is the second leading cause of cancer–related mortality in the U.S. with a dismal 5– year overall survival rate at <15% for those with metastases. Immune checkpoint inhibitors (ICIs) lack therapeutic efficacy in the majority (~95%) of advanced CRC tumors that are microsatellite stable (MSS). Mutations in oncogenic KRAS (~40% of CRC) can further lead to immune evasion in the tumor microenvironment (TME) through the upregulation of immunosuppressive cytokines and impaired antigen presentation through downregulation of HLA I––together resulting in reduced T cell recruitment into the TME. There is thus a critical need for novel treatment strategies to improve ICI responses for patients with mutant KRAS (mKRAS) MSS CRC. We have developed a vaccine that pools six synthetic long peptides (SLPs) targeting the most common KRAS mutations (G12D, G12R, G12V, G12A, G12C, and G13D) –– termed ‘mKRASvax’. We are currently conducting a pilot study testing mKRASvax in combination with anti-PD-1 (nivolumab) and anti-CTLA-4 (ipilimumab) antibodies in PDAC and MSS CRC. Enrolled patients either have: resected pancreatic ductal adenocarcinoma (PDAC; Cohort A) or chemorefractory (two plus lines of chemotherapy) metastatic MSS CRC (Cohort B) [NCT04117087]. We have documented safety and a robust de novo induction of mKRAS–specific CD4 and CD8 T cells. Importantly, interim analysis for our PDAC cohort (Cohort A) shows that high–magnitude T cell responses correlate with greater recurrence–free survival (RFS). In our heavily pre–treated CRC cohort (currently enrolling), we have demonstrated the induction of de novo mKRAS–specific T cells in all the vaccinated patients, with tumor regressions in the first 3 of 9 enrolled patients, including those with liver metastases. Based on this data, we propose to conduct a Phase II clinical trial to study clinical responses (Aim 1) and immunogenicity (Aim 2) induced by mKRASvax in a novel combination with anti-PD-1, balstilimab, and more potent Fc– engineered anti-CTLA-4 antibody, botensilimab, in the first-line, maintenance setting after patients with mKRAS- harboring tumors have received 4 to 6 months of chemotherapy for metastatic CRC. Finally, we hypothesize that the immunomodulatory effects of small molecule inhibitors can be leveraged for more durable anti–tumor immunity when combined with mKRASvax and ICIs. In the true spirit of bidirectional translation, Aim 3 will focus on studying the potential for combining mKRASvax/ICIs and KRAS signaling inhibitors in mice. We will first utilize in vitro human and murine mKRAS CRC cell lines and organoids to assess HLA induction with the RAS(ON) inhibitor RMC-7977, KRASG12D inhibitors MRTX1133 or RM-044, and the MEK inhibitor avutometinib. We will use a KRASG12D–expressing CT26 murine cell hemispleen model to test the effects on tumor size and mouse survival of various combinations of, and therapeutic sequences for, mKRASvax, ICI and KRAS signaling inhibitors. We will also perform imaging mass cytometry (IMC) to study the immune architecture of the TME. Overall, these studies will lead to novel therapeutic mKRAS-based immunotherapies for metastatic CRC.