Mutant KRAS Peptide Vaccine with Dual Checkpoint Blockade in Metastatic Colorectal Cancer: a Phase I Trial

Immune checkpoint inhibitors (ICIs) have limited activity in mismatch repair proficient or microsatellite stable (MMRp/MSS) colorectal cancer (CRC). KRAS mutations, present in approximately 40% of these cancers, can generate neoantigens that are targets for therapeutic vaccines. In this single-arm, phase I study (NCT04117087), we evaluated mKRAS-VAX, a pooled mutant KRAS (mKRAS) peptide vaccine targeting six KRAS mutations with nivolumab and ipilimumab in 13 patients with pretreated metastatic MMRp/MSS CRC. Both primary endpoints of safety and immunogenicity (within 17 weeks post-vaccination) were met. Secondary endpoints included treatment efficacy defined by RECIST v1.1 criteria. All adverse events attributed to mKRAS-VAX were grade 1 or 2, and the addition of mKRAS-VAX did not increase the frequency of severe immune-related adverse events beyond the expected profile of dual ICIs alone. mKRAS-VAX elicited an increase in tumor-specific mKRAS-reactive T-cells in 8/12 biomarker-evaluable patients (75%) by direct ex vivo IFNγ ELISpot and in 12 patients (100%) following in vitro expansion. Our findings support further development of mKRAS vaccines with ICIs for advanced MMRp/MSS CRC.