- Patients with advanced microsatellite-stable colorectal cancer (or MSI-hi disease that is refractory to PD-1 monotherapy) will receive Pexa-Vec, administered IV at a dose of either 3 x 10(8) plaque forming units (pfu) (DL1) or 10(9) pfu (DL2) every 2 weeks for 4 doses, in 2 separate cohorts A and B. The first administration will be on Day (minus) 12, followed by administration on Days 2, 16 and 30 (i.e. 4 doses in total).