A Phase I/II Study of Pexa-Vec Oncolytic Virus in Combination With Immune Checkpoint Inhibition in Refractory Colorectal Cancer

Background: * Immune-based approaches in colorectal cancer have unfortunately with the notable exception of immune checkpoint inhibition in microsatellite instable (MSI-hi) disease been largely unsuccessful. The reasons for this are unclear but no doubt relate to the fact that in advanced disease colorectal cancer appears to be less immunogenic, as evidenced by the lack of infiltrating lymphocytes with advancing T stage * Pexa-Vec (JX-594) is a thymidine kinase gene-inactivated oncolytic vaccinia virus engineered for the expression of transgenes encoding human granulocyte- macrophage colony-stimulating factor (GM-CSF) and beta-galactosidase. Apart from the direct oncolytic activity, oncolytic viruses such as Pexa-Vec have been shown to mediate tumor cell death via the induction of innate and adaptive immune responses * Tremelimumab is a fully human monoclonal antibody that binds to cytotoxic T-lymphocyte-associated protein 4 (CTLA-4) expressed on the surface of activated T lymphocytes and causes inhibition of B7-CTLA-4-mediated downregulation of T-cell activation. Durvalumab is a human monoclonal antibody directed against programmed death-ligand 1 (PD-L1). * The aim of the study is to evaluate whether the anti-tumor immunity induced by Pexa-Vec oncolytic viral therapy can be enhanced by immune checkpoint inhibition. Objective: -To determine the safety, tolerability and feasibility of Pexa-Vec oncolytic virus in combination with immune checkpoint inhibition in patients with refractory metastatic colorectal cancer. Eligibility: * Histologically confirmed metastatic colorectal cancer. * Patients must have progressed on, been intolerant of or refused prior oxaliplatin- and irinotecan-containing, fluorouracil-based, chemotherapeutic regimen and have disease that is not amenable to potentially curative resection. Patients who have a known Kirsten rat sarcoma viral oncogene homolog (KRAS) wild type tumor must have progressed, been intolerant of or refused cetuximab or