- Pexa-Vec will commence on Day (minus) 12 to facilitate tumor biopsies following oncolytic virus alone. All patients will undergo a baseline tumor biopsy and a post treatment biopsy. The timing of the post-treatment biopsy will depend on randomization. The randomization will be balanced with an equal number randomized to each timing group. Patients will be randomized to receive the second biopsy on Day 1 (i.e. after one dose of Pexa-Vec alone) or Day 29 (i.e. after the combination of immune checkpoint inhibition and Pexa-Vec). (Specific dates of biopsy will have a window of 72 hrs to allow for logistical issues; however, the biopsy will not be performed within 48 hrs after Pexa- Vec administration).