 This is a single center, phase I pilot study of sequential ONCOS-102 and pembrolizumab in patients with advanced or unresectable melanoma progressing after PD1 blockade. The primary objective of the study is to determine the safety of sequential treatment with ONCOS-102 followed by pembrolizumab. The protocol aims to enroll 6 patients into each of two cohorts: prior PD1 monotherapy or prior combination PD1 plus ipilimumab combination therapy. 