In Situ Transfection of Interleukin 12 Plasmid Enhances Anti-PD-1 Immune Response in Patients with Immunologically Quiescent Melanoma

Non-response to anti-PD-1 antibodies is characterized by a “cold” tumor microenvironment lacking “exhausted” CD8+ T cells. We report on the first prospective Phase 2 trial of intratumoral plasmid IL-12 and pembrolizumab in advanced melanoma with “exhausted” T-cell (peCTL) poor tumors that had a low pre-test probability of response to anti-PD-1 Ab monotherapy. Patients had a 43% objective response rate (n=23, RECIST 1.1) with 38% complete responses. Interrogation of samples demonstrated that, while intratumoral IL-12 triggered a rapid increase in immune signaling and licensing of the tumor microenvironment, the compensatory adaptive resistance blunted clinical responses. However, by disrupting the PD-1/PD-L1 axis with pembrolizumab, the IL-12/IFN-γ feed-forward cycle was sustained, driving intratumoral cross-presenting DC subsets with increased TIL and TCR clonality and, ultimately, systemic cellular immune responses. Thus, combination of intratumoral plasmid IL-12 with pembrolizumab transformed poorly immunogenic lesions into effective in situ vaccines, attaining clinical activity in low peCTL tumors.