Polycytidine tract deletion from microRNA-detargeted oncolytic Mengovirus optimizes the therapeutic index in a murine multiple myeloma model

Mengovirus is an oncolytic picornavirus whose broad host range allows for testing in immunocompetent cancer models. Two pathogenicity-ablating approaches, polycytidine (polyC) tract truncation and microRNA (miRNA) targets insertion, eliminated the risk of encephalomyocarditis. To investigate whether a polyC truncated, miRNA-detargeted oncolytic Mengovirus might be boosted, we partially or fully rebuilt the polyC tract into the 5′ noncoding region (NCR) of polyC-deleted (MC₀) oncolytic constructs (NC) carrying miRNA target (miRT) insertions to eliminate cardiac/muscular (miR-133b and miR-208a) and neuronal (miR-124) tropisms. PolyC-reconstituted viruses (MC₂₄-NC and MC₃₇-NC) replicated in vitro and showed the expected tropism restrictions, but reduced cytotoxicity and miRT deletions were frequently observed. In the MPC-11 immune competent mouse plasmacytoma model, both intratumoral and systemic administration of MC₀-NC led to faster tumor responses than MC₂₄-NC or MC₃₇-NC, with combined durable complete response rates of 75%, 0.5%, and 30%, respectively. Secondary viremia was higher following MC₀-NC versus MC₂₄-NC or MC₃₇-NC therapy. Sequence analysis of virus progeny from treated mice revealed a high prevalence of miRT sequences loss among MC₂₄- and MC₃₇- viral genomes, but not in MC₀-NC. Overall, MC₀-NC was capable of stably retaining miRT sites and provided a more effective treatment and is therefore our lead Mengovirus candidate for clinical translation.