Utilizing Endogenous MicroRNA Expression for Oncolytic Virotherapy

Recombinant vaccinia virus (rVACV) GLV-1h68 is a promising candidate for oncolytic virotherapy and in current evaluation for human application in clinical trials. Human microRNA let-7a (hsa-miR-let-7a) is strongly expressed in normal tissues and downregulated in most cancers. To increase the safety of rVACVs using endogenous hsa-miR-let-7a, a 4 x hsa-let-7a complementary sequence repeat was linked to the rVACV vital D4R gene using a GLV-1h68 derivative, GLV-1h190, as parental strain yielding GLV-1h250. After recognition and binding of the target structure on the viral mRNA transcript, hsa-miR-let-7a leads to viral mRNA degradation and replication inhibition in normal cells. Oncolytic activity of GLV-1h250 in vitro was studied by infection of A549 lung adenocarcinoma cells, which show a strong downregulation of hsa-miR-let-7a. GLV-1h250 displayed no significant inhibition of replication to GLV-1h190 and GLV-1h68 leading to the conclusion that the oncolytic activity of GLV-1h250 in cancer cells is not impaired. When infecting normal human Endometrial Regenerative Cells, GLV-1h250 displayed a 10-fold reduction in viral replication rate in relation to GLV-1h190 and GLV-1h68 suggesting successful inhibition of viral replication in normal cells. Furthermore, A549 tumor bearing male nude mice were injected with 1 x 107 pfu / mouse to investigate possible toxic effects in vivo. Other than GLV-1h190 and GLV-1h68, replication of GLV-1h250 took place exclusively in the tumor while showing comparable tumor regression. GLV-1h250 displayed solid oncolytic potential and significantly decreased toxicity with a reduced replication in normal cells and tissues as demonstrated in vitro and in vivo in comparison to GLV-1h190 and GLV-1h68.