Engineering and Preclinical Evaluation of Western Reserve Oncolytic Vaccinia Virus Expressing A167Y Mutant Herpes Simplex Virus Thymidine Kinase

Viral replication of thymidine kinase deleted (tk^-) vaccinia virus (VV) is attenuated in resting normal cells, enabling cancer selectivity, however, replication potency of VV-tk^- appears to be diminished in cancer cells. Previously, we found that wild-type herpes simplex virus (HSV)-tk (HSV-tk) disappeared in most of the recombinant VV after multiple screenings, and only a few recombinant VV containing naturally mutated HSV-tk remained stable. In this study, VV-tk of western reserve (WR) VV was replaced by A167Y mutated HSV-tk (HSV-tk_418m), to alter nucleoside selectivity from broad spectrum to purine exclusive selectivity. WOTS-418 remained stable after numerous passages. WOTS-418 replication was significantly attenuated in normal cells, but cytotoxicity was almost similar to that of wild type WR VV in cancer cells. WOTS-418 showed no lethality following a 5 × 10⁸ PFU intranasal injection, contrasting WR VV, which showed 100% lethality at 1 × 10⁵ PFU. Additionally, ganciclovir (GCV) but not BvdU inhibited WOTS-418 replication, confirming specificity to purine nucleoside analogs. The potency of WOTS-418 replication inhibition by GCV was > 10-fold higher than that of our previous truncated HSV-tk recombinant OTS-412. Overall, WOTS-418 demonstrated robust oncolytic efficacy and pharmacological safety which may delegate it as a candidate for future clinical use in OV therapy.