Rad51 Degradation: Role in Oncolytic Virus—Poly(ADP-Ribose) Polymerase Inhibitor Combination Therapy in Glioblastoma

Background: Clinical success of poly(ADP-ribose) polymerase inhibitors (PARPi) has been limited to repair-deficient cancers and by resistance. Oncolytic herpes simplex viruses (oHSVs) selectively kill cancer cells, irrespective of mutation, and manipulate DNA damage responses (DDR). Here, we explore potential synthetic lethal-like interactions between oHSV and PARPi. Methods: The efficacy of combining PARPi, oHSV MG18L, and G47Δ in killing patient-derived glioblastoma stem cells (GSCs) was assessed using cell viability assays and Chou-Talalay synergy analysis. Effects on DDR pathways, apoptosis, and cell cycle after manipulation with pharmacological inhibitors and lentivirus-mediated knockdown or overexpression were examined by immunoblotting and FACS. In vivo efficacy was evaluated in two GSC-derived orthotopic xenograft models (n = 7–8 per group). All statistical tests were two-sided. Results: GSCs are differentially sensitive to PARPi despite uniform inhibition of PARP activity. oHSV sensitized GSCs to PARPi, irrespective of their PARPi sensitivity through selective proteasomal degradation of key DDR proteins; Rad51, mediating the combination effects; and Chk1. Rad51 degradation required HSV DNA replication. This synthetic lethal-like interaction increased DNA damage, apoptosis, and cell death in vitro and in vivo. Combined treatment of mice bearing PARPi-sensitive or -resistant GSC-derived brain tumors greatly extended median survival compared to either agent alone (vs olaparib: P≤.001; vs MG18L: P = .005; median survival for sensitive of 83 [95% CI = 77 to 86], 94 [95% CI = 75 to 107], 102 [95% CI = 85 to 110], and 131 [95% CI = 108 to 170] days and for resistant of 54 [95% CI = 52 to 58], 56 [95% CI = 52 to 61], 62 [95% CI = 56 to 72], and 75 [95% CI = 64 to 90] days for mock, PARPi, oHSV, and combination, respectively). Conclusions: The unique oHSV property to target multiple components of DDR generates cancer selective sensitivity to PARPi. This combination of oHSV with PARPi is a new anticancer strategy that overcomes the clinical barriers of PARPi resistance and DNA repair proficiency and is applicable not only to glioblastoma, an invariably lethal tumor, but also to other tumor types.