Improved radiosensitization of rat glioma cells with adenovirus-expressed mutant herpes simplex virus-thymidine kinase in combination with acyclovir

Adenovirus expressing herpes simplex virus-thymidine kinase (HSV-TK) sensit izes internal rat glioma cells to radiation in combination with acyclovir ( ACV). However, relatively high concentrations of ACV (>10 mu M) are require d to obtain significant radiosensitization. Serum levels rarely reach more than the lower micromolar range, preventing the full use of this genetic ap proach to radiosensitize cells in vivo. To better use the lower concentrati ons of ACV available in sera, we constructed an adenovirus expressing a mut ant HSV-TK ((HSV-TK(75)) isolated for its similar to 20 times greater sensi tivity to ACV than wild-type (wt) HSV-TK. We demonstrate that rat RT2 gliom a cells infected with adenovirus AdCMV-TK(75) and exposed to either ACV or ganciclovir become more sensitive to lower concentrations (1-3 mu M) of the drugs compared with cells infected with AdCMV-TK(wt), which expresses wt H SV-TK. Most importantly, the RT2 cells become more sensitive to low doses ( 2-4 Gy) of Co-60 radiation than cells infected with an adenovirus expressin g wt HSV-TK. This sensitization is accompanied by an increased rate of apop tosis. In summary, we show that infection of rat glioma cells with an adeno virus expressing a mutant HSV-TK sensitizes the cells to low doses of radia tion after exposure to ACV at lower concentrations than those required for wt HSV-TK. This finding suggests that this mutant adenovirus may improve th e in vivo efficacy of HSV-TK-based cancer gene therapy approaches.