Potentiation of ganciclovir toxicity in the herpes simplex virus thymidinekinase/ganciclovir administration system by ponicidin

The herpes simplex virus thymidine kinase (HSV-TK)/ganciclovir (CCV) admini stration system is commonly used in gene therapy trials. We have evaluated the effect of ponicidin, a diterpenoid isolated from a plant, Rabdosia tern ifolia, on the cell-killing activity of the anti-herpes drugs acyclovir (AC V) and CCV. Ponicidin preferentially activated HSV-1-specific TK but not ce llular kinases. In HSV-infected cells, ponicidin significantly accumulated the phosphorylated metabolites of CCV and suppressed the extracellular rele ase of CCV. These data suggested that the cytotoxicities of ACV and CCV in HSV-TK-expressing cells might be potentiated by ponicidin. After transfecte d with the HSV-1 TK gene, COS-1 and several human cancer cells became highl y sensitive to the cytotoxic properties of the nucleoside analogs. When pon icidin at the concentration without antiviral activities (0.2 mu g/mL) was combined with ACV or GCV, the cytotoxic levels in HSV-TK-expressing cells w ere enhanced by 3- to 87-fold and 5- to 52-fold, respectively, compared wit h the nucleoside alone. When the stability of the bioactivity of ponicidin in the blood of mice was evaluated, the substance showed relatively long-la sting effects on the potentiation of the anti-herpetic and cytotoxic activi ties of GCV after intravenous administration. These data suggest that the c ombined use of ponicidin with CCV will be effective for cancer gene therapy , because high cytotoxicity in viral TK-expressing cells should yield more rapid and enhanced tumor elimination.