The effect of the antithyroid drug methimazole (MMI) on cytochrome P450/P450 reductase-dependent activation of the anti-cancer prodrug cyclophosphamide (CPA) was investigated in a rat model of P450 prodrug activation-based cancer gene therapy. MMI treatment decreased the expression of hepatic P450 reductase by approximately 75% but did not alter P450 reductase levels in a 9L gliosarcoma growing in vivo as a subcutaneous solid tumor. In a pharmacokinetic study, MMI treatment significantly decreased the peak plasma concentration of the active, P450-generated metabolite 4-hydroxy-CPA, from 84.1 to 57.8 microM, and substantially prolonged its apparent half-life, from 25.4 to 54.3 minutes. The area under the plasma concentration x time curve and clearance values for 4-hydroxy-CPA were largely unchanged, however, indicating that MMI decreases the rate but not the overall extent of hepatic CPA activation. MMI alleviated some of the systemic toxicities of CPA treatment, as judged by the moderation of CPA-induced body weight loss and hematuria. The impact of MMI on CPA antitumoral activity was evaluated in rats implanted with 9L tumors transduced with P450 reductase in combination with the CPA-activating P450 2B1, which confers the capacity for intratumoral prodrug activation and leads to markedly enhanced chemosensitivity. CPA given as a single, subtherapeutic dose of 75 mg/kg resulted in a 13.8 day growth delay, whereas CPA in combination with MMI increased the growth delay to 17.4 days. By contrast, a tumor growth delay of only 3.4 days was observed in animals bearing 9L wild-type tumors given the same drug combination. We conclude that the selective reduction of liver P450 reductase after MMI treatment decreases the rate of hepatic drug activation and the host toxicity of CPA without loss of the antitumoral effect, thus increasing the therapeutic index of CPA in a P450-based cancer gene therapy model, where CPA undergoes localized drug activation at its intratumoral site of action.