N,N',N''-triethylenethiophosphoramide (thio-TEPA) oxygenation by constitutive hepatic P450 enzymes and modulation of drug metabolism and clearance in vivo by P450-inducing agents.

The cancer chemotherapeutic drug N,N',N''-triethylenephosphoramide (thio-TEPA) is oxidatively desulfurated to yield the active metabolite N,N',N''-triethylenephosphoramide (TEPA) in a reaction catalyzed by the phenobarbital-inducible rat liver P450 enzyme IIB1. In the current study, the role of constitutively expressed P450 enzymes in thio-TEPA metabolism was studied using purified P450s, isolated liver microsomes, and intact rats. Metabolism of thio-TEPA (100 microM) to TEPA by uninduced adult female and male rat liver microsomes proceeded at initial rates of 0.10 and 0.28 nmol TEPA formed/min/mg microsomal protein, respectively. Although these rates are low compared to those catalyzed by phenobarbital-induced liver microsomes (3.5 nmol TEPA/min/mg), they are sufficient to contribute to the systemic metabolism of this drug. Thio-TEPA metabolism catalyzed by uninduced female liver microsomes was approximately 70% inhibitable by antibodies selectively reactive with P450 IIC6. For the uninduced male liver microsomes, which exhibit a severalfold higher rate of thio-TEPA metabolism, enzyme activity was only 15-20% inhibitable by these antibodies but was 80-85% inhibited by an anti-P450 IIC6 monoclonal antibody cross-reactive with P450 IIC11, which is expressed only in the males. Consistent with these observations, purified P450s IIC11 and IIC6 both oxidized thio-TEPA in reconstituted systems (turnover, 1.1 and 0.3 min-1 P450-1, respectively, at 100 microM substrate), while several other constitutive hepatic P450s exhibited significantly lower or undetectable activities (turnover, less than or equal to 0.15 min-1 P450-1). Metabolism of thio-TEPA by purified P450 IIC11 was associated with a time-dependent inactivation of the cytochrome analogous to that previously shown to accompany thio-TEPA metabolism catalyzed by P450 IIB1. Depletion of hepatic P450 IIC11 by cisplatin treatment of adult male rats led to a 70% reduction of TEPA formation catalyzed by the isolated liver microsomes, suggesting that cisplatin may influence thio-TEPA pharmacokinetics when these two drugs are given in combination. The extent to which hepatic P450s contribute to thio-TEPA metabolism and clearance in vivo was assessed by monitoring thio-TEPA and TEPA pharmacokinetics in rats that exhibit widely differing rates of microsomal thio-TEPA metabolism, i.e., uninduced female and male rats, and male rats treated with the P450 IIB1 inducers clofibrate and phenobarbital. In accord with the microsomal activities, conversion of thio-TEPA to TEPA was less extensive and thio-TEPA elimination slower in female than in male rats.(ABSTRACT TRUNCATED AT 400 WORDS)