Abstract

In vitro studies with human liver microsomes and cytochrome P450 (P450) prototype substrates were performed to characterize the selectivity and mechanism of inhibition of P450 by dimethyl-4,4′-dimethoxy-5,6,5′,6′-dimethylenedioxybiphenyl-2,2′-dicarboxylate (DDB). DDB was found to be a strong inhibitor of testosterone 6β-hydroxylation activity (CYP3A4) with aK_i value of 0.27 ± 0.21 μM. At higher concentrations, DDB marginally inhibited caffeineN₃-demethylation (CYP1A2), diclofenac 4′-hydroxylation (CYP2C9), and dextromethorphanO-demethylation (CYP2D6) activities, but this compound had no effect on CYP2A6-, CYP2C19-, and CYP2E1-mediated reactions. Spectral analysis indicated that the formation of metabolite-P450 complex having absorbance at 456 nm was concentration-dependent; 5 to 33% of the total P450 was complexed in rat and human liver microsomes after a 5-min incubation with DDB. In addition, microsomal incubations with DDB in the presence of NADPH resulted in a loss of spectral P450 content, which was restored after adding K₃Fe(CN)₆. This complex formation resulted in a time-dependent loss of CYP3A-catalyzed marker activity (testosterone 6β-hydroxylation) in human liver microsomes. The inhibition was only partially restored upon dialysis. These results collectively suggest that formation of a metabolite-CYP3A complex with DDB was responsible for the CYP3A-selective time-dependent loss of catalytic function of CYP3A.