Effect of cytochromes P450 chemical inhibitors and monoclonal antibodies on human liver microsomal esterase activity.

Selective and nonselective cytochromes P450 (P450) chemical inhibitors and monoclonal antibodies (mAbs) are routinely used to determine the contribution of P450 enzymes involved in the biotransformation of a drug. A fluorometric assay has been established using fluorescein diacetate as a model substrate to determine the effect of some commonly used P450 inhibitors and mAbs on human liver microsomal esterase activity. Of those inhibitors studied, only alpha-naphthoflavone, clotrimazole, ketoconazole, miconazole, nicardipine, and verapamil significantly inhibited human liver microsomal esterase activity, with apparent IC50 values of 18.0, 20.5, 6.5, 15.0, 19.4, and 5.4 microM, respectively. All of these showed > or =20% inhibition of human liver microsomal esterase activity at concentrations typically used for P450 reaction phenotyping studies, with clotrimazole, miconazole, nicardipine, and verapamil showing >60% inhibition. Unlike the chemical inhibitors, no inhibition of human liver microsomal esterase activity was observed in the presence of mAb to CYP1A2, 2C8, 2C9, 2C19, 2D6, and 3A4. These results suggest that P450 chemical inhibitors are capable of inhibiting human liver microsomal esterase activity and should not be used to assess the role of P450 enzymes in the biotransformation of esters. The lack of inhibition of human liver microsomal esterase activity by P450-specific monoclonal antibodies suggests that they may be used to assess the role of P450 enzymes in the biotransformation of esters. Additional experiments to assess the contribution of oxidative enzymes in the metabolism of esters may include incubations in the presence and absence of beta-nicotinamide adenine dinucleotide 2'-phosphate reduced.