Received for publication February 24, 2006.
Revised May 19, 2006.
Accepted for publication May 19, 2006.

EFFECT OF CYTOCHROMES P450 CHEMICAL INHIBITORS AND MONOCLONAL ANTIBODIES ON HUMAN LIVER MICROSOMAL ESTERASE ACTIVITY

Abstract

Selective and non-selective cytochromes P450 (CYP) chemical inhibitors and monoclonal antibodies (mAbs) are routinely used to determine the contribution of CYP enzymes involved in the biotransformation of a drug. A fluorometric assay has been established using fluorescein diacetate (FD) as a model substrate to determine the effect of some commonly used CYP 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 µM, 20.5 µM, 6.5 µM, 15.0 µM, 19.4 µM, and 5.4 µM, respectively. All of these showed 20% inhibition of human liver microsomal esterase activity at concentrations typically used for CYP 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 CYP chemical inhibitors are capable of inhibiting human liver microsomal esterase activity and should not be used to assess the role of CYP enzymes in the biotransformation of esters. The lack of inhibition of human liver microsomal esterase activity by CYP-specific monoclonal antibodies suggests that they may be used to assess the role of CYP 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 -nicotinamide adenine dinucleotide 2'-phosphate reduced (-NADPH).

Key words: carboxylesterases, cytochrome P450, human CYP enzymes, liver microsomes, microsomes

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