IDENTIFICATION OF CYTOCHROME P4503A4 AS THE MAJOR ENZYME RESPONSIBLE FOR THE METABOLISM OF IVERMECTIN BY HUMAN LIVER-MICROSOMES

1. Ivermectin was extensively metabolized by human liver microsomes to at least 10 metabolites. The structure of many of them (mostly hydrox ylated and demethylated) was determined by H-1-NMR and LC/MS. 2. To de termine which human cytochrome P450 isoform(s) is responsible for the metabolism of ivermectin, chemical inhibitors including sulphaphenazol e, quinidine, furafylline, troleandomycin (TAO) and diethyldithiocarba mate (DDC) were used to evaluate their effect on ivermectin metabolism . TAO, a specific inhibitor of cytochrome P4503A4, was the most potent inhibitor, inhibiting the total metabolism as well as formation of ea ch metabolite. Metabolism was also inhibited by an anti-human cytochro me 3A4 antibody by 90%. 3. When ivermectin was incubated with microsom es from cells expressing CYP1A1, 1A2, 2A6, 2B6, 2C8, 2C9, 2C19, 2D6, 2 E1 or 3A4 at 4 mg/ml protein concentrations, metabolic activity was on ly detected with the microsomes containing CYP3A4. The metabolic profi le from cDNA-expressed CYP3A4 microsomes was qualitatively similar to that from human liver microsomes. 4. Thus, cytochrome P4503A4 is the p redominant isoform responsible for the metabolism of ivermectin by hum an liver microsomes.