MIDAZOLAM HYDROXYLATION BY HUMAN LIVER-MICROSOMES IN-VITRO - INHIBITION BY FLUOXETINE, NORFLUOXETINE, AND BY AZOLE ANTIFUNGAL AGENTS

Biotransformation of the imidazobenzodiazepine midazolam to its oc-hyd roxy and 4-hydroxy metabolites was studied in vitro using human liver microsomal preparations, Formation of cu-hydroxy-midazolam was a high- affinity (K-m = 3.3 mu mol/L) Michaelis-Menten process coupled with su bstrate inhibition at high concentrations of midazolam. Formation of 4 -hydroxymidazolam had much lower apparent affinity (57 mu mol/L), with minimal evidence of substrate inhibition. Based on comparison of V-ma x/K-m ratios for the two pathways, alpha-hydroxy-midazolam formation w as estimated to account for 95% of net intrinsic clearance. Three azol e antifungal agents were inhibitors of midazolam metabolism in vitro, with inhibition being largely consistent with a competitive mechanism. Mean competitive inhibition constants (K-i) versus alpha-hydroxy-mida zolam formation were 0.0037 mu mol/L for ketoconazole, 0.27 mu mol/L f or itraconazole, and 1.27 mu mol/L for fluconazole, An in vitro-in viv o scaling model predicted inhibition of oral midazolam clearance due t o coadministration of ketoconazole or itraconazole; the predicted inhi bition was consistent with observed interactions in clinical pharmacok inetic studies. The selective serotonin reuptake inhibitor (SSRI) anti depressant fluoxetine and its principal metabolite, norfluoxetine, als o were inhibitors of both pathways of midazolam biotransformation, wit h norfluoxetine being a much more potent inhibitor than was fluoxetine itself. This finding is consistent with results of other in vitro stu dies and of clinical studies, indicating that fluoxetine, largely via its metabolite norfluoxetine, may impair clearance of P450-3A substrat es.