Triazolam biotransformation by human liver microsomes in vitro: effects of metabolic inhibitors and clinical confirmation of a predicted interaction with ketoconazole

• Published in: The Journal of pharmacology and experimental therapeutics Vol. 276; no. 2; pp. 370 - 379
• Main Authors: von Moltke, L L, Greenblatt, D J, Harmatz, J S, Duan, S X, Harrel, L M, Cotreau-Bibbo, M M, Pritchard, G A, Wright, C E, Shader, R I
• Format: Journal Article
• Language: English
• Published: United States American Society for Pharmacology and Experimental Therapeutics 01.02.1996
• Subjects: Adult; Aged; Animals; Antifungal Agents - pharmacology; Biotransformation; Cytochrome P-450 Enzyme System - physiology; Drug Interactions; Female; Humans; Hypnotics and Sedatives - pharmacokinetics; Ketoconazole - pharmacokinetics; Ketoconazole - pharmacology; Male; Mice; Microsomes, Liver - metabolism; Middle Aged; Triazolam - pharmacokinetics
• ISSN: 0022-3565; 1521-0103

Biotransformation of the triazolobenzodiazepine triazolam to its hydroxylated metabolites, alpha-hydroxy (OH)- and 4-OH-triazolam, was studied in vitro using microsomal preparations of human liver. Mean values of Vmax (10.3 nM/min/mg of protein) and Km (304 microM) for the 4-OH pathway exceeded values for the alpha-OH pathway (2.4 and 74, respectively). However the mean Vmax/Km ratios for the two pathways were nearly identical, indicating that both contribute approximately equally to intrinsic clearance. Ketoconazole was a powerful inhibitor of triazolam biotransformation, having mean competitive Ki values of 0.006 and 0.023 microM for the alpha-OH and 4-OH pathways. This is consistent with the role of P450-3A isoforms in mediating triazolam biotransformation. The serotonin2 antagonist antidepressant nefazodone inhibited the alpha-OH and 4-OH pathways (Ki = 0.6 and 1.7 microM, respectively), but with considerably less activity than ketoconazole. Among six selective serotonin reuptake-inhibitor antidepressants, norfluoxetine was the most potent inhibitor (Ki = 2.7 and 8.0 microM) and fluoxetine itself was the weakest (Ki = 7.0 and 44.3 microM). In a double-blind clinical pharmacokinetic-pharmacodynamic study, administration of triazolam (0.125 mg) preceded by ketoconazole, compared to triazolam preceded by placebo, produced a nearly 9-fold reduction in apparent oral clearance of triazolam (41 vs. 337 ml/min) and a 4-fold prolongation of half-life (13.5 vs. 3.4 hr). Pharmacodynamic testing indicated enhancement of electroencephalographic beta activity, and enhanced decrements in digit-symbol substitution test performance, attributable to coadministration of ketoconazole. Plasma ketoconazole concentrations measured in the clinical study ranged from 0.02 microgram/ml (projected minimum) to 4.95 micrograms/ml (maximum). An in vitro-in vivo scaling model, using these plasma ketoconazole concentrations together with liver partition ratios and the in vitro Ki values, predicted a decrement of triazolam clearance due to ketoconazole coadministration that was consistent with the 88% decrement in clearance actually observed in vivo.