PT  - JOURNAL ARTICLE
AU  - von Moltke, L L
AU  - Greenblatt, D J
AU  - Harmatz, J S
AU  - Duan, S X
AU  - Harrel, L M
AU  - Cotreau-Bibbo, M M
AU  - Pritchard, G A
AU  - Wright, C E
AU  - Shader, R I
TI  - Triazolam biotransformation by human liver microsomes in vitro: effects of metabolic inhibitors and clinical confirmation of a predicted interaction with ketoconazole.
DP  - 1996 Feb 01
TA  - Journal of Pharmacology and Experimental Therapeutics
PG  - 370--379
VI  - 276
IP  - 2
4099  - http://jpet.aspetjournals.org/content/276/2/370.short
4100  - http://jpet.aspetjournals.org/content/276/2/370.full
SO  - J Pharmacol Exp Ther1996 Feb 01; 276
AB  - 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.