Triazolam biotransformation by human liver microsomes in vitro: effects of metabolic inhibitors and clinical confirmation of a predicted interaction with ketoconazole.
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.
von Moltke
L L
LL
Department of Pharmacology and Experimental Therapeutics, Tufts University School of Medicine, Boston, Massachusetts, USA.
Greenblatt
D J
DJ
Harmatz
J S
JS
Duan
S X
SX
Harrel
L M
LM
Cotreau-Bibbo
M M
MM
Pritchard
G A
GA
Wright
C E
CE
Shader
R I
RI
D000328:Adult; D000368:Aged; D000818:Animals; D000935:Antifungal Agents / Q000494:pharmacology*; D001711:Biotransformation; D003577:Cytochrome P-450 Enzyme System / Q000502:physiology; D004347:Drug Interactions; D005260:Female; D006801:Humans; D006993:Hypnotics and Sedatives / Q000493:pharmacokinetics*; D007654:Ketoconazole / Q000493:pharmacokinetics / Q000494:pharmacology*; D008297:Male; D051379:Mice; D008862:Microsomes, Liver / Q000378:metabolism*; D008875:Middle Aged; D014229:Triazolam / Q000493:pharmacokinetics*
D000935:Antifungal Agents; D006993:Hypnotics and Sedatives; D014229:Triazolam; D003577:Cytochrome P-450 Enzyme System; D007654:Ketoconazole