Inhibition of triazolam clearance by macrolide antimicrobial agents: In vitro correlates and dynamic consequences

Background: Macrolide antimicrobial agents may impair hepatic clearance of drugs metabolized by cytochrome P4503A isoforms. Potential interactions of triazolam, a substrate metabolized almost entirely by cytochrome P4503A in humans, with 3 commonly prescribed macrolides were identified using an in vitro metabolic model. The actual interactions, and their pharmacodynamic consequences, were verified in a controlled clinical study. Methods: In an in vitro model using human liver microsomes, 250 mu mol/L triazolam was incubated with ascending concentrations (0 to 250 mu mol/L) of troleandomycin, azithromycin, erythromycin, and clarithromycin. In a randomized, double-blind, 5-trial clinical pharmacokinetic-pharmacodynamic study, 12 volunteers received 0.125 mg triazolam orally, together with placebo, azithromycin, erythromycin, or clarithromycin. In a fifth trial they received placebo plus placebo. Results: Mean 50% inhibitory concentrations versus 4-hydroxytriazolam formation in vitro were as follows: 3.3 mu mol/L troleandomycin, 27.3 mu mol/L erythromycin, 25.2 mu mol/L clarithromycin, and greater than 250 mu mol/L azithromycin, Apparent oral clearance of triazolam when given with placebo or azithromycin was nearly identical (413 and 416 mL/min), as were peak plasma concentrations (1.25 and 1.32 ng/mL) and elimination half-life (2.7 and 2.6 hours). Apparent oral clearance was significantly reduced (P < .05) during erythromycin and clarithromycin trials (146 and 95 mL/min). Peak plasma concentration was correspondingly increased, and elimination half-life was prolonged. The effects of triazolam on dynamic measures were nearly identical when triazolam was given with placebo or azithromycin, but benzodiazepine agonist effects were enhanced during erythromycin and clarithromycin trials. Conclusion: The in vitro model identifies macrolides that may impair triazolam clearance. Anticipated interactions, and their pharmacodynamic consequences in volunteer subjects, were verified in vivo.