• Gerd Mikus, Verena Schöwel, Magdalena Drzewinska, Jens Rengelshausen, Reinhard Ding, Klaus-Dieter Riedel, Jürgen Burhenne, Johanna Weiss, Torben Thomsen, and Walter E Haefeli.
• Department of Internal Medicine VI, Clinical Pharmacology and Pharmacoepidemiology, University of Heidelberg, Heidelberg, Germany. gerd.mikus@med.uni-heidelberg.de
• Clin. Pharmacol. Ther. 2006 Aug 1; 80 (2): 126-35.

ObjectivesCytochrome P450 (CYP) 2C19 and CYP3A4 are the major enzymes responsible for voriconazole elimination. Because the activity of CYP2C19 is under genetic control, the extent of inhibition with a CYP3A4 inhibitor was expected to be modulated by the CYP2C19 metabolizer status. This study thus assessed the effect of the potent CYP3A4 inhibitor ritonavir after short-term administration on voriconazole pharmacokinetics in extensive metabolizers (EMs) and poor metabolizers (PMs) of CYP2C19.MethodsIn a randomized, placebo-controlled crossover study, 20 healthy participants who were stratified according to CYP2C19 genotype received oral ritonavir (300 mg twice daily) or placebo for 2 days. Together with the first ritonavir or placebo dose, a single oral dose of 400 mg voriconazole was administered. Voriconazole was determined in plasma and urine by liquid chromatography-mass spectrometry, and pharmacokinetic parameters were estimated by noncompartmental analysis.ResultsWhen given alone, the apparent oral clearance of voriconazole after single oral dosing was 26%+/-16% (P > .05) lower in CYP2C19*1/*2 individuals and 66%+/-14% (P < .01) lower in CYP2C19 PMs. The addition of ritonavir caused a major reduction in voriconazole apparent oral clearance (354+/-173 mL/min versus 202+/-139 mL/min, P = .0001). This reduction occurred in all CYP2C19 genotypes (463+/-168 mL/min versus 305+/-112 mL/min [P = .023] for *1/*1, 343+/-127 mL/min versus 190+/-93 mL/min [P = .008] for *1/*2, and 158+/-54 mL/min versus 22+/-11 mL/min for *2/*2) and is probably caused by inhibition of CYP3A4-mediated voriconazole metabolism.ConclusionsCoadministration of a potent CYP3A4 inhibitor leads to a higher and prolonged exposure with voriconazole that might increase the risk of the development of adverse drug reactions on a short-term basis, particularly in CYP2C19 PM patients.

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