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WHAT IS ALREADY KNOWN ABOUT THIS SUBJECT • The analgesic activity of tilidine is mediated by its active metabolite, nortilidine, which easily penetrates the blood-brain barrier and binds to the µ-opioid receptor as a potent agonist. • Tilidine undergoes an extensive first-pass metabolism, which has been suggested to be mediated by CYP3A4 and CYP2C19; furthermore, strong inhibition of CYP3A4 and CYP2C19 by voriconazole increased exposure of nortilidine, probably by inhibition of further metabolism. • The novel CYP2C19 gene variant CYP2C19*17 causes ultrarapid drug metabolism, in contrast to the *2 and *3 variants, which result in impaired drug metabolism. WHAT THIS STUDY ADDS • Using a panel study with CYP2C19 ultrarapid and poor metabolizers, a major contribution of polymorphic CYP2C19 on tilidine metabolic elimination can be excluded. • The potent CYP3A4 inhibitor ritonavir alters the sequential metabolism of tilidine, substantially reducing the partial metabolic clearances of tilidine to nortilidine and nortilidine to bisnortilidine, which increases the nortilidine exposure twofold. • The lowest clearance in overall tilidine elimination is the N-demethylation of nortilidine to bisnortilidine. Inhibition of this step leads to accumulation of the active nortilidine. AIMS To investigate in vivo the effect of the CYP2C19 genotype on the pharmacokinetics of tilidine and the contribution of CYP3A4 and CYP2C19 to the formation of nortilidine using potent CYP3A4 inhibition by ritonavir. METHODS Fourteen healthy volunteers (seven CYP2C19 poor and seven ultrarapid metabolizers) received ritonavir orally (300 mg twice daily) for 3 days or placebo, together with a single oral dose of tilidine and naloxone (100 mg and 4 mg, respectively). Blood samples and urine were collected for 72 h. Noncompartmental analysis was performed to determine pharmacokinetic parameters of tilidine, nortilidine, bisnortilidine and ritonavir. RESULTS Tilidine exposure increased sevenfold and terminal elimination half-life fivefold during ritonavir treatment, but no significant differences were observed between the CYP2C19 genotypes. During ritonavir treatment, nortilidine area under the concentration-time curve was on average doubled, with no differences between CYP2C19 poor metabolizers [2242 h ng ml−1 (95% confidence interval 1811-2674) vs. 996 h ng ml−1 (95% confidence interval 872-1119)] and ultrarapid metabolizers [2074 h ng ml−1 (95% confidence interval 1353-2795) vs. 1059 h ng ml−1 (95% confidence interval 789-1330)]. The plasma concentration-time curve of the secondary metabolite, bisnortilidine, showed a threefold increase of time to reach maximal observed plasma concentration; however, area under the concentration-time curve was not altered by ritonavir. CONCLUSIONS The sequential metabolism of tilidine is inhibited by the potent CYP3A4 inhibitor, ritonavir, independent of the CYP2C19 genotype, with a twofold increase in the exposure of the active nortilidine. [ABSTRACT FROM AUTHOR]