Inhibitory effect of stiripentol on carbamazepine and saquinavir metabolism in human.

AIMS: To characterize the in vitro and in vivo inhibitory effect of stiripentol, a new anticonvulsant, on the metabolism of carbamazepine and saquinavir, which are substrates of CYP3A4. METHODS: Human liver microsomes and cDNA-expressed CYP enzymes were used for the in vitro experiments. Pharmacokinetic data from epileptic children and healthy adults were used for the carbamazepine and saquinavir in vivo studies, respectively. RESULTS: Carbamazepine biotransformation to its 10,11-epoxide by human liver microsomes (Vmax = 10.3 nmol min(-1) nmol(-1) P450, apparent Km = 362 microm), cDNA-expressed CYP3A4 (Vmax = 1.17 nmol min(-1) nmol(-1) P450, apparent Km = 119 microm) and CYP2C8 (Vmax = 0.669 nmol min(-1) nmol(-1) P450, apparent Km = 757 microm) was inhibited by stiripentol (IC50 14, 5.1, 37 microM and apparent Ki 3.7, 2.5, 35 microm, respectively). Saquinavir biotransformation to its major metabolite M7 by human liver microsomes (Vmax = 5.7 nmol min(-1) nmol(-1) P450, apparent Km = 0.79 microm) was inhibited by stiripentol (IC50 163 microM, apparent Ki 86 microm). In epileptic children treated with carbamazepine and stiripentol, the plasma concentration ratio of carbamazepine epoxide/carbamazepine was decreased by 65%. The in vivo apparent Ki for stiripentol ranged from 10.5 to 41.4 microm. The pharmacokinetics of saquinavir was not modified by stiripentol in healthy adults. The 95% confidence intervals for the difference for Cmax and AUC of saquinavir between the placebo and stiripentol phase were (-39.8, 39.8) and (-33.2, 112), respectively. CONCLUSIONS: These results showed that stiripentol was a weak inhibitor of saquinavir metabolism both in vitro and in vivo. In contrast, stiripentol is a potent inhibitor of carbamazepine 10,11-epoxide formation in vitro and in vivo in epileptic patients.

Oxidative metabolism of amprenavir in the human liver. Effect of the CYP3A maturation.

Amprenavir is a human immunodeficiency virus-1 (HIV-1) protease inhibitor intended to be used to treat HIV-infected children. Although a pediatric dosage is proposed by the manufacturer, no data are currently available on the pharmacokinetics of amprenavir in neonates and infants. Amprenavir being primarily eliminated after oxidative biotransformation, we explored its in vitro metabolism by cytochrome P450 (P450)-dependent monooxygenases. In our conditions, five metabolites were formed in vitro and subsequently analyzed by liquid chromatography-mass spectrometry; P450-dependent oxidations occurred either on the tetrahydrofuran ring (M3 and M4), the aniline ring (M5), and the aliphatic chain (M2) or resulted from the N-dealkylation and loss of the tetrahydrofuran ring (M1). The two major metabolites, respectively M3 and M2 were formed by human liver microsomes with K(m) between 10 and 70 microM. CYP3A4 and to a lesser extent CYP3A5 were major contributors for the formation of M2, M3, and M5 metabolites, whereas CYP3A7 had no or little activity. This assumption was confirmed by inhibition with ketoconazole and ritonavir (two potent inhibitors of CYP3A) whereas sulfaphenazole (2C9 inhibitor) and quinidine (2D6 inhibitor) were inefficient. The metabolism of amprenavir was negligible in microsomes from either fetuses or neonates and steadily increased after the first weeks of life in relation with the maturation of CYP3A4/5. In conclusion, results demonstrated that the capacity of the human liver to oxidize amprenavir is low during the first weeks after birth and that dosage could be substantially reduced during the early neonatal period.