Several in vivo human studies have produced conflicting
Several in vivo human studies have produced conflicting results for evaluating the inhibition of OCs on the metabolism of CYP3A substrates. Balogh et al.  demonstrated an approximately 25% inhibition of CYP3A activities in vivo after the administration of an OC containing 30μg of ethinylestradiol and 125μg of levonorgestrel in 11 women once per day during a treatment cycle of 21days; the authors compared the AUC values of a CYP3A-dependent nifedipine metabolite, dehydronifedipine, before and after treatments. However, they could not detect the inhibitory effects in 3 of the 11 subjects. One problem is that the AUC value of dehydronifedipine might be influenced by the extent of further metabolism such as the hydrolysis of methylester at C-3 or C-5 and/or the oxidation of a methyl group at C-2 or C-6 of dehydronifedipine . Kuhnz and Löfberg  evaluated the inhibition of CYP3A activities in vivo with several progestins, including levonorgestrel, gestodene, or cyproterone acetate, each in combination with ethinylestradiol or with progestin levonorgestrel or gestodene alone, using the urinary ratio of 6β-hydroxycortisol to ICG-001 (6β-OHF/F) as a non-invasive marker of CYP3A activity. There was either no or only a small decrease in the ratio of 6β-OHF/F, indicating no or minor inhibitory effects of these progestins in vivo. This may be due to the insufficient sensitivity of the urinary ratio 6β-OHF/F for detecting these changes . It should be noted that the urinary ratio 6β-OHF/F is a valid index for in vivo CYP3A activity only when the renal clearance of cortisol is consistent , ,  because the urinary ratio 6β-OHF/F is a function of two independent parameters, the metabolic clearance specific for 6β-hydroxylation and renal clearance of cortisol. McCune et al.  also did not detect the inhibition of CYP3A activities after the administration of an OC containing 30–40–30μg of ethinylestradiol and 50–75–125μg of levonorgestrel based on their analysis of the urinary dextromethorphan ratio (dextromethorphan/3-methoxymorphinan), which could be due to a large degree of intraindividual variability in the urinary dextromethorphan ratio. On the other hand, Slayter et al.  demonstrated 33% inhibition of CYP3A activity in vivo based on the decrease of the clearance of 6α-methylprednisolone as a substrate of CYP3A. The measurement of the total body clearance of 6α-methylprednisolone might lead to an underestimation of the inhibition of CYP3A activity because the total body clearance involves metabolic clearance caused by enzymes other than CYP3A, such as 11β-hydroxysteroid dehydrogenase (11β-HSD)  and UDP-glucuronosyltransferase (UGT) . By using a probe drug for phenotyping, the clearance of the drug should provide the best estimate of the in vivo catalytic activity of the enzyme of interest. If the probe drug has multiple metabolic pathways, the fractional metabolic clearance corresponding to the pathway of interest should be an appropriate measure . Furuta et al. ,  provided evidence for the validity of 6β-hydroxylation clearance of endogenous cortisol (CLm(6β)) as a new index for phenotyping the in vivo CYP3A activity by using stable isotope methodology. The phenotyping can precisely assess the in vivo CYP3A activity even when there are intraindividual and interindividual variations in the cortisol metabolic clearance mediated by enzymes other than CYP3A and in the renal clearance of cortisol. Using Furuta’s method, we measured the 6β-hydroxylation clearances (CLm(6β)) in 49 healthy subjects to evaluate the normal range of in vivo CYP3A activities (CLm(6β), which were 2.40±0.79mL/min) . Furthermore, we evaluated the time courses of the clearance (CLm(6β)) every 2h from 9:00 to 21:00 in 26 healthy subjects, demonstrating that the 6β-hydroxylation clearance (CLm(6β)) in most subjects was within the range of 1.5–3.5mL/min  which is indicated by a shadow zone in Fig. 3A–D. Furthermore, the within-day intraindividual variabilities in the clearance were 1.1–2.5-fold (n=26). This indicated that only one- or two-point measurement of the clearance in a day might mask a weak inhibitory effect of levonorgestrel on the CYP3A activity. Therefore, the present study was undertaken to measure the time course of the 6β-hydroxylation clearance (CLm(6β)) every 2h from 11:00 to 17:00 (days 0 and 28) and from 9:00 to 17:00 (days 1, 2, and 21) in four women after sequential administration of the OC during one menstrual cycle. We used the tri-phasic OC containing ethinylestradiol (30–40–30μg) and levonorgestrel (50–75–125μg). Despite a low dose of the OC, the inhibition of CYP3A activities was estimated to be 43–64% during a 21-day OC treatment regimen, which was almost equal to the inhibitory effects (58%) after the administration of clarithromycin (400mg per day) . Peng et al.  reported that the magnitude of the inhibition of CYP3A activity by itraconazole (400mg) was 34–51% through analysis of the formation clearance of 6β-hydroxycortisol from cortisol or the combined clearance of 6β-hydroxycortisol from cortisol and 6β-hydroxycortisone from cortisone. The in vivo inhibition potencies of the OC (43–64%) obtained in this study were stronger than expected compared with those previously reported in in vitro and in vivo studies , , , , , , .