Cytochrome P450 3A4 (CYP3A4) is present not only in the liver but also in the small intestine, where it functions as a barrier against xenobiotics. Some CYP3A4 substrates exhibit low bioavailability due to intestinal first pass metabolism. The AUCs of such CYP3A4 substrates are remarkably changed by the inhibition, induction, and saturation of CYP3A4 and so prediction of intestinal first-pass metabolism is important. In this article, factors affecting intestinal first-pass metabolism of drugs are reviewed, focusing on the intestinal metabolism by CYP3A. The methods to predict intestinal first-pass metabolism are also reviewed.
Plasma digoxin concentrations are increased by the coadministration of anticholinergic drugs, such as propantheline, which decrease gastrointestinal motility. The present study evaluated the effect of imidafenacin, a novel anticholinergic drug, on the pharmacokinetics of digoxin. The effect of imidafenacin on the pharmacokinetics of digoxin was examined in 14 healthy Japanese male subjects in a single-centre, open-label, randomized, two-way crossover study. Subjects received a daily oral dose of digoxin 0.25 mg on days 1 and 2 and digoxin 0.125 mg on days 3 to 8 (period 1). Following a 2-week washout period, digoxin was administered orally for 8 days in a similar manner (period 2). A twice daily dose of imidafenacin 0.1 mg was concomitantly administered with digoxin for 8 days either in period 1 or 2. The geometric mean ratios [GMR] (90% confidence intervals [CIs]) for digoxin Cmax and AUC0-24 (with/without imidafenacin) at steady state were 0.88 (0.74, 1.04) and 1.00 (0.90, 1.10), respectively. The 90% CIs of GMR for digoxin trough concentration, urinary excretion amount and renal clearance at steady state fell within the range of 0.8 to 1.25. The steady-state pharmacokinetics of digoxin is not affected by concomitant administration of imidafenacin in healthy subjects.
Clomiphene is a first line treatment for anovulation, a common cause of infertility. Response to clomiphene is variable and unpredictable. Tamoxifen is structurally related to clomiphene, and also shows considerable variation in response. CYP2D6 and CYP3A4 are major contributors to the metabolism of tamoxifen. The aim of the present work was to define the role of CYP2D6 and CYP3A4 in the in vitro metabolism of enclomiphene, regarded by some as the more active isomer of clomiphene. Enclomiphene (25 μM) was incubated with human liver microsomes (from 4 extensive (EM) and 1 poor metaboliser with respect to CYP2D6) and with microsomes from lymphoblastoid cells expressing CYP2D6. Microsomes from all the EM livers and recombinant CYP2D6 metabolised enclomiphene (the disappearance of drug ranged from 40-60%). No metabolism was detected in microsomes from the PM liver. Quinidine (1 μM) completely inhibited the metabolism of enclomiphene by all the EM livers and by recombinant CYP2D6 (p<0.001, one way ANOVA). Ketoconazole (2 μM) had no significant effect on enclomiphene metabolism in 3 out of the 4 EM livers. The extent of enclomiphene metabolism was correlated with the amount of CYP2D6 present (p<0.001, Pearson correlation test). The findings indicate that CYP2D6 is primarily responsible for the metabolism of enclomiphene.
Olopatadine, a new second-generation antihistamine, is widely used in the treatment of allergic disorders. The low levels of histamine H1 receptor occupancy in human brain by olopatadine, which is related to its minimal sedation, suggest its low penetration into the brain. The present study evaluates the impact of P-glycoprotein (P-gp) on brain penetration and plasma concentration of olopatadine. The uptake amount of olopatadine in human P-gp transfected LLC-PK1 cells (LLC-GA5-COL150) was lower than that in LLC-PK1. The uptake of olopatadine in LLC-GA5-COL150 was increased in the same level as that in LLC-PK1 in the presence of cyclosporine A, a P-gp inhibitor. After intravenous or oral administration of olopatadine to wild type (WT) and mdr1a/1b knockout (KO) mice at a dose of 1 mg/kg, the brain concentration in KO mice was higher than that in WT mice. On the other hand, the plasma concentration of olopatadine after either route of administration was not different between WT and KO mice. These results suggest that olopatadine is a substrate of P-gp, and that P-gp limits the brain penetration but dose not affect the plasma concentration of olopatadine.
Losartan, a selective angiotensin receptor antagonist, is mainly metabolized by CYP2C9 to an active carboxylic acid, E3174, which is pharmacologically more potent inhibitor than the parent compound. We evaluated the effect of bucolome, a CYP2C9 inhibitor, on the pharmacokinetics of losartan and E3174, which were measured by high performance liquid chromatography in human volunteers and rats. A randomized crossover design study with two phases was done in the volunteer study. In the first phase, the volunteers received losartan 25 mg alone orally (LOS group), and, in the second phase, losartan 25 mg was given after repeated oral administration of 300 mg bucolome for 7 days (LOS+BUC group). In the LOS group, the maximum concentration (Cmax) and area under the concentration curve (AUC) of losartan were significantly higher than in the LOS+BUC group. On the other hand, in the LOS+BUC group, the Cmax and AUC of E3174 were significantly lower than in the LOS group. In the rat study, male Wistar ST rats were used. In the first phase, the rats orally received losartan 10 mg/kg alone or after bucolome was given repeatedly at a dose of 20, 50, or 200 mg/kg for 7 days. In the second phase for steady state, the rats were given losartan 10 mg/kg for 14 days (group A) or losartan 10 mg/kg and bucolome 50 mg/kg for 14 days (Group B). Bucolome at doses 50 and 200 mg/kg significantly increased the AUC losartan and significantly decreased the AUC of 3174. At the steady state, there were no significant differences in AUC of losartan between Group A and B, but the Cmax and AUC of E3174 were significantly lower in Group B than Group A.
High-throughput characterization of drug-drug interactions in plasma protein binding was demonstrated by using a surface plasmon resonance (SPR) biosensor. The method used in this study enabled the discrimination between the two modes of binding inhibition, direct competition and negative allosteric effect, which was difficult in conventional SPR approaches. Two theoretical equations were used representing SPR binding response for directly competitive binding or for independent binding. The experimental binding data for human serum albumin was processed by non-linear least squared regression of the equations. By this approach, drug-drug interactions were classified into three modes, direct competition, independent binding, and allosteric interaction, which were almost consistent with previous reports. In addition, dissociation constants were also estimated roughly for direct competition and for independent binding. The analytical throughput was almost as high as in the previous reports; three minutes per injection. This method is a powerful tool for the characterization of drug-drug interaction at an early stage of new drug development.
In the previous study, we performed a simulation of a clinical pharmacokinetic trial, in which blood was sampled at two time points corresponding to the peak concentration (Cpeak) and trough concentration (Ctrough) following repetitive oral administration at the dose, D, and dosing interval, τ. The approximate oral clearance (CL/Fapprox), estimated as 2·D/(Cpeak·τ+Ctrough·τ), is accurate for drugs with an elimination half-life comparative to or longer than τ; however, it was suggested that we might not use CL/Fapprox for drugs with a considerably short elimination half-life relative to τ. In the present study, we evaluated the accuracy of the alternative oral clearance (CL/Fexp) estimated by the simple monoexponential model. In contrast to CL/Fapprox, CL/Fexp was accurate for drugs with a short elimination half-life relative to τ. The present finding in conjunction with our previous study suggested that the peak-and-trough sampling design is promising for the clinical repeated-dose pharmacokinetic trial for drugs with not only slow but also rapid elimination from the body. We think that the accuracy and precision of the two analysis methods to estimate oral clearance (CL/Fapprox and CL/Fexp) for a target drug should be evaluated carefully before and after a real clinical trial.
We report different effects of administration of proton pump inhibitors on tacrolimus blood concentration in two living-donor liver transplant patients. In case 1, a 51-year-old man with liver cirrhosis due to hepatitis C virus underwent living-donor liver transplantation, and tacrolimus was orally administered. Omeprazole (40 mg/day) was introduced intravenously between postoperative days 5 and 6, and oral lansoprazole (30 mg/day) was introduced from day 6, leading to an increase in the concentration/dose ratio of tacrolimus from day 10. In case 2, a 41-year-old living-donor liver transplant woman received tacrolimus, and co-administered with omeprazole (40 mg/day) intravenously during 7 days immediately after surgery. During this period, trough concentration of tacrolimus was high, but the concentration/dose ratio of tacrolimus was gradually decreasing with time. Switched to rabeprazole (10 mg/day) orally on the postoperative 8th day, the concentration/dose ratio of tacrolimus remained low, indicating little drug-drug interaction between tacrolimus and rabeprazole. In both cases, the genotypes of CYP2C19 and CYP3A5 were defective both in the graft liver and in the native intestine. A drug-drug interaction between rabeprazole and tacrolimus was not observed in this case study presented, suggesting that this combination could be safely used in tacrolimus therapy after liver transplantation.
The multidrug resistance-associated protein 2 (MRP2) encoded by the ABCC2 gene is expressed in the liver, intestine and kidneys and preferentially exports organic anions or conjugates with glucuronide or glutathione. In this study, all 32 exons and the 5′-flanking region of ABCC2 in 236 Japanese were resequenced, and 61 genetic variations including 5 novel nonsynonymous ones were detected. A total of 64 haplotypes were determined/inferred and classified into five *1 haplotype groups (*1A, *1B, *1C, *1G, and *1H) without nonsynonymous substitutions and *2 to *9 groups with nonsynonymous variations. Frequencies of the major 4 haplotype groups *1A (-1774delG), *1B (no common SNP), *1C (-24C>T and 3972C>T), and *2 [1249G>A (Val417Ile)] were 0.331, 0.292, 0.172, and 0.093, respectively. This study revealed that haplotype *1A, which has lowered activity, is quite common in Japanese, and that the frequency of *1C, another functional haplotype, was comparable to frequencies in Asians and Caucasians. In contrast, the haplotypes harboring 3972C>T but not -24C>T (*1G group), which are reportedly common in Caucasians, were minor in Japanese. Moreover, the allele 1446C>T (Thr482Thr), which has increased activity, was not detected in our Japanese population. These findings imply possible differences in MRP2-mediated drug responses between Asians and Caucasians.