Simvastatin does not affect CYP3A activity, quantified by the erythromycin breath test and oral midazolam pharmacokinetics, in healthy male subjects

Potential for inhibition of CYP3A activity by simvastatin, an HMG-CoA reductase inhibitor, was evaluated in 12 healthy male subjects who received placebo or 80 mg of simvastatin, the maximal recommended dose, once daily for 7 consecutive days. On day 7, an intravenous injection of 3 microCi [14C N-methyl]erythromycin for the erythromycin breath test (EBT) was coadministered with a 2 mg oral solution of midazolam. The values for percent 14C exhaled during the first hour (for EBT) and the pharmacokinetic parameters of midazolam (AUC, Cmax, t1/2) were not affected following multiple once-daily oral doses of simvastatin 80 mg. The 95% confidence interval was 0.97 to 1.18 for EBT and 0.99 to 1.23 for midazolam AUC. In addition, the total urinary recoveries of midazolam and its 1'-hydroxy metabolites (free plus conjugate) obtained from both treatments were not statistically different (p > 0.200). These data demonstrate that multiple dosing of simvastatin, at the highest recommended clinical dose, does not significantly alter the in vivo hepatic or intestinal CYP3A4/5 activity as measured by the commonly used EBT and oral midazolam probes.     

Effect of mibefradil on CYP3A4 in vivo

Mibefradil, a calcium channel blocker, was removed from the market because of adverse drug interactions with coadministered CYP3A4 substrates. This study examined the effect of mibefradil on the activity of hepatic and intestinal CYP3A4 in vivo, employing the erythromycin breath test (EBT) and oral midazolam pharmacokinetics. This was a two-period, single-blind, placebo-controlled crossover study in which 8 male volunteers were randomized to the order of receiving placebo and a single 100-mg oral dose of mibefradil. Oral midazolam was coadministered with intravenous [14C N-methyl] erythromycin 1 hour after mibefradil/placebo administration. The EBT was performed 20 minutes following erythromycin administration. Blood and urine were collected during the 36 hours following probe drug administration for analysis of midazolam pharmacokinetics. Coadministration of mibefradil increased the Cmax of midazolam 3-fold, the AUC 8- to 9-fold, and the t1/2 4-fold. Mibefradil coadministration decreased the amount of exhaled 14CO2 in 6 of 8 subjects, with a mean decrease of 25%. It was concluded that a single oral dose of mibefradil significantly inhibits CYP3A4 in intestine and liver. These data support that adverse drug interactions involving mibefradil reflect inhibition of CYP3A4 in intestine and liver. Also, they suggest that the EBT, while a valid probe of in vivo hepatic CYP3A4 activity, is a single time point measurement and may be less sensitive than oral midazolam pharmacokinetics in detecting CYP3A4 inhibition.     

Concurrent administration of the erythromycin breath test (EBT) and oral midazolam as in vivo probes for CYP3A activity

Given the prominent role of CYP3A in the metabolism of drugs, it is important to identify whether new chemical entities will affect this enzyme system and produce clinically relevant drug interactions. This study evaluated concomitant administration of intravenous [14C N-methyl] erythromycin (3 microCi) (erythromycin breath test; EBT) and 2 mg oral midazolam as probes of systemic and of systemic plus presystemic CYP3A activity, respectively. Twelve males received the probes in a two-period crossover fashion: one period included the probes on two occasions, 5 days apart; in the second period, 200 mg ketoconazole was given orally 2 hours prior to the probes. The within-subject CV for EBT (%14CO2/h) and midazolam AUC0-last was 4.9% and 16.9%, respectively. Ketoconazole reduced %14CO2/h by 43% and increased midazolam AUC0-last by approximately fivefold. In a nonrandomized third period (N = 5), ketoconazole was given simultaneously with midazolam (no EBT); midazolam AUC0-last was similar whether ketoconazole was given 2 hours prior to or simultaneously with the midazolam. The low midazolam dose was generally well tolerated; mild sedation was occasionally seen. Concurrent administration of the EBT and oral midazolam is a sensitive and reproducible tool to screen new chemical entities for potentially important CYP3A interactions.     

Effect of grapefruit juice on the disposition of manidipine enantiomers in healthy subjects

AIM: To examine the effect of grapefruit juice, an inhibitor of CYP3A4 in the small intestine, on the disposition of manidipine enantiomers in healthy subjects. METHODS: A randomized cross-over study with at least a 2-week wash-out period was performed. Seven healthy male volunteers received an oral 40-mg dose of racemic manidipine after an overnight fast with either grapefruit juice (GFJ) or water, as a control study. Plasma concentrations of (S)- and (R)-manidipine were monitored up to 10 h after the dosing. RESULTS: The plasma concentrations of (S)-manidipine were significantly higher (P<0.001) than those of (R)-manidipine in the control phase with an S/R ratio for the AUC0-infinity of 1.62 (95% confidence interval 1.52, 1.73). GFJ significantly increased Cmax and AUC0-infinity of (S)-manidipine by 2.4-fold (P<0.01) and 2.3-fold (P<0.01), respectively, and Cmax and AUC0-infinity of (R)-manidipine were increased by 3.4-fold (P<0.01) and 3.0-fold (P<0.01), respectively. There were significant differences (P<0.01) in GFJ-mediated percentage increases in Cmax and AUC0-infinity of (S)-manidipine compared with those of (R)-manidipine. The S/R ratio for AUC0-infinity was significantly decreased from 1.6 to 1.2 during the GFJ phase (P<0.01). CONCLUSION: These results indicate that the stereoselective disposition of manidipine was altered by GFJ, as an inhibitor of CYP3A4. GFJ appears to affect this metabolic disposal of (R)-manidipine to a greater extent than that of (S)-manidipine.     

Pomegranate juice does not impair clearance of oral or intravenous midazolam, a probe for cytochrome P450-3A activity: comparison with grapefruit juice

The effect of pomegranate juice (PJ) or grapefruit juice (GFJ) on CYP3A activity was studied in vitro and in healthy human volunteers. In human liver microsomes, the mean 50% inhibitory concentrations (IC(50)) for PJ and GFJ versus CYP3A (triazolam alpha-hydroxylation) were 0.61% and 0.55%, (v/v) respectively, without preincubation of inhibitor with microsomes. After preincubation, the IC(50) for PJ increased to 0.97% (P < .05), whereas the IC(50) for GFJ decreased to 0.41% (P < .05), suggesting mechanism-based inhibition by GFJ but not PJ. Pretreatment of volunteer subjects (n = 13) with PJ (8 oz) did not alter the elimination half-life, volume of distribution, or clearance of intravenous midazolam (2 mg). Administration of PJ also did not affect C(max), total area under the curve (AUC), or clearance of oral midazolam (6 mg). However, GFJ (8 oz) increased midazolam C(max) and AUC by a factor of 1.3 and 1.5, respectively, and reduced oral clearance to 72% of control values. Thus, PJ does not alter clearance of intravenous or oral midazolam, whereas GFJ impairs clearance and elevates plasma levels of oral midazolam.     

Evaluation of animal models for intestinal first-pass metabolism of drug candidates to be metabolized by CYP3A enzymes via in vivo and in vitro oxidation of midazolam and triazolam

Abstract

1. To search an appropriate evaluation methodology for the intestinal first-pass metabolism of new drug candidates, grapefruit juice (GFJ)- and vehicle (tap water)-pretreated mice or rats were orally administered midazolam (MDZ) or triazolam (TRZ), and blood levels of the parent compounds and their metabolites were measured by liquid chromatography/MS/MS. A significant effect of GFJ to elevate the blood levels was observed only for TRZ in mice.

2. In vitro experiments using mouse, rat and human intestinal and hepatic microsomal fractions demonstrated that GFJ suppressed the intestinal microsomal oxidation of MDZ and especially TRZ. Substrate inhibition by MDZ caused reduction in 1′-hydroxylation but not 4-hydroxylation in both intestinal and hepatic microsomal fractions. The kinetic profiles of MDZ oxidation and the substrate inhibition in mouse intestinal and hepatic microsomal fractions were very similar to those in human microsomes but were different from those in rat microsomes. Furthermore, MDZ caused mechanism-based inactivation of cytochrome P450 3A-dependent TRZ 1′-hydroxylation in mouse, rat and human intestinal microsomes with similar potencies.

3. These results are useful information in the analysis of data obtained in mouse and rat for the evaluation of first-pass effects of drug candidates to be metabolized by CYP3A enzymes.     

Inhibitory effect of docosahexaenoic acid (DHA) on the intestinal metabolism of midazolam: in vitro and in vivo studies in rats

The aim of this study was to evaluate the effects of docosahexaenoic acid (DHA) on the intestinal cytochrome P450 isoenzyme (CYP3A) and P-glycoprotein (P-gp) functions using midazolam and rhodamine-123 as specific substrates of CYP3A and P-gp, respectively. Perfused everted intestinal segments from rats were employed to determine the effects of DHA on midazolam metabolism and rhodamine-123 transport. In addition, the effects of DHA on in vitro midazolam metabolism in rat intestinal microsomes and on midazolam bioavailability in rats were examined. The intestinal extraction ratio (ER G) of midazolam was determined to be 0.43 and decreased significantly to 0.12, 0.07, and 0.06 in the presence of 50, 100, and 200 microM DHA, respectively, in a concentration-dependent manner. The results from an in vitro study using rat intestinal microsomes demonstrated that DHA competitively inhibited the intestinal CYP3A activity with Ki of 15.7 and 27.1 microM for the formations of 1'-OH midazolam and 4-OH midazolam, respectively. Moreover, the oral administration of DHA (100mg/kg) increased the AUC infinity, Cmax, and oral bioavailability (F) of midazolam by about 50% in rats, without affecting the T 1/2, V dss/F, or CL tot/F. In contrast, DHA did not change the serosal-to-mucosal transport of rhodamine-123 in the perfused everted intestine and oral administration of DHA (100mg/kg) had no influence on the pharmacokinetics of intravenously administered midazolam in rats, thus suggesting that DHA has little effect on the intestinal P-gp activity and hepatic clearance of midazolam. This study provided the first direct evidence to show that DHA has an inhibitory effect on the intestinal pre-systemic metabolism of a CYP3A substrate and that DHA has little, if any, effect on the P-gp activity in the gut.     

Effect of grapefruit juice in relation to human pharmacokinetic study

Grapefruit juice (GFJ) interacts with a number of drugs, and can alter pharmacokinetics parameters of the drugs. As for these interactions, most reports have focused on the elevation of drug bioavailability by GFJ, but a few recent reports have indicated that GFJ reduced the absorption of drugs not metabolized by cytochrome P450 (CYP). The predominant mechanisms of GFJ-drug interaction are thought to be due primarily to the inhibition of intestinal CYP3A4 activity without an apparent inhibition of hepatic CYP3A4. GFJ is also an inhibitor of P-glycoprotein, an efflux pump in intestinal cell wall enterocytes, although clinical support for this mechanism remains unclear. In addition, GFJ has recently been shown to be a potent in vitro inhibitor of the organic anion-transporting polypeptides (OATP) 1A2, intestinal uptake transporters of structurally anionic drugs. It is therefore noteworthy that intestinal OATPs-mediated drug uptake are reduced by GFJ. The furanocoumarins, major active ingredients in relation to GFJ-drug interaction, were detected in fresh grapefruit, commercial GFJ and seville orange juice. However, the specific furanocoumarins responsible for the inhibition of CYP3A4 activity in in vitro study have yet to be fully determined and corresponded with GFJ effects in in vivo study. This article summarizes our data concerning GFJ-drug interaction and many GFJ-drug effects, and reviews the mechanism of this interaction, possible active ingredients and clinical implications.     

Impact of curcumin-induced changes in P-glycoprotein and CYP3A expression on the pharmacokinetics of peroral celiprolol and midazolam in rats.

The aim of this study was to evaluate whether curcumin could modulate P-glycoprotein (P-gp) and CYP3A expression, and in turn modify the pharmacokinetic profiles of P-gp and CYP3A substrates in male Sprague-Dawley rats. Intragastric gavage of the rats with 60 mg/kg curcumin for 4 consecutive days led to a down-regulation of the intestinal P-gp level. There was a concomitant upregulation of hepatic P-gp level, but the renal P-gp level was unaffected. Curcumin also attenuated the CYP3A level in the small intestine but induced CYP3A expression in the liver and kidney. Regular curcumin consumption also caused the C(max) and area under the concentration-time curve (AUC(0-8) and total AUC) of peroral celiprolol (a P-gp substrate with negligible cytochrome P450 metabolism) at 30 mg/kg to increase, but the apparent oral clearance (CL(oral)) of the drug was reduced. Similarly, rats treated with curcumin for 4 consecutive days showed higher AUC (AUC(0-4) and total AUC) and lower CL(oral) for peroral midazolam (a CYP3A substrate that does not interact with the P-gp) at 20 mg/kg in comparison with vehicle-treated rats. In contrast, curcumin administered 30 min before the respective drug treatments did not significantly modify the pharmacokinetic parameters of the drugs. Analysis of the data suggests that the changes in the pharmacokinetic profiles of peroral celiprolol and midazolam in the rat model were contributed mainly by the curcumin-mediated down-regulation of intestinal P-gp and CYP3A protein levels, respectively.     

天然维生素E对CYP3A4活性影响的体内研究

目的：阐明天然维生素E在体内对CYP3A4活性的影响。方法：12名健康男性志愿者,随机分两组,采用两阶段双周期交叉设计,一组口服天然维生素E600mg/d,另一组不服药,连续14d。在第15天两组均口服咪达唑仑7.5mg,在0、0.25、0.5、0.75、1、1.5、2、2.5、3、4、6、8、12、24h采外周静脉血5mL,洗脱4周交叉进行下阶段试验。采血后0.5h内进行离心和分离血浆,运用HPLC-MS-MS分别测定血浆中咪达唑仑总药物浓度、1-OH咪达唑仑药物浓度,分析比较不同处理组间药代动力学参数的差异。结果：天然维生素E服用组与对照组咪达唑仑及其代谢产物1-OH咪达唑仑的血浆药物浓度变化相近。天然维生素E服用组与对照组之间咪达唑仑及其代谢产物1-OH咪达唑仑的曲线下面积AUC0-24h和峰浓度Cmax数据相近（P〉0.05）。结论：在本试验中服用天然维生素E并未影响CYP3A4的体内活性。     

The CYP3A4*18 allele, the most frequent coding variant in asian populations, does not significantly affect the midazolam disposition in heterozygous individuals.

The objective of this study was to identify CYP3A4 variants in Koreans and to characterize their functional consequences in vitro and in vivo. Four single nucleotide polymorphisms were identified in 50 Koreans by direct DNA sequencing. In an additional genotyping using 248 subjects, CYP3A4(*)18 was confirmed as the most frequent coding variant in Koreans at 1.7%, and its frequency was similar to that of Asians, suggesting that CYP3A4(*)18 would be the highest coding variant in Asians. The recombinant CYP3A4.18 protein prepared in baculovirus expression system showed 67.4% lower Vmax and 1.8-fold higher K(m) for midazolam 1'-hydroxylation compared with the wild type. The mean values of Cmax and area under the concentration curve (AUC) in the CYP3A4(*)1/(*)18 and CYP3A5(*)1/(*)3 subjects (n = 8) were 63% and 32% higher than in CYP3A4(*)1/(*)1 and CYP3A5(*)1/(*)3 carriers (n = 8), respectively. Although the in vitro assay exhibited a significant reduction of the enzyme activity for midazolam, the in vivo differences associated with the CYP3A4(*)1/(*)18 tend to be low (P < 0.07 in Cmax and P < 0.09 in AUC). In summary, the heterozygous CYP3A4(*)1/(*)18 does not appear to cause a significant change of midazolam disposition in vivo; however, the clinical relevance of CYP3A4(*)18/(*)18 remains to be evaluated.     

Effects of furanocoumarins in Kampo extract-based medicines on rat intestinal absorption of CYP3A and P-glycoprotein substrate drugs in vivo

While a great deal of information of drug-drug interactions is known, most concern Western drugs. Relatively little is known of the interactions between Western drugs and traditional drugs such as Kampo extract medicines (Japanese medicines modified from traditional Chinese medicines). This study investigated the effects of the marketed Kampo extract medicines, Senkyu-cha-cho-san and Sokei-kakketsu-to, on the intestinal absorption of CYP or P-glycoprotein (P-gp) in vivo. Midazolam, a CYP3A substrate drug, or talinolol, a P-gp substrate drug, was orally administered to rats with each of these Kampo extract medicines. Senkyu-cha-chosan or Sokei-kakketsu-to administered as a standard regimen did not obviously affect Cmax and area under the curve (AUC) of midazolam, although both Kampo extract medicines contained notopterol, a potent CYP3A4 inhibitor in vitro. The results implied a lack of potent drug-drug interactions between both Kampo extract medicines and CYP3A substrate drugs. Concomitant administration of each Kampo extract medicine unexpectedly showed the tendency to decrease Cmax and AUC of talinolol. Decreased intestinal absorption of talinolol might be caused, not by the inhibition of P-gp, but by the inhibition of organic anion transporting peptides by both Kampo extract medicines.     

Interaction between grapefruit juice and hypnotic drugs: comparison of triazolam and quazepam

Objective: Grapefruit juice (GFJ) inhibits cytochrome P450 (CYP) 3A4 in the gut wall and increases blood concentrations of CYP3A4 substrates by the enhancement of oral bioavailability. The effects of GFJ on two benzodiazepine hypnotics, triazolam (metabolized by CYP3A4) and quazepam (metabolized by CYP3A4 and CYP2C9), were determined in this study. Methods: The study consisted of four separate trials in which nine healthy subjects were administered 0.25 mg triazolam or 15 mg quazepam, with or without GFJ. Each trial was performed using an open, randomized, cross-over design with an interval of more than 2 weeks between trials. Blood samples were obtained during the 24-h period immediately following the administration of each dose. Pharmacodynamic effects were determined by the digit symbol substitution test (DSST) and utilizing a visual analog scale. Results GFJ increased the plasma concentrations of both triazolam and quazepam and of the active metabolite of quazepam, 2-oxoquazepam. The area under the curve (AUC)(0–24) of triazolam significantly increased by 96% (p<0.05). The AUC(0–24) of quazepam (+38%) and 2-oxoquazepam (+28%) also increased; however, these increases were not significantly different from those of triazolam. GFJ deteriorated the performance of the subjects in the DSST after the triazolam dose (−11 digits at 2 h after the dose, p<0.05), but not after the quazepam dose. Triazolam and quazepam produced similar sedative-like effects, none of which were enhanced by GFJ. Conclusion These results suggest that the effects of GFJ on the pharmacodynamics of triazolam are greater than those on quazepam. These GFJ-related different effects are partly explained by the fact that triazolam is presystemically metabolized by CYP3A4, while quazepam is presystemically metabolized by CYP3A4 and CYP2C9.     

Integrated analysis on the physicochemical properties of dihydropyridine calcium channel blockers in grapefruit juice interactions

Concomitant consumption of grapefruit juice (GFJ) causes increases in the plasma concentration of a variety of drugs due to inhibition of intestinal CYP3A enzyme. Dihydropyridine calcium channel blockers belong to the category of drugs that are most prone to undergo such interaction. Increases in area under the plasma concentration-time curve (AUC) due to GFJ differ greatly depending on the dihydropyridine administered. Therefore, a meta-analysis of each dihydropyridine was performed based on available literature. The criteria for using a publication were: subjects were healthy adults, dihydropyridines were taken with GFJ concomitantly or within one hour after intake of the juice, and the control group administered water in place of GFJ. In these studies, the investigations on GFJ interactions with 13 dihydropyridines such as amlodipine, azelnidipine, benidipine, cilnidipine, efonidipine, felodipine, manidipine, nicardipine, nifedipine, nimodipine, nisoldipine, nitrendipine and pranidipine were reported. As a result of meta-analyses, statistically significant interactions were not identified in amlodipine. Next, correlation analyses between the physicochemical properties and interaction strengths of the dihydropyridines were performed to clarify the cause of the variation in the strengths that was dependent on the dihydropyridine. LogP, molecular weight, topological polar surface area (tPSA), molar refractivity, water diffusion, molecular volume, molecular density, molecular polarizability, and refractive index were calculated from the chemical structures. The interaction strength was defined as the logarithmic values of the increasing AUC ratio. The correlation analyses indicated a relationship of logP and tPSA with the interaction strengths. These findings suggest that the wide difference in the potency of interaction of each dihydropyridine may be explained by the presence of hydrophobic and electrostatic interactions between dihydropyridines and intestinal CYP3A enzyme.     

Effect of an oral contraceptive preparation containing ethinylestradiol and gestodene on CYP3A4 activity as measured by midazolam 1'-hydroxylation

AIMS: To characterize the effect of an oral contraceptive (OC) containing ethinylestradiol and gestodene on the activity of CYP3A4 in vivo as measured by the 1'-hydroxylation of midazolam. METHODS: In this randomised, double-blind, cross-over trial nine healthy female subjects received either a combined OC (30 microg ethinylestradiol and 75 microg gestodene) or placebo once daily for 10 days. On day 10, a single 7.5 mg dose of midazolam was given orally. Plasma concentrations of midazolam and 1'-hydroxymidazolam were determined up to 24 h and the effects of midazolam were measured with three psychomotor tests up to 8 h. RESULTS: The combined OC increased the mean AUC of midazolam by 21% (95% CI 2% to 40%; P = 0.03) and decreased that of 1'-hydroxymidazolam by 25% (95% CI 10% to 41%; P = 0.01), compared with placebo. The metabolic ratio (AUC of 1'-hydroxymidazolam/AUC of midazolam) was 36% smaller (95% CI 19% to 53%; P = 0.01) in the OC phase than in the placebo phase. There were no significant differences in the Cmax, tmax, t(1/2) or effects of midazolam between the phases. CONCLUSIONS: A combined OC preparation caused a modest reduction in the activity of CYP3A4, as measured by the 1'-hydroxylation of midazolam, and slightly increased the AUC of oral midazolam. This study suggests that, at the doses used, ethinylestradiol and gestodene have a relatively small effect on CYP3A4 activity in vivo.     

The effects of an oral contraceptive containing ethinyloestradiol and norgestrel on CYP3A activity

AIMS: To examine the effects of an oral contraceptive containing ethinyloestradiol and norgestrel on intestinal and hepatic CYP3A activity using midazolam as a probe substrate. METHODS: In a nonblinded sequential study, nine healthy women received simultaneous doses of intravenous midazolam (0.05 mg kg(-1)) and oral 15N3-midazolam (3 mg) on days 0, 4, 6, 8, and 14. On study day 5, Ovral(50 microg ethinyloestradiol/500 microg norgestrel) was administered for 10 days. Serum and urine samples were assayed for midazolam, 15N3-midazolam and metabolites by liquid chromatography-mass spectrometry. A Digit Symbol Substitution Test (DSST) was used to assess changes in the pharmacodynamic activity of midazolam. RESULTS: Moderate (% CV 26-46) interindividual variability in the pharmacokinetics of midazolam were observed. Compared with baseline, AUC(0,infinity)iv ratios (95% CIs) after 2, 4, and 10 days treatment with OC were 89% (79, 101), 96% (85, 109), and 88% (77, 99), respectively. The AUC(0,infinity)oral ratios (95% CIs) were 101% (82, 125), 105% (85, 130), and 114% (92, 141), respectively, after 2, 4, and 10 days OC treatment compared with baseline. Concomitant administration of the oral contraceptive, Ovral for 2, 4 or 10 days did not significantly alter the area under the curve, clearance, or half-life of midazolam after either oral or intravenous administration. No alterations in pharmacodynamic effects of midazolam were observed between treatment days. Mean DSST scores strongly correlated with mean total midazolam blood concentrations (r = -0.936). CONCLUSIONS: Administration of Ovral for 10 days had no impact on intestinal or hepatic CYP3A activity as determined by midazolam metabolism.     

临床治疗剂量氯氮平人体内代谢机制研究

目的本课题主要探讨临床治疗剂量氯氮平的代谢机制，为临床合理用药提供指导。方法15例精神分裂症男性住院病人，单用氯氮平治疗，达到稳态浓度后，采用自身前后对照设计试验，研究氟西汀抑制前后氯氮平及其代谢产物药代动力学参数的变化及其与酶活性的相关性。CYP1A2、CYP3A4和CYP2136的活性分别用咖啡因、咪达唑仑和右美沙芬探测。体内氯氮平及其代谢产物、咪哒唑仑及其代谢产物、尿中右美沙芬及其代谢产物用HPLC-MS测定。血中咖啡因及其代谢产物用HPlC-UV测定。数据用SPSS软件进行统计分析。结果氯氮平合用氟西汀后，氯氮平的Cmax、AUC0-24显著增加，t1/2增加趋势，但没有显著性差异。代谢产物去甲氯氮平的AUC0-24显著降低，Cmax和t1／2无显著性差异。N-氧化氯氮平的Cmax和AUC0-24显著性降低，t1／2没有显著性差异。合用氟西汀前后CYP1A2活性无差异，CYP3A4和CYP2D6活性显著降低。合用氟西汀前后CYP1A2活性分别与合用氟西汀前后氯氮平的AUC0-24以及去甲氯氮平的AUC0-24相关，与N-氧化氯氮平的AUC0-24无显著相关。CYP3A4和CYP2D6活性与氯氮平、去甲氯氮平、N-氧化氯氮平的AUC0-24均无显著相关，但是合用氟西汀后CYP3A4活性变化与N-氧化氯氮平的AUC0-24变化显著相关，CYP2D6活性变化与去甲氯氮平的AUC0-24变化显著相关。结论临床剂量氯氮平的代谢途径为去甲基化和N-氧化，其中去甲基代谢为主要代谢途径主要由CYP1A2催化。CYP3A4和CYP2D6不是催化CLZ代谢的主要酶，但是CYP3A4参与了CLZN-氧化代谢，CYP2D6参与去甲基代谢。     

Intestinal first pass metabolism of midazolam in liver cirrhosis --effect of grapefruit juice

AIMS: Grapefruit juice inhibits CYP3A4 in the intestinal wall leading to a reduced intestinal first pass metabolism and thereby an increased oral bioavailability of certain drugs. For example, it has been shown that the oral bioavailability of midazolam, a CYP3A4 substrate, increased by 52% in healthy subjects after ingestion of grapefruit juice. However, this interaction has not been studied in patients with impaired liver function. Accordingly, the effect of grapefruit juice on the AUC of midazolam and the metabolite alpha-hydroxymidazolam was studied in patients with cirrhosis of the liver. METHODS: An open randomized two-way crossover study was performed. Ten patients (3 females, 7 males) with liver cirrhosis based on biopsy or clinical criteria participated. Six patients had a Child-Pugh score of A, one B and three C. Tap water (200 ml) or grapefruit juice were consumed 60 and 15 min before midazolam (15 mg) was administered orally. Plasma samples were analysed for midazolam and alpha-hydroxymidazolam. RESULTS: Grapefruit juice increased the AUC of midazolam by 106% (16, 197%) (mean (95% confidence interval)) and the AUC of the metabolite alpha-hydroxymidazolam decreased to 25% (12, 37%) (P<0.05 for both). The ratio of the AUCs of the metabolite alpha-hydroxymidazolam to midazolam decreased from 0.77 (0.46, 1.07) to 0.11 (0.05, 0.19) (P<0.05). t(1/2) remained unaltered for both drug and metabolite. Midazolam C(max), t(max), and alpha-hydroxymidazolam t(max) increased, but these changes were not statistically significant, whereas C(max) of the metabolite decreased to 30% (14, 47%) (P<0.05). CONCLUSIONS: A marked interaction between oral midazolam and grapefruit juice was found and the data are consistent with a reduced first-pass metabolism of midazolam. This is likely to occur at the intestinal wall inhibition of CYP3A4 activity by grapefruit juice. These results indicate that patients with liver cirrhosis are more dependent on the intestine for metabolism of CYP3A4 substrates than subjects with normal liver function.     

Midazolam metabolism in cytochrome P450 3A knockout mice can be attributed to up-regulated CYP2C enzymes

The cytochrome P450 3A (CYP3A) enzymes represent one of the most important drug-metabolizing systems in humans. Recently, our group has generated cytochrome P450 3A knockout mice to study this drug-handling system in vivo. In the present study, we have characterized the Cyp3a knockout mice by studying the metabolism of midazolam, one of the most widely used probes to assess CYP3A activity. We expected that the midazolam metabolism would be severely reduced in the absence of CYP3A enzymes. We used hepatic and intestinal microsomal preparations from Cyp3a knockout and wild-type mice to assess the midazolam metabolism in vitro. In addition, in vivo metabolite formation was determined after intravenous administration of midazolam. We were surprised to find that our results demonstrated that there is still marked midazolam metabolism in hepatic (but not intestinal) microsomes from Cyp3a knockout mice. Accordingly, we found comparable amounts of midazolam as well as its major metabolites in plasma after intravenous administration in Cyp3a knockout mice compared with wild-type mice. These data suggested that other hepatic cytochrome P450 enzymes could take over the midazolam metabolism in Cyp3a knockout mice. We provide evidence that CYP2C enzymes, which were found to be up-regulated in Cyp3a knockout mice, are primarily responsible for this metabolism and that several but not all murine CYP2C enzymes are capable of metabolizing midazolam to its 1'-OH and/or 4-OH derivatives. These data illustrate interesting compensatory changes that may occur in Cyp3a knockout mice. Such flexible compensatory interplay between functionally related detoxifying systems is probably essential to their biological role in xenobiotic protection.