Inhibition of human CYP3A catalyzed 1'-hydroxy midazolam formation by ketoconazole, nifedipine, erythromycin, cimetidine, and nizatidine

Short-Term Intravenous Administration of Carbon Nano-Onions is Non-Toxic in Female Mice

Background

A nanoscale drug carrier could have a variety of therapeutic and diagnostic uses provided that the carrier is biocompatible in vivo. Carbon nano-onions (CNOs) have shown promising results as a nanocarrier for drug delivery. However, the systemic effect of CNOs in rodents is unknown. Therefore, we investigated the toxicity of CNOs following intravenous administration in female BALB/c mice.

Results

Single or repeated administration of oxi-CNOs (125, 250 or 500 µg) did not affect mouse behavior or organ weight and there was also no evidence of hepatotoxicity or nephrotoxicity. Histological examination of organ slices revealed a significant dose-dependent accumulation of CNO aggregates in the spleen, liver and lungs (p<0.05, ANOVA), with a trace amount of aggregates appearing in the kidneys. However, CNO aggregates in the liver did not affect CYP450 enzymes, as total hepatic CYP450 as well as CYP3A catalytic activity, as meased by erythromycin N-demethylation, and protein levels showed no significant changes between the treatment groups compared to vehicle control. CNOs also failed to act as competitive inhibitors of CYP3A in vitro in both mouse and human liver microsomes. Furthermore, CNOs did not cause oxidative stress, as indicated by the unchanged malondialdehyde levels and superoxide dismutase activity in liver microsomes and organ homogenates.

Conclusion

This study provides the first evidence that short-term intravenous administration of oxi-CNOs is non-toxic to female mice and thus could be a promising novel and safe drug carrier.

Inhibition of CYP3A-mediated Midazolam Metabolism by <i>Kaempferia Parviflora</i>

Kaempferia parviflora (KP) extract has recently attracted attention in Japan as a dietary supplement; however, there is little information regarding food-drug interactions (FDIs). The current study was conducted to clarify the FDI of KP extract via inhibition of cytochrome P450 3A (CYP3A), a typical drug-metabolizing enzyme. The inhibitory effects of KP extract and its main ingredients, 5,7-dimethoxyflavone (5,7-DMF) and 3,5,7,3’,4’-pentamethoxyflavone (3,5,7,3’,4’-PMF), on CYP3A-mediated midazolam 1’-hydroxylation (MDZ 1’-OH) activity were investigated in human liver microsomes. In addition, the effect of a single oral treatment with KP extract (135 mg/kg) on oral MDZ (15 mg/kg) metabolism was investigated in rats. Serum MDZ concentration was analyzed and pharmacokinetic parameters were compared with the control group. KP extract competitively inhibited MDZ 1’-OH activity with an inhibition constant value of 78.14 µg/ml, which was lower than the estimated concentration in the small intestine after ingestion. Furthermore, KP extract, 5,7-DMF, and 3,5,7,3’,4’-PMF inhibited the activity in a time-, NADPH-, and concentration-dependent manner. In vivo study showed that administration of KP extract to rats 2 h before MDZ significantly increased the area under the serum concentration-time curve and the maximum concentration of MDZ significantly by 2.3- and 1.9- fold, respectively (p < 0.05). Conversely, administration of MDZ 18 h after KP extract treatment displayed a weaker effect. These results suggest that KP extract competitively inhibits CYP3A-mediated MDZ metabolism, and that this inhibition may be time-dependent but not irreversible. This work suggests an FDI through CYP3A inhibition by KP extract.

Characterization of the CYP3A4 Enzyme Inhibition Potential of Selected Flavonoids

Acacetin, apigenin, chrysin, and pinocembrin are flavonoid aglycones found in foods such as parsley, honey, celery, and chamomile tea. Flavonoids can act as substrates and inhibitors of the CYP3A4 enzyme, a heme containing enzyme responsible for the metabolism of one third of drugs on the market. The aim of this study was to investigate the inhibitory effect of selected flavonoids on the CYP3A4 enzyme, the kinetics of inhibition, the possible covalent binding of the inhibitor to the enzyme, and whether flavonoids can act as pseudo-irreversible inhibitors. For the determination of inhibition kinetics, nifedipine oxidation was used as a marker reaction. A hemochromopyridine test was used to assess the possible covalent binding to the heme, and incubation with dialysis was used in order to assess the reversibility of the inhibition. All the tested flavonoids inhibited the CYP3A4 enzyme activity. Chrysin was the most potent inhibitor: IC₅₀ = 2.5 ± 0.6 µM, K_i = 2.4 ± 1.0 µM, k_inact = 0.07 ± 0.01 min⁻¹, k_inact/K_i = 0.03 min⁻¹ µM⁻¹. Chrysin caused the highest reduction of heme (94.5 ± 0.5% residual concentration). None of the tested flavonoids showed pseudo-irreversible inhibition. Although the inactivation of the CYP3A4 enzyme is caused by interaction with heme, inhibitor-heme adducts could not be trapped. These results indicate that flavonoids have the potential to inhibit the CYP3A4 enzyme and interact with other drugs and medications. However, possible food–drug interactions have to be assessed clinically.

Midazolam oxidation in cattle liver microsomes: The role of cytochrome P450 3A

In humans, the cytochrome P450 3A (CYP3A) subfamily is involved in midazolam (MDZ) biotransformation into 1'- and 4-hydroxy metabolites, and the former serves as a probe for CYP3A catalytic activity. In veterinary species is still crucial to identify enzyme- and species-specific CYP substrates; thus, the aim of this study was to characterize MDZ oxidation in cattle liver. A HPLC-UV method was used to measure 1'- and 4-hydroxy MDZ (1'- and 4-OHMDZ, respectively) formation in cattle liver microsomes and assess the role of CYP3A by an immunoinhibition study. Moreover, MDZ hydroxylation was evaluated in 300 cattle liver samples and results were correlated with testosterone hydroxylation. Formation of both metabolites conformed to a single-enzyme Michaelis-Menten kinetics. Values of V_max and K_m were 0.67 nmol/min/mg protein and 6.16 μM for 4-OHMDZ, and 0.06 nmol/min/mg protein and 10.08 μM for 1'-OHMDZ. An anti-rat CYP3A1 polyclonal antibody inhibited up to 50% and 94% 1'- and 4-OHMDZ formation, respectively. MDZ oxidation in liver microsomes was poorly correlated with testosterone hydroxylation. In conclusion, cattle metabolized MDZ to 1'-OHMDZ and 4-OHMDZ. The immunoinhibition results indicated a major contribution of CYP3As to 4-OHMDZ formation and the involvement of other CYPs in 1'-OHMDZ production, paving the way for further investigations.

A Mechanistic, Enantioselective, Physiologically Based Pharmacokinetic Model of Verapamil and Norverapamil, Built and Evaluated for Drug-Drug Interaction Studies

The calcium channel blocker and antiarrhythmic agent verapamil is recommended by the FDA for drug–drug interaction (DDI) studies as a moderate clinical CYP3A4 index inhibitor and as a clinical Pgp inhibitor. The purpose of the presented work was to develop a mechanistic whole-body physiologically based pharmacokinetic (PBPK) model to investigate and predict DDIs with verapamil. The model was established in PK-Sim^®, using 45 clinical studies (dosing range 0.1–250 mg), including literature as well as unpublished Boehringer Ingelheim data. The verapamil R- and S-enantiomers and their main metabolites R- and S-norverapamil are represented in the model. The processes implemented to describe the pharmacokinetics of verapamil and norverapamil include enantioselective plasma protein binding, enantioselective metabolism by CYP3A4, non-stereospecific Pgp transport, and passive glomerular filtration. To describe the auto-inhibitory and DDI potential, mechanism-based inactivation of CYP3A4 and non-competitive inhibition of Pgp by the verapamil and norverapamil enantiomers were incorporated based on in vitro literature. The resulting DDI performance was demonstrated by prediction of DDIs with midazolam, digoxin, rifampicin, and cimetidine, with 21/22 predicted DDI AUC ratios or C_trough ratios within 1.5-fold of the observed values. The thoroughly built and qualified model will be freely available in the Open Systems Pharmacology model repository to support model-informed drug discovery and development.

Medical Management of Cushing's Syndrome: Current and Emerging Treatments

Endogenous Cushing's syndrome is a chronic disease associated with increased morbidity and mortality if not appropriately treated. Recurrence and/or persistence of hypercortisolemia after surgical treatment, especially for Cushing's disease, are high, and long-term medical treatment is used to decrease cortisol levels and risk of metabolic comorbidities. Medical treatment is also often required while waiting for radiation effects to take place. In some cases, severe or life-threatening hypercortisolism must be urgently and medically treated, via intravenous medications or with combination therapy, before patients can undergo surgery. In the last decade, medical treatment has progressed from a few steroidogenesis inhibitors to three novel drug groups: new inhibitors for steroidogenic enzymes with possibly fewer side effects, pituitary-directed drugs that aim to inhibit the pathophysiological pathways of Cushing's disease, and glucocorticoid receptor antagonists that block cortisol's action. Understanding the pathophysiology of Cushing's syndrome has also led to the identification of potential targets that may decrease adrenocorticotrophic hormone and/or cortisol excess, and/or decrease tumor cell proliferation, and induce senescence or apoptosis. We provide here a review of current and near-future medical options to treat Cushing's syndrome, and discuss updates on clinical trials and the efficacy and safety of novel or in-development drugs, as well as future potential targets.

The critical role of porcine cytochrome P450 3A46 in the bioactivation of aflatoxin B1

Aflatoxin B₁ (AFB₁) is bioactivated by cytochrome P450 (CYP) 3A isoforms in humans to generate the highly reactive epoxide intermediate AFB₁-8,9-epoxide (AFBO), causing hepatotoxicity and hepatocarcinoma. Due to the unavoidable contamination in their feed, pigs are more likely to be exposed to AFB₁ and indirectly harm human health. Therefore, identifying the porcine CYP3A isoforms involved in AFB₁-8,9-epoxidation is critical. In this study, we used codon optimization and N-terminal coding sequence modification to modify a CYP3A46 recombinant protein that exhibits good structure and catalytic activities and revealed its strong AFB₁-8,9-epoxidase activity for the first time. Site-directed mutagenesis, kinetics and docking analyses were performed and demonstrated that residues Phe-108, Ser-119, Phe-215, Phe-304 and Thr-309 play important roles in AFB₁-8,9-epoxidation and its responsiveness to α-naphthoflavone. Interestingly, we uncovered the dual and reverse roles of Phe-304 in CYP3A46, CYP3A5 and CYP3A4 in AFB₁ oxidation. Unlike the π-π interaction between the Phe-304 phenyl of CYP3A4 and the AFB₁ aromatic ring, Phe-304 of CYP3A46 may function to provide steric hindrance to bind AFB₁. Phe-108 and Phe-215 could stabilize AFB₁ with a potentially productive orientation through van der Waals interactions with AFB₁. Ser-119 and Thr-309 are likely to function to form H-bonds with AFB₁. This study broadens our knowledge of AFB₁ bioactivation in pigs and may contribute to reduce the deleterious effects of AFB₁ in pigs and humans.

Effect of Benifuuki Tea on Cytochrome P450-mediated Metabolic Activity in Rats

BACKGROUND/AIM

Benifuuki tea has recently been used as an alternative therapy for pollinosis, and it may be consumed with pharmaceutical drugs. This study aimed to examine cytochrome P450 (CYP)-mediated food-drug interactions with Benifuuki tea in rats.

MATERIALS AND METHODS

The inhibitory effects of Benifuuki tea and (-)-epigallocatechin-3-O-(3-O-methyl) gallate (EGCG3"Me) on CYP activities were evaluated in vitro. Midazolam pharmacokinetics was investigated after two treatments with Benifuuki tea. In an ex vivo study, CYP activities were determined after 1-week-treatment with the tea.

RESULTS

Benifuuki tea and EGCG3"Me inhibited CYP2D and CYP3A activities in a concentration-dependent manner in vitro. However, MDZ metabolism did not change by Benifuuki treatment in vivo and ex vivo. In contrast, CYP2D activity was decreased ex vivo.

CONCLUSION

Normal intake of Benifuuki tea is not likely to cause food-drug interactions by CYP3A inhibition or induction. In contrast, Benifuuki tea consumption may lead to food-drug interactions through the inhibition of CYP2D.

Drug interactions of meglitinide antidiabetics involving CYP enzymes and OATP1B1 transporter

Meglitinides such as repaglinide and nateglinide are useful to treat type 2 diabetes patients who follow a flexible lifestyle. They are short-acting insulin secretagogues and are associated with less risk of hypoglycemia, weight gain and chronic hyperinsulinemia compared with sulfonylureas. Meglitinides are the substrates of cytochrome P450 (CYP) enzymes and organic anion transporting polypeptide 1B1 (OATP1B1 transporter) and the coadministration of the drugs affecting them will result in pharmacokinetic drug interactions. This article focuses on the drug interactions of meglitinides involving CYP enzymes and OATP1B1 transporter. To prevent the risk of hypoglycemic episodes, prescribers and pharmacists must be aware of the adverse drug interactions of meglitinides.

Physiologically based pharmacokinetic modeling of disposition and drug-drug interactions for atorvastatin and its metabolites

Atorvastatin is the most commonly used of all statins to lower cholesterol. Atorvastatin is extensively metabolized in both gut and liver to produce several active metabolites. The purpose of the present study is to develop a physiologically based pharmacokinetic (PBPK) model for atorvastatin and its two primary metabolites, 2-hydroxy-atorvastatin acid and atorvastatin lactone, using in vitro and in vivo data. The model was used to predict the pharmacokinetic profiles and drug-drug interaction (DDI) effect for atorvastatin and its metabolites in different DDI scenarios. The predictive performance of the model was assessed by comparing predicted results to observed data after coadministration of atorvastatin with different medications such as itraconazole, clarithromycin, cimetidine, rifampin and phenytoin. This population based PBPK model was able to describe the concentration-time profiles of atorvastatin and its two metabolites reasonably well in the absence or presence of those drugs at different dose regimens. The predicted maximum concentration (Cmax), area under the concentration-time curve (AUC) values and between-phase ratios were in good agreement with clinically observed data. The model has also revealed the importance of different metabolic pathways on the disposition of atorvastatin metabolites. This PBPK model can be utilized to assess the safety and efficacy of atorvastatin in the clinic. This study demonstrated the feasibility of applying PBPK approach to predict the DDI potential of drugs undergoing complex metabolism.

Possible drug-drug interaction in dogs and cats resulted from alteration in drug metabolism: A mini review

Pharmacokinetic drug-drug interactions (in particular at metabolism) may result in fatal adverse effects in some cases. This basic information, therefore, is needed for drug therapy even in veterinary medicine, as multidrug therapy is not rare in canines and felines. The aim of this review was focused on possible drug-drug interactions in dogs and cats. The interaction includes enzyme induction by phenobarbital, enzyme inhibition by ketoconazole and fluoroquinolones, and down-regulation of enzymes by dexamethasone. A final conclusion based upon the available literatures and author's experience is given at the end of the review.

Integration of in vitro binding mechanism into the semiphysiologically based pharmacokinetic interaction model between ketoconazole and midazolam

In vitro screening for drug–drug interactions is an integral component of drug development, with larger emphasis now placed on the use of in vitro parameters to predict clinical inhibition. However, large variability exists in K_i reported for ketoconazole with midazolam, a model inhibitor–substrate pair for CYP3A. We reviewed the literature and extracted K_i for ketoconazole as measured by the inhibition of hydroxymidazolam formation in human liver microsomes. The superset of data collected was analyzed for the impact of microsomal binding, using Langmuir and phase equilibrium binding models, and fitted to various inhibition models: competitive, noncompetitive, and mixed. A mixed inhibition model with binding corrected by an independent binding model was best able to fit the data (K_ic = 19.2 nmol/l and K_in = 39.8 nmol/l) and to predict clinical effect of ketoconazole on midazolam area under the concentration–time curve. The variability of reported K_i may partially be explained by microsomal binding and choice of inhibition model.

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.

Chemical inhibitors of cytochrome P450 isoforms in human liver microsomes: a re-evaluation of P450 isoform selectivity

The majority of marketed small-molecule drugs undergo metabolism by hepatic Cytochrome P450 (CYP) enzymes (Rendic 2002). Since these enzymes metabolize a structurally diverse number of drugs, metabolism-based drug-drug interactions (DDIs) can potentially occur when multiple drugs are coadministered to patients. Thus, a careful in vitro assessment of the contribution of various CYP isoforms to the total metabolism is important for predicting whether such DDIs might take place. One method of CYP phenotyping involves the use of potent and selective chemical inhibitors in human liver microsomal incubations in the presence of a test compound. The selectivity of such inhibitors plays a critical role in deciphering the involvement of specific CYP isoforms. Here, we review published data on the potency and selectivity of chemical inhibitors of the major human hepatic CYP isoforms. The most selective inhibitors available are furafylline (in co-incubation and pre-incubation conditions) for CYP1A2, 2-phenyl-2-(1-piperidinyl)propane (PPP) for CYP2B6, montelukast for CYP2C8, sulfaphenazole for CYP2C9, (-)-N-3-benzyl-phenobarbital for CYP2C19 and quinidine for CYP2D6. As for CYP2A6, tranylcypromine is the most widely used inhibitor, but on the basis of initial studies, either 3-(pyridin-3-yl)-1H-pyrazol-5-yl)methanamine (PPM) and 3-(2-methyl-1H-imidazol-1-yl)pyridine (MIP) can replace tranylcypromine as the most selective CYP2A6 inhibitor. For CYP3A4, ketoconazole is widely used in phenotyping studies, although azamulin is a far more selective CYP3A inhibitor. Most of the phenotyping studies do not include CYP2E1, mostly because of the limited number of new drug candidates that are metabolized by this enzyme. Among the inhibitors for this enzyme, 4-methylpyrazole appears to be selective.

Evaluation of the pharmacokinetic interaction of midazolam with ursodeoxycholic acid, ketoconazole and dexamethasone by brain benzodiazepine receptor occupancy

Objectives

To clarify whether alterations in midazolam pharmacokinetics resulting from changes in cytochrome P450 3A (CYP3A) activity lead to changes in its pharmacodynamic effects, benzodiazepine receptor occupancy was measured in the brain of rats after oral administration of midazolam.

Methods

Receptor occupancy was measured by radioligand binding assay in rats pretreated with ursodeoxycholic acid (UDCA), ketoconazole and dexamethasone, and the plasma concentration of midazolam was simultaneously determined.

Key findings

There was a significant increase in the apparent dissociation constant and decrease in the maximum number of binding sites for specific [3H]flunitrazepam binding after oral administration of midazolam at pharmacologically relevant doses, suggesting that midazolam binds significantly to brain benzodiazepine receptors. Pretreatment with UDCA significantly enhanced the binding. This correlated well with significant enhancement by UDCA of the plasma midazolam concentration. The brain benzodiazepine receptor binding of oral midazolam was significantly enhanced by pretreatment with ketoconazole, a potent inhibitor of CYP3A, whereas it was significantly reduced by treatment with dexamethasone, an inducer of this enzyme. These effects paralleled changes in the plasma concentration of midazolam.

Conclusions

The results indicate that pharmacokinetic changes such as altered CYP3A activity significantly influence the pharmacodynamic effect of midazolam by affecting occupancy of benzodiazepine receptors in the brain. They also suggest in-vivo or ex-vivo time-dependent measurements of receptor occupancy by radioligand binding assay to be a tool for elucidating the pharmacokinetic interaction of benzodiazepines with other agents in pre-clinical and clinical evaluations.

A Bayesian meta-analysis on published sample mean and variance pharmacokinetic data with application to drug-drug interaction prediction

In drug-drug interaction (DDI) research, a two-drug interaction is usually predicted by individual drug pharmacokinetics (PK). Although subject-specific drug concentration data from clinical PK studies on inhibitor or inducer and substrate PK are not usually published, sample mean plasma drug concentrations and their standard deviations have been routinely reported. Hence there is a great need for meta-analysis and DDI prediction using such summarized PK data. In this study, an innovative DDI prediction method based on a three-level hierarchical Bayesian meta-analysis model is developed. The three levels model sample means and variances, between-study variances, and prior distributions. Through a ketoconazle-midazolam example and simulations, we demonstrate that our meta-analysis model can not only estimate PK parameters with small bias but also recover their between-study and between-subject variances well. More importantly, the posterior distributions of PK parameters and their variance components allow us to predict DDI at both population-average and study-specific levels. We are also able to predict the DDI between-subject/study variance. These statistical predictions have never been investigated in DDI research. Our simulation studies show that our meta-analysis approach has small bias in PK parameter estimates and DDI predictions. Sensitivity analysis was conducted to investigate the influences of interaction PK parameters, such as the inhibition constant Ki, on the DDI prediction.

Risk management of simvastatin or atorvastatin interactions with CYP3A4 inhibitors

BACKGROUND

Co-administration of cytochrome P450 (CYP) 3A4 inhibitors with simvastatin or atorvastatin is associated with increased risk of developing myopathy or rhabdomyolysis.

OBJECTIVE

To detect co-prescriptions of CYP3A4 inhibitors with simvastatin or atorvastatin in community pharmacies and assess the risk-preventive actions taken by the prescribing physicians who were alerted about the co-prescription by the pharmacist.

METHODS

This naturalistic study was performed during four separate 6-week periods in 2004 and 2005, and involved 110 Norwegian community pharmacists (25-30 in each period). Co-prescription of the selected CYP3A4 inhibitors diltiazem, verapamil, clarithromycin, erythromycin, fluconazole, itraconazole and ketoconazole with either simvastatin or atorvastatin was detected with the aid of a simple computer programme. In instances where the pharmacist alerted the prescribing physician about the co-prescription, information on possible strategies to minimize the risk associated with the interaction was also provided. Odds ratios (ORs) were estimated to describe the associations between prescription variables and frequencies of physician information and prescription change, respectively.

RESULTS

In total, 245 co-prescriptions of CYP3A4 inhibitors with simvastatin (134 events) or atorvastatin (111) were detected. Diltiazem (86 events), verapamil (72), erythromycin (48) and clarithromycin (29) were the most commonly co-prescribed CYP3A4 inhibitors. Physicians were informed in 168 out of 245 cases (68.6%). The prescription was subsequently changed in 100 out of 168 cases (59.5%). Another 50 physicians (29.8%) responded that they would consult the patient and monitor potential adverse effects, while only 18 physicians (10.7%) replied that they had already managed the interactions or considered the issue as irrelevant. The adjusted OR for the informing of the physician was 1.89 (95% CI 0.98, 3.63) in patients receiving a daily HMG-CoA reductase inhibitor ('statin') dose of >or=40 mg compared with patients receiving a statin dose of <40 mg/day. The adjusted OR for prescription change was 4.98 (95% CI 2.36, 10.52) if co-prescription was detected prior to the initiation of concurrent use compared with if it was detected during concurrent use.

CONCLUSION

Nine out of ten physicians changed prescriptions or monitored potential adverse effects when informed by community pharmacists about the risk associated with co-prescription of CYP3A4 inhibitors with simvastatin or atorvastatin. This suggests that an important risk factor for myotoxicity due to these statins could be minimized through interdisciplinary co-operation.

Identification of human liver cytochrome P450 enzymes involved in the metabolism of SCH 351125, a CCR5 antagonist

The identification and relative contribution of human cytochrome P450 enzyme(s) involved in the metabolism of SCH 351125 were investigated. In human liver microsomes, O-deethylation was the major metabolic pathway, whereas aromatization of a piperidine ring to pyridine and the reduction of the N-oxide moiety were minor routes. Recombinant human CYP3A4 and CYP2C9 both exhibited catalytic activity with respect to the formation of rotameric O-deethylated metabolites (M12, M13), the metabolites resulting from aromatization (M22/M24) and N-oxide reduction (M31). Using the relative activity factor (RAF) approach, the relative contributions of CYP3A4 and CYP2C9 to M13 formation were estimated to be 76 and 24%, respectively. There was a high correlation (r>0.96) between the rate of formation of M12 and M13 and 6 beta-hydroxylation of testosterone catalysed by CYP3A4/5. Ketoconazole (2microM) and CYP3A4/5-specific inhibitory monoclonal antibody inhibited the formation of M12 and M13 from human liver microsomes by approximately 60 and 71%, respectively. The results demonstrate that the in vitro metabolism of SCH 351125 is mediated primarily via CYP3A4 and that CYP2C9 plays a minor role. Clinical study designs should encompass these enzymology data to address any potential drug interactions.

Model for the drug–drug interaction responsible for CYP3A enzyme inhibition. II: establishment and evaluation of dexamethasone-pretreated female rats

1. Cytochrome P450 (CYP) 3A catalysis of testosterone 6beta-hydroxylation in female rat liver microsomes was significantly induced, then reached a plateau level after pretreatment with 80 mg kg(-1) day(-1) dexamethasone (DEX) for 3 days. 2. Midazolam was mainly metabolized by CYP3A in DEX-treated female rat liver microsomes from an immuno-inhibition study, and the apparent K(m) was 1.8 microM, similar to that in human microsomes. 3. Ketoconazole and erythromycin, typical CYP3A inhibitors, demonstrated extensive inhibition of midazolam metabolism in DEX-treated female rat liver microsomes, and the apparent K(i) values were 0.088 and 91.2 microM, respectively. The values were similar to those in humans, suggesting that DEX-treated female rat liver microsomes have properties similar to those of humans. 4. After oral administration of midazolam, the plasma midazolam concentration in DEX-treated female rats significantly decreased compared with control female rats. The area under the plasma concentration curve (AUC) and elimination half-life were one-11th and one-20th of those of control female rats, respectively. 5. Using DEX-treated female rats, the effect of CYP3A inhibitors on midazolam pharmacokinetics was evaluated. The AUC and maximum concentration in plasma (C(max)) increased when ketoconazole was co-administered with midazolam. 6. It was shown that the drug-drug interaction that occurs in vitro is also observed in vivo after oral administration of midazolam. In conclusion, the DEX-treated female rat could be a useful model for evaluating drug-drug interactions based on CYP3A enzyme inhibition.

Biotransformation of 3-amino-5,6,7,8-tetrahydro-2-{4-[4-(quinolin-2-yl)piperazin-1-yl]butyl}quinazolin-4(3H)-one (TZB-30878), a novel 5-hydroxytryptamine (5-HT)1A agonist/5-HT3 antagonist, in human hepatic cytochrome P450 enzymes

3-Amino-5,6,7,8-tetrahydro-2-{4-[4-(quinolin-2-yl)piperazin-1-yl]-butyl}quinazolin-4(3H)-one (TZB-30878), a novel 5-hydroxytryptamine (5-HT)(1A) agonist/5-HT(3) antagonist, is currently under development for the treatment of irritable bowel syndrome. The objective of this investigation was to obtain information on the biotransformation of TZB-30878. This compound has quinazoline, piperazine, and quinoline rings. Metabolites of [quinazoline-2-(14)C]TZB-30878 were determined using radio high-performance liquid chromatography on samples obtained after incubation with human hepatic microsomes. Eight metabolites were detected in the microsomal incubation mixture, and their structures were proposed by mass spectrometry techniques using TZB-30878 and two stable labeled TZB-30878 analogs, [quinoline-deuterium (D)(6)]TZB-30878 and [piperazin-D(8)]TZB-30878. Liquid chromatography/tandem mass spectrometry analyses suggested that the eight metabolites consisted of a cyclic metabolite (M6), four hydroxylated metabolites (M1, M2, M3, and M4) (three on quinoline ring and one on quinazoline ring), a deaminated metabolite (M5), and two metabolites (M7 and M8) that were presumably intermediates leading to the formation of the cyclic metabolite M6. Hydroxylation sites in the quinoline and quinazoline rings were predicted by electron density calculations and confirmed by comparison with authentic standards. To the best of our knowledge, N-deamination by microsomes leading to the formation of M5 appears to be novel. In addition, in vitro experiments in human liver microsomes with cytochrome P450 (P450)-specific inhibitors revealed that CYP3A4 was the major enzyme responsible for the metabolism of TZB-30878. Other P450 enzymes, such as a CYP2D6, played a minor role in its metabolism.

Identification of human liver cytochrome P450 enzymes involved in biotransformation of vicriviroc, a CCR5 receptor antagonist

Vicriviroc (SCH 417690), a CCR5 receptor antagonist, is currently under investigation for the treatment of human immunodeficiency virus infection. The objective of this study was to identify human liver cytochrome P450 enzyme(s) responsible for the metabolism of vicriviroc. Human liver microsomes metabolized vicriviroc via N-oxidation (M2/M3), O-demethylation (M15), N,N-dealkylation (M16), N-dealkylation (M41), and oxidation to a carboxylic acid metabolite (M35b/M37a). Recombinant human CYP3A4 catalyzed the formation of all these metabolites, whereas CYP3A5 catalyzed the formation of M2/M3 and M41. CYP2C9 only catalyzed the formation of M15. There was a high correlation between the rates of formation of M2/M3, M15, and M41, which was determined using 10 human liver microsomal samples and testosterone 6beta-hydroxylation catalyzed by CYP3A4/5 (r > or = 0.91). Ketoconazole and azamulin (inhibitors of CYP3A4) were potent inhibitors of the formation of M2/M3, M15, M41, and M35b/M37a from human liver microsomes. A CYP3A4/5-specific monoclonal antibody (1 microg/microg of protein) inhibited the formation of all metabolites from human liver microsomes by 86 to 100%. The results of this study suggest that formation of the major vicriviroc metabolites in human liver microsomes is primarily mediated via CYP3A4. CYP2C9 and CYP3A5 most likely play a minor role in the biotransformation of this compound. These enzymology data will provide guidance to design clinical studies to address any potential drug-drug interactions.

Drug-induced changes in P450 enzyme expression at the gene expression level: a new dimension to the analysis of drug-drug interactions

Drug-drug interactions (DDIs) caused by direct chemical inhibition of key drug-metabolizing cytochrome P450 enzymes by a co-administered drug have been well documented and well understood. However, many other well-documented DDIs cannot be so readily explained. Recent investigations into drug and other xenobiotic-mediated expression changes of P450 genes have broadened our understanding of drug metabolism and DDI. In order to gain additional information on DDI, we have integrated existing information on drugs that are substrates, inhibitors, or inducers of important drug-metabolizing P450s with new data on drug-mediated expression changes of the same set of cytochrome P450s from a large-scale microarray gene expression database of drug-treated rat tissues. Existing information on substrates and inhibitors has been updated and reorganized into drug-cytochrome P450 matrices in order to facilitate comparative analysis of new information on inducers and suppressors. When examined at the gene expression level, a total of 119 currently marketed drugs from 265 examined were found to be cytochrome P450 inducers, and 83 were found to be suppressors. The value of this new information is illustrated with a more detailed examination of the DDI between PPARalpha agonists and HMG-CoA reductase inhibitors. This paper proposes that the well-documented, but poorly understood, increase in incidence of rhabdomyolysis when a PPARalpha agonist is co-administered with a HMG-CoA reductase inhibitor is at least in part the result of PPARalpha-induced general suppression of drug metabolism enzymes in liver. The authors believe this type of information will provide insights to other poorly understood DDI questions and stimulate further laboratory and clinical investigations on xenobiotic-mediated induction and suppression of drug metabolism.

Inhibition of CYP3A4 and CYP3A5 catalyzed metabolism of alprazolam and quinine by ketoconazole as racemate and four different enantiomers

OBJECTIVE

The antifungal drug ketoconazole (KTZ) is known as an inhibitor of, especially, the CYP3A subfamily, which catalyzes the metabolism of a large variety of drugs. Interactions between KTZ and CYP3A substrates have been reported both in vivo and in vitro. Most of them, however, involved the KTZ racemate. KTZ racemate and the separate enantiomers, 2R,4R; 2R,4S; 2S,4S, and 2S,4R, were evaluated for their selectivity in inhibiting alprazolam and quinine metabolism.

METHODS

The inhibition of alprazolam and quinine metabolism was studied in an in vitro system of human liver microsomes (HLM), recombinant of CYP3A4 and CYP3A5. The concentrations of formed 3-hydroxyquinine and 4- and alpha-hydroxyalprazolam were measured by HPLC and LC-MS, respectively.

RESULTS

Quinine 3-hydroxylation was catalyzed to a similar extent by CYP3A4 and CYP3A5. The formation rate of 4-hydroxyalprazolam was higher than that of alpha-hydroxyalprazolam for each HLM, CYP3A4 and CYP3A5. KTZ racemate and enantiomers showed differential inhibitory effects of quinine and alprazolam metabolism. Quinine metabolism catalyzed by HLM, CYP3A4 and CYP3A5 was potently inhibited by the trans-enantiomer KTZ 2S,4S, with IC(50) value of 0.16 microM for HLM, 0.04 microM for CYP3A4 and 0.11 microM for CYP3A5. The same enantiomer showed the lowest IC(50) values of 0.11 microM for HLM and 0.04 microM for CYP3A5 with respect to alprazoalm 4-hydroxylation and also the same pattern for alprazolamalpha-hydroxylation, 0.13 microM for HLM and 0.05 microM for CYP3A5. Alprazolam metabolism (both alpha- and 4- hydroxylations) catalyzed by CYP3A4 was inhibited potently by the cis-enantiomer KTZ 2S,4R, with IC(50) values of 0.03 microM.

CONCLUSIONS

Alprazolam and quinine metabolism is catalyzed by both CYP3A4 and CYP3A5. The present study showed that different KTZ enantiomers inhibit CYP3A4 and CYP3A5 to different degrees, indicating that structural differences among the enantiomers would be related to their inhibitory potency on these two enzymes.

Effect of oral ketoconazole on intestinal first-pass effect of midazolam and fexofenadine in cynomolgus monkeys

Because the expression of drug-metabolizing enzymes and drug efflux transporters has been shown in the intestine, the contribution of this tissue to the first-pass effect has become of significant interest. Consequently, a comprehensive understanding of the absorption barriers in key preclinical species would be useful for the precise characterization of drug candidates. In the present investigation, we evaluated the intestinal first-pass effect of midazolam (MDZ) and fexofenadine (FEX), typical substrates for CYP3A and P-glycoprotein (P-gp), respectively, with ketoconazole (KTZ) as a potent dual CYP3A/P-gp inhibitor in cynomolgus monkeys. When MDZ or FEX was administered i.v. at doses of 0.3 or 1 mg/kg, respectively, the plasma concentration-time profiles were not influenced by p.o. coadministration of KTZ (20 mg/kg). On the other hand, when MDZ or FEX was administered p.o. at doses of 1 or 5 mg/kg, respectively, concomitant with a dose p.o. of KTZ (20 mg/kg), significant increases were observed in the area under the plasma concentration-time curves of MDZ or FEX (22-fold in MDZ and 3-fold in FEX). These findings indicate that both CYP3A and P-gp play a key role in the intestinal barrier and that inhibition of intestinal CYP3A/P-gp activities contributes exclusively toward the drug-drug interactions (DDI) with KTZ. Additionally, the K(i) values of the antifungal agents, KTZ, itraconazole, and fluconazole, for MDZ 1'-hydroxylation in monkey intestinal and liver microsomes were comparable with those in the respective human samples. These results suggest that monkeys may be an appropriate animal species for evaluating the intestinal first-pass effect of p.o. administered drugs and predicting intestinal DDI related to CYP3A4 and P-gp in humans.

Cytochrome P450-dependent metabolism of midazolam in hepatic microsomes from chickens, turkeys, pheasant and bobwhite quail

In vitro putative cytochrome P450 3A mediated activity, and inhibition thereof, were measured in four avian species using midazolam (MDZ) as a substrate and ketoconazole as an inhibitor. All species produced 1-hydroxymidazolam (1-OH MDZ) to a much greater extent than 4-hydroxymidazolam (4-OH MDZ). Calculated Vmaxapparent values for formation of 1-OH MDZ were 117+/-17, 239+/-108, 437+/-168, and 201+/-55 pmol/mg protein*min and Kmapparent values were 2.1+/-0.8, 2.4+/-1.6, 6.7+/-5.1 and 3.2+/-2.1 microm for chicken, turkey, pheasant and bobwhite quail, respectively. For the formation of 4-OH MDZ the Vmaxapparent values were 21+/-10, 94+/-46, 144+/-112, and 68+/-30 pmol/mg protein*min and Kmapparent values for 4-OH MDZ formation were 12.4+/-10.1, 18.0+/-10.8, 38.6+/-34.7 and 29.1+/-10.1 microm for chicken, turkey, pheasant and bobwhite quail, respectively. In all four species, ketoconazole inhibited the production of both major metabolites of MDZ, with 4-OH MDZ formation more sensitive to inhibition than 1-OH MDZ. Pheasant and bobwhite quail appeared most sensitive to ketoconazole inhibition.

Inhibition of UDP-glucuronosyltransferase 2b7-catalyzed morphine glucuronidation by ketoconazole: dual mechanisms involving a novel noncompetitive mode

Glucuronidation of morphine in humans is predominantly catalyzed by UDP-glucuronosyltransferase 2B7 (UGT2B7). Since our recent research suggested that cytochrome P450s (P450s) interact with UGT2B7 to affect its function [Takeda S et al. (2005) Mol Pharmacol 67:665-672], P450 inhibitors are expected to modulate UGT2B7-catalyzed activity. To address this issue, we investigated the effects of P450 inhibitors (cimetidine, sulfaphenazole, erythromycin, nifedipine, and ketoconazole) on the UGT2B7-catalyzed formation of morphine-3-glucuronide (M-3-G) and morphine-6-glucuronide (M-6-G). Among the inhibitors tested, ketoconazole was the most potent inhibitor of both M-3-G and M-6-G formation by human liver microsomes. The others were less effective except that nifedipine exhibited an inhibitory effect on M-6-G formation comparable to that by ketoconazole. Neither addition of NADPH nor solubilization of liver microsomes affected the ability of ketoconazole to inhibit morphine glucuronidation. In addition, ketoconazole had an ability to inhibit morphine UGT activity of recombinant UGT2B7 freed from P450. Kinetic analysis suggested that the ketoconazole-produced inhibition of morphine glucuronidation involves a mixed-type mechanism. Codeine potentiated inhibition of morphine glucuronidation by ketoconazole. In contrast, addition of another substrate, testosterone, showed no or a minor effect on ketoconazole-produced inhibition of morphine UGT. These results suggest that 1) metabolism of ketoconazole by P450 is not required for inhibition of UGT2B7-catalyzed morphine glucuronidation; and 2) this drug exerts its inhibitory effect on morphine UGT by novel mechanisms involving competitive and noncompetitive inhibition.

Effect of azole antifungals ketoconazole and fluconazole on the pharmacokinetics of dexloxiglumide

AIMS

Dexloxiglumide is a new CCK(1) receptor antagonist under investigation for treatment of functional gastrointestinal disorders and is metabolized by CYP3A4 and CYP2C9. The objectives of these two separate randomized, two-period, two-treatment crossover studies were to investigate the effects of steady-state ketoconazole, a model CYP3A4 inhibitor (Study 1), and steady-state fluconazole, a model CYP2C9 inhibitor (Study 2), on the pharmacokinetics of dexloxiglumide in healthy subjects.

METHODS

Plasma samples were analysed for dexloxiglumide and its primary metabolites: O-demethyl dexloxiglumide (ODM; Study 1 and 2) and dexloxiglumide carboxylic acid (DCA; Study 2).

RESULTS

Following ketoconazole coadministration, dexloxiglumide C(max) increased by 32% (90% confidence intervals (CI) 112-154), with unchanged ODM C(max); AUC of dexloxiglumide and ODM increased by 36% (90% CI 124-140 and 128-142, respectively). No changes were observed in dexloxiglumide or ODM t((1/2)). Fluconazole coadministration caused a 77% increase (90% CI 154-204) in dexloxiglumide C(max), no change in ODM C(max) and a 32% decrease (90% CI 62-75) in DCA C(max). Fluconazole coadministration resulted in a 2.5-fold increase (90% CI 235-267) in dexloxiglumide AUC, 40% increase (90% CI 136-156) in ODM AUC and an 18% decrease (90% CI 82-94) in DCA AUC. The t((1/2)) of all three analytes increased by approximately 2-fold with fluconazole coadministration (P-value < 0.05).

CONCLUSIONS

Ketoconazole caused a minimal increase while fluconazole caused a moderate increase in dexloxiglumide systemic exposure with no change in the adverse event profile of dexloxiglumide.

IDENTIFICATION OF HUMAN LIVER CYTOCHROME P450 ENZYMES RESPONSIBLE FOR THE METABOLISM OF LONAFARNIB (SARASAR)

Lonafarnib (Sarasar), a farnesyl transferase inhibitor, is currently under development for the treatment of solid tumors. Incubation of lonafarnib with human liver microsomes resulted in the formation of four oxidative metabolites (M1, M2, M3, and M4). Minor to trace levels of these metabolites were detected in humans after multiple-dose administration of lonafarnib. Liquid chromatography-mass spectrometry analyses exhibited a mass to charge ratio (m/z) for the (M+H)(+) ion of M1, M2, M3, and M4 at 653, 635, 669, and 653 Th, respectively. These metabolites, respectively, resulted from changes of +O, -2H, +2O, and +O relative to lonafarnib. Recombinant human CYP3A4 and CYP3A5 exhibited catalytic activity with respect to the formation of M1, M2, and M3, whereas CYP2C8 exhibited catalytic activity with respect to the formation of M4. There was a high correlation between the formation of M1, determined in 10 human liver microsomal samples, and 6beta-hydroxylation of testosterone catalyzed by CYP3A4/5 (r = 0.93). The IC(50) values of ketoconazole for inhibition of M1 and M2 were 0.61 and 0.92 microM, respectively. The formation of M4 by human liver microsomes was inhibited 72% by 50 microM quercetin, suggesting that the formation of M4 was mediated via CYP2C8. A CYP3A4/5-specific inhibitory monoclonal antibody inhibited the formation of M1, M2, and M3 by 85, 75, and 100%, respectively. In conclusion, the formation of metabolites M1, M2, and M3 from lonafarnib was mediated via CYP3A4 and CYP3A5.

Mechanism-based inhibition of cytochrome P450 3A4 by therapeutic drugs

Consistent with its highest abundance in humans, cytochrome P450 (CYP) 3A is responsible for the metabolism of about 60% of currently known drugs. However, this unusual low substrate specificity also makes CYP3A4 susceptible to reversible or irreversible inhibition by a variety of drugs. Mechanism-based inhibition of CYP3A4 is characterised by nicotinamide adenine dinucleotide phosphate hydrogen (NADPH)-, time- and concentration-dependent enzyme inactivation, occurring when some drugs are converted by CYP isoenzymes to reactive metabolites capable of irreversibly binding covalently to CYP3A4. Approaches using in vitro, in silico and in vivo models can be used to study CYP3A4 inactivation by drugs. Human liver microsomes are always used to estimate inactivation kinetic parameters including the concentration required for half-maximal inactivation (K(I)) and the maximal rate of inactivation at saturation (k(inact)). Clinically important mechanism-based CYP3A4 inhibitors include antibacterials (e.g. clarithromycin, erythromycin and isoniazid), anticancer agents (e.g. tamoxifen and irinotecan), anti-HIV agents (e.g. ritonavir and delavirdine), antihypertensives (e.g. dihydralazine, verapamil and diltiazem), sex steroids and their receptor modulators (e.g. gestodene and raloxifene), and several herbal constituents (e.g. bergamottin and glabridin). Drugs inactivating CYP3A4 often possess several common moieties such as a tertiary amine function, furan ring, and acetylene function. It appears that the chemical properties of a drug critical to CYP3A4 inactivation include formation of reactive metabolites by CYP isoenzymes, preponderance of CYP inducers and P-glycoprotein (P-gp) substrate, and occurrence of clinically significant pharmacokinetic interactions with coadministered drugs. Compared with reversible inhibition of CYP3A4, mechanism-based inhibition of CYP3A4 more frequently cause pharmacokinetic-pharmacodynamic drug-drug interactions, as the inactivated CYP3A4 has to be replaced by newly synthesised CYP3A4 protein. The resultant drug interactions may lead to adverse drug effects, including some fatal events. For example, when aforementioned CYP3A4 inhibitors are coadministered with terfenadine, cisapride or astemizole (all CYP3A4 substrates), torsades de pointes (a life-threatening ventricular arrhythmia associated with QT prolongation) may occur.However, predicting drug-drug interactions involving CYP3A4 inactivation is difficult, since the clinical outcomes depend on a number of factors that are associated with drugs and patients. The apparent pharmacokinetic effect of a mechanism-based inhibitor of CYP3A4 would be a function of its K(I), k(inact) and partition ratio and the zero-order synthesis rate of new or replacement enzyme. The inactivators for CYP3A4 can be inducers and P-gp substrates/inhibitors, confounding in vitro-in vivo extrapolation. The clinical significance of CYP3A inhibition for drug safety and efficacy warrants closer understanding of the mechanisms for each inhibitor. Furthermore, such inactivation may be exploited for therapeutic gain in certain circumstances.

Enhancement of nifedipine-induced gingival overgrowth by concomitant ketoconazole in rats

Nifedipine (NIF), a calcium channel blocker, is well known to induce gingival overgrowth (GO) as an adverse effect, and ketoconazole (KTZ), an azole antifungal agent, has been implicated in various drug interactions. We here examined the effects of concomitant KTZ on NIF-induced GO in a rat model. Fifteen-day-old male Fischer rats were fed chow with NIF, KTZ, or both. After 41 days, gingival conditions were evaluated and the concentration of NIF in serum was measured. Rats fed with NIF alone had induced GO and suppressed growth, while those given KTZ alone did not. Compared to the administration of NIF alone, concomitant KTZ significantly increased the severity of GO and the concentration of NIF in serum. These results suggest that concomitant KTZ increases the serum concentration of NIF and enhances the severity of GO.

Model for the drug-drug interaction responsible for CYP3A enzyme inhibition. I: evaluation of cynomolgus monkeys as surrogates for humans

1. Anti-human cytochrome P450 (CYP) 3A4 antiserum completely inhibited midazolam metabolism in monkey liver microsomes, suggesting that midazolam was mainly metabolized by CYP3A enzyme(s) in monkey liver microsomes. 2. Midazolam metabolism was also inhibited in vitro by typical chemical inhibitors of CYP3A, such as ketoconazole, erythromycin and diltiazem, and the apparent K(i) values for ketoconazole, erythromycin and diltiazem were 0.127, 94.2 and 29.6 microM, respectively. 3. CYP3A inhibitors increased plasma midazolam concentrations when midazolam and CYP3A inhibitors were co-administered orally. However, the pharmacokinetic parameters of midazolam were not changed by treatment with CYP3A inhibitors when midazolam was given intravenously. This suggests that CYP3A inhibitors modified the first-pass metabolism in the liver and/or intestine, but not systemic metabolism. 4. The drug-drug interaction responsible for CYP3A enzyme(s) inhibition was observed when midazolam and inhibitors were co-administrated orally. Therefore, it was concluded that monkeys given midazolam orally could be useful models for predicting drug-drug interactions in man based on CYP3A enzyme inhibition.

Determination of the absolute configuration and solution conformation of the antifungal agents ketoconazole, itraconazole, and miconazole with vibrational circular dichroism

The absolute configuration assignments of three antifungal agents, (+)-(2R,4S)-ketoconazole, (+)-(2R,4S)-itraconazole (with (S)-configuration at the sec-butyl group) and (+)-(S)-miconazole nitrate have been confirmed by using vibrational circular dichroism (VCD). For these three antifungal drugs, this study also provides evidence for the most abundant conformations of miconazole and for the relative conformations of the azole, dichlorophenyl, and methoxyphenyl groups in ketoconazole and itraconazole, in chloroform solution.

In Vitro Metabolism of a New Neuroprotective Agent, KR-31543 in the Human Liver Microsomes: Identification of Human Cytochrome P450

KR-31543, (2S,3R,4S)-6-amino-4-[N-(4-chlorophenyl)-N-(2-methyl-2H-tetrazol-5-ylmethyl)amino]-3,4-dihydro-2-dimethoxymethyl-3-hydroxy-2-methyl-2H-1-benzopyran, is a new neuroprotective agent for preventing ischemia-reperfusion damage. This study was performed to identify the metabolic pathway of KR-31543 in human liver microsomes and to characterize cytochrome P450 (CYP) enzymes that are involved in the metabolism of KR-31543. Human liver microsomal incubation of KR-31543 in the presence of NADPH resulted in the formation of two metabolites, M1 and M2. M1 was identified as N-(4-chlorophenyl)-N-(2-methyl-2H-tetrazol-5-ylmethyl)amine on the basis of LC/MS/MS analysis with a synthesized authentic standard, and M2 was suggested to be hydroxy-KR-31543. Correlation analysis between the known CYP enzyme activities and the rates of the formation of M1 and M2 in the 12 human liver microsomes have showed significant correlations with testosterone 6beta-hydroxylase activity (a marker of CYP3A4). Ketoconazole, a selective inhibitor of CYP3A4, and anti-CYP3A4 monoclonal antibodies potently inhibited both N-hydrolysis and hydroxylation of KR-31543 in human liver microsomes. These results provide evidence that CYP3A4 is the major isozyme responsible for the metabolism of KR-31543 to M1 and M2.

Factors affecting the clinical development of cytochrome p450 3A substrates

The objective of this review is to evaluate the risks associated with the discovery and development of cytochrome p450 (CYP) 3A substrates. CYP3A is the most abundant p450 enzyme in human liver and is highly expressed in the intestinal tract. The enzyme contributes substantially to metabolism of approximately 50% of currently marketed drugs that undergo oxidative metabolism. As a result, drug-drug interactions involving inhibitors of CYP3A-mediated metabolism can be of great clinical consequence. It is the position of the authors that, because of the factors responsible for the broad substrate specificity of CYP3A, discovery and development of compounds across a large and broad portfolio that are completely devoid of CYP3A metabolism is not feasible. Thus, it is important that scientifically valid approaches to the discovery and development of compounds metabolised by CYP3A be realised. The clinical relevance of CYP3A metabolism is dependent on a multitude of factors that include the degree of intestinal and hepatic CYP3A-mediated first-pass extraction, the therapeutic index of the compound and the adverse event associated with inhibition of CYP3A metabolism. Thus, a better understanding of the disposition of a CYP3A-metabolised compound relative to the projected or observed therapeutic index (or safety margin) can provide ample evidence to support the continued development of a CYP3A substrate. This document will highlight current practices as well as the benefits and risks associated with those practices.

Inhibitory Effect of Antituberculosis Drugs on Human Cytochrome P450-Mediated Activities

The potential for drug-drug interactions mediated by the inhibition of cytochrome P-450 (CYP) were concerned during antituberculosis therapy. However, the information regarding human CYP inhibition by antituberculosis drugs is limited to isoniazid. In the current study, we examined the inhibitory effects of pyrazinamide and ethionamide, both of which are chemically related to isoniazid, on the CYP-mediated activities in human liver microsomes and compared them to that of isoniazid. No remarkable effects on any CYP activities were observed by pyrazinamide and ethionamide. In contrast, in addition to the reported inhibitory effect of isoniazid on CYP1A2, CYP2A6, CYP2C19, and CYP3A activities, our results newly showed its effect on CYP2C9 and CYP2E1 activities. Isoniazid showed potent direct inhibitory effect on S-warfarin 7-hydroxylation, while a preincubation step in the presence of NADPH was needed to inhibit chlorzoxazone 6-hydroxylation. Furthermore, irreversible inhibition of CYP2C19 activity by isoniazid was also observed in the dilution study. These results suggested that pyrazinamide and ethionamide did not seem to cause drug interactions mediated by the inhibition of CYP. In contrast, isoniazid might contribute to the severe drug interactions by a different inhibitory mechanism depending on each of the CYP isozymes, in addition to the reported observations.

Enantioselectivity of inhibition of cytochrome P450 3A4 (CYP3A4) by ketoconazole: Testosterone and methadone as substrates

Racemic ketoconazole (KTZ) was the first orally active azole antifungal agent used in clinical practice and has become widely used in the treatment of mucosal fungal infections associated with AIDS immunosuppression and cancer chemotherapy. However, the use of KTZ has been limited because of adverse drug-drug interactions. KTZ blocks ergosterol biosynthesis by inhibiting the fungal cytochrome P450 (CYP51). KTZ is also a potent inhibitor of human cytochrome P450 3A4 (CYP3A4) enzyme, the major drug-metabolizing CYP isozyme in the human liver. We examined the enantioselective differences of KTZ in the inhibition of human CYP3A4 and in antifungal action. Dextro- and levo-KTZ exhibited modest enantioselective differences with respect to CYP3A4 inhibition of testosterone and methadone metabolism. For both substrates levo-KTZ was approximately a 2-fold more potent inhibitor. We examined the enantioselective differences in the in vitro activity of KTZ against medically relevant species of Candida and Aspergillus, as well as Cryptococcus neoformans. Overall, levo-KTZ was 2-4-fold more active than dextro-KTZ. Therefore, levo-KTZ is a more potent inhibitor of CYP3A4 and has stronger in vitro antifungal activity. Chirality 16:79-85, 2004.