Ketoconazole and sulphaphenazole as the respective selective inhibitors of P4503A and 2C9

In vitro inhibition and induction of human liver cytochrome P450 enzymes by a novel anti-fibrotic drug fluorofenidone

Fluorofenidone (AKF-PD) is an analog of pirfenidone and shows stronger antifibrotic effect and lower toxicity compared to pirfenidone in preclinical studies. However, the inhibitory and inducible effects of AKF-PD on human CYP450s are unclear. The aim of this study was to evaluate the ability of AKF-PD to inhibit and induce CYP450s in vitro.In inhibition study, the inhibitory effects of CYP1A2, CYP3A4, CYP2C9, CYP2E1, CYP2C19 and CYP2D6 by AKF-PD were evaluated with the metabolic rate of probe drug of each enzyme in pooled human liver microsomes. The enzyme inducible potential of AKF-PD was evaluated by the mRNA expression and enzyme activity of CYP1A2, CYP2B6 and CYP3A4 in human hepatocytes. The results suggested that AKF-PD produced weak inhibition on CYP1A2 and CYP2C19, while no inhibitory effects were found on the other enzymes. Since the plasma concentration of AKF-PD is much lower than the IC₅₀ values of both CYP1A2 and CYP2C19, the inhibitory effects can be reasonably ignored.On the other hand, AKF-PD showed no inducible effects on CYP1A2 while showed potential inducible ability on CYP2B6 and CYP3A4 in some test groups. Further study of this novel anti-fibrotic drug should take into account in clinical therapies.

Molecular Docking as a Promising Predictive Model for Silver Nanoparticle-Mediated Inhibition of Cytochrome P450 Enzymes

Cytochrome P450 (CYP) enzymes are responsible for oxidative metabolisms of a large number of xenobiotics. In this study, we investigated interactions of silver nanoparticles (AgNPs) and silver ions (Ag⁺) with six CYP isoforms, namely, CYP1A2, CYP2C9, CYP2C19, CYP2D6, CYP2E1, and CYP3A4, within CYP-specific inhibitor-binding pockets by molecular docking and quantum mechanical (QM) calculations. The docking results revealed that the Ag₃ cluster, not Ag⁺, interacted with key amino acids of CYP2C9, CYP2C19, and CYP2D6 within a distance of about 3 Å. Moreover, the QM analysis confirmed that the amino acid residues of these CYP enzymes strongly interacted with the Ag₃ cluster, providing more insight into the mechanism of the potential inhibition of CYP enzyme activities. Interestingly, these results are consistent with previous in vitro data indicating that AgNPs inhibited activities of CYP2C and CYP2D in rat liver microsomes. It is suggested that the Ag₃ cluster is a minimal unit of AgNPs for in silico modeling. In summary, we demonstrated that molecular docking, together with QM analysis, is a promising tool to predict AgNP-mediated CYP inhibition. These methods are useful for deeper understanding of reaction mechanisms and could be used for other nanomaterials.

Interspecies Variation in NCMN-O-Demethylation in Liver Microsomes from Various Species

NCMN (N-(3-carboxy propyl)-4-methoxy-1,8-naphthalimide), a newly developed ratiometric two-photon fluorescent probe for human Cytochrome P450 1A (CYP1A), shows the best combination of specificity and reactivity for real-time detection of the enzymatic activities of CYP1A in complex biological systems. This study aimed to investigate the interspecies variation in NCMN-O-demethylation in commercially available liver microsomes from human, mouse, rat, beagle dog, minipig and cynomolgus monkey. Metabolite profiling demonstrated that NCMN could be O-demethylated in liver microsomes from all species but the reaction rate varied considerably. CYP1A was the major isoform involved in NCMN-O-demethylation in all examined liver microsomes based on the chemical inhibition assays. Furafylline, a specific inhibitor of mammalian CYP1A, displayed differential inhibitory effects on NCMN-O-demethylation in all tested species. Kinetic analyses demonstrated that NCMN-O-demethylation in liver microsomes form rat, minipig and cynomolgus monkey followed biphasic kinetics, while in liver microsomes form human, mouse and beagle dog obeyed Michaelis-Menten kinetics, the kinetic parameters from various species are much varied, while NCMN-O-demethylation in MLM exhibited the highest similarity of specificity, kinetic behavior and intrinsic clearance as that in HLM. These findings will be very helpful for the rational use of NCMN as a practical tool to decipher the functions of mammalian CYP1A or to study CYP1A associated drug-drug interactions in vivo.

Human CYP3A4-mediated toxification of the pyrrolizidine alkaloid lasiocarpine

Pyrrolizidine alkaloids (PA) are widely distributed phytotoxins contaminating food and feed. Hepatic enzymes are considered to bioactivate PA. Previous studies showed differences in the metabolism rate in liver homogenates of different species. Thus, uncertainty remains with respect to the relevance of human metabolism. Our study aimed to analyze whether the PA representative lasiocarpine is toxified by human cytochrome P450 (CYP) enzymes. We compared the metabolic elimination of lasiocarpine in the presence of rat and human S9 fractions and liver microsomes. Experiments with the potent CYP3A/Cyp3a inhibitor ketoconazole and supersomes containing individual human and rat CYPs revealed that enzymes of the CYP3A/Cyp3a family of both species are of major relevance for lasiocarpine metabolism. To assess if metabolism by human CYP3A4 results in a toxification of lasiocarpine we performed experiments with V79 cells. γH2AX and micronucleus formation were analyzed as endpoints for genotoxicity. No effects were observed in the wildtype cells, which lack CYP activity. By contrast, a V79 clone engineered for expression of human CYP3A4 showed concentration-dependent γH2AX and micronucleus formation. Concluding, our results showed the CYP3A4-dependent formation of genotoxic metabolites of lasiocarpine. The results confirm previous data indicating the need to include metabolism of PA for human risk assessment.

Enantiospecific Pharmacogenomics of Fluvastatin

The aim of this study was to investigate how variability in multiple genes related to pharmacokinetics affects fluvastatin exposure. We determined fluvastatin enantiomer pharmacokinetics and sequenced 379 pharmacokinetic genes in 200 healthy volunteers. CYP2C9*3 associated with significantly increased area under the plasma concentration-time curve (AUC) of both 3R,5S-fluvastatin and 3S,5R-fluvastatin (by 67% and 94% per variant allele copy, P = 3.77 × 10^-9 and P = 3.19 × 10^-12 ). In contrast, SLCO1B1 c.521T>C associated with increased AUC of active 3R,5S-fluvastatin only (by 34% per variant allele copy; P = 8.15 × 10^-8 ). A candidate gene analysis suggested that CYP2C9*2 also affects the AUC of both fluvastatin enantiomers and that SLCO2B1 single-nucleotide variations may affect the AUC of 3S,5R-fluvastatin. Thus, SLCO transporters have enantiospecific effects on fluvastatin pharmacokinetics in humans. Genotyping of both CYP2C9 and SLCO1B1 may be useful in predicting fluvastatin efficacy and myotoxicity.

Factors associated with postoperative complications in healthy horses after general anesthesia for ophthalmic versus non-ophthalmic procedures: 556 cases (2012-2014)

OBJECTIVE To compare complications between healthy horses undergoing general anesthesia for ophthalmic versus non-ophthalmic procedures and identify potential risk factors for the development of complications. DESIGN Retrospective case series. ANIMALS 502 horses (556 anesthetic procedures). PROCEDURES Medical records from January 2012 through December 2014 were reviewed to identify horses undergoing general anesthesia. Signalment, body weight, drugs administered, patient positioning, procedure type (ophthalmic, orthopedic, soft tissue, or diagnostic imaging), specific procedure, procedure time, anesthesia time, recovery time, recovery quality, and postoperative complications were recorded. RESULTS Patients underwent general anesthesia for ophthalmic (n = 106), orthopedic (246), soft tissue (84), diagnostic imaging (110), or combined (10) procedures. Mean procedure, anesthesia, and recovery times were significantly longer for patients undergoing ophthalmic versus non-ophthalmic procedures. Excluding diagnostic imaging procedures, there was a significant positive correlation between surgery time and recovery time. Within ophthalmic procedures, surgery time, anesthesia time, and recovery time were significantly greater for penetrating keratoplasty versus other ophthalmic procedures. There was a significantly higher rate of postoperative colic following penetrating keratoplasty, compared with all other ophthalmic procedures. CONCLUSIONS AND CLINICAL RELEVANCE Results suggested that in healthy horses, duration of general anesthesia should be minimized to decrease the risk of postanesthetic complications. Judicious use of orally administered fluconazole is recommended for horses undergoing general anesthesia. For horses undergoing a retrobulbar nerve block during general anesthesia, use of the lowest effective volume is suggested.

Identification of canine cytochrome P-450s (CYPs) metabolizing the tramadol (+)-M1 and (+)-M2 metabolites to the tramadol (+)-M5 metabolite in dog liver microsomes

We previously showed that (+)-tramadol is metabolized in dog liver to (+)-M1 exclusively by CYP2D15 and to (+)-M2 by multiple CYPs, but primarily CYP2B11. However, (+)-M1 and (+)-M2 are further metabolized in dogs to (+)-M5, which is the major metabolite found in dog plasma and urine. In this study, we identified canine CYPs involved in metabolizing (+)-M1 and (+)-M2 using recombinant enzymes, untreated dog liver microsomes (DLMs), inhibitor-treated DLMs, and DLMs from CYP inducer-treated dogs. A canine P-glycoprotein expressing cell line was also used to evaluate whether (+)-tramadol, (+)-M1, (+)-M2, or (+)-M5 are substrates of canine P-glycoprotein, thereby limiting their distribution into the central nervous system. (+)-M5 was largely formed from (+)-M1 by recombinant CYP2C21 with minor contributions from CYP2C41 and CYP2B11. (+)-M5 formation in DLMs from (+)-M1 was potently inhibited by sulfaphenazole (CYP2C inhibitor) and chloramphenicol (CYP2B11 inhibitor) and was greatly increased in DLMs from phenobarbital-treated dogs. (+)-M5 was formed from (+)-M2 predominantly by CYP2D15. (+)-M5 formation from (+)-M1 in DLMs was potently inhibited by quinidine (CYP2D inhibitor) but had only a minor impact from all CYP inducers tested. Intrinsic clearance estimates showed over 50 times higher values for (+)-M5 formation from (+)-M2 compared with (+)-M1 in DLMs. This was largely attributed to the higher enzyme affinity (lower Km) for (+)-M2 compared with (+)-M1 as substrate. (+)-tramadol, (+)-M1, (+)-M2, or (+)-M5 were not p-glycoprotein substrates. This study provides a clearer picture of the role of individual CYPs in the complex metabolism of tramadol in dogs.

Variation in the Response of Clozapine Biotransformation Pathways in Human Hepatic Microsomes to CYP1A2- and CYP3A4-selective Inhibitors

The atypical antipsychotic agent clozapine (CLZ) is effective in many patients who are resistant to conventional antipsychotic drugs. Cytochromes P450 (CYPs) 1A2 and 3A4 oxidize CLZ to norCLZ and CLZ N-oxide in human liver. Concurrent treatment with inducers and inhibitors of CYP1A2 modulates CLZ elimination that disrupts therapy. Drug-drug interactions involving CYP3A4 are also significant but less predictable. To further characterize the factors underlying these interactions, we used samples from a cohort of human livers to assess variation in CLZ oxidation pathways in relation to intrinsic CYP3A4 and CYP1A2 activities and the effects of the corresponding selective inhibitors ketoconazole (0.2 and 2 μM) and fluvoxamine (1 and 10 μM). The CYP3A4-selective inhibitor ketoconazole (2 μM) impaired CLZ N-oxide formation in all 14 of the livers used in inhibition studies (≥50% inhibition) while the CYP1A2-selective inhibitor fluvoxamine (10 μM) decreased norCLZ formation in nine. Ketoconazole effectively inhibited CLZ metabolism in five of seven livers that catalysed CYP3A4-dependent testosterone 6β-hydroxylation at or above the median rate and in four other livers with lower intrinsic CYP3A4 activity. Similarly, fluvoxamine (10 μM) readily inhibited CLZ oxidation in seven livers with high CYP1A2-mediated 7-ethoxyresorufin O-deethylation activity (at or above the median) and three livers with lower intrinsic CYP1A2 activity. In three livers, CLZ biotransformation was impaired by both ketoconazole and fluvoxamine, consistent with a major role for both CYPs. These findings suggest that the intrinsic activities of CYPs 1A2 and 3A4 are unrelated to the response to CYP-selective inhibitors and that assessment of the activities in vivo may not assist the prediction of drug-drug interactions.

Synergism between macrolide antibiotics and the azole fungicide ketoconazole in growth inhibition testing of the green alga Pseudokirchneriella subcapitata

Macrolide antibiotics and azole fungicides are detected widely in the aquatic environment as a result of their increased use in humans and animal livestock disease and their incomplete removal by wastewater treatment plants. In most cases, ecotoxicological tests are performed by using individual chemical substances, but because of the coexistence of a number of chemicals in the environment, organisms are exposed to many chemicals simultaneously. Therefore, it is important to evaluate effects of chemical interactions, adding to potential hazards of individual chemical. Here, we investigated the synergetic effects of combined chemicals (the azole fungicide ketoconazole and either of two macrolide antibiotics, erythromycin and clarithromycin) in growth inhibition testing using Pseudokirchneriella subcapitata according to OECD Test guideline 201. Combination index plots, isobolograms, and curve-shift analyses revealed that the combination of macrolide antibiotic and ketoconazole at various ratios resulted in strong synergism that enhanced growth inhibition of P. subcapitata, suggesting the necessity of investigating potential hazard of combined chemicals for regulatory purposes.

Ketoconazole Stereoisomers Differentially Induce Cytochrome P450 1A1 Between Human Hepatoma HepG2 and Mouse Hepatoma Hepa1c1c7 Cells

Ketoconazole (KTZ) has 2 chiral centers with the therapeutically active form being a racemic mixture of 2 cis-enantiomers, namely, (2R,4S)-(+)-KTZ and (2S,4R)-(-)-KTZ. The aims of the present study were to examine the effects of (+)-KTZ, (-)-KTZ, and (±)-KTZ on aryl hydrocarbon receptor activation and subsequently CYP1A1 induction in both human HepG2 and murine Hepa1c1c7 hepatoma cells, and to further test their inhibitory effect using recombinant human and mouse CYP1A1 enzymes. Our results demonstrated that (+)-KTZ induced human CYP1A1 more than (-)-KTZ, whereas on the other hand (-)-KTZ induced murine Cyp1a1 more than (+)-KTZ at the mRNA, and activity levels. Human CYP1A1 showed higher affinity to 7ER compared with murine Cyp1a1 (Km values 13.29 nM for human vs. 168.1 nM for murine). The intrinsic clearance values for human and murine CYP1A1 were 194.1 and 87.6 μL/pmol P450/min, respectively, whereas, Vmax values were 2.58 and 14.73 pmol/pmol P450/min, respectively. (+)-KTZ and (-)-KTZ directly inhibited CYP1A1 activity by noncompetitive mechanism. The affinity of (-)-KTZ to interact with human CYP1A1 and murine Cyp1a1 was significantly different from (+)-KTZ, as the Ki values for human CYP1A1 and murine Cyp1a1 were 199.4 and 413.7 nM, respectively, for (+)-KTZ, and 269.3 and 230.8 nM, respectively, for (-)-KTZ.

In vitro metabolism of an estrogen-related receptor γ modulator, GSK5182, by human liver microsomes and recombinant cytochrome P450s

GSK5182 (4-[(Z)-1-[4-(2-dimethylaminoethyloxy)phenyl]-hydroxy-2-phenylpent-1-enyl]phenol) is a specific inverse agonist for estrogen-related receptor γ, a member of the orphan nuclear receptor family that has important functions in development and homeostasis. This study was performed to elucidate the metabolites of GSK5182 and to characterize the enzymes involved in its metabolism. Incubation of human liver microsomes with GSK5182 in the presence of NADPH resulted in the formation of three metabolites, M1, M2 and M3. M1 and M3 were identified as N-desmethyl-GSK5182 and GSK5182 N-oxide, respectively, on the basis of liquid chromatography-tandem mass spectrometric (LC-MS/MS) analysis. M2 was suggested to be hydroxy-GSK5182 through interpretation of its MS/MS fragmentation pattern. In addition, the specific cytochrome P450 (P450) and flavin-containing monooxygenase (FMO) isoforms responsible for GSK5182 oxidation to the three metabolites were identified using a combination of correlation analysis, chemical inhibition in human liver microsomes and metabolism by expressed recombinant P450 and FMO isoforms. GSK5182 N-demethylation and hydroxylation is mainly mediated by CYP3A4, whereas FMO1 and FMO3 contribute to the formation of GSK5182 N-oxide from GSK5182. The present data will be useful for understanding the pharmacokinetics and drug interactions of GSK5182 in vivo.

Cytochrome P450-mediated bioactivation of mefenamic acid to quinoneimine intermediates and inactivation by human glutathione S-transferases

Mefenamic acid (MFA) has been associated with rare but severe cases of hepatotoxicity, nephrotoxicity, gastrointestinal toxicity, and hypersensitivity reactions that are believed to result from the formation of reactive metabolites. Although formation of protein-reactive acylating metabolites by phase II metabolism has been well-studied and proposed to be the cause of these toxic side effects, the oxidative bioactivation of MFA has not yet been competely characterized. In the present study, the oxidative bioactivation of MFA was studied using human liver microsomes (HLM) and recombinant human P450 enzymes. In addition to the major metabolite 3'-OH-methyl-MFA, resulting from the benzylic hydroxylation by CYP2C9, 4'-hydroxy-MFA and 5-hydroxy-MFA were identified as metabolites resulting from oxidative metabolism of both aromatic rings of MFA. In the presence of GSH, three GSH conjugates were formed that appeared to result from GSH conjugation of the two quinoneimines formed by further oxidation of 4'-hydroxy-MFA and 5-hydroxy-MFA. The major GSH conjugate was identified as 4'-OH-5'-glutathionyl-MFA and was formed at the highest activity by CYP1A2 and to a lesser extent by CYP2C9 and CYP3A4. Two minor GSH conjugates resulted from secondary oxidation of 5-hydroxy-MFA and were formed at the highest activity by CYP1A2 and to a lesser extent by CYP3A4. Additionally, the ability of seven human glutathione S-transferases (hGSTs) to catalyze the GSH conjugation of the quinoneimines formed by P450s was also investigated. The highest increase of total GSH conjugation was observed with hGSTP1-1, followed by hepatic hGSTs hGSTA2-2 and hGSTM1-1. The results of this study show that, next to phase II metabolites, reactive quinoneimines formed by oxidative bioactivation might also contribute to the idiosyncratic toxicity of MFA.

Hepatic cytochrome P450s play a major role in monocrotaline-induced renal toxicity in mice

AIM

Monocrotaline (MCT) in plants of the genus Crotalaria induces significant toxicity in multiple organs including the liver, lung and kidney. Metabolic activation of MCT is required for MCT-induced toxicity. In this study, we attempted to determine whether the toxicity of MCT in kidney was a consequence of the metabolic activation of MCT in the liver.

METHODS

Liver-specific cytochrome P450 reductase-null (Null) mice, wild-type (WT) mice and CYP3A inhibitor ketoconazole-pretreated WT (KET-WT) mice were examined. The mice were injected with MCT (300, 400, or 500 mg/kg, ip), and hepatotoxicity and nephrotoxicity were examined 24 h after MCT treatment. The levels of MCT and its metabolites in the blood, liver, lung, kidney and bile were determined using LC-MS analysis.

RESULTS

Treatment of WT mice with MCT increased the serum levels of alanine aminotransferase, hyaluronic acid, urea nitrogen and creatinine in a dose-dependent manner. Histological examination revealed that MCT (500 mg/kg) caused severe liver injury and moderate kidney injury. In contrast, these pathological abnormalities were absent in Null and KET-WT mice. After injection of MCT (400 and 500 mg/kg), the plasma, liver, kidney and lung of WT mice had significantly lower MCT levels and much higher N-oxide metabolites contents in compared with those of Null and KET-WT mice. Furthermore, WT mice had considerably higher levels of tissue-bound pyrroles and bile GSH-conjugated MCT metabolites compared with Null and KET-WT mice.

CONCLUSION

Cytochrome P450s in mouse liver play a major role in the metabolic activation of MCT and thus contribute to MCT-induced renal toxicity.

Characterization of human cytochrome P450s involved in the bioactivation of clozapine

Clozapine is known to cause hepatotoxicity in a small percentage of patients. Oxidative bioactivation to reactive intermediates by hepatic cytochrome P450s (P450s) has be proposed as a possible mechanism. However, in contrast to their role in formation of N-desmethylclozapine and clozapine N-oxide, the involvement of individual P450s in the bioactivation to reactive intermediates is much less well characterized. The results of the present study show that 7 of 14 recombinant human P450s were able to bioactivate clozapine to a glutathione-reactive nitrenium ion. CYP3A4 and CYP2D6 showed the highest specific activity. Enzyme kinetical characterization of these P450s showed comparable intrinsic clearance of bioactivation, implicating that CYP3A4 would be more important because of its higher hepatic expression, compared with CYP2D6. Inhibition experiments using pooled human liver microsomes confirmed the major role of CYP3A4 in hepatic bioactivation of clozapine. By studying bioactivation of clozapine in human liver microsomes from 100 different individuals, an 8-fold variability in bioactivation activity was observed. In two individuals bioactivation activity exceeded N-demethylation and N-oxidation activity. Quinidine did not show significant inhibition of bioactivation in any of these liver fractions, suggesting that CYP2D6 polymorphism is not an important factor in determining susceptibility to hepatotoxicity of clozapine. Therefore, interindividual differences and drug-drug interactions at the level of CYP3A4 might be factors determining exposure of hepatic tissue to reactive clozapine metabolites.

Ilaprazole, a new proton pump inhibitor, is primarily metabolized to ilaprazole sulfone by CYP3A4 and 3A5

Ilaprazole is a new proton pump inhibitor, designed for treatment of gastric ulcers, and developed by Il-Yang Pharmaceutical Co (Seoul, Korea). It is extensively metabolised to the major metabolite ilaprazole sulfone. In the present study, several in vitro approaches were used to identify the cytochrome P450 (CYP) enzymes responsible for ilaprazole sulfone formation. Concentrations of ilaprazole sulfone were determined by liquid chromatography-tandem mass spectrometry (LC-MS/MS). Incubation of ilaprazole with cDNA-expressed recombinant CYPs indicated that CYP3A was the major enzyme that catalyses ilaprozole to ilaprazole sulfone. This reaction was inhibited significantly by ketoconazole, a CYP3A inhibitor, and azamulin, a mechanism-based inhibitor of CYP3A, while no substantial effect was observed using selective inhibitors for eight other P450s (CYP1A2, CYP2A6, CYP2B6, CYP2C8, CYP2C9, CYP2C19, CYP2D6, and CYP2E1). In addition, the formation of ilaprazole sulfone correlated well with CYP3A-catalysed testosterone 6β-hydroxylation and midazolam 1'-hydroxylation in 20 different human liver microsome panels. The intrinsic clearance of the formation of ilaprazole sulfone by CYP3A4 was 16-fold higher than that by CYP3A5. Collectively, these results indicate that the formation of the major metabolite of ilaprazole, ilaprazole sulfone, is predominantly catalysed by CYP3A4/5.

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.

Human cytochrome P450: metabolism of testosterone by CYP3A4 and inhibition by ketoconazole

This unit describes methods for measuring CYP3A4 activity using testosterone as a specific substrate, and for measuring CYP3A4 inhibition using ketoconazole as a selective inhibitor of testosterone oxidation. CYP3A4 is one of the most important and most abundant drug-metabolizing CYP isoforms in human liver microsomes (∼40% of total CYP), and it has the broadest substrate specificity. It is important to determine whether CYP3A4 is involved in its metabolism.

Inhibition of cytochrome P450 2A participating in coumarin 7-hydroxylation in pig liver microsomes

Five commonly used human cytochrome P450 (CYP) inhibitors were examined for their effects on coumarin 7-hydroxylase (CYP2A) activity in pig liver microsomes. The K(m) and V(max) values for coumarin 7-hydroxylation in pig liver microsomes were estimated to be 1 μm and 0.26 nmol·mg/min, respectively. The following human CYP inhibitors caused little or no inhibition of CYP2A as defined by a K(i) > 200 μm: quinidine (CYP2D6), troleandomycin (CYP3A4), and sulfaphenazole (CYP2C9). The other two human CYP inhibitors were classified as strong inhibitors of CYP2A: 8-methoxypsoralen (CYP2A6) and α-naphthoflavone (CYP1A1/2). In the absence of a preincubation period, 8-MOP inhibited the 7-hydroxylation of coumarin with a K(i) value of 1.1 μm, which decreased to 0.1 μm when 8-MOP was preincubated with pig liver microsomes for 3 min. α-Naphthoflavone inhibited the 7-hydroxylation of coumarin with a K(i) value of 32 μm, which did not increase ability to inhibitor CYP2A when α-naphthoflavone was preincubated with pig liver microsomes for 3 min. These results of this study suggest that 8-MOP is a potent, mechanism-based inhibitor of pig CYP2A activity in pig liver microsomes.

Characterisation of zuclopenthixol metabolism by in vitro and therapeutic drug monitoring studies

OBJECTIVE

Zuclopenthixol pharmacokinetics is incompletely characterised. We investigated potential interactions mediated through cytochrome P450 enzymes.

METHOD

In vitro, we examined the impact of CYP2D6 and CYP3A4 inhibitors on zuclopenthixol metabolism in microsomes from six human livers. Subsequently, we compared dose-corrected serum zuclopenthixol concentrations in 923 samples from a therapeutic drug monitoring database from patients prescribed oral (n = 490) or injected (n = 423) zuclopenthixol alone or with fluoxetine, paroxetine, levomepromazine or carbamazepine.

RESULTS

In vitro fluoxetine, paroxetine, ketoconazole and quinidine all significantly inhibited zuclopenthixol metabolism. Ketoconazole and quinidine together abolished zuclopenthixol disappearance. Clinically, dose-corrected oral zuclopenthixol serum concentrations increased significantly, after adjustment, by 93%, 78% and 46% during co-treatment with fluoxetine, paroxetine and levomepromazine and decreased 67% with carbamazepine. Carbamazepine caused dose-dependent reductions in the oral zuclopenthixol concentration-dose ratio (P < 0.001), fluoxetine (P < 0.001) and paroxetine (P = 0.011) dose-dependent increases and levomepromazine an increase related to its serum concentration (P < 0.001). Results for injected zuclopenthixol were similar but not all reached statistical significance.

CONCLUSION

The In vitro study suggests zuclopenthixol is metabolised primarily by CYP2D6 and CYP3A4. The clinical study supports this, demonstrating the impact of co-prescribed inhibitors or inducers. Guidelines should incorporate these interactions noting the potential for zuclopenthixol-related toxicity or treatment failure.

Contributions of human cytochrome P450 enzymes to glyburide metabolism

Glyburide (GLB) is a widely used oral sulfonylurea for the treatment of gestational diabetes. The therapeutic use of GLB is often complicated by a substantial inter-individual variability in the pharmacokinetics and pharmacodynamics of the drug in human populations, which might be caused by inter-individual variations in factors such as GLB metabolism. Therefore, there has been a continued interest in identifying human cytochrome P450 (CYP) isoforms that play a major role in the metabolism of GLB. However, contrasting data are available in the present literature in this regard. The present study systematically investigated the contributions of various human CYP isoforms (CYP3A4, CYP3A5, CYP2C8, CYP2C9 and CYP2C19) to in vitro metabolism of GLB. GLB depletion and metabolite formation in human liver microsomes were most significantly inhibited by the CYP3A inhibitor ketoconazole compared with the inhibitors of other CYP isoforms. Furthermore, multiple correlation analysis between GLB depletion and individual CYP activities was performed, demonstrating a significant correlation between GLB depletion and the CYP3A probe activity in 16 individual human liver microsomal preparations, but not between GLB depletion and the CYP2C19, CYP2C8 or CYP2C9 probe activity. By using recombinant supersomes overexpressing individual human CYP isoforms, it was found that GLB could be depleted by all the enzymes tested; however, the intrinsic clearance (V(max)/K(m)) of CYP3A4 for GLB depletion was 4-17 times greater than that of other CYP isoforms. These results confirm that human CYP3A4 is the major enzyme involved in the in vitro metabolism of GLB.

OPRM1 and CYP2B6 gene variants as risk factors in methadone-related deaths

Methadone is a medication valued for its effectiveness in the treatment of heroin addiction; however, many fatal poisonings associated with its use have been reported over the years. We have examined the association between CYP2B6 and micro-opioid receptor (OPRM1) gene variations and apparent susceptibility to methadone poisoning. Genomic DNA was extracted from postmortem whole blood of 40 individuals whose deaths were attributed to methadone poisoning. The presence of CYP2B6*4,*9, and *6 alleles and the OPRM1 A118G variant was determined by SNP genotyping. CYP2B6 *4, *9, and *6 alleles were found to be associated with higher postmortem methadone concentrations in blood (P < or = 0.05). OPRM1 A118G was also associated with higher postmortem methadone concentrations in blood but not to a level of statistical significance (P = 0.39). In these methadone-related deaths, OPRM1 118GA was associated with higher postmortem benzodiazepine concentrations (P = 0.04), a finding not associated with morphine-related deaths. The risk of a methadone-related fatality during treatment may be evaluated in part by screening for CYP2B6*6 and A118G.

Genotoxicity of a freshwater cyanotoxin, cylindrospermopsin, in two human cell lines: Caco-2 and HepaRG

Cylindrospermopsin (CYN), a cyanotoxin produced by certain freshwater cyanobacteria, causes human intoxications and animal mortalities. CYN is a potent inhibitor of protein- and glutathione-synthesis. Preliminary evidence for in vivo tumor initiation has been found in mice but the mechanism remains unclear. Several in vitro and in vivo studies demonstrate that CYN is genotoxic and requires metabolic activation. In the present study, the genotoxicity of CYN was assessed in human hepatocyte and enterocyte cell lines, which are models for CYN target organs. The cytokinesis-block micronucleus assay was conducted on liver-derived HepaRG cells and colon-derived Caco-2 cells. Each cell-type was exposed to CYN in both the differentiated and the undifferentiated states, and both with and without the cytochrome P450 inhibitor, ketoconazole, to determine the involvement of metabolism in CYN genotoxicity. CYN increased the frequency of micronuclei in binucleated cells (MNBNC) in both Caco-2 and HepaRG cells. Moreover, ketoconazole reduced both the genotoxicity and cytotoxicity caused by CYN. Our results confirm the involvement of metabolic activation of CYN in mediating its toxicity and suggest that CYN is progenotoxic.

Roles of different CYP enzymes in the formation of specific fluvastatin metabolites by human liver microsomes

Fluvastatin has been considered to be metabolised to 5-hydroxy fluvastatin (M-2), 6-hydroxy fluvastatin (M-3) and N-desisopropyl fluvastatin (M-5) in human liver microsomes by primarily CYP2C9. To elucidate the contribution of different CYP enzymes on fluvastatin metabolism, we examined the effect of CYP inhibitors and CYP2C-specific monoclonal antibodies on the formation of fluvastatin metabolites in human liver microsomes. Human liver microsomes were incubated with fluvastatin with or without pre-treatment with CYP inhibitors or monoclonal antibodies. Selective inhibitors of CYP2C9 (sulfaphenazole), CYP3A (ketoconazole) and CYP2C8 (quercetin) were employed and monoclonal antibodies were against CYP2C8, CYP2C9, CYP2C19 and CYP2C8/9/18/19. According to the amount of fluvastatin metabolites produced, the formation of M-3 was found to be major pathway of fluvastatin metabolism (the relative contribution was calculated to be more than 80%). Sulfaphenazole inhibited the formation of M-2 largely, but had little effect on the formation of M-3. It also inhibited the formation of M-5. Ketoconazole markedly inhibited the formation of M-3, but did not inhibit the formation of M-2 and M-5. Quercetin had a moderate inhibitory effect on the formation of all three fluvastatin metabolites. Monoclonal antibodies against CYP2C9 and CYP2C8/9/18/19 markedly inhibited the formation of M-2 and M-5. None of monoclonal antibodies showed clear inhibition on the formation of M-3. In contrast to previous published work, our results suggest that M-2 and M-5 are formed preferentially by CYP2C9, and that M-3 is mainly formed by CYP3A. In summary, the results contribute to a better understanding of the drug-drug interaction potential for fluvastatin in vivo.

Cranberry juice suppressed the diclofenac metabolism by human liver microsomes, but not in healthy human subjects

AIM

To investigate a potential interaction between cranberry juice and diclofenac, a substrate of CYP2C9.

METHODS

The inhibitory effect of cranberry juice on diclofenac metabolism was determined using human liver microsome assay. Subsequently, we performed a clinical trial in healthy human subjects to determine whether the repeated consumption of cranberry juice changed the diclofenac pharmacokinetics.

RESULTS

Cranberry juice significantly suppressed diclofenac metabolism by human liver microsomes. On the other hand, repeated consumption of cranberry juice did not influence the diclofenac pharmacokinetics in human subjects.

CONCLUSIONS

Cranberry juice inhibited diclofenac metabolism by human liver microsomes, but not in human subjects. Based on the present and previous findings, we think that although cranberry juice inhibits CYP2C9 activity in vitro, it does not change the pharmacokinetics of medications metabolized by CYP2C9 in clinical situations.