cDNA-expressed Human Cytochrome P450 Research Articles

Cytochrome P450 Mediated Bioactivation of Rutaevin, a Bioactive and Potentially Hepatotoxic Component of Evodia Rutaecarpa.

Rutaevin is one of the major bioactive constituents isolated from Evodia rutaecarpa, a well-known herbal medicine that has been widely prescribed for the treatment of gastrointestinal disorders in China. However, oral administration of rutaevin has been shown to cause hepatotoxicity in mice. Bioactivation was suggested to be involved in rutaevin-induced hepatotoxicity. The aim of this study was to investigate the bioactivation of rutaevin in rat and human liver microsomes fortified with NADPH. Rutaevin was metabolized into the reactive intermediate cis-butene-1,4-dial (BDA) that was dependent on NADPH. The rutaevin-derived BDA intermediate was trapped by nucleophiles such as glutathione (GSH), N-acetyl-lysine (NAL), and methoxylamine (MOA) in the microsomal incubation system. A total of 10 conjugates resulting from the conjugation of the intermediate with GSH, NAL, or MOA were detected and structurally characterized by liquid chromatography combined with high-resolution tandem mass spectrometry. M1, structurally confirmed by NMR spectroscopic analysis, was identified as a cyclic mono(GSH) conjugate of the BDA intermediate, which was also found in the biliary samples of rutaevin-treated rats. Further inhibitory experiments suggested that ketoconazole showed strong inhibitory effect on the formation of the rutaevin-derived BDA intermediate. CYP3A4 was demonstrated to be the major enzyme responsible for rutaevin bioactivation by using cDNA-expressed human recombinant cytochrome P450 enzymes. Additionally, it was found that rutaevin was a mechanism-based inactivator of CYP3A4, with inactivation parameters of KI = 15.98 μM, kinact = 0.032 min-1, and t1/2inact = 21.65 min. In summary, these findings are of great significance in understanding the bioactivation mechanism of rutaevin, the potential mechanism of rutaevin-caused hepatotoxicity, and the drug-drug interactions associated with rutaevin mainly via CYP3A4 inactivation.

In vitro inhibition of human cytochrome P450 enzymes by the novel atypical antipsychotic drug asenapine: a prediction of possible drug\u2013drug interactions

BackgroundInhibition of cytochrome P450 (CYP) enzymes is the most common cause of harmful drug–drug interactions. The present study aimed at examining the inhibitory effect of the novel antipsychotic drug asenapine on the main CYP enzymes in human liver.MethodsThe experiments were performed in vitro using pooled human liver microsomes and the human cDNA-expressed CYP enzymes: CYP1A2, CYP2C9, CYP2C19, CYP2D6, and CYP3A4 (Supersomes). Activities of CYP enzymes were determined using the CYP-specific reactions: caffeine 3-N-demethylation (CYP1A2), diclofenac 4′-hydroxylation (CYP2C9), perazine N-demethylation (CYP2C19), bufuralol 1′-hydroxylation (CYP2D6), and testosterone 6β-hydroxylation (CYP3A4). The rates of the CYP-specific reactions were assessed in the absence and presence of asenapine using HPLC.ResultsThe obtained results showed that both in human liver microsomes and Supersomes asenapine potently and to a similar degree inhibited the activity of CYP1A2 via a mixed mechanism (Ki = 3.2 μM in liver microsomes and Supersomes) and CYP2D6 via a competitive mechanism (Ki = 1.75 and 1.89 μM in microsomes and Supersomes, respectively). Moreover, asenapine attenuated the CYP3A4 activity via a non-competitive mechanism (Ki = 31.3 and 27.3 μM in microsomes and Supersomes, respectively). In contrast, asenapine did not affect the activity of CYP2C9 or CYP2C19.ConclusionThe potent inhibition of CYP1A2 and CYP2D6 by asenapine, demonstrated in vitro, will most probably be observed also in vivo, since the calculated Ki values are close to the presumed concentration range for asenapine in the liver in vivo. Therefore, pharmacokinetic interactions involving asenapine and CYP2D6 or CYP1A2 substrates are likely to occur during their co-administration to patients.

In Vitro Metabolism of 25B-NBF, 2-(4-Bromo-2,5-Dimethoxyphenyl)-N-(2-Fluorobenzyl)ethanamine, in Human Hepatocytes Using Liquid Chromatography⁻Mass Spectrometry.

25B-NBF, 2-(4-bromo-2,5-dimethoxyphenyl)-N-(2-fluorobenzyl)ethanamine, is a new psychoactive substance classified as a phenethylamine. It is a potent agonist of the 5-hydroxytryptamine receptor, but little is known about its metabolism and elimination properties since it was discovered. To aid 25B-NBF abuse screening, the metabolic characteristics of 25B-NBF were investigated in human hepatocytes and human cDNA-expressed cytochrome P450 (CYP) and UDP-glucuronosyltransferase (UGT) enzymes using liquid chromatography–high resolution mass spectrometry. At a hepatic extraction ratio of 0.80, 25B-NBF was extensively metabolized into 33 metabolites via hydroxylation, O-demethylation, bis-O-demethylation, N-debenzylation, glucuronidation, sulfation, and acetylation after incubation with pooled human hepatocytes. The metabolism of 25B-NBF was catalyzed by CYP1A1, CYP1A2, CYP2B6, CYP2C9, CYP2C19, CYP2D6, CYP2J2, CYP3A4, and UGT2B7 enzymes. Based on these results, it is necessary to develop a bioanalytical method for the determination of not only 25B-NBF but also its metabolites in biological samples for the screening of 25B-NBF abuse.

In vitro study of modulatory effects of extracts of Strobilanthes crispus on human cDNA-expressed cytochrome P450 2A6 (CYP2A6)

In vitro study of modulatory effects of extracts of Strobilanthes crispus on human cDNA-expressed cytochrome P450 2A6 (CYP2A6)

Metabolic characterization of (1-(5-fluoropentyl)-1H-indol-3-yl)(4-methyl-1-naphthalenyl)-methanone (MAM-2201) using human liver microsomes and cDNA-overexpressed cytochrome P450 enzymes.

MAM-2201 is a synthetic cannabinoid that is increasingly found in recreational drug abusers and cases of severe intoxication. Thus, characterization of the metabolic pathways of MAM-2201 is necessary to predict individual pharmacokinetics and toxicity differences, and to avoid toxic drug-drug interactions. Collectively, 19 phase 1 metabolites of MAM-2201 were identified using liquid chromatography-Orbitrap mass spectrometry following human liver microsomal incubations in the presence of NADPH: 7 hydroxy-MAM-2201 (M1-M7), 4 dihydroxy-MAM-2201 (M8-M11), dihydrodiol-MAM-2201 (M12), N-(5-hydroxypentyl)-MAM-2201 (M13), hydroxy-M13 (M14), N-dealkyl-MAM-2201 (M15), 2 hydroxy-M15 (M16, M17), MAM-2201 N-pentanoic acid (M18), and hydroxy-M18 (M19). On the basis of intrinsic clearance values in human liver microsomes, hydroxy-MAM-2201 (M1), N-(5-hydroxypentyl)-MAM-2201 (M13), and hydroxy-M13 (M14) were the major metabolites. Based on an enzyme kinetics study using human cDNA-expressed cytochrome P450 (CYP) enzymes and an immunoinhibition study using selective CYP antibodies in human liver microsomes, CYP1A2, CYP2B6, CYP2C8, CYP2C9, CYP2C19, CYP2D6, and CYP3A4 enzymes were responsible for MAM-2201 metabolism. The CYP3A4 enzyme played a prominent role in MAM-2201 metabolism, and CYP1A2, CYP2B6, CYP2C8, and CYP2C9 enzymes played major roles in the formation of some metabolites. MAM-2201 is extensively metabolized by multiple CYP enzymes, indicating that MAM-2201 and its metabolites should be used as markers of MAM-2201 abuse and toxicity. Graphical abstract In vitro metabolic pathways of MAM-2201 were characterized in human liver microsomes and recombinant CYPs using LC-HRMS analysis. Total 19 phase I metabolites were identified with predominant contribution of CYP3A4.

Biotransformation of Cobicistat: Metabolic Pathways and Enzymes.

Cobicistat (COBI) is a pharmacoenhancer for antiretroviral therapy. The current study was designed to profile the metabolic pathways of COBI and to determine the enzymes that contribute to COBI metabolism. We screened COBI metabolites in mice and human liver microsomes. We also used cDNAexpressed human cytochromes P450 (CYPs) to explore the role of human enzymes in COBI metabolism. Twenty new and three known metabolites of COBI were identified in mouse urine and feces. These new metabolic pathways of COBI include glycine conjugation, N-acetyl cysteine conjugation, morpholine ring-opening, and thiazole ring-opening. Twelve of COBI metabolites were further confirmed in mouse and human liver microsomes, including nine new metabolites. Consistent with the previous report, CYP3A4 and CYP2D6 were determined as the major enzymes that contribute to COBI metabolism. This study provided a full map of COBI metabolism. These results can be used to manage CYP-mediated drug-drug interactions and adverse drug reactions that are associated with COBI-containing regimens in human.

In vitro inhibitory effect of luotonin A on human CYP1A

Luotonin A, a pyrroloquinolinequinoline alkaloid, is a natural inhibitor of topoisomerase I. In the present study, cytochrome P450 (CYP) inhibition by luotonin A was examined in pooled human liver microsomes (HLMs) and human recombinant cDNA-expressed human CYPs using a cocktail probe assay to investigate potential drug-drug interactions. Luotonin A selectively inhibited CYP1A2-catalyzed phenacetin O-deethylation with an IC(50) of 6.3 μM in HLMs, and strongly decreased CYP1A2-catalyzed phenacetin O-deethylation dose-dependently in HLMs, but did not inhibit it time-dependently. Furthermore, the Lineweaver-Burk and secondary plots for the inhibition of CYP1A2 in HLMs well fitted competitive inhibition mode. Luotonin A showed the selectivity of inhibitory effects on CYP1A1 and CYP1A2 in human recombinant cDNA-expressed CYP 1A1 and 1A2, respectively. Luotonin A was found to be a potent CYP1A inhibitor that might cause drug-drug interactions when co-administrated with CYP1A substrates.

CPY3A4-Mediated Lopinavir Bioactivation and Its Inhibition by Ritonavir

The combination of lopinavir (LPV) and ritonavir (RTV) is one of the preferred regimens for the treatment of HIV infection with confirmed efficacy and relatively low toxicity. LPV alone suffers the poor bioavailability due to its rapid and extensive metabolism. RTV boosts the plasma concentration of LPV by suppressing its metabolism and thus increasing LPV efficacy. In the current study, we found that RTV also inhibits LPV bioactivation. LPV bioactivation was investigated in human liver microsomes and cDNA-expressed human cytochromes P450. Twelve GSH-trapped reactive metabolites of LPV were identified by using a metabolomic approach. Semicarbazide-trapped reactive metabolites of LPV were also detected. RTV effectively suppressed all pathways of LPV bioactivation via CYP3A4 inhibition. Our data together with previous reports suggest that LPV plus RTV is an ideal combination because RTV not only boosts LPV plasma concentration, but it decreases LPV bioactivation.

In Vitro Characterization of the Metabolic Pathways and Cytochrome P450 Inhibition and Induction Potential of BMS-690514, an ErbB/Vascular Endothelial Growth Factor Receptor Inhibitor

(3R,4R)-4-Amino-1-((4-((3-methoxyphenyl)amino)pyrrolo[2,1-f][1,2,4]triazin-5-yl)methyl)-3-piperidinol (BMS-690514) is a potent inhibitor of ErbB human epidermal growth factor receptors (HER1, 2, and 4) and vascular endothelial growth factor receptors 1 to 3 that has been under clinical development for solid tumor malignancies. BMS-690514 is primarily cleared by metabolism with the primary metabolic pathways being direct glucuronidation (M6), hydroxylation (M1, M2, and M37), and O-demethylation (M3). In the current investigation, the metabolic drug-drug interaction potential of BMS-690514 was evaluated in a series of in vitro studies. Reaction phenotyping experiments with cDNA-expressed human cytochrome P450 (P450) and UDP-glucuronosyltransferase (UGT) enzymes and human liver microsomes (HLM) in the presence of P450 or UGT inhibitors suggested that CYP3A4, CYP2D6, and CYP2C9 were the major enzymes responsible for the oxidative metabolism of BMS-690514, whereas both UGT2B4 and UGT2B7 were responsible for the formation of M6. BMS-690514 did not cause direct or time-dependent inhibition of P450 enzymes (IC(50) values ≥40 μM) in incubations with HLM and probe substrates of CYP1A2, 2A6, 2B6, 2C8, 2C9, 2C19, 2D6, or 3A4. The compound also did not substantially induce CYP1A1, CYP1A2, CYP2B6, CYP3A4, or UGT1A1 at concentrations up to 10 μM in cultured human hepatocytes. Considering the submicromolar plasma C(max) concentration at the anticipated clinical dose of 200 mg, BMS-690514 is unlikely to cause clinically relevant drug-drug interactions when coadministered with other medications. In addition, because multiple enzymatic clearance pathways are available for the compound, inhibition of an individual metabolic pathway either via coadministered drugs or gene polymorphisms is not expected to cause pronounced (>2-fold) increases in BMS-690514 exposure.

In vitro determination of the effect of Andrographis paniculata extracts and andrographolide on human hepatic cytochrome P450 activities

We investigated the effects of Andrographis paniculata (AP) extracts and andrographolide on the catalytic activity of three human cDNA-expressed cytochrome P450 enzymes: CYP2C9, CYP2D6 and CYP3A4. In vitro probe-based high performance liquid chromatography assays were developed to determine CYP2C9-dependent tolbutamide methylhydroxylation, CYP2D6-dependent dextromethorphan O-demethylation and CYP3A4-dependent testosterone 6β-hydroxylation activities in the presence and absence of AP extracts and andrographolide. Our results indicate that AP ethanol and methanol extracts inhibited CYP activities more potently than aqueous and hexane extracts across the three isoforms. Potent inhibitory effects were observed on CYP3A4 and CYP2C9 activities (K (i) values below 20μg/ml). Andrographolide was found to exclusively but weakly inhibit CYP3A4 activity. In conclusion, data presented in this study suggest that AP extracts have the potential to inhibit CYP isoforms in vitro. There was, however, variation in the potency of inhibition depending on the extracts and the isoforms investigated.

In vitro metabolism of magnolin and characterization of cytochrome P450 enzymes responsible for its metabolism in human liver microsomes

Magnolin is a major bioactive component found in Shin-i, the dried flower buds of Magnolia fargesii; it has anti-inflammatory and anti-histaminic activities. Incubation of magnolin in human liver microsomes with an nicotinamide adenine dinucleotide phosphate-generating system resulted in the formation of five metabolites, namely, O-desmethyl magnolin (M1 and M2), didesmethylmagnolin (M3), and hydroxymagnolin (M4 and M5).In this study, we characterized the human liver cytochrome P450 (CYP) enzymes responsible for the biotransformation of three major metabolites—M1, M2, and M4—of magnolin. CYP2C8, CYP2C9, CYP2C19, and CYP3A4 were identified as the major enzymes responsible for the formation of the two O-desmethyl magnolins (M1 and M2), on the basis of a combination of correlation analysis and experiments, including immunoinhibition of magnolin in human liver microsomes and metabolism of magnolin by human cDNA-expressed CYP enzymes. CYP2C8 played a predominant role in the formation of hydroxymagnolin (M4).These results suggest that the pharmacokinetics of magnolin may not be affected by CYP2C8, CYP2C9, CYP2C19, and CYP3A4 responsible for the metabolism of magnolin or by the co-administration of appropriate CYP2C8, CYP2C9, CYP2C19, and CYP3A4 inhibitors or inducers due to the involvement of multiple CYP enzymes in the metabolism of magnolin.

In vitro effects of active constituents and extracts of Orthosiphon stamineus on the activities of three major human cDNA-expressed cytochrome P450 enzymes

Orthosiphon stamineus (OS) has been traditionally used to treat diabetes, kidney and urinary disorders, high blood pressure and bone or muscular pain. To assess the possibility of drug–herb interaction via interference of metabolism, effects of four OS extracts of different polarity and three active constituents (sinensetin, eupatorin and rosmarinic acid) on major human cDNA-expressed cytochrome P450 (CYP) enzymes were investigated. Three substrate-probe based high-performance liquid chromatography (HPLC) assays were established to serve as activity markers for CYP2C9, CYP2D6 and CYP3A4. Our results indicate that OS extracts and constituents exhibited differential modulatory effects on different CYPs. While none of the OS components showed significant inhibition on CYP2C9, eupatorin strongly and uncompetitively inhibited CYP2D6 activity with a Ki value of 10.2μM. CYP3A4 appeared to be the most susceptible enzyme to OS inhibitory effects. It was moderately inhibited by OS dichloromethane and petroleum ether extract with mixed-type and noncompetitive inhibitions (Ki=93.7 and 44.9μg/mL), respectively. Correlation study indicated that the inhibition was accounted for by the presence of eupatorin in the extracts. When IC50 values of these extracts were expressed in volume per dose unit to reflect inhibitory effect at recommended human doses from commercially available products, moderate inhibition was also observed. In addition, CYP3A4 was strongly and noncompetitively inhibited by eupatorin alone, with a Ki value of 9.3μM. These findings suggest that co-administration of OS products, especially those with high eupatorin content, with conventional drugs may have the potential to cause drug–herb interactions involving inhibition of major CYP enzymes.

Main contribution of the cytochrome P450 isoenzyme 1A2 (CYP1A2) to N-demethylation and 5-sulfoxidation of the phenothiazine neuroleptic chlorpromazine in human liver—A comparison with other phenothiazines

The aim of the present study was to identify cytochrome P450 (CYP) isoenzymes involved in the 5-sulfoxidation, mono-N-demethylation and di-N-demethylation of the aliphatic-type phenothiazine neuroleptic chlorpromazine in human liver. Experiments were performed in vitro using cDNA-expressed human CYP isoforms (Supersomes 1A2, 2A6, 2B6, 2C8, 2C9, 2C19, 2D6, 2E1, 3A4), liver microsomes from different donors and CYP-selective inhibitors. The obtained results indicate that CYP1A2 is the only CYP isoform that catalyzes the mono-N-demethylation and di-N-demethylation of chlorpromazine (100%) and is the main isoform responsible for chlorpromazine 5-sulfoxidation (64%) at a therapeutic concentration of the drug (10 μM). CYP3A4 contributes to a lesser degree to chlorpromazine 5-sulfoxidation (34%). The role of CYP2B6, CYP2C19 and CYP2D6 in catalyzing of the latter reaction is negligible (0.1–2%). Similar results were obtained at a higher, non-therapeutic concentration of the drug (100 μM); however, the contribution of CYP1A2 to chlorpromazine mono-N-demethylation was noticeably lower (75%), mostly in favour of CYP2C19 and CYP3A4 (about 12% each). The obtained results indicate that the catalysis of chlorpromazine N-demethylation and 5-sulfoxidation in humans exhibits a stricter CYP1A2 preference compared to the previously tested phenothiazines (promazine, perazine, and thioridazine). Hence pharmacokinetic interactions involving chlorpromazine and CYP1A2 substrates and inhibitors are likely to occur. Considering strong dopaminergic D 2, noradrenergic α 1 and cholinergic M 1 receptor blocking properties of chlorpromazine and some of its metabolites, as well as their serious side effects, the obtained results may be of pharmacological and clinical importance.

In vitro modulatory effects on three major human cytochrome P450 enzymes by multiple active constituents and extracts of Centella asiatica

Ethnopharmacological relevance Centella asiatica (CA) has been widely cultivated as a vegetable or spice in China, Southeast Asia, India, Sri Lanka, Africa, and Oceanic countries and traditionally used for wound healing and maintaining normal blood pressure. Aim of the study The present study was carried out to examine the potential modulatory effects of three commercially available active components (asiaticoside, asiatic acid and madecassic acid) and four extracts (aqueous, ethanol, dichloromethane and hexane) of CA on three major cDNA-expressed human cytochrome P450 (CYP) isoforms. Materials and methods High-performance liquid chromatography (HPLC)-based enzyme assays, namely tolbutamide 4-methyhydroxylase, dextromethorphan O-demethylase and testosterone 6β-hydroxylase assays were developed to probe activities of CYP2C9, CYP2D6 and CYP3A4, respectively. Probe substrates were incubated with or without each active component and extract for each isoform, followed by examination of the kinetics parameters, IC 50 and K i, to characterize modulatory effects. Results CYP2C9 was more susceptible to inhibitory effects by CA extracts compared to CYP2D6 and CYP3A4. Moderate degree of inhibition was observed in ethanol ( K i = 39.1 μg/ml) and dichloromethane ( K i = 26.6 μg/ml) extracts implying potential risk of interaction when CYP2C9 substrates are consumed with CA products. The two extracts however showed negligible inhibition towards CYP2D6 and CYP3A4 (IC 50's of 123.3 μg/ml and above). Similarly CA aqueous and hexane extracts did not significantly inhibit all three isoforms investigated (IC 50's of 117.9 μg/ml and above). Among the active constituents investigated, asiatic acid and madecassic acid appeared to selectively inhibit CYP2C9 and CYP2D6 more than CYP3A4. Of particular interest is the potent inhibitory effect of asiatic acid on CYP2C9 ( K i = 9.1 μg/ml). This signifies potential risk of interaction when substrates for this isoform are taken together with CA products with high asiatic acid content. Inhibitions of asiatic acid with the other isoforms and that of madecassic acid with all isoforms were only moderate ( K i's ranged from 17.2 to 84.4 μg/ml). On the other hand, the IC 50 values for asiaticoside were high (1070.2 μg/ml or above) for all three isoforms, indicating negligible or low potential of this compound to modulate CYP enzymatic activity. Conclusion Centella asiatica extracts and active constituents inhibited CYP2C9, CYP2D6 and CYP3A4 activities with varying potency with CYP2C9 being the most susceptible isoform to inhibition. Significant inhibition was observed for asiatic acid and CA ethanol and dichloromethane extracts, implying involvement of semipolar constituents from CA in the effect. This study suggested that CA could cause drug–herb interactions through CYP2C9 inhibition.

Investigations on the cytochrome P450 (CYP) isoenzymes involved in the metabolism of the designer drugs N-(1-phenyl cyclohexyl)-2-ethoxyethanamine and N-(1-phenylcyclohexyl)-2-methoxyethanamine

Investigations using insect cell microsomes with cDNA-expressed human cytochrome P450 (CYP)s and human liver microsomes (HLM) are reported on the CYP isoenzymes involved in the metabolism of the designer drugs N-(1-phenylcyclohexyl)-2-ethoxyethanamine (PCEEA) to O-deethyl PCEEA and N-(1-phenylcyclohexyl)-2-methoxyethanamine (PCMEA) to O-demethyl PCMEA. Gas chromatography–mass spectrometry or liquid chromatography–mass spectrometry was used for the analysis of the incubation samples. PCEEA O-deethylation was catalyzed by CYP2B6, CYP2C9, CYP2C19, and CYP3A4, while PCMEA O-demethylation was catalyzed only by CYP2B6 and CYP2C19. Considering the relative activity factor approach, these enzymes accounted for 53%, 25%, 4%, and 18% of net clearance for PCEEA and 91% and 9% of net clearance for PCMEA, respectively. The chemical CYP2B6 inhibitor 4-(4-chlorobenzyl)pyridine (CBP) reduced the metabolite formation in pooled HLM by 63% at 1 μM PCEEA. At 10 μM PCEEA, CBP reduced metabolite formation by 61%, while inhibition of CYP3A4 by ketoconazole and inhibition of CYP2C9 by sulfaphenazole showed no inhibitory effect. At 1 μM PCMEA, CBP reduced metabolite formation in pooled HLM by 70% and at 10 μM PCMEA by 78%, respectively. In conclusion, the main metabolic step of both studied drugs was catalyzed by different CYPs.

Research Highlights

Research Highlights

Identification of Human Liver Cytochrome P450 Isoforms Involved in Autoinduced Metabolism of the Antiangiogenic Agent (Z)-5-[(1,2-Dihydro-2-oxo-3H-indol-3-ylidene)methyl]-2,4-dimethyl-1H-pyrrole-3-propanoic Acid (TSU-68)

(Z)-5-[(1,2-Dihydro-2-oxo-3H-indol-3-ylidene)methyl]-2,4-dimethyl-1H-pyrrole-3-propanoic acid (TSU-68) is a new anticancer drug that inhibits angiogenic receptor tyrosine kinases, which play a crucial role in tumor-induced vascularization. TSU-68 undergoes hepatic oxidation and glucuronidation. Incubation of TSU-68 with human liver microsomes in the presence of NADPH resulted in the formation of three major metabolites: 5-, 6-, and 7-hydroxyindolinone derivatives. The 5-, 6-, and 7-hydroxylation followed simple Michaelis-Menten kinetics with V(max)/K(m) values (an indicator of intrinsic clearance) of 13, 25, and 6 microl/min/mg, respectively. Of the 10 cDNA-expressed human cytochrome P450 isoforms examined, only CYP1A1 and CYP1A2 exhibited appreciable TSU-68 hydroxylation activity. Inhibition studies with alpha-naphthoflavone (a selective CYP1A2 inhibitor) and anti-CYP1A2 antibody also indicated the almost exclusive role of CYP1A2 in microsomal TSU-68 hydroxylation. Treatment of human hepatocytes with 10 microM TSU-68 resulted in a 28- to 140-fold increase in CYP1A1/2-mediated ethoxyresorufin O-deethylase activity. The protein levels of CYP1A2 were increased in TSU-68-treated hepatocytes, and those of CYP1A1, which were undetectable in control hepatocytes, were also increased to detectable levels in the TSU-68-treated hepatocytes. Thus, TSU-68 was shown to induce CYP1A1/2 expression, which was responsible for its hydroxylation. The observation that TSU-68 treatment resulted in a 10- to 45-fold increase in 5-, 6-, and 7-hydroxylation directly demonstrated the autoinduced hydroxylation of TSU-68. In conclusion, TSU-68 has the potential to cause induction of its own CYP1A1/2-mediated oxidative metabolism in humans. This autoinductive effect provides a clear explanation for the clinically observed decrease in TSU-68 plasma concentrations during repeated administration of the drug.

In Vitro Metabolism of the Analgesic Bicifadine in the Mouse, Rat, Monkey, and Human

The in vitro metabolism of [(14)C]bicifadine by hepatic microsomes and hepatocytes from mouse, rat, monkey, and human was compared using radiometric high-performance liquid chromatography and liquid chromatography/tandem mass spectrometry. Two main metabolic pathways were identified in all four species. One pathway was an NADPH-dependent pathway in which the methyl group was oxidized to form a hydroxymethyl metabolite (M2). Its formation was inhibited in human microsomes only by quinidine, a CYP2D6 inhibitor. In incubations with individual cDNA-expressed human cytochromes P450, M2 was formed only by CYP2D6 and CYP1A2, with CYP2D6 activity 6-fold greater than that of CYP1A2. M2 was oxidized further to the carboxylic acid metabolite (M3) by hepatocytes from all four species. The second major metabolic pathway was an NADPH-independent oxidation at the C2 position of the pyrrolidine ring, forming a lactam metabolite (M12). This reaction was almost completely inhibited in human hepatic microsomes and mitochondria by the monoamine oxidase (MAO)-B-specific inhibitor selegiline. Clorgyline, a specific inhibitor of MAO-A, was less effective in inhibiting M12 formation. Other metabolic pathways of variable significance among the four species included the formation of carbamoyl-O-glucuronide, hydroxymethyl lactam, and carboxyl lactam. Overall, the data indicate that the primary enzymes responsible for the primary metabolism of bicifadine in humans are MAO-B and CYP2D6.

In vitro metabolism of eupatilin by multiple cytochrome P450 and UDP-glucuronosyltransferase enzymes

Eupatilin, a pharmacologically active flavone derived from Artemisia plants, is extensively metabolized to eupatilin glucuronide, 4-O-desmethyleupatilin and 4-O-desmethyleupatilin glucuronide in human liver microsomes. This study characterized the human liver cytochrome P450 (CYP) and UDP-glucuronosyltransferase (UGT) enzymes responsible for the metabolism of eupatilin. The specific CYPs responsible for O-demethylation of eupatilin to the major metabolite, 4-O-desmethyleupatilin were identified using a combination of correlation analysis, immuno-inhibition, chemical inhibition in human liver microsomes and metabolism by human cDNA-expressed CYP enzymes. UGT enzymes involved in the eupatilin glucuronidation were identified using pooled human liver microsomes and human cDNA-expressed UGT enzymes. Eupatilin was predominantly metabolized by CYP1A2 and, to a lesser extent, CYP2C8 mediated O-demethylation of eupatilin to 4-O-desmethyleupatilin. Eupatilin glucuronidation was catalysed by UGT1A1, UGT1A3, UGT1A7, UGT1A8, UGT1A9, and UGT1A10.

Characterization of Human Liver Cytochrome P-450 Enzymes Involved in the O-demethylation of a New P-glycoprotein Inhibitor HM-30181*

HM-30181, 4-oxo-4H-chromene-2-carboxylic acid [2-(2-{4-[2-(6,7-dimethoxy-3,4-dihydro-1H-isoquinolin-2-yl)-ethyl]-phenyl}-2H-tetrazol-5-yl)-4,5-dimethoxy-phenyl]-amide, is a new P-glycoprotein inhibitor with the potential to increase the cytotoxic activity of orally coadministered paclitaxel. This study was performed to characterize human cytochrome P-450 (CYP) enzymes involved in the metabolism of HM-30181 to 4- or 5-O-desmethyl-HM-30181 (M2) and 6- or 7-O-desmethyl-HM-30181 (M3) and to investigate the inhibitory potential of HM-30181 on CYP enzymes in human liver microsomes. CYP3A4 was identified as the major isozyme responsible for the O-demethylation of HM-30181 to M2 and M3 based on the correlation analysis, chemical inhibition and immuno-inhibition study and metabolism in cDNA-expressed human CYP isozymes. HM-30181 itself had no inhibitory effects on CYPs 1A2, 2A6, 2C8, 2C9, 2C19, 2D6, and 3A4 in human liver microsomes, suggesting the possibility that the pharmacokinetics of HM-30181 could be changed with coadministration of known CYP3A4 inducers or inhibitors.