Oxidation and rearrangements of flavanones by mammalian cytochrome P450

1. To clarify the metabolic pathways of flavanones in mammals, the metabolism of (±)-flavanone and (±)-4′-methoxyflavanone by rat liver microsomes and recombinant human P450s in which structural changes are readily identifiable were examined.

2. The β-nicotinamide adenine dinucleotide phosphate (NADPH)-dependent formation of flavone plus (±)-2,3-trans-flavanonol and of 4′-methoxyflavone plus (±)-2,3-trans-4′-methoxyflavanonol, respectively, by rat liver microsomes was observed.

3. The same metabolites were generated by recombinant human P450s in addition to the formation of isoflavone from (±)-flavanone.

4. The kinetic isotope effects in these reactions were examined using deuterated (±)-flavanone and (±)-4′-methoxyflavanone. There was a strong isotope effect in the production of flavanonols, but the isotope effect in the production of flavones was small. The results indicated that the P450-mediated conversion of (±)-flavanone and of (±)-4′-methoxyflavanone to the corresponding metabolites proceeded via abstraction of a hydrogen radical from the C-2- or C-3-position of the flavanone skeleton.

5. The antioxidant properties of flavanone and its metabolites were examined by measuring superoxide-scavenging activity in a xanthine–xanthine oxidase-cytochrome c system. (±)-2,3-trans-Flavanonol had higher activity than that of other flavonoids.

6. Flavanones are metabolized by mammalian P450s, providing important information relevant to the metabolism and pharmacological action of dietary flavanones.

     

Deactivation of anti-cancer drug letrozole to a carbinol metabolite by polymorphic cytochrome P450 2A6 in human liver microsomes

1. Cytochromes P450 (P450) involved in letrozole metabolism were investigated. Among 13 recombinant P450 forms examined, only P450 2A6 and 3A4 showed activities in transforming letrozole to its carbinol metabolite with small K_m and high V_max values yielding apparent V_max/K_m values of 0.48 and 0.24 nl min^?1 nmol^?1 P450, respectively.

2. The metabolic activities of individual human liver microsomes showed a significant correlation with coumarin 7-hydroxylase activities (P450 2A6 marker) at a letrozole concentration of 0.5 μM, while a good correlation was also seen with testosterone 6β-hydroxylase activities (P450 3A4 marker) at 5 μM substrate concentration with different inhibition by 8-methoxypsolaren.

3. Significantly low carbinol-forming activities were seen in human liver microsomes from individuals possessing CYP2A6*4/*4 (whole CYP2A6 gene deletion) at a letrozole concentration of 0.5 μM. A V_max/K_m value measured for CYP2A6.7 (amino acid substitution type) in human liver microsomes, in the presence of anti-P450 3A4 antibodies, was approximately seven-fold smaller than that for CYP2A6.1 (wild-type).

4. These results demonstrate that P450 2A6 and 3A4 catalyse the conversion of letrozole to its carbinol metabolite in vitro at low and high concentrations of letrozole. Polymorphic variation of CYP2A6 is considered to be relevant to inter-subject variation in therapeutic exposure of letrozole.

     

Cytochrome P450 2A6 and other human P450 enzymes in the oxidation of flavone and flavanone

1.?We previously reported that flavone and flavanone interact spectrally with cytochrome P450 (P450 or CYP) 2A6 and 2A13 and other human P450s and inhibit catalytic activities of these P450 enzymes. In this study, we studied abilities of CYP1A1, 1A2, 1B1, 2A6, 2A13, 2C9 and 3A4 to oxidize flavone and flavanone.

2.?Human P450s oxidized flavone to 6- and 5-hydroxylated flavones, seven uncharacterized mono-hydroxylated flavones, and five di-hydroxylated flavones. CYP2A6 was most active in forming 6-hydroxy- and 5-hydroxyflavones and several mono- and di-hydroxylated products.

3.?CYP2A6 was also very active in catalyzing flavanone to form 2′- and 6-hydroxyflavanones, the major products, at turnover rates of 4.8?min^?1 and 1.3?min^?1, respectively. Other flavanone metabolites were 4′-, 3′- and 7-hydroxyflavanone, three uncharacterized mono-hydroxylated flavanones and five mono-hydroxylated flavones, including 6-hydroxyflavone. CYP2A6 catalyzed flavanone to produce flavone at a turnover rate of 0.72?min^?1 that was ～3-fold higher than that catalyzed by CYP2A13 (0.29?min^?1).

4.?These results indicate that CYP2A6 and other human P450s have important roles in metabolizing flavone and flavanone, two unsubstituted flavonoids, present in dietary foods. Chemical mechanisms of P450-catalyzed desaturation of flavanone to form flavone are discussed.     

Development of new Coumarin-based profluorescent substrates for human cytochrome P450 enzymes

1. Cytochrome P450 (CYP) enzymes constitute an essential xenobiotic metabolizing system that regulates the elimination of lipophilic compounds from the body. Convenient and affordable assays for CYP enzymes are important for assessing these metabolic pathways.

2. In this study, 10 novel profluorescent coumarin derivatives with various substitutions at carbons 3, 6 and 7 were developed. Molecular modeling indicated that 3-phenylcoumarin offers an excellent scaffold for the development of selective substrate compounds for various human CYP forms, as they could be metabolized to fluorescent 7-hydroxycoumarin derivatives. Oxidation of profluorescent coumarin derivatives to fluorescent metabolites by 13 important human liver xenobiotic-metabolizing CYP forms was determined by enzyme kinetic assays.

3. Four of the coumarin derivatives were converted to fluorescent metabolites by CYP1 family enzymes, with 6-methoxy-3-(4-trifluoromethylphenyl)coumarin being oxidized selectively by CYP1A2 in human liver microsomes. Another set of four compounds were metabolized by CYP2A6 and CYP1 enzymes. 7-Methoxy-3-(3-methoxyphenyl)coumarin was oxidized efficiently by CYP2C19 and CYP2D6 in a non-selective fashion.

4. The advantages of the novel substrates were (1) an excellent signal-to-background ratio, (2) selectivity for CYP1 forms, and (3) convenient multiwell plate measurement, allowing for precise determination of potential inhibitors of important human hepatic forms CYP1A2, CYP2C19 and CYP2D6.

     

Regioselectivity significantly impacts microsomal glucuronidation efficiency of R/S-6, 7-, and 8-hydroxywarfarin

1. Coumadin (R/S-warfarin) metabolism plays a critical role in patient response to anticoagulant therapy. Several cytochrome P450s oxidize warfarin into R/S-6-, 7-, 8-, 10, and 4′-hydroxywarfarin that can undergo subsequent glucuronidation by UDP-glucuronosyltransferases (UGTs); however, current studies on recombinant UGTs cannot be adequately extrapolated to microsomal glucuronidation capacities for the liver.

2. Herein, we estimated the capacity of the average human liver to glucuronidate hydroxywarfarin and identified UGTs responsible for those metabolic reactions through inhibitor phenotyping. There was no observable activity toward R/S-warfarin, R/S-10-hydroxywarfarin or R/S-4′-hydroxywarfarin.

3. The observed metabolic efficiencies (V_max/K_m) toward R/S-6-, 7-, and especially 8-hydroxywarfarin indicated a high glucuronidation capacity to metabolize these compounds.

4. UGTs demonstrated strong regioselectivity toward the hydroxywarfarins. UGT1A6 and UGT1A1 played a major role in R/S-6- and 7-hydroxywarfarin glucuronidation, respectively, whereas UGT1A9 accounted for almost all of the generation of the R/S-8-hydroxywarfarin glucuronide.

5. In summary, these studies expanded insights to glucuronidation of hydroxywarfarins by pooled human liver microsomes, novel roles for UGT1A6 and 1A9, and the overall degree of regioselectivity for the UGT reactions.

     

Characterization of in vitro and in vivo metabolism of leelamine using liquid chromatography-tandem mass spectrometry

1. Leelamine is a diterpene compound found in the bark of pine trees and has garnered considerable interest owing to its potent anticancer properties. The aim of the present study was to investigate the metabolic profile of leelamine in human liver microsomes (HLMs) and mice using liquid chromatography-tandem mass spectrometry (LC-MS/MS).

2. We found that leelamine undergoes only Phase I metabolism, which generates one metabolite that is mono-hydroxylated at the C9 carbon of the octahydrophenanthrene ring (M1) both in vitro and in vivo. The structure and metabolic pathway of M1 were determined from the MSⁿ fragmentation obtained by collision-induced dissociation using LC-MS/MS in HLMs.

3. Cytochrome p450 (CYP) 2D6 was found to be the dominant CYP enzyme involved in the biotransformation of leelamine to its hydroxylated metabolite, whereas CYP2C19, CYP1A1, and CYP3A4 contributed to some extent.

4. Moreover, we identified only one metabolite M1, in the urine, but none in the feces. In conclusion, leelamine was metabolized to a mono-hydroxyl metabolite by CYP2D6 and mainly excreted in the urine.

     

Minor contribution of CYP3A5 to the metabolism of hepatitis C protease inhibitor paritaprevir in vitro

1. Paritaprevir (PTV) is a non-structural protein 3/4A protease inhibitor developed for the treatment of hepatitis C disease as a fixed dose combination of ombitasvir (OBV) and ritonavir (RTV) with or without dasabuvir.

2. The aim of this study was to evaluate the effects of cytochrome P450 (CYP) 3A5 on in vitro PTV metabolism using human recombinant CYP3A4, CYP3A5 (rCYP3A4, rCYP3A5) and human liver microsomes (HLMs) genotyped as either CYP3A5*1/*1, CYP3A5*1/*3 or CYP3A5*3/*3.

3. The intrinsic clearance (CL_int, V_max/K_m) for the production of a metabolite from PTV in rCYP3A4 was 1.5 times higher than that in rCYP3A5. The PTV metabolism in CYP3A5*1/*1 and CYP3A5*1/*3 HLMs expressing CYP3A5 was comparable to that in CYP3A5*3/*3 HLMs, which lack CYP3A5.

4. CYP3A4 expression level was significantly correlated with PTV disappearance rate and metabolite formation. In contrast, there was no such correlation found for CYP3A5 expression level.

5. This study represents that the major CYP isoform involved in PTV metabolism is CYP3A4, with CYP3A5 having a minor role in PTV metabolism. The findings of the present study may provide foundational information on PTV metabolism, and may further support dosing practices in HCV-infected patients prescribed PTV-based therapy.

     

Marmoset cytochrome P450 2B6, a propofol hydroxylase expressed in liver

1. The common marmoset (Callithrix jacchus) is a useful experimental animal to evaluate the pharmacokinetics of drug candidates. Cytochrome P450 (P450) 2B enzyme in marmoset livers has been identified; however, only limited information on the enzymatic properties and distribution has been available.

2. Marmoset P450 2B6 amino acids showed high sequence identities (>86%) with those of primates including humans and cynomolgus monkeys. Phylogenetic analysis using amino acid sequences indicated that marmoset P450 2B6 was closer to human and cynomolgus monkey P450 2B6 than to P450 2B orthologs of other species, including pigs, dogs, rabbits and rodents.

3. Quantitative polymerase chain reaction analysis using specific primers showed P450 2B6 mRNA predominantly expressed in livers among the five marmoset tissues, similar to those of humans and cynomolgus monkeys.

4. Marmoset P450 2B6 heterologously expressed in Escherichia coli membranes oxidized 7-ethoxycoumarin, pentoxyresorufin, propofol and testosterone, at roughly similar rates to those of humans and/or cynomolgus monkeys. A high capacity of marmoset P450 2B6 with propofol 4-hydroxylation (at low ionic strength conditions) with a low K_m value was relatively comparable to that for marmoset livers.

5. These results collectively indicated a high propofol 4-hydroxylation activity of P450 2B6 expressed in marmoset livers.

     

Metabolism of deltamethrin and cis- and trans-permethrin by human expressed cytochrome P450 and carboxylesterase enzymes

1. The metabolism of the pyrethroids deltamethrin (DLM), cis-permethrin (CPM) and trans-permethrin (TPM) was studied in human expressed cytochrome P450 (CYP) and carboxylesterase (CES) enzymes.

2. DLM, CPM and TPM were metabolised by human CYP2B6 and CYP2C19, with the highest apparent intrinsic clearance (CL_int) values for pyrethroid metabolism being observed with CYP2C19. Other CYP enzymes contributing to the metabolism of one or more of the three pyrethroids were CYP1A2, CYP2C8, CYP2C9*1, CYP2D6*1, CYP3A4 and CYP3A5. None of the pyrethroids were metabolised by CYP2A6, CYP2E1, CYP3A7 or CYP4A11.

3. DLM, CPM and TPM were metabolised by both human CES1 and CES2 enzymes.

4. Apparent CL_int values for pyrethroid metabolism by CYP and CES enzymes were scaled to per gram of adult human liver using abundance values for microsomal CYP enzymes and for CES enzymes in liver microsomes and cytosol. TPM had the highest and CPM the lowest apparent CL_int values for total metabolism (CYP and CES enzymes) per gram of adult human liver.

5. Due to their higher abundance, all three pyrethroids were extensively metabolised by CES enzymes in adult human liver, with CYP enzymes only accounting for 2%, 10% and 1% of total metabolism for DLM, CPM and TPM, respectively.

     

Determination of cytochrome P450 enzymes involved in the metabolism of (−)-terpinen-4-ol by human liver microsomes

1. The in vitro metabolism of (?)-terpinen-4-ol was examined in human liver microsomes and recombinant enzymes.

2. The biotransformation of (?)-terpinen-4-ol was investigated by gas chromatography–mass spectrometry. (?)-Terpinen-4-ol was found to be oxidized to (?)-(1S,2R,4R)-1,2-epoxy-p-menthan-4-ol, major metabolic product by human liver microsomal P450 enzymes. The formation of metabolites of (?)-terpinen-4-ol was determined by relative abundance of mass fragments and retention times on GC.

3. CYP2A6 in human liver microsomes was a major enzyme involved in the oxidation of (?)-terpinen-4-ol by human liver microsomes, based on the following lines of evidence. First, of 11 recombinant human P450 enzymes tested, CYP2A6 had the highest activity for oxidation of (?)-terpinen-4-ol. Second, oxidation of (?)-terpinen-4-ol was inhibited by (+)-menthofuran. Finally, there was a good correlation between CYP2A6 maker activity and (?)-terpinen-4-ol oxidation activities in liver microsomes of 10 human samples.

4. Kinetic analysis showed that the V_max/K_m values for (?)-(1S,2R,4R)-1,2-epoxy-p-menthan-4-ol catalysed by liver microsomes of human sample HH-18 was 2.49 μL/min/nmol.

5. Human recombinant CYP2A6 catalysed (?)-(1S,2R,4R)-1,2-epoxy-p-menthan-4-ol with V_max values of 13.9 nmol/min/nmol P450 and apparent K_m values of 91 μM.

     

Methoxsalen as an in vitro phenotyping tool in comparison with 1-aminobenzotriazole

1. The objective is to evaluate methoxsalen as an in vitro phenotyping tool in comparison to ABT as a nonspecific inactivator of P450 mediated metabolism.

2. The reversible inhibition of methoxsalen and ABT against the P450, FMO, AO, MAO-A and -B, enzymes were evaluated using standard marker probe reactions. The time-dependent inhibition of P450 enzymes was evaluated in human liver microsomes. CES1 activities were determined by monitoring the depletion of known substrate, the clopidogrel. The metabolism of P450 substrates in the presence and absence of methoxsalen or ABT was evaluated in human liver microsomes.

3. Methoxsalen is a direct inhibitor and inhibited the activities (>90%) of all enzymes at a concentration of 300?µM except for CYP2C9. Methoxsalen is also a potent time-dependent inhibitor of all P450 enzymes except for CYP2C19 (moderate) at a concentration of 300?µM. Methoxsalen inhibited the metabolism of P450 substrates in the pre-incubation mode. ABT is a potent TDI of several P450 except for CYP2C19 (47%) and CYP2C9 (27%).

4. The results indicate that methoxsalen is a potent pan P450 inhibitor than ABT and can be a better tool in distinguishing P450 mediated metabolism form non-P450 metabolism in human liver microsomes.

     

Predicted contributions of cytochrome P450s to drug metabolism in human liver microsomes using relative activity factor were dependent on probes

1. Contributions of cytochrome P450 (CYP450) isoforms to drug metabolism are often predicted using relative activity factor (RAF) method, assuming RAF values were independent of probe. We aimed to report probe-dependent characteristic of RAF values using CYP3A4 or CYP2C9 probes.

2. Metabolism of four CYP3A4 probes (testosterone, midazolam, verapamil and atorvastatin) and three CYP2C9 probes (tolbutamide, diclofenac and S-warfarin) in human liver microsomes (HLM) and cDNA-expressed recombinant CYP450 (Rec-CYP450) systems were characterized and RAF_CL value was estimated as ratio of probe intrinsic clearance in HLM to that in Rec-CYP450. CYP450i contributions to metabolic reaction of a probe were predicted using other probes and compared with data from specific inhibitions. Contributions of CYP3A4 and CYP2C9 to metabolism of deoxypodophyllotoxin and nateglinide were also predicted.

3. RAF values were dependent on probes, leading to probe-dependently predicted contributions. Predicted contributions of CYP3A4 to formations of 6β-hydroxytestosterone, 1′-hydroxymidazolam, norverapamil, ortho-hydroxyatorvastatin and para-hydroxyatorvastatin using other probes were 47.46–219.46%, 21.62–98.87%, 186.49–462.44%, 21.87–101.15% and 53.62–247.97%, respectively. Predicted contributions of CYP3A4 and CYP2C9 to nateglinide metabolism were 8.18–37.84% and 36.08–94.04%, separately.

4. In conclusion, CYP450i contribution to drug metabolism in HLM estimated using RAF approach were probe-dependent. Therefore, contribution of each isoform must be confirmed by multiple probes.

     

Expression and inducibility of cytochrome P450s in human hepatocytes isolated from chimeric mice with humanised livers

1. The evaluation of drug-mediated cytochrome P450 (P450) induction using human hepatocytes is important for predicting drug interactions. In this study, we prepared hepatocytes from chimeric mice with humanised livers (Hu-Liver mice) and evaluated the expression and inducibility of P450s in these hepatocytes.

2. Up to 95% of the Hu-Liver cells stained positive for human leukocyte antigen and the mean viability exceeded 85% (n?=?10). Monolayer-cultured Hu-Liver cells displayed a similar morphology to cultures of the corresponding human hepatocytes used as transplantation donors.

3. The mRNA expression levels in Hu-Liver cells of 16 P450 forms belonging to P450 subfamilies 1–4 correlated well with the expression levels of the same enzymes in human hepatocytes. The variations in individual P450 mRNA levels between Hu-Liver cells and the corresponding human hepatocytes were within five-fold for 13 P450 forms.

4. The production of 6β-hydroxytestosterone in Hu-Liver cells was significantly increased (p?<?.05) following treatment with the CYP3A inducer, rifampicin.

5. Hu-Liver cells have characteristics similar to those of human hepatocytes in terms of mRNA expression levels and the inducibility of the various P450 forms. Thus, Hu-Liver cells can potentially be used for in vitro drug-mediated induction assays of human hepatic P450s.

     

Suitable albumin concentrations for enhanced drug oxidation activities mediated by human liver microsomal cytochrome P450 2C9 and other forms predicted with unbound fractions and partition/distribution coefficients of model substrates

1. Albumin has reportedly enhanced cytochrome P450 (P450)-mediated drug oxidation rates in human liver microsomes. Consequently, measurements of clearances and fractions metabolized could vary depending on the experimental albumin concentrations used.

2. In this study, the oxidation rates of diclofenac and warfarin by human liver microsomes were significantly enhanced in the presence of 0.10% (w/v) bovine serum albumin, whereas those of tolbutamide and phenytoin required 1.0% and 2.0% of albumin for significant enhancement. Values of the fractions metabolized by P450 2C9 for four substrates did not markedly change in the presence of albumin at the above-mentioned concentrations.

3. The oxidation rates of bupropion, omeprazole, chlorzoxazone and phenacetin in human liver microsomes were reportedly enhanced by 0.5%, 1%, 2% and 2% of albumin, respectively. Analysis of reported intrinsic clearance values and suitable albumin concentrations for the currently analyzed substrates and the reported substrates revealed an inverse correlation, with warfarin as an outlier.

4. Suitable albumin concentrations were multivariately correlated with physicochemical properties, that is, the plasma unbound fractions, octanol–water partition coefficient and acid dissociation constant (r?=?0.98, p<.0001, n?=?10). Therefore, multiple physicochemical properties may be determinants of suitable albumin concentrations for substrate oxidations in human liver microsomes.

     

Effects of CYP3A4*1G and CYP3A5*3 polymorphisms on pharmacokinetics of tylerdipine hydrochloride in healthy Chinese subjects

1. The aim of this analysis was to explore the influence of CYP3A4*1G and CYP3A5*3 polymorphisms on the pharmacokinetics of tylerdipine in healthy Chinese subjects.

2. A total of 64 and 63 healthy Chinese subjects were included and identified as the genotypes of CYP3A4*1G and CYP3A5*3, respectively. Plasma samples were collected for up to 120?h post-dose to characterize the pharmacokinetic profile following single oral dose of the drug (5, 15, 20, 25 and 30?mg). Plasma levels were measured by a high-performance liquid chromatography-mass spectrometry (LC-MS/MS). The pharmacokinetic parameters were calculated using non-compartmental method. The maximum concentration (C_max) and the area under the curve (AUC_0–24?h) were all corrected by the dose given.

3. In the wild-type group, the mean dose-corrected AUC_0–24?h was 1.35-fold larger than in CYP3A4*1G carriers (p?=?.018). Among the three CYP3A5 genotypes, there showed significantly difference (p?=?.008) in the t_1/2, but no significant difference was observed for the AUC_0–24?h and C_max. In subjects with the CYP3A5*3/*3 genotype, the mean t_1/2 was 1.35-fold higher than in CYP3A5*1/*1 group (p?=?.007). And the t_1/2 in CYP3A5*3 carriers also was 1.32-fold higher than in the wild-type group (p?=?.004).

4. CYP3A4*1G and CYP3A5*3 polymorphisms may influence tylerdipine pharmacokinetic in healthy Chinese subjects.

     

In vitro and in vivo pharmacokinetic characterization of mavacamten,a first-in-class small molecule allosteric modulator of beta cardiac myosin

1. Mavacamten is a small molecule modulator of cardiac myosin designed as an orally administered drug for the treatment of patients with hypertrophic cardiomyopathy. The current study objectives were to assess the preclinical pharmacokinetics of mavacamten for the prediction of human dosing and to establish the potential need for clinical pharmacokinetic studies characterizing drug–drug interaction potential.

2. Mavacamten does not inhibit CYP enzymes, but at high concentrations relative to anticipated therapeutic concentrations induces CYP2B6 and CYP3A4 enzymes in vitro. Mavacamten showed high permeability and low efflux transport across Caco-2 cell membranes. In human hepatocytes, mavacamten was not a substrate for drug transporters OATP, OCT and NTCP. Mavacamten was determined to have minimal drug–drug interaction risk.

3. In vitro mavacamten metabolite profiles included phase I- and phase II-mediated metabolism cross-species. Major pathways included aromatic hydroxylation (M1), aliphatic hydroxylation (M2); N-dealkylation (M6), and glucuronidation of the M1-metabolite (M4). Reaction phenotyping revealed CYPs 2C19 and 3A4/3A5 predominating.

4. Mavacamten demonstrated low clearance, high volume of distribution, long terminal elimination half-life and excellent oral bioavailability cross-species.

5. Simple four-species allometric scaling led to predicted plasma clearance, volume of distribution and half-life of 0.51?mL/min/kg, 9.5?L/kg and 9?days, respectively, in human.

     

The in vitro metabolism of bupropion revisited: concentration dependent involvement of cytochrome P450 2C19

1. The involvement of cytochrome P450 2B6 (CYP2B6) to the in vitro and in vivo metabolism of bupropion has been well studied. In these investigations we performed a detailed in vitro phenotyping study to characterize isoforms other than CYP2B6.

2. A total of nine metabolites were identified (M1–M9) in the incubations with cDNA-expressed P450s (rhCYP) and human liver microsomes (HLM).

3. Incubations in rhCYP identified CYP2B6 as the isoform responsible for the formation of hydroxybupropion (M3). CYP2C19 was involved in bupropion metabolism primarily through alternate hydroxylation pathways (M4–M6) with higher activity at lower substrate concentrations, near 1 µM.

4. The results from HLM inhibition studies using CYP2B6 and CYP2C19 inhibitory antibodies indicated that CYP2B6 contributed to approximately 90% of M3 formation, and CYP2C19 contributed to approximately 70–90% of M4, M5, and M6 formation.

5. Studies using single donor HLM with varying degrees of CYP2B6 and CYP2C19 activities showed a good relationship between M3 formation and CYP2B6 activity and M4/M5 formation and CYP2C19 activity.

6. These results confirmed the principle role of CYP2B6 in hydroxybupropion formation, as a selective CYP2B6 probe. In addition, the new findings revealed that CYP2C19 also contributes to bupropion metabolism through alternate hydroxylation pathways.

     

Hepatic CYP2B6 is altered by genetic,physiologic, and environmental factors but plays little role in nicotine metabolism

1. Human cytochrome P4502B6 (CYP2B6) is predominantly expressed in the liver and it plays a major role in the metabolism of several therapeutically important drugs and environmental toxicants.

2. The objective was twofold: (1) to determine the role of genetic, physiological, and environmental factors in predicting hepatic CYP2B6 protein expression; and (2) to investigate the role of CYP2B6 in nicotine C-oxidation.

3. Human livers (n?=?40) were assessed for CYP2B6 protein and genotype.

4. Linear regression analyses indicated that CYP2B6 genotype (10%), gender (14%), and exposure to inducers (21%), but not age, were predictors of CYP2B6 protein amounts. Livers with at least one CYP2B6*5 or *6 allele were associated with lower CYP2B6. Female livers and livers exposed to inducers (phenobarbital and/or dexamethasone) were associated with higher CYP2B6.

5. A weak correlation between CYP2B6 and nicotine C-oxidation activity was observed, which was abrogated when controlling for CYP2A6 protein levels. CYP2B6*6 was not associated with different nicotine kinetics.

6. In summary, CYP2B6 protein expression was associated with genotype, gender, and exposure to inducers, but not with nicotine C-oxidation activity.