Comparison of CYP2A6 catalytic activity on coumarin 7-hydroxylation in human and monkey liver microsomes

Fluorescence-Based High-Throughput Assays for Investigating Cytochrome P450 Enzyme-Mediated Drug-Drug Interactions

The cytochrome P450 enzymes (CYPs), a group of heme-containing enzymes, catalyze oxidative metabolism of a wide range of drugs and xenobiotics, as well as different endogenous molecules. Strong inhibition of human CYPs is the most common cause of clinically associated pharmacokinetic drug-drug/herb-drug interactions (DDIs/HDIs), which may result in serious adverse drug reactions, even toxicity. Accurate and rapid assessing of the inhibition potentials on CYP activities for therapeutic agents is crucial for the prediction of clinically relevant DDIs/HDIs. Over the past few decades, significant efforts have been invested into developing optical substrates for the human CYPs, generating a variety of powerful tools for high-throughput assays to detect CYP activities in biologic specimens and for screening of CYP inhibitors. This minireview focuses on recent advances in optical substrates developments for human CYPs, as well as their applications in screening CYP inhibitors and DDIs/HDIs studies. The examples for rational design and optimization of highly specific optical substrates for the target CYP enzyme, as well as applications in investigating CYP-mediated DDIs, are illustrated. Finally, the challenges and future perspectives in this field are proposed. Collectively, this review summarizes the reported optical-based biochemical assays for highly efficient CYP activities detection, which strongly facilitated the discovery of CYP inhibitors and the investigations on CYP-mediated DDIs. SIGNIFICANCE STATEMENT: Optical substrates for cytochrome P450 enzymes (CYPs) have emerged as powerful tools for the construction of high-throughput assays for screening of CYP inhibitors. This mini-review covers the advances and challenges in the development of highly specific optical substrates for sensing human CYP isoenzymes, as well as their applications in constructing fluorescence-based high-throughput assays for investigating CYP-mediated drug-drug interactions.

Bioactivation of the tobacco carcinogens 4-aminobiphenyl (4-ABP) and 2-amino-9H-pyrido[2,3-b]indole (AαC) in human bladder RT4 cells

Occupational and tobacco exposure to aromatic amines (AAs) including 4-aminobiphenyl (4-ABP) and 2-naphthylamine (2-NA) are associated with bladder cancer (BC) risk. Several epidemiological studies have also reported a possible role for structurally related heterocyclic aromatic amines (HAAs) formed in tobacco smoke or cooked meats with BC risk. We had screened for DNA adducts of 4-ABP, 2-NA, and several prominent HAAs formed in tobacco smoke or grilled meats including 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP), 2-amino-3,8-dimethylmidazo[4,5-f]quinoxaline (MeIQx), and 2-amino-9H-pyrido[2,3-b]indole (AαC) in the bladder DNA of BC patients, using liquid chromatography/mass spectrometry. We detected DNA adducts of 4-ABP, but not adducts of the other carcinogens. In this study, we have examined the capacity of RT4 cells, an epithelial human bladder cell line, to bioactivate AAs and HAAs to DNA damaging agents, which may contribute to BC. 4-ABP and AαC formed DNA adducts, but DNA adducts of 2-NA, PhIP, and MeIQx were not detected. 4-ABP DNA adducts were formed at tenfold higher levels than AαC adducts. Pretreatment of RT4 cells with α-naphthoflavone (1-10 µM), a specific cytochrome P450 1 (CYP1) inhibitor, decreased AαC adduct formation by 50% but did not affect the level of 4-ABP adducts. However, cell pretreatment with 8-methoxypsoralen (0.1-1 µM), a potent inhibitor of CYP2A, resulted in a 90% decrease of 4-ABP DNA adducts levels. These data signify that CYP2A and CYP1A isoforms expressed in the target urothelium bioactivate 4-ABP and AαC, respectively, and may be a critical feature of aromatic amine-induced urinary bladder carcinogenesis. The bioactivation of other tobacco and environmental AAs by bladder CYPs and their ensuing bladder DNA damage warrants further study.

Predictors of Variation in CYP2A6 mRNA, Protein, and Enzyme Activity in a Human Liver Bank: Influence of Genetic and Nongenetic Factors

Cytochrome P450 2A6 CYP2A6: metabolizes several clinically relevant substrates, including nicotine, the primary psychoactive component in cigarette smoke. Smokers vary widely in their rate of inactivation and clearance of nicotine, altering numerous smoking phenotypes. We aimed to characterize independent and shared impact of genetic and nongenetic sources of variation in CYP2A6 mRNA, protein, and enzyme activity in a human liver bank (n = 360). For the assessment of genetic factors, we quantified levels of CYP2A6, cytochrome P450 oxidoreductase (POR), and aldo-keto reductase 1D1 (AKR1D1) mRNA, and CYP2A6 and POR proteins. CYP2A6 enzyme activity was determined through measurement of cotinine formation from nicotine and 7-hydroxycoumarin formation from coumarin. Donor DNA was genotyped for CYP2A6, POR, and AKR1D1 genetic variants. Nongenetic factors assessed included gender, age, and liver disease. CYP2A6 phenotype measures were positively correlated to each other (r values ranging from 0.47-0.88, P < 0.001). Female donors exhibited higher CYP2A6 mRNA expression relative to males (P < 0.05). Donor age was weakly positively correlated with CYP2A6 protein (r = 0.12, P < 0.05) and activity (r = 0.20, P < 0.001). CYP2A6 reduced-function genotypes, but not POR or AKR1D1 genotypes, were associated with lower CYP2A6 protein (P < 0.001) and activity (P < 0.01). AKR1D1 mRNA was correlated with CYP2A6 mRNA (r = 0.57, P < 0.001), protein (r = 0.30, P < 0.001), and activity (r = 0.34, P < 0.001). POR protein was correlated with CYP2A6 activity (r = 0.45, P < 0.001). Through regression analyses, we accounted for 17% (P < 0.001), 37% (P < 0.001), and 77% (P < 0.001) of the variation in CYP2A6 mRNA, protein, and activity, respectively. Overall, several independent and shared sources of variation in CYP2A6 activity in vitro have been identified, which could translate to variable hepatic clearance of nicotine.

Functioning of drug-metabolizing microsomal cytochrome P450s: In silico probing of proteins suggests that the distal heme 'active site' pocket plays a relatively 'passive role' in some enzyme-substrate interactions

PURPOSE

The currently held mechanistic understanding of microsomal cytochrome P450s (CYPs) seeks that diverse drug molecules bind within the deep-seated distal heme pocket and subsequently react at the heme centre. To explain a bevy of experimental observations and meta-analyses, we indulge a hypothesis that involves a "diffusible radical mediated" mechanism. This new hypothesis posits that many substrates could also bind at alternate loci on/within the enzyme and be reacted without the pertinent moiety accessing a bonding proximity to the purported catalytic Fe-O enzyme intermediate.

METHODS

Through blind and heme-distal pocket centered dockings of various substrates and non-substrates (drug molecules of diverse sizes, classes, topographies etc.) of microsomal CYPs, we explored the possibility of access of substrates via the distal channels, its binding energies, docking orientations, distance of reactive moieties (or molecule per se) to/from the heme centre, etc. We investigated specific cases like- (a) large drug molecules as substrates, (b) classical marker drug substrates, (c) class of drugs as substrates (Sartans, Statins etc.), (d) substrate preferences between related and unrelated CYPs, (e) man-made site-directed mutants' and naturally occurring mutants' reactivity and metabolic disposition, (f) drug-drug interactions, (g) overall affinities of drug substrate versus oxidized product, (h) meta-analysis of in silico versus experimental binding constants and reaction/residence times etc.

RESULTS

It was found that heme-centered dockings of the substrate/modulator drug molecules with the available CYP crystal structures gave poor docking geometries and distances from Fe-heme centre. In conjunction with several other arguments, the findings discount the relevance of erstwhile hypothesis in many CYP systems. Consequently, the newly proposed hypothesis is deemed a viable alternate, as it satisfies Occam's razor.

CONCLUSIONS

The new proposal affords expanded scope for explaining the mechanism, kinetics and overall phenomenology of CYP mediated drug metabolism. It is now understood that the heme-iron and the hydrophobic distal pocket of CYPs serve primarily to stabilize the reactive intermediate (diffusible radical) and the surface or crypts of the apoprotein bind to the xenobiotic substrate (and in some cases, the heme distal pocket could also serve the latter function). Thus, CYPs enhance reaction rates and selectivity/specificity via a hitherto unrecognized modality.

CYP2D6 and CYP2A6 biotransform dietary tyrosol into hydroxytyrosol

The dietary phenol tyrosol has been reported to be endogenously transformed into hydroxytyrosol, a potent antioxidant with multiple health benefits. In this work, we evaluated whether tyrosine hydroxylase (TH) and cytochrome P450s (CYPs) catalyzed this process. To assess TH involvement, Wistar rats were treated with α-methyl-L-tyrosine and tyrosol. Tyrosol was converted into hydroxytyrosol whilst α-methyl-L-tyrosine did not inhibit the biotransformation. The role of CYP was assessed in human liver microsomes (HLM) and tyrosol-to-hydroxytyrosol conversion was observed. Screening with selective enzymatic CYP inhibitors identified CYP2A6 as the major isoform involved in this process. Studies with baculosomes further demonstrated that CYP2D6 and CYP3A4 could transform tyrosol into hydroxytyrosol. Experiments using human genotyped livers showed an interindividual variability in hydroxytyrosol formation and supported findings that CYP2D6 and CYP2A6 mediated this reaction. The dietary health benefits of tyrosol-containing foods remain to be evaluated in light of CYP pharmacogenetics.

Evaluation of early fetal exposure to vaginally-administered metronidazole in pregnant cynomolgus monkeys

Given concern about potential embryo-fetal harm following seminal exposure to drugs with teratogenic potential, pharmaceutical companies use theoretical calculations to estimate seminal concentrations, maternal exposure, and distribution across the placenta to the embryo-fetal compartment for risk assessment. However, it is plausible that there are additional mechanisms whereby the conceptus is exposed. In order to determine if theoretical calculations are sufficiently conservative to predict embryo-fetal exposure from drugs in semen, pregnant cynomolgus monkeys were given a vaginal dose of metronidazole during the early fetal period and cesarean-sectioned. Maternal, fetal, and amniotic fluid samples were analyzed for metronidazole and 2-hydroxymetronidazole. Exposure to metronidazole and its metabolite were comparable in all matrices. These data demonstrated no preferential transfer mechanism to conceptus following intravaginal administration of a small molecule drug; and therefore, suggest that traditional modeling for embryo-fetal exposure to drugs in semen in support of risk assessment for pharmaceutical agents is sufficiently conservative.

Differential effects of nicotine treatment and ethanol self-administration on CYP2A6, CYP2B6 and nicotine pharmacokinetics in African green monkeys

In primates, nicotine is metabolically inactivated in the liver by CYP2A6 and possibly CYP2B6. Changes in the levels of these two enzymes may affect nicotine pharmacokinetics and influence smoking behaviors. This study investigated the independent and combined effects of ethanol self-administration and nicotine treatment (0.5 mg/kg b.i.d. s.c.) on hepatic CYP2A6 and CYP2B6 levels (mRNA, protein, and enzymatic activity), in vitro nicotine metabolism, and in vivo nicotine pharmacokinetics in monkeys. CYP2A6 mRNA and protein levels and in vitro coumarin (selective CYP2A6 substrate) and nicotine metabolism were decreased by nicotine treatment but unaffected by ethanol. CYP2B6 protein levels and in vitro bupropion (selective CYP2B6 substrate) metabolism were increased by ethanol but unaffected by nicotine treatment; CYP2B6 mRNA levels were unaltered by either treatment. Combined ethanol and nicotine exposure decreased CYP2A6 mRNA and protein levels, as well as in vitro coumarin and nicotine metabolism, and increased CYP2B6 protein levels and in vitro bupropion metabolism, with no change in CYP2B6 mRNA levels. Chronic nicotine resulted in higher nicotine plasma levels achieved after nicotine administration, consistent with decreased CYP2A6. Ethanol alone, or combined with nicotine, resulted in lower nicotine plasma levels by a mechanism independent of the change in these enzymes. Thus, nicotine can decrease hepatic CYP2A6, reducing the metabolism of its substrates, including nicotine, whereas ethanol can increase hepatic CYP2B6, increasing the metabolism of CYP2B6 substrates. In vivo nicotine pharmacokinetics are differentially affected by ethanol and nicotine, but when both drugs are used in combination the effect more closely resembles ethanol alone.

7-Ethynylcoumarins: selective inhibitors of human cytochrome P450s 1A1 and 1A2

To discover new selective mechanism-based P450 inhibitors, eight 7-ethynylcoumarin derivatives were prepared through a facile two-step synthetic route. Cytochrome P450 activity assays indicated that introduction of functional groups in the backbone of coumarin could enhance the inhibition activities toward P450s 1A1 and 1A2, providing good selectivity against P450s 2A6 and 2B1. The most potent product 7-ethynyl-3,4,8-trimethylcoumarin (7ETMC) showed IC(50) values of 0.46 μM and 0.50 μM for P450s 1A1 and 1A2 in the first six minutes, respectively, and did not show any inhibition activity for P450s 2A6 and 2B1 even at the dose of 50 μM. All of the inhibitors except 7-ethynyl-3-methyl-4-phenylcoumarin (7E3M4PC) showed mechanism-based inhibition of P450s 1A1 and 1A2. In order to explain this mechanistic difference in inhibitory activities, X-ray crystallography data were used to study the difference in conformation between 7E3M4PC and the other compounds studied. Docking simulations indicated that the binding orientations and affinities resulted in different behaviors of the inhibitors on P450 1A2. Specifically, 7E3M4PC with its two-plane structure fits into the P450 1A2's active site cavity with an orientation leading to no reactive binding, causing it to act as a competitive inhibitor.

Polymorphism of human cytochrome P450 enzymes and its clinical impact

Pharmacogenetics is the study of how interindividual variations in the DNA sequence of specific genes affect drug response. This article highlights current pharmacogenetic knowledge on important human drug-metabolizing cytochrome P450s (CYPs) to understand the large interindividual variability in drug clearance and responses in clinical practice. The human CYP superfamily contains 57 functional genes and 58 pseudogenes, with members of the 1, 2, and 3 families playing an important role in the metabolism of therapeutic drugs, other xenobiotics, and some endogenous compounds. Polymorphisms in the CYP family may have had the most impact on the fate of therapeutic drugs. CYP2D6, 2C19, and 2C9 polymorphisms account for the most frequent variations in phase I metabolism of drugs, since almost 80% of drugs in use today are metabolized by these enzymes. Approximately 5-14% of Caucasians, 0-5% Africans, and 0-1% of Asians lack CYP2D6 activity, and these individuals are known as poor metabolizers. CYP2C9 is another clinically significant enzyme that demonstrates multiple genetic variants with a potentially functional impact on the efficacy and adverse effects of drugs that are mainly eliminated by this enzyme. Studies into the CYP2C9 polymorphism have highlighted the importance of the CYP2C9*2 and *3 alleles. Extensive polymorphism also occurs in other CYP genes, such as CYP1A1, 2A6, 2A13, 2C8, 3A4, and 3A5. Since several of these CYPs (e.g., CYP1A1 and 1A2) play a role in the bioactivation of many procarcinogens, polymorphisms of these enzymes may contribute to the variable susceptibility to carcinogenesis. The distribution of the common variant alleles of CYP genes varies among different ethnic populations. Pharmacogenetics has the potential to achieve optimal quality use of medicines, and to improve the efficacy and safety of both prospective and currently available drugs. Further studies are warranted to explore the gene-dose, gene-concentration, and gene-response relationships for these important drug-metabolizing CYPs.

Inhibition and induction of human cytochrome P450 enzymes: current status

Variability of drug metabolism, especially that of the most important phase I enzymes or cytochrome P450 (CYP) enzymes, is an important complicating factor in many areas of pharmacology and toxicology, in drug development, preclinical toxicity studies, clinical trials, drug therapy, environmental exposures and risk assessment. These frequently enormous consequences in mind, predictive and pre-emptying measures have been a top priority in both pharmacology and toxicology. This means the development of predictive in vitro approaches. The sound prediction is always based on the firm background of basic research on the phenomena of inhibition and induction and their underlying mechanisms; consequently the description of these aspects is the purpose of this review. We cover both inhibition and induction of CYP enzymes, always keeping in mind the basic mechanisms on which to build predictive and preventive in vitro approaches. Just because validation is an essential part of any in vitro-in vivo extrapolation scenario, we cover also necessary in vivo research and findings in order to provide a proper view to justify in vitro approaches and observations.

Involvement of CYP2A6 in the formation of a novel metabolite, 3-hydroxypilocarpine, from pilocarpine in human liver microsomes

Pilocarpine is a cholinergic agonist that is metabolized to pilocarpic acid by serum esterase. In this study, we discovered a novel metabolite in human urine after the oral administration of pilocarpine hydrochloride, and we investigated the metabolic enzyme responsible for the metabolite formation. The structure of the metabolite was identified as 3-hydroxypilocarpine by liquid chromatography-tandem mass spectrometry and NMR analyses and by comparing to the authentic metabolite. To clarify the human cytochrome P450 (P450) responsible for the metabolite formation, in vitro experiments using P450 isoform-selective inhibitors, cDNA-expressed human P450s (Supersomes; CYP1A2, -2A6, -2B6, -2C9, -2C19, -2D6, -2E1, and -3A4), and liver microsomes from different donors were conducted. The formation of 3-hydroxypilocarpine in human liver microsomes was strongly inhibited (>90%) by 200 microM coumarin. Other selective inhibitors of CYP1A2 (furafylline and alpha-naphthoflavone), CYP2C9 (sulfaphenazole), CYP2C19 [(S)-mephenytoin], CYP2E1 (4-methylpyrazole), CYP2D6 (quinidine), and CYP3A4 (troleandomycin) had a weak inhibitory effect (<20%) on the formation. The highest formation activity was expressed by recombinant CYP2A6. The K(m) value for recombinant CYP2A6 was 3.1 microM, and this value is comparable with that of human liver microsomes (1.5 microM). The pilocarpine 3-hydroxylation activity was correlated with coumarin 7-hydroxylation activity in 16 human liver microsomes (r = 0.98). These data indicated that CYP2A6 is the main enzyme responsible for the 3-hydroxylation of pilocarpine. In conclusion, we identified a novel metabolite of pilocarpine, 3-hydroxypilocarpine, and we clarified the involvement of CYP2A6 in the formation of this molecule in human liver microsomes.

Validated assays for human cytochrome P450 activities

The measurement of the effect of new chemical entities on human cytochrome P450 marker activities using in vitro experimentation represents an important experimental approach in drug development. In vitro drug interaction data can be used in guiding the design of clinical drug interaction studies, or, when no effect is observed in vitro, the data can be used in place of an in vivo study to claim that no interaction will occur in vivo. To make such a claim, it must be assured that the in vitro experiments are performed with absolute confidence in the methods used and data obtained. To meet this need, 12 semiautomated assays for human P450 marker substrate activities have been developed and validated using approaches described in the GLP (good laboratory practices) as per the code of U.S. Federal Regulations. The assays that were validated are: phenacetin O-deethylase (CYP1A2), coumarin 7-hydroxylase (CYP2A6), bupropion hydroxylase (CYP2B6), amodiaquine N-deethylase (CYP2C8), diclofenac 4'-hydroxylase and tolbutamide methylhydroxylase (CYP2C9), (S)-mephenytoin 4'-hydroxylase (CYP2C19), dextromethorphan O-demethylase (CYP2D6), chlorzoxazone 6-hydroxylase (CYP2E1), felodipine dehydrogenase, testosterone 6 beta-hydroxylase, and midazolam 1'-hydroxylase (CYP3A4 and CYP3A5). High-pressure liquid chromatography-tandem mass spectrometry, using stable isotope-labeled internal standards, has been applied as the analytical method. This analytical approach, through its high sensitivity and selectivity, has permitted the use of very low incubation concentrations of microsomal protein (0.01-0.2 mg/ml). Analytical assay accuracy and precision values were excellent. Enzyme kinetic and inhibition parameters obtained using these methods demonstrated high precision and were within the range of values previously reported in the scientific literature. These methods should prove useful in the routine assessments of the potential for new drug candidates to elicit pharmacokinetic drug interactions via inhibition of cytochrome P450 activities.

Synthesis of N-acetyl-S-(3-coumarinyl)-cysteine methyl ester and HPLC analysis of urinary coumarin metabolites

N-Acetyl-S-(3-coumarinyl)cysteine, a metabolite of coumarin in rodents, has been synthesized as methyl ester. A new synthetic route to prepare N-acetyl-S-(3-coumarinyl)-D,L-cysteine methyl ester comprises reaction of 3-mercaptocoumarin with N-acetyl-3-chloro-D,L-alanine methyl ester. N-acetyl-S-(4-coumarinyl)-L-cysteine was obtained by reaction of 3-bromocoumarin and N-acetyl-L-cysteine. A method for the determination of N-acetyl-S-(3-coumarinyl)cysteine as its methyl ester in urine by HPLC has been developed.

Down-regulation of hepatic nicotine metabolism and a CYP2A6-like enzyme in African green monkeys after long-term nicotine administration

Nicotine metabolism is decreased in smokers compared with nonsmokers, but the mechanism(s) responsible for the slower metabolism are unknown. Nicotine is inactivated to cotinine by CYP2A6 in human liver [nicotine C-oxidation (NCO)]. CYP2B6 also metabolizes nicotine to cotinine but with lower affinity than CYP2A6. To evaluate the effects of long-term nicotine treatment on hepatic levels of CYP2A6 and CYP2B6, and nicotine metabolism, an African green monkey (AGM) model was developed. As in humans, approximately 80 to 90% of in vitro hepatic NCO is mediated by a CYP2A6-like protein (CYP2A6agm) in this species, as determined by inhibition studies. Male AGM (n = 6 per group) were treated for 3 weeks with nicotine (s.c., 0.3 mg/kg, b.i.d.), phenobarbital (oral, 20 mg/kg, as a positive control for P450 induction), and/or saline (s.c., b.i.d.). Immunoblotting demonstrated a 59% decrease (p < 0.05) in hepatic CYP2A6agm protein in nicotine-treated animals. A CYP2B6-like protein (CYP2B6agm) was modestly and insignificantly decreased (14%, p = 0.11). In vitro NCO was decreased by 41% in the nicotine-treated group (p < 0.05), mediated by a decrease in CYP2A6agm, as demonstrated using inhibitory antibodies. CYP2A6agm mRNA (33%, P < or = 0.05) and CYP2B6agm (35%, p < 0.01) mRNA were also significantly decreased in the nicotine-treated group. Phenobarbital-treated animals demonstrated an increase in CYP2B6agm (650%, p < 0.001), but not CYP2A6agm (20%, p = 0.49). NCO was increased in the phenobarbital-treated group (55%, p < 0.05) by an increase in CYP2B6agm-mediated NCO. Consistent with the slower nicotine metabolism observed in smokers, nicotine may decrease its own metabolism in primates by decreasing the expression of the primary nicotine-metabolizing enzyme CYP2A6.

Evaluation of cytochrome P450 probe substrates commonly used by the pharmaceutical industry to study in vitro drug interactions

Pharmaceutical industry investigators routinely evaluate the potential for a new drug to modify cytochrome p450 (p450) activities by determining the effect of the drug on in vitro probe reactions that represent activity of specific p450 enzymes. The in vitro findings obtained with one probe substrate are usually extrapolated to the compound's potential to affect all substrates of the same enzyme. Due to this practice, it is important to use the right probe substrate and to conduct the experiment under optimal conditions. Surveys conducted by reviewers in CDER indicated that the most common in vitro probe reactions used by industry investigators include the following: phenacetin O-deethylation for CYP1A2, coumarin 7-hydroxylation for CYP2A6, 7-ethoxy-4-trifluoromethyl coumarin O-dealkylation for CYP2B6, tolbutamide 4'-hydroxylation for CYP2C9, S-mephenytoin 4-hydroxylation for CYP2C19, bufuralol 1'-hydroxylation for CYP2D6, chlorzoxazone 6-hydroxylation for CYP2E1, and testosterone 6 beta-hydroxylation for CYP3A4. We reviewed the validation information in the literature on these reactions and other frequently used reactions, including caffeine N3-demethylation for CYP1A2, S-mephenytoin N-demethylation for CYP2B6, S-warfarin 7'-hydroxylation for CYP2C9, dextromethorphan O-demethylation for CYP2D6, and midazolam 1'-hydroxylation for CYP3A4. The available information indicates that we need to continue the search for better probe substrates for some enzymes. For CYP3A4-based drug interactions it may be necessary to evaluate two or more probe substrates. In many cases, the probe reaction represents a particular enzyme activity only under specific experimental conditions. Investigators must consider appropriateness of probe substrates and experimental conditions when conducting in vitro drug interaction studies and when extrapolating the results to in vivo situations.

CYP2A6 genetic variation and potential consequences

Human cytochrome P450 2A6 (CYP2A6) has been shown to have large interindividual and interethnic variability in levels of expression and activity. This is thought to be largely due to genetic polymorphisms. In recent years, 13 genetic variants (CYP2A6*1-*11 and the gene duplication, *1 x 2) of CYP2A6 have been identified and a number of these have been shown to result in altered CYP2A6 enzyme activity. For example, there are alleles which result in variants that are in inactive (e.g. due to a gene deletion), have decreased activity (e.g. altered enzyme structure or transcriptional activity) or have increased activity (e.g. due to gene duplications). The resulting interindividual variation in metabolic activity may affect the metabolism of CYP2A6 substrates including nicotine, cotinine (the major metabolite of nicotine), several tobacco-specific procarcinogens, coumarin and many toxins. The frequencies of the CYP2A6 alleles vary considerably among different ethnic populations, which may partially explain the interethnic variability found in CYP2A6-related metabolic activity (e.g. nicotine metabolism), behaviors (i.e. smoking) and disease (i.e. lung cancer). Investigations of the genetic variation of CYP2A6 and its resulting effects on metabolism and health consequences are still fairly early; this review summarizes what is presently known about CYP2A6, its genetic variants and their clinical consequences.

Inhibition of cytochromes P450 by antifungal imidazole derivatives

The interactions of a panel of antifungal agents with cytochromes P450 (P450s), as a means of predicting potential drug-drug interactions, have not yet been investigated. The objective of this study was to evaluate the specificity and selectivity of five antifungal agents using selective probe reactions for each of the eight major P450s. The index reactions used were phenacetin O-deethylation (for CYP1A2), coumarin 7-hydroxylation (CYP2A6), diclofenac 4'-hydroxylation (CYP2C9), omeprazole 5-hydroxylation (CYP2C19), dextromethorphan O-demethylation (CYP2D6), 7-ethoxy-4-trifluoromethylcoumarin deethylation (CYP2B6), chlorzoxazone 6-hydroxylation (CYP2E1), and omeprazole sulfonation (CYP3A4). Five antifungal agents that include an imidazole moiety (clotrimazole, miconazole, sulconazole, tioconazole, and ketoconazole) were examined in cDNA-expressing microsomes from human lymphoblast cells or human liver microsomes. All inhibitors studied demonstrated nonselective inhibition of P450s. Ketoconazole seemed to be the most selective for CYP3A4, although it also inhibited CYP2C9. High-affinity inhibition was seen for CYP1A2 (sulconazole and tioconazole K(i), 0.4 microM), CYP2B6 (miconazole K(i), 0.05 microM; sulconazole K(i), 0.04 microM), CYP2C19 (miconazole K(i), 0.05 microM; sulconazole K(i), 0.008 microM; tioconazole K(i), 0.04 microM), CYP2C9 (sulconazole K(i), 0.01 microM), CYP2D6 (miconazole K(i), 0.70 microM; sulconazole K(i), 0.40 microM), CYP2E1 (tioconazole K(i), 0.4 microM), and CYP3A4 (clotrimazole K(i), 0.02 microM; miconazole K(i), 0.03 microM; tioconazole K(i), 0.02 microM). Therefore, this class of compounds is likely to result in significant drug-drug interactions in vivo.

Evidence for Singlet Oxygen Involvement in Rat and Human Cytochrome P450-dependent Substrate Oxidations

Recently, we proposed that singlet oxygen ((1)O(2)) plays an essential role in microsomal cytochrome P450 (P450)-dependent p-hydroxylation of aniline and O-deethylation of 7-ethoxycoumarin. We then examined whether the role of (1)O(2) is general in the P450-dependent substrate oxidations. In the present study, we examined omega- and (omega-1)-hydroxylations of lauric acid, O-demethylation of p-nitroanisole, and N-demethylation of aminopyrine in rat liver microsomes. The addition of beta-carotene and NaN(3) significantly suppressed these reactions in a concentration-dependent manner, and (1)O(2) during the reactions was detected by ESR spin-trapping using 2,2,6,6-tetramethyl-4-piperidone (TMPD) as a (1)O(2)-spin trapping reagent, where the addition of (1)O(2) quenchers, SKF-525A as a P450 inhibitor, or p-nitroanisole decreased ESR signal intensities due to TMPD-(1)O(2) adduct. Next, we examined the effect of (1)O(2) quenchers on P450-dependent reactions in the human liver microsomes, and (1)O(2) was also indicated to be an active species in substrate hydroxylations and dealkylations such as nifedipine oxidation by CYP3A4. On the basis of the results, we concluded that (1)O(2) is an essentially important active oxygen species in both rat and human P450-dependent substrate oxidations.

Molecular cloning, expression, and characterization of CYP2D17 from cynomolgus monkey liver

The cynomolgus monkey is a species used in drug-safety evaluation and biotransformation studies by the pharmaceutical industry. Relatively little is known, however, about the catalytic activities and specificities of cytochromes P450 (CYP) in this species. As a first step in characterizing monkey CYPs, a cDNA was cloned by reverse-transcriptase PCR from cynomolgus monkey liver mRNA using oligonucleotide primers based on the human CYP2D6 sequence. The full-length cDNA (called CYP2D17) encoded a 497-amino-acid protein that is 93% identical to human CYP2D6 and 90% identical to marmoset CYP2D19. The CYP2D17 cDNA was cloned into a baculovirus expression vector, and microsomes prepared from CYP2D17-infected insect cells were used to determine the catalytic properties of the recombinant enzyme. The recombinant CYP2D17 results were compared to data generated with monkey liver microsomes, human liver microsomes, and recombinant CYP2D6 and demonstrated catalytic similarity using probe substrates and inhibitors. Recombinant CYP2D17 catalyzed the oxidation of bufuralol to 1'-hydroxybufuralol and dextromethorphan to dextrorphan, reactions shown to be mediated by CYP2D6 in humans; the apparent K(m) values for bufuralol and dextromethorphan were 1 and 0.8 microM, respectively. Moreover, both of these reactions were more strongly inhibited by quinidine than by quinine. A more complete understanding of the substrate specificities and activities of monkey CYPs will be advantageous in delineating species differences in metabolite profiles and metabolic activation of new chemical entities in the pharmaceutical industry.

A comparison of basal and induced hepatic microsomal cytochrome P450 monooxygenase activities in the cynomolgus monkey (Macaca fascicularis) and man

1. The specific activities of hepatic microsomal cortisol 6beta-hydroxylase, coumarin 7-hydroxylase, S-mephenytoin 4'-hydroxylase and phenoxazone hydroxylase and the O-dealkylations of seven homologous alkoxyresorufins were < 3-fold different between the untreated (UT) cynomolgus monkey and man. 2. Heptoxy- and octoxyresorufin O-dealkylase, S-mephenytoin N-demethylase and dextromethorphan O-demethylase specific activities were > 6-fold higher, whereas tolbutamide hydroxylase was almost 5-fold lower in the UT monkey than in man. 3. Phenobarbitone induced (2-6-fold) coumarin 7-hydroxylase, cortisol 6beta-hydroxylase, S-mephenytoin N-demethylase, phenoxazone hydroxylase and benzyloxyresorufin O-dealkylase activities, but not the O-dealkylations of pentoxyresorufin or other alkoxyresorufins, in monkey. 4. Rifampicin induced (2-3-fold) cortisol 6beta-hydroxylase, S-mephenytoin 4'-hydroxylase, S-mephenytoin N-demethylase and tolbutamide hydroxylase activities, the O-dealkylations of methoxy-, ethoxy- and propoxyresorufin and CYP2C- and CYP3A-immunorelated proteins in monkey. 5. Dextromethorphan O-demethylase was significantly reduced by both phenobarbitone and rifampicin treatment in monkey. 6. Beta-naphthoflavone induced (8-39-fold) the O-dealkylations of several alkoxyresorufins, the greatest effect being on propoxyresorufin, but had no effect on the other activities measured in monkey. 7. Constitutive hepatic microsomal CYP2D6-immunorelated proteins were expressed at apparently much higher levels in monkey than in man.