Role of cytochrome P450 2C9 and an allelic variant in the 4'-hydroxylation of (R)- and (S)-flurbiprofen

Predicting drug metabolism: a site of metabolism prediction tool applied to the cytochrome P450 2C9

The aim of the present study is to develop a method for predicting the site at which molecules will be metabolized by CYP 2C9 (cytochrome P450 2C9) using a previously reported protein homology model of the enzyme. Such a method would be of great help in designing new compounds with a better pharmacokinetic profile, or in designing prodrugs where the compound needs to be metabolized in order to become active. The methodology is based on a comparison between alignment-independent descriptors derived from GRID Molecular Interaction Fields for the CYP 2C9 active site, and a distance-based representation of the substrate. The predicted site of metabolism is reported as a ranking list of all the hydrogen atoms of each substrate molecule. Eighty-seven CYP 2C9-catalyzed oxidative reactions reported in the literature have been analyzed. In more than 90% of these cases, the hydrogen atom ranked at the first, second, or third position was the experimentally reported site of oxidation.

Differences in flurbiprofen pharmacokinetics between CYP2C9*1/*1, *1/*2, and *1/*3 genotypes

OBJECTIVE

This study was conducted to examine differences in flurbiprofen metabolism among individuals with the CYP2C9*1/*1, *1/*2, and *1/*3 genotypes.

METHODS

Fifteen individuals with the CYP2C9*1/*1 ( n=5), *1/*2 ( n=5), and *1/*3 ( n=5) genotypes received a single 50-mg oral dose of flurbiprofen. Plasma and urine samples were collected over 24 h, and flurbiprofen and 4'-hydroxyflurbiprofen pharmacokinetic data were compared across genotypes.

RESULTS

CYP2C9 genotype was a significant predictor of flurbiprofen metabolism and accounted for 59% of the variability in flurbiprofen AUC(0- infinity ), and approximately 50% of the variability in flurbiprofen oral clearance, formation clearance to 4'-hydroxyflurbiprofen, and the 0 to 24-h urinary metabolic ratio of flurbiprofen to 4'-hydroxyflurbiprofen. Flurbiprofen AUC(0- infinity )was significantly higher and all measures of flurbiprofen clearance were significantly lower in the CYP2C9*1/*3 individuals than in those with *1/*1. Significant differences in these parameters were not detected between *1/*2 subjects and *1/*1 subjects.

CONCLUSIONS

CYP2C9 genotype is a significant predictor of flurbiprofen disposition in humans by altering CYP2C9-mediated metabolism and reducing systemic clearance. The effects are most pronounced in individuals carrying the *3 allele.

Activation of cytochrome P450 2C9-mediated metabolism: mechanistic evidence in support of kinetic observations

Studies were designed to investigate the possible mechanisms associated with the kinetic observation of CYP2C9 activation by dapsone and its phase I metabolite, N-hydroxydapsone. Kinetic studies suggested that dapsone activated CYP2C9-mediated flurbiprofen 4(')-hydroxylation by decreasing the K(m) (alpha=0.2) and increasing the V(max) (beta=1.9). Interestingly, N-hydroxydapsone also activated flurbiprofen 4(')-hydroxylation by increasing V(max) (beta=1.5) but had no effect on K(m) (alpha=0.98). To study the effects of these modulators on the binding affinity of flurbiprofen, spectral binding studies were performed. In the presence of dapsone, the spectral binding constant (K(s)) for flurbiprofen was reduced from 14.1 to 2.1 microM, while in the presence of N-hydroxydapsone, the K(s) remained unchanged (14.0 microM), which suggests that dapsone causes an increase in the affinity of flurbiprofen for CYP2C9, whereas N-hydroxydapsone does not. Additionally, stoichiometry measurements under activation conditions in the presence of dapsone resulted in a doubling of both NADPH and oxygen consumption for flurbiprofen 4(')-hydroxylation, with an overall increase in metabolite formation and a decrease in formation of peroxide and excess water. Interestingly, the presence of N-hydroxydapsone generally caused the same effects on stoichiometry as those of flurbiprofen 4(')-hydroxylation but failed to reduce excess water formation, which suggests that, while N-hydroxydapsone activates CYP2C9, it does so less efficiently and possibly through a mechanism different from that of dapsone.

Tolbutamide, flurbiprofen, and losartan as probes of CYP2C9 activity in humans

The metabolic activity of CYP2C9 in 16 subjects expressing four different genotypes (CYP2C9*1/*1, *1/*2, *1/*3, and *2/*2) was evaluated. Single oral doses of tolbutamide, flurbiprofen, and losartan were administered in a randomized, crossover design. Plasma and urine were collected over 24 hours. The urinary metabolic ratio and amount of metabolite(s) excreted were correlated with formation clearance. The formation clearance of tolbutamide to its CYP2C9-mediated metabolites demonstrated a stronger association with genotype compared to flurbiprofen and losartan, respectively (r2 = 0.64 vs. 0.53 vs. 0.42). A statistically significant correlation was observed between formation clearance of tolbutamide and the 0- to 12-hour urinary amount of 4'-hydroxytolbutamide and carboxytolbutamide (r = 0.84). Compared to tolbutamide, the correlations observed between the respective measures of flurbiprofen and losartan metabolism were not as strong. Tolbutamide is a better CYP2C9 probe than flurbiprofen and losartan, and the 0- to 12-hour amount of 4'-hydroxytolbutamide and carboxytolbutamide is the best urinary measure of its metabolism.

Essential requirements for substrate binding affinity and selectivity toward human CYP2 family enzymes

A detailed analysis of substrate selectivity within the cytochrome P450 2 (CYP2) family is reported. From a consideration of specific interactions between drug substrates for human CYP2 family enzymes and the putative active sites of CYP2A6, CYP2B6, CYP2C8, CYP2C9, CYP2C19, CYP2D6, and CYP2E1, it is likely that the number and disposition of hydrogen bond donor/acceptors and aromatic rings within the various P450 substrate molecules determines their enzyme selectivity and binding affinity, together with directing their preferred routes of metabolism by the CYP2 enzymes concerned. Although many aliphatic residues are present in most P450 active sites, it would appear that their main contribution centers around hydrophobic interactions and desolvation processes accompanying substrate binding. Molecular modeling studies based on the recent CYP2C5 crystal structure appear to show close agreement with site-directed mutagenesis experiments and with information on substrate metabolism and selectivity within the CYP2 family.

Polymorphic variants (CYP2C9*3 and CYP2C9*5) and the F114L active site mutation of CYP2C9: effect on atypical kinetic metabolism profiles

CYP2C9 wild-type protein has been shown to exhibit atypical kinetic profiles of metabolism that may affect in vitro-in vivo predictions made during the drug development process. Previous work suggests a substrate-dependent effect of polymorphic variants of CYP2C9 on the rate of metabolism; however, it is hypothesized that these active site amino acid changes will affect the kinetic profile of a drug's metabolism as well. To this end, the kinetic profiles of three model CYP2C9 substrates (flurbiprofen, naproxen, and piroxicam) were studied using purified CYP2C9*1 (wild-type) and variants involving active site amino acid changes, including the naturally occurring variants CYP2C9*3 (Leu359) and CYP2C9*5 (Glu360) and the man-made mutant CYP2C9 F114L. CYP2C9*1 (wild-type) metabolized each of the three compounds with a distinctive profile reflective of typical hyperbolic (flurbiprofen), biphasic (naproxen), and substrate inhibition (piroxicam) kinetics. CYP2C9*3 metabolism was again hyperbolic for flurbiprofen, of a linear form for naproxen (no saturation noted), and exhibited substrate inhibition with piroxicam. CYP2C9*5-mediated metabolism was hyperbolic for flurbiprofen and piroxicam but linear with respect to naproxen turnover. The F114L mutant exhibited a hyperbolic kinetic profile for flurbiprofen metabolism, a linear profile for naproxen metabolism, and a substrate inhibition kinetic profile for piroxicam metabolism. In all cases except F114L-mediated piroxicam metabolism, turnover decreased and the K(m) generally increased for each allelic variant compared with wild-type enzyme. It seems that the kinetic profile of CYP2C9-mediated metabolism is dependent on both substrate and the CYP2C9 allelic variant, thus having potential ramifications on drug disposition predictions made during the development process.

Cytochrome P450 2C9 polymorphisms: a comprehensive review of the in-vitro and human data

The discovery of six distinct polymorphisms in the genetic sequence encoding for the cytochrome P450 2C9 (CYP2C9) protein has stimulated numerous investigations in an attempt to characterize their population distribution and metabolic activity. Since the CYP2C9*1, *2 and *3 alleles were discovered first, they have undergone more thorough investigation than the recently identified *4, *5 and *6 alleles. Population distribution data suggest that the variant *2 and *3 alleles are present in approximately 35% of Caucasian individuals; however, these alleles are significantly less prevalent in African-American and Asian populations. In-vitro data have consistently demonstrated that the CYP2C9*2 and *3 alleles are associated with significant reductions in intrinsic clearance of a variety of 2C9 substrates compared with CYP2C9*1; however, the degree of these reductions appear to be highly substrate-dependent. In addition, multiple in-vivo investigations and clinical case reports have associated genotypes expressing the CYP2C9*2 and *3 alleles with significant reductions in both the metabolism and daily dose requirements of selected CYP2C9 substrates. Individuals expressing these variant genotypes also appear to be significantly more susceptible to adverse events with the narrow therapeutic index agents warfarin and phenytoin, particularly during the initiation of therapy. These findings have subsequently raised numerous questions regarding the potential clinical utility of genotyping for CYP2C9 prior to initiation of therapy with these agents. However, further clinical investigations evaluating the metabolic consequences in individuals expressing the CYP2C9*2, *3, *4, *5, or *6 alleles are required before large-scale clinical genotyping can be recommended.

Summary of information on human CYP enzymes: human P450 metabolism data

This chapter is an update of the data on substrates, reactions, inducers, and inhibitors of human CYP enzymes published previously by Rendic and DiCarlo (1), now covering selection of the literature through 2001 in the reference section. The data are presented in a tabular form (Table 1) to provide a framework for predicting and interpreting the new P450 metabolic data. The data are formatted in an Excel format as most suitable for off-line searching and management of the Web-database. The data are presented as stated by the author(s) and in the case when several references are cited the data are presented according to the latest published information. The searchable database is available either as an Excel file (for information contact the author), or as a Web-searchable database (Human P450 Metabolism Database, www.gentest.com) enabling the readers easy and quick approach to the latest updates on human CYP metabolic reactions.

Pharmacogenetics of warfarin elimination and its clinical implications

Warfarin is one of the most widely prescribed oral anticoagulants. However, optimal use of the drug has been hampered by its >10-fold interpatient variability in the doses required to attain therapeutic responses. Pharmacogenetic polymorphism of cytochrome P450 (CYP) may be associated with impaired elimination of warfarin and exaggerated anticoagulatory responses to the drug in certain patients. Clinically available warfarin is a racemic mixture of (R)- and (S)-warfarin, and the (S)-enantiomer has 3 to 5 times greater anticoagulation potency than its optical congener. Both enantiomers are eliminated extensively via hepatic metabolism with low clearance relative to hepatic blood flow. CYP2C9 is almost exclusively responsible for the metabolism of the pharmacologically more active (S)-enantiomer. Several human allelic variants of CYP2C9 have been cloned, designated as CYP2C9*1 (reference sequence or wild-type allele), CYP2C9*2, CYP2C9*3 and CYP2C9*4, respectively. The allelic frequencies for these variants differ considerably among different ethnic populations. Caucasians appear to carry the CYP 2C9*2 (8 to 20%) and CYP2C9*3 (6 to 10%) variants more frequently than do Asians (0% and 2 to 5%, respectively). The metabolic activities of the CYP2C9 variants have been investigated in vitro. The catalytic activity of CYP2C9*3 expressed from cDNA was significantly less than that of CYP2C9*1. Human liver microsomes obtained from individuals heterozygous for CYP2C9*3 showed significantly reduced (S)-warfarin 7-hydroxylation as compared with those obtained from individuals genotyped as CYP2C9*1. The influence of the CYP2C9*3 allele on the in vivo pharmacokinetics of (S)-warfarin has been studied in Japanese patients. Patients with the homozygous CYP2C9*3 genotype, as well as those with the heterozygous CYP2C9*1/*3 genotype, had significantly reduced clearance of (S)-warfarin (by 90 and 60%, respectively) compared with those with homozygous CYP2C9*1. The maintenance dosages of warfarin required in Japanese patients with heterozygous and homozygous CYP2C9*3 mutations were significantly lower than those in patients with CYP2C9*1/*1. In addition, 86% of British patients exhibiting adequate therapeutic responses with lower maintenance dosages of warfarin (<1.5 mg/day) had either the CYP2C9*2 or CYP2C9*3 mutation singly or in combination, whereas only 38% of randomly selected patients receiving warfarin carried the corresponding mutations. Furthermore, the former group showed more frequent episodes of major bleeding associated with warfarin therapy. These data indicate that the CYP2C9*3 allele may be associated with retarded elimination of (S)-warfarin and the resulting clinical effects. However, relationships between CYP2C9 genotype, enzyme activity, metabolism of probe substrates, dosage requirements and bleeding complications should be interpreted with caution, and further studies are required.

Minimal in vivo activation of CYP2C9-mediated flurbiprofen metabolism by dapsone

Dapsone has been shown to activate flurbiprofen 4'-hydroxylation by expressed CYP2C9 enzyme and in human liver microsomes. It has been suggested that this observation is due to substrate cooperativity on enzyme activity; however, the in vivo relevance of this observation is unknown. Thus, the purpose of this study was to evaluate whether dapsone can act cooperatively with flurbiprofen to activate the in vivo metabolism of flurbiprofen to 4'-hydroxyflurbiprofen. Twelve healthy subjects received single-dose flurbiprofen 50 mg on three occasions: alone (visit A); 2 h after a single dapsone 100-mg dose (visit B); and 2 h after the seventh daily dose of dapsone 100 mg (visit C). Concentrations of flurbiprofen and 4'-hydroxy flurbiprofen in plasma and urine and dapsone and N-acetyldapsone in plasma were determined by HPLC. Flurbiprofen pharmacokinetic parameters for the three visits were estimated by non-compartmental methods and compared in the absence and presence of dapsone. Flurbiprofen apparent oral clearance was increased by approximately 11% (P < 0.02) after dapsone 100 mg for 7 days. Dapsone plasma concentrations averaged 5 +/- 2 microM after a single dose and 11 +/- 4 microM after seven daily 100 mg doses. These dapsone plasma concentrations were within the range of concentrations producing activation of flurbiprofen metabolism by CYP2C9 in vitro. These results are consistent with the hypothesis that dapsone does influence flurbiprofen metabolism in vivo in a cooperative way to enhance metabolism. However, the magnitude of effect is substantially less than observed in vitro.

Effects of oral vitamin K on S- and R-warfarin pharmacokinetics and pharmacodynamics: enhanced safety of warfarin as a CYP2C9 probe

Evidence for the selectivity of S-warfarin metabolism by CYP2C9 is substantial, suggesting that warfarin may be a potential CYP2C9 phenotyping probe. It is, however, limited by its ability to elevate the international normalized ratio (INR) and potentially cause bleeding. The effect of vitamin K to attenuate the elevation of INR may enable the safe use of warfarin as a probe. The objective of this study was to investigate the pharmacokinetics and pharmacodynamics of S- and R-warfarin in plasma following the administration of warfarin alone versus warfarin and vitamin K in CYP2C9*1 homozygotes. Healthy adults received, in a randomized crossover fashion in a fasted state, warfarin 10 mg orally or warfarin 10 mg plus vitamin K 10 mg orally. Blood samples were obtained over 5 days during each phase. INR measurements were obtained at baseline and day 2 in each phase. INR, AUC0-infinity, and t1/2 of plasma S- and R-warfarin were examined. Eleven CYP2C9*1 homozygotes (3 men, 8 women) were enrolled. INR at day 2 following warfarin 10 mg was 1.18 +/- 0.19, which differed significantly from baseline (INR = 1.00 +/- 0.05) and warfarin with vitamin K (INR = 1.06 +/- 0.07). INR at baseline was not significantly different from warfarin with vitamin K. t1/2 and AUC0-infinity of both enantiomers did not significantly differ between the phases. It was concluded that INR is apparently attenuated by concomitant administration of a single dose of vitamin K without affecting the pharmacokinetics of either warfarin stereoisomer. Warfarin 10 mg may be safely used as a CYP2C9 probe in *1 homozygotes when given concomitantly with 10 mg of oral vitamin K.

Interindividual variability in inhibition and induction of cytochrome P450 enzymes

Drug interactions have always been a major concern in medicine for clinicians and patients. Inhibition and induction of cytochrome P450 (CYP) enzymes are probably the most common causes for documented drug interactions. Today, many pharmaceutical companies are predicting potential interactions of new drug candidates. Can in vivo drug interactions be predicted accurately from in vitro metabolic studies? Should the prediction be qualitative or quantitative? Although some scientists believe that quantitative prediction of drug interactions is possible, others are less optimistic and believe that quantitative prediction would be very difficult. There are many factors that contribute to our inability to quantitatively predict drug interactions. One of the major complicating factors is the large interindividual variability in response to enzyme inhibition and induction. This review examines the sources that are responsible for the interindividual variability in inhibition and induction of cytochrome P450 enzymes.

Inhibition of cytochrome P450 2C9 activity in vitro by 5-hydroxytryptamine and adrenaline

In the present study, the occurrence of a modulatory effect of 14 neurotransmitters, precursors and metabolites on the cytochrome P450 2C9 (CYP2C9) enzyme activity, as determined by diclofenac 4-hydroxylation, was studied in human liver microsomes. Two indoleamines, 5-hydroxytryptamine (5-HT) and adrenaline, showed a non-competitive-type inhibitory effect of approximately 90% of the diclofenac 4-hydroxylase activity, with Ki values of 63.5 (0.7 and 156 (89.3 microM, respectively. The rest of substances analysed were weak inhibitors or had no inhibitory effect. CYP2C subfamily is present in human brain, although CYP2C9 isozyme has not yet been identified in this tissue, and CYP2C9 is involved in the metabolism of psychoactive drugs. Therefore, the fact that endogenous compounds could modulate the CYP2C9 activity, suggests that an hypothetical local activity of brain CYP2C9 might be susceptible to regulatory mechanisms. The possible clinical implications of this modulation are discussed.

Sensitive and specific high-performance liquid chromatographic assay for 4'-hydroxyflurbiprofen and flurbiprofen in human urine and plasma

A high-performance liquid chromatographic assay has been developed for the simultaneous quantitation of flurbiprofen and its major metabolite, 4'-hydroxyflurbiprofen, in urine and plasma. No extraction procedure was necessary for analysis of these compounds, which reduced time involved in sample preparation. The analytes were separated on a Brownlee Spheri-5 C18 column with a mobile phase of acetonitrile-20 mM dibasic potassium phosphate pH 3 buffer (40:60, v/v). Fluorescence detection was utilized with an excitation wavelength of 260 nm and an emission wavelength of 320 nm, providing excellent sensitivity. The limit of quantitation for 4'-hydroxyflurbiprofen and flurbiprofen was 0.25 microg/ml in urine and 0.05 microg/ml and 0.25 microg/ml, respectively, in plasma. All components were eluted within 16 min. Intra-day, inter-day, freeze-thaw, and in process stability were tested for both compounds and the coefficient of variation was less than 14% in all cases. This method provides a sensitive and specific assay for the detection of flurbiprofen and 4'-hydoxyflurbiprofen in urine and plasma and is suitable for use in in vivo studies evaluating the regulation of CYP2C9 activity.

Validation of methods for CYP2C9 genotyping: frequencies of mutant alleles in a Swedish population

Cytochrome P450 2C9 (CYP2C9) catalysis the metabolism of important drugs such as phenytoin, S-warfarin, tolbutamide, losartan, torasemide, and nonsteroidal anti-inflammatory drugs. A functional polymorphism of the CYP2C9 gene has been described. The variant alleles include CYP2C9*2 having a point mutation in exon 3 causing an Arg144Cys exchange, and CYP2C9*3 with a point mutation in exon 7 resulting in an Ile359Leu exchange. Genotyping of these variant forms was carried out in 430 Swedish healthy volunteers and three different methods were compared. Sequence analysis of the different PCR products revealed that other genes in the CYP2C locus were co-amplified in one of the methods applied, whereas the other two methods were specific for CYP2C9. The frequencies of the CYP2C9*1, CYP2C9*2 and CYP2C9*3 alleles in the population examined were found to be 0.819, 0.107, and 0.074, respectively. The need for careful evaluation of the genotyping procedure by sequence analysis of PCR products is emphasised.

Comparative studies on the catalytic roles of cytochrome P450 2C9 and its Cys- and Leu-variants in the oxidation of warfarin, flurbiprofen, and diclofenac by human liver microsomes

S-Warfarin 7-hydroxylation, S-flurbiprofen 4'-hydroxylation, and diclofenac 4'-hydroxylation activities were determined in liver microsomes of 30 humans of which 19 were wild-type (Arg144.Ile359), 8 were heterozygous Cys (Cys144.Ile359), and 3 were heterozygous Leu (Arg144.Leu359) allelic variants of the cytochrome P450 2C9 (CYP2C9) gene. All of the human samples examined contained P450 protein(s) immunoreactive with anti-CYP2C9 antibodies in liver microsomes. Individuals with the Cys144 allele of CYP2C9 had similar, but slightly lower, activities for the oxidations of these substrates than those of wild-type CYP2C9. One of the three human samples heterozygous for the Leu359 allele had very low Vmax and high Km values for the oxidation of three substrates examined, while the other two individuals gave kinetic parameters comparable to those seen in the wild-type and Cys144 CYP2C9. Reverse transcriptase-polymerase chain reaction analysis, however, showed that all of the three human samples with the heterozygous Leu359 variant were found to express both Ile359 and Leu359 variants at relatively similar extents in liver RNA of three humans. These results suggest that the Cys144 variant of CYP2C9 catalyzes the CYP2C9 substrates at rates comparative to, but slightly lower than, those of wild-type CYP2C9, while the Leu359-allelic variant has slower rates for the oxidation of these drug substrates. Activities for the oxidation of these CYP2C9 substrates in humans with heterozygous Leu359 allele is likely to be dependent on the levels of expression of each of the wild- and Leu-variants in the livers. However, one of the humans with a heterozygous Leu allele was found to have very low activities towards the oxidation of CYP2C9 substrates. The basis of this defect in catalytic functions towards CYP2C9 substrates is unknown.

Relative quantities of catalytically active CYP 2C9 and 2C19 in human liver microsomes: application of the relative activity factor approach

The relative catalytic activities of CYP2C9 and CYP2C19 in human liver microsomes has been determined using the approach of relative activity factors (RAFs). Tolbutamide methylhydroxylation and S-mephenytoin 4'-hydroxylation were used as measures of CYP2C9 and CYP2C19 activity, respectively. The kinetics of these reactions were studied in human liver microsomes, in microsomes from human lymphoblastoid cells, and in insect cells expressing CYP2C9 and CYP2C19. RAFs were calculated as the ratio of Vmax (reaction velocity at saturating substrate concentrations) in human liver microsomes of the isoform-specific index reaction divided by the Vmax of the reaction catalyzed by the cDNA expressed isoform. RAFs were also determined for SUPERMIX, a commercially available mixture of cDNA expressed human drug metabolizing CYPs formulated to achieve a balance of enzyme activities similar to that found in human liver microsomes. Lymphoblast RAF2C9 in human liver microsomes ranged from 54 to 145 pmol CYP/mg protein (mean value: 87), while a value of 251 pmol CYP/mg protein was obtained for SUPERMIX. Insect cell RAF2C9 in human liver microsomes ranged from 1.6 to 143 pmol CYP/mg protein (mean value: 49), while a value of 201 pmol CYP/mg protein was obtained for SUPERMIX. Both lymphoblast and insect cell RAF2C19 in human liver microsomes ranged from 4 to 45 pmol CYP/mg protein (mean values: 29 and 28, respectively), while a value of 29 pmol CYP/mg protein was obtained for SUPERMIX. The nature of the cDNA expression system used had no effect on the kinetic parameters of CYP2C9 as a tolbutamide methylhydroxylase, or of CYP2C19 as a S-mephenytoin hydroxylase. However insect cell expressed CYP2C19 (which includes oxidoreductase) had substantially greater activity as a tolbutamide methylhydroxylase when compared to lymphoblast expressed CYP2C19. The ratio of mean lymphoblast-determined RAF2C9 to RAF2C19 in human livers was 3.0 (range 1.6-17.9; n = 10), while this ratio for SUPERMIX was 8.6. The ratio of mean insect cell-determined RAF2C9 to RAF2C19 in human livers was 1.7 (range 0.04-16.2; n = 10), while this ratio for SUPERMIX was 7.0. Neither ratio is in agreement with the 20:1 ratio of immunoquantified levels of CYP2C9 and 2C19 in human liver microsomes reported in previous studies. SUPERMIX may contain catalytically active CYP2C9 in levels higher than those in human liver microsomes.

Cytochrome P4502C9: an enzyme of major importance in human drug metabolism

Accumulating evidence indicates that CYP2C9 ranks amongst the most important drug metabolizing enzymes in humans. Substrates for CYP2C9 include fluoxetine, losartan, phenytoin, tolbutamide, torsemide, S-warfarin, and numerous NSAIDs. CYP2C9 activity in vivo is inducible by rifampicin. Evidence suggests that CYP2C9 substrates may also be induced variably by carbamazepine, ethanol and phenobarbitone. Apart from the mutual competitive inhibition which may occur between alternate substrates, numerous other drugs have been shown to inhibit CYP2C9 activity in vivo and/or in vitro. Clinically significant inhibition may occur with coadministration of amiodarone, fluconazole, phenylbutazone, sulphinpyrazone, sulphaphenazole and certain other sulphonamides. Polymorphisms in the coding region of the CYP2C9 gene produce variants at amino acid residues 144 (Arg144Cys) and 359 (Ile359Leu) of the CYP2C9 protein. Individuals homozygous for Leu359 have markedly diminished metabolic capacities for most CYP2C9 substrates, although the frequency of this allele is relatively low. Consistent with the modulation of enzyme activity by genetic and other factors, wide interindividual variability occurs in the elimination and/or dosage requirements of prototypic CYP2C9 substrates. Individualisation of dose is essential for those CYP2C9 substrates with a narrow therapeutic index.

Characterization of CYP2C19 and CYP2C9 from human liver: respective roles in microsomal tolbutamide, S-mephenytoin, and omeprazole hydroxylations

Individuals with drug metabolism polymorphisms involving CYP2C enzymes exhibit deficient oxidation of important therapeutic agents, including S-mephenytoin, omeprazole, warfarin, tolbutamide, and nonsteroidal anti-inflammatory drugs. While recombinant CYP2C19 and CYP2C9 proteins expressed in yeast or Escherichia coli have been shown to oxidize these agents, the capacity of the corresponding native P450s isolated from human liver to do so is ill defined. To that end, we purified CYP2C19, CYP2C9, and CYP2C8 from human liver samples using conventional chromatographic techniques and examined their capacity to oxidize S-mephenytoin, omeprazole, and tolbutamide. Upon reconstitution, CYP2C19 metabolized S-mephenytoin and omeprazole at rates that were 11- and 8-fold higher, respectively, than those of intact liver microsomes, whereas neither CYP2C9 nor CYP2C8 displayed appreciable metabolic activity with these substrates. CYP2C19 also proved an efficient catalyst of tolbutamide metabolism, exhibiting a turnover rate similar to CYP2C9 preparations (2.0-6.4 vs 2.4-4.3 nmol hydroxytolbutamide formed/min/nmol P450). The kinetic parameters of CYP2C19-mediated tolbutamide hydroxylation (Km = 650 microM, Vmax = 3.71 min-1) somewhat resembled those of the CYP2C9-catalyzed reaction (Km = 178-407 microM, Vmax = 2.95-7.08 min-1). Polyclonal CYP2C19 antibodies markedly decreased S-mephenytoin 4'-hydroxylation (98% inhibition) and omeprazole 5-hydroxylation (85% inhibition) by human liver microsomes. CYP2C19 antibodies also potently inhibited (>90%) microsomal tolbutamide hydroxylation, which was similar to the inhibition (>85%) observed with antibodies to CYP2C9. Moreover, excellent correlations were found between immunoreactive CYP2C19 content, S-mephenytoin 4'-hydroxylase activity (r = 0.912; P < 0. 001), and omeprazole 5-hydroxylase activity (r = 0.906; P < 0.001) in liver samples from 13-17 different subjects. A significant relationship was likewise observed between microsomal tolbutamide hydroxylation and CYP2C9 content (r = 0.664; P < 0.02) but not with CYP2C19 content (r = 0.393; P = 0.184). Finally, immunoquantitation revealed that in these human liver samples, expression of CYP2C9 (88. 5 +/- 36 nmol/mg) was 5-fold higher than that of CYP2C19 (17.8 +/- 14 nmol/mg) and nearly 8-fold higher than that of CYP2C8 (11.5 +/- 12 nmol/mg). Our results, like those obtained with recombinant CYP2C enzymes, indicate that CYP2C19 is a primary determinant of S-mephenytoin 4'-hydroxylation and low-Km omeprazole 5-hydroxylation in human liver. Despite its tolbutamide hydroxylase activity, the low levels of hepatic CYP2C19 expression (relative to CYP2C9) may preclude an important role for this enzyme in hepatic tolbutamide metabolism and any polymorphisms thereof.

Baculovirus-mediated expression of cytochrome P4502C8 and human NADPH-cytochrome P450 reductase: optimization of protein expression

1. High expression levels of cytochrome P450 (CYP) 2C8 and NADPH-cytochrome P450 oxidoreductase (OxR) in Spodoptera frugiperda (Sf21) cells have been achieved using the baculovirus expression system. 2. The baculovirus dual expression plasmid, pAcUW31, was used to insert CYP2C8 and OxR cDNAs downstream of the polyhedrin (polh) or p10 promoters, either separately or together, generating four recombinant baculoviruses; two expressing single proteins (CYP2C8 driven by the p10 promoter, bVp10.2C8 or OxR driven by the polh promoter, bVpolh.OxR) with another two coexpressing both CYP2C8 and OxR under reciprocal control of the polh and p10 promoters (bVpolh.OxR-p10.2C8 and bVpolh.2C8-p10.OxR). 3. High levels of singly expressed CYP2C8 and OxR were achieved from bVp10.2C8 and bVpolh.OxR, with levels of 0.7-1.2 nmol CYP/mg protein and 400-500 nmol cytochrome c reduced/min/mg protein respectively. 4. The two dual gene clones (bVpolh.OxR-p10.2C8 and bVpolh.2C8-p10.OxR) showed, in general, greater variation in CYP content and OxR activity than single gene clones. Screening was therefore necessary for the selection of dual gene clones expressing both proteins optimally. 5. Sf21 microsomes infected by selected dual gene clones were, on average, 14 times more active in tolbutamide hydroxylase activity than those expressing CYP2C8 alone, with a mean spectral CYP content of 79 pmol/mg cell lysate protein and a mean OxR level of 600 nmol/min/mg cell lysate protein.

Roles of two allelic variants (Arg144Cys and Ile359Leu) of cytochrome P4502C9 in the oxidation of tolbutamide and warfarin by human liver microsomes

1. Tolbutamide methyl hydroxylation and racemic warfarin 7-hydroxylation activities were determined in liver microsomes of 39 Japanese and 45 Caucasians genotyped for the cytochrome P450 (P450 or CYP) 2C9 gene into three groups, namely the wild-type (Arg144.Ile359), and two heterozygous Cys allele (Cys144.Ile359) and Leu allele (Arg144.Leu359) variants. 2. Good correlations were found between tolbutamide methyl hydroxylation and racemic warfarin 7-hydroxylation activities in liver microsomes of Japanese and Caucasians. Humans with the Cys allele CYP2C9 variant, which was detected in 22% of Caucasians, were found to have similar catalytic rates to those of the wild-type in the oxidations of tolbutamide and racemic warfarin, whereas humans with the Leu allele, which was detected in 8% Japanese and 7% Caucasian samples, had lower catalytic rates than those of other two groups. 3. The rates of 6- and 7-hydroxylation of racemic warfarin were correlated well with those of S-warfarin, but not R-warfarin, in human liver microsomes. 4. Both human liver microsomes and recombinant CYP2C9 catalysed 7-hydroxylation of S-warfarin more extensively than those of R-warfarin. K(m)'s for the 7-hydroxylation of S-warfarin were not very different in liver microsomes of humans with these three genotypes. Anti-CYP2C9 antibodies and sulphaphenazole inhibited the 6- and 7-hydroxylation of S-warfarin, but not R-warfarin, by > 90% and the methyl hydroxylation of tolbutamide by about 50%. 5. These results suggest that humans with Leu allele of CYP2C9 have lower Vmax's for S-warfarin 7-hydroxylation and tolbutamide methyl hydroxylation than those with wild-type and Cys allele CYP2C9, although the K(m)'s are not very different in liver microsomes of these three groups of humans. R-warfarin hydroxylation may be catalysed by P450 enzymes other than CYP2C9 in man.

Warfarin analog inhibition of human CYP2C9-catalyzed S-warfarin 7-hydroxylation

Human metabolism of the S-warfarin enantiomer is catalyzed primarily by cytochrome P4502C9 (CYP2C9), which, because of the enzyme's broad drug substrate specificity, leads to drug-S-warfarin interactions. Several warfarin analogs have been synthesized and used to determine whether they exhibit diminished interactions with CYP2C9. The kinetics of the warfarin analogs' inhibition of human liver microsomal CYP2C9 catalyzed metabolism of S-warfarin to S-7-hydroxywarfarin have been investigated. R- and S-7-fluorowarfarin were both predominantly competitive inhibitors, whereas racemic 6-fluorowarfarin and racemic 6,7,8-trifluorowarfarin were predominantly mixed inhibitors with some competitive inhibition. For the alcohols produced by reductive methylation of the side chain of R- and S-warfarin, the R-enantiomer did not inhibit S-warfarin metabolism, whereas the S-enantiomer was primarily a competitive inhibitor. The fluorine substituted warfarins and the S-warfarin alcohol apparently bind with high affinity to CYP2C9. Thus their use clinically (if efficacious) would not prevent CYP2C9 associated warfarin-drug interactions. The R-warfarin alcohol did not inhibit CYP2C9 catalyzed metabolism of S-warfarin and is less likely than warfarin to participate in CYP2C9 associated warfarin-drug interactions.

The effect of sulfaphenazole and sulfadoxine on tolbutamide disposition in dwarf goats (Caprus hircus aegagrus)

The aim of the present study was to investigate the effects of intravenously administered sulfadoxine (5 mg kg-1 bodyweight) or sulfaphenazole (5 mg kg-1 bodyweight) on the in vivo elimination of i.v. tolbutamide (5 mg kg-1 bodyweight), as both compounds were shown to inhibit tolbutamide hydroxylation in vitro. It was shown that relative large differences in tolbutamide clearance exist among goats (n = 6). A high correlation was seen between tolbutamide and sulfadoxine clearances. Tolbutamide clearance was significantly reduced by concommitant administration of sulfaphenazole. Sulfadoxine (SDX) had a less consistent effect. Mean tolbutamide plasma clearance was not significantly affected due to the fact that three animals showed an inhibition, whereas three others apparently did not respond. A negative correlation was found between the amount of N4-acetyl SDX in urine and the SDX clearance. Approximately 93 per cent of tolbutamide was bound to plasma proteins. However, there was no evidence for displacement of tolbutamide from its protein binding sites by sulfaphenazole or sulfadoxine. The results described in the present study confirm previous in vitro data obtained with goat hepatocytes. Although quantitative differences in inhibition exist between in vivo and in vitro results, hepatocytes are a good model to study potential drug-drug interactions at the level of biotransformation processes.

Differential inhibitory effects of phenytoin, diclofenac, phenylbutazone and a series of sulfonamides on hepatic cytochrome P4502C activity in vitro, and correlation with some molecular descriptors in the dwarf goat (Caprus hircus aegagrus)

1. The aim of the present study was to investigate the potency of various sulfonamides to inhibit tolbutamide hydroxylation (a CYP2C activity) in hepatic microsomal fractions and hepatocytes of the dwarf goat. Also a number of suggested substrates for human CYP2C9 was investigated. 2. From Dixon plots (microsomal fractions) it was observed that all compounds were competitive inhibitors of tolbutamide hydroxylation. Phenytoin (PT) showed the lowest Ki. Ki for the sulfonamides ranged between 205 and 4546 microM, sulfadoxine having the lowest Ki followed by sulfadimethoxine, sulfamoxole, sulfadimidine and sulfaphenazole. 3. In hepatocytes sulfaphenazole and diclofenac were the most potent inhibitors. 4. Out data indicate that PT, diclofenac (DF) and phenylbutazone (PBZ) are relative strong competitive inhibitors of tolbutamide hydroxylation and they are probably also substrates for the same enzyme. Differential inhibition of tolbutamide hydroxylation by sulfonamides was observed. 5. Correlation of structural parameters with the inhibition constant or the inhibition in hepatocytes showed that molecular volume, polarizability and molecular surface area are important parameters in determining the rate of inhibition of tolbutamide hydroxylation by sulfonamides in both microsomes and hepatocytes. In addition, log Poct are also involved in determining inhibition constants in microsomal fractions.

Inhibition of tolbutamide 4-methylhydroxylation by a series of non-steroidal anti-inflammatory drugs in V79-NH cells expressing human cytochrome P4502C10

1. To study the role of cytochrome P4502C10 in the metabolism of the non-steroidal antiinflammatory drugs (NSAIDs) diclofenac, phenylbutazone, fenoprofen, ibuprofen, flurbiprofen, ketoprofen and naproxen, a cell line was developed stably expressing CYP2C10 cDNA. A retroviral vector construct, containing a human CYP2C10 cDNA, was transfected in V79-NH Chinese hamster lung cells by calcium phosphate co-precipitation. Sublines stably expressing human cytochrome P450 cDNA were established by selection with the neomycin analogue G418. 2. Enzymatic activity of CYP2C10 was detected by 4-methylhydroxylation of tolbutamide. This activity was inhibited to background levels by preincubation with the CYP2C9/10 inhibitor sulphaphenazole. 3. Preincubations with the NSAIDs ketoprofen, phenylbutazone, flurbiprofen and diclofenac (all 250 microM) caused a decrease in 4-methylhydroxylation of tolbutamide (500 microM), significantly different from control values (p < 0.05). Inhibition of this activity was not seen in preincubations with the NSAIDs fenoprofen, ibuprofen and naproxen (250 microM). 4. The V79-NH CYP2C10 cell line we have developed has been shown to be a useful tool to predict drug-drug interactions.

Studies of flurbiprofen 4'-hydroxylation. Additional evidence suggesting the sole involvement of cytochrome P450 2C9

Flurbiprofen, a non-steroidal anti-inflammatory drug (NSAID), is metabolized by both oxidation via the cytochrome P450 system and by glucuronidation. The major oxidative pathway in flurbiprofen metabolism is to a 4'-hydroxy metabolite, and recently we demonstrated that cytochrome P450 2C9 and its R144C variant were involved in this process (Tracy et al., Biochem Pharmacol 49: 1269-1275, 1995). Using complementary DNA (cDNA)-expressed cell systems, it has been demonstrated that at physiological concentrations of flurbiprofen there is a lack of involvement of P450s 1A2, 2C8, 2E1, and 3A4. In evaluating flurbiprofen as a potential probe for cytochrome P450 2C9, it is important to assess the involvement of additional P450s in this process. To this end, further studies were undertaken using specific inhibitors of P450 2C9 and P450 cDNA-expressed microsomes for P450 1A1, 2A6, 2B6, 2C19, and 2D6 to assess their potential involvement. We observed the inhibition of (R)- and (S)-flurbiprofen 4'-hydroxylation by an inhibitor of P450 2C9, sulfaphenazole (Ki = 0.07 and 0.06 microM, respectively), and the NSAID piroxicam (Ki = 10 and 7 microM, respectively). Furthermore, using microsomes from a lymphoblastoid cell line, we found that P450s 1A1, 2A6, 2B6, 2C19, and 2D6 were not involved in flurbiprofen hydroxylation at physiological concentrations of flurbiprofen. This finding is particularly important due to the sequence homology and potential substrate overlap of P450 2C9 and 2C19. These studies then provide additional evidence to suggest that P450 2C9 may be the only isoform involved to any substantial degree in flurbiprofen 4'-hydroxylation, and thus this reaction is useful as an in vitro probe for this particularly cytochrome P450 isoform and may be useful as an in vivo probe.

Cytochromes P450, 1A2, and 2C9 are responsible for the human hepatic O-demethylation of R- and S-naproxen

A preliminary report implicated cytochrome P450 (CYP) 2C9 in the human liver microsomal O-demethylation of S-naproxen, suggesting that this pathway may be suitable for investigation of human hepatic CYP2C9 in vitro. Kinetic and inhibitor studies with human liver microsomes and confirmatory investigations with cDNA-expressed enzymes were undertaken here to define the role of CYP2C9 and other isoforms in the O-demethylation of R- and S-naproxen. All studies utilised a newly developed sensitive and specific HPLC assay that measured the respective O-desmethyl metabolites of R- and S-naproxen in incubations of human liver microsomes and in COS cell lysates. Microsomal R- and S-naproxen O-demethylation kinetics followed Michaelis-Menten kinetics, with respective mean apparent Km values of 123 microM and 143 microM. Sulfaphenazole, a specific inhibitor of CYP2C9, reduced the microsomal O-demethylation of R- and S-naproxen by 43% and 47%, respectively, and the CYP1A2 inhibitor furafylline decreased R- and S-naproxen O-demethylation by 38% and 28%, respectively. R,S-Mephenytoin was a weak inhibitor of R- and S-naproxen O-demethylation, but other CYP isoform specific inhibitors (e.g., coumarin, diethyldithiocarbamate, quinidine, troleandomycin) had little or no effect on these reactions. cDNA-expressed CYP2C9 and CYP1A2 were both shown to O-demethylate R- and S-naproxen. Apparent Km values (92-156 microM) for the reactions catalysed by the recombinant enzymes were similar to those observed for human liver microsomal R- and S-naproxen O-demethylation. The data demonstrate that CYP2C9 and CYP1A2 together account for the majority of human liver R- and S-naproxen O-demethylation, precluding the use of either R- or S-naproxen as a CYP isoform-specific substrate in vitro and in vivo.