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

Metabolite Measurement in Index Substrate Drug Interaction Studies: A Review of the Literature and Recent New Drug Application Reviews

BACKGROUND/OBJECTIVES

Index substrates are used to understand the processes involved in pharmacokinetic (PK) drug-drug interactions (DDIs). The aim of this analysis is to review metabolite measurement in clinical DDI studies, focusing on index substrates for cytochrome P450 (CYP) enzymes, including CYP1A2 (caffeine), CYP2B6 (bupropion), CYP2C8 (repaglinide), CYP2C9 ((S)-warfarin, flurbiprofen), CYP2C19 (omeprazole), CYP2D6 (desipramine, dextromethorphan, nebivolol), and CYP3A (midazolam, triazolam).

METHODS

All data used in this evaluation were obtained from the Certara Drug Interaction Database. Clinical index substrate DDI studies with PK data for at least one metabolite, available from literature and recent new drug application reviews, were reviewed. Further, for positive DDI studies, a correlation analysis was performed between changes in plasma exposure of index substrates and their marker metabolites.

RESULTS

A total of 3261 individual index DDI studies were available, with 45% measuring at least one metabolite. The occurrence of metabolite measurement in clinical DDI studies varied widely between index substrates and enzymes.

DISCUSSION AND CONCLUSIONS

For substrates such as caffeine, bupropion, omeprazole, and dextromethorphan, the use of the metabolite/parent area under the curve ratio can provide greater sensitivity to DDI or reduce intrasubject variability. In some cases (e.g., omeprazole, repaglinide), the inclusion of metabolite measurement can provide mechanistic insights to understand complex interactions.

Pharmacogenetic Variation in Neanderthals and Denisovans and Implications for Human Health and Response to Medications

Modern humans carry both Neanderthal and Denisovan (archaic) genome elements that are part of the human gene pool and affect the life and health of living individuals. The impact of archaic DNA may be particularly evident in pharmacogenes-genes responsible for the processing of exogenous substances such as food, pollutants, and medications-as these can relate to changing environmental effects, and beneficial variants may have been retained as modern humans encountered new environments. However, the health implications and contribution of archaic ancestry in pharmacogenes of modern humans remain understudied. Here, we explore 11 key cytochrome P450 genes (CYP450) involved in 75% of all drug metabolizing reactions in three Neanderthal and one Denisovan individuals and examine archaic introgression in modern human populations. We infer the metabolizing efficiency of these 11 CYP450 genes in archaic individuals and find important predicted phenotypic differences relative to modern human variants. We identify several single nucleotide variants shared between archaic and modern humans in each gene, including some potentially function-altering mutations in archaic CYP450 genes, which may result in altered metabolism in living people carrying these variants. We also identified several variants in the archaic CYP450 genes that are novel and unique to archaic humans as well as one gene, CYP2B6, that shows evidence for a gene duplication found only in Neanderthals and modern Africans. Finally, we highlight CYP2A6, CYP2C9, and CYP2J2, genes which show evidence for archaic introgression into modern humans and posit evolutionary hypotheses that explain their allele frequencies in modern populations.

A Sensitive UPLC-MS/MS Method for the Determination of Flurbiprofen in Rat Plasma: Application to Real Sample

For the quantification of flurbiprofen in rat plasma, a simple UPLC-MS/MS method with high sensitivity and short retention time for flurbiprofen was developed and validated using specific parameters. Etodolac was used as internal standard. The transitions (precursor to the product) of flurbiprofen and internal standard were obtained using the electrospray ionization in the negative ion multiple reaction monitoring mode, 243.2 → 199.2, 286.2 → 212.1, respectively. For chromatographic separation, C18 column was used for the stationary phase and gradient elution was used for the mobile phase. This mobile phase consisted of a methanol (A) and a 5 mM ammonium formate solution (B), which varied at a flow rate of 0.4 mL/min. For flurbiprofen, LLOQ was determined as 5 ng/mL. Quantification of flurbiprofen in the rat plasma with a linear calibration curve of 5-5000 ng/mL (r > 0.9991 for plasma) is possible with a retention time of 1.89 min. The total analysis time of the method was 3 min. The proposed method was validated. The intraday and inter-day precision (RSD%) and accuracy (RE%) were within 10% in all cases for flurbiprofen. The stability of flurbiprofen was evaluated under conditions such as short-term, long-term, autosampler and freeze/thaw. After method validation, flurbiprofen was succesfully quantified in real rat plasma samples.

Pharmacokinetics and phenotyping properties of the Basel phenotyping cocktail combination capsule in healthy male adults

AIMS

We compared the phenotyping metrics of a combination capsule formulation to its individual components of the newly composed Basel phenotyping cocktail. Moreover, we investigated a reduced sampling regimen for clinical applications.

METHODS

We performed in vitro experiments and a crossover pharmacokinetic study in twelve healthy male subjects to compare the Basel phenotyping cocktail capsule containing 6 cytochrome P450 (CYP) probe drugs with individual administration of the same drugs. Parent compounds and selected metabolites were determined by liquid chromatography-tandem mass spectrometry. Metabolic ratios (MR) for are under the curve (AUC) and single time point measurements and their correlation were determined.

RESULTS

Experiments with human liver microsomes and primary human hepatocytes in 3D co-culture confirmed that flurbiprofen is a suitable CYP2C9 substrate. Both cocktail formulations (capsule and individual probe drug administration) were well-tolerated and yielded reproducible MRs, which were almost identical. Correlations between single time point ratios and the corresponding AUC ratios depended on the sampling time point and the concentration time curve of the probe drugs. The MR of the capsule (Spearman rank correlation coefficient, R_s : 0.77-0.97) as well as the individual components (R_s : 0.69-0.99) correlated best at 6 h post-treatment considering all 6 CYPs. Moreover, the 2-h time points of the capsule agreed suitably with the AUC; however, the MR of omeprazole could not be determined for 10 out of 12 subjects.

CONCLUSION

The capsule is easy to swallow, well tolerated and provides reliable estimates for CYP activity. The optimal sampling point for the capsule formulation is 6 h after intake.

CYP2C19 and 3A4 Dominate Metabolic Clearance and Bioactivation of Terbinafine Based on Computational and Experimental Approaches

Lamisil (terbinafine) is an effective, widely prescribed antifungal drug that causes rare idiosyncratic hepatotoxicity. The proposed toxic mechanism involves a reactive metabolite, 6,6-dimethyl-2-hepten-4-ynal (TBF-A), formed through three N-dealkylation pathways. We were the first to characterize them using in vitro studies with human liver microsomes and modeling approaches, yet knowledge of the individual enzymes catalyzing reactions remained unknown. Herein, we employed experimental and computational tools to assess terbinafine metabolism by specific cytochrome P450 isozymes. In vitro inhibitor phenotyping studies revealed six isozymes were involved in one or more N-dealkylation pathways. CYP2C19 and 3A4 contributed to all pathways, and so, we targeted them for steady-state analyses with recombinant isozymes. N-Dealkylation yielding TBF-A directly was catalyzed by CYP2C19 and 3A4 similarly. Nevertheless, CYP2C19 was more efficient than CYP3A4 at N-demethylation and other steps leading to TBF-A. Unlike microsomal reactions, N-denaphthylation was surprisingly efficient for CYP2C19 and 3A4, which was validated by controls. CYP2C19 was the most efficient among all reactions. Nonetheless, CYP3A4 was more selective at steps leading to TBF-A, making it more effective in terbinafine bioactivation based on metabolic split ratios for competing pathways. Model predictions did not extrapolate to quantitative kinetic constants, yet some results for CYP3A4 and CYP2C19 agreed qualitatively with preferred reaction steps and pathways. Clinical data on drug interactions support the CYP3A4 role in terbinafine metabolism, while CYP2C19 remains understudied. Taken together, knowledge of P450s responsible for terbinafine metabolism and TBF-A formation provides a foundation for investigating and mitigating the impact of P450 variations in toxic risks posed to patients.

Polymorphic analysis of CYP2C9 gene in Vietnamese population

Genetic variations in CYP2C9 are associated to inter-individual variability of drugs metabolism and response. The only report has been done previously mainly focusing on the common variant alleles of CYP2C9 in Vietnamese Kinh subjects. However, little is known about the complete spectrum of this gene polymorphism in different ethnic groups of Vietnam. We sequenced the promoter region and all exons of CYP2C9 in 100 healthy unrelated Vietnamese Kinh subjects. Additionally, common CYP2C9 variants, *2 and *3, were also analyzed by RFLP-PCR in extra 194 Kinh subjects and 279 of other four ethnic groups in Vietnam. The results of these common variants observed from five ethnic groups were compared with other populations in the world. Seven previously reported alleles and two genotypes were determined in Kinh subjects. The percentage of CYP2C9*1 and CYP2C9*3 alleles are 96.5 and 3.5%, respectively. We found one novel non-synonymous variant in exon 7 leading to amino acid change at 363 position from proline to histidine. Functional analysis by SIFT and Polyphen-2 indicated that this mutation is intolerant and probably damaging. Prevalence of CYP2C9*2 observed in Vietnamese population was significantly lower compared with that of other populations in the South and West of Asia as well as in Europe. This study provides information of genetic distribution pattern of CYP2C9 in Vietnamese, which would be useful for optimizing drug therapies in Vietnam.

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

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. 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. 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. 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.

P450 Pharmacogenetics in Indigenous North American Populations

Indigenous North American populations, including American Indian and Alaska Native peoples in the United States, the First Nations, Métis and Inuit peoples in Canada and Amerindians in Mexico, are historically under-represented in biomedical research, including genomic research on drug disposition and response. Without adequate representation in pharmacogenetic studies establishing genotype-phenotype relationships, Indigenous populations may not benefit fully from new innovations in precision medicine testing to tailor and improve the safety and efficacy of drug treatment, resulting in health care disparities. The purpose of this review is to summarize and evaluate what is currently known about cytochrome P450 genetic variation in Indigenous populations in North America and to highlight the importance of including these groups in future pharmacogenetic studies for implementation of personalized drug therapy.

Biotransformation of NSAIDs by pig liver microsomes in vitro: Kinetics, metabolites identification and toxicity prediction

Non-steroidal anti-inflammatory drugs (NSAIDs) are one of the most frequently used pharmaceuticals in animals. In the current study, the biotransformation of five NSAIDs by pig liver microsomes (PLMs) was studied. The pseudo-first-order kinetics mode was obtained for the metabolization of the studied NSAIDs by PLMs in vitro. The metabolites were identified by high performance liquid chromatography with a high-resolution LTQ-Orbitrap mass spectrometry. The hydroxylation of benzene was confirmed to be the dominating metabolic pathway. Finally, the toxicity of the metabolites was predicted by the Estimation Programs Interface Suite software based on quantitative structure-activity relationships. Decreased toxicity was expected for the most metabolites of the studied NSAIDs except flurbiprofen, whose main metabolite exhibited slightly more toxicity. The present study provided a preliminary foundation to understand the metabolites of some NSAIDs and their toxicity, which was of great significance in animal food safety.

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.

Evaluation of cytochrome P450 2C9 activity in normal, healthy, adult Western Indian population by both phenotyping and genotyping

Objectives:

Cytochrome P450 2C9 (CYP2C9) is a member of cytochrome P450 (CYP) family that accounts for nearly 18% of the total CYP protein content in the human liver microsomes and catalyzes almost 15–20% of the drugs. Considering the paucity of data on the polymorphisms of CYP2C9 in Western Indian population, the present study was conducted to evaluate the prevalence of CYP2C9 polymorphisms (*1, *2 and *3) and correlate it with the activity using flurbiprofen (FLB) as a probe drug.

Materials and Methods:

A 100 mg FLB capsule was administered to 298 healthy adult participants. Venous blood samples were analyzed at 2 h postdose for the estimation of FLB and 4-hydroxy FLB. Metabolic ratio (MR) was calculated to determine the extent of poor metabolizer (PM) and rapid metabolizer status using probit plot. Genotyping of CYP2C9 polymorphism was performed using polymerase chain reaction-restriction fragment length polymorphism technique.

Results:

Of the total 298 participants, phenotype was assessable in 288 and genotype was performed in 289 participants. The median (range) MR of the study population was 6.6 (1.65–66.05). Five participants were found to be PMs by phenotype. Of the total 289 participants, 209 (72.3%) (66.7, 77.2) had CYP2C9*1/*1, 25 (8.7%) (5.8, 12.7) with CYP2C9*1/*2, 55 (19%) (14.8, 24.1) had CYP2C9*1/*3, 3 (1%) (0.3, 3.3) had CYP2C9*2/*3 genotype. A significant association between phenotype and genotype was observed.

Conclusion:

To conclude, the present study found significant association of CYP2C9 activity by both phenotype and genotype and these findings have to be corroborated in different kinds of patients.

Cyclosporin A-sensitive cytotoxicity of flurbiprofen non-stereoselectively mediated by cytochrome P450 metabolism in three-dimensional cultured rat hepatocytes

OBJECTIVES

2-Arylpropionic acid (profen) drugs are associated with severe hepatotoxicity; however, risk factors are still poorly understood. Acyl-coenzyme A (acyl-CoA) thioesters of profen drugs play a more important role in the covalent binding to rat hepatocyte proteins than the respective acyl-glucuronides. Therefore, we examined whether acyl-glucuronides, acyl-CoA thioesters and oxidative metabolites of profen drugs stereoselectively participated in liver damage.

METHODS

Cytotoxicity was determined by measuring lactate dehydrogenase (LDH) leakage from three-dimensional cultured rat hepatocytes.

KEY FINDINGS

LDH leakage was not induced by R-2-phenylpropionic acid and R-ibuprofen greatly forming acyl-CoA thioesters. S-Naproxen metabolized mainly by Uridine 5'-diphosphate (UDP)-glucuronosyl-transferase did not enhance LDH leakage. However, flurbiprofen (FLP) induced LDH leakage. A selective cytochrome P450 (CYP) 2C11 inhibitor suppressed 40-50% of the R-FLP and S-FLP-induced cytotoxicity. Borneol non-stereoselectively accelerated the FLP-induced cytotoxicity. The R-FLP-induced cytotoxicity decreased intracellular adenosine triphosphate (ATP) levels to 50% of untreated hepatocytes. An inhibitor of mitochondrial permeability transition pore, cyclosporin A (Cys A), rescued ATP levels and LDH leakage back to control levels.

CONCLUSION

The reactive acyl-CoA thioesters and acyl-glucuronides were not associated with liver damage, denying one of the leading hypotheses. CYP metabolism of FLP non-stereoselectively participated in Cys A-sensitive cytotoxicity, suggesting mitochondrial injury.

CYP2C9 genotype vs. metabolic phenotype for individual drug dosing--a correlation analysis using flurbiprofen as probe drug

Currently, genotyping of patients for polymorphic enzymes responsible for metabolic elimination is considered a possibility to adjust drug dose levels. For a patient to profit from this procedure, the interindividual differences in drug metabolism within one genotype should be smaller than those between different genotypes. We studied a large cohort of healthy young adults (283 subjects), correlating their CYP2C9 genotype to a simple phenotyping metric, using flurbiprofen as probe drug. Genotyping was conducted for CYP2C9*1, *2, *3. The urinary metabolic ratio MR (concentration of CYP2C9-dependent metabolite divided by concentration of flurbiprofen) determined two hours after flurbiprofen (8.75 mg) administration served as phenotyping metric. Linear statistical models correlating genotype and phenotype provided highly significant allele-specific MR estimates of 0.596 for the wild type allele CYP2C9*1, 0.405 for CYP2C9*2 (68 % of wild type), and 0.113 for CYP2C9*3 (19 % of wild type). If these estimates were used for flurbiprofen dose adjustment, taking 100 % for genotype *1/*1, an average reduction to 84 %, 60 %, 68 %, 43 %, and 19 % would result for genotype *1/*2, *1/*3, *2/*2, *2/*3, and *3/*3, respectively. Due to the large individual variation within genotypes with coefficients of variation ≥ 20 % and supposing the normal distribution, one in three individuals would be out of the average optimum dose by more than 20 %, one in 20 would be 40 % off. Whether this problem also applies to other CYPs and other drugs has to be investigated case by case. Our data for the given example, however, puts the benefit of individual drug dosing to question, if it is exclusively based on genotype.

Pharmacogenetics of P450 oxidoreductase: implications in drug metabolism and therapy

The redox reaction of cytochrome P450 enzymes (CYP) is an important physiological and biochemical reaction in the human body, as it is involved in the oxidative metabolism of both endogenous and exogenous substrates. Cytochrome P450 oxidoreductase (POR) is the only obligate electron donor for all of the hepatic microsomal CYP enzymes. It plays a crucial role in drug metabolism and treatment by not only acting as an electron donor involved in drug metabolism mediated by CYP enzymes but also by directly inducing the transformation of some antitumor precursors. Studies have found that the gene encoding human POR is highly polymorphic, which is of considerable clinical significance as it affects the metabolism and curative effects of clinically used drugs. This review aims to discuss the effect of POR and its genetic polymorphisms on drug metabolism and therapy, as well as the potential mechanisms of POR pharmacogenetics.

Are there differences in the catalytic activity per unit enzyme of recombinantly expressed and human liver microsomal cytochrome P450 2C9? A systematic investigation into inter-system extrapolation factors

The 'relative activity factor' (RAF) compares the activity per unit of microsomal protein in recombinantly expressed cytochrome P450 enzymes (rhCYP) and human liver without separating the potential sources of variation (i.e. abundance of enzyme per mg of protein or variation of activity per unit enzyme). The dimensionless 'inter-system extrapolation factor' (ISEF) dissects differences in activity from those in CYP abundance. Detailed protocols for the determination of this scalar, which is used in population in vitro-in vivo extrapolation (IVIVE), are currently lacking. The present study determined an ISEF for CYP2C9 and, for the first time, systematically evaluated the effects of probe substrate, cytochrome b5 and methods for assessing the intrinsic clearance (CL(int) ). Values of ISEF for S-warfarin, tolbutamide and diclofenac were 0.75 ± 0.18, 0.57 ± 0.07 and 0.37 ± 0.07, respectively, using CL(int) values derived from the kinetic values V(max) and K(m) of metabolite formation in rhCYP2C9 + reductase + b5 BD Supersomes™. The ISEF values obtained using rhCYP2C9 + reductase BD Supersomes™ were more variable, with values of 7.16 ± 1.25, 0.89 ± 0.52 and 0.50 ± 0.05 for S-warfarin, tolbutamide and diclofenac, respectively. Although the ISEF values obtained from rhCYP2C9 + reductase + b5 for the three probe substrates were statistically different (p < 0.001), the use of the mean value of 0.54 resulted in predicted oral clearance values for all three substrates within 1.4 fold of the observed literature values. For consistency in the relative activity across substrates, use of a b5 expressing recombinant system, with the intrinsic clearance calculated from full kinetic data is recommended for generation of the CYP2C9 ISEF. Furthermore, as ISEFs have been found to be sensitive to differences in accessory proteins, rhCYP system specific ISEFs are recommended.

Structural features of cytochromes P450 and ligands that affect drug metabolism as revealed by X-ray crystallography and NMR

Cytochromes P450 (P450s) play a major role in the clearance of drugs, toxins, and environmental pollutants. Additionally, metabolism by P450s can result in toxic or carcinogenic products. The metabolism of pharmaceuticals by P450s is a major concern during the design of new drug candidates. Determining the interactions between P450s and compounds of very diverse structures is complicated by the variability in P450-ligand interactions. Understanding the protein structural elements and the chemical attributes of ligands that dictate their orientation in the P450 active site will aid in the development of effective and safe therapeutic agents. The goal of this review is to describe P450-ligand interactions from two perspectives. The first is the various structural elements that microsomal P450s have at their disposal to assume the different conformations observed in X-ray crystal structures. The second is P450-ligand dynamics analyzed by NMR relaxation studies.

Biotransformation of flurbiprofen by Cunninghamella species

The biotransformation of the fluorinated anti-inflammatory drug flurbiprofen was investigated in Cunninghamella spp. Mono- and dihydroxylated metabolites were detected using gas chromatography-mass spectrometry and fluorine-19 nuclear magnetic resonance spectroscopy, and the major metabolite 4'-hydroxyflurbiprofen was isolated by preparative high-pressure liquid chromatography (HPLC). Cunninghamella elegans DSM 1908 and C. blakesleeana DSM 1906 also produced a phase II (conjugated) metabolite, which was identified as the sulfated drug via deconjugation experiments.

Effect of tarenflurbil on cognitive decline and activities of daily living in patients with mild Alzheimer disease: a randomized controlled trial

CONTEXT

Amyloid-beta peptide (Abeta(42)) has been implicated in the pathogenesis of Alzheimer disease (AD). Tarenflurbil, a selective Abeta(42)-lowering agent, demonstrated encouraging results on cognitive and functional outcomes among mildly affected patients in an earlier phase 2 trial.

OBJECTIVE

To determine the efficacy, safety, and tolerability of tarenflurbil.

DESIGN, SETTING, AND PATIENTS

A multicenter, randomized, double-blind, placebo-controlled trial enrolling patients with mild AD was conducted at 133 trial sites in the United States between February 21, 2005, and April 30, 2008. Concomitant treatment with cholinesterase inhibitors or memantine was permitted.

INTERVENTION

Tarenflurbil, 800 mg, or placebo, administered twice a day.

MAIN OUTCOME MEASURES

Co-primary efficacy end points were the change from baseline to month 18 in total score on the subscale of the Alzheimer Disease Assessment Scale-Cognitive Subscale (ADAS-Cog, 80-point version) and Alzheimer Disease Cooperative Studies-activities of daily living (ADCS-ADL) scale. Additional prespecified slope analyses explored the possibility of disease modification.

RESULTS

Of the 1684 participants randomized, 1649 were included in the analysis, and 1046 completed the trial. Tarenflurbil had no beneficial effect on the co-primary outcomes (difference in change from baseline to month 18 vs placebo, based on least squares means: 0.1 for ADAS-Cog; 95% CI, -0.9 to 1.1; P = .86 and -0.5 for ADCS-ADL; 95% CI, -1.9 to 0.9; P = .48) using an intent-to-treat analysis. No significant differences occurred in the secondary outcomes. The ADAS-Cog score decreased by 7.1 points over 18 months. The tarenflurbil group had a small increase in frequency of dizziness, anemia, and infections.

CONCLUSION

Tarenflurbil did not slow cognitive decline or the loss of activities of daily living in patients with mild AD.

TRIAL REGISTRATION

clinicaltrials.gov Identifier: NCT00105547.

Prediction of activation energies for aromatic oxidation by cytochrome P450

We have estimated the activation energy for aromatic oxidation by compound I in cytochrome P450 for a diverse set of 17 substrates using state-of-the-art density functional theory (B3LYP) with large basis sets. The activation energies vary from 60 to 87 kJ/mol. We then test if these results can be reproduced by computationally less demanding methods. The best methods (a B3LYP calculation of the activation energy of a methoxy-radical model or a partial least-squares model of the semiempirical AM1 bond dissociation energies and spin densities of the tetrahedral intermediate for both a hydroxyl-cation and a hydroxyl-radical model) give correlations with r(2) of 0.8 and mean absolute deviations of 3 kJ/mol. Finally, we apply these simpler methods on several sets of reactions for which experimental data are available and show that we can predict the reactive sites by combining calculations of the activation energies with the solvent-accessible surface area of each site.

Polymorphisms of human cytochrome P450 2C9 and the functional relevance

Human cytochrome P450 2C9 (CYP2C9) accounts for ∼20% of hepatic total CYP content and metabolizes ~15% clinical drugs such as phenytoin, S-warfarin, tolbutamide, losartan, and many nonsteroidal anti-inflammatory agents (NSAIDs). CYP2C9 is highly polymorphic, with at least 33 variants of CYP2C9 (*1B through *34) being identified so far. CYP2C9*2 is frequent among Caucasians with ~1% of the population being homozygous carriers and 22% are heterozygous. The corresponding figures for the CYP2C9*3 allele are 0.4% and 15%, respectively. There are a number of clinical studies addressing the impact of CYP2C9 polymorphisms on the clearance and/or therapeutic response of therapeutic drugs. These studies have highlighted the importance of the CYP2C9*2 and *3 alleles as a determining factor for drug clearance and drug response. The CYP2C9 polymorphisms are relevant for the efficacy and adverse effects of numerous NSAIDs, sulfonylurea antidiabetic drugs and, most critically, oral anticoagulants belonging to the class of vitamin K epoxide reductase inhibitors. Warfarin has served as a practical example of how pharmacogenetics can be utilized to achieve maximum efficacy and minimum toxicity. For many of these drugs, a clear gene-dose and gene-effect relationship has been observed in patients. In this regard, CYP2C9 alleles can be considered as a useful biomarker in monitoring drug response and adverse effects. Genetic testing of CYP2C9 is expected to play a role in predicting drug clearance and conducting individualized pharmacotherapy. However, prospective clinical studies with large samples are warranted to establish gene-dose and gene-effect relationships for CYP2C9 and its substrate drugs.

Rapid Single-Nucleotide Polymorphism Detection of Cytochrome P450 (CYP2C9) and Vitamin K Epoxide Reductase (VKORC1) Genes for the Warfarin Dose Adjustment by the SMart-Amplification Process Version 2

Background: Polymorphisms of the CYP2C9 (cytochrome P450, family 2, subfamily C, polypeptide 9) gene (CYP2C9*2, CYP2C9*3) and the VKORC1 (vitamin K epoxide reductase complex, subunit 1) gene (−1639G&gt;A) greatly impact the maintenance dose for the drug warfarin. Prescreening patients for their genotypes before prescribing the drug facilitates a faster individualized determination of the proper maintenance dose, minimizing the risk for adverse reaction and reoccurrence of thromboembolic episodes. With current methodologies, therapy can be delayed by several hours to 1 day if genotyping is to determine the loading dose. A simpler and more rapid genotyping method is required.

Methods: We developed a single-nucleotide polymorphism (SNP)-detection assay based on the SMart Amplification Process version 2 (SMAP 2) to analyze CYP2C9*2, CYP2C9*3, and VKORC1 −1639G&gt;A polymorphisms. Blood from consenting participants was used directly in a closed-tube real-time assay without DNA purification to obtain results within 1 h after blood collection.

Results: We analyzed 125 blood samples by both SMAP 2 and PCR-RFLP methods. The results showed perfect concordance.

Conclusions: The results validate the accuracy of the SMAP 2 for determination of SNPs critical to personalized warfarin therapy. SMAP 2 offers speed, simplicity of sample preparation, the convenience of isothermal amplification, and assay-design flexibility, which are significant advantages over conventional genotyping technologies. In this example and other clinical scenarios in which genetic testing is required for immediate and better-informed therapeutic decisions, SMAP 2–based diagnostics have key advantages.

Enantioselective Pharmacokinetics of Ibuprofen and Involved Mechanisms

Although dexibuprofen (S-ibuprofen) was marketed in Austria and Switzerland, the racemate at various formulations is still extensively used worldwide, and there are no indications that the racemate will be replaced by the single enantiomer. Thus, elucidation of the characteristics and involved mechanisms of the chiral pharmacokinetics of racemic ibuprofen is of special importance for the understanding of the pharmacological and toxicological consequences, and for prediction of the clinically potential drug interactions and influence of the pathological states. Stereoselective pharmacokinetics and metabolism are common features for chiral nonsteroidal antiinflammatory drugs (NSAIDs) and especially for 2-arylpropionic acid derivatives characterized with a chiral center adjacent to the carboxyl group. Although the enantioselective pharmacokinetic characteristics of different NSAIDs should be treated case by case, they share similar mechanisms underlying the protein binding, metabolism and chiral inversion. Ibuprofen was the most extensively researched drug in terms of chiral characteristics and mechanisms. Therefore, elucidation of the mechanisms derived from research on ibuprofen may provide better understanding and prediction of other chiral drugs. This article attempts to elucidate the chiral pharmacokinetics and involved mechanisms of ibuprofen in comparison with other NSAIDs based on recent developments. Topics on history of ibuprofen, enantioselective analysis method, absorption, protein binding, conventional metabolism, metabolic chiral inversion, gene polymorphism, and biochemical developments were included. It is worth mentioning that some underlying biochemical mechanisms, especially for the metabolic chiral inversion and ethnic differences still remain to be seen. Further research is required to develop human-resourced researching model and to provide more evidence concerning the site of inversion, species variation, CYP450 gene polymorphisms, and biochemical mechanisms.

Disposition of flurbiprofen in man: influence of stereochemistry and age

1. The stereoselective metabolism and pharmacokinetics of the enantiomers of flurbiprofen were investigated following the oral administration of the racemic drug (100 mg) to four young and four elderly healthy volunteers (two males and two females per group). 2. The stereochemical composition of the drug and the 4'-hydroxy- metabolite in serum and the drug, 4'-hydroxy- and 3'-hydroxy-4'-methoxy- metabolites, both free and conjugated, in urine were determined by a direct chromatographic method of enantiomeric analysis. 3. Modest enantioselectivity in clearance (CL S/R: young, 0.86; elderly, 0.88) was largely responsible for the apparent elimination half-life of (S)-flurbiprofen being significantly greater (p<0.01) than that of the R-enantiomer in both age groups (young, S: 5.2 +/- 0.7 versus R: 4.5 +/- 0.6 h; elderly, S: 9.6 +/- 1.2 versus R: 7.1 +/- 1.0 h). The serum concentrations of 4'-hydroxyflurbiprofen were five- to 20-fold lower than those of the corresponding drug enantiomers, stereoselective disposition being evident in the significantly greater (p<0.05) apparent half-lives of the S- compared with the R-enantiomer in both groups (young, S: 10.6 +/- 2.4 versus R: 6.7 +/- 1.1 h; elderly, S: 13.7 +/- 1.7 versus R: 10.2 +/- 1.2 h). 4. Some 60 and 72% of the dose was excreted in 24-h urine in elderly and young volunteers, respectively, a significantly greater (p<0.05) proportion of which was of the R-configuration in both age groups (S/R: young, 0.87; elderly, 0.81). The major urinary excretion products were flurbiprofen and 4'-hydroxyflurbiprofen, and their acyl-conjugates in both groups. 5. Age-associated differences in the pharmacokinetics of flurbiprofen occurred in a non-stereoselective manner and were primarily as a consequence of a significant approximately 40% decrease (p<0.01) in clearance of both enantiomers in the elderly due to reduced metabolic activity. Consequently, the elderly had greater exposure to both enantiomers, as reflected by the AUCs(0-inf) being significantly higher (p<0.05), by 60%, in this age group compared with the young. 6. The findings suggest that age-related alterations in the disposition of flurbiprofen could have significant implications for the use of the drug in the elderly.

Differential genotype dependent inhibition of CYP2C9 in humans

The effects of genetic polymorphisms in drug-metabolizing enzymes (e.g., CYP2C9(*)3) on drug clearance have been well characterized but much less is known about whether these polymorphisms alter susceptibility to drug-drug interactions. Previous in vitro work has demonstrated that genotype-dependent inhibition of CYP2C9 mediated flurbiprofen metabolism, suggesting the possibility of genotype-dependent inhibition interactions in vivo. In the current study, flurbiprofen was used as a probe substrate and fluconazole as a prototypical inhibitor to investigate whether genotype-dependent inhibition of CYP2C9 occurs in vivo. From 189 healthy volunteers who were genotyped for CYP2C9 polymorphisms, 11 control subjects (CYP2C9(*)1/(*)1), 9 heterozygous and 2 homozygous for the CYP2C9(*)3 allele participated in the pharmacokinetic drug interaction study. Subjects received a single 50-mg oral dose of flurbiprofen alone or after administration of either 200 or 400 mg of fluconazole for 7 days using an open, randomized, crossover design. Flurbiprofen and fluconazole plasma concentrations along with flurbiprofen and 4'-hydroxyflurbiprofen urinary excretion were monitored. Flurbiprofen apparent oral clearance differed significantly among the three genotype groups (p < 0.05) at baseline but not after pretreatment with 400 mg of fluconazole for 7 days. Changes in flurbiprofen apparent oral clearance after fluconazole coadministration were gene dose-dependent, with virtually no change occurring in (*)3/(*)3 subjects. Analysis of fractional clearances suggested that the fraction metabolized by CYP2C9, as influenced by genotype, determined the degree of drug interaction observed. In summary, the presence of CYP2C9(*)3 alleles (either one or two alleles) can alter the degree of drug interaction observed upon coadministration of inhibitors.

Functional evolution of the vitamin D and pregnane X receptors

Background

The vitamin D receptor (VDR) and pregnane X receptor (PXR) are nuclear hormone receptors of the NR1I subfamily that show contrasting patterns of cross-species variation. VDR and PXR are thought to have arisen from duplication of an ancestral gene, evident now as a single gene in the genome of the chordate invertebrate Ciona intestinalis (sea squirt). VDR genes have been detected in a wide range of vertebrates including jawless fish. To date, PXR genes have not been found in cartilaginous fish. In this study, the ligand selectivities of VDRs were compared in detail across a range of vertebrate species and compared with those of the Ciona VDR/PXR. In addition, several assays were used to search for evidence of PXR-mediated hepatic effects in three model non-mammalian species: sea lamprey (Petromyzon marinus), zebrafish (Danio rerio), and African clawed frog (Xenopus laevis).

Results

Human, mouse, frog, zebrafish, and lamprey VDRs were found to have similar ligand selectivities for vitamin D derivatives. In contrast, using cultured primary hepatocytes, only zebrafish showed evidence of PXR-mediated induction of enzyme expression, with increases in testosterone 6β-hydroxylation activity (a measure of cytochrome P450 3A activity in other species) and flurbiprofen 4-hydroxylation activity (measure of cytochrome P450 2C activity) following exposure to known PXR activators. A separate assay in vivo using zebrafish demonstrated increased hepatic transcription of another PXR target, multidrug resistance gene (ABCB5), following injection of the major zebrafish bile salt, 5α-cyprinol 27-sulfate. The PXR target function, testosterone hydroxylation, was detected in frog and sea lamprey primary hepatocytes, but was not inducible in these two species by a wide range of PXR activators in other animals. Analysis of the sea lamprey draft genome also did not show evidence of a PXR gene.

Conclusion

Our results show tight conservation of ligand selectivity of VDRs across vertebrate species from Agnatha to mammals. Using a functional approach, we demonstrate classic PXR-mediated effects in zebrafish, but not in sea lamprey or African clawed frog liver cells. Using a genomic approach, we failed to find evidence of a PXR gene in lamprey, suggesting that VDR may be the original NR1I gene.

Predominant contribution of UDP-glucuronosyltransferase 2B7 in the glucuronidation of racemic flurbiprofen in the human liver

Flurbiprofen is a nonsteroidal anti-inflammatory drug used as a racemic mixture. Although glucuronidation is one of its elimination pathways, the role of UDP-glucuronosyltransferase (UGT) in this process remains to be investigated. Thus, the kinetics of the stereoselective glucuronidation of racemic (R,S)-flurbiprofen by recombinant UGT isozymes and human liver microsomes (HLMs) were investigated, and the major human UGT isozymes involved were identified. UGT1A1, 1A3, 1A9, 2B4, and 2B7 showed glucuronidation activity for both (R)- and (S)-glucuronide, with UGT2B7 possessing the highest activity. UGT2B7 formed the (R)-glucuronide at a rate 2.8-fold higher than that for (S)-glucuronide, whereas the other UGTs had similar formation rates. The glucuronidation of racemic flurbiprofen by HLMs also resulted in the formation of (R)-glucuronide as the dominant form, which occurred to a degree similar to that by recombinant UGT2B7 (2.1 versus 2.8). The formation of (R)-glucuronide correlated significantly with morphine 3-OH glucuronidation (r = 0.96, p < 0.0001), morphine 6-OH glucuronidation (r = 0.91, p < 0.0001), and 3'-azido-3'-deoxythymidine glucuronidation (r = 0.85, p < 0.0001), a reaction catalyzed mainly by UGT2B7, in individual HLMs. In addition, the formation of both glucuronides correlated significantly (r = 0.99, p < 0.0001). Mefenamic acid inhibited the formation of both (R)- and (S)-glucuronide in HLMs with similar IC(50) values (2.0 and 1.7 muM, respectively), which are close to those in recombinant UGT2B7. In conclusion, these findings suggest that the formation of (R)- and (S)-glucuronide from racemic flurbiprofen is catalyzed by the same UGT isozyme, namely UGT2B7.

Enzyme source effects on CYP2C9 kinetics and inhibition

When choosing a recombinant cytochrome P450 (P450) enzyme system for in vitro studies, it is critical to understand the strengths, limitations, and applicability of the enzyme system to the study design. Although literature kinetic data may be available to assist in enzyme system selection, comparison of data from separate laboratories is often confounded by differences in experimental conditions and bioanalytical techniques. We measured the Michaelis-Menten kinetic parameters for four CYP2C9 substrates (diclofenac, (S)-warfarin, tolbutamide, and (S)-flurbiprofen) using four recombinant CYP2C9 enzyme systems (Supersomes, Baculosomes, RECO system, and in-house purified, reconstituted enzyme) to determine whether the enzyme systems exhibited kinetic differences in metabolic product formation rates under uniform experimental conditions. The purified, reconstituted enzyme systems exhibited higher K(m) values, reduced substrate affinity, and lower calculated intrinsic clearance values compared with baculovirus microsomal preparations. Six- to 25-fold differences in predicted intrinsic clearance values were calculated for each substrate depending on the enzyme system-substrate combination. Results suggest that P450 reductase interactions with the CYP2C9 protein and varying ratios of CYP2C9/P450 reductase in the enzyme preparations may play a role in these observed differences. In addition, when (S)-flurbiprofen was used as a substrate probe to determine CYP2C9 inhibition with a set of 12 inhibitors, decreased inhibition potency was observed across 11 of those inhibitors in the RECO purified, reconstituted enzyme compared with the Supersomes baculovirus microsomal preparation and pooled human liver microsomes. Considering these differences, consistent use of an enzyme source is an important component in producing comparable and reproducible kinetics and inhibition data with CYP2C9.

Evaluation of flurbiprofen urinary ratios as in vivo indices for CYP2C9 activity

AIMS

We investigated flurbiprofen pharmacokinetics in 12 volunteers to develop a phenotypic trait measure that correlates with the fractional clearance to 4'-hydroxyflurbiprofen. The effect of the CYP2C9 inhibitor fluconazole on flurbiprofen metabolism was also evaluated.

METHODS

Flurbiprofen pharmacokinetics were evaluated before and after the first and seventh doses of fluconazole. The urinary recovery ratio was calculated as FLRR = 4'-OHF/[4'-OHF + F(tot)] and the urinary metabolic ratio was calculated as FLMR = 4'-OHF/F(tot), where 4'-OHF and F(tot) represent total (conjugated and unconjugated) amounts recovered in urine.

RESULTS

There was a statistically significant relationship between the 4'-OHF formation clearance (4OHCLf) and both the 8-h FLRR and the 8-h FLMR with and without administration of fluconazole. The flurbiprofen apparent oral clearance (CL/F) was decreased by 53% [90% confidence interval (CI) -58, -48] and 64% (90% CI -69, -59), respectively, after administration of one and seven doses of fluconazole when compared with administration of flurbiprofen alone; similarly, the 4OHCLf decreased by 69% (90% CI -74, -64) and 78% (90% CI -83, -73), the 8-h FLRR decreased by 35% (90% CI -41, -29) and 40% (90% CI -46, -35) and the 8-h FLMR decreased by 61% (90% CI -65, -58) and 67% (90% CI -70, -63). The magnitude of decrease in CL/F and 4OHCLf was greater after seven doses compared with after one dose of fluconazole (P < 0.005).

CONCLUSIONS

This study provides strong evidence that both the 8-h FLRR and the 8-h FLMR are suitable phenotypic indices for CYP2C9 activity.

Pharmacokinetic Parameters of (R)-(−) and (S)-(+)-Flurbiprofen in Dairy Bovines

The aim of the study was to evaluate the pharmacokinetics of flurbiprofen (FBP) in different age groups and physiological status groups in dairy cattle. Ten Argentine Holstein bovines were divided into three different groups: 3 cows in early lactation, 3 cows in gestation and 4 newborn calves. Based on previous experience, all the animals received racemic FBP (50:50) at a dose of 0.5 mg/kg by intravenous administration. Blood samples were taken at predetermined times after administration of flurbiprofen. Plasma enantiomer concentrations were measured by HPLC. Total body clearance (ClB) of (S)-(+)-FBP was higher in calves than in cows (114.5, 136.4, 121.4, 128.9 microg/ml vs 22.0, 24.2, 46.5 microg/ml and 27.6, 25.3, 34.6 microg/ml). In calves the disposition kinetics showed stereoselective behaviour. Area under the concentration-time curve (AUC) was higher and Cl(B) and steady-state volume of distribution (V(ss)) were lower for (R)-(-)-FBP than for (S)-(+)-FBP. In cows, stereoselectivity was observed in Cl(B) and elimination half-life (t(1)/2) only in the early lactation group. In this study, enantioselective metabolic behaviour of FBP under the physiological situations studied was found. Hence, it is possible that both enantiomers of flurbiprofen may contribute to the drug's therapeutic effects, but further studies with the administration of separate enantiomers will be required to elucidate their metabolism.

Enantiomer/enantiomer interactions between the S- and R- isomers of omeprazole in human cytochrome P450 enzymes: major role of CYP2C19 and CYP3A4

We investigated the enzyme kinetic basis for the stereoselective disposition of R- and S-omeprazole (OME) and racemic OME in human liver microsomes. OME is primarily metabolized by the hepatic cytochrome P450 enzyme system (CYP2C19 and 3A4). The metabolism of each enantiomer and pseudoracemic OME was studied using unlabeled and (13)C(7)-labeled enantiomers. The enantiomers inhibited each other's metabolism competitively in human liver microsomes and in recombinant CYP2C19 and 3A4. The results obtained with the individual enantiomers allowed successful prediction of the enzyme kinetics for the pseudoracemate. The intrinsic clearance of each enantiomer in a pseudoracemic mixture remained the same as those of the individually incubated enantiomers, although K(m) and V(max) decreased. In the pseudoracemate, the relative contribution of CYP2C19 and 3A4 to 5-hydroxylation and 5'-O-demethylation of R-OME was comparable to that obtained for the incubation of R-OME alone. For S-OME, however, the presence of its antipode greatly increased the contribution of CYP3A4, with increasing concentrations, compared with that obtained when incubating S-OME alone. The results of our in vitro study clearly show metabolic interactions between the OME enantiomers, which may also occur in vivo. Because the enantiomers of OME produce similar pharmacological effects, the enantiomer interactions should not significantly affect the pharmacodynamics. On the other hand, the use of the S-enantiomer results in less complex enzyme kinetics than those of the racemate; thus, the outcome of its clinical use is more predictable.

Impact of CYP2C9 genotype on pharmacokinetics: are all cyclooxygenase inhibitors the same?

The market withdrawals of rofecoxib (Vioxx) and valdecoxib (Bextra) have focused considerable attention on the side effect profiles of cyclooxygenase (COX) inhibitors. As a result, attempts will be made to identify risk factors in the hope that physicians might be able to ensure patient safety. At first glance, CYP2C9 genotype might be considered a risk factor because many COX inhibitors are CYP2C9 substrates in vitro. This observation has led some to hypothesize that a reduction in clearance, in subjects expressing variant forms of the enzyme (e.g., CYP2C9*1/*3 or CYP2C9*3/*3 genotype), will lead to increased exposure and a greater risk of cardiovascular or gastrointestinal side effects. For any drug, however, one has to consider all clearance pathways. Therefore, a number of COX inhibitors were surveyed and it was determined that CYP2C9 plays a relatively minor role in the overall clearance ( Collapse

Key Words Collapse

MESH Headings

• Animals
• Aryl Hydrocarbon Hydroxylases/genetics
• Aryl Hydrocarbon Hydroxylases/metabolism
• Cardiovascular System/drug effects
• Cyclooxygenase 2 Inhibitors/pharmacokinetics
• Cyclooxygenase Inhibitors/pharmacokinetics
• Cytochrome P-450 CYP2C8
• Cytochrome P-450 CYP2C9
• Cytochrome P-450 CYP3A
• Cytochrome P-450 Enzyme System/genetics
• Cytochrome P-450 Enzyme System/metabolism
• Gastrointestinal Tract/drug effects
• Genotype
• Humans
• Polymorphism, Genetic
• Risk Factors
• Substrate Specificity

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Affiliation(s)

A David Rodrigues Drug Metabolosm, Bristol-Myers Squibb, Princeton, NJ 08543, USA.

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CYP2C9 genotype-dependent effects on in vitro drug-drug interactions: switching of benzbromarone effect from inhibition to activation in the CYP2C9.3 variant

The CYP2C9.3 variant exhibits marked decreases in substrate turnover compared with the wild-type enzyme, but little is known regarding the effect this variant form may have on the occurrence of drug-drug interactions. To examine this possibility, the effect of the potent CYP2C9 inhibitor, benzbromarone, was studied with regard to CYP2C9.1- and CYP2C9.3-mediated flurbiprofen metabolism to evaluate whether the variant enzyme exhibits differential inhibition kinetics. Although benzbromarone inhibited CYP2C9.1 activity as expected, CYP2C9.3-mediated flurbiprofen 4'-hydroxylation was activated in the presence of benzbromarone. T1 relaxation studies revealed little change in distances of flurbiprofen protons from the heme iron of either CYP2C9.1 or CYP2C9.3 in the presence of benzbromarone compared with flurbiprofen alone. Spectral binding studies were also performed to investigate whether benzbromarone affected substrate binding, with the addition of benzbromarone having little effect on flurbiprofen-binding affinity in both CYP2C9.1 and CYP2C9.3. Docking studies with the 2C9.1 structure crystallized with a closed active site identified multiple but overlapping subsites with sufficient space for benzbromarone binding in the enzyme when flurbiprofen was positioned closest to the heme. If the closed conformation of 2C9.3 is structurally similar to 2C9.1, as expected for the conservative I359L mutation, then the dynamics of benzbromarone binding may account for the switching of drug interaction effects. In conclusion, the I359L amino acid substitution found in CYP2C9.3 not only reduces metabolism compared with CYP2C9.1 but can also dramatically alter inhibitor effects, suggesting that differential degrees of drug inhibition interactions may occur in individuals with this variant form of CYP2C9.

Active-site characteristics of CYP2C19 and CYP2C9 probed with hydantoin and barbiturate inhibitors

Three series of N-3 alkyl substituted phenytoin, nirvanol, and barbiturate derivatives were synthesized and their inhibitor potencies were tested against recombinant CYP2C19 and CYP2C9 to probe the interaction of these ligands with the active sites of these enzymes. All compounds were found to be competitive inhibitors of both enzymes, although the degree of inhibitory potency was generally much greater towards CYP2C19. Inhibitor stereochemistry did not markedly influence K(i) towards CYP2C9, and log P adequately predicted inhibitor potency for this enzyme. In contrast, stereochemistry was an important factor in determining inhibitor potency towards CYP2C19. (S)-(+)-N-3-Benzylnirvanol and (R)-(-)-N-3-benzylphenobarbital emerged as the most potent and selective CYP2C19 inhibitors, with K(i) values of < 250nM--at least two orders of magnitude greater inhibitor potency than towards CYP2C9. Both inhibitors were metabolized preferentially at their C-5 phenyl substituents, indicating that CYP2C19 prefers to orient the N-3 substituents away from the active oxygen species. These features were incorporated into expanded CoMFA models for CYP2C9, and a new, validated CoMFA model for CYP2C19.

The structure of human cytochrome P450 2C9 complexed with flurbiprofen at 2.0-A resolution

The structure of human P450 2C9 complexed with flurbiprofen was determined to 2.0 A by x-ray crystallography. In contrast to other structurally characterized P450 2C enzymes, 2C5, 2C8, and a 2C9 chimera, the native catalytic domain of P450 2C9 differs significantly in the conformation of the helix F to helix G region and exhibits an extra turn at the N terminus of helix A. In addition, a distinct conformation of the helix B to helix C region allows Arg-108 to hydrogen bond with Asp-293 and Asn-289 on helix I and to interact directly with the carboxylate of flurbiprofen. These interactions position the substrate for regioselective oxidation in a relatively large active site cavity and are likely to account for the high catalytic efficiency exhibited by P450 2C9 for the regioselective oxidation of several anionic non-steroidal anti-inflammatory drugs. The structure provides a basis for interpretation of a number of observations regarding the substrate selectivity of P450 2C9 and the observed effects of mutations on catalysis.

Induction and inhibition of cytochromes P450 by the St. John's wort constituent hyperforin in human hepatocyte cultures

St. John's wort extract (SJW) (Hypericum perforatum L.) is among the most commonly used herbal medications in the United States. The predominance of clinical reports indicates that SJW increases the activity of cytochrome P450 3A4 (CYP3A4) enzyme and reduces plasma concentrations of certain drugs. Although the inductive effect of SJW on CYP3A4 is clear, other reports indicate that SJW constituents may have, to a small degree, some enzyme inhibitory effects. Therefore, we sought to study the induction and inhibition effects of the constituents of SJW on CYP3A4 in the human hepatocyte model. Moreover, most research has focused on the induction of CYP3A4 by SJW with little attention paid to other prominent drug-metabolizing enzymes such as CYP1A2, CYP2C9, and CYP2D6. To examine the effects of SJW on CYP1A2, CYP2C9, CYP2D6, as well as CYP3A4, hepatocytes were exposed to hyperforin and hypericin, the primary constituents of SJW extract. Hepatocytes treated with hypericin or hyperforin were exposed to probe substrates to determine enzyme activity and protein and RNA harvested. Hyperforin treatment resulted in significant increases in mRNA, protein, and activity of CYP3A4 and CYP2C9, but had no effect on CYP1A2 or CYP2D6. Acute administration of hyperforin at 5 and 10 microM 1 h before and along with probe substrate inhibited CYP3A4 activity. Hypericin had no effect on any of the enzymes tested. These results demonstrate that with chronic exposure, the inductive effect of SJW on drug-metabolizing enzymes predominates, and human hepatocyte cultures are a versatile in vitro tool for screening the effect of herbal products on cytochrome P450 enzymes.