CATALYTIC ROLES OF CYP2C9 AND ITS VARIANTS (CYP2C9*2 AND CYP2C9*3) IN LORNOXICAM 5′-HYDROXYLATION

Functional Measurement of CYP2C9 and CYP3A4 Allelic Polymorphism on Sildenafil Metabolism

Aim

We aimed to systematically examine the effects of enzymatic activity of 38 human CYP2C9 alleles and 21 human CYP3A4 alleles, including wild-type CYP2C9.1 and CYP3A4.1, which contain the 24 CYP2C9 novel alleles (*36–*60) and 6 CYP3A4 novel alleles (*28–*34) newly found in the Chinese population, on sildenafil metabolism through in vitro experiment.

Methods

The recombinant cytochrome P450 alleles protein of CYP2C9 and CYP3A4 expressed in insect baculovirus expression system were reacted with 10–500 µM sildenafil for 30 minutes at 37°C, and the reaction was terminated by cooling to −80°C immediately. Next, we used ultra-performance liquid chromatography tandem mass spectrometry (UHPLC-MS/MS) detection system to detect sildenafil and its active metabolite N-desmethyl sildenafil.

Results

The intrinsic clearance (Vmax/Km) values of most CYP2C9 variants were significantly altered when compared with the wild-type CYP2C9*1, with most of these variants exhibiting either reduced Vmax and/or increased Km values. Four alleles (CYP2C9*11, *14, *31, *49) exhibited no markedly decreased relative clearance (1-fold). The relative clearance of the remaining thirty-three variants exhibited decrease in different levels, ranging from 1.81% to 88.42%. For the CYP3A4 metabolic pathway, when compared with the wild-type CYP3A4*1, the relative clearance values of four variants (CYP3A4*3, *10, *14 and *I335T) showed significantly higher relative clearance (130.7–134.9%), while five variants (CYP3A4*2, *5, *24, *L22V and *F113I) exhibited sharply reduced relative clearance values (1.80–74.25%), and the remaining nine allelic variants showed no statistical difference. In addition, the kinetic parameters of two CYP3A4 variants (CYP3A4*17 and CYP3A4*30) could not be detected, due to the defect of the CYP3A4 gene.

Conclusion

These findings were the first evaluation of all these infrequent CYP2C9 and CYP3A4 alleles for sildenafil metabolism; when treating people who carry these CYP2C9 and CYP3A4 variants, there should be more focus on the relation of dose intensity, side effects and therapeutic efficacy when administering sildenafil. The study will provide fundamental data on effect of CYP2C9 and CYP3A4 allelic variation on sildenafil metabolism for further clinical research.

Polymorphism of human cytochrome P450 enzymes and its clinical impact

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

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.

Pharmacogenetics of analgesics: toward the individualization of prescription

The use of analgesics is based on the empiric administration of a given drug with clinical monitoring for efficacy and toxicity. However, individual responses to drugs are influenced by a combination of pharmacokinetic and pharmacodynamic factors that can sometimes be regulated by genetic factors. Whereas polymorphic drug-metabolizing enzymes and drug transporters may affect the pharmacokinetics of drugs, polymorphic drug targets and disease-related pathways may influence the pharmacodynamic action of drugs. After a usual dose, variations in drug toxicity and inefficacy can be observed depending on the polymorphism, the analgesic considered and the presence or absence of active metabolites. For opioids, the most studied being morphine, mutations in the ABCB1 gene, coding for P-glycoprotein (P-gp), and in the µ-opioid receptor reduce morphine potency. Cytochrome P450 (CYP) 2D6 mutations influence the analgesic effect of codeine and tramadol, and polymorphism of CYP2C9 is potentially linked to an increase in nonsteroidal anti-inflammatory drug-induced adverse events. Furthermore, drug interactions can mimic genetic deficiency and contribute to the variability in response to analgesics. This review summarizes the available data on the pharmacokinetic and pharmacodynamic consequences of known polymorphisms of drug-metabolizing enzymes, drug transporters, drug targets and other nonopioid biological systems on central and peripheral analgesics.

Polymorphic variants of CYP2C9: mechanisms involved in reduced catalytic activity

CYP2C9 catalyzes the demethylation of the biphasic kinetics substrate (S)-naproxen, and the CYP2C9*2 (R144C) and CYP2C9*3 (I359L) variants are associated with lower rates of (S)-naproxen demethylation. To assess the reasons for these reductions in catalytic activity of the two variants and potential substrate concentration-dependent differences in a biphasic kinetics substrate, cytochrome P450 (P450) cycle coupling and uncoupling were monitored during coincubation of (S)-naproxen and CYP2C9 over a range of P450 reductase concentrations. Coupling was greatest in the CYP2C9.1 enzyme, followed by CYP2C9.2, and then CYP2C9.3. Uncoupling in CYP2C9.1 and CYP2C9.3 was primarily to H(2)O(2). In contrast, CYP2C9.2 uncoupled to excess water preferentially. The conversion of enzyme to the high spin state was similar in CYP2C9.1 and CYP2C9.2, but lower in CYP2C9.3. It is noteworthy that neither altered substrate binding nor altered interaction with reductase seemed to be involved in reduced catalysis. These results suggest that in addition to coupling differences, differential uncoupling to shunt products and differences in spin state help explain the reduced catalytic activity in these enzymes.

Effect of the CYP2C9*3 allele on lornoxicam metabolism

BACKGROUND

We investigated whether the CYP2C9 genotypes would affect lornoxicam metabolism in healthy volunteers.

METHODS

Twelve healthy volunteers who had been genotyped for CYP2C9 gene were selected to participate in our study. After 8 mg lornoxicam was taken, blood samples were drawn from 0 to 36 h. The plasma concentrations of lornoxicam and 5'-hydroxylornoxicam were determined by HPLC method. 5'-hydroxylornoxicam was purified from rabbits'urine by semi-preparative HPLC.

RESULTS

Lornoxicam and 5'-hydroxylornoxicam both exhibit CYP2C9 genotype-dependent pharmacokinetic profiles. The area under the plasma concentration-time curve (AUC) of lornoxicam increased by 60 +/- 9.78% (P <0.05) and the AUC of 5'-hydroxylornoxicam decreased by 65 +/- 11.75% (p <0.001) in heterozygous CYP2C9*1/*3 subjects (n=6) compared with CYP2C9*1/*1 group (n=6). t1/2 value of lornoxicam and 5'-hydroxylornoxicam prolonged by 39 +/- 8.35% and curtailed by 59 +/- 6.83% respectively in CYP2C9*1/*3 subjects. But no significant differences in Tmax of lornoxicam and 5'-hydroxylornoxicam were observed between these 2 genotypes. In addition, for the first time we exploit the purification method for 5'-hydroxylornoxicam from rabbits' urine.

CONCLUSION

The CYP2C9*3 allele significantly affected the metabolism of lornoxicam. The pharmacokinetic parameters of both lornoxicam and 5'-hydroxylornoxicam were significantly different between these 2 genotypes.

IMPACT OF CYP2C9 GENOTYPE ON PHARMACOKINETICS: ARE ALL CYCLOOXYGENASE INHIBITORS THE SAME?: TABLE 1

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 (<or=20% of the dose) of sulindac, naproxen, ketoprofen, diclofenac, rofecoxib, and etoricoxib. CYP2C9 genotype would have no clinically meaningful impact on the pharmacokinetics of these drugs. In contrast, CYP2C9 genotype is expected to impact the clearance of ibuprofen, indomethacin, flurbiprofen, celecoxib, valdecoxib, lornoxicam, tenoxicam, meloxicam, and piroxicam. However, even when CYP2C9 is a major determinant of clearance, it is necessary to consider CYP2C8 genotype (e.g., ibuprofen) and, possibly, CYP3A4 activity (e.g., celecoxib, valdecoxib, and meloxicam) also.

Lornoxicam pharmacokinetics in relation to cytochrome P450 2C9 genotype

AIMS

To investigate the pharmacokinetics of lornoxicam and the relationship with CYP2C9 polymorphism in healthy Chinese subjects.

METHODS

A single oral dose of 8 mg lornoxicam was administered to 18 healthy Chinese male subjects. Plasma was sampled for 24 h post dose, and plasma concentrations of lornoxicam were measured using a validated LC/MS/MS method. CYP2C9 genotype was determined by polymerase chain reaction-based restriction fragment length polymorphism or by direct sequencing of the coding region of the CYP2C9 gene.

RESULTS

Of the 18 subjects, one subject was found to be a very poor metabolizer of lornoxicam with a long t(1/2) of 106 h, a low CL/F of 0.71 ml min(-1), and a high AUC(0-infinity) of 187.6 microg ml(-1) h. Genotyping studies revealed that this subject was heterozygous for CYP2C9*3 and a new variant CYP2C9 allele. Of the other 17 subjects, 13 were *1/*1 carriers, three were *1/*3 carriers, and one was a *1/*2 carrier. Mean AUC(0-infinity) values (95% confidence intervals) of lornoxicam were 9.25 (6.55, 11.95) vs. 4.75 (3.55, 5.95) microg ml(-1) h in *1 heterozygotes vs.*1 homozygotes, and mean CL/F values were 14.8 (10.2, 19.4) vs. 32.9 (24.5, 41.3) ml min(-1), respectively (P < 0.05 for both AUC and CL/F).

CONCLUSIONS

The results show that the pharmacokinetics of lornoxicam are dependent on CYP2C9 polymorphism. In particular, the presence of the CYP2C9*3 allele impairs the oral clearance of lornoxicam.

Upstream and coding region CYP2C9 polymorphisms: correlation with warfarin dose and metabolism

OBJECTIVES

To assess whether CYP2C9 alleles other than CYP2C9*2 and *3 are associated with a low-warfarin dose requirement and the relevance of upstream CYP2C9 polymorphisms to dose requirement and metabolism.

METHODS

CYP2C9 exons, intron-exon boundaries and 3 kb of upstream sequence in 20 patients requiring <or= 1.5 mg warfarin per day and with apparently homozygous wild-type or heterozygous CYP2C9*2 genotypes were screened for novel polymorphisms by single-strand conformational polymorphism analysis. PCR-based genotyping assays for novel upstream and other known polymorphisms were used to screen a larger patient population of known CYP2C9*2 and *3 genotype requiring a range of warfarin doses.

RESULTS

Polymorphisms at eight different upstream sites were found, five of which were already described. We found that the majority of the upstream polymorphisms were in complete linkage disequilibrium with previously described coding region polymorphisms. However, two polymorphisms, T-1188C and the novel DeltaG-2664DeltaT-2665, occurred both in individuals who were otherwise wild-type and in individuals positive for coding region polymorphisms. Evidence for 11 haplotypes, including 8 with frequencies >or= 0.01, was obtained. In individuals negative for coding region polymorphisms, neither individual genotypes for T-1188C or DeltaG-2664DeltaT-2665 or particular combinations of haplotype pairs were predictive of dose requirement or S-warfarin total clearance, suggesting neither upstream polymorphism was functionally significant. Dose requirements in CYP2C9*11 heterozygotes were not statistically significantly different from homozygous wild-type individuals.

CONCLUSIONS

The coding region non-synonymous polymorphisms associated with the CYP2C9*2 and CYP2C9*3 alleles are the major CYP2C9-related factor affecting warfarin dose in UK Caucasians. Upstream CYP2C9 polymorphisms do not appear to be important independent determinants of dose requirement.

ROLE OF CYP2C9 AND ITS VARIANTS (CYP2C9*3 AND CYP2C9*13) IN THE METABOLISM OF LORNOXICAM IN HUMANS

CYP2C9 is an important member of the cytochrome P450 enzyme superfamily with some 12 CYP2C9 alleles (*1-*12) being previously reported. Recently, we identified a new CYP2C9 allele with a Leu90Pro mutation in a Chinese poor metabolizer of lornoxicam [Si D, Guo Y, Zhang Y, Yang L, Zhou H, and Zhong D (2004) Pharmacogenetics 14:465-469]. The new allele, designated CYP2C9*13, was found to occur in approximately 2% of the Chinese population. To examine enzymatic activity of the CYP2C9*13 allele, kinetic parameters for lornoxicam 5'-hydroxylation were determined in COS-7 cells transiently transfected with pcDNA3.1 plasmids carrying wild-type CYP2C9*1, variant CYP2C9*3, and CYP2C9*13 cDNA. The protein levels of cDNA-expressed CYP2C9*3 and *13 in postmitochondrial supernatant (S9) from transfected cells were lower than those from wild-type CYP2C9*1. Mean values of Km and Vmax for CYP2C9*1, *3, and *13 were 1.24, 1.61, and 2.79 microM and 0.83, 0.28, and 0.22 pmol/min/pmol, respectively. Intrinsic clearance values (Vmax/Km) for variant CYP2C9*3 and CYP2C9*13 on the basis of CYP2C9 protein levels were separately decreased to 28% and 12% compared with wild type. In a subsequent clinical study, the AUC of lornoxicam was increased by 1.9-fold and its oral clearance (CL/F) decreased by 44% in three CYP2C9*1/*13 subjects, compared with CYP2C9*1/*1 individuals. This suggests that the CYP2C9*13 allele is associated with decreased enzymatic activity both in vitro and in vivo.

Role of Human Liver Cytochrome P450 2C9 in the Metabolism of a Novel α4β1/α4β7 Dual Antagonist, TR-14035

The metabolism of a novel dual antagonist for alpha4beta1/alpha4beta7 integrin, TR-14035, and the role of polymorphic enzyme responsible for this metabolism were investigated. Human liver microsomes catalyzed the NADPH-dependent metabolism of TR-14035 to a primary metabolite, O-desmethyl TR-14035. This formation was completely blocked by both sulfaphenazole, a selective CYP2C9 inhibitor, and CYP2C9 antibody, whereas potent inhibitors selective for other CYPs exhibited little effects. Of 12 recombinant CYPs examined, O-desmethyl metabolite was principally formed by CYP2C9. CYP1A1, an extrahepatic enzyme, also had this activity (about one-fourth of CYP2C9). Utilizing recombinant CYP2C9*1, K(m) and V(max)/K(m) values of 23.3 microM and 0.284 microL/min/pmol CYP2C9, respectively, were obtained for the O-desmethyl formation, which were quite similar to those in CYP2C9*2 enzyme. In contrast, V(max)/K(m) value in recombinant CYP2C9*3 was approximately one-sixth of CYP2C9*1 and *2. In agreement, kinetics studies using human liver microsomes with CYP2C9*1/*1, *2/*2 and *3/*3 genotypes revealed that the V(max)/K(m) value in *2/*2 microsomes was comparable to that in wild type microsomes, in contrast, that in *3/*3 microsomes was reduced. These results demonstrate CYP2C9 is a primary enzyme mediating the O-desmethylation of TR-14035 in human liver. In homozygotes of CYP2C9*3, the metabolic clearance of TR-14035 should be decreased compared with homozygotes of CYP2C9*1 or 2.

The CYP2C9 polymorphism: from enzyme kinetics to clinical dose recommendations

CYP2C9 is the major human enzyme of the cytochrome P450 2C subfamily and metabolizes approximately 10% of all therapeutically relevant drugs. Two inherited SNPs termed CYP2C9*2 (Arg144Cys) and *3 (Ile359Leu) are known to affect catalytic function. Numerous rare or functionally silent polymorphisms have been identified. About 35% of the Caucasian population carries at least one *2 or *3 allele. CYP2C9 metabolizes several oral hypoglycemics, oral anticoagulants, non-steroidal anti-inflammatory drugs and other drugs, including phenytoin, losartan, fluvastatin, and torsemide. In vitro studies with several drugs indicate that the Cys144 (.2) and Leu359 (.3) variants confer only about 70 and 10% of the intrinsic clearance of the wild-type protein (.1), respectively. The clinical pharmacokinetic implications of these polymorphisms vary depending on the enzymes contribution to total oral clearance. Several studies demonstrated that the CYP2C9 polymorphisms are medically important for non-steroidal anti-inflammatory drugs, for oral hypoglycemics, vitamin K antagonistic oral anticoagulants, and phenytoin. In particular, CYP2C9 polymorphisms should be routinely considered in therapy with oral anticoagulants where severe adverse events at initiation of therapy might be reduced by genotyping. CYP2C9 polymorphisms were also clinically associated with side effects of phenytoin, with gastric bleeding during therapy with non-steroidals and with hypoglycemia under oral hypoglycemic drugs. Data appear mature enough for the routine consideration of CYP2C9 genotypes in therapy with acenocoumarol, phenytoin, warfarin, and some other drugs. Nevertheless, it is advisable before the routine clinical use of these genotype data to rigorously test the benefits of genotype-based therapeutic recommendations by randomized controlled clinical trials.