Discovery of novel functional variants and extensive evaluation of CYP2D6 genetic polymorphisms in Koreans

Relation between hydroxychloroquine dose and continuation rate in patients with systemic lupus erythematosus

OBJECTIVES

Hydroxychloroquine (HCQ) is recommended at a target dose of 5 mg/kg per actual body weight to reduce the risk of retinopathy in systemic lupus erythematosus (SLE). However, the efficacy of HCQ has been established at doses of 6.5 mg/kg per ideal body weight. This study aimed to clarify the effects of the HCQ dose on the continuation rate in Japanese patients, who generally have a lower body mass index than Western patients.

METHODS

This retrospective single-centre observational study enrolled patients with SLE on HCQ therapy. Patients were divided into two groups with a dose per actual body weight [the low-dose (<5 mg/kg) group and the high-dose (≥5 mg/kg) group], and continuation rates were compared. The efficacy of 1-year HCQ therapy was assessed in patients without additional immunosuppressive agents and biologics.

RESULTS

Of the 231 patients enrolled, 48 (20.8%) discontinued HCQ. The HCQ dose per actual body weight was identified as an independent risk factor for discontinuation. The low-dose group showed a significantly higher 1-year HCQ continuation rate than the high-dose group (83.2% vs. 72.8%, respectively). Both groups showed reductions in glucocorticoid requirement and serological activity after 1-year HCQ therapy.

CONCLUSIONS

HCQ <5 mg/kg per actual body weight may facilitate greater continuation.

Epigenetic Mechanisms Contribute to Intraindividual Variations of Drug Metabolism Mediated by Cytochrome P450 Enzymes

Significant interindividual and intraindividual variations on cytochrome P450 (CYP)-mediated drug metabolism exist in the general population globally. Genetic polymorphisms are one of the major contribution factors for interindividual variations, but epigenetic mechanisms mainly contribute to intraindividual variations, including DNA methylation, histone modifications, microRNAs, and long non-coding RNAs. The current review provides analysis of advanced knowledge in the last decade on contributions of epigenetic mechanisms to intraindividual variations on CYP-mediated drug metabolism in several situations, including (1) ontogeny, the developmental changes of CYP expression in individuals from neonates to adults; (2) increased activities of CYP enzymes induced by drug treatment; (3) increased activities of CYP enzymes in adult ages induced by drug treatment at neonate ages; and (4) decreased activities of CYP enzymes in individuals with drug-induced liver injury (DILI). Furthermore, current challenges, knowledge gaps, and future perspective of the epigenetic mechanisms in development of CYP pharmacoepigenetics are discussed. In conclusion, epigenetic mechanisms have been proven to contribute to intraindividual variations of drug metabolism mediated by CYP enzymes in age development, drug induction, and DILI conditions. The knowledge has helped understanding how intraindividual variation are generated. Future studies are needed to develop CYP-based pharmacoepigenetics to guide clinical applications for precision medicine with improved therapeutic efficacy and reduced risk of adverse drug reactions and toxicity. SIGNIFICANCE STATEMENT: Understanding epigenetic mechanisms in contribution to intraindividual variations of CYP-mediated drug metabolism may help to develop CYP-based pharmacoepigenetics for precision medicine to improve therapeutic efficacy and reduce adverse drug reactions and toxicity for drugs metabolized by CYP enzymes.

Association between CYP2D6 phenotype and recurrence of Plasmodium vivax infection in south Korean patients

Background

Primaquine is activated by CYP2D6 in the hepatocytes. In Korea, primaquine is the only hypnozoitocidal agent used for patients with vivax malaria. Thus, patients with poor CYP2D6 activity could have an increased risk of primaquine failure and subsequent relapse. The study sought to identify the association between CYP2D6 phenotype and recurrence of malaria in Korean patients.

Methods

A total of 102 patients with vivax malaria were prospectively enrolled from eight institutions in Korea. An additional 38 blood samples from patients with recurred vivax malaria were provided by the Korea Disease Control and Prevention Agency. Malaria recurrence was defined as more than one episode of vivax malaria in the same or consecutive years. CYP2D6 star alleles, phenotypes, and activity scores were examined.

Results

Genotyping for CYP2D6 was successful in 101 of the prospectively enrolled patients and 38 samples from the Korea Disease Control and Prevention Agency, of which 91 were included in the no-recurrence group and 48 were included in the recurrence group. Reduced CYP2D6 activity (intermediate metabolizer) phenotype was more common in the recurrence group than in the no-recurrence group (OR, 2.33 (95% CI, 1.14–4.77); p = 0.02). Patients with lower CYP2D6 activity had a higher probability of recurrence (p = 0.029).

Conclusion

This study suggests that CYP2D6 polymorphism may affect primaquine efficacy and thus Plasmodium vivax recurrence in Korea.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12936-022-04311-6.

Flares after hydroxychloroquine reduction or discontinuation: results from the Systemic Lupus International Collaborating Clinics (SLICC) inception cohort

OBJECTIVES

To evaluate systemic lupus erythematosus (SLE) flares following hydroxychloroquine (HCQ) reduction or discontinuation versus HCQ maintenance.

METHODS

We analysed prospective data from the Systemic Lupus International Collaborating Clinics (SLICC) cohort, enrolled from 33 sites within 15 months of SLE diagnosis and followed annually (1999-2019). We evaluated person-time contributed while on the initial HCQ dose ('maintenance'), comparing this with person-time contributed after a first dose reduction, and after a first HCQ discontinuation. We estimated time to first flare, defined as either subsequent need for therapy augmentation, increase of ≥4 points in the SLE Disease Activity Index-2000, or hospitalisation for SLE. We estimated adjusted HRs (aHRs) with 95% CIs associated with reducing/discontinuing HCQ (vs maintenance). We also conducted separate multivariable hazard regressions in each HCQ subcohort to identify factors associated with flare.

RESULTS

We studied 1460 (90% female) patients initiating HCQ. aHRs for first SLE flare were 1.20 (95% CI 1.04 to 1.38) and 1.56 (95% CI 1.31 to 1.86) for the HCQ reduction and discontinuation groups, respectively, versus HCQ maintenance. Patients with low educational level were at particular risk of flaring after HCQ discontinuation (aHR 1.43, 95% CI 1.09 to 1.87). Prednisone use at time-zero was associated with over 1.5-fold increase in flare risk in all HCQ subcohorts.

CONCLUSIONS

SLE flare risk was higher after HCQ taper/discontinuation versus HCQ maintenance. Decisions to maintain, reduce or stop HCQ may affect specific subgroups differently, including those on prednisone and/or with low education. Further study of special groups (eg, seniors) may be helpful.

Comprehensive analysis of important pharmacogenes in Koreans using the DMET™ platform

Genetic polymorphisms of enzymes and transporters associated with the absorption, distribution, metabolism, and elimination (ADME) of drugs are one of the major factors that contribute to interindividual variations in drug response. In the present study, we aimed to elucidate the pharmacogenetic profiles of the Korean population using the Affymetrix Drug Metabolizing Enzyme and Transporters (DMET™) platform. A total of 1,012 whole blood samples collected from Korean subjects were genotyped using the DMET™ plus microarray. In total, 1,785 single nucleotide polymorphism (SNP) markers for 231 ADME genes were identified. The genotype and phenotype of 13 clinically important ADME genes implemented in the Clinical Pharmacogenetics Implementation Consortium guidelines were compared among different ethnic groups. Overall, the genotype frequencies of the Korean population were similar to those of the East Asian population. Several genes, notably CYP2C19 and VKORC1, showed marked differences in Koreans compared to Europeans (EURs) or Africans (AFRs). The percentage of CYP2C19 poor metabolizers was 15% in Koreans and less than 3% in EURs or AFRs. The frequencies of causative SNPs of the VKORC1 gene for the low warfarin dose phenotype were 90%, 60%, and 10% in Koreans, EURs and AFRs, respectively. Our findings can be utilized for optimal pharmacotherapy in Korean patients.

Pharmacogenetics of tamoxifen therapy in Asian populations: from genetic polymorphism to clinical outcomes

BACKGROUND

Compared with western countries, Asian breast cancer patients have unique pathological and biological characteristics. Most of them are premenopausal women with HR positive. Tamoxifen as the first-line drug for premenopausal women with HR+ is involved in multiple enzymes and transporters during metabolizing and transporting process. Variants that cause decreased or inactive gene products leading to abnormal responses in tamoxifen therapy have well been studied in western countries, whereas such information is much less reported in Asian populations.

OBJECTIVE

In order to elucidate the relationship between genetic variants and tamoxifen-induced individual drug reactions in different Asian populations and further identify genotypes/phenotypes with potential therapeutic significance.

METHODS

We reviewed the frequencies of genetic variants in major enzymes and transporter genes involved in the metabolism and transport of tamoxifen across Asian populations as well as significant correlations between genotypes/metabolic phenotypes and metabolites concentrations or BC clinical outcomes.

RESULTS

Significant inter-ethnic differences in allele frequencies was found among Asian populations, such as CYP2D6*4, *10, *41, CYP2C9*2, ABCB1 C3435T and SLCO1B1*5, and CYP2D6*10/*10 is the most common genotype correlated with adverse clinical outcomes. Moreover, we summarized the barriers and controversies of implementing pharmacogenetics in tamoxifen therapy and concluded that more population-specific pharmacogenetic studies are needed in the future.

CONCLUSION

This review revealed more systematic pharmacogenomics of genes involved in the metabolism and transport besides CYP2D6, are required to optimize the genotyping strategies and guide the personalized tamoxifen therapy in Asian populations.

CYP2D6 Genotype-Based Dose Recommendations for Risperidone in Asian People

The aim of this study was to provide dose recommendations for risperidone in Asian people based on cytochrome P450 enzyme CYP2D6 genotype. First, we investigated the influence of CYP2D6 polymorphism on the pharmacokinetics of risperidone in Chinese patients with schizophrenia. Then, we performed a search for studies covering the relationship between pharmacokinetic parameters of risperidone and CYP2D6 genotype. Pooled pharmacokinetic parameters were meta-analyzed using a random-effects model. Lastly, we calculated the dose adjustment for risperidone based on CYP2D6 genotype for white and Asian people. Significant differences between the extensive metabolizer and intermediate metabolizer groups were observed for dose-adjusted risperidone level, 9-hydroxyrisperidone level, and risperidone/9-hydroxyrisperidone ratio, but not for the total active moiety. Meta-analysis showed that significant differences were observed among the four phenotype groups, including steady state concentration, peak risperidone concentration, and the area under the curve, using the Kruskal-Wallis test. No differences were found in oral clearance. For risperidone, dose recommendations for poor and ultrarapid metabolizers of CYP2D6 for Asians were different compared to that for white people for poor metabolizers (dose adjustment around 45% for white people, while for Asians the risperidone dose should be reduced by 26%). For ultrarapid metabolizers, risperidone dose should be increased by about 33% for white people and 30% for Asians. This was a first attempt to apply pharmacogenetics to suggest dose-regimens for Asian people; further research to replicate and extend these findings is recommended.

Identification and functional characterization of CYP4V2 genetic variants exhibiting decreased activity of lauric acid metabolism

The objectives of the present study were to identify CYP4V2 genetic variants and characterize their functional consequences. A total of 26CYP4V2 genetic variants were identified, including seven novel variants in 60 randomly selected healthy subjects. Six protein-coding variants were studied, including three novel variants (L22V, R287T, and G410C) and three previously reported variants (R36S, Q259K, and H331P). The cDNA sequences encoding each amino acid variant and the wild-type CYP4V2 protein were cloned into the pcDNA/PDEST40 expression vector and transfected into eukaryotic 293T cells for overexpression of the CYP4V2 coding variants. CYP4V2 H331P and CYP4V2 G410C exhibited significant decreases in activity for lauric acid oxidation (20-30% of wild-type activity), when compared to the wildtype, which was correlated with low expression of CYP4V2 H331P and G410C substituted proteins. The other four CYP4V2 amino variants were comparable to wild-type CYP4V2 for lauric acid metabolism. The CYP4V2 H331P and G410C substitutions were predicted to cause a structural change through in silico analysis. In conclusion, the present study provides functional information about CYP4V2 genetic variants. These findings will be valuable for interpreting individual variations in phenotypes associated with CYP4V2 function in the clinical setting.

Inhibition of 20-hydroxyeicosatetraenoic acid (20-HETE) glucuronidation by non-steroidal anti-inflammatory drugs in human liver microsomes and recombinant UDP-glucuronosyltransferase enzymes

20-hydroxyeicosatetraenoic acid (20-HETE) is an arachidonic acid metabolite which is known to increase platelet aggregation and cardiovascular risk. In this study, nine non-steroidal anti-inflammatory drugs (NSAIDs) selected by chemical structures were screened to determine their effects on the glucuronidation of 20-HETE using human liver microsomes (HLMs). Then, the combined effects of the selected NSAID and genetic polymorphisms in UDP-glucuronosyltransferase (UGT) were investigated. Among the tested NSAIDs, diclofenac was the strongest inhibitor of 20-HETE glucuronidation with an IC₅₀ value of 3.5 μM. Celecoxib, naproxen, mefenamic acid, ibuprofen, and indomethacin showed modest inhibition with IC₅₀ values of 77, 91, 190, 208, and 220 μM, respectively, while acetylsalicylic acid, rofecoxib, and meloxicam did not inhibit 20-HETE glucuronidation. Glucuronidation of 20-HETE by UGT2B7 and UGT1A9 recombinant enzymes was significantly inhibited by indomethacin, mefanemic acid, diclofenac, ibuprofen, naproxen, and celecoxib (P < 0.001). In addition, diclofenac exhibited a competitive inhibition mechanism with the K_m value of 20-HETE glucuronidation increasing from 23.5 μM to 62 μM in the presence of 3.5 μM diclofenac. Diclofenac further decreased 20-HETE glucuronidation in HLMs carrying UGT2B7*2 alleles compared with the wild-type HLMs. The results from this study would be useful in understanding the alteration of 20-HETE levels in relation to NSAID and UGT genetic polymorphisms.

Genetic and epigenetic polymorphisms of eNOS and CYP2D6 in mainland Chinese Tibetan, Mongolian, Uygur, and Han populations

Cytochrome P450 2D6 (CYP2D6) and endothelial nitric oxide synthase (eNOS) are important in the cardiovascular disease susceptibility and drug response. The methylation of cytosine in the promoter of genes could influence expression. The interindividual differences of drug response could only be explained partly by the polymorphisms of drug disposition genes. This study was aimed to investigate the genetic and epigenetic polymorphisms of CYP2D6 and eNOS in Chinese Tibetan, Mongolian, Uygur, and Han volunteers. Blood samples were collected from 842 unrelated Chinese healthy subjects (323 Tibetan, 134 Mongolian, 162 Uygur, and 223 Han) for genotyping and part of the DNA samples were subjected to cytosine methylation analysis. Significant genetic and epigenetic interethnic polymorphisms of eNOS and CYP2D6 were found in the four Chinese groups. The 4b-G-T wild-type haplotype of eNOS was the most common in Chinese Tibetan (87.1%), Mongolian (79.9%), Uygur (76.4%), and Han (79.5%), respectively. eNOS T-786C creates a new CpG site and a significantly higher methylation level was found in -786CC than that in -786TC in Chinese Han (P = 0.0485) and total Chinese (P = 0.0104). CYP2D6 A-678 (rs28633410) makes the symmetrical CpG site changed to be an asymmetrical CpA site, and the methylation level of cytosine in the CpG context of G-678A was significantly higher than that in the CpA. The results of the present study could be useful for understanding the interindividual disparity of cardiovascular disease susceptibility and drug response among different Chinese ethnic groups and would provide more evidences for the precise medication of Chinese.

Characterization of Reference Materials for Genetic Testing of CYP2D6 Alleles: A GeT-RM Collaborative Project

Pharmacogenetic testing increasingly is available from clinical and research laboratories. However, only a limited number of quality control and other reference materials currently are available for the complex rearrangements and rare variants that occur in the CYP2D6 gene. To address this need, the Division of Laboratory Systems, CDC-based Genetic Testing Reference Material Coordination Program, in collaboration with members of the pharmacogenetic testing and research communities and the Coriell Cell Repositories (Camden, NJ), has characterized 179 DNA samples derived from Coriell cell lines. Testing included the recharacterization of 137 genomic DNAs that were genotyped in previous Genetic Testing Reference Material Coordination Program studies and 42 additional samples that had not been characterized previously. DNA samples were distributed to volunteer testing laboratories for genotyping using a variety of commercially available and laboratory-developed tests. These publicly available samples will support the quality-assurance and quality-control programs of clinical laboratories performing CYP2D6 testing.

Influences of cytochrome b5 expression and its genetic variant on the activity of CYP2C9, CYP2C19 and CYP3A4

The objective of the present study was to investigate the effects of cytochrome b5 (cytb5) on the drug metabolism catalyzed by CYP2C9, CYP2C19 and CYP3A4. Activities of CYP2C9, CYP2C19, and CYP3A4 were determined by using the prototypical substrates tolbutamide, omeprazole and midazolam, respectively. Cytb5 protein and mRNA contents showed large inter-individual variations with 11- and 6-fold range, respectively. All of three P450s showed an increased activity in proportion to the amount of cytb5 expression. Particularly, CYP3A4 showed the strongest correlation between cytb5 protein amount and the activity, followed by CYP2C9 and CYP2C19. The putative splicing variant, c.288G>A (rs7238987) was identified and was screened in 36 liver tissues by direct DNA sequencing. Liver tissues having a splicing variant exhibited unexpected sizes of cytb5 mRNA and a decreased expression tendency of cytb5 protein compared to the wild-type. A decreased activity in the metabolism of the CYP2C19 substrate omeprazole was observed in liver tissues carrying the splicing variant when compared to the wild-type Cytb5 (P < 0.05). The present results propose that different expression of cytb5 can cause variations in CYP mediated drug metabolism, which may explain, at least in part, the inter-individual difference in drug responses in addition to the CYP genetic polymorphisms.

Real time PCR detection of common CYP2D6 genetic variants and its application in a Karen population study

BACKGROUND

Plasmodium vivax malaria is characterized by relapses arising from the hypnozoite stages in the liver. The only currently registered drug for radical treatment to prevent relapse is primaquine. Primaquine, a prodrug, requires metabolism through the liver cytochrome CYP2D6 isoenzyme to its active metabolite. Mutations in the CYP2D6 gene may thus affect primaquine efficacy. A SNPs genotyping technique was developed to characterize the CYP2D6 genetic variants and tested this in the patients with Plasmodium vivax infection collected in a Karen population on the Thailand-Myanmar border, where P. vivax malaria is endemic.

METHODS

Direct sequencing of PCR-reamplified products (DSP) was used to uncover exonic CYP2D6 sequence variations. Subsequently, an allele-specific oligonucleotide probe real-time SNPs genotyping (ASO) assay was developed for rapid detection of the four clinically relevant CYP2D6 variants occurring in this population. These two in-house developed assays were used to genotype CYP2D6 mutations in blood samples obtained from 70 Karen adults.

RESULTS

Results showed a high degree of concordance between the DSP and ASO methods. Six CYP2D6 point mutations were identified within the Karen population: C100T, C1039T, G1661C, G1846A, C2850T and G4180C, at frequencies of 0.43, 0.43, 0.76, 0.02, 0.32 and 0.76, respectively. The CYP2D6*2, *4, *5, *10 and *36 allelic frequencies were 0.33, 0.02, 0.03, 0.40 and 0.01, respectively. Alleles conferring an intermediate CYP2D6 metabolizer phenotype comprised 46% of the total number of alleles.

CONCLUSION

The newly developed ASO assay is a reliable and rapid tool for large-scale CYP2D6 genotyping. The high frequency of the CYP2D6*10 allele in the Karen population warrants further assessment of its association with the radical curative efficacy of primaquine.

CYP2D6 Protein Level Is the Major Contributor to Interindividual Variability in CYP2D6-Mediated Drug Metabolism in Healthy Human Liver Tissue

CYP2D6 genetic polymorphisms are considered a major contributor to the large interindividual variability in CYP2D6-mediated drug metabolism, but fail to explain a significant portion of the variability. The aim of this study was to assess the ability of the CYP2D6 activity score (AS) estimated from CYP2D6 genotype to predict CYP2D6 expression and enzyme activity. The CYP2D6 gene region was sequenced in 115 healthy human liver tissue samples to determine their CYP2D6 AS. Additionally, CYP2D6 enzyme activity, protein, and mRNA levels were estimated. CYP2D6 AS explained 23% of the interindividual variability in CYP2D6 activity, but only 7.5% in tissues assigned AS 1-2. The CYP2D6 protein level was found to be the major determinant of CYP2D6 activity, explaining 59% of variability. These findings suggest that while CYP2D6 AS is a good predictor of poor metabolizer phenotype, additional nongenetic factors may govern the rate of CYP2D6-mediated metabolism in those without the poor metabolizer phenotype.

CYP2D6 allele frequencies in Korean population, comparison with East Asian, Caucasian and African populations, and the comparison of metabolic activity of CYP2D6 genotypes

Cytochrome P450 (CYP) 2D6 is present in less than about 2% of all CYP enzymes in the liver, but it is involved in the metabolism of about 25% of currently used drugs. CYP2D6 is the most polymorphic among the CYP enzymes. We determined alleles and genotypes of CYP2D6 in 3417 Koreans, compared the frequencies of CYP2D6 alleles with other populations, and observed the differences in pharmacokinetics of metoprolol, a prototype CYP2D6 substrate, depending on CYP2D6 genotype. A total of 3417 unrelated healthy subjects were recruited for the genotyping of CYP2D6 gene. Among them, 42 subjects with different CYP2D6 genotypes were enrolled in the pharmacokinetic study of metoprolol. The functional allele *1 and *2 were present in frequencies of 34.6 and 11.8%, respectively. In decreased functional alleles, *10 was the most frequent with 46.2% and *41 allele was present in 1.4%. The nonfunctional alleles *5 and *14 were present at 4.5 and 0.5% frequency, respectively. The *X × N allele was present at a frequency of 1.0%. CYP2D6*1/*1, *1/*2 and *2/*2 genotypes with normal enzyme activity were present in 12.1%, 8.6% and 1.4% of the subjects, respectively. CYP2D6*5/*5, *5/*14, and *14/*14 genotypes classified as poor metabolizer were only present in 4, 2, and 1 subjects, respectively. Mutant genotypes with frequencies of more than 1% were CYP2D6*1/*10 (32.0%), *10/*10 (22.3%), *2/*10 (11.7%), *5/*10 (3.7%), *1/*5 (2.5%), and *10/*41 (1.2%). The relative clearance of metoprolol in CYP2D6*1/*10, *1/*5, *10/*10, *5/*10, and *5/*5 genotypes were 69%, 57%, 24%, 14% and 9% of CYP2D6*wt/*wt genotype, respectively. These results will be very useful in establishing a strategy for precision medicine related to the genetic polymorphism of CYP2D6.

Effects of CYP2D6 and CYP3A5 genetic polymorphisms on steady-state pharmacokinetics and hemodynamic effects of tamsulosin in humans

PURPOSE

Tamsulosin is one of the most potent drugs currently available to treat benign prostatic hyperplasia. Cytochrome P450 (CYP) 2D6 and CYP3A are the two major enzymes responsible for tamsulosin metabolism. The purpose of this study was to evaluate the effects of CYP2D6 and CYP3A5 genetic polymorphisms on the pharmacokinetics and hemodynamic effects of tamsulosin in humans.

METHODS

Twenty-nine male subjects were enrolled and their CYP2D6 (*2,*4,*5,*10,*14,*21,*41, and *xN) and CYP3A5 (*5) genotypes were screened. Tamsulosin was administered daily for 6 days to assess its steady-state pharmacokinetics and hemodynamic effects according to CYP2D6 and CYP3A5 genotypes.

RESULTS

CYP2D6 group 3 (with genotype *10/*10 or *5/*10) exhibited higher plasma levels than CYP2D6 group 1 (with genotype *1/*1,*1/*2,*1/*2xN, or *2/*10xN) or CYP2D6 group 2 (with genotype *1/*10,*1/*41, or *2/*5) (trough concentrations for groups 1, 2, and 3: 1.3, 1.8, and 3.8 ng/mL, respectively [P < 0.001]; peak concentrations for groups 1, 2, 3: 8.3, 10.0, and 13.8 ng/mL, respectively [P < 0.005]). Similarly, CYP2D6 genotypes influenced the hemodynamic effects of tamsulosin based on systolic and diastolic blood pressures. However, the CYP3A5*3 polymorphism did not affect tamsulosin plasma levels and its hemodynamic effects.

CONCLUSION

The CYP2D6 but not the CYP3A5 genetic polymorphisms affected the pharmacokinetics and the hemodynamic effects of tamsulosin.

Prediction and visualization of CYP2D6 genotype-based phenotype using clustering algorithms

This study focused on the role of cytochrome P450 2D6 (CYP2D6) genotypes to predict phenotypes in the metabolism of dextromethorphan. CYP2D6 genotypes and metabolic ratios (MRs) of dextromethorphan were determined in 201 Koreans. Unsupervised clustering algorithms, hierarchical and k-means clustering analysis, and color visualizations of CYP2D6 activity were performed on a subset of 130 subjects. A total of 23 different genotypes were identified, five of which were observed in one subject. Phenotype classifications were based on the means, medians, and standard deviations of the log MR values for each genotype. Color visualization was used to display the mean and median of each genotype as different color intensities. Cutoff values were determined using receiver operating characteristic curves from the k-means analysis, and the data were validated in the remaining subset of 71 subjects. Using the two highest silhouette values, the selected numbers of clusters were three (the best) and four. The findings from the two clustering algorithms were similar to those of other studies, classifying *5/*5 as a lowest activity group and genotypes containing duplicated alleles (i.e., CYP2D6*1/*2N) as a highest activity group. The validation of the k-means clustering results with data from the 71 subjects revealed relatively high concordance rates: 92.8% and 73.9% in three and four clusters, respectively. Additionally, color visualization allowed for rapid interpretation of results. Although the clustering approach to predict CYP2D6 phenotype from CYP2D6 genotype is not fully complete, it provides general information about the genotype to phenotype relationship, including rare genotypes with only one subject.

Genetic Variants of Cytochrome CYP2D6 in Two Mixed Chilean Populations

OBJECTIVES

It is known that the interindividual and interethnic variability of the genetic polymorphisms of CYP2D6 plays an important role in the presentation of adverse drug reactions and concerning lack of therapeutic effects in humans. However, there are few data available from mixed populations of Latin America, including the Chilean. The aim of this study was therefore to estimate the frequencies of CYP2D6 variants in two samples of hospitals from the northern (Hospital San José, HSJ) and eastern (Clínica Las Condes, CLC) parts of Santiago, Chile, with different degrees of Amerindian admixture (HSJ: 34.5%; CLC: 15.9%).

METHODS

We used polymerase chain reaction followed by restriction endonuclease digestion (PCR-RFLP) to genotype 7 CYP2D6 alleles in 250 healthy unrelated individuals of Chilean Mestizo background. The detection of allele CYP2D6*5 and the duplication of this gene was performed by long-PCR.

RESULTS

The degrees of Amerindian admixture are reflected in the observed frequencies of the CYP2D6*1 (HSJ: 58.26%; CLC: 41.06%), CYP2D6*2 (HSJ: 28.10%; CLC: 40.65%), and CYP2D6*4 (HSJ: 8.26%; CLC: 12.60%) alleles; the frequencies of CYP2D6*1 (p = 0.0002) and CYP2D6*2 (p = 0.0036) are significantly different between the samples. Four individuals (CLC: 0.41%; HSJ: 1.24%) could not be assigned to a genotype. We identified 3.25% of the genotypes which predict a poor metabolizer phenotype in CLC and 1.65% in HSJ.

CONCLUSION

Our data indicate ethnic group-dependent genetic differences in the vulnerability to treatment with the large variety of drugs metabolized by the enzyme CYP2D6.

A Study on CYP2C19 and CYP2D6 Polymorphic Effects on Pharmacokinetics and Pharmacodynamics of Amitriptyline in Healthy Koreans

We performed a double-blinded, genotype-based stratification study to explore the pharmacokinetics and pharmacodynamics of amitriptyline according to CYP2C19 and CYP2D6 genotype in Korean subjects. Twenty-four healthy adults were grouped by genotype of CYP2C19 and CYP2D6. After a single dose of 25 mg of amitriptyline, blood samples were collected and anticholinergic effects were measured. The extent of N-demethylation of amitriptyline significantly decreased in subjects carrying two nonfunctional alleles of CYP2C19. The extent of hydroxylation of amitriptyline or nortriptyline was significantly reduced in subjects carrying two CYP2D6 decreased functional alleles compared with those with no or one decreased functional allele. The overall metabolic pathway of amitriptyline was more likely to be dominated by CYP2C19 than CYP2D6. The gene variations of CYP2C19 and CYP2D6 did not change the pharmacodynamic effect. The findings of this study will provide useful information on individualized drug treatment with amitriptyline considering both CYP2D6 and CYP2C19 gene variations.

Effect of 22 CYP2D6 variants found in the Chinese population on tolterodine metabolism in vitro

Cytochrome P450 2D6 (CYP2D6) is an important member of the cytochrome P450 enzyme superfamily. We recently identified 22 novel variants in the Chinese population using PCR and bidirectional sequencing methods. The aim of this study is to characterize the enzymatic activity of these variants and their effects on the metabolism of the antimuscarinic drug tolterodine in vitro. A baculovirus-mediated expression system was used to express wild-type CYP2D6 and 24 variants (CYP2D6*2, CYP2D6*10, and 22 novel CYP2D6 variants) at high levels. The insect microsomes expressing CYP2D6 proteins were incubated with 0.1-50 μM tolterodine at 37 °C for 30 min and the metabolites were analyzed by high-performance liquid chromatography-tandem mass spectrometry system. Of the 24 CYP2D6 variants tested, 2 variants (CYP2D6*92 and CYP2D6*96) were found to be catalytically inactive, 4 variants (CYP2D6*94, F164L, F219S and D336N) exhibited markedly increased intrinsic clearance values (V_max/K_m) compared with the wild-type (from 66.34 to 99.79%), whereas 4 variants (CYP2D6*10, *93, *95 and E215K) exhibited significantly decreased values (from 49.02 to 98.50%). This is the first report of all these rare alleles for tolterodine metabolism and these findings suggest that more attention should be paid to subjects carrying these infrequent CYP2D6 alleles when administering tolterodine in the clinic.

Gene polymorphisms and contents of cytochrome P450s have only limited effects on metabolic activities in human liver microsomes

Extensive inter-individual variations in pharmacokinetics are considered as a major reason for unpredictable drug responses. As the most important drug metabolic enzymes, inter-individual variations of cytochrome P450 (CYP) activities are not clear in human liver. In this paper, metabolic activities, gene polymorphisms and protein contents of 10 CYPs were determined in 105 human normal liver microsomes. The results indicated substantial inter-individual variations in CYP activities, with the greatest being CYP2C19 activity (>600-fold). Only half of 10 CYP isoforms and 26 gene polymorphism sites had limited effects on metabolic activities, such as CYP2A6, CYP2B6, CYP2C9, CYP2D6 and CYP3A4/5, others had almost no effects. Compared with their respective wild type, Km, Vmax, and CLint decreased by 51.6%, 88.7% and 70.7% in CYP2A6*1/*4 genotype, Vmax and CLint decreased by 32.8% and 60.2% in CYP2C9*1/*3 genotype, Km increased by 118.4% and CLint decreased by 65.2% in CYP2D6 100TT genotype, respectively. Moreover, there were only 4 CYP isoforms, CYP1A2, CYP2A6, CYP2E1 and CYP3A5, which had moderate or weak correlations between Vmax values and corresponding contents. In conclusions, the genotypes and contents of some CYPs have only limited effects on metabolic activities, which imply that there are other more important factors to influence inter-individual variations.

High frequency of CYP2D6 ultrarapid metabolizers in Spain: controversy about their misclassification in worldwide population studies

A high frequency (7-10%) of CYP2D6 ultrarapid metabolizers estimated from the genotype (gUMs) has been claimed to exist among Spaniards and Southern Europeans. However, methodological aspects such as the inclusion of individuals carrying non-active multiplied alleles as gUMs may have led to an overestimation. Thus, this study aimed to analyze the gUM frequency (considering only those carrying more than two active genes) in 805 Spanish healthy volunteers studied for CYP2D6*2, *3, *4, *5, *6, *10, *17, *35, *41, and multiplications. Second, all worldwide studies reporting gUM frequencies were reviewed in order to evaluate potential misclassifications. The gUM frequency in this Spanish population was 5.34%, but increased to 8.3% if all individuals with CYP2D6 multiplications were classified as gUMs without considering the activity of the multiplied alleles. Moreover, among all reviewed worldwide studies only 55.6% precisely determined whether the multiplied alleles were active. Present results suggest that the evaluation of CYP2D6 ultrarapid metabolism should be standarized, and that the frequency of gUMs should be reconsidered in Spaniards and globally.

A new therapeutic option for postoperative pain management with oxycodone HCI injection

Fentanyl is the most commonly used opioid analgesic in intravenous patient-controlled analgesia (IV PCA) in Korea. IV oxycodone was approved for postoperative IV PCA by the Ministry of Food and Drug Safety of Korea in 2013. The approved dosage regimen for postoperative pain relief with IV oxycodone is IV bolus loading of 2 mg followed by PCA composed of demand boluses of 1 mg and no background infusion with an oxycodone concentration of 1 mg/ml. However, a simulation study indicated that the minimum effective analgesic concentration (MEAC, as indicated by relief of pain by administering rescue analgesics) of oxycodone was reached most quickly with a higher loading dose of 0.1 mg/kg and IV PCA with background infusion. Oxycodone is a therapeutic option as an analgesic for postoperative pain management. It is necessary to reduce the analgesic dose of oxycodone in elderly patients because metabolic clearance decreases with age.

Genetic polymorphisms analysis of CYP2D6 in the Uygur population

BACKGROUND

This study aimed to investigate genetic polymorphisms of CYP2D6 among healthy Uygur individuals. Genetic polymorphisms of CYP2D6 could greatly affect CYP2D6 activity and lead to differences among individuals in drug efficacy or side effects. To investigate genetic polymorphisms of CYP2D6 in the Uygur population, we directly sequenced the whole gene in 96 unrelated, healthy Uygur volunteers from the Xinjiang Uygur Autonomous Region and screened for genetic variants in the promoter, intron, exons, and 3'UTR.

RESULTS

We detected 62 genetic polymorphisms of CYP2D6, 16 of which were novel SNP with three novel non-synonymous mutations detected for the first time. The allelic frequencies of CYP2D6*1, *10, *39, and *48 were 0.542, 0.156, 0.068, 0.229, and 0.073, respectively. The frequency of CYP2D6*1/*10 which decreased CYP2D6 enzyme activity was 31.3 %.

CONCLUSIONS

Our results provided basic information about CYP2D6 polymorphisms, suggested that the enzymatic activities of CYP2D6 might be different within the Uygur ethnic group, and provide a basis for safer drug administration and better therapeutic treatment of Uygur individuals.

Assessment of 25 CYP2D6 alleles found in the Chinese population on propafenone metabolism in vitro

Cytochrome P450 enzyme 2D6 (CYP2D6) is an important member of the cytochrome P450 enzyme superfamily, with more than 100 CYP2D6 allelic variants being previously reported. The aim of this study was to assess the catalytic characteristics of 25 alleles (CYP2D6.1 and 24 CYP2D6 variants) and their effects on the metabolism of propafenone in vitro. Twenty-five CYP2D6 alleles were expressing in 21 Spodoptera frugiperda (Sf) insect cells, and each variant was evaluated using propafenone as the substrate. Reactions were performed at 37 °C with 1-100 μmol/L propafenone for 30 min. After termination, the product 5-OH-propafenone was extracted and used for signal collection by ultra-performance liquid chromatography (UPLC). Compared with wild type CYP2D6.1, the intrinsic clearance (Vmax and Km) values of all variants were significantly altered. Three variants (CYP2D6.87, CYP2D6.90, CYP2D6.F219S) exhibited markedly increased intrinsic clearance values (129% to 165%), whereas 21 variants exhibited significantly decreased values (16% to 85%) due to increased Km and (or) decreased Vmax values. These results indicated that the majority of tested alleles had significantly altered catalytic activity towards propafenone hydroxylation in this expression system. Attention should be paid to subjects carrying these rare alleles when treated with propafenone.

Physiological Content and Intrinsic Activities of 10 Cytochrome P450 Isoforms in Human Normal Liver Microsomes

Due to a lack of physiologic cytochrome P450 (P450) isoform content, P450 activity is typically only determined at the microsomal level (per milligram of microsomal protein) and not at the isoform level (per picomole of P450 isoform), which could result in the misunderstanding of variations in P450 activity between individuals and further hinder development of personalized medicine. We found that there were large variations in protein content, mRNA levels, and intrinsic activities of the 10 P450s in 100 human liver samples, in which CYP2E1 and CYP2C9 showed the highest expression levels. P450 gene polymorphisms had different effects on activity at two levels: CYP3A5*3 and CYP2A6*9 alleles conferred increased activity at the isoform level but decreased activity at the microsomal level; CYP2C9*3 had no effect at the isoform level but decreased activity at the microsomal level. The different effects at each level stem from the different effects of each polymorphism on the resulting P450 protein. Individuals with CYP2A6*1/*4, CYP2A6*1/*9, CYP2C9*1/*3, CYP2D6 100C>T TT, CYP2E1 7632T>A AA, CYP3A5*1*3, and CYP3A5*3*3 genotypes had significantly lower protein content, whereas CYP2D6 1661G>C mutants had a higher protein content. In conclusion, we first offered the physiologic data of 10 P450 isoform contents and found that some single nucleotide polymorphisms had obvious effects on P450 expression in human normal livers. The effects of gene polymorphisms on intrinsic P450 activity at the isoform level were quite different from those at the microsomal level, which might be due to changes in P450 protein content.

Effects of polymorphisms in CYP2D6 and ABC transporters and side effects induced by gefitinib on the pharmacokinetics of the gefitinib metabolite, O-desmethyl gefitinib

We investigated the effects of polymorphisms in CYP2D6, ABCB1, and ABCG2 and the side effects induced by gefitinib on the pharmacokinetics of O-desmethyl gefitinib, the active metabolite of gefitinib. On day 14 after beginning therapy with gefitinib, plasma concentrations of gefitinib and O-desmethyl gefitinib were measured. Patients were grouped into three groups according to their combination of CYP2D6 alleles: homozygous extensive metabolisers (EMs; *1/*1, *1/*2, and *2/*2; n = 13), heterozygous EMs (*1/*5, *2/*5, *1/*10, and *2/*10; n = 18), and intermediate metabolisers (IMs; *5/*10 and *10/*10; n = 5). The median AUC0-24 of O-desmethyl gefitinib in CYP2D6 IMs was 1460 ng h/mL, whereas that in homozygous EMs was 12,523 ng h/mL (P = 0.021 in univariate analysis). The median AUC ratio of O-desmethyl gefitinib to gefitinib differed among homozygous EMs, heterozygous EMs, and IMs at a ratio of 1.41:0.86:0.24 (P = 0.030). On the other hand, there were no significant differences in the AUC0-24 of O-desmethyl gefitinib between ABCB1 and ABCG2 genotypes. In a multivariate analysis, CYP2D6 homozygous EMs (P = 0.012) were predictive for a higher AUC0-24 of O-desmethyl gefitinib. The side effects of diarrhoea, skin rash, and hepatotoxicity induced by gefitinib were unrelated to the AUC0-24 of O-desmethyl gefitinib. CYP2D6 polymorphisms were associated with the formation of O-desmethyl gefitinib from gefitinib. In CYP2D6 homozygous EMs, the plasma concentrations of O-desmethyl gefitinib were higher over 24 h after taking gefitinib than those of the parent compound; however, side effects induced by gefitinib were unrelated to O-desmethyl gefitinib exposure.

Association of Polymorphisms of Cytochrome P450 2D6 With Blood Hydroxychloroquine Levels in Patients With Systemic Lupus Erythematosus

OBJECTIVE

To evaluate associations of genetic polymorphisms in cytochrome P450 (CYP) isoforms 2D6, 3A5, and 3A4 with blood concentrations of hydroxychloroquine (HCQ) and its metabolite, N-desethyl HCQ (DHCQ), in patients with systemic lupus erythematosus (SLE).

METHODS

SLE patients taking HCQ for >3 months were recruited and were genotyped for 4 single-nucleotide polymorphisms in CYP2D6*10, CYP3A5*3, and CYP3A4*18B. Blood HCQ and DHCQ concentrations ([HCQ] and [DHCQ]) were measured and their association with corresponding genotypes was investigated.

RESULTS

A total of 194 patients were included in the analysis. CYP2D6*10 polymorphisms (rs1065852 and rs1135840) were significantly associated with the [DHCQ]:[HCQ] ratio after adjustment for age, sex, dose per weight per day, and SLE Disease Activity Index score (P = 0.03 and P < 0.01, respectively). In adjusted models, the [DHCQ]:[HCQ] ratio was highest in patients with the G/G genotype of the CYP2D6*10 (rs1065852) polymorphism and lowest in those with the A/A genotype (P = 0.03). Similarly, the [DHCQ]:[HCQ] ratio was highest in patients with the C/C genotype of the CYP2D6*10 (rs1135840) polymorphism and lowest in those with the G/G genotype (P < 0.01). The CYP2D6*10 (rs1065852) polymorphism was significantly related to the [DHCQ] (P = 0.01). However, the polymorphisms of CYP3A5*3 and CYP3A4*18B did not show any significant association with the [HCQ], [DHCQ], or [DHCQ]:[HCQ] ratio.

CONCLUSION

Our study showed that the [DHCQ]:[HCQ] ratio was related to CYP2D6 polymorphisms in Korean lupus patients taking oral HCQ. CYP polymorphisms may explain why there is wide variation in blood HCQ concentrations. The role of an individual's CYP polymorphisms should be considered when prescribing oral HCQ.

Genetic Polymorphism of CYP2D6 and Clomiphene Concentrations in Infertile Patients with Ovulatory Dysfunction Treated with Clomiphene Citrate

CYP2D6 is primarily responsible for the metabolism of clomiphene citrate (CC). The purpose of the present study was to investigate the relationship between CYP2D6 genotypes, concentrations of CC and its major metabolites and drug response in infertility patients. We studied 42 patients with ovulatory dysfunction treated with only CC. Patients received a dose of 100 mg/day CC on days 3-7 of the menstrual cycle. CYP2D6 genotyping and measurement of CC and the major metabolite concentrations were performed. Patients were categorized into CC responders or non-responders according to one cycle response for the ovulation. Thirty-two patients were CC responders and 10 patients were non-responders with 1 cycle treatment. The CC concentrations were highly variable within the same group, but non-responders revealed significantly lower (E)-clomiphene concentration and a trend of decreased concentrations of active metabolites compared to the responders. Nine patients with intermediate metabolizer phenotype were all responders. We confirmed that the CC and the metabolite concentrations were different according to the ovulation status. However, our results do not provide evidence for the contribution of CYP2D6 polymorphism to either drug response or CC concentrations.

Resolution of a clinical AmpliChip CYP450 Test™ no call: discovery and characterization of novel CYP2D6*1 haplotypes

A Han Chinese patient failed CYP2D6 genotype analysis with the AmpliChip CYP450 Test™. The CYP2D6 gene locus of the patient and her son were extensively genotyped including copy number variation and gene resequencing. Two SNPs were discovered on the patient's CYP2D6*1 allele, -498C>A and 1661G>C, while the son's CYP2D6*1 allele had -498C>A only. AmpliChip failure was attributed to the presence of a CYP2D6*1 allele carrying the 1661G>C SNP. Functional analyses of -498C>A did not reveal altered activity in vitro or in vivo suggesting that both novel CYP2D6*1 subvariants are functional. The implementation of pharmacogenetics-guided drug therapy relies on accurate clinical-grade genotype analysis. Although the AmpliChip is a reliable platform, numerous allelic (sub)variants and gene arrangements are not detected or may trigger no calls. While such cases may be rare, the clinical/genetic testing community must be aware of the challenges of CYP2D6 testing on the AmpliChip platform and implications regarding accuracy of test results.

Effect of the potent CYP2D6 inhibitor sarpogrelate on the pharmacokinetics and pharmacodynamics of metoprolol in healthy male Korean volunteers

1. Recently, we demonstrated that sarpogrelate is a potent and selective CYP2D6 inhibitor in vitro. Here, we evaluated the effect of sarpogrelate on the pharmacokinetics and pharmacodynamics of metoprolol in healthy subjects. 2. Nine healthy male subjects genotyped for CYP2D6*1/*1 or *1/*2 were included in an open-label, randomized, three treatment-period and crossover study. A single oral dose of metoprolol (100 mg) was administered with water (treatment A) and sarpogrelate (100 mg bid.; a total dose of 200 mg and treatment B), or after pretreatment of sarpogrelate for three days (100 mg tid.; treatment C). Plasma levels of metoprolol and α-hydroxymetoprolol were determined using a validated LC-MS/MS method. Changes in heart rate and blood pressure were monitored as pharmacodynamic responses to metoprolol. 3. Metoprolol was well tolerated in the three treatment groups. In treatment B and C groups, the AUCt of metoprolol increased by 53% (GMR, 1.53; 90% CI, 1.17-2.31) and by 51% (1.51; 1.17-2.31), respectively. Similar patterns were observed for the increase in Cmax of metoprolol by sarpogrelate. However, the pharmacodynamics of metoprolol did not differ significantly among the three treatment groups. 4. Greater systemic exposure to metoprolol after co-administration or pretreatment with sarpogrelate did not result in clinically relevant effects. Co-administration of both agents is well tolerated and can be employed without the need for dose adjustments.

Determination of major UDP-glucuronosyltransferase enzymes and their genotypes responsible for 20-HETE glucuronidation

The compound 20-HETE is involved in numerous physiological functions, including blood pressure and platelet aggregation. Glucuronidation of 20-HETE by UDP-glucuronosyltransferases (UGTs) is thought to be a primary pathway of 20-HETE elimination in humans. The present study identified major UGT enzymes responsible for 20-HETE glucuronidation and investigated their genetic influence on the glucuronidation reaction using human livers (n = 44). Twelve recombinant UGTs were screened to identify major contributors to 20-HETE glucuronidation. Based on these results, UGT2B7, UGT1A9, and UGT1A3 exhibited as major contributors to 20-HETE glucuronidation. The Km values of 20-HETE glucuronidation by UGT1A3, UGT1A9, and UGT2B7 were 78.4, 22.2, and 14.8 μM, respectively, while Vmax values were 1.33, 1.78, and 1.62 nmol/min/mg protein, respectively. Protein expression levels and genetic variants of UGT1A3, UGT1A9, and UGT2B7 were analyzed in human livers using Western blotting and genotyping, respectively. Glucuronidation of 20-HETE was significantly correlated with the protein levels of UGT2B7 (r(2) = 0.33, P < 0.001) and UGT1A9 (r(2) = 0.31, P < 0.001), but not UGT1A3 (r(2) = 0.02, P > 0.05). A correlation between genotype and 20-HETE glucuronidation revealed that UGT2B7 802C>T, UGT1A9 -118T9>T10, and UGT1A9 1399T>C significantly altered 20-HETE glucuronide formation (P < 0.05-0.001). Increased levels of 20-HETE comprise a risk factor for cardiovascular diseases, and the present data may increase our understanding of 20-HETE metabolism and cardiovascular complications.

Cytochrome P450 pharmacogenetics in African populations: implications for public health

INTRODUCTION

Africa harbors a disproportionate burden of disease when taking into account the triple challenge caused by HIV/AIDS, tuberculosis (TB) and malaria, against a backdrop of an increasing burden of noncommunicable diseases. More than 80% of therapeutic drugs used in the management of these diseases/conditions are metabolized by CYP enzymes that exhibit genetic polymorphisms.

AREAS COVERED

There is variability in the expression and activities of CYPs resulting in interindividual differences in the response to standard doses of therapeutic drugs, due to genetic polymorphisms, which exhibit both quantitative and qualitative differences between racial and between ethnic groups. The review aims to evaluate the implications of the genetic variation in CYPs on the public health of Africans. The CYPs reviewed here metabolize most of the commonly used therapeutic drugs and include CYP1A2, 2A6, 2B6, 2C8, 2C9, 2C19, 2D6, 3A4 and 3A5. Allele frequencies are compared between African ethnic groups and among populations of African, Asian and European origin. Data are obtained from our own studies and literature.

EXPERT OPINION

The variability in the pattern of genetic variation between populations translates into differences in drug response. Understanding CYP variability improves rational drug use and has public health significance.

Genetic variations of human CYP2D6 in the Chinese Han population

Aim: The purpose of this study was to determine the genetic polymorphisms of the CYP2D6 gene and to elucidate the allele distribution pattern in the Chinese Han population. Materials & methods: We used PCR and bidirectional sequencing methods to analyze all nine exons of the CYP2D6 gene in 2129 unrelated, healthy Chinese Han subjects from two geographical locations in China: the northern and southern regions. Results: In total, 165 mutated sites were detected in 2129 participants, of which 67 sites were reported for the first time. Among these novel mutation sites, 22 were nonsynonymous and 12 were named as novel alleles (*87–*93, *94A, *94B and *95–*98) by the Human CYP Allele Nomenclature Committee. In addition, 29 previously reported alleles and 84 genotypes were also detected in 1954 volunteers. Functional prediction of novel variants revealed that eight variants might have a deleterious effect on CYP2D6. Linkage disequilibrium analysis and tagSNP selection were performed separately. By using these methods, distinct differences were found between the two regions. Conclusion: This study provides the most comprehensive data concerning CYP2D6 polymorphisms in the Chinese Han population to date and increases the number of known alleles; these findings may greatly contribute to the development of personalized medicine for the Chinese Han population.

Original submitted 1 January 2013; Revision submitted 14 August 2013

Complexities of CYP2D6 gene analysis and interpretation

Cytochrome P450 2D6 (CYP2D6) plays an important role in the metabolism and bioactivation of about 25% of clinically used drugs including many antidepressants, antipsychotics and opioids. CYP2D6 activity is highly variably ranging from no activity in so-called poor metabolizers to ultrarapid metabolism at the other end of the extreme of the activity distribution. A large portion of this variability can be explained by the highly polymorphic nature of the CYP2D6 gene locus for which > 100 variants and subvariants identified to date. Allele frequencies vary markedly between ethnic groups; some have exclusively or predominantly only been observed in certain populations. Pharmacogenetic testing holds the promise of individualizing drug therapy by identifying patients with CYP2D6 diplotypes that puts them at an increased risk of experiencing dose-related adverse events or therapeutic failure. Inferring a patient's CYP2D6 metabolic capacity, or phenotype, however, is a challenging task due to the complexity of the CYP2D6 gene locus. Allelic variation includes SNPs, small insertions and deletions, gene copy number variation and rearrangements with CYP2D7, a highly related non-functional gene. This review provides a summary of the intricacies of CYP2D6 variation and genotype analysis, knowledge that is invaluable for the translation of genotype into clinically useful information.

Identification of cytochrome P450s involved in the metabolism of arachidonic acid in human platelets

Although cytochrome P450s (CYPs) have been identified in most human cells, identification of CYPs in human platelets remains poorly explored. CYP expressions in human platelets were screened by using reverse transcriptase-polymerase chain reaction and western blot analysis followed by functional assays using arachidonic acid (ARA). CYP1A1, 2U1, 2J2, 4A11, 4F2, and 5A1 were expressed as both proteins and mRNAs in platelets. Ethoxyresorufin-O-deethylase activity was observed in platelets and this activity was significantly decreased after treatment with the general P450 inhibitor SKF-525A and the CYP1A inhibitor, α-naphthoflavone (40-45%, P<0.001). Seventeen ARA metabolites were detected in ARA-treated platelets. Among these, the levels of 20-hydroxyeicosatetraenoic acid and epoxyeicosatrienoic acids were significantly decreased with the treatment of the P450 ω-hydroxylase inhibitor 17-octadecynoic acid (P<0.05-0.001). In summary, multiple ARA-metabolizing P450s were identified in human platelets. These findings may provide an important resource for understanding physiological function of platelet.

Functional influence of human CYP2D6 allelic variations: P34S, E418K, S486T, and R296C

CYP2D6 is responsible for the oxidative metabolism of 20-25 % of clinical drugs and its genetic polymorphisms can significantly influence the drug metabolism. In this study, we analyzed the functional activities of four nonsynonymous single nucleotide polymorphisms from CYP2D6*52 allele, which were recently found, and one found frequently in CYP2D6 alleles. Recombinant variant enzymes of E418K, S486T, and R296C were successfully expressed in Escherichia coli and purified. However, a CYP holoenzyme spectrum of P34S variant was not detected in E. coli whole cell level. Structural analysis indicated that P34S mutation seemed to perturb a highly conserved proline-rich N-terminus of CYP2D6. Steady state kinetic analyses showed the significant reductions of enzymatic activities in E418K and R296C variants. In the case of bufuralol 1'-hydroxylation, a novel mutant, E418K, showed 32 % decrease in catalytic efficiency (k cat/K m) mainly due to the decrease of k cat value. R296C showed much greater reduction in the catalytic efficiency (9 % of wild-type) due to both of a decrease of k cat value and an increase of K m value. In the case of dextromethorphan O-demethylation, E418K showed both of a decrease of k cat value and an increase K m value to result in ~43 % reduction of catalytic efficiency. A highly decreased catalytic efficiency (~6 % of wild-type) in the mutant of R296C also was observed mainly due to the dramatic change of k cat value of dextromethorphan O-demethylation. These results suggested that individuals carrying these allelic variants are likely to have the altered metabolic abilities of many clinical drugs therefore, these polymorphisms of CYP2D6 should be much concerned for reliable drug treatment.

Influence of ABCB1 genetic polymorphisms on the pharmacokinetics of risperidone in healthy subjects with CYP2D6*10/*10

BACKGROUND AND PURPOSE

The objective of this study was to investigate the combined influence of genetic polymorphisms in ABCB1 and CYP2D6 genes on risperidone pharmacokinetics.

EXPERIMENTAL APPROACH

Seventy-two healthy Korean volunteers receiving a single oral dose of 2 mg risperidone were included in this study.

KEY RESULTS

Significant differences were observed between the ABCB1 3435C>T genotypes for the pharmacokinetic parameters (peak serum concentration) of risperidone and the active moiety (risperidone and its main metabolite, 9-hydroxyrisperidone). There were no significant differences in the area under the serum concentration-time curves of risperidone and the active moiety among the ABCB1 2677G>T/A and 3435C>T genotypes. However, the peak serum concentration and area under the serum concentration-time curves were significantly different among the ABCB1 3435C>T genotypes in CYP2D6*10/*10.

CONCLUSIONS AND IMPLICATIONS

These findings indicate that polymorphisms of ABCB1 3435C>T in individuals with CYP2D6*10/*10, which has low metabolic activity, could play an important role in the potential adverse effects or toxicity of risperidone.

Pharmacogenomics of CYP2D6: molecular genetics, interethnic differences and clinical importance

CYP2D6 has received intense attention since the beginning of the pharmacogenetic era in the 1970s. This is because of its involvement in the metabolism of more than 25% of the marketed drugs, the large geographical and inter-ethnic differences in the genetic polymorphism and possible drug-induced toxicity. Many interesting reviews have been published on CYP2D6 and this review aims to reinstate the importance of the genetic polymorphism of CYP2D6 in different populations as well as some clinical implications and important drug interactions.

Polymorphic metabolism by functional alterations of human cytochrome P450 enzymes

The study of cytochrome P450 pharmacogenomics is of particular interest because of its promise in the development of rational means to optimize drug therapy with respect to patient's genotype to ensure maximum efficacy with minimal adverse effects. Drug metabolizing P450 enzymes are polymorphic and are the main phase I enzymes responsible for the metabolism of clinical drugs. Therefore, polymorphisms in the P450s have the most impact on the fate of clinical drugs in phase I metabolism since almost 80% of drugs in use today are metabolized by these enzymes. Predictive genotyping for P450 enzymes for a more effective therapy will be routine for specific drugs in the future. In this review, we discuss the current knowledge of polymorphic metabolism by functional alterations in nonsynonymous SNPs of P450 1A2, 2A6, 2C8, 2C9, 2C19, 2D6, and 3A4 enzymes.