Catalytic roles of CYP2D6.10 and CYP2D6.36 enzymes in mexiletine metabolism: In vitro functional analysis of recombinant proteins expressed in Saccharomyces cerevisiae

Cytochrome P450 2D6 (CYP2D6) and glucose-6-phosphate dehydrogenase (G6PD) genetic variations in Thai vivax malaria patients: Implications for 8-aminoquinoline radical cure

BACKGROUND

Primaquine and tafenoquine are the only licensed drugs that effectively kill the hypnozoite stage and are used to prevent Plasmodium vivax malaria relapse. However, both primaquine and tafenoquine can cause acute hemolysis in glucose-6-phosphate dehydrogenase (G6PD)-deficient people with varying degrees of severity depending on G6PD variants. Additionally, primaquine efficacy against malaria parasites was decreased in individuals with impaired cytochrome P450 2D6 (CYP2D6) activity due to genetic polymorphisms. This study aimed to characterize G6PD and CYP2D6 genetic variations in vivax malaria patients from Yala province, a malaria-endemic area along the Thai-Malaysian border, and determine the biochemical properties of identified G6PD variants.

METHODOLOGY/PRINCIPLE FINDINGS

Multiplexed high-resolution melting assay and DNA sequencing detected five G6PD variants, including G6PD Kaiping, G6PD Vanua Lava, G6PD Coimbra, G6PD Mahidol, and G6PD Kerala-Kalyan. Biochemical and structural characterization revealed that G6PD Coimbra markedly reduced catalytic activity and structural stability, indicating a high susceptibility to drug-induced hemolysis. While Kerala-Kalyan had minor effects, it is possible to develop mild adverse effects when receiving radical treatment. CYP2D6 genotyping was performed using long-range PCR and DNA sequencing, and the phenotypes were predicted using the combination of allelic variants. Decreased and no-function alleles were detected at frequencies of 53.4% and 14.2%, respectively. The most common alleles were CYP2D6*36+*10 (25.6%), *10 (23.9%), and *1 (22.2%). Additionally, 51.1% of the intermediate metabolizers showed CYP2D6*10/*36+*10 as the predominant genotype (15.9%).

CONCLUSIONS/SIGNIFICANCE

Our findings provide insights about genetic variations of G6PD and CYP2D6 in 88 vivax malaria patients from Yala, which may influence the safety and effectiveness of radical treatment. Optimization of 8-aminoquinoline administration may be required for safe and effective treatment in the studied population, which could be a significant challenge in achieving the goal of eliminating malaria.

Substrate specificity of CYP2D6 genetic variants

Genetic variation in the gene encoding CYP2D6 is used to guide drug prescribing in clinical practice. However, genetic variants in CYP2D6 show substrate-specific effects that are currently not accounted for. With a systematic literature, we retrieved 22 original studies describing in vitro experiments focusing on CYP2D6 alleles (CYP2D6*1, *2, *10 and *17) and substrates. Allele activity (clearance of the allele of interest divided by the clearance of the wildtype) was extracted. The results support the hypothesis of the existence of substrate specificity of the CYP2D6*17-allele (higher debrisoquine clearance), a subtle effect of the CYP2D6*10-allele (lower dextromethorphan clearance) but no substrate-specific effect of the CYP2D6*2-allele. Although our results support substrate specificity, for most substrates data are too sparse and require further studies.

Points-to-consider documents: Scientific information on the evaluation of genetic polymorphisms during non-clinical studies and phase I clinical trials in the Japanese population

Pharmacotherapy shows striking individual differences in pharmacokinetics and pharmacodynamics, involving drug efficacy and adverse reactions. Recent genetic research has revealed that genetic polymorphisms are important intrinsic factors for these inter-individual differences. This pharmacogenomic information could help develop safer and more effective precision pharmacotherapies and thus, regulatory guidance/guidelines were developed in this area, especially in the EU and US. The Project for the Promotion of Progressive Medicine, Medical Devices, and Regenerative Medicine by the Ministry of Health, Labour and Welfare, performed by Tohoku University, reported scientific information on the evaluation of genetic polymorphisms, mainly on drug metabolizing enzymes and transporters, during non-clinical studies and phase I clinical trials in Japanese subjects/patients. We anticipate that this paper will be helpful in drug development for the regulatory usage of pharmacogenomic information, most notably pharmacokinetics.

Ten Years' Experience with the CYP2D6 Activity Score: A Perspective on Future Investigations to Improve Clinical Predictions for Precision Therapeutics

The seminal paper on the CYP2D6 Activity Score (AS) was first published ten years ago and, since its introduction in 2008, it has been widely accepted in the field of pharmacogenetics. This scoring system facilitates the translation of highly complex CYP2D6 diplotype data into a patient’s phenotype to guide drug therapy and is at the core of all CYP2D6 gene/drug pair guidelines issued by the Clinical Pharmacogenetics Implementation Consortium (CPIC). The AS, however, only explains a portion of the variability observed among individuals and ethnicities. In this review, we provide an overview of sources in addition to CYP2D6 genotype that contribute to the variability in CYP2D6-mediated drug metabolism and discuss other factors, genetic and non-genetic, that likely contribute to the observed variability in CYP2D6 enzymatic activity.

DDPH, a novel antihypertensive agent, is a potential dual inhibitor of hepatic CYP2D and CYP3A

DDPH (1-(2, 6-dimethylphenoxy)-2-(3, 4-dimethoxyphenylethylamino) propane hydrochloride) is a promising novel antihypertensive agent, with potent antihypertensive, neuroprotective and cardioprotective effects. This study aimed to investigate the effects of DDPH on the expression and activity of hepatic cytochrome P450 (CYP) isoforms and evaluate the metabolic drug-drug interactions of DDPH with propafenone. Our results showed that orally administered DDPH (12.5-50 mg/kg/d) for 7 days significantly inhibited CYP2D1 and CYP3A1 activity and mRNA and protein expression but weakly increased CYP1A2 activity and expression in rats. Enzyme kinetics studies showed that DDPH was a competitive inhibitor of CYP2D1 and mixed inhibitor of CYP3A1 in rat liver microsomes with Ki values of 3.70 ± 0.42 μM and 4.79 ± 1.10 μM respectively. With human liver microsomes, DDPH was a noncompetitive inhibitor of CYP2D6 (Ki = 0.85 ± 0.06 μM) and mixed inhibitor of CYP3A (Ki = 2.15 ± 0.41 μM). Further in vivo study showed that oral administration of DDPH (12.5-50 mg/kg/d) for 7 days in rats significantly increased the area under the plasma concentration-time curve (AUC) of propafenone by 25.4%-63.9%, with a concomitant decrease in the plasma clearance. In conclusion, the results indicated that DDPH inhibited CYP2D and CYP3A activities and down-regulated their protein expression and mRNA transcription. DDPH might cause metabolic drug-drug interactions through modulation of the activity and expression of CYP2D and CYP3A. This information could be important in the prediction of possible drug-drug interactions as well as for the effective therapy and the limitation of toxicity of DDPH in clinical practice.

Functional characterization of CYP2D6 novel allelic variants identified in the Chinese Han population

AIM

This study was aimed to functionally characterize four novel CYP2D6 alleles identified in Chinese Han population.

MATERIALS & METHODS

CYP2D6 proteins of wild-type and the four novel variants along with CYP2D6.2 and CYP2D6.10 were heterologously expressed in yeast cells and the kinetic parameters were determined.

RESULTS

Compared with CYP2D6.1 (frequency in Chinese 24.65%), CYP2D6.X (1.63%), CYP2D6.Y (1.50%), CYP2D6.Z (0.81%), CYP2D6.10 (52.53%) and CYP2D6.75 (0.13%) exhibited low activity at different degrees, whereas the kinetic parameters of CYP2D6.2 (11.06%) were much the same with CYP2D6.1. The novel allele CYP2D6.75 showed decreased enzyme activity.

CONCLUSION

This is the first study to conduct functional analysis of CYP2D6 four novel alleles in Chinese Han population, which might be helpful for optimizing pharmacotherapy and the design of personalized medicine.

Nanocomposites of graphene and cytochrome P450 2D6 isozyme for electrochemical-driven tramadol metabolism

Cytochrome P450 enzymes (cyt P450s) with an active center of iron protoheme are involved in most clinical drugs metabolism process. Herein, an electrochemical platform for the investigation of drug metabolism in vitro was constructed by immobilizing cytochrome P450 2D6 (CYP2D6) with cyt P450 reductase (CPR) on graphene modified glass carbon electrode. Direct and reversible electron transfer of the immobilized CYP2D6 with the direct electron transfer constant of 0.47 s(-1) and midpoint potential of -0.483 V was obtained. In the presence of substrate tramadol, the electrochemical-driven CYP2D6 mediated catalytic behavior toward the conversion of tramadol to o-demethyl-tramadol was confirmed. The Michaelis-Menten constant (Km(app)) and heterogeneous reaction rate constant during the metabolism of tramadol were calculated to be 23.85 μM and 1.96 cm s(-1), respectively. The inhibition effect of quinidine on CYP2D6 catalyze-cycle was also investigated. Furthermore, this system was applied to studying the metabolism of other drugs.

In vitro assessment of the allelic variants of cytochrome P450

The cytochrome P450 (CYP) superfamily is one of the most important groups of enzymes involved in drug metabolism. It is responsible for the metabolism of a large number of drugs. Many CYP isoforms are expressed polymorphically, and catalytic alterations of allelic variant proteins can affect the metabolic activities of many drugs. The CYP2D6, CYP2C9, CYP2C19, and CYP2B6 genes are particularly polymorphic, whereas CYP1A1, CYP1A2, CYP2E1, and CYP3A4 are relatively well conserved without common functional polymorphisms. In vitro studies using cDNA expression systems are useful tools for evaluating functional alterations of the allelic variants of CYP, particularly for low-frequency alleles. Recombinant CYPs have been successfully expressed in bacteria, yeast, baculoviruses, and several mammalian cells. Determination of CYP variant-mediated kinetic parameters (Km and Vmax) in vitro can be useful for predicting drug dosing and clearance in humans. This review focuses on the advantages and disadvantages of the various cDNA-expression systems used to determine the kinetic parameters for CYP allelic variants, the methods for determining the kinetic parameters, and the findings of in vitro studies on highly polymorphic CYPs, including CYP2D6, CYP2C9, CYP2C19, and CYP2B6.

CYP2D7-2D6 hybrid tandems: identification of novel CYP2D6 duplication arrangements and implications for phenotype prediction

AIMS

Allelic variants of cytochrome P450 CYP2D6 (CYP2D6), such as gene deletion, duplication, multiplication and conversion, contribute to the wide range of CYP2D6 activity. Novel gene arrangements were discovered and characterized.

MATERIALS & METHODS

DNA from 32 Caucasian and 59 African-American duplication-positive subjects were analyzed by long-range PCR and genotyping to detect CYP2D7-2D6 hybrid tandem alleles. Novel allelic variants were sequenced and a strategy for the detection and analysis of hybrid genes was refined.

RESULTS

CYP2D7-2D6 hybrid tandem alleles were identified in one African-American and four Caucasian subjects. Three novel hybrid genes were found on CYP2D6*1 and CYP2D6*2 duplication backgrounds and designated CYP2D6*76, *77 and *78. CYP2D7 to 2D6 conversion occurred in introns 1 and 4, and exon 9. All carried a T-insertion in exon 1 abolishing activity. In Caucasians, four out of 33 (12%) of the duplication-positive alleles were hybrid tandems, three CYP2D6*77 + *2 and one CYP2D6*78 + *2. By contrast, in African-Americans only one of 60 duplication-positive alleles was identified as a hybrid tandem. This allele was designated CYP2D6*76 + *1.

CONCLUSION

Hybrid tandem alleles occur infrequently (<0.25%) in Caucasians, but may explain why not every subject with a CYP2D6 duplication presents with an ultrarapid metabolizer phenotype.

The mechanism causing the difference in kinetic properties between rat CYP2D4 and human CYP2D6 in the oxidation of dextromethorphan and bufuralol

The capacity to oxidize bufuralol (BF) and dextromethorphan (DEX) was compared kinetically between human CYP2D6 and four rat CYP2D (CYP2D1, -2D2, -2D3 and -2D4) isoenzymes in a yeast cell expression system. In BF 1''-hydroxylation and DEX O-demethylation, only CYP2D4 showed hook-shaped Eadie-Hofstee plots, the other four CYP2D enzymes exhibiting linear plots. In DEX N-demethylation, rat CYP2D2 did not show any detectable activity under the conditions used, whereas the other four enzymes yielded linear Eadie-Hofstee plots. To elucidate the mechanisms causing the nonlinear kinetics, four CYP2D4 mutants, CYP2D4-F109I, -V123F, -L216F and -A486F, were prepared. CYP2D4-V123F, -L216F and -A486F yielded linear or linear-like Eadie-Hofstee plots for BF 1''-hydroxylation, whereas only CYP2D4-A486F exhibited linear plots for DEX O-demethylation. The substitution of Phe-109 by isoleucine did not have any effect on the oxidative capacity of CYP2D4 for either BF or DEX. These results suggest that the introduction of phenylalanine in the active-site cavity of CYP2D4 simplifies complicated interactions between the substrates and the amino acid residues, but the mechanisms causing the simplification differ between BF and DEX.

Comparative metabolic capabilities and inhibitory profiles of CYP2D6.1, CYP2D6.10, and CYP2D6.17

Polymorphisms in the cytochrome P450 2D6 (CYP2D6) gene are a major cause of pharmacokinetic variability in human. Although the poor metabolizer phenotype is known to be caused by two null alleles leading to absence of functional CYP2D6 protein, the large variability among individuals with functional alleles remains mostly unexplained. Thus, the goal of this study was to examine the intrinsic enzymatic differences that exist among the several active CYP2D6 allelic variants. The relative catalytic activities (enzyme kinetics) of three functionally active human CYP2D6 allelic variants, CYP2D6.1, CYP2D6.10, and CYP2D6.17, were systematically investigated for their ability to metabolize a structurally diverse set of clinically important CYP2D6-metabolized drugs [atomoxetine, bufuralol, codeine, debrisoquine, dextromethorphan, (S)-fluoxetine, nortriptyline, and tramadol] and the effects of various CYP2D6-inhibitors [cocaine, (S)-fluoxetine, (S)-norfluoxetine, imipramine, quinidine, and thioridazine] on these three variants. The most significant difference observed was a consistent but substrate-dependent decease in the catalytic efficiencies of cDNA-expressed CYP2D6.10 and CYP2D6.17 compared with CYP2D6.1, yielding 1.32 to 27.9 and 7.33 to 80.4% of the efficiency of CYP2D6.1, respectively. The most important finding from this study is that there are mixed effects on the functionally reduced allelic variants in enzyme-substrate affinity or enzyme-inhibitor affinity, which is lower, higher, or comparable to that for CYP2D6.1. Considering the rather high frequencies of CYP2D6*10 and CYP2D6*17 alleles for Asians and African Americans, respectively, these data provide further insight into ethnic differences in CYP2D6-mediated drug metabolism. However, as with all in vitro to in vivo extrapolations, caution should be applied to the clinical consequences.

Cytochrome P4502D6 (CYP2D6) Gene Locus Heterogeneity: Characterization of Gene Duplication Events

Duplications and multiplications of active CYP2D6 genes can cause ultrarapid drug metabolism and lead to therapeutic failure. Multiple functional and non-functional duplication alleles have been further characterized. Duplications were detected by long-range polymerase chain reaction (PCR), PCR-restriction fragment length polymorphism, and sequence analysis. A PCR fragment encompassing the entire duplicated gene was utilized for detailed characterization. Duplications occurred at 1.3, 5.75, and 2.0% in Caucasian, African American, and racially mixed populations, respectively (n=887 total). Of those 28, 47, and 17% were non-functional CYP2D6*4 x N. Twelve unique duplication alleles were detected: *1 x N, *2 x N, *4 x N, *6 x N, *10 x N, *17 x N, *17 x N[spacer], *29 x N, *35 x N, *43 x N, *45 x N, and a novel non-functional tandem arrangement of a chimeric 2D7/2D6 and *1 gene. All novel duplications except *35 x N were found in African Americans. Accurate identification of gene duplication events is essential to avoid false-positive ultrarapid metabolism assignments and thus, overestimation of predicted activity and increased risk for unwanted adverse events.

Diverse Structures of Chimeric CYP-REP7/6-Containing CYP2D6 and a Novel Defective CYP2D6 Haplotype Harboring Single-type *36 and CYP-REP7/6 in Japanese

Chimeric REP7/6 has been used as a marker of CYP2D6 deletion, such as for CYP2D6*5. However, the CYP2D6*10D (*10D) haplotype found in a Japanese population consist of CYP2D6*10B, CYP2D7P-derived 3'-flanking region, and a chimeric repetitive sequence, CYP-REP7/6 (REP7/6) (Ishiguro et al. Clin. Chim. Acta. 2004: 347, 217-221). From our analysis, REP7/6 was found in 26 out of 254 Japanese subjects. Thus, the REP7/6-containing CYP2D6 genes (2D6-REP7/6) were analyzed in detail. In order to specifically detect the 2D6-REP7/6 structure, primers were designed in CYP2D6 intron 6 and the REP7/6 3'-flanking region. Among 26 subjects analyzed by PCR, 5 had 2D6-REP7/6. The other 21 subjects were confirmed to have *5 by another *5-specific primer set. Three out of five subjects with 2D6-REP7/6 had the *10D structure. However, further analysis by PCR and sequencing revealed that their haplotypes were further divided into tandem-type *36-*10D (n=2) and single-type *10D (n=1). The remaining two subjects had a novel type of a *36-containing defective structure that consists of CYP2D6*36 and 3'-flanking REP7/6 (single-type *36-REP7/6). Then, REP7/6 sequences in *5, *10D, *36-*10D, and single-type *36 were determined and classified into 5 types: types A to D for *5, type E for *10D and *36-*10D, and type F for *36. These findings could be useful for accurate determination of *5 and REP7/6-harboring aberrant CYP2D6 haplotypes.