Methadone enantiomer plasma levels, CYP2B6, CYP2C19, and CYP2C9 genotypes, and response to treatment

BACKGROUND AND OBJECTIVE: Recent in vitro studies have suggested an important role of cytochrome P450 (CYP) 2B6 and CYP2C19 in methadone metabolism. We aimed to determine the influence of CYP2B6, CYP2C9, and CYP2C19 genetic polymorphism on methadone pharmacokinetics and on the response to treatment. METHODS: We included 209 patients in methadone maintenance treatment on the basis of their response to treatment and their daily methadone dose. Patients were genotyped for CYP2B6, CYP2C9, and CYP2C19. Steady-state trough and peak (R)-, (S)-, and (R,S)-plasma levels and peak-to-trough plasma level ratios were measured. RESULTS: CYP2B6 genotype influences (S)-methadone and, to a lesser extent, (R)-methadone plasma levels, with the median trough (S)-methadone plasma levels being 105, 122, and 209 ng . kg/mL . mg for the noncarriers of allele *6, heterozygous carriers, and homozygous carriers (*6/*6), respectively (P = .0004). CYP2C9 and CYP2C19 genotypes do not influence methadone plasma levels. Lower peak and trough plasma levels of methadone and higher peak-to-trough ratios were measured in patients considered as nonresponders [median (R,S)-methadone trough plasma levels of 183 and 249 ng . kg/mL . mg (P = .0004) and median peak-to-trough ratios of 1.82 and 1.58 for high-dose nonresponders and high-dose responders, respectively (P = .0003)]. CONCLUSION: Although CYP2B6 influences (S)-methadone plasma levels, given that only (R)-methadone contributes to the opioid effect of this drug, a major influence of CYP2B6 genotype on response to treatment is unlikely and has not been shown in this study. Lower plasma levels of methadone in nonresponders, suggesting a higher clearance, and higher peak-to-trough ratios, suggesting a shorter elimination half-life, are in agreement with the usual clinical measures taken for such patients, which are to increase methadone dosages and to split the daily dose into several intakes.     

Stereoselective block of hERG channel by (S)-methadone and QT interval prolongation in CYP2B6 slow metabolizers

Methadone inhibits the cardiac potassium channel hERG and can cause a prolonged QT interval. Methadone is chiral but its therapeutic activity is mainly due to (R)-methadone. Whole-cell patch-clamp experiments using cells expressing hERG showed that (S)-methadone blocked the hERG current 3.5-fold more potently than (R)-methadone (IC50s (half-maximal inhibitory concentrations) at 37 degrees C: 2 and 7 microM). As CYP2B6 slow metabolizer (SM) status results in a reduced ability to metabolize (S)-methadone, electrocardiograms, CYP2B6 genotypes, and (R)- and (S)-methadone plasma concentrations were obtained for 179 patients receiving (R,S)-methadone. The mean heart-rate-corrected QT (QTc) was higher in CYP2B6 SMs (*6/*6 genotype; 439+/-25 ms; n=11) than in extensive metabolizers (non *6/*6; 421+/-25 ms; n=168; P=0.017). CYP2B6 SM status was associated with an increased risk of prolonged QTc (odds ratio=4.5, 95% confidence interval=1.2-17.7; P=0.03). This study reports the first genetic factor implicated in methadone metabolism that may increase the risk of cardiac arrhythmias and sudden death. This risk could be reduced by the administration of (R)-methadone.     

Long-term efavirenz autoinduction and its effect on plasma exposure in HIV patients

We investigated the influence of gender and pharmacogenetic variations on long-term efavirenz autoinduction and disposition among patients with HIV in Tanzania (N = 129). Plasma concentrations (at 16?h) of efavirenz and 8-hydroxyefavirenz were quantified at weeks 4 and 16 of therapy. Genotyping was performed to identify cytochrome P450 (CYP) 2B6*6, CYP3A5*3, *6, and *7, and ABCB1-3435?C/T genotypes. There were reductions in the median efavirenz concentration (Wilcoxon matched-pair test P < 0.001) and efavirenz/8-hydroxyefavirenz ratio (P < 0.001) by 19 and 32%, respectively, at week 16 as compared with week 4. The proportion of patients with efavirenz concentration <1?μg/ml at week 16 was higher by 67, 25, and 5% in CYP2B6*1/*1, *1/*6, and *6/*6 genotypes, respectively. The defined therapeutic range based on observed plasma concentrations is affected by the time point of sampling and the CYP2B6 genotype. The effect of efavirenz autoinduction on reducing plasma exposure continues up to week 16 and predominantly affects CYP2B6 extensive metabolizers. Among CYP2B6 slow metabolizers, the presence of a CYP3A5 genotype allele is associated with greater effects of efavirenz autoinduction on plasma concentrations of the drug. The cumulative induction may influence the long-term antiretroviral therapy outcome, particularly in CYP2B6*1 carriers.     

Activity levels of tamoxifen metabolites at the estrogen receptor and the impact of genetic polymorphisms of phase I and II enzymes on their concentration levels in plasma

The therapeutic effect of tamoxifen depends on active metabolites, e.g., cytochrome P450 2D6 (CYP2D6) mediated formation of endoxifen. To test for additional relationships, 236 breast cancer patients were genotyped for CYP2D6, CYP2C9, CYP2B6, CYP2C19, CYP3A5, UGT1A4, UGT2B7, and UGT2B15; also, plasma concentrations of tamoxifen and 22 of its metabolites, including the (E)-, (Z)-, 3-, and 4'-hydroxymetabolites as well as their glucuronides, were quantified using liquid chromatography-tandem mass spectrometry (MS). The activity levels of the metabolites were measured using an estrogen response element reporter assay; the strongest estrogen receptor inhibition was found for (Z)-endoxifen and (Z)-4-hydroxytamoxifen (inhibitory concentration 50 (IC50) 3 and 7?nmol/l, respectively). CYP2D6 genotypes explained 39 and 9% of the variability of steady-state concentrations of (Z)-endoxifen and (Z)-4-hydroxytamoxifen, respectively. Among the poor metabolizers, 93% had (Z)-endoxifen levels below IC90 values, underscoring the role of CYP2D6 deficiency in compromised tamoxifen bioactivation. For other enzymes tested, carriers of reduced-function CYP2C9 (*2, *3) alleles had lower plasma concentrations of active metabolites (P < 0.004), pointing to the role of additional pathways.     

Genetic polymorphisms of the CYP3A4, CYP3A5, and MDR-1 genes and pharmacokinetics of the calcineurin inhibitors cyclosporine and tacrolimus

BACKGROUND: The calcineurin inhibitors cyclosporine (INN, cyclosporin) and tacrolimus have a narrow therapeutic index and show considerable interindividual variability in their pharmacokinetics. The low oral bioavailability of calcineurin inhibitors is thought to result from the actions of the metabolizing enzymes cytochrome P450 (CYP) 3A4 and CYP3A5 and the multidrug efflux pump P-glycoprotein, encoded by MDR-1. OBJECTIVE: Our objective was to determine the role of genetic polymorphisms in CYP3A4, CYP3A5, and MDR-1 with respect to interindividual variability in cyclosporine and tacrolimus pharmacokinetics. METHODS: Kidney transplant recipients receiving cyclosporine (n = 110) or tacrolimus (n = 64) were genotyped for CYP3A4*1B and *3, CYP3A5*3 and *6, and MDR-1 C3435T. Dose-adjusted trough levels were determined and correlated with the corresponding genotype. RESULTS: Tacrolimus dose-adjusted trough levels were higher in CYP3A5*3/*3 patients (n = 45) than in *1/*3 plus *1/*1 patients (n = 17), as follows: median and range, 94 (34-398) ng/mL per mg/kg versus 61 (37-163) ng/mL per mg/kg (P <.0001, Mann-Whitney test). CYP3A4*1B allele carriers (n = 10) had lower tacrolimus dose-adjusted trough levels compared with those in patients with the wild-type (*1/*1) genotype (n = 54): median and range, 57 (40-163) ng/mL per mg/kg versus 89 (34-398) ng/mL per mg/kg) (P =.003, Mann-Whitney test). No evidence was found supporting a role for the MDR-1 C3435T polymorphism in tacrolimus dose requirement. None of the polymorphisms studied correlated with cyclosporine dose-adjusted predose concentrations. CONCLUSION: As a group, patients with the CYP3A5*3/*3 genotype require less tacrolimus to reach target predose concentrations compared with CYP3A5*1 allele carriers, whereas CYP3A4*1B carriers require more tacrolimus to reach target trough concentrations compared with CYP3A4*1 homozygotes.     

Tacrolimus pharmacokinetics and pharmacogenetics: influence of adenosine triphosphate-binding cassette B1 (ABCB1) and cytochrome (CYP) 3A polymorphisms

Tacrolimus, an immunosuppressant used after organ transplantation, has a narrow therapeutic range and its pharmacokinetic variability complicates its daily dose assessment. P-glycoprotein (P-gp), encoded by the adenosine triphosphate-binding cassette B1 (ABCB1) and the cytochrome (CYP) 3A4 and 3A5 enzymes appears to play a role in the tacrolimus metabolism. In the present study, two different renal transplant recipient groups were used to examine the influence of ABCB1 and CYP3A polymorphisms on the daily tacrolimus dose and several pharmacokinetic parameters. In total 63 Caucasian renal transplant recipients divided into 26 early [median (range) of the days since transplantation - 16 (3-74)] and 37 late [median (range) of the days since transplantation - 1465 (453-4128)] post-transplant recipients were genotyped for ABCB1 and CYP3A polymorphisms. The pharmacokinetic parameters of tacrolimus were determined for all renal transplant recipients and correlated with their corresponding genotypes. A significant difference in allele frequencies of the CYP3A4*1B (P = 0.028) and CYP3A5*1 (P = 0.022) alleles was observed between the early and late post-transplant recipient groups. Significantly higher dose-normalized trough levels (dnC(0)), dose-normalized area under the curve (dnAUC(0-12)), and dose-normalized maximum concentration (dnC(max)) were observed for carriers of the CYP3A5*3 variant allele in both renal transplant patient groups. Except for the daily tacrolimus dose (P = 0.025) no significant differences were observed for carriers of the CYP3A4*1B variant allele. Neither the individual ABCB1 polymorphisms nor the ABCB1 haplotypes were associated with any pharmacokinetic parameter. We noticed that patients carrying a CYP3A5*1 allele require a twofold higher tacrolimus dose compared with homozygous carriers of the CYP3A5*3 variant allele to maintain the target dnAUC(0-12). Therefore, genotyping for the CYP3A5*3 variant allele can contribute to a better and more individualized immunosuppressive therapy in transplant patients.     

The role of common variants of ABCB1, CYP3A4, and CYP3A5 genes in lipid-lowering efficacy and safety of simvastatin treatment

OBJECTIVE: Our objective was to investigate the interactions between common polymorphisms in ABCB1, CYP3A4, and CYP3A5 genes and the lipid-lowering efficacy and safety of the 3-hydroxy-3-methylglutaryl-coenzyme A reductase inhibitor simvastatin. METHODS: One hundred sixteen hypercholesterolemic patients were prospectively screened by physical examination, medical history, and clinical laboratory evaluation and were included in this study. Subjects entering the study were treated with 20 mg/d simvastatin. Plasma lipid and lipoprotein levels were measured before treatment, after 2 months of treatment, and after 6 months of treatment. Ninety-nine patients completed the 6-month follow-up and were included in the association analysis for treatment efficacy. Seventeen subjects who had adverse drug reactions (ADRs) to simvastatin (ADR group) could not complete the 6-month follow-up and were included in the association analyses for safety. Myalgia was observed in 15 of 17 subjects and was the only ADR included in the association analyses, but other common ADRs were also observed. Myalgia was defined as proximal or diffuse muscle pain, tenderness, or weakness, or both pain and weakness, with normal or slightly increased serum creatine phosphokinase levels. ABCB1 (1236C>T, 2677G>A/T, and 3435C>T), CYP3A4 (-392A>G), and CYP3A5 (6986A>G) allele variants were determined by polymerase chain reaction and restriction mapping. RESULTS: After adjustment for covariates, carriers of the ABCB1 1236T variant allele had a greater reduction in total cholesterol and low-density lipoprotein cholesterol with simvastatin treatment, as compared with homozygotes with the wild-type allele (-29.0% [95% confidence interval (CI), -25.9 to -32.5] versus -24.2% [95% CI, -19.0 to -29.3] [P = .042] and -39.6% [95% CI, -35.8 to -44.0] versus -33.8% [95% CI, -27.4 to -40.2] [P = .042], respectively). Similar results were observed for the 2677G>A/T polymorphism and haplotype data. The 1236T, 2677non-G, and 3435T alleles were less frequent in ADR cases than in the non-ADR group (P < .05 for all single-nucleotide polymorphisms). Haplotype analyses also demonstrated a reduction of the T-non-G-T haplotype frequency (20%) in patients in whom myalgia developed during simvastatin treatment, as compared with the non-ADR group (41.4%) (P = .03). No significant associations were observed between the CYP3A4 -392A>G and CYP3A5*3 allele variants and the efficacy or tolerability of simvastatin. CONCLUSIONS: Our data suggest an association of ABCB1 gene polymorphisms and the efficacy and safety of simvastatin.     

Enantiomeric metabolic interactions and stereoselective human methadone metabolism

Methadone is administered as a racemate, although opioid activity resides in the R-enantiomer. Methadone disposition is stereoselective, with considerable unexplained variability in clearance and plasma R/S ratios. N-Demethylation of methadone in vitro is predominantly mediated by cytochrome P450 CYP3A4 and CYP2B6 and somewhat by CYP2C19. This investigation evaluated stereoselectivity, models, and kinetic parameters for methadone N-demethylation by recombinant CYP2B6, CYP3A4, and CYP2C19, and the potential for interactions between enantiomers during racemate metabolism. CYP2B6 metabolism was stereoselective. CYP2C19 was less active, and stereoselectivity was opposite that for CYP2B6. CYP3A4 was not stereoselective. With all three isoforms, enantiomer N-dealkylation rates in the racemate were lower than those of (R)-(6-dimethyamino-4,4-diphenyl-heptan-3-one) hydrochloride (R-methadone) or (S)-(6-dimethyamino-4,4-diphenyl-heptan-3-one) hydrochloride (S-methadone) alone, suggesting an enantiomeric interaction and mutual metabolic inhibition. For CYP2B6, the interaction between enantiomers was stereoselective, with S-methadone as a more potent inhibitor of R-methadone N-demethylation than R-of S-methadone. In contrast, enantiomer interactions were not stereoselective with CYP2C19 or CYP3A4. For all three cytochromes P450, methadone N-demethylation was best described by two-site enzyme models with competitive inhibition. There were minor model differences between cytochromes P450 to account for stereoselectivity of metabolism and enantiomeric interactions. Changes in plasma R/S methadone ratios observed after rifampin or troleandomycin pretreatment in humans in vivo were successfully predicted by CYP2B6- but not CYP3A4-catalyzed methadone N-demethylation. CYP2B6 is a predominant catalyst of stereoselective methadone metabolism in vitro. In vivo, CYP2B6 may be a major determinant of methadone metabolism and disposition, and CYP2B6 activity and stereoselective metabolic interactions may confer variability in methadone disposition.     

ABCB1 single-nucleotide polymorphisms determine tacrolimus response in patients with ulcerative colitis

Tacrolimus (Tac) is effective in the treatment of steroid-refractory ulcerative colitis (UC); however, nonresponse and unpredictable side effects are major limitations. Because Tac response in patients who have undergone solid-organ transplantation has been associated with the presence of variants in CYP3A and ABCB1, we elucidated the contributions of CYP3A4*1B and CYP3A5*3 and of ABCB1 1236C>T, 2677G>T,A, and 3435C>T polymorphisms to Tac response in 89 patients with UC. Short-term remission and response were achieved in 61 and 14% of the patients, respectively, and were associated with colectomy-free survival. In a linear logistic regression model, patients with homozygous variants for one of the three ABCB1 alleles showed significantly higher short-term remission rates as compared with those of other genotypes. The effects held true after multivariate analysis including multiple comparisons and were more pronounced after correction for dose-adjusted Tac blood trough levels. We suggest that ABCB1, but not CYP3A5, may predict short-term remission of Tac in steroid-refractory UC.     

Prediction of codeine toxicity in infants and their mothers using a novel combination of maternal genetic markers

Substantial variation exists in response to standard doses of codeine ranging from poor analgesia to life-threatening central nervous system (CNS) depression. We aimed to discover the genetic markers predictive of codeine toxicity by evaluating the associations between polymorphisms in cytochrome P450 2D6 (CYP2D6), UDP-glucuronosyltransferase 2B7 (UGT2B7), P-glycoprotein (ABCB1), mu-opioid receptor (OPRM1), and catechol O-methyltransferase (COMT) genes, which are involved in the codeine pathway, and the symptoms of CNS depression in 111 breastfeeding mothers using codeine and their infants. A genetic model combining the maternal risk genotypes in CYP2D6 and ABCB1 was significantly associated with the adverse outcomes in infants (odds ratio (OR) 2.68; 95% confidence interval (CI) 1.61-4.48; P(trend) = 0.0002) and their mothers (OR 2.74; 95% CI 1.55-4.84; P(trend) = 0.0005). A novel combination of the genetic and clinical factors predicted 87% of the infant and maternal CNS depression cases with a sensitivity of 80% and a specificity of 87%. Genetic markers can be used to improve the outcome of codeine therapy and are also probably important for other opioids sharing common biotransformation pathways.     

Genetic polymorphisms and the impact of a higher clopidogrel dose regimen on active metabolite exposure and antiplatelet response in healthy subjects

A double-blind crossover study was conducted in four CYP2C19 genotype-defined metabolizer groups to assess whether increase in clopidogrel dosing can overcome reduced pharmacodynamic response in CYP2C19 poor metabolizers (PMs). Ten healthy subjects in each of four metabolizer groups were randomized to a clopidogrel regimen of a 300-mg loading dose (LD) and a 75-mg/day maintenance dose (MD) for 4 days followed by 600-mg LD and 150 mg/day MD, or vice versa. The exposure levels of clopidogrel's active metabolite H4 (clopi-H4) in PMs were 71% lower on the 75-mg/day regimen and 64% lower on the 150-mg/day regimen than the corresponding exposure levels in extensive metabolizers (EMs). In PMs, the maximal platelet aggregation (MPA) induced by adenosine diphosphate (ADP) 5 μmol/l was 10.5% lower on the 75-mg/day regimen and 7.9% lower on the 150-mg/day regimen than the corresponding values in EMs. PMs who were on the clopidogrel regimen of 600-mg LD/150 mg/day MD showed clopi-H4 exposure and MPA levels similar to those in EMs who were on the regimen of 300-mg LD/75 mg/day MD. In a pooled analysis evaluating CYP1A2, CYP2B6, CYP2C9, CYP2C19, CYP3A5, CYP2D6, ABCB1, and P2RY12 polymorphisms (N = 396 healthy subjects), only CYP2C19 had a significant impact on antiplatelet response. In healthy CYP2C19 PMs, a clopidogrel regimen of 600-mg LD/150 mg/day MD largely overcomes diminished clopi-H4 exposure and antiplatelet response, as assessed by MPA levels.     

Pharmacogenetics of acenocoumarol: CYP2C9, CYP2C19, CYP1A2, CYP3A4, CYP3A5 and ABCB1 gene polymorphisms and dose requirements

BACKGROUND AND OBJECTIVE: Acenocoumarol (AC) is a coumarin derivative, vitamin K antagonist anticoagulant drug. It has a narrow therapeutic index and shows large pharmacokinetic and pharmacodynamic interindividual variability. Our objective was to investigate the association between AC dose requirements to achieve a target level of anticoagulation and genetic polymorphisms of genes possibly associated with its metabolism (CYP1A2, CYP2C9, CYP2C19, CYP3A4, CYP3A5) and transport (ABCB1). METHODS: Ninety-six Bulgarian patients treated orally with AC for at least 3 months were included. They were separated into three groups according to their AC dose requirement, i.e. low, medium and high. RESULTS AND DISCUSSION: CYP2C9*1/*3 (associated with an intermediate CYP2C9 activity), CYP2C9*2/*2, and CYP2C9*2/*3 genotypes (associated with a low CYP2C9 activity) were more prevalent in the group with low dose requirement of AC compared with the other two groups (P = 0.003). The frequency of CYP2C9*1/*1 genotype, which is associated with an extensive CYP2C9 activity, was higher in the group of patients with high dose requirements (79%), compared with the groups of the medium and low dose requirements (67% and 21% respectively). In addition, the ABCB1 2677GG/3435CC haplotype was associated with use of lower AC dose, whereas the 2677TT/3435TT and 2677GT/3435TT haplotypes were associated with use of higher AC dose (P = 0.03). The distribution of polymorphisms of other genes did not show significant differences between the three groups. CONCLUSION: In vivo, cytochromes P450 isoforms other than CYP2C9 [corrected] were not significantly associated with dose requirement of AC. In our Bulgarian patients, the presence of CYP2C9*2 or/and CYP2C9*3 alleles, as well as the ABCB1 2677GG/3435CC haplotype were associated with low dose requirement of AC.     

Pharmacokinetics and toxicity of docetaxel: role of CYP3A, MDR1, and GST polymorphisms

OBJECTIVE: Patients initiating docetaxel chemotherapy were genotyped for CYP3A4, CYP3A5, MDR1, GSTM1, GSTT1, GSTM3, and GSTP1 to identify variability factors of docetaxel pharmacokinetics and toxicity. METHODS: Genotyping was performed by direct sequencing (CYP3A4), real-time polymerase chain reaction (CYP3A5), and polymerase chain reaction-restriction fragment length polymorphism (MDR1 and GST). The clearance and area under the curve of docetaxel were calculated by use of a Bayesian approach. Absolute neutrophil count was recorded twice weekly. RESULTS: With regard to the pharmacokinetic analysis, 58 patients were included. CYP3A4*1B carriers (*1A/*1B, n=4), who are also CYP3A5*1/*3 carriers, had a significantly higher clearance and lower dose-normalized area under the curve of docetaxel than those with the wild genotype (*1A/*1A, n=53): 55.2+/-13.5 L/h versus 37.3+/-11.7 L/h (P=.01) and 31.4+/-6.2 (microg . h/L)/(mg/m(2)) versus 52.7+/-18.2 (microg . h/L)/(mg/m(2)) (P=.005), respectively. No influence of MDR1 was evidenced. With regard to the pharmacodynamic analysis, febrile neutropenia occurred more frequently in GSTP1*A/*B carriers (31.6% versus 3.7% in *A/*A carriers and 0% in *A/*C, *B/*B, and *B/*C carriers) (P=.037). Grade 3 neutropenia occurred more frequently in 3435TT MDR1 genotype carriers: TT, 100%; CT, 77.3%; and CC, 54.5% (P=.046). No influence of GSTM1, GSTT1, or GSTM3 polymorphisms was evidenced on docetaxel toxicity. CONCLUSIONS: Patients carrying the CYP3A*1B allele may have enhanced docetaxel clearance and may be underexposed, whereas those carrying GSTP1*A/*B and 3435TT genotypes may have excessive hematologic toxicity. Further studies are warranted to determine the usefulness of genotyping before docetaxel treatment.     

CYP3A5 genotype markedly influences the pharmacokinetics of tacrolimus and sirolimus in kidney transplant recipients

It is currently not clear whether the concentration-time curves of the immunosuppressants differ with respect to the CYP3A5, MDR1, or MRP2 genotype in dose-adapted stable kidney transplant patients. Dose/trough concentration ratios were obtained in 134 tacrolimus and 20 sirolimus-treated patients, and plasma concentration-time profiles were obtained from 16 (tacrolimus) and 10 (sirolimus) patients. Genotyping was carried out for CYP3A5 6986A>G; ABCB1 2677G>T/A, 3435C>T and ABCC2 -24C>T; 1249G>A; 3972C>T. Dose/trough concentration ratios were 0.67+/-0.3 and 1.36+/-0.73 x 10(3) l (P<0.00001) for tacrolimus and 0.42+/-0.17 and 0.84+/-0.46 x 10(3) l (P=0.18) for sirolimus in CYP3A5 non-expressors and expressors. The unadjusted tacrolimus area under curve (AUC)(0-12) was 106.8+/-17.5 ng/ml x h compared with 133.3+/-42.2 ng/ml x h (P=0.37) without affecting serum creatinine. Mean unadjusted AUC(0-24) of sirolimus did not differ significantly either. Therefore, CYP3A5 expressor status and not transporter variants is a main determinant of oral clearance, particularly for tacrolimus. Dose adaptation according to trough levels, however, appears to be sufficient to maintain similar concentration-time profiles.     

ABCB1 genetic variability and methadone dosage requirements in opioid-dependent individuals

BACKGROUND AND OBJECTIVES: The most common treatment for opioid dependence is substitution therapy with another opioid such as methadone. The methadone dosage is individualized but highly variable, and program retention rates are low due in part to nonoptimal dosing resulting in withdrawal symptoms and further heroin craving and use. Methadone is a substrate for the P-glycoprotein transporter, encoded by the ABCB1 gene, which regulates central nervous system exposure. This retrospective study aimed to investigate the influence of ABCB1 genetic variability on methadone dose requirements. METHODS: Genomic deoxyribonucleic acid was isolated from opioid-dependent subjects (n = 60) and non-opioid-dependent control subjects (n = 60), and polymerase chain reaction-restriction fragment length polymorphism and allele-specific polymerase chain reaction were used to determine the presence of single nucleotide polymorphisms at positions 61, 1199, 1236, 2677, and 3435. ABCB1 haplotypes were inferred with PHASE software (version 2.1). RESULTS: There were no significant differences in the allele or genotype frequencies of the individual single nucleotide polymorphisms or haplotypes between the 2 populations. ABCB1 genetic variability influenced daily methadone dose requirements, such that subjects carrying 2 copies of the wild-type haplotype required higher doses compared with those with 1 copy and those with no copies (98.3 +/- 10.4, 58.6 +/- 20.9, and 55.4 +/- 26.1 mg/d, respectively; P = .029). In addition, carriers of the AGCTT haplotype required significantly lower doses than noncarriers (38.0 +/- 16.8 and 61.3 +/- 24.6 mg/d, respectively; P = .04). CONCLUSION: Although ABCB1 genetic variability is not related to the development of opioid dependence, identification of variant haplotypes may, after larger prospective studies have been performed, provide clinicians with a tool for methadone dosage individualization.     

Genetic polymorphism of cytochrome P450s and P-glycoprotein in the Finnish population

The objective of this study was to investigate the genetic polymorphism of selected cytochrome P450 (CYP) enzymes and ABCB1 (encoding P-glycoprotein) of central importance with regard to the disposition of clinically used drugs in the Finnish population and to compare the results to pre-existing data from Caucasian populations. A random sample of 449 Finns was studied. Single nucleotide polymorphisms (SNPs) were genotyped using blood-derived genomic DNA and 5'-nuclease assays. We found that the allele frequencies of CYP1A2 SNP g.-163C>A, CYP2C8*3, CYP2C9*2, CYP2C9*3 and CYP2C19*2 were similar to those seen in other Caucasian populations. However, the allelic frequency of the variant ABCB1 SNP c.3435C>T allele was lower than previously reported. The frequency of the homozygous CYP3A5*1 expression was significantly higher than expected based on Hardy-Weinberg calculations (observed n = 8 vs. expected n = 3, P = 0.01). Other genotype frequencies corresponded to the expected values. The strong linkage between the CYP2C8*3 and the CYP2C9*2 alleles was confirmed in this study and the number of individuals with the rare haplotype CYP2C8*3*3/CYP2C9*2*2 was higher than expected. We conclude that the frequency of mutated CYP alleles in Finns were in agreement with earlier findings in Caucasian populations, but a lower frequency of the ABCB1 variant allele 3435T corresponding to that reported in Asian populations was found. The higher than expected frequency of the CYP3A5*1*1 genotype and the CYP2C8*3*3/CYP2C9*2*2 haplotype may influence the response to treatment with drugs metabolized by these enzymes.     

Polymorphic organic anion transporting polypeptide 1B1 is a major determinant of repaglinide pharmacokinetics

BACKGROUND AND OBJECTIVE: A large interindividual variability exists in the plasma concentrations of repaglinide. Our aim was to investigate possible associations between the pharmacokinetics of repaglinide and single nucleotide polymorphisms (SNPs) in the genes encoding for the drug transporters organic anion transporting polypeptide 1B1 (OATP1B1) (SLCO1B1 ) and P-glycoprotein ( MDR1 , ABCB1 ) and the drug-metabolizing enzymes cytochrome P450 (CYP) 2C8 and CYP3A5. METHODS: A total of 56 healthy volunteers ingested a single 0.25-mg dose of repaglinide. Plasma repaglinide and blood glucose concentrations were measured for up to 7 hours. All subjects were genotyped for the -11187G>A and 521T>C SNPs in SLCO1B1 and the 3435C>T and 2677G>T/A SNPs in ABCB1 , as well as for the CYP2C8*3 (416G>A, 1196A>G), CYP2C8*4 (792C>G), and CYP3A5*3 (6986A>G) alleles. RESULTS: The area under the plasma concentration-time curve from time 0 to infinity [AUC(0-infinity)] and peak concentration in plasma (Cmax) of repaglinide varied 16.9-fold and 10.7-fold, respectively, between individual subjects. Multiple regression analyses indicated that the SLCO1B1 521T>C SNP and the CYP2C8*3 allele were independent predictors of the AUC(0-infinity) and Cmax of repaglinide (adjusted multiple R2 = 45% and 36%, respectively). In subjects with the SLCO1B1 521CC genotype, the AUC(0-infinity) of repaglinide was 107% and 188% higher, respectively, than in subjects with the SLCO1B1 521TC or 521TT (reference) genotype (P < .0001). In subjects with the CYP2C8*1/*3 genotype, the AUC(0-infinity) and Cmax of repaglinide were 48% and 44% lower, respectively, than in those with the CYP2C8*1/*1 genotype (P < .05). The pharmacokinetics of repaglinide was not associated with the studied ABCB1 SNPs or the CYP3A5*3 allele. The elimination half-life of repaglinide was not associated with any SNP. Only the SLCO1B1 -11187GA genotype was significantly associated with an enhanced effect of repaglinide on blood glucose. CONCLUSIONS: Genetic polymorphism in SLCO1B1 is a major determinant of interindividual variability in the pharmacokinetics of repaglinide. The effect of SLCO1B1 polymorphism on the pharmacokinetics of repaglinide may be clinically important.     

Inhibition of cytochrome P4502D6 activity with paroxetine normalizes the ultrarapid metabolizer phenotype as measured by nortriptyline pharmacokinetics and the debrisoquin test.

BACKGROUND: The ultrarapid metabolizer phenotype of the cytochrome P4502D6 (CYP2D6) enzyme has been considered a relevant cause of nonresponse to antidepressant drug therapy. Prescribing high doses of antidepressants to such patients leads to high concentrations of potentially toxic metabolites and an increased risk for adverse reactions. Normalization of the metabolic status of ultrarapid metabolizers by inhibition of CYP2D6 activity could offer a clinically acceptable method to successfully treat such patients with antidepressants. METHODS: Five ultrarapid metabolizers with a CYP2D6 gene duplication or triplication were treated with 25 mg nortriptyline twice a day for 3 consecutive weeks, alone during the first week and concomitantly with the CYP2D6 inhibitor paroxetine 10 mg or 20 mg twice a day, respectively, during the second and third weeks. After the third week, nortriptyline was discontinued and the subjects were treated with paroxetine 20 mg twice a day during the fourth study week. At the end of each study week, the steady-state pharmacokinetic parameters of nortriptyline or paroxetine were determined within the dose interval. In addition, the CYP2D6 phenotype was determined by debrisoquin (INN, debrisoquine) test at baseline and at the end of each study phase. Treatment-related adverse events were recorded during drug administration and for 1 week thereafter. RESULTS: All 5 subjects had very low (subtherapeutic) nortriptyline concentrations after 7 days' treatment with nortriptyline only. Addition of paroxetine 10 mg twice a day to the nortriptyline regimen resulted in a change in all individuals to the "normal" extensive debrisoquine metabolizer phenotype, and therapeutic plasma nortriptyline concentrations were achieved in 4 of 5 subjects after a 3 times mean increase in nortriptyline trough concentration (P =.0011). Doubling the paroxetine dose caused a 15 times mean increase in paroxetine trough concentration (P <.001), indicating strong inhibition by paroxetine of its own metabolism. The high paroxetine concentrations in 2 subjects caused them to have the poor debrisoquine metabolizer phenotype and resulted in a further increase in plasma nortriptyline trough concentration (P =.0099). A strong correlation (rank correlation coefficient [r(s)] = 0.89; P <.0001) was observed between paroxetine and nortriptyline trough concentrations. Paroxetine also significantly decreased the fluctuation of nortriptyline concentrations within the dose interval. One subject discontinued the study after the second study week because of adverse effects; otherwise, the study drugs were well tolerated. CONCLUSIONS: Paroxetine, with a daily dosage from 20 to 40 mg, is an effective tool in normalizing the metabolic status of CYP2D6 ultrarapid metabolizers.     

Relationship of paroxetine disposition to metoprolol metabolic ratio and CYP2D6*10 genotype of Korean subjects

OBJECTIVE: To evaluate the relationship between the metabolic ratio (MR) of metoprolol, CYP2D6*10B genotype, and the disposition of paroxetine in Korean subjects. METHODS: A single 40-mg dose of paroxetine was administered orally to one poor metabolizer and 15 healthy subjects recruited from 223 Korean extensive metabolizers whose phenotypes were predetermined by use of the metoprolol MR. Genotypes were determined by allele-specific polymerase chain reaction and the GeneChip microarray technique. Pharmacokinetic parameters were estimated from plasma concentrations of paroxetine for more than 240 hours after the oral dose. RESULTS: The oral clearance and area under the plasma concentration versus time curve (AUC) of paroxetine were best described by a nonlinear relationship with metoprolol MR at correlation coefficients of 0.82 and 0.91, respectively (P < .05). Nine extensive metabolizer who were either homozygous or heterozygous for CYP2D6*10B had significantly lower oral clearance values of paroxetine than six extensive metabolizers with CYP2D6*1/*1. The AUC of paroxetine in subjects who were homozygous for CYP2D6*10B (666.4 +/- 169.4 ng/mL x h) was significantly greater than that of subjects who were homozygous for the wild type (194.5 +/- 55.9 ng/mL x h). Unexpectedly, the average AUC of subjects who were heterozygous for CYP2D6*10B was greater with wide variation (789.8 +/- 816.9 ng/mL x h) than that of subjects who were homozygous CYP2D6*10B/*10B mainly because of two atypical subjects whose metoprolol MR was not associated with the CYP2D6*10B genotype and who showed greater AUC and lower oral clearance than subjects with homozygous CYP2D6*10B. CONCLUSIONS: The CYP2D6 activity measured by metoprolol MR was a strong predictor of paroxetine disposition in Korean extensive metabolizers. In general, the extensive metabolizers with the CYP2D6*10B allele seemed to have higher plasma concentrations of paroxetine than extensive metabolizers with the wild-type CYP2D6 genotype. However, quantitative prediction of paroxetine disposition from the CYP2D6*10B genotype alone was not perfect because several Korean extensive metabolizers had metoprolol MRs that were not associated with the genotype.     

Influence of Genetic Polymorphisms on the Response to Tramadol,Ibuprofen, and the Combination in Patients With Moderate to Severe Pain After Dental Surgery

PurposeWe aimed to elucidate the influence on analgesic effect of genetic polymorphisms in enzymes responsible for biotransformation of tramadol and ibuprofen or other possible genes involved in their mechanism of action.MethodsThe study population comprised 118 patients from a multicenter, randomized, double-blind, placebo-controlled, Phase III clinical trial that assessed the analgesic efficacy and tolerability of a single dose of ibuprofen (arginine)/tramadol 400/37.5 mg compared with ibuprofen arginine 400 mg alone, tramadol 50 mg alone, and placebo in patients with moderate to severe pain after dental surgery. We analyzed 32 polymorphisms in the cytochrome P450 (CYP) enzymes COMT, ABCB1, SLC22A1, OPRM1, and SLC22A1.FindingsWe did not find any statistically significant difference among CYP2C9 phenotypes related to ibuprofen response, although CYP2C9 poor metabolizers had a longer effect (higher pain relief at 6 hours). Likewise, we did not find any statistically significant difference among PTGS2 genotypes, contradicting previously publications.ImplicationsThere was not a clear effect of CYP2D6 phenotype on tramadol response, although CYP2D6 poor metabolizers had a slower analgesic effect. Concerning the transport of CYP2D6, we observed a better response in individuals carrying ABCB1 mutated alleles, which might correlate with higher tramadol plasma levels. Finally, we found a statistically significant better response in patients carrying the OPRM1 A118G G allele, which contradicts the previous reports. Measuring the active metabolite O-desmethyl-tramadol formation would be of great importance to better evaluate this association because O-desmethyl-tramadol has a higher μ-opioid receptor affinity compared with the parent drug. EudraCT.ema.europa.eu identifier: 2013-004637-33.