Impact of CYP2D6, CYP3A5, CYP2C9 and CYP2C19 polymorphisms on tamoxifen pharmacokinetics in Asian breast cancer patients

AIM

To investigate the impact of genetic polymorphisms in CYP2D6, CYP3A5, CYP2C9 and CYP2C19 on the pharmacokinetics of tamoxifen and its metabolites in Asian breast cancer patients.

METHODS

A total of 165 Asian breast cancer patients receiving 20 mg tamoxifen daily and 228 healthy Asian subjects (Chinese, Malay and Indian; n = 76 each) were recruited. The steady-state plasma concentrations of tamoxifen and its metabolites were quantified using high-performance liquid chromatography. The CYP2D6 polymorphisms were genotyped using the INFINITI™ CYP450 2D6I assay, while the polymorphisms in CYP3A5, CYP2C9 and CYP2C19 were determined via direct sequencing.

RESULTS

The polymorphisms, CYP2D6*5 and *10, were significantly associated with lower endoxifen and higher N-desmethyltamoxifen (NDM) concentrations. Patients who were *1/*1 carriers exhibited 2.4- to 2.6-fold higher endoxifen concentrations and 1.9- to 2.1-fold lower NDM concentrations than either *10/*10 or *5/*10 carriers (P < 0.001). Similarly, the endoxifen concentrations were found to be 1.8- to 2.6-times higher in *1/*5 or *1/*10 carriers compared with *10/*10 and *5/*10 carriers (P≤ 0.001). Similar relationships were observed between the CYP2D6 polymorphisms and metabolic ratios of tamoxifen and its metabolites. No significant associations were observed with regards to the polymorphisms in CYP3A5, CYP2C9 and CYP2C19.

CONCLUSIONS

The present study in Asian breast cancer patients showed that CYP2D6*5/*10 and *10/*10 genotypes are associated with significantly lower concentrations of the active metabolite of tamoxifen, endoxifen. Identifying such patients before the start of treatment may be useful in optimizing therapy with tamoxifen. The role of CYP3A5, CYP2C9 and CYP2C19 seem to be minor.     

Impact of genetic factors (VKORC1, CYP2C9, CYP4F2 and EPHX1) on the anticoagulation response to fluindione

AIM

Genetic variants of the enzyme that metabolizes warfarin, cytochrome P-450 2C9 (CYP2C9) and of a key pharmacologic target of vitamin K antagonists, vitamin K epoxide reductase (VKORC1), contribute to differences in patients'' responses to coumarin derivatives. The role of these variants in fluindione response is unknown. Our aim was to assess whether genetic factors contribute to the variability in the response to fluindione.

METHODS

Four hundred sixty-five patients with a venous thromboembolic event treated by fluindione for at least 3 months with a target international normalized ratio (INR) of 2.0 to 3.0 were studied. VKORC1, CYP2C9, CYP4F2 and EPHX1 genotypes were assessed. INR checks, fluindione doses and bleeding events were collected.

RESULTS

VKORC1 genotype had a significant impact on early anticoagulation (INR value ≥2 after the first two intakes) (P < 0.0001), on the time required to reach a first INR within the therapeutic range (P < 0.0001) and on the time to obtain a first INR value > 4 (P = 0.0002). The average daily dose of fluindione during the first period of stability was significantly associated with the VKORC1 genotype: 19.8 mg (±5.5) for VKORC1 CC, 14.7 mg (±6.2) for VKORC1 CT and 8.2 mg (±2.5) for VKORC1 TT (P < 0.0001). CYP2C9, CYP4F2 and EPHX1 genotypes did not significantly influence the response to fluindione.

CONCLUSIONS

VKORC1 genotype strongly affected anticoagulation induced by fluindione whereas CYP2C9, CYP4F2 and EPHX1 genotypes seemed less determining.     

Expansion of a PBPK model to predict disposition in pregnant women of drugs cleared via multiple CYP enzymes,including CYP2B6, CYP2C9 and CYP2C19

Aim

Conducting PK studies in pregnant women is challenging. Therefore, we asked if a physiologically-based pharmacokinetic (PBPK) model could be used to predict the disposition in pregnant women of drugs cleared by multiple CYP enzymes.

Methods

We expanded and verified our previously published pregnancy PBPK model by incorporating hepatic CYP2B6 induction (based on in vitro data), CYP2C9 induction (based on phenytoin PK) and CYP2C19 suppression (based on proguanil PK), into the model. This model accounted for gestational age-dependent changes in maternal physiology and hepatic CYP3A, CYP1A2 and CYP2D6 activity. For verification, the pregnancy-related changes in the disposition of methadone (cleared by CYP2B6, 3A and 2C19) and glyburide (cleared by CYP3A, 2C9 and 2C19) were predicted.

Results

Predicted mean post-partum to second trimester (PP : T₂) ratios of methadone AUC, C_max and C_min were 1.9, 1.7 and 2.0, vs. observed values 2.0, 2.0 and 2.6, respectively. Predicted mean post-partum to third trimester (PP : T₃) ratios of methadone AUC, C_max and C_min were 2.1, 2.0 and 2.4, vs. observed values 1.7, 1.7 and 1.8, respectively. Predicted PP : T₃ ratios of glyburide AUC, C_max and C_min were 2.6, 2.2 and 7.0 vs. observed values 2.1, 2.2 and 3.2, respectively.

Conclusions

Our PBPK model integrating prior physiological knowledge, in vitro and in vivo data, allowed successful prediction of methadone and glyburide disposition during pregnancy. We propose this expanded PBPK model can be used to evaluate different dosing scenarios, during pregnancy, of drugs cleared by single or multiple CYP enzymes.     

Influence of CYP2B6 and ABCB1 SNPs on nevirapine plasma concentrations in Burundese HIV-positive patients using dried sample spot devices

AIMS

The pharmacokinetics (PK) and pharmacogenetics (PG) of nevirapine have been studied in rich and limited-resource countries. CYP2B6 single nucleotide polymorphisms (SNPs) have been associated with decreased drug clearance. We evaluated the PG determinants of nevirapine trough concentrations in a rural cohort in Burundi using easy to store and transport dried sample spot devices.

METHODS

A cross-sectional analysis in HIV-positive nevirapine-treated patients in Kiremba, north of Burundi, was performed in 2009. After blood withdrawal whole blood was stored on dried blood spots and plasma (after centrifugation) was placed on dried plasma spot devices and stored at room temperature. Nevirapine plasma and dried sample spot concentrations were compared to test the clinical usefulness of this method. SNPs in CYP2B6 and ABCB1 (using a real time PCR technique) were analyzed and associated with nevirapine plasma trough concentrations.

RESULTS

Nevirapine concentrations measured on dried plasma spot devices were highly related to plasma concentrations in 60 patients, although a negative bias was observed (−18%). Nevirapine trough concentrations were above the target concentration (3000 ng ml⁻¹) in 84% of patients and they were associated with CYP2B6 SNPs (both at position 516 and 983). No effect of ABCB1 SNPs was noted.

CONCLUSIONS

Dried plasma spot devices are accurate tools for measuring nevirapine concentrations in rural settings where refrigeration is not available, despite a moderate underestimation bias. They allowed the evaluation of nevirapine concentrations in a cohort of HIV-infected people in rural Burundi, confirming very good exposure and correlation with PG polymorphisms in the CYP2B6 encoding gene.     

Pharmacogenetic markers of CYP2B6 associated with efavirenz plasma concentrations in HIV-1 infected Thai adults

Aims

To investigate the frequency of CYP2B6 polymorphisms and the influence of haplotype structure on plasma efavirenz concentrations in Thai adults with HIV-1 infection.

Methods

Genotyping of nine single nucleotide polymorphisms (SNPs, c.64C>T, c.499C>G, c.516G>T, c.785A>G, c.1375A>G, c.1459C>T, g.3003T>C, g.18492C>T and g.21563C>T) of CYP2B6 were performed using real-time PCR-based allelic discrimination on blood samples from 52 HIV-infected adults who had received an efavirenz-based regimen. Plasma efavirenz concentrations were measured by high performance liquid chromatography.

Results

The minor allele frequencies for c.64C>T, c.516G>T, c.785A>G, g.3003C>T, g.18492T>C and g.21563C>T were 0.087, 0.365, 0.413, 0.308 and 0.356, respectively. However, no variant alleles were identified for three SNPs (c.499 C>G, c.1375 A>G and c.1459 C>T). Efavirenz plasma concentrations were significantly associated with c.516G>T (P= 0.0095), c.785A>G (P= 0.0017), g.21563C>T (P= 0.0036) and g.18492C>T (P= 0.0011). The composite CYP2B6 of three SNPs (c.516G ≥ T, c.785A ≥ G and g.21563C ≥ T) genotypes were significantly associated with higher efavirenz concentrations.

Conclusions

Our data indicate that the GAC-CYP2B6 haplotype is associated with higher plasma efavirenz concentrations in HIV-infected Thai adults.     

CYP4F2 rs2108622: a minor significant genetic factor of warfarin dose in Han Chinese patients with mechanical heart valve replacement

AIMS

The objective of this study was to assess the effect of the CYP4F2 on the daily stable warfarin dose requirement in Han Chinese patients with mechanical heart valve replacement (MHVR).

METHODS

From March 2007 to November 2008, 222 Han Chinese MHVR patients were recruited in our study. VKORC1 3673G>A, 5417G>T, CYP2C9*3 and CYP4F2 rs2108622 were genotyped by using the polymerase chain reaction restriction fragment length polymorphism method (PCR-RFLP). Polymorphisms of VKORC1 9041G>A were detected by direct sequencing. Multiple linear regression analysis was used to investigate the contribution of CYP4F2.

RESULTS

The CYP4F2 rs2108622 CT/TT group took a significantly higher stable warfarin dose (3.2 mg day⁻¹) than the CC group (2.9 mg day⁻¹, 95% CI 0.2, 1.0, P= 0.033). The multiple linear regression model included VKORC1 3673G>A, CYP2C9, CYP4F2 genotypes and clinical characteristics. The model could explain 56.1% of the variance in stable warfarin dose in Han Chinese patients with MHVR. CYP4F2 contributed about 4% to the variance in the warfarin dose. There was no variation in the SNPs of VKORC1 5417G>T.

CONCLUSION

CYP4F2 is a minor significant factor of individual variability in the stable warfarin dose in Han Chinese patients with MHVR. The effect of CYP2C9 and VKORC1 genotypes on variability in the stable warfarin dose had also been confirmed.     

A haplotype of CYP2C9 associated with warfarin sensitivity in mechanical heart valve replacement patients

AIMS

The objectives of this study were to determine the distribution of CYP2C9 variants in Koreans and investigate their association with warfarin dose requirements in patients who received MHVRs.

METHODS

All nine exons, intron–exon junction, and promoter region of CYP2C9 were amplified and directly sequenced in 50 healthy normal Koreans. Additional direct DNA sequencing of the CYP2C9 gene was conducted in 36 of the 267 MHVR patients who required low maintenance warfarin doses without carrying CYP2C9*3 and VKORC1 1173T mutations. The effects of CYP2C9 genetics on warfarin maintenance dose were assessed in 267 MHVR patients.

RESULTS

Thirty-nine single nucleotide polymorphisms (SNPs) including seven previously unidentified SNPs were identified in 50 Koreans by direct DNA sequencing. One of the CYP2C9 haplotypes exhibited an association with warfarin low dose requirement. The adjusted odds ratio for the haplotype between the low dose group and the normal subjects was 2.5 (95% confidence interval 1.05, 6.16). This haplotype consisting of -1565C>T, -1188T>C, IVS3+197G>A, IVS3-334C>T, IVS3-65G>C, IVS4-115A>G, and IVS5-73A>G was found in 15% of 36 MHVR patients who required low warfarin doses, while 4% of 50 normal healthy subjects exhibited this haplotype. One of the SNPs comprising this haplotype, -1565C>T, apparently changed a protein binding pattern as observed in electrophoretic mobility shift assay.

CONCLUSION

The haplotype including -1565C>T, -1188T>C, IVS3+197G>A, IVS3-334C>T, IVS3-65G>C, IVS4-115A>G, and IVS5-73A>G seems to be associated with low warfarin dose requirement and this haplotype could be considered in the development of a warfarin dose prediction model for Asian populations.     

Inhibition of CYP2D6-mediated tramadol O-demethylation in methadone but not buprenorphine maintenance patients

AIMS

To compare the O- (CYP2D6 mediated) and N- (CYP3A4 mediated) demethylation metabolism of tramadol between methadone and buprenorphine maintained CYP2D6 extensive metabolizer subjects.

METHODS

Nine methadone and seven buprenorphine maintained subjects received a single 100 mg dose of tramadol hydrochloride. Blood was collected at 4 h and assayed for tramadol, methadone, buprenorphine and norbuprenorphine (where appropriate) and all urine over 4 h was assayed for tramadol and its M1 and M2 metabolites.

RESULTS

The urinary metabolic ratio [median (range)] for O-demethylation (M1) was significantly lower (P= 0.0002, probability score 1.0) in the subjects taking methadone [0.071 (0.012–0.103)] compared with those taking buprenorphine [0.192 (0.108–0.392)], but there was no significant difference (P= 0.21, probability score 0.69) in N-demethylation (M2). The percentage of dose [median (range)] recovered as M1 was significantly lower in subjects taking methadone compared with buprenorphine (0.069 (0.044–0.093) and 0.126 (0.069–0.187), respectively, P= 0.04, probability score 0.19), M2 was significantly higher in subjects taking methadone compared with buprenorphine (0.048 (0.033–0.085) and 0.033 (0.014–0.049), respectively, P= 0.04, probability score 0.81). Tramadol was similar (0.901 (0.635–1.30) and 0.685 (0.347–1.04), respectively, P= 0.35, probability score 0.65).

CONCLUSIONS

Methadone inhibited the CYP2D6-mediated metabolism of tramadol to M1. Hence, as the degree of opioid analgesia is largely dependent on M1 formation, methadone maintenance patients may not receive adequate analgesia from oral tramadol.     

CYP2B6 (c.516G→T) and CYP2A6 (*9B and/or *17) polymorphisms are independent predictors of efavirenz plasma concentrations in HIV-infected patients

AIMS

Interindividual variability in efavirenz pharmacokinetics is not entirely explained by the well-recognized CYP2B6 516G→T single nucleotide polymorphism. The aim of this study was to determine whether polymorphisms in the CYP2A6 gene can be used to enhance the predictability of efavirenz concentrations in human immunodeficiency virus (HIV)-infected native African patients.

METHODS

Mid-dose efavirenz plasma concentrations were determined at 4 and 8 weeks following initiation of antiretroviral therapy in 65 HIV-infected Ghanaian patients. Selected CYP2B6 and CYP2A6 genotypes were determined by commercial 5′-nuclease assays. Relationships between averaged 4- and 8-week mid-dose efavirenz concentrations, demographic variables and genotypes were evaluated by univariate and multivariate statistical approaches including gene–gene interactions.

RESULTS

CYP2B6 c.516G→T, CYP2B6 c.983T→C, CYP2A6*9B and CYP2A6*17 allele frequencies were 45, 4, 5 and 12%, respectively. Rifampicin therapy, gender, age and body mass index had no significant influence on efavirenz mid-dose concentrations. Median efavirenz concentrations were more than five times higher (P < 0.001) in patients with CYP2B6 c.516TT genotype compared with GG and GT genotypes. Although none of the CYP2A6 genotypes was associated with altered efavirenz concentrations individually, CYP2A6*9B and/or CYP2A6*17 carriers showed a 1.8 times higher median efavirenz concentration (P= 0.017) compared with noncarriers. Multiple linear regression analysis indicated that the CYP2B6 c.516G→T polymorphism and CYP2A6 slow-metabolizing variants accounted for as much as 36 and 12% of the total variance in efavirenz concentrations, respectively.

CONCLUSIONS

Our findings support previous work showing efavirenz oxidation by CYP2A6, and suggest that both CYP2A6 and CYP2B6 genotyping may be useful for predicting efavirenz plasma concentrations.     

Pharmacokinetic assessment of a five-probe cocktail for CYPs 1A2, 2C9, 2C19, 2D6 and 3A

AIMS

To assess the pharmacokinetics (PK) of selective substrates of CYP1A2 (caffeine), CYP2C9 (S-warfarin), CYP2C19 (omeprazole), CYP2D6 (metoprolol) and CYP3A (midazolam) when administered orally and concurrently as a cocktail relative to the drugs administered alone.

METHODS

This was an open-label, single-dose, randomized, six-treatment six-period six-sequence William''s design study with a wash-out of 7 or 14 days. Thirty healthy male subjects received 100 mg caffeine, 100 mg metoprolol, 0.03 mg kg⁻¹ midazolam, 20 mg omeprazole and 10 mg warfarin individually and in combination (cocktail). Poor metabolizers of CYP2C9, 2C19 and 2D6 were excluded. Plasma samples were obtained up to 48 h for caffeine, metoprolol and omeprazole, 12 h for midazolam, 312 h for warfarin and the cocktail. Three different validated liquid chromatography tandem mass spectrometry methods were used. Noncompartmental PK parameters were calculated. Log-transformed C_max, AUC_last and AUC for each analyte were analysed with a linear mixed effects model with fixed term for treatment, sequence and period, and random term for subject within sequence. Point estimates (90% CI) for treatment ratios (individual/cocktail) were computed for each analyte C_max, AUC_last and AUC.

RESULTS

There was no PK interaction between the probe drugs when administered in combination as a cocktail, relative to the probes administered alone, as the 90% CI of the PK parameters was within the prespecified bioequivalence limits of 0.80, 1.25.

CONCLUSION

The lack of interaction between probes indicates that this cocktail could be used to evaluate the potential for multiple drug–drug interactions in vivo.     

Exposure to oral oxycodone is increased by concomitant inhibition of CYP2D6 and 3A4 pathways, but not by inhibition of CYP2D6 alone

AIM

The aim of this study was to find out whether the inhibition of cytochrome P450 2D6 (CYP2D6) with paroxetine or concomitant inhibition of CYP2D6 and CYP3A4 with paroxetine and itraconazole, altered the pharmacokinetics and pharmacological response of orally administered oxycodone.

METHODS

A randomized placebo-controlled cross-over study design with three phases was used. Eleven healthy subjects ingested 10 mg of oral immediate release oxycodone on the fourth day of pre-treatment with either placebo, paroxetine (20 mg once daily) or paroxetine (20 mg once daily) and itraconazole (200 mg once daily) for 5 days. The plasma concentrations of oxycodone and its oxidative metabolites were measured for 48 h, and pharmacological (analgesic and behavioural) effects were evaluated.

RESULTS

Paroxetine alone reduced the area under concentration–time curve (AUC(0,0–48 h)) of the CYP2D6 dependent metabolite oxymorphone by 44% (P < 0.05), but had no significant effects on the plasma concentrations of oxycodone or its pharmacological effects when compared with the placebo phase. When both oxidative pathways of the metabolism of oxycodone were inhibited with paroxetine and itraconazole, the mean AUC(0,∞) of oxycodone increased by 2.9-fold (P < 0.001), and its C_max by 1.8-fold (P < 0.001). Visual analogue scores for subjective drug effects, drowsiness and deterioration of performance were slightly increased (P < 0.05) after paroxetine + itraconazole pre-treatment when compared with placebo.

CONCLUSIONS

Drug interactions arising from CYP2D6 inhibition most likely have minor clinical importance for oral oxycodone if the function of the CYP3A4 pathway is normal. When both CYP2D6 and CYP3A4 pathways are inhibited, the exposure to oral oxycodone is increased substantially.     

Effect of CYP2C19*2 and *17 mutations on pharmacodynamics and kinetics of proton pump inhibitors in Caucasians

AIMS

To investigate the impact of CYP2C19 mutations *2-*6 and *17 on acid-inhibition and pharmacokinetics of lansoprazole (L15), omeprazole (O10, O20) and pantoprazole (P40) in Caucasians.

METHODS

CYP2C19 genotyping for *2–*6 and *17 mutations was assessed in subjects who were H. pylori negative in two randomized crossover trials. The influence of CYP2C19 mutations on single and repeated administration of L15 and O10 (study A) and O20 and P40 (study B) was investigated. Pharmacokinetics and the cumulative percentage of time with intragastric pH above 4 (% > pH 4) were assessed on day 1 and 6.

RESULTS

For study A CYP2C19 genotyping found five *1/*1, four *1/*2, one *1/*17 and one *2/*17. For study B the results were six *1/*1, two *1/*2, six *1/*17, one *2/*2 and one *2/*17. For all PPIs AUC was highest in *2/*2 and lowest in *1/*17. On day 1, all PPIs significantly increased percentage >pH 4 compared with baseline. *1/*1 genotype showed no significant acid-inhibition after L15, O10 and O20. *1/*17 genotype showed no significant acid-inhibition after O20 and P40. *1/*2 genotype showed significant acid-inhibition after L15 and O10. On day 6, all four PPIs showed significantly increased acid-inhibition. *1/*1 and *1/*17 showed a significantly increased percentage > pH 4 after treatment with O20 and P40. However, in *1/*1 subjects percentage > pH 4 was not significantly increased after L15 and O10. *1/*2 genotype showed a significant acid-inhibitory effect after repeated dosing with L15 and O10.

CONCLUSIONS

Caucasian subjects with *1/*1 and *1/*17 genotype need stronger acid-suppression therapy, especially during the first days of treatment or with on-demand therapy.

WHAT IS ALREADY KNOWN ABOUT THIS SUBJECT

• The influence of CYP2C19 on the kinetics and dynamics of omeprazole, lansoprazole and rabeprazole has been studied in Japanese subjects.
• It has been suggested that subjects with *1/*1 genotype might need stronger acid suppression than *1/*2 and *2/*2 subjects. This suggestion comes from data in Japanese subjects and has not been confirmed in Caucasians.
• Furthermore, a novel CYP2C19 mutation, *17, which mainly occurs in Caucasians has been discovered. This mutation has been associated with clinical failure, but its relevance for therapy with PPIs has not been studied yet.

WHAT THIS STUDY ADDS

• In this study, the influence of CYP2C19 on both the pharmacokinetics and dynamics in Caucasian subjects after single and repeated dosing has been investigated.
• This is the first study showing that Caucasian subjects with *1/*1 and *1/*17 mutations need stronger acid-inhibition. In this study three proton pump inhibitors (omeprazole, lansoprazole and pantoprazole, in different doses) were studied of which pantoprazole had not been studied before in this setting, not even in Japanese.

     

Contribution of CYP2C19 and CYP3A4 to the formation of the active nortilidine from the prodrug tilidine

AIMS

To investigate in vivo the effect of the CYP2C19 genotype on the pharmacokinetics of tilidine and the contribution of CYP3A4 and CYP2C19 to the formation of nortilidine using potent CYP3A4 inhibition by ritonavir.

METHODS

Fourteen healthy volunteers (seven CYP2C19 poor and seven ultrarapid metabolizers) received ritonavir orally (300 mg twice daily) for 3 days or placebo, together with a single oral dose of tilidine and naloxone (100 mg and 4 mg, respectively). Blood samples and urine were collected for 72 h. Noncompartmental analysis was performed to determine pharmacokinetic parameters of tilidine, nortilidine, bisnortilidine and ritonavir.

RESULTS

Tilidine exposure increased sevenfold and terminal elimination half-life fivefold during ritonavir treatment, but no significant differences were observed between the CYP2C19 genotypes. During ritonavir treatment, nortilidine area under the concentration–time curve was on average doubled, with no differences between CYP2C19 poor metabolizers [2242 h ng ml⁻¹ (95% confidence interval 1811–2674) vs. 996 h ng ml⁻¹ (95% confidence interval 872–1119)] and ultrarapid metabolizers [2074 h ng ml⁻¹ (95% confidence interval 1353–2795) vs. 1059 h ng ml⁻¹ (95% confidence interval 789–1330)]. The plasma concentration–time curve of the secondary metabolite, bisnortilidine, showed a threefold increase of time to reach maximal observed plasma concentration; however, area under the concentration–time curve was not altered by ritonavir.

CONCLUSIONS

The sequential metabolism of tilidine is inhibited by the potent CYP3A4 inhibitor, ritonavir, independent of the CYP2C19 genotype, with a twofold increase in the exposure of the active nortilidine.     

Pharmacogenetics of CYP2C19: functional and clinical implications of a new variant CYP2C19*17

AIMS

Cytochrome P450 2C19 metabolizes many important drugs. In 2006, a variant allele (CYP2C19*17) associated with increased activity was discovered, but its likely clinical significance is controversial. Investigators disagree about the phenotype to be assigned to the two CYP2C19*17 genotypes. The aim of this study was to provide a critical summary, helpful to prescribers.

METHODS

We searched MEDLINE for papers on the allele from 2006 and then undertook historical searches through the reference lists of papers retrieved. The relevant information was critically assessed and summarized.

RESULTS

CYP2C19*17 was associated with increased enzymic activity. Substrates studied were omeprazole, pantoprazole, escitalopram, sertraline, voriconazole, tamoxifen and clopidogrel. Most studies used pharmacokinetic variables as outcome measure. For clopidogrel, activated by CYP2C19, pharmacodynamic consequences focused on platelet aggregation. While for most pharmacokinetic parameters of the substrates studied the average value was altered, the range of values showed mostly complete overlap for CYP2C19*1/*17 heterozygotes and wild-type homozygotes. Even for CYP2C19*17 homozygotes, the absolute effect was modest compared with the effect of previously identified loss-of-function alleles. In Helicobacter pylori eradication CYP2C19*2 carriage was associated with an altered eradication rate (odds ratio 4.20, 95% confidence interval 1.23, 16.44) relative to the wild-type, but CYP2C19*17 homozygosity was not. Prevalence of the variant allele was typically <5% in Asians and about four times higher in White and African populations.

CONCLUSIONS

Assignment of CYP2C19*17 homozygotes as extensive metabolizers rather than ultrarapid metabolizers is adequate. CYP2C19*17 genotyping is unlikely to have clinical utility except for drugs with very narrow therapeutic indices.     

Effect of CYP2C19 polymorphism on nelfinavir to M8 biotransformation in HIV patients

Aims

To evaluate the effect of CYP2C19 polymorphism on nelfinavir and M8 pharmacokinetic variability in human immunodeficiency virus-infected patients and to study the link between pharmacokinetic exposure and short-term efficacy and toxicity.

Methods

Nelfinavir (n = 120) and M8 (n = 119) concentrations were measured in 34 protease inhibitor-naïve patients. Two weeks after initiating the treatment, blood samples were taken before, 1, 3 and 6 h after drug administration. Genotyping for CYP3A4, 3A5, 2C19 and MDR1 was performed. A population pharmacokinetic model was developed to describe nelfinavir-M8 concentration time-courses and to estimate interpatient variability. The influence of individual characteristics and genotypes were tested using a likelihood ratio test. Estimated mean (C_mean), maximal (C_max) and trough (C_trough) nelfinavir and M8 concentrations were correlated to short-term virological efficacy and tolerance using Spearman nonparametric correlation tests.

Results

A one-compartment model with first-order absorption, elimination and metabolism to M8 best described nelfinavir data. M8 was modelled by an additional compartment. Mean pharmacokinetic estimates and the corresponding intersubject variabilities were: absorption rate 0.17 h⁻¹ (99%), absorption lag time 0.82 h, apparent nelfinavir total clearance 52 l h⁻¹ (49%), apparent nelfinavir volume of distribution 191 l, M8 elimination rate constant 1.76 h⁻¹ and nelfinavir to M8 0.39 h⁻¹ (59%) in *1/*1 patients and 0.20 h⁻¹ in *1/*2 or *2/*2 patients for CYP2C19*2. Nelfinavir C_mean was positively correlated to glycaemia and triglyceride increases (P = 0.02 and P = 0.04, respectively).

Conclusions

The rate of metabolism of nelfinavir to M8 was reduced by 50% in patients with *1/*2 or *2/*2 genotype for CYP2C19 compared with those with *1/*1 genotype.

What is already known about this subject

• Nelfinavir is an HIV protease inhibitor, substrate of the transporter P-glycoprotein and metabolized via CYP2C19, CYP3A4 and CYP3A5 enzymes.
• Pharmacokinetic studies have shown wide interindividual variability of nelfinavir concentrations, some of this variability perhaps caused by variant drug metabolism or transporter genes.
• For CYP3A4*1B and CYP3A5*3 polymorphism, results from three studies are in agreement, showing no difference in nelfinavir concentrations between patients with these different genotypes.
• However, for MDR1 and CYP2C19 polymorphism, there have been contradictory studies, showing either no impact on nelfinavir concentration or modified concentrations which could influence virological response.

What this study adds

• Patients with an *1/*2 or *2/*2 genotype for CYP2C19 had a nelfinavir to M8 biotransformation divided by 2 compared with *1/*1 patients.
• No evidence of any influence of MDR1 polymorphism on nelfinavir absorption could be detected.

     

Influences of different proton pump inhibitors on the anti-platelet function of clopidogrel in relation to CYP2C19 genotypes

AIMS

The efficacy of clopidogrel is influenced by CYP2C19 genotypes and substrates of CYP2C19, such as proton pump inhibitors (PPIs). We assessed the influence of three different PPIs on the anti-platelet function of clopidogrel in relation to CYP2C19 genotype status.

METHODS

Thirty-nine healthy volunteers with different CYP2C19 genotypes took clopidogrel 75 mg with or without omeprazole 20 mg, lansoprazole 30 mg or rabeprazole 20 mg in the morning for 7 days. The influence of the three PPIs on the anti-platelet function of clopidogrel was determined. A less than 30% inhibition of platelet aggregation (IPA) during clopidogrel dosing was defined as a ‘low responder’. We also examined whether evening dosing of omeprazole could prevent the interaction with clopidogrel dosed in the morning.

RESULTS

In rapid metabolizers (RMs, *1/*1, n = 15) of CYP2C19, omeprazole and rabeprazole significantly attenuated the anti-platelet function of clopidogrel. In decreased metabolizers (DMs, carriers of *2 and/or *3, n = 24), there was a large variation in IPA and there was a trend but no significant decrease in IPA when placed on a concomitant PPI. Some DMs became ‘low-responders’ when placed on a concomitant PPI. Evening omeprazole dose in RMs did not seem to cause a significant decrease in IPA in contrast to morning dosing, but did so in DMs.

CONCLUSIONS

The three PPIs affected the efficacy of clopidogrel to different degrees. Both omeprazole and rabeprazole significantly decreased IPA in RMs but not DMs, although there was a trend towards lower IPA in DMs. Morning and evening dosing of omeprazole were both associated with lower IPA in DMs.     

CYP2C8 but not CYP3A4 is important in the pharmacokinetics of montelukast

AIM

According to product information, montelukast is extensively metabolized by CYP3A4 and CYP2C9. However, CYP2C8 was also recently found to be involved. Our aim was to study the effects of selective CYP2C8 and CYP3A4 inhibitors on the pharmacokinetics of montelukast.

METHODS

In a randomized crossover study, 11 healthy subjects ingested gemfibrozil 600 mg, itraconazole 100 mg (first dose 200 mg) or both, or placebo twice daily for 5 days, and on day 3, 10 mg montelukast. Plasma concentrations of montelukast, gemfibrozil, itraconazole and their metabolites were measured up to 72 h.

RESULTS

The CYP2C8 inhibitor gemfibrozil increased the AUC(0,∞) of montelukast 4.3-fold and its t_1/2 2.1-fold (P < 0.001). Gemfibrozil impaired the formation of the montelukast primary metabolite M6, reduced the AUC and C_max of the secondary (major) metabolite M4 by more than 90% (P < 0.05) and increased those of M5a and M5b (P < 0.05). The CYP3A4 inhibitor itraconazole had no significant effect on the pharmacokinetic variables of montelukast or its M6 and M4 metabolites, but markedly reduced the AUC and C_max of M5a and M5b (P < 0.05). The effects of the gemfibrozil-itraconazole combination on the pharmacokinetics of montelukast did not differ from those of gemfibrozil alone.

CONCLUSIONS

CYP2C8 is the dominant enzyme in the biotransformation of montelukast in humans, accounting for about 80% of its metabolism. CYP3A4 only mediates the formation of the minor metabolite M5a/b, and is not important in the elimination of montelukast. Montelukast may serve as a safe and useful CYP2C8 probe drug.     

Prediction of tacrolimus metabolism and dosage requirements based on CYP3A4 phenotype and CYP3A5*3 genotype in Chinese renal transplant recipients

Aim:

To examine how the endogenous CYP3A4 phenotype and CYP3A5^*3 genotype of Chinese renal transplant recipients influenced the dose-corrected trough concentration (C₀/D) and weight-corrected daily dose (D/W) of tacrolimus.

Methods:

A total of 101 medically stable kidney transplant recipients were enrolled, and their blood and urine samples were gathered. The endogenous CYP3A4 phenotype was assessed by the ratio of 6β-hydroxycortisol and 6β-hydroxycortisone to cortisol and cortisone in urine. CYP3A5^*3 genotype was determined using PCR-RELP.

Results:

In overall renal transplant recipients, a multiple regression analysis including the endogenous CYP3A4 phenotype, CYP3A5^*3 genotype and post-operative period accounted for 60.1% of the variability in C₀/D ratio; a regression equation consisting of the endogenous CYP3A4 phenotype, post-operative period, body mass index, CYP3A5^*3 genotype, gender, total bilirubin and age explained 61.0% of the variability in D/W ratio. In CYP3A5^*3/^*3 subjects, a combination of the endogenous CYP3A4 phenotype, post-operative period and age was responsible for 65.3% of the variability in C₀/D ratio; a predictive equation including the endogenous CYP3A4 phenotype, post-operative period, body mass index, gender and age explained 61.2% of the variability in the D/W ratio. Base on desired target range of tacrolimus trough concentrations, individual daily dosage regimen was calculated, and all the observed daily doses were within the predicted range.

Conclusion:

This study provides the equations to predict tacrolimus metabolism and dosage requirements based on the endogenous CYP3A4 phenotype, CYP3A5^*3 genotype and other non-genetic variables.     

In silico prediction of efavirenz and rifampicin drug-drug interaction considering weight and CYP2B6 phenotype

AIMS

This study aimed to test whether a pharmacokinetic simulation model could extrapolate nonclinical drug data to predict human efavirenz exposure after single and continuous dosing as well as the effects of concomitant rifampicin and further to evaluate the weight-based dosage recommendations used to counteract the rifampicin–efavirenz interaction.

METHODS

Efavirenz pharmacokinetics were simulated using a physiologically based pharmacokinetic model implemented in the Simcyp™ population-based simulator. Physicochemical and metabolism data obtained from the literature were used as input for prediction of pharmacokinetic parameters. The model was used to simulate the effects of rifampicin on efavirenz pharmacokinetics in 400 virtual patients, taking into account bodyweight and CYP2B6 phenotype.

RESULTS

Apart from the absorption phase, the simulation model predicted efavirenz concentration–time profiles reasonably well, with close agreement with clinical data. The simulated effects of rifampicin co-administration on efavirenz treatment showed only a minor decrease of 16% (95% confidence interval 13–19) in efavirenz area under the concentration–time curve, of the same magnitude as what has been clinically observed (22%). Efavirenz exposure depended on CYP2B6 phenotype and bodyweight. Increasing the efavirenz dose during concomitant rifampicin was predicted to be most successful in patients over 50 kg regardless of CYP2B6 status.

CONCLUSIONS

Our findings, although based on a simulation approach using limited in vitro data, support the current recommendations for using a 50 kg bodyweight cut-off for efavirenz dose increment when co-treating with rifampicin.     

Effects of CYP2D6 genotypes on age-related change of flecainide metabolism: involvement of CYP1A2-mediated metabolism

AIMS

The aim of this study was to clarify the effects of CYP2D6 genotype on age-related change in flecainide metabolism in patients with supraventricular tachyarrhythmias. An in vitro study using microsomes was performed to identify other CYPs responsible for age-related change in flecainide metabolism.

METHODS

The study population comprised 111 genotyped patients: CYP2D6-homozygous extensive metabolizers (hom-EMs, n= 34), heterozygous EMs (het-EMs, n= 56), and intermediate and poor metabolizers (IMs/PMs, n= 21). Serum concentrations of flecainide and its metabolites [m-O-dealkylated flecainide (MODF) and m-O-dealkylated lactam of flecainide] were determined by use of a high-performance liquid chromatography. Metabolic ratio (MR) was expressed as serum concentrations of flecainide to its metabolites. In vitro formation of MODF was examined in human liver microsomes and cDNA-expressed CYP isoforms.

RESULTS

MR was higher in elderly patients (≥70 years) than in middle-aged patients (<70 years). The increase of MR in elderly patients differed among CYP2D6 genotypes: 1.6-fold in het-EMs [4.3, 95% confidence interval (CI) 2.8, 5.7 vs. 2.7, 95% CI 2.3, 3.1, P < 0.05], 1.5-fold in IMs/PMs (6.0, 95% CI 4.5, 7.6 vs. 4.1, 95% CI 2.9, 5.4, P < 0.05), and no change in hom-EMs. The in vitro study using microsomes revealed that both CYP2D6 and CYP1A2 were involved in the formation of MODF. MODF formation in CYP2D6 PM microsomes increased as CYP1A2 activity increased.

CONCLUSIONS

The results suggest that patients with poor CYP2D6-mediated metabolism (het-EMs and IMs/PMs) showed age-related reduction in flecainide metabolism because metabolism was taken over by CYP1A2, whose activity decreases with age.