Extensive Metabolizer Subjects Research Articles

Evaluation of the effect of CYP2D6*3, *4,*10, and *17 polymorphisms on the pharmacokinetic of tamoxifen and its metabolites in patients with hormone-positive breast cancer

Background and objectiveA high rate of interindividual variability in response to tamoxifen (TAM) in breast cancer patients with CYP2D6 polymorphism has been reported, which affects the patient’s therapeutic outcome. The objective of this study was to investigate the pharmacogenomics of CYP2D6 genotyping in Iranian patients with breast cancer treated with adjuvant TAM. MethodsA peripheral blood sample was obtained to determine the steady-state plasma concentrations of TAM and its metabolites (Endoxifen (EN) and 4-Hydroxytamoxifen (4-OHT)) using high-performance liquid chromatography with fluorescence detection (HPLC-FLU) assay. We detected CYP2D6 * 3, * 4, * 10, and * 17 single nucleotide polymorphisms via polymerase chain reaction and restriction fragment length polymorphism (PCR-RFLP) method. ResultsA total of 84 Iranian estrogen receptor‑positive breast cancer patients receiving the daily dose of 20 mg tamoxifen were recruited. Although a consequent decrease in the median EN and 4-OHT concentrations was observed by comparing poor or intermediate metabolizer patients with an extensive metabolizer population, this difference did not reach a significant level. The mean plasma EN concentrations in poor and intermediate metabolizers were 46.1% (95% CI, 7.4–27.8%) and 59.4% (95% CI, 11.9–37.3%) of extensive metabolizer subjects, respectively. Poor and intermediate metabolizers had the mean plasma 4-OHT concentrations that were 46.6% (95% CI, 0.9–61.7%) and 73.2% (95% CI, 2.7–93.1%) of those of subjects who were extensive metabolizer, respectively. ConclusionsThe possible role of genotyping in Iranian patients' response to treatment may explain inter-individual differences in the plasma concentrations of active metabolites of TAM.

The Pharmacokinetics of Gefitinib in a Chinese Cancer Population Group: A Virtual Clinical Trials Population Study.

Gefitinib, a selective inhibitor of the epidermal growth factor receptor (EGFR) tyrosine kinase, is used to treat non-small-cell lung cancer (NSCLC). Lung cancer rates are high in China and are expected to increase over the next decade. CYP 2D6 intermediate metaboliser (IM) phenotypes are more prevalent in the Chinese population compared to Caucasians; the increased risk of drug-drug interactions (DDI) with chemotherapy polypharmacy may lead to different clinical pharmacokinetics outcomes for Chinese patients. This study developed and validated a virtual Chinese cancer population for the pragmatic assessment of gefitinib DDI as a victim drug in Chinese and Caucasian cancer populations. When assessing the impact of 2D6 phenotypes on bupropion mediated CYP 2D6 DDI in Chinese cancer population, we found that AUC increased by at least 60% in extensive metabolizers (EM) and 30% in IM. As a result, fmCYP2D6 was reduced by 15% in IM in the presence of bupropion, translating into > 70% of EM subjects and > 48% of IM subjects with trough concentrations at steady state (Ctrough,ss) below the gefitinib target trough level. The PBPK model predicted that a 500mg once daily dose in both EM and IM subjects successfully reduced the percent of subjects below the Ctrough,ss. Such changes in Ctrough,ss warrant further investigation and highlight the ability of pharmacokinetic modelling to investigate populations that may be difficult to recruit for traditional clinical studies.

Effect of CYP2D6 Poor Metabolizer Phenotype on Stereoselective Nebivolol Pharmacokinetics.

Nebivolol is a drug available as a racemate of d-nebivolol (SRRR) and lnebivolol (RSSS). In human liver microsomes, CYP2D6 mainly catalyses the metabolism of lnebivolol, while CYP2C19 catalyses the metabolism of d-nebivolol. Nebivolol stereoselective pharmacokinetics has been described only for extensive metabolizers (EM). To describe the stereoseletive nebivolol pharmacokinetics in CYP2D6 poor metabolizers (PM) and to assess whether the phenotype has an impact on nebivolol pharmacokinetics. Three healthy volunteers PM phenotyped (ratios of 20.1, 220 and 244 for the 8 h urinary excretion of metoprolol/alfa-hydroxymetoprolol) received a single oral dose of racemic nebivolol and sequential blood samples were collected between zero (predose) and 48 h. The obtained data were compared to 22 EM subjects with normal kidney function enrolled in our previous study. For both isomers, Cmax, Tmax and AUC0-48 were significantly greater in the PM group compared to the EMs (p = 0.006 - 0.001). For d-nebivolol, Cmax, Tmax and AUC0-48 were, on average, 5.9, 2.7 and 15.0 larger in PMs. The corresponding values for l-nebivolol were 4.4, 2.7 and 17.5. The decline in plasma concentration of both nebivolol isomers in PM phenotypes, especially those with MR of 220 and 244, which indicate minimal or absent CYP2D6 activity, points to alternative mechanisms for nebivolol elimination. Collectively, our results are the first to show the significant impact of CYP2D6 PM phenotype on the metabolic disposition and in vivo exposure to both nebivolol isomers.

Biosynthesis and Identification of Metabolites of Maraviroc and Their Use in Experiments to Delineate the Relative Contributions of Cytochrome P4503A4 versus 3A5

Maraviroc (MVC) is a CCR5 coreceptor antagonist indicated in combination with other antiretroviral agents for the treatment of CCR5-tropic human immunodefinciency virus-1 infection. In this study, the metabolism of MVC was investigated in human liver microsomes to delineate the relative roles of CYP3A4 and CYP3A5. MVC is metabolized to five hydroxylated metabolites, all of which were biosynthesized and identified using mass and NMR spectroscopy. The sites of metabolism were the 2- and 3-positions of the 4,4-difluorocyclohexyl moiety and the methyl of the triazole moiety. Absolute configurations were ultimately ascertained by comparison to authentic standards. The biosynthesized metabolites were used for quantitative in vitro experiments in liver microsomes using cyp3cide, a selective inactivator of CYP3A4. (1S,2S)-2-OH-MVC was the main metabolite representing approximately half of the total metabolism, and CYP3A5 contributed approximately 40% to that pathway in microsomes from CYP3A5*1/*1 donors. The other four metabolites were almost exclusively metabolized by CYP3A4. (1S,2S)-2-hydroxylation also correlated to T-5 N-oxidation, a CYP3A5-specific activity. These data are consistent with clinical pharmacokinetic data wherein CYP3A5 extensive metabolizer subjects showed a modestly lower exposure to MVC.

CYP2C19 Genotype-Dependent Pharmacokinetic Drug Interaction Between Voriconazole and Ritonavir-Boosted Atazanavir in Healthy Subjects.

Voriconazole, a broad-spectrum triazole antifungal agent, is metabolized by cytochrome P450 (CYP) 2C19 and, to a lesser extent, by CYP3A. Genetic polymorphism of CYP2C19 not only plays a prominent role in its disposition but may also influence potential drug interactions with CYP450 modulators such as ritonavir. This study assessed 2-way drug interactions of voriconazole added on to ritonavir-boosted atazanavir in both CYP2C19 extensive-metabolizer (EM) and poor-metabolizer (PM) healthy subjects. Each subject received voriconazole alone on days 1-3, followed by a 7-day washout. Atazanavir/ritonavir 300/100mg once daily was given on days 11-30 and voriconazole on days 21-30. Voriconazole doses were 200mg (400mg on days 1 and 21) twice daily and 50mg (100mg on days 1 and 21) twice daily for CYP2C19 EM and PM subjects, respectively. On coadministration, voriconazole AUC and Cmin decreased by 33% (90%CI, 22%-42%) and 39% (90%CI, 28%-49%), respectively, in CYP2C19 EMs, whereas voriconazole Cmax and AUC increased 4.4-fold (90%CI, 3.6-fold to 5.4-fold) and 5.6-fold (90%CI, 4.5-fold to 7.0-fold), respectively, in PMs. Adding voriconazole resulted in a 20%-30% decrease in atazanavir Cmin in both EMs and PMs. Ritonavir exposure was generally unchanged in either population. The safety and tolerability profiles of the combination were comparable with atazanavir/ritonavir and voriconazole administered alone. The most frequent adverse events with voriconazole were visual disturbance and headache. Coadministration of voriconazole and atazanavir/ritonavir is not recommended unless the benefit/risk to the patient justifies the use of the combination.

The Use of In Vitro Data and Physiologically-Based Pharmacokinetic Modeling to Predict Drug Metabolite Exposure: Desipramine Exposure in Cytochrome P4502D6 Extensive and Poor Metabolizers Following Administration of Imipramine.

Major circulating drug metabolites can be as important as the drugs themselves in efficacy and safety, so establishing methods whereby exposure to major metabolites following administration of parent drug can be predicted is important. In this study, imipramine, a tricyclic antidepressant, and its major metabolite desipramine were selected as a model system to develop metabolite prediction methods. Imipramine undergoes N-demethylation to form the active metabolite desipramine, and both imipramine and desipramine are converted to hydroxylated metabolites by the polymorphic enzyme CYP2D6. The objective of the present study is to determine whether the human pharmacokinetics of desipramine following dosing of imipramine can be predicted using static and dynamic physiologically-based pharmacokinetic (PBPK) models from in vitro input data for CYP2D6 extensive metabolizer (EM) and poor metabolizer (PM) populations. The intrinsic metabolic clearances of parent drug and metabolite were estimated using human liver microsomes (CYP2D6 PM and EM) and hepatocytes. Passive diffusion clearance of desipramine, used in the estimation of availability of the metabolite, was predicted from passive permeability and hepatocyte surface area. The predicted area under the curve (AUCm/AUCp) of desipramine/imipramine was 12- to 20-fold higher in PM compared with EM subjects following i.v. or oral doses of imipramine using the static model. Moreover, the PBPK model was able to recover simultaneously plasma profiles of imipramine and desipramine in populations with different phenotypes of CYP2D6. This example suggested that mechanistic PBPK modeling combined with information obtained from in vitro studies can provide quantitative solutions to predict in vivo pharmacokinetics of drugs and major metabolites in a target human population.

The Pharmacokinetics and Pharmacodynamics of Prasugrel and Clopidogrel in Healthy Japanese Volunteers.

The dosing regimen of prasugrel adjusted for Japanese patients was compared with that of clopidogrel by analyzing the pharmacokinetics and pharmacodynamics in 40 healthy Japanese subjects in a randomized, single‐blind crossover study. In period 1, the subjects received either 300 mg clopidogrel or 20 mg prasugrel; after a >2‐week interval (period 2), the drug was switched. Blood samples of 36 of the 40 subjects were collected for analysis of pharmacokinetics, pharmacodynamics, and CYP2C19 genotypes. The plasma concentration of the active metabolite of prasugrel increased rapidly and reached its peak 30 minutes postadministration, whereas that of the active metabolite of clopidogrel reached its peak 1 hour postadministration. The mean AUC and Cmax of the active metabolite of clopidogrel, but not those of prasugrel, were CYP2C19 genotype dependent. Prasugrel rapidly inhibited platelet aggregation, reaching its maximum effect 1 hour postadministration. Clopidogrel, on the other hand, showed maximum inhibition 2 hours postadministration. Platelet aggregation inhibition by clopidogrel was significantly lower in the poor‐metabolizer subjects than in the extensive‐metabolizer subjects. Overall, prasugrel inhibited platelet aggregation more rapidly and more effectively in healthy Japanese subjects than was observed for clopidogrel.

Contribution of ABCB1 and CYP2D6 genotypes to the outcome of tamoxifen adjuvant treatment in premenopausal women with breast cancer.

Recent pre-clinical evidence suggests that the active metabolite of tamoxifen, endoxifen, is a substrate for efflux pump P-glycoprotein. The aim of our study was to evaluate, if the polymoprhisms within ABCB1 gene alter tamoxifen adjuvant treatment efficacy in premenopausal women. Totally 71 premenopausal women with estrogen receptor positive breast cancer indicated for tamoxifen adjuvant treatment were followed retrospectively for median period of 56 months. The gentic polymorphisms of CYP2D6 and ABCB1 were analyzed and potential covariates as tumor grading, staging, age at the diagnosis, comedication, quantitative positivity of ER or PR were also evaluated. Cox proportional-hazards regression model indicated that patients carrying at least one variant allele in ABCB1 rs1045642 had significantly longer time to event survival compared to wild type subjects. Non-significant trend was noted for better treatment outcome of patients carrying at least one variant allele in the SNP rs2032582, while for the CYP2D6 polymorphism poor metabolizer phenotype resulted in worse outcome in comparison to extensive metabolizers subjects with HR of 4.04 (95 % CI 0.31-52.19). Similarly, patients using CYP2D6 inhibitors had non-significantly shorter time-to-event as compared to never users resulting in hazard ratio of 2.06 (95 % CI 0.40-10.63). ABCB1 polymorphisms may affect outcome of tamoxifen adjuvant treatment in premenopausal breast cancer patiens. This factor should be taken into account in addition to the CYP2D6 polymorphism or phenotypic inhibition of CYP2D6 activity.

Evidence of Sex-related Differences in Cytochrome P450 2C19 and 3A4 Mediated Metabolism of Omeprazole: Observations in an Understudied Population in South-East Anatolia in Turkey

Sex-dependent and population differences in the activities of cytochrome P450 enzymes affect the metabolism of many xenobiotics, including drugs. The proton pump inhibitor omeprazole is widely prescribed in many countries that call for population studies of its disposition. Omeprazole is metabolized by CYP2C19 and CYP3A4, resulting in the formation of 5-hydroxyomeprazole and omeprazole sulfone, respectively. This study examined the sex-differences in CYP2C19-, and CYP3A4-mediated metabolism of omeprazole in an understudied population in Turkey. Data were obtained from 107 healthy unrelated subjects (51 females and 56 males; heterozygous and homozygous extensive metabolizers (EMs) in regard to CYP2C19) from a recent study on the relationship between CYP2C19 genotype and phenotype, using a single oral dose of 60 mg R,S-omeprazole. The differences in plasma concentrations of racemic omeprazole, its R- and S-enantiomers, 5-hydroxyomeprazole, and omeprazole sulfone as well as the metabolic ratios of omeprazole and its enantiomers were compared between females and males. We also examined the parameters mentioned above in homozygous EM (CYP2C19*1/*1) subjects (n=90). No significant differences of metabolic ratios (MRs) (omeprazole/5-hydroxyomeprazole (O/OHO), omeprazole/omeprazole sulfone (O/OS), omeprazole plus 5-hydroxyomeprazole/ omeprazole sulfone (O+OHO/OS), omeprazole plus omeprazole sulfone/5-hydroxyomeprazole (O+OS/OHO), and the same ratios for its S-, and R- enantiomers) were found between females and males in the homozygous EM group, except a trend for difference of the O+OS/OHO (P=0.061), RO+OS/OHO (P=0.047) and SO+OS/OHO (P=0.072) MRs, which was higher in females. In the heterozygous plus homozygous EMs with regard to CYP2C19, the MR of O+OS/OHO (P=0.059) was borderline higher in females, while the O+OHO/OS (P=0.009) MR was higher in males. Similar results were found for MRs of R-, and S-omeprazole (RO+OS/OHO, P=0.040; SO+OS/OHO P=0.018; RO+OHO/OS, P=0.005; SO+OHO/OS, P=0.007). There are sex-related differences in the MRs of omeprazole and its R-, and S- enantiomers, also suggesting that females have a higher CYP3A4, but a slightly lower CYP2C19 activity compared to males in healthy Turkish subjects.

Clopidogrel Pharmacokinetics and Pharmacodynamics Vary Widely Despite Exclusion or Control of Polymorphisms (CYP2C19, ABCB1, PON1), Non-Compliance, Diet, Smoking, Co-Medications (including Proton Pump Inhibitors), and Pre-Existent Variability in Platelet Function

AbstractAbstract 4356 Background:Platelet inhibition by clopidogrel is highly variable. Patients with reduced inhibition have increased risk for major adverse cardiovascular events. The aim of this study was to determine whether known genetic, drug, dietary, compliance and lifestyle factors affecting clopidogrel absorption and metabolism fully account for the variability in clopidogrel pharmacokinetics and pharmacodynamics. Methods and Results:Healthy subjects (n=160; age 20–53; homozygous CYP2C19 extensive metabolizer genotype; no nicotine 6 weeks, prescription drugs 4 weeks, over-the-counter drugs 2 weeks, caffeine and alcohol 72 hours; confined; restricted diet) received clopidogrel 75 mg daily for 9 days. At steady-state, clopidogrel active metabolite (clopidogrelAM) pharmacokinetics varied widely between subjects (CVs 33.8% and 40.2% for clopidogrelAM AUCt and Cmax, respectively). On-treatment pharmacodynamic endpoints VASP P2Y12 platelet reactivity index (PRI), maximal platelet aggregation (MPA) to ADP, and VerifyNow P2Y12 platelet response units (PRU) also varied widely (CVs 32 – 53%). All identified factors together accounted for only 18% of inter-subject variation in pharmacokinetic parameters and 32 – 64% of inter-subject variation in PRI, MPA, and PRU. High on-treatment platelet reactivity was present in 45% of healthy, homozygous CYP2C19 extensive metabolizer subjects, free of nicotine, alcohol, and co-medications, with witnessed clopidogrel 75 mg/d treatment for 9 days. Conclusions:Clopidogrel pharmacokinetics and pharmacodynamics vary widely despite rigorous exclusion or control of known disease, polymorphisms (CYP2C19, CYP3A5, ABCB1, PON1), non-compliance, co-medications, diet, smoking, alcohol, demographics, and pre-treatment platelet hyperreactivity. Thus, as yet unidentified factors contribute to high on-treatment platelet reactivity with its known increased risk of major adverse cardiovascular events. Disclosures:Frelinger:Takeda Pharmaceuticals: Research Funding; GLSynthesis: Research Funding. Bhatt:Medscape Cardiology: Membership on an entity’s Board of Directors or advisory committees; Boston VA Research Institute: Membership on an entity’s Board of Directors or advisory committees; Society of Chest Pain Centers: Membership on an entity’s Board of Directors or advisory committees; American Heart Association Get With The Guidelines Science Subcommittee: Chair, Chair Other; American College of Cardiology: Editor, Clinical Trials, Cardiosource Other; Duke Clinical Research Institute: clinical trial steering committees, clinical trial steering committees Other; Slack Publications: Chief Medical Editor, Cardiology Today Intervention Other; WebMD: CME steering committees, CME steering committees Other; Amarin, AstraZeneca, Bristol-Myers Squibb, Eisai, Ethicon, Medtronic, Sanofi Aventis, The Medicines Company: Research Funding; PLx Pharma, Takeda: unfunded research Other. Lee:Takeda Global Research & Development Center, Inc: Employment. Mulford:Takeda Global Research & Development Center, Inc: Employment. Wu:Takeda Global Research & Development Center, Inc: Employment. Nudurupati:Takeda Global Research & Development Center, Inc: Employment. Michelson:Eli Lilly: Data monitoring committee and idependent external monitor of clinical trials, Research Funding; Takeda: Research Funding; Oxygen Biotherapeutics: Research Funding; Alexion: Research Funding; Omthera: Research Funding.

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

Management of pain in opioid dependent individuals is problematic due to numerous issues including cross-tolerance to opioids. Hence there is a need to find alternative analgesics to classical opioids and tramadol is potentially one such alternative. Methadone inhibits CYP2D6 in vivo and in vitro. We aimed to investigate the effect of methadone on the pathways of tramadol metabolism: O-demethylation (CYP2D6) to the opioid-active metabolite M1 and N-demethylation (CYP3A4) to M2 in subjects maintained on methadone or buprenorphine as a control. Compared with subjects on buprenorphine, methadone reduced the clearance of tramadol to active O-desmethyl-tramadol (M1) but had no effect on N-desmethyltramadol (M2) formation. Similar to other analgesics whose active metabolites are formed by CYP2D6 such as codeine, reduced formation of O-desmethyltramadol (M1) is likely to result in reduced analgesia for subjects maintained on methadone. Hence alternative analgesics whose metabolism is independent of CYP2D6 should be utilized in this patient population. 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. 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). 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.

Absorption, Distribution, Metabolism and Excretion of [14C]Dexlansoprazole in Healthy Male Subjects

The proton pump inhibitor dexlansoprazole is a modified-release formulation of dexlansoprazole, an enantiomer of lansoprazole, which employs a Dual Delayed Release™ (DDR) delivery system. This study was conducted in healthy subjects to assess the absorption, distribution, metabolism and excretion of a 60 mg dose of [14C]dexlansoprazole. After multiple daily doses of dexlansoprazole DDR for 4 days followed by a single dose of [14C]dexlansoprazole on day 5, absorption, distribution, metabolism and elimination of [14C]dexlansoprazole were assessed in six healthy male subjects whose CYP (cytochrome P450) 2C19 metabolizer status was also determined. Five subjects were phenotyped as extensive metabolizers (EMs) and one subject was a poor metabolizer (PM). Recovery of radioactivity in urine and faeces averaged 98% after 7 days (51% in urine and 48% in faeces) post-14C dosing. In plasma, dexlansoprazole was the largest component detected, with the main metabolites in the EM subjects being 5-glucuronyloxy dexlansoprazole and 5-hydroxy dexlansoprazole (CYP2C19 mediated), whereas the PM subject had greater amounts of dexlansoprazole sulfone (CYP3A mediated). Dexlansoprazole was not detected in urine; six metabolites were identified accounting for an average of 86% of the urinary radioactivity, with 5-glucuronyloxy dexlansoprazole, 5-glucuronyloxy dexlansoprazole sulfide, 2-S-N-acetylcysteinyl benzimidazole and 5-sulfonyloxy dexlansoprazole sulfide being the primary metabolites. In faeces, parent drug and six identified metabolites accounted for 23% and 72%, respectively, of the faecal radioactivity, with 5-hydroxy dexlansoprazole sulfide and dexlansoprazole sulfide being predominant. Overall, the results indicate that [14C]dexlansoprazole was well absorbed and extensively metabolized by oxidation, reduction and conjugation to 13 identified metabolites.

Abstract 760: A randomized, crossover, phase 1 study to evaluate the effect of a strong CYP3A4 inhibitor on tivantinib (ARQ 197) pharmacokinetics in healthy subjects

Abstract Background: Tivantinib is a selective, oral, non-ATP-competitive, small-molecule inhibitor of c-MET. In vitro data suggest that CYP2C19 and CYP3A4 are the major enzymes involved in tivantinib metabolism. Therefore, there is potential that CYP3A4 inhibitors may affect tivantinib pharmacokinetics (PK) leading to increased exposure. During the course of their disease, cancer patients receive multiple drugs, some of which may be strong CYP3A4 inhibitors. This open-label, 2-stage study evaluated the effect of ketoconazole, a strong inhibitor of CYP3A4, on tivantinib metabolism in adults with different CYP2C19 activity. Methods: Healthy adults (age 18 to 45 years) with no clinical conditions were eligible. In stage 1, healthy subjects who were either an extensive metabolizer (EM) or intermediate metabolizer (IM) based on CYP2C19 activity were randomized to 1 of 2 sequences, each consisting of two 8-day treatment periods under fed conditions with a 14-day washout between periods. In sequence 1, tivantinib (120 mg) was administered alone on day 4 of period 1 followed by ketoconazole (400 mg/day) with concomitant tivantinib (120 mg) on day 4 of period 2. The treatments were reversed in sequence 2. If there was no interaction, poor metabolizer (PM) subjects were to be analyzed in stage 2. Serial blood samples were collected for 96 hours after tivantinib administration, and area under the curve (AUC), maximum concentration (Cmax), and time to peak concentration (tmax) were calculated and statistically compared. Results: Sixteen subjects were randomized equally to 1 of 2 treatment sequences. When tivantinib was combined with ketoconazole, total exposure to tivantinib (AUC) was increased 2.1-fold and Cmax was increased 1.4-fold compared with tivantinib alone. Median tmax was similar for ketoconazole + tivantinib and tivantinib alone (4.75 vs 4.50 hours, respectively). Exposure to tivantinib was 2- to 3-fold higher in CYP2C19 IM subjects compared with CYP2C19 EM subjects, with or without ketoconazole. The median tmax for tivantinib was comparable for IM and EM subjects with or without ketoconazole (tivantinib, 4.50 vs 4.49 hours, respectively and ketoconazole + tivantinib, 5.00 vs 4.50 hours, respectively). A total of 10 adverse events (AEs) were reported in 5 subjects (31%); 4 AEs were considered related to ketoconazole and the remaining AEs were unrelated to study treatment. Two subjects discontinued the study because of elevated creatinine (related to ketoconazole) and elevated neutrophils (unrelated to treatment), respectively. Stage 2 assessment in CYP2C19 PM subjects was not performed. Conclusions: Concurrent administration of ketoconazole substantially increased tivantinib exposure. Therefore, caution should be exercised when strong CYP3A4 inhibitors are coadministered with tivantinib. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 760. doi:1538-7445.AM2012-760

Methadone inhibits CYP2D6 and UGT2B7/2B4 in vivo: a study using codeine in methadone‐ and buprenorphine‐maintained subjects

To compare the O-demethylation (CYP2D6-mediated), N-demethylation (CYP3A4-mediated) and 6-glucuronidation (UGT2B4/7-mediated) metabolism of codeine between methadone- and buprenorphine-maintained CYP2D6 extensive metabolizer subjects. Ten methadone- and eight buprenorphine-maintained subjects received a single 60 mg dose of codeine phosphate. Blood was collected at 3 h and urine over 6 h and assayed for codeine, norcodeine, morphine, morphine-3- and -6-glucuronides and codeine-6-glucuronide. The urinary metabolic ratio for O-demethylation was significantly higher (P= 0.0044) in the subjects taking methadone (mean ± SD, 2.8 ± 3.1) compared with those taking buprenorphine (0.60 ± 0.43), likewise for 6-glucuronide formation (0.31 ± 0.24 vs. 0.053 ± 0.027; P < 0.0002), but there was no significant difference (P= 0.36) in N-demethylation. Similar changes in plasma metabolic ratios were also found. In plasma, compared with those maintained on buprenorphine, the methadone-maintained subjects had increased codeine and norcodeine concentrations (P < 0.004), similar morphine (P= 0.72) and lower morphine-3- and -6- and codeine-6-glucuronide concentrations (P < 0.008). Methadone is associated with inhibition of CYP2D6 and UGTs 2B4 and 2B7 reactions in vivo, even though it is not a substrate for these enzymes. Plasma morphine was not altered, owing to the opposing effects of inhibition of both formation and elimination; however, morphine-6-glucuronide (analgesically active) concentrations were substantially reduced. Drug interactions with methadone are likely to include drugs metabolized by various UGTs and CYP2D6.

Atomoxetine pharmacokinetics in healthy Chinese subjects and effect of the CYP2D6*10 allele

To characterize atomoxetine pharmacokinetics, explore the effect of the homozygous CYP2D6*10 genotype on atomoxetine pharmacokinetics and evaluate the tolerability of atomoxetine, in healthy Chinese subjects. Twenty-four subjects, all CYP2D6 extensive metabolizers (EM), were randomized to receive atomoxetine (40 mg qd for 3 days, then 80 mg qd for 7 days) or matching placebo (2 : 1 ratio) in a double-blind fashion. Atomoxetine serum concentrations were measured following single (40 mg) and multiple (80 mg) doses. Adverse events, clinical safety laboratory data and vital signs were assessed during the study. Atomoxetine was rapidly absorbed with median time to maximum serum concentrations of approximately 1.5 h after single and multiple doses. Atomoxetine concentrations appeared to decrease monoexponentially with a mean apparent terminal half-life (t(1/2)) of approximately 4 h. The apparent clearance, apparent volume of distribution and t(1/2) following single and multiple doses were similar, suggesting linear pharmacokinetics with respect to time. Homozygous CYP2D6*10 subjects had 50% lower clearances compared with other EM subjects, resulting in twofold higher mean exposures. No clinically significant changes or abnormalities were noted in laboratory data and vital signs. The pharmacokinetics of atomoxetine in healthy Chinese subjects appears comparable to other ethnic populations. Multiple dosing of 80 mg qd atomoxetine was well tolerated in this study.

Metabolism, Pharmacokinetics, and Excretion of the Substance P Receptor Antagonist CP-122,721 in Humans: Structural Characterization of the Novel Major Circulating Metabolite 5-Trifluoromethoxy Salicylic Acid by High-Performance Liquid Chromatography-Tandem Mass Spectrometry and NMR Spectroscopy

The metabolism, pharmacokinetics, and excretion of a potent and selective substance P receptor antagonist, CP-122,721 [(+)-(2S,3S)-3-(2-methoxy-5-trifluoromethoxybenzylamino)-2-phenylpiperidine], have been studied in six healthy male human subjects [four extensive metabolizers (EMs) and two poor metabolizers (PMs) of CYP2D6) following oral administration of a single 30-mg dose of [14C]CP-122,721. Approximately 84% of the administered radioactivity was recovered from the urine and feces of the subjects over a period of 312 h. Approximately 80% of the dose for EM subjects was recovered within 48 h. PM subjects, however, excreted only about 45% of the dose in 48 h and required the full 312 h to achieve nearly 80% recovery. Absorption of CP-122,721 was rapid in both extensive and poor metabolizers, as indicated by the rapid appearance of radioactivity in serum. The serum concentrations of total radioactivity were always much greater than those of unchanged drug indicating early formation of metabolites. The average CP-122,721 t1/2 was 6.7 h and 45.0 h for EM and PM subjects, respectively. The serum concentrations of CP-122,721 reached a peak of 7.4 and 69.8 ng/ml for extensive and poor metabolizers, respectively. The major metabolic pathways of CP-122,721 were due to O-demethylation, aromatic hydroxylation, and indirect glucuronidation. The minor metabolic pathways included aliphatic oxidation at the piperidine moiety, O-dealkylation of the trifluoromethoxy group, N-dealkylation, and oxidative deamination. In addition to the major human circulating metabolite 5-trifluoromethoxy salicylic acid (TFMSA), all other circulating metabolites of CP-122,721 were glucuronide conjugates of oxidized metabolites. TFMSA was identified using high pressure liquid chromatography/tandem mass spectrometry and NMR and mechanisms were proposed for its formation. There are no known circulating active metabolites of CP-122,721.

Miotic action of tramadol is determined by CYP2D6 genotype

Polymorphic CYP2D6 is the enzyme that activates the opioid analgesic tramadol by O-demethylation to its active metabolite O-demethyltramadol (M1). Our objective was to determine the opioid effects measured by pupillary response to tramadol of CYP2D6 genotyped volunteers in relation to the disposition of tramadol and M1 in plasma. Tramadol displayed phenotypic pharmacokinetics and it was possible to identify poor metabolizers (PM) with >99% confidence from the metabolic ratio (MR) in a single blood sample taken between 2.5 and 24 h post-dose. Homozygous extensive metabolizers (EM) differed from PM subjects by an almost threefold greater (P=0.0014) maximal pupillary constriction (Emax). Significant correlations between the AUC and Cmax values of M1 versus pupillary constriction were found. The corresponding correlations of pharmacokinetic parameters for tramadol itself were weaker and negative. The strongest correlations were for the single-point metabolic ratios at all sampling intervals versus the effects, with rs ranging from 0.85 to 0.89 (p<0.01). It is concluded that the concept of dual opioid/non-opioid action of the drug, though considerably stronger in EMs, is valid for both EM and PM subjects. This is the theoretical basis for the frequent use and satisfactory efficacy of tramadol in clinical practice when given to genetically non-selected population.

Minimizing Polymorphic Metabolism in Drug Discovery: Evaluation of the Utility of in Vitro Methods for Predicting Pharmacokinetic Consequences Associated with CYP2D6 Metabolism

Minimizing interindividual variability in drug exposure is an important goal for drug discovery. The reliability of the selective CYP2D6 inhibitor quinidine was evaluated in a retrospective analysis using a standardized approach that avoids laboratory-to-laboratory variation. The goal was to evaluate the reliability of in vitro metabolism studies for predicting extensive metabolizer (EM)/poor metabolizer (PM) exposure differences. Using available literature, 18 CYP2D6 substrates were selected for further analysis. In vitro microsomal studies were conducted at 1 microM substrate and 0.5 microM P450 to monitor substrate depletion. An estimate of the fraction metabolized by CYP2D6 in microsomes was derived from the rate constant determined with and without 1 microM quinidine for 11 substrates. Clearance in EM and PM subjects and fractional recovery of metabolites were taken from the literature. A nonlinear relationship between the contribution of CYP2D6 and decreased oral clearance for PMs relative to EMs was evident. For drugs having <60% CYP2D6 involvement in vivo, a modest difference between EM and PM exposure was observed (<2.5-fold). For major CYP2D6 substrates (>60%), more dramatic exposure differences were observed (3.5- to 53-fold). For compounds primarily eliminated by hepatic P450 and with sufficient turnover to be evaluated in vitro, the fraction metabolized by CYP2D6 in vitro compared favorably with the in vivo data. The in vitro estimation of fraction metabolized using quinidine as a specific inhibitor provided an excellent predictive tool. Results from microsomal substrate depletion experiments can be used with confidence to select compounds in drug discovery using a cutoff of >60% metabolism by CYP2D6.

Measurement of CYP2D6 and CYP3A4 activity in vivo with dextromethorphan: sources of variability and predictors of adverse effects in 419 healthy subjects

Dextromethorphan (DEM) shares part of the adverse event profile of opioids and is widely used as a probe drug for CYP2D6 phenotyping and for the assessment of CYP2D6 activity. It has also been used to assess CYP3A4 activity. This study examined the influence of anthropometric variables, oral contraceptives, smoking habits, mu-opioid receptor and MDR1 genetic polymorphisms and components of the DEM ratios on the variability of CYP2D6 and CYP3A4 metabolic ratios and on the occurrence of adverse events following DEM administration. This was a retrospective analysis of a database in 419 healthy subjects. CYP2D6 and CYP3A4 metabolic ratios were measured as the log of the ratios of the amount of DEM to the amount of dextrorphan (DOR) and of the amount of DEM to the amount of 3-methoxy-morphinan (MET) excreted in urine during a 12-h time period, respectively, following the oral administration of 80 mg of dextromethorphan hydrobromide. Logistic regression was performed to examine the factors associated with changes in metabolic ratios and with the occurrence of adverse events. The CYP2D6 metabolic ratio allowed identification of extensive and poor metabolizers of DEM. The CYP2D6 and CYP3A4 metabolic ratios were not strictly independent one from each other. Based on multivariate analysis, the CYP2D6 metabolic ratio was a stronger independent predictor of adverse events (p<0.0001) than the CYP2D6 phenotype (p=0.05). Anthropometric variables, oral contraceptives, smoking habits, mu-opioid receptor and MDR1 genetic polymorphisms did not significantly contribute to changes in metabolic ratios or to the occurrence of adverse events. Dextromethorphan can be used for CYP2D6 phenotyping, but the CYP2D6 and CYP3A4 metabolic ratios are not strictly independent one from each other. The CYP2D6 metabolic ratio predicts adverse events to DEM as does CYP2D6 phenotype, and extensive metabolizer subjects are not protected against adverse events.

, but not , polymorphisms affect the pharmacokinetics and pharmacodynamics of glyburide in Chinese subjects

Although cytochrome P450 (CYP) 2C9 was thought to be the main pathway for glyburide (INN, glibenclamide) metabolism in vivo, studies in vitro indicated that CYP2C19 had a more dominant effect. This study investigated the relative influence of CYP2C9 and CYP2C19 genotypes on the pharmacokinetics and pharmacodynamics of glyburide in Chinese subjects. Three groups of healthy male Chinese subjects (n=6 per group) were enrolled, as follows: group I, CYP2C9*1/*1 and CYP2C19 extensive metabolizers (EMs); group II, CYP2C9*1/*1 and CYP2C19 poor metabolizers (PMs); and group III, CYP2C9*1/*3 and CYP2C19 EMs. Subjects received single oral doses of 5 mg glyburide. Multiple blood samples were collected, and the plasma glyburide concentrations were determined by an HPLC method. The plasma glucose and insulin concentrations were also measured up to 2 hours after dosing. No significant differences in glyburide pharmacokinetics were observed between CYP2C19 EM and PM subjects who had the CYP2C9*1/*1 genotype (group I versus group II). Their respective values for area under the plasma concentration-time curve from time 0 to infinity (AUC0-infinity) and elimination half-life (t1/2) were 0.46+/-0.13 microg.h/mL versus 0.57+/- 0.11 microg.h/mL (P=.569) and 2.09+/-0.22 hours versus 2.24+/- 0.27 hours (P=.721). However, significant increases in AUC(0-infinity) (125% and 82%; P=.008 and .024, respectively) and t1/2 (71% and 60%; P=.003 and .007, respectively) were observed when CYP2C9*1/*3 subjects (group III) were compared with CYP2C9*1/*1 subjects in group I or II. Blood glucose reductions at 2 hours after dosing were 41.8%, 23.9%, and 27.7% in groups I, II, and III, respectively (P=.029), and hypoglycemia developed in 3 of 6 CYP2C9*1/*3 carriers and 2 of 12 CYP2C9*1/*1 carriers. CYP2C9, but not CYP2C19, polymorphism appears to exert a dominant influence on glyburide pharmacokinetics and pharmacodynamics in vivo. Further studies in diabetic patients with long-term dosing are warranted to confirm these findings.