Cytochrome P450 (P450) enzymes are major catalysts involved in the oxidations of most drugs, steroids, carcinogens, fat-soluble vitamins, and natural products. The binding of substrates to some of ...the 57 human P450s and other mammalian P450s is more complex than a two-state system and has been proposed to involve mechanisms such as multiple ligand occupancy, induced-fit, and conformational-selection. Here, we used kinetic analysis of binding with multiple concentrations of substrates and computational modeling of these data to discern possible binding modes of several human P450s. We observed that P450 2D6 binds its ligand rolapitant in a mechanism involving conformational-selection. P450 4A11 bound the substrate lauric acid via conformational-selection, as did P450 2C8 with palmitic acid. Binding of the steroid progesterone to P450 21A2 was also best described by a conformational-selection model. Hexyl isonicotinate binding to P450 2E1 could be described by either a conformational-selection or an induced-fit model. Simulation of the binding of the ligands midazolam, bromocriptine, testosterone, and ketoconazole to P450 3A4 was consistent with an induced-fit or a conformational-selection model, but the concentration dependence of binding rates for varying both P450 3A4 and midazolam concentrations revealed discordance in the parameters, indicative of conformational-selection. Binding of the P450s 2C8, 2D6, 3A4, 4A11, and 21A2 was best described by conformational-selection, and P450 2E1 appeared to fit either mode. These findings highlight the complexity of human P450-substrate interactions and that conformational-selection is a dominant feature of many of these interactions.
Safe and effective use of drugs requires an understanding of metabolism and transport. We identified the 100 most prescribed drugs in six countries and conducted a literature search on in vitro data ...to assess contribution of Phase I and II enzymes and drug transporters to metabolism and transport.
Eighty‐nine of the 100 drugs undergo drug metabolism or are known substrates for drug transporters. Phase I enzymes are involved in metabolism of 67 drugs, while Phase II enzymes mediate metabolism of 18 drugs. CYP3A4/5 is the most important Phase I enzyme involved in metabolism of 43 drugs followed by CYP2D6 (23 drugs), CYP2C9 (23 drugs), CYP2C19 (22 drugs), CYP1A2 (14 drugs) and CYP2C8 (11 drugs). More than half of the drugs (54 drugs) are known substrates for drug transporters. P‐glycoprotein (P‐gp) is known to be involved in transport of 30 drugs, while breast cancer resistance protein (BCRP) facilitates transport of 11 drugs. A considerable proportion of drugs are subject to a combination of Phase I metabolism, Phase II metabolism and/or drug transport.
We conclude that the majority of the most frequently prescribed drugs depend on drug metabolism or drug transport. Thus, understanding variability of drug metabolism and transport remains a priority.
Background: Tramadol is used off-label for medically supervised opioid withdrawal. Tramadol is metabolized by CYP2D6 to an active metabolite with significantly more pharmacologic activity compared to ...the parent compound.
Objectives: The objective of this study is to evaluate the effects of CYP2D6 inhibitors on patient response to a tramadol taper for medically supervised opioid withdrawal.
Methods: A retrospective chart review of patients who received a tramadol taper for medically supervised opioid withdrawal was conducted comparing patients who received concomitant moderate-to-strong CYP2D6 inhibitors to patients without concomitant therapy. The primary outcome was the change in Clinical Institute Narcotic Assessment (CINA) scores from baseline to discharge. Secondary outcomes included area under the curve of CINA scores over time, additional CINA outcomes, length of stay, and readmissions.
Results: Of 100 charts reviewed, 30 patients received a concomitant moderate-to-strong CYP2D6 inhibitor. There were no statistically significant differences between the baseline demographics of the two groups. Change from baseline CINA to discharge did not differ significantly between the Non-2D6 group and the 2D6 group (-4.0 ± 3.83 and −4.5 ± 4.48 respectively; p = 0.606). The average CINA score for nausea and vomiting was significantly higher in the Non-2D6 group compared to the 2D6 group (0.34 ± 0.35 and 0.18 ± 0.33 respectively; p = 0.019). Otherwise there were no significant differences found in any secondary outcomes.
Conclusions: Based on these results, moderate-to-strong CYP2D6 inhibitors do not appear to have a significant impact on the withdrawal course for patients treated with a high-dose tramadol taper.
Objective
β‐Blockers require careful initiation and titration when used in patients with heart failure. Some patients tolerate β‐blocker therapy initiation without difficulty, whereas in other ...patients this period presents clinical challenges. We tested the hypothesis that polymorphisms at codons 389 (Arg389Gly) and 49 (Ser49Gly) of the β1‐adrenergic receptor would be associated with differences in initial tolerability of β‐blocker therapy in patients with heart failure. We also tested whether polymorphisms in the β2‐adrenergic receptor, G‐protein αs subunit (Gsα), and cytochrome P450 (CYP) 2D6 genes or S‐metoprolol plasma concentrations were associated with β‐blocker tolerability.
Methods
Sixty‐one β‐blocker‐naive patients with systolic heart failure were prospectively enrolled. Patients began taking 12.5 to 25 mg metoprolol controlled release/extended release with titration every 2 weeks (as tolerated) to 200 mg/d or the maximum tolerated dose over a period of 8 to 10 weeks. Decompensation was the composite of death, heart failure hospitalization, increase in other heart failure medications, or need to discontinue metoprolol. End points were assessed during the titration period.
Results
The overall rate of decompensation was not different between the codon 49 or 389 genotypes. However, a significantly greater percentage of patients with the Gly389 variant required increases in heart failure medications as compared with Arg389 homozygotes (48% versus 14%, respectively; P = .006). Similarly, patients with the Ser49 homozygous genotype were significantly more likely to require increases in concomitant heart failure therapy as compared with Gly49 carriers (41% versus 11%, respectively; P = .03). Neither CYP2D6 genotypes nor metoprolol pharmacokinetics differed between patients with and those without a decompensation event. There was no association between the β2‐adrenergic receptor or Gsα polymorphisms with decompensated heart failure.
Conclusions
Patients with the Gly389 variant and Ser49Ser genotype were significantly more likely to require increases in heart failure medications during β‐blocker titration and thus may require more frequent follow‐up during titration.
Clinical Pharmacology & Therapeutics (2005) 77, 127–137; doi: 10.1016/j.clpt.2004.10.006
We analyzed the polymorphisms of 7 antihypertensive drugs-related genes and the factors associated with hypertension in hypertensive patients of Han ethnicity in Qingyang, China. A total of 354 ...hypertensive patients of Han ethnicity were enrolled from Qingyang, China. The ACE (I/D), ADRB1 (1165G > C), AGTR1 (1166A > C), CYP2C9*3, CYP2D6*10, CYP3A5*3 and NPPA (T2238C) polymorphisms were assessed. Clinical data of patients was also obtained. The influencing factors of hypertension were evaluated. The genotype frequencies of ACE, ADRB1, AGTR1, CYP2C9, CYP3A5 and NPPA loci were in Hardy-Weinberg equilibrium, with mutation frequencies of 39.27%, 74.29%, 6.21%, 4.80%, 72.46% and 0.71%, respectively. CYP2D6 locus was not in Hardy-Weinberg equilibrium. There was no statistical difference in allele frequencies between different genders (P > .05). There was significant difference in the frequencies of ACE (I/D) and NPPA (T2238C) loci among different regions of China (P < .05). Gender, ACE (I/D) and ADRB1 (1165G > C) gene polymorphism, smoking, homocysteine and HDL levels were associated hypertension. The mutation frequencies of ADRB1 (1165G > C) and CYP3A5*3 were high in hypertensive patients of Han ethnicity in Qingyang, suggesting these patients may be more sensitive to beta-blockers and calcium ion antagonists. Meanwhile, hypertension was associated with gender, ACE (I/D) and ADRB1 (1165G > C) gene polymorphisms, smoking, homocysteine and HDL levels.
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.
Background
Many drugs with dose-dependent effects on hemodynamic variables are metabolized by cytochrome P450 2D6 (CYP2D6). The aim of this study was to compare prescribed dosages and hemodynamic ...responses of such drugs in relation to pharmacogenetic variability in CYP2D6 metabolism among patients aged ≥ 70 years exposed to polypharmacy.
Materials and Methods
We included 173 patients with detailed information about drug use. The patients were retrospectively subjected to
CYP2D6
genotyping, which comprised the most common variant alleles encoding reduced, absent, or increased CYP2D6 metabolism. In order to compare dosages across different CYP2D6-metabolized drugs, all prescribed daily doses were harmonized to the ‘percent of a daily defined dose’ (DDD). The mean harmonized DDD was compared between genotype-predicted normal metabolizers (NMs) and patients with reduced or absent CYP2D6 enzyme activity, defined as intermediate or poor metabolizers (IMs/PMs). Blood pressure, pulse, and patient proportions with orthostatism and bradycardia were also compared between genotype subgroups.
Results
The genotype-predicted phenotype subgroups comprised 79 NMs (45.7%), 75 IMs (43.4%), and 16 PMs (9.2%). There were no differences in dosing of CYP2D6 substrates between NMs and IMs/PMs (
p
= 0.76). A higher proportion of CYP2D6 IMs/PMs experienced orthostatism (
p
= 0.03), while there were no significant subgroup differences for the other hemodynamic variables.
Conclusion
In this real-life clinical setting of patients aged ≥ 70 years, dosing of CYP2D6 substrates were not adjusted according to genotype-predicted CYP2D6 metabolism. The increased occurrence of orthostatism in patients with reduced/absent CYP2D6 metabolism may indicate that individualized dosing based on genotype has the potential to prevent adverse effects in these vulnerable patients.
Cytochrome P450 (CYP) enzymes are useful biocatalysts for the pharmaceutical and biotechnological industries. A high‐throughput method for quantification of CYP expression in yeast is needed in order ...to fully exploit the yeast expression system. Carbon monoxide (CO) difference spectra of whole cells have been successfully used for the quantification of heterologous CYP expressed in Escherichia coli in the 96‐well format; however, very few researchers have shown whole‐cell CO difference spectra with yeast cells using 1‐cm path length. Spectral interference from the native hemoproteins often obscures the P450 peak, challenging functional CYP quantification in whole yeast cells. For the first time, we describe the high‐throughput determination of CO difference spectra using whole cells in the 96‐well format for the quantification of CYP genes expressed in Pichia pastoris. Very little interference from the hemoproteins of P. pastoris enabled CYP quantification even at relatively low expression levels. P. pastoris strains carrying a single copy or three copies of both hCPR and CYP2D6 integrated into the chromosomal DNA were used to establish the method in 96‐well format, allowing to detect quantities of CYP2D6 as low as 6 nmol gCDW–1 and 12 pmol per well. Finally, the established method was successfully demonstrated and used to screen P. pastoris clones expressing Candida CYP52A13.
Cytochrome P450 (CYP) enzymes are useful biocatalysts for the pharmaceutical and biotechnological industries. In this study, the authors describe, for the first time, the high‐throughput quantification of heterologous CYP genes expressed in Pichia pastoris, using carbon monoxide difference spectra. This is a widely applicable method for rapid screening of mutant libraries.
Conducting pharmacokinetic (PK) studies in pregnant women is challenging. Therefore, we asked if a physiologically based pharmacokinetic (PBPK) model could be used to evaluate different dosing ...regimens for pregnant women. We refined and verified our previously published pregnancy PBPK model by incorporating cytochrome P450 CYP1A2 suppression (based on caffeine PK) and CYP2D6 induction (based on metoprolol PK) into the model. This model accounts for gestational age-dependent changes in maternal physiology and hepatic CYP3A activity. For verification, the disposition of CYP1A2-metabolized drug theophylline (THEO) and CYP2D6-metabolized drugs paroxetine (PAR), dextromethorphan (DEX), and clonidine (CLO) during pregnancy was predicted. Our PBPK model successfully predicted THEO disposition during the third trimester (T3). Predicted mean postpartum to third trimester (PP:T3) ratios of THEO area under the curve (AUC), maximum plasma concentration, and minimum plasma concentration were 0.76, 0.95, and 0.66 versus observed values 0.75, 0.89, and 0.72, respectively. The predicted mean PAR steady-state plasma concentration (Css) ratio (PP:T3) was 7.1 versus the observed value 3.7. Predicted mean DEX urinary ratio (UR) (PP:T3) was 2.9 versus the observed value 1.9. Predicted mean CLO AUC ratio (PP:T3) was 2.2 versus the observed value 1.7. Sensitivity analysis suggested that a 100% induction of CYP2D6 during T3 was required to recover the observed PP:T3 ratios of PAR Css, DEX UR, and CLO AUC. Based on these data, it is prudent to conclude that the magnitude of hepatic CYP2D6 induction during T3 ranges from 100 to 200%. Our PBPK model can predict the disposition of CYP1A2, 2D6, and 3A drugs during pregnancy.
Few studies of the effect of the dynamic physiologic changes during pregnancy on plasma concentrations of fluoxetine (FLX) have been published.
We determined the change in concentration to dose (C/D) ...ratios of R- and S-FLX and R- and S-norfluoxetine monthly during pregnancy and postpartum, assessed their relationships to cytochrome P450 (CYP) 2D6 and CYP2C9 metabolizer phenotypes, and evaluated the course of their depressive and anxiety symptoms.
In this observational study, 10 FLX-treated pregnant individuals provided blood samples at steady state every 4 weeks during pregnancy and once postpartum for measurement of plasma FLX and norfluoxetine enantiomer concentrations. Participants were genotyped for variants in CYP2C9 and CYP2D6 using commercial assays with Taqman probes. At each assessment, depressive and anxiety symptoms were quantified.
The C/D ratios of all FLX and norfluoxetine enantiomers, and the active moiety, decreased steadily through pregnancy and rose after birth. In the final trimester, the mean C/D ratio of the active moiety was 24.9% lower compared with the mean nonpregnant, 12-week postpartum C/D ratio. One individual with CYP2D6 ultrarapid metabolizer status was prescribed the highest FLX dose among participants. In these treated individuals, the mean depressive and anxiety symptoms remained in the mild range across the perinatal period.
These data do not support a recommendation for routine plasma concentration monitoring or CYP2D6 pharmacogenetic testing for pregnant people treated with FLX; however, monitoring for symptom relapse is recommended because of declining plasma drug concentrations.
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