Defective metabolism of metoprolol in poor hydroxylators of debrisoquine

Pharmacogenetic factors affecting β-blocker metabolism and response

INTRODUCTION

β-blockers are among the most widely prescribed of all drugs, used for treatment of a large number of cardiovascular diseases. Herein we evaluate literature pertaining to pharmacogenetics of β-blocker therapy, provide insight into the robustness of the genetic associations, and determine the appropriateness for translating these genetic associations into clinical practice.

AREAS COVERED

A literature search was conducted using PubMed to collate evidence on associations between CYP2D6, ADRB1, ADRB2, and GRK5 genetic variation and drug-response outcomes in the presence of β-blocker exposure. Pharmacokinetic, pharmacodynamic, and clinical outcomes studies were included if genotype data and β-blocker exposure were documented.

EXPERT OPINION

Substantial data suggest that specific ADRB1 and GRK5 genotypes are associated with improved β-blocker efficacy and have potential for use to guide therapy decisions in the clinical setting. While the data do not justify ordering a CYP2D6 pharmacogenetic test, if CYP2D6 genotype is available in the electronic health record, there may be clinical utility for understanding dosing of β-blockers.

P450 Pharmacogenetics in Indigenous North American Populations

Indigenous North American populations, including American Indian and Alaska Native peoples in the United States, the First Nations, Métis and Inuit peoples in Canada and Amerindians in Mexico, are historically under-represented in biomedical research, including genomic research on drug disposition and response. Without adequate representation in pharmacogenetic studies establishing genotype-phenotype relationships, Indigenous populations may not benefit fully from new innovations in precision medicine testing to tailor and improve the safety and efficacy of drug treatment, resulting in health care disparities. The purpose of this review is to summarize and evaluate what is currently known about cytochrome P450 genetic variation in Indigenous populations in North America and to highlight the importance of including these groups in future pharmacogenetic studies for implementation of personalized drug therapy.

Development of metoprolol tartrate-loaded sustained-release pellets: effect of talc on the mechanism of drug release

Talc is one of the most commonly used antiadherents in the coating film. However, the mechanism of influence of talc on drug release has yet to be fully understood. In this study, metoprolol tartrate (MT)-loaded Eudragit NE 30 D-coated sustained-release (SR) pellets were prepared using talc as an antiadherent in the layering and coating processes. Talc significantly reduced the stickiness of the layered or coated substrates, thus enhancing the process smoothness. Moreover, the incorporation of talc into the coating film significantly affected drug release. The water vapor permeability and drug permeability of free films increased as the concentration of talc increased. Importantly, talc had a dynamic effect on the drug release. The drug release rate of the pellets in the initial stage (within 2 h) increased with increasing talc concentrations, which exceeded the critical pigment volume concentration resulted in leaks formation in the coated film. However, subsequent swelling of the membrane and expansion of the copolymer network eliminated the influence of talc and the drug release was then controlled by the polymeric membrane. These results suggest that talc contributed to the reduction of the sticking of layered or coated substrates, and facilitated the manufacturing process and drug release properties.

The Clinical Significance of the Pharmacogenetics of Cardiovascular Medications

Inter-individual variability in the response to numerous drugs can be traced to a number of sources. One source of variability in drug response is the variability associated with the metabolic capacity of an individual. The component of metabolic capacity that will be the focus of this article is that determined by heredity. Pharmacogenetics is frequently referred to as the study of the effects of heredity on the disposition and response to medications. This article will review the pharmacokinetic and pharmacodynamic significance of pharmacogenetics as it pertains to a select number of cardiovascular agents. The enzyme systems responsible for drug metabolism discussed in this article will be limited to the P-450IID6 and N-acetylation pathways. Given the extensive use of cardiovascular agents in clinical practice that are affected by this genetic polymorphism, it is important for the practicing pharmacist to be aware of this phenomenon and its implications. Hopefully, the knowledge gained from this article will help practicing pharmacists to appreciate the clinical significance of polymorphic drug metabolism and provide a basis for the application of this knowledge to a variety of practice settings.

Personalized medicine: importance of clinical interpretative skills for real-world patient care

Sequencing of the human genome led to great hopes and expectations for a 'genomics revolution' where disease diagnosis as well as therapies would be based on an individual's genetic makeup. Although significant progress has been made, a number of challenging hurdles must be overcome prior to the broader adoption and implementation of pharmacogenomics and personalized medicine as a part of standard patient care. One aspect of pharmacogenomics-based personalized medicine that has not garnered as much attention, a key focus of this perspective, is the importance of interpreting pharmacogenomic test results in a patient-specific clinical context, and expert physicians and other allied health care providers with the requisite expertise in clinical pharmacology and genomics who are able to provide such services.

Bilayer mucoadhesive microparticles for the delivery of metoprolol succinate: Formulation and evaluation

Metoprolol succinate is a very potent drug for the treatment of hypertension but suffers from poor bioavailability due to its erratic absorption in lower GI tract. Therefore, in the present study, it was hypothesized that by formulating mucoadhesive particles, the residence time in the GIT and release of drug may be prolonged that will enhance the bioavailability of metoprolol succinate. Metoprolol succinate loaded chitosan microparticles were prepared by ionic gelation method. The optimized microparticles were coated with sodium alginate to form a layer over chitosan microparticles to increase the mucoadhesive strength and to release the drug in controlled manner. Coated and uncoated microparticles were evaluated for particle size, zeta potential, morphology, entrapment efficiency, drug loading and in vitro drug release. The coated microparticles showed comparatively less drug release in the 0.1 N HCl while sustained release in PBS (pH 6.8) as compared to uncoated microparticles. The in vivo study on albino rats demonstrated an increase in bioavailability of the coated microparticles as compared to marketed formulation. From the study it can be concluded that alginate coated chitosan microparticles could be a useful carrier for the oral delivery of metoprolol succinate.

Genetic Polymorphisms of Cytochrome P450 Enzymes and the Effect on Interindividual, Pharmacokinetic Variability in Extensive Metabolizers

Genetic polymorphisms of cytochrome P450 (CYP) enzymes are one of the factors that contribute to the pharmacokinetic (PK) variability of drugs. PK variability is observed in the bimodal distribution between extensive metabolizers (EMs) and poor metabolizers (PMs). PK variability may also exist between individuals genotyped as homozygous EMs and heterozygous EMs. This may carry implications for drug dosing and drug response (e.g., risk of therapeutic failure or drug toxicity). Studies have reported significant PK differences between homozygous and heterozygous EMs. Some literature suggests that this distinction may be of clinical relevance. Due to study design limitations and data that are either sparse or conflicting, generalizations regarding the potential impact of the CYP genotype, within EMs, are difficult. Optimally designed clinical trials are needed. This review evaluates the potential impact of CYP genetic polymorphisms on interindividual PK variability of drugs within an EM population.

Cardiovascular pharmacogenomics: the future of cardiovascular therapeutics?

Responses to drug therapy vary from benefit to no effect to adverse effects which can be serious or occasionally fatal. Increasing evidence supports the idea that genetic variants can play a major role in this spectrum of responses. Well-studied examples in cardiovascular therapeutics include predictors of steady-state warfarin dosage, predictors of reduced efficacy among patients receiving clopidogrel for drug eluting stents, and predictors of some serious adverse drug effects. This review summarizes contemporary approaches to identifying and validating genetic predictors of variability in response to drug treatment. Approaches to incorporating this new knowledge into clinical care, and the barriers to this concept, are addressed.

Hydroxylation of Debrisoquine Using Perfused Liver Isolated from Sprague Dawley and DA Rats: Comparison With In-vivo Results

The hydroxylation of debrisoquine was investigated in Sprague-Dawley (SD) and Dark-Agouti (DA) rats. Female and male rats were phenotyped in-vivo with debrisoquine six times during their growth. The ratios debrisoquine/4-hydroxydebrisoquine of the female DA rats increased until the 15th week and then decreased; but the values of the metabolic ratios never exceeded 2. Female DA rats cannot be considered as genetically deficient for hydroxylation of debrisoquine in regard to the metabolic ratio, but the percentage of debrisoquine excretion is up to ten fold higher than that in the other strains. Therefore SD and DA rat livers were perfused for 2 h when the clearance of debrisoquine was significantly lower in the female DA group than in the other groups. 4-Hydroxydebrisoquine in the perfusate increased with time, but the amount after 120 min was 12 fold lower in the female DA rat group than in the female SD rat group. The results of the male DA group fell between. This study confirms that female DA rats present a lower debrisoquine 4-hydroxylating capacity than other rats but shows that urinary metabolic ratio is not sufficient to assess the deficiency of debrisoquine hydroxylation.

Polymorphism of human cytochrome P450 enzymes and its clinical impact

Pharmacogenetics is the study of how interindividual variations in the DNA sequence of specific genes affect drug response. This article highlights current pharmacogenetic knowledge on important human drug-metabolizing cytochrome P450s (CYPs) to understand the large interindividual variability in drug clearance and responses in clinical practice. The human CYP superfamily contains 57 functional genes and 58 pseudogenes, with members of the 1, 2, and 3 families playing an important role in the metabolism of therapeutic drugs, other xenobiotics, and some endogenous compounds. Polymorphisms in the CYP family may have had the most impact on the fate of therapeutic drugs. CYP2D6, 2C19, and 2C9 polymorphisms account for the most frequent variations in phase I metabolism of drugs, since almost 80% of drugs in use today are metabolized by these enzymes. Approximately 5-14% of Caucasians, 0-5% Africans, and 0-1% of Asians lack CYP2D6 activity, and these individuals are known as poor metabolizers. CYP2C9 is another clinically significant enzyme that demonstrates multiple genetic variants with a potentially functional impact on the efficacy and adverse effects of drugs that are mainly eliminated by this enzyme. Studies into the CYP2C9 polymorphism have highlighted the importance of the CYP2C9*2 and *3 alleles. Extensive polymorphism also occurs in other CYP genes, such as CYP1A1, 2A6, 2A13, 2C8, 3A4, and 3A5. Since several of these CYPs (e.g., CYP1A1 and 1A2) play a role in the bioactivation of many procarcinogens, polymorphisms of these enzymes may contribute to the variable susceptibility to carcinogenesis. The distribution of the common variant alleles of CYP genes varies among different ethnic populations. Pharmacogenetics has the potential to achieve optimal quality use of medicines, and to improve the efficacy and safety of both prospective and currently available drugs. Further studies are warranted to explore the gene-dose, gene-concentration, and gene-response relationships for these important drug-metabolizing CYPs.

CYP2D6 genotype and its relationship with metoprolol dose, concentrations and effect in patients with systolic heart failure

The aims of this study were to examine the relationships between CYP2D6 genotype and metoprolol dose, S- and R-metoprolol concentrations and clinical effects in patients with systolic heart failure. Data were obtained for 52 subjects, of which 27 had 2 functional alleles (24/27, CYP2D6*1/*1), 22 had 1 functional allele (18/22, CYP2D6*1/*4) and 3 had no functional alleles (CYP2D6*4/*4). Median dose-adjusted concentrations of S-metoprolol (active) were 6.3- and 3.2-fold higher in subjects with zero or one functional allele (P=0.016 and P=0.006), respectively, compared with subjects with two functional alleles. For the R-enantiomer (inactive), these concentrations were 10.7- and 3.7-fold higher (P=0.013 and P=0.003), respectively. Despite clear gene-concentration differences, no relationships between CYP2D6 genotype and dose or clinical effects could be shown. Although the number with no functional alleles was too small (n=3) to show effects, in patients with 1 functional allele other sources of variance are likely to be obscuring differences in clinical effects.

The clinical role of genetic polymorphisms in drug-metabolizing enzymes

For most drug-metabolizing enzymes (DMEs), the functional consequences of genetic polymorphisms have been examined. Variants leading to reduced or increased enzymatic activity as compared to the wild-type alleles have been identified. This review tries to define potential fields in the therapy of major medical conditions where genotyping (or phenotyping) of genetically polymorphic DMEs might be beneficial for drug safety or therapeutic outcome. The possible application of genotyping is discussed for depression, cardiovascular diseases and thromboembolic disorders, gastric ulcer, malignant diseases and tuberculosis. Some drugs used for relief of these ailments are metabolized with participation of genetically polymorphic DMEs including CYP2D6, CYP2C9, CYP2C19, thiopurine-S-methyltransferase, dihydropyrimidine dehydrogenase, uridine diphosphate glucuronosyltransferase and N-acetyltransferase type 2. Current evidence suggests that taking genetically determined metabolic capacities of DMEs into account has the potential to improve individual risk/benefit relationship. However, more prospective studies with clinical endpoints are needed before the paradigm of 'personalized medicine' based on DME variants can be established.

Effect of Honey on CYP3A4, CYP2D6 and CYP2C19 Enzyme Activity in Healthy Human Volunteers

Honey is a common food supplement but not many studies have studied honey and drug interaction. This study investigates the influence of 7 days of honey administration on the activity of CYP3A4, CYP2D6 and CYP2C19 drug-metabolizing enzymes in healthy volunteers by using appropriate biomarker and probe drugs. A within-group pharmacokinetic study was done in 12 healthy volunteers. Urine samples (0-8 hr) were collected after administration of 30 mg of oral dextromethorphan (probe drug for CYP2D6) for analysis of dextromethorphan and dextrorphan. A plasma sample (4 hr) was collected after administration of 200 mg of oral proguanil (probe drug for CYP2C19) for the analysis of proguanil and cycloguanil. Urine samples (0-24 hr) were collected for the analysis of 6beta-hydroxycortisol (biomarker for CYP3A4). The volunteers were administered honey for 7 days. Subsequently blood and urine samples were collected after drug dosing as before. These samples were analysed for drug and metabolite concentrations in urine and plasma using high performance liquid chromatography method. Seven days of honey administration resulted in statistically significant increase in 24-hr urinary excretion of 6beta-hydroxycortisol. However, the metabolic ratios of dextromethorphan and proguanil were not significantly altered after 7 days of honey administration. Honey obtained from Western Ghats of southern India may induce CYP3A4 enzyme activity but not CYP2D6 and CYP2C19 enzyme activities.

Pharmacogenetics, drug-metabolizing enzymes, and clinical practice

The application of pharmacogenetics holds great promise for individualized therapy. However, it has little clinical reality at present, despite many claims. The main problem is that the evidence base supporting genetic testing before therapy is weak. The pharmacology of the drugs subject to inherited variability in metabolism is often complex. Few have simple or single pathways of elimination. Some have active metabolites or enantiomers with different activities and pathways of elimination. Drug dosing is likely to be influenced only if the aggregate molar activity of all active moieties at the site of action is predictably affected by genotype or phenotype. Variation in drug concentration must be significant enough to provide "signal" over and above normal variation, and there must be a genuine concentration-effect relationship. The therapeutic index of the drug will also influence test utility. After considering all of these factors, the benefits of prospective testing need to be weighed against the costs and against other endpoints of effect. It is not surprising that few drugs satisfy these requirements. Drugs (and enzymes) for which there is a reasonable evidence base supporting genotyping or phenotyping include suxamethonium/mivacurium (butyrylcholinesterase), and azathioprine/6-mercaptopurine (thiopurine methyltransferase). Drugs for which there is a potential case for prospective testing include warfarin (CYP2C9), perhexiline (CYP2D6), and perhaps the proton pump inhibitors (CYP2C19). No other drugs have an evidence base that is sufficient to justify prospective testing at present, although some warrant further evaluation. In this review we summarize the current evidence base for pharmacogenetics in relation to drug-metabolizing enzymes.

The relevance of CYP2D6 genetic polymorphism on chronic metoprolol therapy in cardiovascular patients

BACKGROUND AND OBJECTIVE

CYP2D6 polymorphisms are well described in normal populations but there are few data on its clinical significance. We therefore investigated the influence of CYP2D6 polymorphism on steady-state plasma concentrations and apparent oral clearance of metoprolol in patients with cardiovascular diseases.

METHODS

Ninety-one patients on metoprolol were recruited. Plasma concentrations of metoprolol and alpha-hydroxy metoprolol were measured at 4-h post-dose. CYP2D6 genotyping (*1, *3, *4, *5, *9, *8, *10, *17 and duplication) were performed on the DNA extracted. Ratio of plasma concentrations of metoprolol and alpha-hydroxy metoprolol and the apparent oral clearance of metoprolol were calculated. The influences of CYP2D6 genotypes were investigated.

RESULTS

A 100-fold variation was noted for both plasma concentrations of metoprolol and alpha-hydroxy metoprolol. There was a weak correlation between the total daily doses and plasma concentrations of both. Plasma concentrations were found to be higher in patients with genotypes that predicted lower enzyme activity. One patient homozygous for CYP2D6*4 had the highest metoprolol concentration per unit dose. With an antimode of 10. Two patients were identified as poor metabolizers (PMs) (2.1%; 95% CI: 2.9). The PMs who had a plasma metabolic ratio (pMR) of 37.8 was homozygous CYP2D6*4 whereas the other with pMR 14.5 was genotyped CYP2D6 *4/*10. There was a 36-fold difference between the highest and lowest clearance values. Large overlaps in the clearance values were noted between most of the genotypes.

CONCLUSIONS

Our data suggest that pharmacogenetic measures could be used to design a more individualized metoprolol dosage regimen for patients.

Assessment of in vivo CYP2D6 activity: differential sensitivity of commonly used probes to urine pH

Drug/metabolite ratios (MRs) are used as in vivo markers of enzyme activity. The ratios are potentially confounded by the renal clearance of the drug (urine-based MRs) or metabolite (plasma-based MRs). The authors have investigated the relative sensitivity of urinary MR of 3 in vivo probe substrates of CYP2D6 debrisoquine (DB), dextromethorphan (DM), and metoprolol (MP) to changes in urine pH. Three groups of healthy volunteers each comprising 12 individuals were given DB (10 mg), DM (25 mg), or MP (100 mg) on 3 occasions. In 1 study arm, urine was acidified by the oral intake of ammonium chloride; in another, it was alkalinized by intake of sodium bicarbonate; and in the third, urine pH was uncontrolled. Urinary MP/alpha-hydroxy-MP, DM/dextrorphan, and DB/4-hydroxy-DB ratios were calculated. The mean(geo) MR for DB was not significantly different in any of the study arms, whereas those for MP and DM were significantly different under acidified and alkalinized urine conditions compared to uncontrolled urine pH (P < .01) and were correlated with urine pH (P < .001). Without control of urine pH, in vivo estimates of CYP2D6 metabolic activity are likely to be less precise using DM or MP as probe substrates compared to DB. Although this is unlikely to cause any problem in distinguishing the large functional differences in CYP2D6 in poor metabolizer (PM) and extensive metabolizer (EM) phenotypes, this may contribute to difficulties in differentiating in vivo metabolic activity among allelic variants within the overall CYP2D6 EM phenotype using MP or DM. However, because DB is not available in many countries (eg, United States), alternative in vivo markers of CYP2D6 with low sensitivity to urine pH should be sought.

Polymorphic cytochrome P450 2D6: humanized mouse model and endogenous substrates

Cytochrome P450 2D6 (CYP2D6) is the first well-characterized polymorphic phase I drug-metabolizing enzyme, and more than 80 allelic variants have been identified for the CYP2D6 gene, located on human chromosome 22q13.1. Human debrisoquine and sparteine metabolism is subdivided into two principal phenotypes--extensive metabolizer and poor metabolizer--that arise from variant CYP2D6 genotypes. It has been estimated that CYP2D6 is involved in the metabolism and disposition of more than 20% of prescribed drugs, and most of them act in the central nervous system or on the heart. These drug substrates are characterized as organic bases containing one nitrogen atom with a distance about 5, 7, or 10 A from the oxidation site. Aspartic acid 301 and glutamic acid 216 were determined as the key acidic residues for substrate-enzyme binding through electrostatic interactions. CYP2D6 transgenic mice, generated using a lambda phage clone containing the complete wild-type CYP2D6 gene, exhibits enhanced metabolism and disposition of debrisoquine. This transgenic mouse line and its wild-type control are models for human extensive metabolizers and poor metabolizers, respectively, and would have broad application in the study of CYP2D6 polymorphism in drug discovery and development, and in clinical practice toward individualized drug therapy. Endogenous 5-methoxyindole- thylamines derived from 5-hydroxytryptamine were identified as high-affinity substrates of CYP2D6 that catalyzes their O-demethylations with high enzymatic capacity and specificity. Thus, polymorphic CYP2D6 may play an important role in the interconversions of these psychoactive tryptamines, including a crucial step in a serotonin-melatonin cycle.

Pharmacokinetic evaluation of guar gum-based three-layer matrix tablets for oral controlled delivery of highly soluble metoprolol tartrate as a model drug

The objective of the present study is to carry out pharmacokinetic evaluation of oral controlled release formulation (guar gum-based three-layer matrix tablets) containing highly soluble metoprolol tartrate as a model drug. Six healthy volunteers participated in the study, and a two-way crossover design was followed. The plasma concentration of metoprolol tartrate was estimated by reverse-phase HPLC. The pharmacokinetic parameters were calculated from the plasma concentration of metoprolol tartrate versus time data. The delayed T(max) lower C(max) decreased K(a) unaltered bioavailability and prolonged t(1/2) indicated a slow and prolonged release of metoprolol tartrate from guar gum three-layer matrix tablets in comparison with the immediate release tablet dosage form. The results of the study indicated that guar gum three-layer matrix tablets were able to provide oral controlled delivery of highly water-soluble drug such as metoprolol tartrate in humans.

Human variability in polymorphic CYP2D6 metabolism: is the kinetic default uncertainty factor adequate?

Human variability in the kinetics of CYP2D6 substrates has been quantified using a database of compounds metabolised extensively (>60%) by this polymorphic enzyme. Published pharmacokinetic studies (after oral and intravenous dosing) in non-phenotyped healthy adults, and phenotyped extensive (EMs), intermediate or slow-extensive (SEMs) and poor metabolisers (PMs) have been analysed using data for parameters that relate primarily to chronic exposure (metabolic and total clearances, area under the plasma concentration time-curve) and primarily to acute exposure (peak concentration). Similar analyses were performed with the available data for subgroups of the population (age, ethnicity and disease). Interindividual differences in kinetics for markers of oral exposure were large for non-phenotyped individuals and for EMs (coefficients of variation were 67-71% for clearances and 54-63% for C(max)), whereas the intravenous data indicated a lower variability (34-38%). Comparisons between EMs, SEMs and PMs revealed an increase in oral internal dose for SEMs and PMs (ratio compared to EMs=3 and 9-12, respectively) associated with lower variability than that for non-phenotyped individuals (coefficients of variation were 32-38% and 30% for SEMs and PMs, respectively). In relation to the uncertainty factors used for risk assessment, most subgroups would not be covered by the kinetic default of 3.16. CYP2D6-related factors necessary to cover 95-99% of each subpopulation ranged from 2.7 to 4.1 in non-phenotyped healthy adults and EMs to 15-18 in PMs and 22-45 in children. An exponential relationship (R(2)=0.8) was found between the extent of CYP2D6 metabolism and the uncertainty factors. The extent of CYP2D6 involvement in the metabolism of a substrate is critical in the estimation of the CYP2D6-related factor. The 3.16 kinetic default factor would cover PMs for substrates for which CYP2D6 was responsible for up to 25% of the metabolism in EMs.

Drug interaction studies with esomeprazole, the (S)-isomer of omeprazole

Esomeprazole, the (S)-isomer of omeprazole, is the first proton pump inhibitor (PPI) developed as a single isomer for the treatment of patients with acid related diseases. Because of the extensive use of PPIs, the documentation of the potential for drug interactions with esomeprazole is of great importance. Altered absorption or metabolism are 2 of the major mechanisms for drug-drug interactions. Since intragastric pH will increase with esomeprazole treatment, it can be hypothesised that the absorption of drugs with pH-sensitive absorption (e.g. digoxin and ketoconazole) may be affected. Esomeprazole does not seem to have any potential to interact with drugs that are metabolised by cytochrome P450 (CYP) 1 A2, 2A6, 2C9, 2D6 or 2E1. In drug interaction studies with diazepam, phenytoin and (R)-warfarin, it was shown that esomeprazole has the potential to interact with CYP2C19. The slightly altered metabolism of cisapride was also suggested to be the result of inhibition of a minor metabolic pathway for cisapride mediated by CYP2C19. Esomeprazole did not interact with the CYP3A4 substrates clarithromycin (2 studies) or quinidine. Since the slightly increased area under the concentration-time curve (AUC) of cisapride could be explained as an inhibition of CYP2C19, the data on these 3 CYP3A4 substrates indicate that esomeprazole does not have the potential to inhibit this enzyme. The minor effects reported for diazepam, phenytoin, (R)-warfarin, and cisapride are unlikely to be of clinical relevance. Clarithromycin interacts with the metabolism of esomeprazole resulting in a doubling of the AUC of esomeprazole. The increased plasma concentrations of esomeprazole are unlikely to have any safety implications. It can be concluded that the potential for drug-drug interactions with esomeprazole is low, and similar to that reported for omeprazole.

Metoprolol CR/XL in the Treatment of Chronic Heart Failure

Metoprolol CR/XL (metoprolol succinate extended-release tablets) is a beta1-selective agent that improved survival and reduced hospitalization among patients with New York Heart Association class II-IV heart failure in a randomized trial. Metoprolol CR/XL differs from conventional metoprolol tartrate with respect to pharmacokinetic and pharmacodynamic properties that may be clinically important in patients with heart failure. A thorough patient evaluation should be performed to determine optimal dosage and titration of this drug, as with any beta-blocker, and to assess the potential for drug-drug or drug-disease interactions. By applying knowledge of drug-specific characteristics and designing therapy for each individual patient, improvement in patient outcomes can be realized with metoprolol CR/XL.

Antihypertensive pharmacogenetics: getting the right drug into the right patient

Pharmacogenetic investigation seeks to identify genetic factors that contribute to interpatient and interdrug variation in responses to antihypertensive drug therapy. Classical studies have characterized single gene polymorphisms of drug metabolizing enzymes that are responsible for large interindividual differences in pharmacokinetic responses to several antihypertensive drugs. Progress is being made using candidate gene and genome scanning approaches to identify and characterize many additional genes influencing pharmacodynamic mechanisms that contribute to interindividual differences in responses to antihypertensive drug therapy. Knowledge of polymorphic variation in these genes will help to predict individual patients' blood pressure responses to antihypertensive drug therapy and may also provide new insights into molecular mechanisms responsible for elevation of blood pressure.

Effect of metoprolol and verapamil administered separately and concurrently after single doses on liver blood flow and drug disposition

Nine healthy males participated in a double-blind, placebo-controlled, randomized, crossover study to determine the effects of verapamil and metoprolol administered alone and concurrently on blood flow through the hepatic artery and portal and hepatic veins and to detect a possible drug interaction between the two agents. Single oral doses of placebo/placebo, metoprolol (50 mg)/placebo, verapamil (80 mg)/placebo, or verapamil/metoprolol were separated by at least 14 days. Liver blood flow through individual hepatic vessels was measured up to 8 hours after dosage administration using a duplex Doppler ultrasound technique. Cardiac output, heart rate, blood pressure, stroke volume, and total peripheral resistance were measured for 3 hours after drug doses were given. In 5 subjects, pharmacokinetic parameters for total drug as well as S- and R-enantiomers were also measured. Verapamil given alone caused a rapid and intense increase in liver blood flow (hepatic artery = 50%, portal vein = 42%, hepatic vein = 55%) 0.75 to 1 hour after administration because of a decrease in total peripheral resistance and an increase in heart rate, stroke volume, and cardiac output. Metoprolol given alone caused a slow but prolonged decrease in liver blood flow (maximum decrease: hepatic artery = -54%, portal vein = -21%, hepatic vein = -27%) 4 hours after administration because of a decrease in heart rate and cardiac output. When the two agents were given together, a composite of the changes noted after separate administration was noted: a brief peak increase in liver blood flow at 0.33 to 1 hour followed by a slow, prolonged decrease that reached its maximum decline 4 to 5 hours postdose. During the combined phase, metoprolol and its enantiomers had an increased AUC and Cmax, while verapamil and its enantiomers had an increased AUC and t1/2. These pharmacokinetic changes were consistent with the magnitude and time course of liver blood flow changes through the hepatic artery and portal or hepatic veins.

The relative bioavailability of metoprolol following oral and rectal administration to volunteers and patients

The pharmacokinetics, efficacy and safety of metoprolol tartrate 25 mg fatty suppositories were studied in 5 healthy volunteers and in 8 patients suffering from instable angina pectoris. Metoprolol 25 mg capsules were used as a control oral dosage form. Metoprolol showed a considerable rectal bioavailability (AUC, C max) and was absorbed quickly from the rectum (T max). In both groups rectal bioavailability was comparable. However, oral bioavailability was much lower in the volunteer group than in the patient group. Furthermore, ratios of metoprolol/alpha-OH-metoprolol concentrations in plasma and urine gave an indication for a partial avoidance of the first pass effect after rectal administration. Further research is necessary to define an exact rectal dosage of metoprolol. In all patients, a substantial drop in heart rate, systolic and diastolic blood pressure was seen after administration of the first suppository. Metoprolol suppositories appear to be an effective, safe and suitable alternative for patients who are in need for beta blocking medication and who are unable to take oral medication for a certain amount of time.

Pharmacokinetic and bioequivalent study of a generic Metoprolol tablet preparation

A study was conducted to compare the in vivo bioavailability of a generic metoprolol tablet preparation (Metoprolol) with that of the innovator product, Betaloc. Both preparations have a labeled dose of 100 mg metoprolol tartrate. Twelve healthy adult male volunteers participated in the study, which was conducted according to a standard two-way crossover design with a washout period of 1 week. The bioavailability was compared using the total area under the plasma level versus time curve (AUC0-infinity), peak plasma concentration (Cmax), and time to reach peak plasma concentration (Tmax). No statistically significant difference was observed between the logarithmically transformed AUC0-infinity values or the logarithmically transformed Cmax values of the two preparations. However, a statistically significant difference was observed between the Tmax values, but may not be therapeutically significant or important. Moreover, the 90% confidence interval (CI) for the ratio of the logarithmically transformed AUC0-infinity values of Metoprolol over those of Betaloc was calculated to be between 0.94 and 1.02, while that of Cmax was between 0.98 and 1.01, both of which are within the acceptable limit of 0.80-1.25. From the data obtained, it was also observed that a high proportion of our volunteers of Asian origin appeared to be poor metabolizers of metoprolol, which was consistent with what had been observed in our previous study of another preparation of metoprolol.

Age might influence the frequency distribution of metoprolol hydroxylation polymorphism in a Chinese population

The frequency distribution of the 8-hour urinary ratio of metoprolol/alpha-hydroxymetoprolol (MR) was investigated in 206 healthy rural Chinese volunteers. The frequency of the poor metabolizer phenotype of metoprolol alpha-hydroxylation was 0% in this Chinese population. When the 206 subjects were subgrouped into four groups by age, the excretion of alpha-hydroxymetoprolol was significantly lower in the subjects with age older than 41 years compared to two younger groups (16-30 years), and the metoprolol hydroxylation metabolic ratio(metoprolol/a-hydroxymetoprolol, MR) was higher in these middle-aged subjects, indicating that age may affect the frequency distribution of the 8-hour urinary MR or log MR, although multiple regression analysis did not show a significant correlation between age, urine volume and MR. Also, a significant correlation was found between age and the excretion (percentage of dose) of alpha-hydroxymetoprolol in 8-hour urine (r = 0.259, P < 0.001). On the other hand, both multiple regression analysis and nonparametric tests showed that the larger the amount of urine excreted, the more metoprolol was recovered, demonstrating that the urinary excretion of unchanged metoprolol is renal flow-limited.

Drug interactions with proton pump inhibitors

Omeprazole, lansoprazole and pantoprazole are all mainly metabolised by the polymorphically expressed cytochrome P450 (CYP) isoform CYP2C19 (S-mephenytoin hydroxylase). All 3 proton pump inhibitors have a very limited potential for drug interactions at the CYP level. Small effects on CYP reported for these compounds are usually of no clinical relevance. No dose related adverse effects have been identified, suggesting that the small proportion of slow metabolisers is at no additional risk for clinically important drug interactions. The absorption of some compounds, e.g. benzylpenicillin (penicillin G), are altered during treatment with proton pump inhibitors as a result of the increased intragastric pH. A synergy has been confirmed between omeprazole and amoxicillin or clarithromycin in the antibacterial effect against Helicobacter pylori.

Pharmacokinetics, metabolism and interactions of acid pump inhibitors. Focus on omeprazole, lansoprazole and pantoprazole

This review updates and evaluates the currently available information regarding the pharmacokinetics, metabolism and interactions of the acid pump inhibitors omeprazole, lansoprazole and pantoprazole. Differences and similarities between the compounds are discussed. Omeprazole, lansoprazole and pantoprazole are all mainly metabolished by the polymorphically expressed cytochrome P450 (CYP) isoform S-mephenytoin hydroxylase (CYP2C19), which means that within a population a few individuals (3% of Caucasians) metabolise the compounds slowly compared with the majority of the population. For all 3 compounds, the area under the plasma concentration-versus-time curve (AUC) for a slow metaboliser is, in general, approximately 5 times higher than that in an average patient. Since all 3 compounds are considered safe and well tolerated, and no dosage-related adverse drug reactions have been identified, this finding seems to be of no clinical relevance. The acid pump inhibitors seem to be similarly handled in the elderly, where a somewhat slower elimination can be demonstrated compared with young individuals. In patients with renal insufficiency, omeprazole is eliminated as in healthy individuals, whereas the data on lansoprazole and pantoprazole are unresolved. In patients with hepatic insufficiency, as expected, the elimination rates of all 3 compounds are substantially decreased. No clinically relevant effects on specific endogenous glandular functions, such as the adrenal (cortisol), the gonads or the thyroid, were demonstrated for omeprazole and pantoprazole, whereas a few minor concerns have been raised regarding lansoprazole. The absorption of some compounds, e.g. digoxin, might be altered as a result of the increased gastric pH obtained during treatment with acid pump inhibitors, and, accordingly, similar effects are expected irrespective of which acid pump inhibitor is given. The effect of the acid pump inhibitors on enzymes in the liver has been intensely debated, and some authors have claimed that lansoprazole and pantoprazole have less potential than omeprazole to interact with other drugs metabolised by CYP. However, after assessment of available data in this area, the conclusion is that all 3 acid pump inhibitors have a very limited potential for drug interactions at the CYP level. In addition, the small effects on CYP reported for these compounds are rarely of any clinical relevance, considering the normal intra- (and inter-)individual variations in metabolism observed for most drugs. In conclusion, omeprazole, lansoprazole and pantoprazole are structurally very similar, and an evaluation of available data indicates that also with respect to pharmacokinetics, metabolism and interactions in general they demonstrate very similar properties, even though omeprazole has been more thoroughly studied with regard to different effects.

"It's the genes, stupid". Molecular bases and clinical consequences of genetic cytochrome P450 2D6 polymorphism

In this review we highlight the information available on the genetic polymorphism of cytochrome P4502D6 expression in man. An absent function of this enzyme is observed in 7-10 percent of the Caucasian population which are referred to as Poor metabolizers as opposed to the remainder of the population (Extensive Metabolizers). More than 30 widely used drugs have been identified as substrates for CYP2D6. Disposition and action of these compounds depend on the individual phenotype. Both the molecular bases of the variable enzyme activity and the consequences for drug therapy are outlined. While mutations on the DNA level have been investigated in great detail larger scale clinical trials are lacking and information on therapeutic consequences of CYP2D6 mediated polymorphic drug oxidation is restricted to case reports. Besides these implications for drug metabolism several lines of evidence indicate that CYP2D6 could be involved in biotransformation of endogenous compounds.

Metoprolol alpha-hydroxylation is a poor probe for debrizoquine oxidation (CYP2D6) polymorphism in Jordanians

The frequency distribution of the 8-h urinary ratio of log metoprolol/alpha-hydroxymetoprolol was assessed in 65 healthy, unrelated Jordanian volunteers. There was no apparent bimodality in the frequency distribution of this ratio among the subjects studied. The frequency of the poor metabolizer phenotype of metoprolol alpha-hydroxylation was 1.5% (one subject). There was a significant correlation (r = 0.61, P < 0.05, n = 39) between the log metoprolol/alpha-hydroxymetoprolol and the log debrisoquine/4-hydroxydebrisoquine ratios. However, the frequency of poor metabolizer status of debrisoquine among the 39 subjects was 7.7% (three subjects). Only one of the poor metabolizer of metoprolol alpha-hydroxylation. These findings indicate that metoprolol alpha-hydroxylation by CYP2D6 represents a poor probe for studying debrisoquine polymorphism in Jordanians.

Metoprolol: a pharmacoeconomic and quality-of-life evaluation of its use in hypertension, post-myocardial infarction and dilated cardiomyopathy

Metoprolol is a beta 1-selective adrenoceptor antagonist that is widely used in several indications. A recent investigation has also highlighted a potential role for metoprolol in selected patients with idiopathic dilated cardiomyopathy. Pharmacoeconomic and quality-of-life data for metoprolol are limited to the areas of hypertension, post-myocardial infarction and idiopathic dilated cardiomyopathy. In these settings, metoprolol has shown beneficial effects on morbidity and mortality, or closely-related end-points. Controlled release formulations offer the potential to maximise the confirmed antihypertensive benefits of metoprolol by maintaining clinically effective plasma drug concentrations within a narrow range over a 24-hour interval between doses. Recent data support the use of controlled release metoprolol at the low dose of 50 mg/day. Metoprolol is at least as effective as many other antihypertensive drugs, although compared with thiazide diuretics at relatively high doses in the MAPHY (Metoprolol Atherosclerosis Prevention in Hypertensives) trial, metoprolol was associated with a more favourable effect on mortality. Pharmacoeconomic analysis, also based on the MAPHY trial, indicates that metoprolol is more cost effective than high dose thiazide diuretics in middle-aged men with mild to moderate hypertension. However, the advantage for beta-blockade in this trial is not supported by results of other studies, and the applicability of these data to current medical practice using lower thiazide doses is therefore questionable. Quality of life in patients with mild to moderate hypertension did not deteriorate in most investigations with metoprolol. Furthermore, quality of life was similar for controlled release metoprolol and atenolol. With conventional/matrix-based sustained release metoprolol, quality of life was less satisfactory than with lisinopril but was only marginally different from that with diltiazem (at lower than usual therapeutic doses). Nevertheless, these newer agents have no proven beneficial effect on mortality, and further studies are also warranted with controlled release metoprolol 50 mg/day. When administered post-myocardial infarction, conventional metoprolol was associated with significant improvements in quality of life and was cost saving over a 3-year period. Significant improvements in quality of life were also evident for metoprolol-treated patients with idiopathic dilated cardiomyopathy. In summary, available data support the continued extensive usage of metoprolol as treatment for hypertension and as therapy post-myocardial infarction. Pharmacoeconomic data supporting an advantage for metoprolol over high dose thiazides in hypertension needs further assessment in settings reflecting usual general practice approaches to managing patients with hypertension, while differences in quality of life between metoprolol and other antihypertensive agents appear to be marginal.(ABSTRACT TRUNCATED AT 400 WORDS)

Inter- and intra-subject variability of metoprolol kinetics after intravenous administration

The pharmacokinetics of metoprolol were assessed from the data of 3 experiments in which intravenous deuterated metoprolol was given concomitantly with various oral metoprolol formulations to healthy volunteers. The plasma levels after intravenous administration were well described by a biexponential equation. However, a deviation from a pure biphasic decline was observed over a short period of time, immediately after the rapid elimination phase. The inter- and intra-subject variability of the parameters describing the two-compartment model was large. The intra-subject variability of the terminal half-life and the plasma clearance was lower than their inter-subject variability. The plasma clearance was linearly related to 1/t1/2. Contrarily to the other parameters of the model, t1/2 appeared to be a characteristic of the subject.

Interactions of bupranolol with the polymorphic debrisoquine/sparteine monooxygenase (CYP2D6)

The beta-adrenoceptor blocker bupranolol turned out to be a competitive inhibitor of the polymorphic cytochrome P450 CYP2D6 of which sparteine is a substrate. There was stereo-selectivity of bupranolol involved: (-)-bupranolol was the weakest inhibitor with an apparent Ki value of 1.32 microM, (+)-bupranolol was the most potent with an apparent Ki value of 0.55 microM, while the therapeutically used racemic bupranolol had an intermediate value of 0.88 microM. A 10 min pre-incubation of 5 microM bupranolol with the enzyme preparation prior to the addition of substrate, reduced the inhibition of sparteine metabolism from 52 to about 25%. This suggests that--during these inhibition studies--bupranolol was much more rapidly metabolized than was sparteine, so that the measured Ki values must represent overestimates. The enzyme catalysing bupranolol metabolism was CYP2D6: microsomes from a liver with the genetic enzyme deficiency did not metabolize bupranolol; in microsomes from livers containing the enzyme and 10 microM bupranolol, 5 microM quinidine caused a 72% inhibition of bupranolol metabolism. Although our methods were not sufficiently sensitive to measure the Km of bupranolol directly, it is undoubtedly the beta-adrenoceptor blocker with the highest-known apparent affinity for CYP2D6. High affinity and rapid metabolism are infrequent combinations in enzymology.

Dextromethorphan O-demethylation polymorphism in Jordanians

The O-demethylation of dextromethorphan (DMT) to dextrorphan (DRP) was studied in 241 unrelated, healthy Jordanian volunteers (171 males, 70 females). Urine was collected for 8 h following a single oral dose of DMT bromhydrate 30 mg. A thin-layer chromatographic (TLC) technique was used to identify the metaboliser phenotype. The frequency of the poor metaboliser phenotype was found to be 2.9% (approximate 95% confidence interval 0.8-5.0%). Applying the Hardy-Weinberg Law, the frequency of the recessive autosomal gene controlling poor metabolism was 0.17 (95% confidence interval 0.108-0.232).

Oxidative metabolism of cinnarizine in rat liver microsomes

The oxidative metabolism of cinnarizine (CZ) [1-(diphenylmethyl)-4-(3-phenyl-2-propenyl)-piperazine] to 1-(diphenylmethyl)piperazine (M-1), 1-(diphenylmethyl)-4-[3-(4'-hydroxyphenyl)-2-propenyl]piperazine (M-2), benzophenone (M-3) and 1-[4'-hydroxyphenyl)-phenylmethyl]-4-(3- phenyl-2-propenyl)piperazine (M-4) has been studied in rat liver microsomes. In Wistar rats, kinetic analysis revealed sex differences (male > female) in the Km values for formation of all the metabolites and the Vmax values for the formation of M-1, M-3 and M-4. The reactions required NADPH, and were inhibited by carbon monoxide and SKF 525-A. Only M-2 formation was suppressed by sparteine or metoprolol, and was significantly lower in female Dark Agouti rats than in Wistar rats of both sexes. The results suggest that CZ is oxidized by cytochrome P450, and M-2 formation is related to debrisoquine/sparteine-type polymorphic drug oxidation.

Controlled release metoprolol formulations. A review of their pharmacodynamic and pharmacokinetic properties, and therapeutic use in hypertension and ischaemic heart disease

Conventional formulations of metoprolol have become well established in cardiovascular medicine and are particularly useful in the management of hypertension and ischaemic heart disease. Recently developed controlled release metoprolol delivery systems (metoprolol CR/ZOK and metoprolol OROS) were designed to overcome the drug delivery problems of matrix-based sustained release forms by releasing the drug at a relatively constant rate over a 24-hour period, and thus producing sustained and consistent metoprolol plasma concentrations and beta 1-blockade while retaining the convenience of once daily administration. Clinically and statistically significant reductions in blood pressure have been observed with metoprolol CR/ZOK and metoprolol OROS 24 hours after administration in mildly or moderately hypertensive patients. Studies in patients with mild to moderate hypertension have demonstrated that a similar or higher percentage of patients achieved a goal response with metoprolol CR/ZOK compared with matrix-based sustained release formulations of metoprolol, or conventional atenolol or bisoprolol, while metoprolol OROS achieved an equal or greater response rate compared with conventional or matrix-based sustained release metoprolol preparations. In patients with stable effort angina pectoris, once daily administration of metoprolol CR/ZOK provided at least equal antianginal efficacy as conventional metoprolol in divided doses, while metoprolol OROS reduced the mean number of anginal attacks by the same margin as atenolol. Controlled release metoprolol formulations have been well tolerated in clinical trials. Metoprolol CR/ZOK was associated with a similar or lesser degree of adverse effects related to the central nervous system compared with atenolol or long acting propranolol. Metoprolol CR/ZOK also demonstrated less pronounced beta 2-mediated bronchoconstrictor effects than atenolol in asthmatics, and less general fatigue and leg fatigue in healthy subjects. Metoprolol OROS produced less pronounced bronchoconstrictor effects than atenolol, matrix-based sustained release metoprolol or long acting propranolol in patients with asthma or obstructive airways disease, and healthy volunteers. These results are presumably due to the beta 1-selectivity of metoprolol in addition to the relatively low plasma concentrations maintained by metoprolol CR/ZOK and metoprolol OROS, and the avoidance of high peak plasma concentrations with these agents. Despite the relative safety of the controlled release forms of metoprolol, the use of all beta-adrenoceptor antagonists should be avoided in patients with a history of bronchospasm. Thus, controlled release metoprolol formulations offer the potential to maximise the confirmed benefits of this agent in the management of hypertension and angina, by maintaining clinically effective plasma concentrations within a narrow therapeutic range over a 24-hour dose interval.

Ethnic differences in drug disposition and responsiveness

Interethnic differences are important factors accounting for interindividual variations in drug responsiveness. However, these differences in drug response have been a relatively neglected area of investigation, so that similar doses are prescribed to different ethnic populations without consideration of interethnic pharmacokinetic and pharmacodynamic variation. With the increased recognition of genetically determined polymorphism in metabolising ability as an important factor in drug disposition, concern has developed for the importance of individualising drug dose to account for racial differences. The recognition of these differences in drug disposition and responses calls into question the failure of drug licensing authorities to demand information on dosage, efficacy and toxicity in different ethnic groups, and to accept data from limited ethnic groups such as Caucasians. This article reviews the evidence for ethnic differences in drug disposition and sensitivity and should encourage further investigations to elucidate the extent of such differences, their causes and their therapeutic impact.

Clinical consequences of polymorphic drug oxidation

Many characters are genetically regulated as polymorphisms. This means that discrete groups are seen within the distribution of a certain character. Drug metabolism is no exception and the polymorphism of acetylation is recognised since the 50's. Polymorphic drug oxidation was discovered in the 70's and has been extensively studied. There are two fully established polymorphisms in drug oxidation named as the debrisoquine/sparteine and the s-mephenytoin hydroxylation polymorphisms. The metabolism of a number of important drugs cosegregates with that of debrisoquine. Among these drugs are beta-blockers, antiarrhythmics, tricyclic antidepressants and neuroleptics. Apart from accumulation of parent drug and active metabolite, also reduced formation of active metabolite occur for some drugs in slow metabolisers. There are, however, few cases where the presence of polymorphic drug metabolism is of significant disadvantage. The polymorphisms will add to variability in drug clearance but the potential clinical importance should be evaluated for each drug. The cytochrome P-450 isozyme responsible for debrisoquine hydroxylation is of high affinity-low capacity character, which means that it can be saturated under certain circumstances. This will decrease the difference in drug metabolic rate between rapid and low metabolisers as will inhibitors of the debrisoquine isozyme like cimetidine, quinidine and propafenone. The debrisoquine isozyme is not readily inducible. In cases where a major metabolic route or the formation of an active metabolite are polymorphically controlled, knowledge about a patient's oxidator status might be of practical value for dose adjustments especially if there is a narrow therapeutic ratio or an established concentration-effect relationship. For some drugs it is difficult to differentiate between insufficient therapeutic effect and symptoms of overdosage. Tricyclic antidepressants and neuroleptics meet some of these criteria and patients who get recurrent treatment may benefit if the physician has knowledge about debrisoquine metabolic phenotype. Otherwise, the clinical consequences of polymorphisms in drug oxidation seem so far to be limited, considering that a number of disease conditions have not shown any clear association with oxidation status. The polymorphisms in drug metabolism should be considered as a part of natural variability which could in fact be larger with other drugs that do not show polymorphic elimination.

Lack of effect of omeprazole treatment on steady-state plasma levels of metoprolol

In a randomised double-blind crossover study, seven healthy males were concomitantly given metoprolol 100 mg o.d. as a controlled release formulation, and omeprazole 40 mg o.d. or placebo, for 8 days. Plasma levels of the R- and S-enantiomers of metoprolol were determined on the 8th day of each treatment. The subjects were also characterised by their metabolic capacity to hydroxylate debrisoquine. Concomitant omeprazole treatment had no significant influence on the steady-state plasma levels of the two enantiomers of metoprolol. All subjects were characterised by extensive debrisoquine hydroxylation, i.e. extensive metoprolol metabolism. As metoprolol is metabolised to a great extent by debrisoquine hydroxylase (IID6), it is concluded that concomitant omeprazole treatment will probably have a negligible influence on the metabolism of the relatively large number of drugs mainly metabolised by this isoenzyme of the cytochrome P450 family.

Enantioselective and diastereoselective aspects of the oxidative metabolism of metoprolol

Enantio- and diastereoselective aspects of oxidative metabolism of metoprolol (1) were examined in the presence of rat liver and human liver microsomes using a pseudoracemate of 1, made up of equal molar (2R)-1-d0 and (2S)-1-d2, as substrate. Both O-demethylation and alpha-hydroxylation showed only slight enantioselectivity, 2R/2S ratios being 1.18 and 0.93 for these pathways in rat liver microsomes and 1.09 and 0.92 in human liver microsomes. In the presence of the rat liver microsomal fraction, alpha-hydroxylation yielded predominantly the 1'R-hydroxy product, 1'R/1'S ratio greater than 12, regardless of the stereochemistry of the side chain. In humans (extensive metabolizers) administered a single 50 mg oral dose of pseudoracemic metoprolol tartrate, urinary alpha-hydroxymetoprolol (2) accounted for 9.3 +/- 2.4% of the dose, 2R/2S ratio 0.85 +/- 0.14, and the carboxylic acid metabolite 4, accounted for 52.7 +/- 6.8% of the dose, 2R/2S ratio 1.15 +/- 0.09. The data suggested that preferential O-demethylation of the (2R)-enantiomer of 1 could contribute to the 2S greater than 2R plasma ratio of metoprolol enantiomers observed in this population.

A pharmacokinetic and pharmacodynamic comparison of metoprolol CR/ZOK with a conventional slow release preparation

The plasma concentration-time profile and hemodynamic effects of metoprolol after the administration of metoprolol CR/ZOK, a multiple-unit controlled release formulation and a conventional slow release formulation, metoprolol Durules (Astra, AB Hässle, Mölndal, Sweden) once daily, were investigated in 12 healthy men. Data were collected over one 24-hour dose interval at steady state after 5 days of treatment. The study was randomized, three-way, crossover double-blind comparison of metoprolol CR/ZOK 200 mg, metoprolol Durules 200 mg and placebo. The reduction in exercise heart rate compared with placebo treatment was used as a measure of beta 1-blockade. The metoprolol plasma concentration-time profile during treatment with metoprolol CR/ZOK was smooth and uniform, showing a more constant release profile than that obtained with metoprolol Durules. This was demonstrated by the significantly longer time period during which the plasma concentration exceeded 75% of the maximum concentration (T75), for metoprolol CR/ZOK compared with metoprolol Durules (P less than .01). The Fluctuation Index of plasma metoprolol concentration was significantly smaller for metoprolol CR/ZOK than for metoprolol Durules (P less than .001). The pharmacokinetic differences between the formulations also produced differences in the time profiles of exercise heart rate. The percentage fluctuation in exercise heart rate over the dose interval tended to be smaller for metoprolol CR/ZOK. At the start and the end of the dosing interval, the CR/ZOK formulation was significantly more effective (P less than .01). These results indicate that Metoprolol CR/ZOK has a more sustained time profile of beta 1-blockade at steady state.(ABSTRACT TRUNCATED AT 250 WORDS)

Debrisoquine oxidative phenotyping and psychiatric drug treatment

The debrisoquine/sparteine phenotype was determined in 51 patients with depression, who were subdivided into 3 groups in terms of their drug treatment. Log (MR) for each group was compared. Patients treated with benzodiazepines had the same distribution of log (MR) as the healthy population, but the distribution was shifted towards higher values in patients treated with neuroleptics and antidepressants. It appears that the phenotypic expression of debrisoquine oxidation may be modified by drugs whose metabolism follows the same route as debrisoquine. The debrisoquine test must be carefully interpreted in patients receiving several drugs in the same time.