Timolol and atenolol: relationships between oxidation phenotype, pharmacokinetics and pharmacodynamics

The cardiac work-loop technique: An in vitro model for identifying and profiling drug-induced changes in inotropy using rat papillary muscles

The cardiac work-loop technique closely mimics the intrinsic in vivo movement and characteristics of cardiac muscle function. In this study, six known inotropes were profiled using the work-loop technique to evaluate the potential of this method to predict inotropy. Papillary muscles from male Sprague-Dawley rats were mounted onto an organ bath perfused with Krebs-Henseleit buffer. Following optimisation, work-loop contractions were performed that included an initial stabilisation period followed by vehicle control or drug administration. Six known inotropes were tested: digoxin, dobutamine, isoprenaline, flecainide, verapamil and atenolol. Muscle performance was evaluated by calculating power output during work-loop contraction. Digoxin, dobutamine and isoprenaline caused a significant increase in power output of muscles when compared to vehicle control. Flecainide, verapamil and atenolol significantly reduced power output of muscles. These changes in power output were reflected in alterations in work loop shapes. This is the first study in which changes in work-loop shape detailing for example the activation, shortening or passive re-lengthening have been linked to the mechanism of action of a compound. This study has demonstrated that the work-loop technique can provide an important novel method with which to assess detailed mechanisms of drug-induced effects on cardiac muscle contractility.

[Pharmacogenetics of timolol]

In recent years, β-adrenergic blockers have become the first choice drugs for glaucoma treatment. Timolol holds the main position among them, being a part of most combined antiglaucoma preparations. The use of timolol maleate in clinical practice may be accompanied by severe side effects affecting different organs and systems. The fact that cells with β-adrenergic receptors are widely common within the human body explains pharmacodynamic effects of timolol maleate. Because of these undesirable side effects, timolol maleate often evokes negative reaction from doctors and patients, which to certain extent limits its usage in ophthalmological practice. Obviously, efficacy and safety of timolol administration depends on individual characteristics making personalized approach necessary for every patient. Such particular approach, being the foundation of personalized medicine, increases efficacy and safety of timolol while reducing costs by using targeted doses.

Cardiac safety of ophthalmic timolol

INTRODUCTION

It is generally believed that topical administration of eye drops safeguards against harmful systemic effects. However, about 80% of the drug in the ophthalmic products is systemically absorbed and the first-pass metabolism is avoided. Ophthalmic timolol is widely prescribed in the treatment of glaucoma either alone or in the combination eye drop products, many of which have been launched fairly recently. Ophthalmic timolol may cause serious adverse effects such as symptomatic bradycardia, various conduction disorders in the heart, orthostatic hypotension, syncope and falls. Areas covered: In this review we document a number of factors associated with the properties of ophthalmic timolol and specific features of a patient, which may jeopardize patient's cardiac safety even after topical treatment. Expert opinion: Plasma timolol levels are correlated with cardiovascular adverse effects in patients, since timolol is mainly metabolized by cytochrome P450 2D6 (CYP2D6) enzyme in the liver. Patients who are lacking the functional CYP2D6 or who are concomitantly using potent CYP2D6 inhibitor drugs (e.g. paroxetine or fluoxetine) or verapamil or other beta-blockers are at risk of getting serious cardiac adverse effects. Prior to treatment initiation, ECG should be always performed and CYP2D6 genotyping should be considered, if routinely available.

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.

Paroxetine markedly increases plasma concentrations of ophthalmic timolol; CYP2D6 inhibitors may increase the risk of cardiovascular adverse effects of 0.5% timolol eye drops

Although ophthalmic timolol is generally well tolerated, a significant fraction of topically administered timolol can be systemically absorbed. We investigated the effect of the strong CYP2D6 inhibitor paroxetine on the pharmacokinetics of timolol after ophthalmic administration. In a four-phase crossover study, 12 healthy volunteers ingested either paroxetine (20 mg) or placebo daily for 3 days. In phases 1-2, timolol 0.1% gel, and in phases 3-4, timolol 0.5% drops were administered to both eyes. Paroxetine increased the plasma concentrations of timolol with both timolol formulations to a similar degree. The geometric mean ratio (95% confidence interval) of timolol peak concentration was 1.53-fold (1.23-1.91) with 0.1% timolol and 1.49-fold (0.94-2.36) with 0.5% timolol, and that of timolol area under the plasma concentration-time curve (AUC) from time 0 to 12 hours was 1.61-fold (1.26- to 2.06-fold) and 1.78-fold (1.21-2.62), respectively. During paroxetine administration, six subjects on 0.5% timolol drops, but none on 0.1% timolol gel, had plasma timolol concentrations exceeding 0.7 ng/ml, which can cause systemic adverse effects in patients at risk. There was a positive correlation between the AUC from time 0 to 13 hours of paroxetine and the placebo phase AUC from time 0 to infinity of timolol after timolol 0.5% drops (P < 0.05), and a nonsignificant trend after timolol 0.1% gel, consistent with the role of CYP2D6 in the metabolism of both agents. In the orthostatic test, heart rate immediately after upright standing was significantly lower (P < 0.05) during the paroxetine phase than during the placebo phase at 1 and 3 hours after 0.5% timolol dosing. In conclusion, paroxetine and other CYP2D6 inhibitors can have a clinically important interaction with ophthalmic timolol, particularly when patients are using 0.5% timolol formulations.

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.

Polymorphisms of genes CYP2D6, ADRB1 and GNAS1 in pharmacokinetics and systemic effects of ophthalmic timolol. A pilot study

OBJECTIVE

To test the hypotheses that (1) CYP2D6 genotype is associated with pharmacokinetics of ophthalmic timolol and (2) variation in genotypes of ADRB1 (beta(1)-adrenoceptor) and GNAS1 (alpha-subunit of G-protein) modulate heart rate (HR), and systolic (SAP) and diastolic (DAP) arterial pressure responses to timolol.

METHODS

Nineteen glaucoma patients and eighteen healthy volunteers were treated with 0.5% aqueous and 0.1% hydrogel formulations of ophthalmic timolol using a randomised cross-over design. The participants conducted head-up tilt and maximum exercise test at four visits. Plasma concentration of timolol was measured twice for glaucoma patients and ten times for healthy volunteers on each visit. Also, the genotypes for CYP2D6, ADRB1 and GNAS1 were determined.

RESULTS

Among healthy volunteers using aqueous timolol, poor metabolisers (PMs, n=2) of CYP2D6 had higher maximum plasma concentrations (C(max), values 2.63 and 2.94 ng/ml), longer elimination half-lives ( T(1/2), 5.49 and 6.75 h), and higher area-under-curve (AUC, 19.54 and 23.25 ng.h/ml) than intermediate [IMs, n=6, mean+/-SD 1.73+/-0.59 ng/ml (not significant), 3.30+/-0.48 h, 11.32+/-3.72 ng.h/ml], extensive (EMs, n=8, 1.60+/-0.72 ng/ml, 3.24+/-1.24 h, 8.52+/-6.12 ng.h/ml) and ultra-rapid (UMs, n=2, values 1.23 and 1.67 ng/ml, 2.22 and 2.52 h, 6.16 and 6.94 ng.h/ml) metabolisers. The IMs, EMs and UMs did not differ from each other for any of the kinetic variables. Also, the elevation of HR from rest to maximum level tended to differ between PMs and IMs, and between PMs and UMs. The pharmacokinetics and pharmacodynamics between the CYP2D6 groups did not differ with statistical significance when hydrogel timolol was used. Upon head-up tilt, the Ser49 homozygotes (n=26) had higher SAP (P=0.03) and DAP (P<0.01) than the Gly carriers (n=11). The change in DAP from rest to maximum during exercise was lower (P<0.01) in subjects with CC alleles of GNAS1 (n=13) than those with at least one T allele (n=24).

CONCLUSION

The CYP2D6 poor metabolisers may be more prone to systemic adverse events with aqueous timolol than extensive metabolisers. Since CYP2D6 genotyping is not routine clinical practice, using 0.1% timolol hydrogel instead of 0.5% aqueous preparation will increase patient safety.

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.

Gender, ethnicity, and genes in cardiovascular disease. Part 2: implications for pharmacotherapy

Women are underrepresented in clinical trials. Lower doses of beta-blockers are required for Southeast Asians. ACE and ARB's are teratogenic in the second trimester. Torsades de Pointes is more common in women related to a longer QT-interval. Lower dose OCPs decrease the risk of MI, stroke and thrombosis. HRTs are not effective for CAD prevention.

Genetic polymorphisms of human N-acetyltransferase, cytochrome P450, glutathione-S-transferase, and epoxide hydrolase enzymes: relevance to xenobiotic metabolism and toxicity

In this review, an overview is presented of the current knowledge of genetic polymorphisms of four of the most important enzyme families involved in the metabolism of xenobiotics, that is, the N-acetyltransferase (NAT), cytochrome P450 (P450), glutathione-S-transferase (GST), and microsomal epoxide hydrolase (mEH) enzymes. The emphasis is on two main topics, the molecular genetics of the polymorphisms and the consequences for xenobiotic metabolism and toxicity. Studies are described in which wild-type and mutant alleles of biotransformation enzymes have been expressed in heterologous systems to study the molecular genetics and the metabolism and pharmacological or toxicological effects of xenobiotics. Furthermore, studies are described that have investigated the effects of genetic polymorphisms of biotransformation enzymes on the metabolism of drugs in humans and on the metabolism of genotoxic compounds in vivo as well. The effects of the polymorphisms are highly dependent on the enzyme systems involved and the compounds being metabolized. Several polymorphisms are described that also clearly influence the metabolism and effects of drugs and toxic compounds, in vivo in humans. Future perspectives in studies on genetic polymorphisms of biotransformation enzymes are also discussed. It is concluded that genetic polymorphisms of biotransformation enzymes are in a number of cases a major factor involved in the interindividual variability in xenobiotic metabolism and toxicity. This may lead to interindividual variability in efficacy of drugs and disease susceptibility.

Use of in vitro and in vivo data to estimate the likelihood of metabolic pharmacokinetic interactions

This article reviews the information available to assist pharmacokineticists in the prediction of metabolic drug interactions. Significant advances in this area have been made in the last decade, permitting the identification in early drug development of dominant cytochrome P450 (CYP) isoform(s) metabolising a particular drug as well as the ability of a drug to inhibit a specific CYP isoform. The major isoforms involved in human drug metabolism are CYP3A, CYP2D6, CYP2C, CYP1A2 and CYP2E1. Often patients are taking multiple concurrent medications, and thus an assessment of potential drug-drug interactions is imperative. A database containing information about the clearance routes for over 300 drugs from multiple therapeutic classes, including analgesics, anti-infectives, psychotropics, anticonvulsants, cancer chemotherapeutics, gastrointestinal agents, cardiovascular agents and others, was constructed to assist in the semiquantitative prediction of the magnitude of potential interactions with drugs under development. With knowledge of the in vitro inhibition constant of a drug (Ki) for a particular CYP isoform, it is theoretically possible to assess the likelihood of interactions for a drug cleared through CYP-mediated metabolism. For many agents, the CYP isoform involved in metabolism has not been identified and there is substantial uncertainty given the current knowledge base. The mathematical concepts for prediction based on competitive enzyme inhibition are reviewed in this article. These relationships become more complex if the inhibition is of a mixed competitive/noncompetitive nature. Sources of uncertainty and inaccuracy in predicting the magnitude of in vivo inhibition includes the nature and design of in vitro experiments to determine Ki, inhibitor concentration in the hepatic cytosol compared with that in plasma, prehepatic metabolism, presence of active metabolites and enzyme induction. The accurate prospective prediction of drug interactions requires rigorous attention to the details of the in vitro results, and detailed information about the pharmacokinetics and metabolism of the inhibitor and inhibited drug. With the discussion of principles and accompanying tabulation of literature data concerning the clearance of various drugs, a framework for reasonable semiquantitative predictions is offered in this article.

"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.

Pharmacokinetics of the newer antidepressants: clinical relevance

The newer antidepressants are a diverse group of compounds with distinct pharmacokinetic properties. The selective serotonin reuptake inhibitors (SSRIs)--paroxetine, sertraline, and fluvoxamine--have elimination half-lives of 15-26 hours. The extended half-life of fluoxetine (4-6 days) and its active metabolite, norfluoxetine (4-16 days), results in an extended time to steady-state and a prolonged washout period when dosing is discontinued. The SSRIs are administered as a single daily dose. Venlafaxine and nefazodone have short half-lives, 2-5 hours, and are dosed > or = 2 times daily. The newer antidepressants are all highly cleared from the body through hepatic metabolism. The biotransformation of all the drugs except paroxetine and fluvoxamine results in the formation of pharmacologically active metabolites. The newer antidepressants display a broad variability similar to the tricyclic antidepressants (TCAs) in steady-state drug concentrations. Due largely to a safer toxicity profile, the variability in clearance is of lesser importance with the newer antidepressants than with the TCAs. No useable concentration versus therapeutic effect relationship has been found with the newer drugs, and widely varying concentrations appear to have little relationship to adverse effects. Knowledge of kinetic characteristics is important for designing dosage regimens and avoiding potentially serious drug-drug interactions that are mediated through inhibition of specific hepatic cytochrome P450 enzyme pathways. Each of the SSRIs inhibits at least one cytochrome P450 enzyme, and all of the SSRIs increase serum concentrations of concomitantly administered TCAs.

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.

Genetically determined adverse drug reactions involving metabolism

Genetic factors represent an important source of interindividual variation in drug response. Relatively few adverse drug effects with a pharmacodynamic basis are known, and most of the well characterised inherited traits take the form of genetic polymorphisms of drug metabolism. Monogenic control of N-acetylation, S-methylation and cytochrome P450-catalysed oxidation of drugs can have important clinical consequences. Individuals who inherit an impaired ability to perform one or more of these reactions may be at an increased risk of concentration-related toxicity. There is a strong case for phenotyping before starting treatment with a small number of drugs that are polymorphically N-acetylated or S-methylated. However, the issue of clinical significance is perhaps most relevant for the debrisoquine oxidation polymorphism, which is mediated by cytochrome CYP2D6 and which determines the pharmacokinetics of many commonly used drugs. Phenotypic poor metabolisers of debrisoquine (8% of Caucasian populations) taking standard doses of some tricyclic antidepressants, neuroleptics or antiarrhythmic drugs may be particularly prone to adverse reactions. Similarly, clinically relevant drug interactions between these drugs and other substrates of cytochrome CYP2D6 may occur in the majority of the population who are extensive metabolisers. However, it is clear that in the majority of cases there is a need for controlled prospective studies to determine clinical significance. Accordingly, routine debrisoquine phenotyping or genotyping before beginning drug treatment is difficult to justify at present, although it may be helpful in individual cases. When prescribing drugs whose metabolism is polymorphic alone or in combination, careful titration of the dose in both phenotypic groups is prudent. In some cases it will be preferable to use alternative therapy to avoid the risk of adverse drug reactions.

Clinical significance of genetic influences on cardiovascular drug metabolism

Inherited differences in metabolism may be responsible for individual variability in the efficacy of drugs and the occurrence of adverse drug reactions. Among the cardiovascular drugs reported to exhibit genetic polymorphism are debrisoquine, sparteine, some beta-adrenoceptor antagonists, flecainide, encainide, propafenone, nifedipine, procainamide, and hydralazine. The implications of genetic differences in the metabolism of these drugs for cardiovascular therapeutics is the subject of this review.

Duration and long-term efficacy of phenylephrine-induced reduction in the systemic absorption of ophthalmic timolol in rabbits

Co-administration of phenylephrine decreases systemic timolol absorption after a single topical ocular dose of timolol in rabbits. This is probably due to vasoconstriction in the conjunctiva of the eye and nasal mucosa. In this study, we evaluated the duration of action and long-term efficacy of phenylephrine in reducing the systemic absorption of ophthalmic timolol in pigmented rabbits. Although co-administered phenylephrine had a short duration of interaction with systemic timolol absorption (20-60 min), its effect on systemic timolol absorption was substantial. The long-term vasoconstrictive effect of phenylephrine was studied by administering timolol-phenylephrine eyedrops into the eyes of rabbits once a day for two months. Systemic peak concentrations of timolol following timolol-phenylephrine eyedrop administration remained unchanged throughout the study. Phenylephrine may be useful additive in decreasing the systemic concentrations of ophthalmic drugs.

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.

Influence of debrisoquine oxidation phenotype on exercise tolerance and subjective fatigue after metoprolol and atenolol in healthy subjects

1. The effects of single doses of metoprolol 50 mg, metoprolol 100 mg and atenolol 100 mg on exercise tolerance were compared with placebo in a double-blind random cross-over study in 12 healthy subjects. Nine subjects were extensive metabolisers of debrisoquine, and three were poor metabolisers. 2. Three hours after dosing beta-adrenoceptor blocker treatments significantly reduced exercise heart rate, prolonged time to complete exercise, and increased subjective fatigue measured by visual analogue scale. 3. Scores for subjective fatigue did not correlate with reduction in exercise heart rate or prolongation of exercise time. Exercise time prolongation was weakly but not significantly correlated with exercise heart rate reduction. 4. When compared with placebo, prolongation of exercise time and increased fatigue with metoprolol were not significantly related to debrisoquine oxidation phenotype or to the debrisoquine/4-hydroxydebrisoquine (D/4OH-D) ratio. 5. When metoprolol responses were compared with those for atenolol, changes in exercise time and fatigue scores were significantly related to oxidation phenotype. For metoprolol 100 mg, poor metabolisers required 20.8 s longer to complete exercise (P less than 0.05) and had higher fatigue scores by 78% (P less than 0.05) as compared with extensive metabolisers.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Systemic absorption of ocular timolol in poor and extensive metabolizers of debrisoquine

The oxidation of timolol exhibits genetic polymorphism of debrisoquine type. After oral administration, poor metabolizers have high timolol concentrations in plasma and show an intensified systemic beta-blockade. Since the contribution of debrisoquine metabolizer status on timolol eyedrop therapy is not known we determined the systemic absorption of ocularly applied timolol in healthy subjects classified either extensive or poor metabolizers. Unlike after oral timolol ocular drug administration caused higher peak drug concentrations in plasma in extensive metabolizers. The variation in the systemic absorption of ocular timolol contributed more than the debrisoquine oxidation phenotype to timolol plasma levels after a single ocular timolol application.

Recent developments in hepatic drug oxidation. Implications for clinical pharmacokinetics

Cytochrome P450 (P450) is the collective term for a group of related enzymes or isozymes which are responsible for the oxidation of numerous drugs and other foreign compounds, as well as many endogenous substrates including prostaglandins, fatty acids and steroids. Each P450 is encoded by a separate gene, and a classification system for the P450 gene superfamily has recently been proposed. The P450 genes are assigned to families and subfamilies according to the degree of similarity of the amino acid sequences of the protein part of the encoded P450 isozymes. It is estimated that there are between 20 and 200 different P450 genes in humans. The human P450IID6 is a particular isozyme which has been extensively studied over the past 10 years. The P450IID6 is the target of the sparteine/debrisoquine drug oxidation polymorphism. Between 5 and 10% of Caucasians are poor metabolisers, and it has recently been shown that the P450IID6 enzyme is absent in the livers of these individuals. The defect has also been characterised at the DNA and messenger RNA (mRNA) level, and to date 3 different forms of incorrectly spliced P450IID6 pre-mRNAs have been identified in the livers of poor metabolisers. The P450IID6 has a broad substrate specificity and is known to oxidise 15 to 20 commonly used drugs. The metabolism of these drugs is therefore subjected to the sparteine/debrisoquine oxidation polymorphism. The clinical significance of this polymorphism for a particular drug is defined according to the usefulness of phenotyping patients before treatment. It is concluded that this strategy would be of potential value for tricyclic antidepressants, some neuroleptics (e.g. perphenazine and thioridazine) and some anti-arrhythmics (e.g. propafenone and flecainide). The P450IID6 displays markedly stereoselective metabolism and appears uninducible by common inducers like rifampicin and phenazone (antipyrine). With some substrates, such as imipramine, desipramine and propafenone, P450IID6 becomes saturated at therapeutic doses. Finally, its function is potently inhibited by many commonly used drugs, for example, quinidine. The most clinically relevant interaction in relation to P450IID6 function appears to be the potent inhibition by some neuroleptics of the metabolism of tricyclic antidepressants. No drug-metabolising P450 has been so well characterised at the gene, protein and functional levels as the P450IID6. This development is based on an extensive use of specific model drugs, the oxidation of which in vitro and in vivo is dependent on the function of P450IID6; it can be expected that other human drug-metabolising P450s will be similarly characterised in future.

The genetic polymorphism of debrisoquine/sparteine metabolism--clinical aspects

It has been established that the metabolism of more than twenty drugs, including antiarrhythmics, beta-adrenoceptor antagonists, antidepressants, opiates and neuroleptics is catalyzed by cytochrome P-450dbl. The activity of this P-450 isozyme is under genetic rather than environmental control. This article discusses the therapeutic implications for each of the classes of drugs affected by this genetic polymorphism in drug metabolism. Not only are the problems associated with poor metabolizers who are unable to metabolize the compounds discussed, but it is also emphasized that it is difficult to attain therapeutic plasma concentrations for some drugs in high activity extensive metabolizers.

Timolol metabolism and debrisoquine oxidation polymorphism: a population study

1. The metabolism of orally administered timolol (T) to its ring cleavage ethanolamine (TE) and glycine (TG) products was studied in 108 unrelated hypertensive patients. 2. Statistically significant correlations between the 0-8 h urinary debrisoquine/4-hydroxy-debrisoquine ratio and the T/TE (rs = 0.74, P less than 0.001), T/TG (rs = 0.42, P less than 0.001) and T/TE + TG (rs = 0.49, P less than 0.001) ratios were found. 3. The log10 T/TE, T/TG and T/TE + TG ratios from poor metabolisers of debrisoquine (PMs) were grouped at the upper end of a unimodal distribution. 4. These results indicate that timolol metabolism is partly under monogenic control of the debrisoquine-type. 5. The mean +/- s.d. plasma timolol concentration in PMs (82 +/- 43 ng ml-1) was double that in extensive metabolisers (45 +/- 19 ng ml-1) (P = 0.011). The clinical significance of this observation remains to be established.

Clinical significance of the sparteine/debrisoquine oxidation polymorphism

The sparteine/debrisoquine oxidation polymorphism results from differences in the activity of one isozyme of cytochrome P450, the P450db1 (P450 IID1). The oxidation of more than 20 clinically useful drugs has now been shown to be under similar genetic control to that of sparteine/debrisoquine. The clinical significance of this polymorphism may be defined by the value of phenotyping patients before treatment. The clinical significance of such polymorphic elimination of a particular drug can be analyzed in three steps: first, does the kinetics of active principle of a drug depend significantly on P450db1?; second, is the resulting pharmacokinetic variability of any clinical importance?; and third, can the variation in response be assessed by direct clinical or paraclinical measurements? It is concluded from such an analysis that, in general, the sparteine/debrisoquine oxidation polymorphism is of significance in patient management only for those drugs for which plasma concentration measurements are considered useful and for which the elimination of the drug and/or its active metabolite is mainly determined by P450db1. At present, this applies to tricyclic antidepressants and to certain neuroleptics (e.g. perphenazine and thioridazine) and antiarrhythmics (e.g. propafenone and flecainide). Phenotyping should be introduced in to clinical routine under strictly controlled conditions to afford a better understanding of its potentials and limitations. The increasing knowledge of specific substrates and inhibitors of P450db1 allows precise predictions of drug-drug interactions. At present, the strong inhibitory effect of neuroleptics on the metabolism of tricyclic antidepressants represents the best clinically documented and most relevant example of such an interaction.

[Systemic complications of beta-blocking eyedrops. Apropos of 6 cases]

Six cases of systemic reactions to topical treatment with beta-blocking eyedrops are reported, bradycardia and faintness due to an overdosage of ophthalmic timolol; decompensated heart failure one month after the prescription of carteolol eyedrops: bronchospasm after two weeks of treatment with metipranolol eyedrops; crippling Raynaud's phenomenon of otherwise unknown origin, which had begun with timolol eyedrops, continued with carteolol eyedrops and regressed after discontinuation of ophthalmic beta-blockers; aggravation of an anaphylactoid shock in a patient treated with ophthalmic timolol, and myocardial infarction possibly due to the abrupt withdrawal of timolol eyedrops. It cannot be overstressed that the rules governing the prescription of oral beta-blockers also apply to ophthalmic preparations of these drugs: respect of contra-indications, strict adherence to the dosage recommended, gradual drug withdrawal and regular supervision. Only controlled studies and long-term follow-up will be able to demonstrate differences in safety between the five beta-blockers commercialized as eyedrops in this country.

Prevalence of debrisoquine oxidation phenotypes in glaucoma patients

The oxidation of debrisoquine, a sympatholytic antihypertensive agent, exhibits genetic polymorphism. The debrisoquine/4-OH-debrisoquine metabolic ratio (MR) separates the population to poor (PM, MR greater than 12.6) and extensive (EM, MR less than 12.6) metabolizers. 5-10% of the caucasians belong to the PM phenotype. The oxidation of many other drugs, like timolol, correlates with the debrisoquine phenotype. We determined the debrisoquine phenotype in 102 glaucoma patients. The majority of the patients was treated with ophthalmic timolol. Five patients were classified as PMs. However, two of them were on quinidine, a well known inhibitor of debrisoquine oxidation. The prevalence of debrisoquine PM phenotype in glaucoma patients was 2.9% (excluding patients on quinidine) or 4.9% (with patients on quinidine). The figures are slightly lower than the mean value reported for the normal Finnish population. However, both figures lay within the 95% confidence limits of the prevalence of PM phenotype in the normal Finnish population. The beta-blocking activity of oral timolol is increased in PMs. The significance of timolol oxidation phenotype during ocular timolol therapy warrants further investigation.

Lack of relationship between debrisoquine oxidation phenotype and the pharmacokinetics and first dose effect of prazosin

The relationship between debrisoquine oxidation phenotype and the pharmacokinetics and pharmacodynamics of a single oral dose of prazosin has been studied in eight hypertensive patients (four extensive and four poor metabolisers). No significant differences between the two phenotypes were observed in either the area under the plasma prazosin concentration-time curve, the terminal half-life or the first-dose effect of prazosin.

Enzymatic basis of the debrisoquine/sparteine-type genetic polymorphism of drug oxidation. Characterization of bufuralol 1'-hydroxylation in liver microsomes of in vivo phenotyped carriers of the genetic deficiency

The genetically controlled polymorphic oxidation of debrisoquine and sparteine is caused by the absence or functional deficiency of a cytochrome P-450 isozyme. In order to elucidate the mechanisms underlying the differences in cytochrome P-450 function we have studied the 1'-hydroxylation of the prototype drug bufuralol in human liver microsomes of individuals phenotyped in vivo as extensive metabolizers (EM, N = 10), poor metabolizers (PM, N = 5) and in subjects with an intermediate rate of metabolism (IM, N = 4). PM- as compared to EM-microsomes were characterized by a decreased Vmax for (+)-bufuralol 1'-hydroxylation (7.51 +/- 2.03 nmol X mg-1 X hr-1 vs 11.95 +/- 4.80 nmol X mg-1 X hr-1) but not for (-)-bufuralol 1'-hydroxylation (4.72 +/- 0.87 nmol X mg-1 X hr-1 vs 5.55 +/- 1.49 nmol X mg-1 X hr-1). The apparent Km for (+)-bufuralol 1'-hydroxylation was increased in PM microsomes (118 +/- 84.9 microM vs 17.9 +/- 6.30 microM). Inhibition of bufuralol 1'-hydroxylation by quinidine was biphasic in EM microsomes, providing further support for the involvement of at least two cytochrome P-450 isozymes. Quinidine acted as a competitive inhibitor of only the high affinity/stereoselectivity component of the reaction. Our data suggest that the debrisoquine/sparteine type of oxidation polymorphism is caused by an almost complete loss of a minor cytochrome P-450 isozyme which has a high affinity and stereoselectivity for (+)-bufuralol and a high sensitivity to inhibition by quinidine.

Systemic reactions to ophthalmic drug preparations

Adverse systemic reactions associated with the use of topical ophthalmic timolol, chloramphenicol, phenylephrine and cyclopentolate are surveyed, with special emphasis on precautions and contraindications for these ophthalmic drug preparations. Systemic reactions secondary to timolol, a beta-adrenergic antagonist indicate that it should be used with caution in patients with asthma or a history of asthma, chronic obstructive pulmonary disease or cardiovascular disease and in those patients receiving systemic administration of beta-blockers or verapamil. Because significant blood dyscrasias or aplastic anaemia have been reported following topical ophthalmic chloramphenicol, the only absolute indication in ocular conditions is an organism that is resistant to all other antibiotics. Both 2.5% and 10% phenylephrine have been associated with cardiovascular effects and should be used with caution in selected patients on monoamine oxidase inhibitors, tricyclic antidepressants or atropine or in those with hypertension, advanced arteriosclerotic changes, aneurysms, orthostatic hypotension, long-standing insulin-dependent diabetes and in children with low bodyweights. Central nervous system toxicity secondary to cyclopentolate is dose-related and can be avoided by use of minimal concentrations and avoidance of unnecessary repetition of administration. Occlusion of the nasolacrimal passage with finger pressure immediately after instillation of any eyedrop also decreases the amount of drug that is absorbed systemically.

Beta-blocker therapy and the risk of anaphylaxis

Beta-blocker therapy is associated with an increase in the severity and, possibly, the incidence of acute anaphylaxis. The population at risk consists of people with allergic conditions who are given a beta-blocker for an unrelated condition. Anaphylaxis under these conditions may be severe, protracted and resistant to conventional treatment because of the beta-adrenergic blockade. Severe or fatal attacks have been triggered by insect stings, the ingestion of allergenic foods or drugs, and injections of radiocontrast media, antisera or immunotherapy antigens. These occurrences are probably infrequent, but their incidence is unknown. At least two fatal cases have recently occurred in Canada. Clinical allergists, internists and family practitioners in particular should be aware of the need for aggressive and prolonged support in patients who experience anaphylaxis while receiving beta-blocker therapy and should report all such occurrences to the federal registry of adverse drug reactions. Allergy skin testing or immunotherapy is inadvisable in patients who take a beta-blocker orally or in the form of ophthalmic eyedrops. The list of relative contraindications to beta-blocker use should be extended to include susceptibility to recurrent anaphylaxis, whether it is idiopathic or due to an identifiable cause.

Twelve hour (trough) plasma nifedipine concentrations during chronic treatment with nifedipine retard

Variation in plasma nifedipine concentrations have been reported and one study has suggested the existence of a subpopulation of poor metabolisers of this drug. We have studied the variability of 12 h plasma nifedipine concentrations in 64 hypertensive patients on long-term nifedipine Retard 20 mg twice daily. A slightly skewed unimodal distribution with a modal concentration of 15 to 30 ng/ml was obtained. No relationship between 12-h plasma levels and debrisoquine hydroxylation phenotype was found.

Systemic side effects from ophthalmic timolol and their prevention

Topically applied ophthalmic drugs can occasionally produce adverse systemic effects due to systemic absorption of the drug or impediment of the drug's metabolism. Increasing evidence points to significant adverse systemic effects from topical ocular administration of timolol, a beta-adrenergic blocking agent marketed for the treatment of glaucoma. Various methods to decrease or avoid unwanted systemic effects from eyedrops are discussed, including avoidance of overdosage and how to apply eye medications.

Adverse reactions and interactions with beta-adrenoceptor blocking drugs

beta-Blocking drugs are widely used throughout the world and serious adverse reactions are relatively uncommon. Most of those which do occur are pharmacologically predictable and may be avoided by ensuring that patients who are to be given beta-blockers do not have a predisposition to the development of bronchospasm, cardiac failure or peripheral ischaemia. In some situations, the use of a beta 1-selective blocking drug may reduce the risk of a severe adverse reaction, but there is little evidence that other ancillary properties such as partial agonist activity are of relevance in this context. Long term experience with many of the beta-blockers in current use suggests that unpredictable major adverse reactions such as the practolol oculomucocutaneous syndrome are unlikely to be repeated, although some of these drugs may be associated with immunological disturbances and some have been implicated in the development of retroperitoneal fibrosis. beta-Blocking drugs appear to be associated with a number of subjective side effects including muscle fatigue, peripheral coldness and some neurological symptoms. These side effects are highly subjective and are therefore difficult to quantify and it is not known whether they are of major importance in terms of their effect upon patients' overall well-being. It cannot be assumed that simply because such side effects can be elicited that they do, in fact, matter. However, because beta-blockers are often prescribed for patients who have no symptoms and for whom the benefits of therapy are generally small, such side effects would be of considerable importance if they had an overall effect upon quality of life. There are theoretical reasons to suppose that the incidence and severity of such side effects may be related to the ancillary properties of the individual drugs, but there is little evidence that parameters such as beta 1-selectivity, or partial agonist activity are clinically important determinants of the severity of these side effects. Lipophilicity, however, may be associated with an increased incidence of neurological symptoms. beta-Blocking drugs may cause a variety of metabolic disturbances including an increase in serum VLDL-cholesterol concentrations. However, long term studies have not shown that such disturbances are associated with an increased risk of cardiovascular disease, indicating that such metabolic changes may not be of major importance in practice. beta-Blocking drugs may be involved in a number of interactions with other drugs, but few of these have been shown to be of clinical significance.(ABSTRACT TRUNCATED AT 400 WORDS)

Sparteine oxidation is practically abolished in quinidine-treated patients

In eight patients a sparteine-test was carried out immediately before and after 1 week of treatment with quinidine 600-800 mg day-1. Before treatment one patient was classified as a poor metaboliser (metabolic ratio: greater than or equal to 20), and seven patients as extensive metabolisers. During quinidine treatment, the formation of sparteine metabolites (2- and 5-dehydrosparteine) was practically abolished. Patients initially classified as extensive metabolisers thus exhibited the phenotype of poor metabolisers during quinidine treatment.

A simple borohydride/GC method for measuring sparteine metabolites in man

A simple borohydride/GC method was developed for phenotyping sparteine oxidation in man. The major metabolites of sparteine found in human urine, 2- and 5-dehydrosparteine, were converted quantitatively back to sparteine by sodium borohydride reduction. The amount of sparteine metabolites can be estimated from the difference of sparteine concentrations between the borohydride-treated and untreated urine samples. The coefficient of variation of this assay was estimated from repeated analyses to be +/- 3% within a day (intra-assay) and +/- 8% between days (inter-assay).

Debrisoquine polymorphism and the metabolism and action of metoprolol, timolol, propranolol and atenolol

The contribution of debrisoquine polymorphism to the metabolism and action of beta-adrenoceptor antagonists (beta-blockers) varies widely between drugs. Oxidation phenotype is a major determinant of the metabolism, pharmacokinetics and some of the pharmacological actions of metoprolol, bufuralol and timolol. The poor metabolizer phenotype is associated with an increased area under the plasma drug concentration vs. time curve, a prolongation of elimination half-life and a more intense and sustained beta-blockade. The stereoselective metabolism of metoprolol also displays phenotypic differences, which should be taken into account when interpreting plasma concentration vs. response relationships. Studies in vivo and in vitro have identified some of the metabolic pathways which are subject to this defect, namely the alpha-hydroxylation and the O-demethylation of metoprolol and the 1'-hydroxylation of bufuralol. In contrast, the pharmacokinetics and pharmacodynamics of propranolol, which is also extensively oxidized, are not related to debrisoquine polymorphism, although 4'-hydroxypropranolol formation is deficient in the poor metabolizer phenotype. The disposition of atenolol, which is almost completely eliminated unchanged by renal and faecal excretion, is independent of oxidation phenotype. If standard doses of some beta-blockers are used in poor metabolizers, these patients may be susceptible to concentration-related adverse reactions and they may also require lower and less frequent dosing for control of angina pectoris.

The polymorphic oxidation of beta-adrenoceptor antagonists. Clinical pharmacokinetic considerations

Wide variability in response to some drugs such as debrisoquine can be attributed largely to genetic polymorphism of their oxidative metabolism. Most beta-blockers undergo extensive oxidation. Anecdotal reports of high plasma concentrations of certain beta-blockers in poor metabolisers (PMs) of debrisoquine have claimed that the oxidation of these drugs is under polymorphic control. Subsequently, controlled studies have shown that debrisoquine oxidation phenotype is a major determinant of the metabolism, pharmacokinetics and some of the pharmacological actions of metoprolol, bufuralol, timolol and bopindolol. The poor metaboliser phenotype is associated with increased plasma drug concentrations, a prolongation of elimination half-life and more intense and sustained beta-blockade. Phenotypic differences have also been observed in the pharmacokinetics of the enantiomers of metoprolol and bufuralol. In vivo and in vitro studies have identified some of the metabolic pathways which are subject to the defect, viz. alpha-hydroxylation and O-demethylation of metoprolol and 1'- and possibly 4- and 6-hydroxylation of bufuralol. In contrast, the overall pharmacokinetics and pharmacodynamics of propranolol, which is also extensively oxidised, are not related to debrisoquine polymorphism, although 4'-hydroxypropranolol formation is lower in poor metabolisers. As anticipated, the disposition of atenolol which is eliminated predominantly unchanged by the kidney and in the faeces, is unrelated to debrisoquine phenotype. The clinical significance of impaired elimination of beta-blockers is not clear. If standard doses of beta-blockers are used in poor metabolisers, these subjects may be susceptible to concentration-related adverse reactions and they may also require less frequent dosing for control of angina pectoris.