Drug interaction between propafenone and metoprolol

Clinical Pharmacokinetics of Metoprolol: A Systematic Review

BACKGROUND

Metoprolol is recommended for therapeutic use in multiple cardiovascular conditions, thyroid crisis, and circumscribed choroidal hemangioma. A detailed systematic review on the metoprolol literature would be beneficial to assess all pharmacokinetic parameters in humans and their respective effects on patients with hepatic, renal, and cardiovascular diseases. This review combines all the pharmacokinetic data on metoprolol from various accessible studies, which may assist in clinical decision making.

METHODOLOGY

The Google Scholar and PubMed databases were searched to screen articles associated with the clinical pharmacokinetics of metoprolol. The comprehensive literature search retrieved 41 articles including data on plasma concentration-time profiles after intravenous and oral (immediate-release, controlled-release, slow-release, or extended-release) routes of administration, and at least one pharmacokinetic parameter was reported in all studies included.

RESULTS

Out of 41 retrieved articles, six were after intravenous and 12 were after oral administration in healthy individuals. The oral studies depict a dose-dependent increase in maximum plasma concentration (C_max), time to reach maximum plasma concentration (T_max), and area under the concentration-time curve (AUC). Two studies were conducted in R- and S-enantiomers, in which one study reported the gender differences, depicting greater C_max and AUC among women, whereas in another study S-metoprolol was found to have higher values of C_max, T_max, and AUC in comparison with R-metoprolol. Results in different diseases depicted that after IV administration of 20 mg, patients with renal impairment showed an increase in clearance (CL) (60 L/h vs 48 L/h) compared with healthy subjects, whereas a decrease in CL (36.6 ± 7.8 L/h vs 48 ± 6.6 L/h) was seen in patients with hepatic cirrhosis at a similar dose. In comparison with a single oral dose following administration of 15 mg IV in three divided doses, patients having an acute myocardial infarction (AMI) showed an increase in C_max (823 nmol/L vs 248 nmol/L) at a steady state. Twenty different studies have reported significant changes in CL, C_max, and AUC of metoprolol when it is co-administered with other drugs. One study has reported a drug-food interaction for metoprolol but no significant changes were seen in the C_max and AUC.

CONCLUSION

This review summarizes all the pharmacokinetic parameters of metoprolol after pooling up-to-date data from all the studies available. The summarized pharmacokinetic data presented in this review can assist in developing and evaluating pharmacokinetic models of metoprolol. Moreover, this data can provide practitioners with an insight into dosage adjustments among the diseased populations and can assist in preventing potential adverse drug reactions. This review can also help avoid side effects and drug-drug interactions.

Application of unbound liver-to-plasma concentration ratio to quantitative projection of cytochrome P450-mediated drug-drug interactions using physiologically based pharmacokinetic modelling approach

1. This study evaluated the prediction accuracy of cytochrome P450 (CYP)-mediated drug-drug interaction (DDI) using minimal physiologically-based pharmacokinetic (PBPK) modelling incorporating the hepatic accumulation factor of an inhibitor (i.e. unbound liver/unbound plasma concentration ratio [K_p,uu,liver]) based on 22 clinical DDI studies. 2. K_p,uu,liver values were estimated using three methods: (1) ratio of cell-to-medium ratio in human cryopreserved hepatocytes (C/M_u) at 37 °C to that on ice (K_p,uu,C/M), (2) multiplication of total liver/unbound plasma concentration ratio (K_p,u,liver) estimated from C/M_u at 37 °C with unbound fraction in human liver homogenate (K_p,uu,cell) and (3) observed K_p,uu,liver in rats after intravenous infusion (K_p,uu,rat). 3. PBPK model using each K_p,uu,liver projected the area under the curve (AUC) increase of substrates more accurately than the model assuming a K_p,uu,liver of 1 for the average fold error and root mean square error did. Particularly, the model with a K_p,uu,liver of 1 underestimated the AUC increase of triazolam following co-administration with CYP3A4 inhibitor itraconazole by five-fold, whereas the AUC increase projected using the model incorporating the K_p,uu,C/M, K_p,uu,cell, or K_p,uu,rat of itraconazole and hydroxyitraconazole was within approximately two-fold of the actual value. 4. The results indicated that incorporating K_p,uu,liver into the PBPK model improved the accuracy of DDI projection.

Pharmacokinetic Variability of Drugs Used for Prophylactic Treatment of Migraine

In this review, we evaluate the variability in the pharmacokinetics of 11 drugs with established prophylactic effects in migraine to facilitate 'personalized medicine' with these drugs. PubMed was searched for 'single-dose' and 'steady-state' pharmacokinetic studies of these 11 drugs. The maximum plasma concentration was reported in 248 single-dose and 115 steady-state pharmacokinetic studies, and the area under the plasma concentration-time curve was reported in 299 single-dose studies and 112 steady-state pharmacokinetic studies. For each study, the coefficient of variation was calculated for maximum plasma concentration and area under the plasma concentration-time curve, and we divided the drug variability into two categories; high variability, coefficient of variation >40%, or low or moderate variability, coefficient of variation <40%. Based on the area under the plasma concentration-time curve in steady-state studies, the following drugs have high pharmacokinetic variability: propranolol in 92% (33/36), metoprolol in 85% (33/39), and amitriptyline in 60% (3/5) of studies. The following drugs have low or moderate variability: atenolol in 100% (2/2), valproate in 100% (15/15), topiramate in 88% (7/8), and naproxen and candesartan in 100% (2/2) of studies. For drugs with low or moderate pharmacokinetic variability, treatment can start without initial titration of doses, whereas titration is used to possibly enhance tolerability of topiramate and amitriptyline. The very high pharmacokinetic variability of metoprolol and propranolol can result in very high plasma concentrations in a small minority of patients, and those drugs should therefore be titrated up from a low initial dose, depending mainly on the occurrence of adverse events.

Cytochrome P450-2D6 Genotype Definition May Improve Therapy for Paroxysmal Atrial Fibrillation A Case of Syncope Following "Pill-in-the-Pocket" Quinidine plus Propafenone

Classes 1A, 1C and III anti-arrhythmics may be ineffective or induce adverse events including potentially fatal arrhythmias when administered in recommended doses. Serum levels of these medications vary widely during conventional dosing due in large part to variations in cytochrome P450-2D6 isoenzyme activity which metabolizes most antiarrhythmics in addition to over 25% of other commonly prescribed medications. 2D6 activity is also profoundly inhibited by some antiarrhythmics and other commonly used medications and varies widely between the individuals of all populations, a pattern which has resulted in separation of subjects into 4 phenotypes and genotypes consisting of poor metabolizers (PM), intermediate metabolizers (IM), efficient metabolizers (EM), and ultra-rapid metabolizers (UM). Patients with a phenotype PM classification almost universally are also genotype PM due to the possession of two inactive 2D6 alleles, with this PM pattern often inducing supratherapeutic and toxic antiarrhythmic blood levels during conventional antiarrhythmic therapy. UM individuals have supranormal levels of 2D6 activity often created by the presence of 3 or more active alleles which often induce subtherapeutic and ineffective drug levels during antiarrhythmic administration in conventional doses. We searched for evidence relating Cytochrome P450-2D6 phenotypes or genotypes to antiarrhythmic metabolism in order to judge whether this analysis might contribute to improved safety and effectiveness of antiarrhythmic medications commonly utilized in the treatment of atrial fibrillation. The available evidence strongly supported these possibilities. We also describe a patient in whom knowledge of his IM/PM CYP2D6 genotype might have prevented the only episode of syncope and myocardial stunning which developed during his 28 years of "Pill-in-a-Pocket" therapy.

Pharmacokinetic drug-drug interaction and their implication in clinical management

Drug-drug interactions (DDIs) are one of the commonest causes of medication error in developed countries, particularly in the elderly due to poly-therapy, with a prevalence of 20-40%. In particular, poly-therapy increases the complexity of therapeutic management and thereby the risk of clinically important DDIs, which can both induce the development of adverse drug reactions or reduce the clinical efficacy. DDIs can be classify into two main groups: pharmacokinetic and pharmacodynamic. In this review, using Medline, PubMed, Embase, Cochrane library and Reference lists we searched articles published until June 30 2012, and we described the mechanism of pharmacokinetic DDIs focusing the interest on their clinical implications.

Importance of multi-p450 inhibition in drug-drug interactions: evaluation of incidence, inhibition magnitude, and prediction from in vitro data

Drugs that are mainly cleared by a single enzyme are considered more sensitive to drug-drug interactions (DDIs) than drugs cleared by multiple pathways. However, whether this is true when a drug cleared by multiple pathways is coadministered with an inhibitor of multiple P450 enzymes (multi-P450 inhibition) is not known. Mathematically, simultaneous equipotent inhibition of two elimination pathways that each contribute half of the drug clearance is equal to equipotent inhibition of a single pathway that clears the drug. However, simultaneous strong or moderate inhibition of two pathways by a single inhibitor is perceived as an unlikely scenario. The aim of this study was (i) to identify P450 inhibitors currently in clinical use that can inhibit more than one clearance pathway of an object drug in vivo and (ii) to evaluate the magnitude and predictability of DDIs caused by these multi-P450 inhibitors. Multi-P450 inhibitors were identified using the Metabolism and Transport Drug Interaction Database. A total of 38 multi-P450 inhibitors, defined as inhibitors that increased the AUC or decreased the clearance of probes of two or more P450s, were identified. Seventeen (45%) multi-P450 inhibitors were strong inhibitors of at least one P450, and an additional 12 (32%) were moderate inhibitors of one or more P450s. Only one inhibitor (fluvoxamine) was a strong inhibitor of more than one enzyme. Fifteen of the multi-P450 inhibitors also inhibit drug transporters in vivo, but such data are lacking on many of the inhibitors. Inhibition of multiple P450 enzymes by a single inhibitor resulted in significant (>2-fold) clinical DDIs with drugs that are cleared by multiple pathways such as imipramine and diazepam, while strong P450 inhibitors resulted in only weak DDIs with these object drugs. The magnitude of the DDIs between multi-P450 inhibitors and diazepam, imipramine, and omeprazole could be predicted using in vitro data with similar accuracy as probe substrate studies with the same inhibitors. The results of this study suggest that inhibition of multiple clearance pathways in vivo is clinically relevant, and the risk of DDIs with object drugs may be best evaluated in studies using multi-P450 inhibitors.

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.

Role of cytochrome P450 in drug interactions

Drug-drug interactions have become an important issue in health care. It is now realized that many drug-drug interactions can be explained by alterations in the metabolic enzymes that are present in the liver and other extra-hepatic tissues. Many of the major pharmacokinetic interactions between drugs are due to hepatic cytochrome P450 (P450 or CYP) enzymes being affected by previous administration of other drugs. After coadministration, some drugs act as potent enzyme inducers, whereas others are inhibitors. However, reports of enzyme inhibition are very much more common. Understanding these mechanisms of enzyme inhibition or induction is extremely important in order to give appropriate multiple-drug therapies. In future, it may help to identify individuals at greatest risk of drug interactions and adverse events.

Pharmacokinetics and interactions of headache medications, part II: prophylactic treatments

The present part II review highlights pharmacokinetic drug-drug interactions (excluding those of minor severity) of medications used in prophylactic treatment of the main primary headaches (migraine, tension-type and cluster headache). The principles of pharmacokinetics and metabolism, and the interactions of medications for acute treatment are examined in part I. The overall goal of this series of two reviews is to increase the awareness of physicians, primary care providers and specialists regarding pharmacokinetic drug-drug interactions (DDIs) of headache medications. The aim of prophylactic treatment is to reduce the frequency of headache attacks using beta-blockers, calcium-channel blockers, antidepressants, antiepileptics, lithium, serotonin antagonists, corticosteroids and muscle relaxants, which must be taken daily for long periods. During treatment the patient often continues to take symptomatic drugs for the attack, and may need other medications for associated or new-onset illnesses. DDIs can, therefore, occur. As a whole, DDIs of clinical relevance concerning prophylactic drugs are a limited number. Their effects can be prevented by starting the treatment with low dosages, which should be gradually increased depending on response and side effects, while frequently monitoring the patient and plasma levels of other possible coadministered drugs with a narrow therapeutic range. Most headache medications are substrates of CYP2D6 (e.g., beta-blockers, antidepressants) or CYP3A4 (e.g., calcium-channel blockers, selective serotonin re-uptake inhibitors, corticosteroids). The inducers and, especially, the inhibitors of these isoenzymes should be carefully coadministered.

Prediction of differences in in vivo oral clearance of N,N-dipropyl-2-[4-methoxy-3-(2-phenylethoxy)phenyl] ethylamine monohydrochloride (NE-100) between extensive and poor metabolizers from in vitro metabolic data in human liver microsomes lacking CYP2D6 activity and recombinant CYPs

1. It has previously been reported that N,N-dipropyl-2-[4-methoxy-3-(2-phenylethoxy)phenyl]-ethylamine monohydrochloride (NE-100) was predominantly metabolized by cytochrome P450 (CYP) 2D6 in human liver microsomes (HLM). In the present study, the contribution of CYP forms involved in the formation of the major metabolites of NE-100 in human liver lacking CYP2D6 activity (PM-HLM) has been predicted by use of in vitro kinetic data on recombinant CYPs microsomes (rCYPs). 2. In PM-HLM, NE-100 is predicted to be metabolized to N-despropyl-NE-100 (NE-098), p-hydroxy-NE-100 (NE-152) and m-hydroxyl-NE-100 (NE-163), but not to O-demethy-NE-100 (NE-125), which is a major metabolite in pooled human liver microsomes (EM-HLM). The relative activity factor approach assumed that NE-098 formation is predominantly catalysed by CYP3A4 and CYP2C9 and the NE-152+163mix (a mixture of two hydroxylated metabolites, NE-152 and NE-163) formation is only catalysed by CYP3A4. 3. The predicted contribution rates of CYP3A4 and CYP2C9 for NE-098 formation were 58.1 and 34.6%, respectively, in PM-HLM. These predicted results were strongly supported by kinetic and inhibition studies using PM-HLM. The intrinsic clearance of NE-100 predicted from rCYPs (the predicted CLint-HLM-total) corresponded to those observed from EM- and PM-HLM (the observed CLint-HLM). 4. The in vivo oral clearance (CLoral) of NE-100 in extensive metabolizers and poor metabolizers of CYP2D6 was predicted to be 50times higher in extensive metabolizers than poor metabolizers using in vitro-in vivo scaling method based on the dispersion model. These data suggest that polymorphism of CYP2D6 might greatly affect NE-100 metabolism in vivo.

Effect of amiodarone on the plasma levels of metoprolol

On average, metoprolol plasma concentration is doubled after an amiodarone loading dose (1.2 g/day over a period of 6 days). However, the individual amount of this drug interaction depends on the CYP2D6 genotype.

Extended-release metoprolol succinate in chronic heart failure

OBJECTIVE

To review the pharmacology, pharmacokinetics, efficacy, and tolerability of extended-release (ER) metoprolol succinate and its role in the management of chronic heart failure.

DATA SOURCES

A MEDLINE search of English-language literature (1990-October 2002) was conducted using congestive heart failure and metoprolol CR/XL or metoprolol CR/ZOK as search terms to identify pertinent studies.

STUDY SELECTION/DATA EXTRACTION

All of the articles identified from the data sources were evaluated, with priority given to randomized, double-blind, placebo-controlled studies.

DATA SYNTHESIS

ER metoprolol succinate is a controlled-release tablet designed to produce even and consistent beta(1)-blockade throughout the 24-hour dosing interval, with less fluctuation in metoprolol plasma concentrations compared with immediate-release metoprolol. Three randomized, double-blind, placebo-controlled trials have evaluated the efficacy of ER metoprolol succinate in the treatment of patients with chronic heart failure. The MERIT-HF (Metoprolol CR/XL Randomized Intervention Trial in Congestive Heart Failure) study, the largest of these trials and the largest randomized mortality trial with beta-blockers in heart failure to date, demonstrated that ER metoprolol succinate reduced the relative risk of all-cause mortality by 34% versus placebo. Furthermore, the relative risk of the combined endpoint of mortality plus all-cause hospitalizations was reduced by 19% and sudden death was reduced by 41%. The benefits of therapy were evident in various patient subgroups, including elderly patients and those with diabetes mellitus. ER metoprolol succinate was generally well tolerated, with a similar proportion of patients discontinuing therapy due to adverse events relative to placebo (9.8% and 11.7%, respectively).

CONCLUSIONS

ER metoprolol succinate therapy provides substantial mortality and morbidity benefits in patients with New York Heart Association class II and III heart failure who are stabilized on angiotensin-converting enzyme inhibitors and diuretics. ER metoprolol succinate is administered once daily, is well tolerated, and provides consistent beta(1)-blockade over the 24-hour dosing interval.

Effect of the CYP2D6 genotype on metoprolol metabolism persists during long-term treatment

The beta1 selective beta-blocker metoprolol is metabolized predominantly but not exclusively by CYP2D6. Due to the polymorphism of the CYP2D6 gene, CYP2D6 activity varies markedly between individuals. Consequently, after short-term administration metoprolol plasma concentrations were found to be several fold higher in poor metabolizers than in extensive metabolizers. However, it is currently not known, whether the impact of the CYP2D6 polymorphism persists during long-term therapy, since alternate mechanisms of elimination or metabolism could be effective in this setting. The study comprised 91 Caucasian patients on long-term treatment with metoprolol (median duration of treatment 12.6 months; median daily drug dose: 47.5 mg/day). Metoprolol and alpha-OH-metoprolol plasma concentrations were assessed by HPLC. Genotyping detected the null alleles (*0): *3, *4, *5, *6, *7, *8, *12, *14, *15, the alleles *9, *10 and *41 associated with reduced enzymatic activity as well as the fully functional alleles *1 and *2. Genotype and allele frequencies were in accordance with published frequencies for the German population. The plasma metabolic ratio of metoprolol/alpha-OH-metoprolol was markedly affected by the genotype (P < 0.0001). In accordance, median adjusted metoprolol plasma concentrations were 6.2- and 3.9-fold higher in patients with *0/*0 genotypes (n = 8) and intermediate genotypes (n = 10), respectively, as compared to those with two fully functional alleles (n = 31; P < 0.01). In summary, the pronounced effect of the CYP2D6 genotype persists during long-term therapy, affecting both metabolic ratio and metoprolol plasma concentration.

Cardiovascular drug-drug interactions

The drug-drug interactions discussed in this article have either documented or suspected clinical relevance for patients with cardiovascular disease and the clinician involved in the care of these patients. Oftentimes, drug-drug interactions are difficult, if not impossible, to predict because of the high degree of interpatient variability in drug disposition. Certain drug-drug interactions, however, may be avoided through knowledge and sound clinical judgment. Every clinician should maintain a working knowledge of reported drug-drug interactions and an understanding of basic pharmacokinetic and pharmacodynamic principles to help predict and minimize the incidence and severity of drug-drug interactions.

Enantioselectivity in the steady-state pharmacokinetics of metoprolol in hypertensive patients

In the present study we investigated the enantioselectivity in the pharmacokinetics of metoprolol administered in a multiple-dose regimen as the racemate. The study was conducted on 10 patients of both sexes with mild to severe essential hypertension, aged 28 to 76 years, with normal hepatic and renal function and phenotyped as extensive metabolizers of debrisoquine (urine debrisoquine to 4-hydroxydebrisoquine ratios of 0.28 to 6.56). The patients were treated with racemic metoprolol (two 100 mg tablets every 24 h) for 7 days. Serial blood samples were collected at times zero, 0.5, 1, 1.5, 2, 3, 4, 6, 8, 10, 12, 16, 20, 22, and 24 h and urine at each 6 h period until 24 h after metoprolol administration. The plasma concentrations of the (-)-(S)- and (+)-(R)-metoprolol enantiomers were determined by HPLC using a chiral stationary phase (Chiralpak AD, 4.6 x 250 mm) and fluorescence detection. The enantiomeric ratios differing from one were evaluated by the paired t test and the results are reported as means (95% CI). No differences were observed between metoprolol enantiomers in half-lives and absorption, distribution and elimination rate constants. However, the following differences (p < 0.05) were observed between the (-)-(S) and (+)-(R) enantiomers: maximum plasma concentration, C(max), 179.99 (123. 33-236.64) versus 151.30 (95.04-207.57) ng/mL; area under the plasma concentration versus time curve, AUC(0-24)(SS), 929.85 (458.02-1401. 70) versus 782.11 (329.80-1234.40) ng h/mL; apparent total clearance, Cl(T)/f, 1.70 (0.79-2.61) versus 2.21 (1.06-3.36) L/h/kg, apparent distribution volume, Vd/f, 10.51 (6.35-14.68) versus 13.80 (6.93-20. 68) L/kg, and renal clearance, Cl(R), 0.06 (0.05-0.08) versus 0.07 (0.05-0.09) L/kg. The enantiomeric ratios AUC((-)-(S))/AUC((+)-(R)) ranged from 1.14 to 1.44, with a mean of 1.29. The data obtained demonstrate enantioselectivity in the kinetic disposition of metoprolol, with plasma accumulation of the pharmacologically more active (-)-(S)-metoprolol enantiomer in hypertensive patients phenotyped as extensive metabolizers of debrisoquine.

Clinically important pharmacokinetic drug-drug interactions: role of cytochrome P450 enzymes

Drug-drug interactions have become an important issue in health care. It is now realized that many drug-drug interactions can be explained by alterations in the metabolic enzymes that are present in the liver and other extra-hepatic tissues and many of the major pharmacokinetic interactions between drugs are due to hepatic cytochrome P450 (P450 or CYP) enzymes being affected by previous administration of other drugs. After coadministration, some drugs act as potent enzyme inducers, whereas others are inhibitors. However, reports of enzyme inhibition are very much more common. Understanding these mechanisms of enzyme inhibition or induction is extremely important in order to give appropriate multiple-drug therapies. In the future, it may help to identify individuals at greatest risk of drug interactions and adverse events.

beta-blockers. Drug interactions of clinical significance

The clinician prescribing beta-blockers for his or her patients is faced with an often difficult situation. There are many beta-blockers, each with its own pharmacological profile. Patients are often taking multiple medications, thus increasing the risk of both anticipated and unexpected drug interactions. Reports of drug interactions are frequently anecdotal. The prescriber may not be aware of the patient's other medications or lifestyle habits. Pharmacokinetic and pharmacodynamic drug interactions involving beta-blockers are documented in the literature, but these studies often examine small numbers of patients. For these reasons, it is difficult for the practitioner to distill guidelines for the administration of beta-blockers in conjunction with other medication. In general, beta-blockers are well tolerated, and symptomatic drug interactions are relatively infrequent. It is incumbent upon the clinical practitioner to have knowledge of his or her patient's drug profile and to be aware of the various drug interactions as well as each patient's unique pathophysiological profile when prescribing any medication, including beta-blockers. beta-Blockers may interact with a large number of commonly prescribed drugs, including antihypertensive and antianginal drugs, inotropic agents, anti-arrhythmics, NSAIDs, psychotropic drugs, anti-ulcer medications, anaesthetics, HMG-CoA reductase inhibitors, warfarin, oral hypoglycaemics and rifampicin (rifampin).

Genetic differences in drug disposition

Genetic polymorphisms of drug metabolizing enzymes are well recognized. This review presents molecular mechanisms, ontogeny and clinical implications of genetically determined intersubject variation in some of these enzymes. Included are the polymorphic enzymes N-acetyl transferase, cytochromes P4502D6 and 2C, which have been well described in humans. Information regarding other Phase I and Phase II polymorphic pathways, such as glutathione and methyl conjugation and alcohol and acetaldehyde oxidation continues to increase and are also discussed. Genetic factors effecting enzyme activity are frequently important determinants of the disposition of drugs and their efficacy and toxicity. In addition, associations between genetic differences in these enzymes and susceptibility to carcinogens and teratogens have been reported. Ultimately, the application of knowledge regarding these genetic factors of enzyme activity may guide medical therapy and minimize xenobiotic-induced disease.

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.

Propafenone. A reappraisal of its pharmacology, pharmacokinetics and therapeutic use in cardiac arrhythmias

Propafenone is an orally active sodium channel blocking agent with beta-adrenoceptor antagonist and weak calcium antagonist activity. The pharmacokinetic profile of propafenone is complex, characterised as typically nonlinear, saturable, stereoselective and dependent on both dose and debrisoquin metaboliser phenotype; individualised dosage titration is required. Both placebo- and drug-controlled studies have confirmed the efficacy of propafenone in the treatment of premature ventricular complexes, ventricular couplets and nonsustained ventricular tachycardia; in a large meta-analysis, propafenone together with amiodarone, flecainide and encainide were significantly more effective in the control of ventricular ectopy than other antiarrhythmic agents. However, the use of propafenone in these indications, like that of other antiarrhythmic agents, is likely to be limited to patients with a favourable risk-to-benefit ratio. Propafenone has also demonstrated efficacy in the treatment of malignant ventricular arrhythmias (ventricular fibrillation and sustained ventricular tachycardia); preliminary mortality data obtained with propafenone have been encouraging in this patient group. In addition, propafenone has a favourable noncardiac tolerability profile and beta-adrenoceptor antagonist activity, which may offer advantages in some specific patient groups. The area of research concerning propafenone which has shown the greatest expansion over the past 5 years is in the treatment of supraventricular arrhythmias. Propafenone has marked efficacy in patients with Wolff-Parkinson-White syndrome and has been recommended as a first-line prophylactic agent in those with rapid anterograde conduction. Propafenone is also effective in the conversion of atrial fibrillation to sinus rhythm, although comparative studies are required to determine advantages over more established agents. Propafenone use has been successfully extended to children with limited data demonstrating consistent efficacy in the control of junctional ectopic tachycardia. As with all antiarrhythmic agents, propafenone has the potential to induce arrhythmias. Comparative studies are required to assess in more detail the cardiac tolerability profile of propafenone against other class Ic agents. In conclusion, propafenone offers a broad spectrum of activity in the treatment of cardiac arrhythmias, although its use in patients with potentially malignant arrhythmias will remain limited for the present. Due to its unique pharmacodynamic profile, propafenone deserves consideration as an individual agent.

Genetically determined differences in drug metabolism as a risk factor in drug toxicity

Drug metabolizing enzymes are of paramount importance in drug detoxification as well as chemical mutagenesis, carcinogenesis and toxicity via metabolic activation. Thus genetically determined differences in the activity of these enzymes can influence individual susceptibility to adverse drug reactions, drug induced diseases and certain types of chemically induced cancers. The genetic polymorphisms of three human drug metabolizing enzymes, namely N-acetyltransferase and two cytochrome P-450 isozymes (P-4502D6: debrisoquine/sparteine polymorphism, P-4502C8-10: mephenytoin polymorphism) have been firmly established. Based on the metabolic handling of certain probe drugs, the population can be divided into two phenotypes: the rapid acetylator/extensive metabolizer and slow acetylator/poor metabolizer. These polymorphisms have provided useful tools to study the relationship between genetically determined differences in the activity of drug metabolizing enzymes and the risk for adverse drug reactions and certain types of chemically-induced diseases and cancers. With regard to the susceptibility of the two phenotypes, drug mediated toxicity for the following scenarios can be anticipated. (1) The toxicity of the drug is caused by the parent compound and the elimination of the drug proceeds exclusively via the polymorphic enzyme. No alternate pathways of biotransformation are available. Thus the slow acetylator/poor metabolizer phenotype will be more prone to such a type of toxicity since, at the same level of exposure, this phenotype will accumulate the drug as a result of impaired metabolism (e.g. isoniazid polyneuropathy, perhexiline polyneuropathy, pesticide induced Parkinsons disease). (2) The polymorphic pathway is a major route of detoxification. Impairment of this pathway shifts the metabolism to an alternate pathway via which a reactive intermediate is being formed. In such a situation the slow acetylator/poor metabolizer phenotype constitutes a major risk factor for toxicity (e.g. isoniazid hepatotoxicity). (3) The toxicity is mediated by a reactive intermediate generated by a polymorphic enzyme. Hence extensive metabolizers are at a much higher risk than poor metabolizers to develop toxicity or cancer (e.g. bronchial carcinoma in smokers, not chemically induced aggressive bladder cancer).

Omeprazole drug interaction studies

This review examines the literature on drug interactions with omeprazole. Different mechanisms have been proposed as potential causes for such interactions. First, the absorption of some drugs might be altered due to the decreased intragastric acidity resulting from omeprazole treatment. There was no effect of omeprazole on the absorption of amoxycillin, bacampicillin and alcohol, while the amount of digoxin and nifedipine absorbed was increased by 10 and 21%, respectively, both increases probably being of no clinical significance. Secondly, the metabolism of high clearance drugs might be altered by changes in liver blood flow, although that is not affected by omeprazole, as indicated by the unchanged elimination of indocyanine green. In addition, the clearance of intravenously administered lidocaine (lignocaine) [a high clearance drug] was unaffected by omeprazole, further indicating that the latter does not alter liver blood flow. Thirdly, since omeprazole is a substituted benzimidazole, it might have the potential to interfere with the metabolism of other drugs by altering the activity of drug metabolising enzymes in the cytochrome P450 system, through either induction or inhibition. There is no indication of induction of this enzyme system in any interaction study with omeprazole. As regards inhibition, on the other hand, there is now considerable information available which indicates that omeprazole has the potential to partly inhibit the metabolism of drugs metabolised to a great extent by the cytochrome P450 enzyme subfamily IIC (diazepam, phenytoin), but not of those metabolised by subfamilies IA (caffeine, theophylline), IID (metoprolol, propranolol) and IIIA (cyclosporin, lidocaine, quinidine). Since relatively few drugs are metabolised mainly by IIC compared with IID and IIIA, the potential for omeprazole to interfere with the metabolism of other drugs appears to be limited.

Clinical pharmacokinetics of propafenone

Propafenone is a class 1C antiarrhythmic agent which is administered as a racemate of S(+)- and R(-)-enantiomers. It is well absorbed and is predominantly bound to alpha 1-acid glycoprotein in the plasma. The enantiomers display stereoselective disposition characteristics, the R-enantiomer being cleared more quickly. The hepatic metabolism of propafenone is polymorphic and genetically determined: about 10% of Caucasians have a reduced capacity to hydroxylate the drug. This polymorphic metabolism accounts for the marked interindividual variability in the relationships between dose and concentration, and between concentration and pharmacodynamic effects. During long term administration, the metabolism is saturable in patients with the 'extensive metaboliser' phenotype, leading to accumulation of the parent compound. Propafenone blocks fast inward sodium channels in a frequency-dependent manner, and also has moderate beta-blocking effects. Both the enantiomers and the 5-OH metabolite have a potency to block sodium channels comparable with that of the parent compound. The S-enantiomer is a more potent beta-antagonist than the R-enantiomer. Propafenone typically slows conduction markedly but only modestly prolongs refractoriness. These cardiac effects are determined by the extent of its myocardial accumulation. The drug should be used with caution in patients with serious structural heart disease, as it may cause or aggravate life-threatening arrhythmias. Significant interactions occur when propafenone is coadministered with other drugs. It increases the plasma concentrations of digoxin, warfarin, metoprolol and propranolol as well as enhancing their respective pharmacodynamic effects. Doses of these drugs should therefore be decreased if they are coadministered with propafenone.

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.

The effects of encainide versus diltiazem on the oxidative metabolic pathways of antipyrine

The effects of diltiazem and encainide on the pharmacokinetics and metabolism of antipyrine were compared in nine healthy male volunteers. Diltiazem 90 mg every 8 hours for 5 days decreased the oral clearance of antipyrine from 2.34 to 1.86 L/hour (p less than 0.05) and increased half-life from 12.7 to 15.9 hours (p less than 0.05). Diltiazem reduced the formation rate constants for 3-hydroxymethylantipyrine by 27% (p less than 0.05) and 4-hydroxyantipyrine by 37% (p less than 0.05). There was also a 21% reduction in the formation rate constant for norantipyrine (0.05 less than p less than 0.10). Encainide 25 mg every 8 hours for 5 days had no apparent effect on the oral clearance or half-life of antipyrine, or on the formation rate constants for metabolites of antipyrine. In contrast to a previously published report in rats, encainide, unlike diltiazem, does not inhibit the oxidative metabolism of antipyrine in humans.

Interaction between propranolol and propafenone in healthy volunteers

The effects of propafenone on the pharmacokinetics and pharmacodynamics of propranolol were evaluated in 12 healthy male subjects. Both propafenone and propranolol were each administered alone for one week followed by concomitant administration for an additional week. Blood samples, obtained at steady-state, were analyzed for propafenone and its two metabolites as well as for propranolol and 4-hydroxypropranolol. Left ventricular function, exercise performance and electrocardiographic intervals were assessed. Coadministration of propranolol did not produce any significant change in propafenone kinetics including peak plasma concentration (Cmax), time to peak plasma concentration (Tmax), elimination rate constant (t1/2), mean steady-state plasma concentration (Css), or area under the concentration vs time curves. However, concomitant propafenone administration significantly increased Cmax (83%), Tmax (55%), t1/2 (30%), and Css (213%) which were accompanied by significant decreases in plasma levels of 4-hydroxy-propranolol. Propafenone and propranolol significantly reduced supine systolic and diastolic blood pressure by 2.5 to 15.4%. The combination did not reduce diastolic blood pressure further (64.0 +/- 2.8 to 59.7 +/- 1.7 mmHg) nor did it produce a supplemental reduction in heart rate (12% reduction with propranolol, 10% reduction with concomitant administration). Propranolol, but not propafenone, significantly decreased end-diastolic volume index (13%), stroke volume index (15%), and velocity of circumferential fiber shortening (19%). The combination did not cause any further changes in echocardiographic measurements. Electrocardiographic intervals were not altered by either drug use alone or in combination.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Effects of coadministration of propafenone on the pharmacokinetics of digoxin in healthy volunteer subjects

Previous reports have suggested an interaction between propafenone and digoxin. We investigated the pharmacokinetics of IV digoxin when given alone (Phase I), after pretreatment with propafenone 150 mg every 8 hours for seven days (Phase II), and after propafenone 300 mg every 8 hours for 7 days (Phase III). The total body clearance of digoxin during Phase I was 2.45 ml/min/kg and was 2.17 ml/min/kg during Phase II (NS) and decreased to 1.92 ml/min/kg during Phase III (P less than 0.05). The renal clearance and half-life of digoxin were not significantly altered by propafenone. There was a trend towards a decrease in the volume of distribution of digoxin from 9.43 L/kg in Phase I, to 9.33 L/kg in Phase II, and 8.02 L/kg in Phase III. Similarly there was a trend towards a decreased nonrenal clearance of digoxin from 1.21 ml/min/kg during Phase I to 1.01 ml/min/kg during Phase II and to 0.75 ml/min/kg during Phase III. The changes in volume of distribution and nonrenal clearance parallel each other resulting in no change in the elimination half-life of digoxin. It is postulated that the mechanism of this interaction is due to decreases in the volume of distribution and nonrenal elimination of digoxin by propafenone. The degree of this interaction was related to the dose of propafenone. The magnitude of this interaction may be greater in patients and, thus, may require a reduction in the digoxin dose.

Severe complications of antianginal drug therapy in a patient identified as a poor metabolizer of metoprolol, propafenone, diltiazem, and sparteine

A 47-year-old patient suffering from coronary artery disease was admitted to the CCU in shock with III. AV block, severe hypotension, and impairment of ventricular function. One week prior to admission a therapy with standard doses of metoprolol (100 mg t.i.d. and then 100 mg b.i.d.) had been initiated. Two days before admission diltiazem (60 mg b.i.d.) was prescribed in addition. Analyses of a blood sample revealed unusually high plasma concentrations of metoprolol (greater than 3000 ng/ml) and diltiazem (526 ng/ml). The patient recovered within 1 week following discontinuation of antianginal therapy. Three months later the patient was exposed to a single dose of metoprolol, diltiazem, propafenone (since he had received this drug in the past), and sparteine (as a probe for the debrisoquine/sparteine type polymorphism of oxidative drug metabolism). It was found that he was a poor metabolizer of all four drugs, indicating that their metabolism is under the same genetic control. Therefore, patients belonging to the poor-metabolizer phenotype of sparteine/debrisoquine polymorphism in drug metabolism, which constitutes 6.4% of the German population, may experience adverse drug reactions when treated with standard doses of one of these drugs alone. Moreover, the coadministration of these frequently used drugs is expected to be especially harmful in this subgroup of patients.