The effect of enzyme induction on the metabolism of disopyramide in man

Model-based comparative analysis of rifampicin and rifabutin drug-drug interaction profile

Rifamycins are widely used for treating mycobacterial and staphylococcal infections. Drug-drug interactions (DDI) caused by rifampicin (RIF) is a major issue. We used a model-based approach to predict the magnitude of DDI with RIF and rifabutin (RBT) for 217 cytochrome P450 (CYP) substrates. On average, DDI caused by low-dose RIF were twice more potent than those caused by RBT. Contrary to RIF, RBT appears unlikely to cause severe DDI, even with sensitive CYP substrates.

Tuberculosis and pharmacological interactions: A narrative review

Even if major improvements in therapeutic regimens and treatment outcomes have been progressively achieved, tuberculosis (TB) remains the leading cause of death from a single infectious microorganism. To improve TB treatment success as well as patients' quality of life, drug-drug-interactions (DDIs) need to be wisely managed. Comprehensive knowledge of anti-TB drugs, pharmacokinetics and pharmacodynamic (PK/PD) parameters, potential patients' changes in absorption and distribution, possible side effects and interactions, is mandatory to built effective anti-TB regimens. Optimization of treatments and adherence to international guidelines can help bend the curve of TB-related mortality and, ultimately, decrease the likelihood of treatment failure and drop-out during anti-TB treatment. Aim of this paper is to describe the most relevant DDIs between anti-TB and other drugs used in daily clinical practice, providing an updated and "easy-to-use" guide to minimize adverse effects, drop-outs and, in the long run, increase treatment success.

Pharmacokinetic interactions with rifampicin : clinical relevance

The antituberculosis drug rifampicin (rifampin) induces a number of drug-metabolising enzymes, having the greatest effects on the expression of cytochrome P450 (CYP) 3A4 in the liver and in the small intestine. In addition, rifampicin induces some drug transporter proteins, such as intestinal and hepatic P-glycoprotein. Full induction of drug-metabolising enzymes is reached in about 1 week after starting rifampicin treatment and the induction dissipates in roughly 2 weeks after discontinuing rifampicin. Rifampicin has its greatest effects on the pharmacokinetics of orally administered drugs that are metabolised by CYP3A4 and/or are transported by P-glycoprotein. Thus, for example, oral midazolam, triazolam, simvastatin, verapamil and most dihydropyridine calcium channel antagonists are ineffective during rifampicin treatment. The plasma concentrations of several anti-infectives, such as the antimycotics itraconazole and ketoconazole and the HIV protease inhibitors indinavir, nelfinavir and saquinavir, are also greatly reduced by rifampicin. The use of rifampicin with these HIV protease inhibitors is contraindicated to avoid treatment failures. Rifampicin can cause acute transplant rejection in patients treated with immunosuppressive drugs, such as cyclosporin. In addition, rifampicin reduces the plasma concentrations of methadone, leading to symptoms of opioid withdrawal in most patients. Rifampicin also induces CYP2C-mediated metabolism and thus reduces the plasma concentrations of, for example, the CYP2C9 substrate (S)-warfarin and the sulfonylurea antidiabetic drugs. In addition, rifampicin can reduce the plasma concentrations of drugs that are not metabolised (e.g. digoxin) by inducing drug transporters such as P-glycoprotein. Thus, the effects of rifampicin on drug metabolism and transport are broad and of established clinical significance. Potential drug interactions should be considered whenever beginning or discontinuing rifampicin treatment. It is particularly important to remember that the concentrations of many of the other drugs used by the patient will increase when rifampicin is discontinued as the induction starts to wear off.

Clinically important drug interactions in epilepsy: interactions between antiepileptic drugs and other drugs

Antiepileptic drugs (AEDs) are commonly prescribed for long periods, up to a lifetime, and many patients will require treatment with other agents for the management of concomitant or intercurrent conditions. When two or more drugs are prescribed together, clinically important interactions can occur. Among old-generation AEDs, carbamazepine, phenytoin, phenobarbital, and primidone are potent inducers of hepatic enzymes, and decrease the plasma concentration of many psychotropic, immunosuppressant, antineoplastic, antimicrobial, and cardiovascular drugs, as well as oral contraceptive steroids. Most new generation AEDs do not have clinically important enzyme inducing effects. Other drugs can affect the pharmacokinetics of AEDs; examples include the stimulation of lamotrigine metabolism by oral contraceptive steroids and the inhibition of carbamazepine metabolism by certain macrolide antibiotics, antifungals, verapamil, diltiazem, and isoniazid. Careful monitoring of clinical response is recommended whenever a drug is added or removed from a patient's AED regimen.

Clinically significant interactions with drugs used in the treatment of tuberculosis

Clinically significant interactions occurring during antituberculous chemotherapy principally involve rifampicin (rifampin), isoniazid and the fluoroquinolones. Such interactions between the antituberculous drugs and coadministered agents are definitely much more important than among antituberculous drugs themselves. These can be associated with consequences even amounting to therapeutic failure or toxicity. Most of the interactions are pharmacokinetic rather than pharmacodynamic in nature. The cytochrome P450 isoform enzymes are responsible for many interactions (especially those involving rifampicin and isoniazid) during drug biotransformation (metabolism) in the liver and/or intestine. Generally, rifampicin is an enzyme inducer and isoniazid acts as an inhibitor. The agents interacting significantly with rifampicin include anticoagulants, anticonvulsants, anti-infectives, cardiovascular therapeutics, contraceptives, glucocorticoids, immunosuppressants, psychotropics, sulphonylureas and theophyllines. Isoniazid interacts principally with anticonvulsants, theophylline, benzodiapines, paracetamol (acetaminophen) and some food. Fluoroquinolones can have absorption disturbance due to a variety of agents, especially the metal cations. Other important interactions of fluoroquinolones result from their enzyme inhibiting potential or pharmacodynamic mechanisms. Geriatric and immunocompromised patients are particularly at risk of drug interactions during treatment of their tuberculosis. Among the latter, patients who are HIV infected constitute the most important group. This is largely because of the advent of new antiretroviral agents such as the HIV protease inhibitors and the non-nucleoside reverse transcriptase inhibitors in the armamenterium of therapy. Compounding the complexity of drug interactions, underlying medical diseases per se may also contribute to or aggravate the scenario. It is imperative for clinicians to be on the alert when treating tuberculosis in patients with difficult co-morbidity requiring polypharmacy. With advancement of knowledge and expertise, it is hoped that therapeutic drug monitoring as a new paradigm of care can enable better management of these drug interactions.

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.

Antiarrhythmic agents: drug interactions of clinical significance

The management of cardiac arrhythmias has grown more complex in recent years. Despite the recent focus on nonpharmacological therapy, most clinical arrhythmias are treated with existing antiarrhythmics. Because of the narrow therapeutic index of antiarrhythmic agents, potential drug interactions with other medications are of major clinical importance. As most antiarrhythmics are metabolised via the cytochrome P450 enzyme system, pharmacokinetic interactions constitute the majority of clinically significant interactions seen with these agents. Antiarrhythmics may be substrates, inducers or inhibitors of cytochrome P450 enzymes, and many of these metabolic interactions have been characterised. However, many potential interactions have not, and knowledge of how antiarrhythmic agents are metabolised by the cytochrome P450 enzyme system may allow clinicians to predict potential interactions. Drug interactions with Vaughn-Williams Class II (beta-blockers) and Class IV (calcium antagonists) agents have previously been reviewed and are not discussed here. Class I agents, which primarily block fast sodium channels and slow conduction velocity, include quinidine, procainamide, disopyramide, lidocaine (lignocaine), mexiletine, flecainide and propafenone. All of these agents except procainamide are metabolised via the cytochrome P450 system and are involved in a number of drug-drug interactions, including over 20 different interactions with quinidine. Quinidine has been observed to inhibit the metabolism of digoxin, tricyclic antidepressants and codeine. Furthermore, cimetidine, azole antifungals and calcium antagonists can significantly inhibit the metabolism of quinidine. Procainamide is excreted via active tubular secretion, which may be inhibited by cimetidine and trimethoprim. Other Class I agents may affect the disposition of warfarin, theophylline and tricyclic antidepressants. Many of these interactions can significantly affect efficacy and/or toxicity. Of the Class III antiarrhythmics, amiodarone is involved in a significant number of interactions since it is a potent inhibitor of several cytochrome P450 enzymes. It can significantly impair the metabolism of digoxin, theophylline and warfarin. Dosages of digoxin and warfarin should empirically be decreased by one-half when amiodarone therapy is added. In addition to pharmacokinetic interactions, many reports describe the use of antiarrhythmic drug combinations for the treatment of arrhythmias. By combining antiarrhythmic drugs and utilising additive electrophysiological/pharmacodynamic effects, antiarrhythmic efficacy may be improved and toxicity reduced. As medication regimens grow more complex with the aging population, knowledge of existing and potential drug-drug interactions becomes vital for clinicians to optimise drug therapy for every patient.

Treatment of concomitant illnesses in patients receiving anticonvulsants: drug interactions of clinical significance

As epilepsy often is a chronic condition requiring prolonged therapy with anticonvulsants, patients being treated for epilepsy can be at risk when they are prescribed other drugs for concomitant diseases. Pharmacokinetic interactions can occur at each step of drug disposition (absorption, distribution, metabolism and elimination). Although such interactions may occur frequently with some drugs, only some will be clinically relevant. Alterations in the hepatic biotransformation of metabolised drugs due to hepatic isoenzyme induction or inhibition is of particular concern. The consequences of pharmacokinetic interactions are either accumulation of the drug leading to toxicity, or lowering of plasma concentrations resulting in reduced efficacy. Clinically relevant interactions depend on the structure, dosage and duration of administration of interacting agents, and on the individual's genetic make-up. In the past, drug interactions have been analysed empirically. At present, at least for interactions between drugs that are biotransformed in the liver, the risk should be predicted by considering the individual cytochrome P450 isoforms involved in the metabolism of coadministered drugs. Although drug-drug interactions can be predicted, their extent cannot be due to large interindividual variability. Even if nearly all drug combinations could be used with close clinical surveillance and blood concentration determinations, drugs that are not metabolised and are not highly protein bound, as are several of the new anticonvulsants, such as gabapentin, lamotrigine and vigabatrin, have a clear advantage in terms of a lower interaction potential.

Adverse effects of class I antiarrhythmic drugs

Class I antiarrhythmic drugs are characterised by their ability to block the fast inward sodium current in cardiac muscle tissue. However, at the same time, they can be responsible for various effects involving other organs and systems. Although some of these effects can be helpful in specific situations, most of them, such as their pro-arrhythmic propensity, are deleterious. Some of the adverse effects of class I antiarrhythmic drugs are directly linked to sodium-channel blockade (conduction disorders haemodynamic perturbations, and digestive and neurological effects), while others are linked to other specific pharmacological properties (e.g. atropinic, or alpha- or beta-adrenergic blockade) or to nonspecific properties (idiosyncratic hypersensitivity, and haematological, dermatological or hepatic reactions). Other adverse effects are associated with complex interactions between class I antiarrhythmics and individual predisposing factors, trigger mechanisms and physiological factors (including concomitant drug treatment). These numerous variations and interactions within a specific environment and underlying disorder might be of pharmacological or/and pharmacokinetic origin, making analysis of the true liability of the class I drugs very difficult when adverse effects occur.

Pharmacokinetic drug interactions with rifampicin

Rifampicin, an antituberculosis drug, is usually administered for 4 to 12 months with other antituberculosis drugs or medications from other classes. A potential for drug interactions often exists because rifampicin is a potent inducer of hepatic drug metabolism, as evidenced by a proliferation of smooth endoplasmic reticulum and an increase in the cytochrome P450 content in the liver. The induction is a highly selective process and not every drug metabolised via oxidation is affected. Case reports and studies have demonstrated enhanced metabolism of several drugs; most of these interactions are clinically important. At the start of rifampicin treatment, and again at the end, clinicians must check the dosages of any accompanying medications with which rifampicin may potentially interact. Monitoring of clinical response and blood drug concentrations is essential to adjust the drug dosage during rifampicin therapy. Rifampicin also interacts with cholephils such as bilirubin and bromosulphthalein. Its pharmacokinetics are reported to be altered by ethambutol, p-aminosalicylic acid (through its excipient component), ketoconazole, cyclosporin, clofazimine, probenecid and phenobarbital through one or other of the following mechanisms--impaired absorption of rifampicin, competition between the drug and rifampicin for hepatic uptake and altered hepatic metabolism of rifampicin. Most interactions affecting rifampicin have been relatively minor or are not expected to alter its therapeutic efficacy.

Cytochrome P-450 isozyme pattern is related to individual susceptibility to diethylnitrosamine-induced liver cancer in rats

Differences in susceptibility to chemical carcinogenesis between rodent strains and species have been linked to variations in genetically-determined mixed function oxidase activities. In order to verify whether such variations also determine the susceptibility of individual animals of the same strain to a chemical carcinogen, outbred male Wistar rats were administered diethylnitrosamine (DEN) (1, 2, or 3 mg/kg) five times a week for 20 weeks. The relationship was examined between the outcome (i.e., presence or absence of liver tumors, and latency period) and the hepatic activities of mixed function oxidases and conjugating enzymes, as well as of O6-methylguanine-DNA-methyltransferase, measured before the carcinogen treatment. In addition, the metabolic profiles of two model drugs, antipyrine and disopyramide, in the urine were analyzed and correlated with the carcinogen susceptibility. The length of the latency period of hepatocellular tumors in individual rats was negatively related to the activities of hepatic dimethylnitrosamine N-demethylase, aryl hydrocarbon hydroxylase and epoxide hydrolase and positively related to the amount of microsomal protein. Consistent relationships between the other 10 measured parameters and the susceptibility to DEN-induced carcinogenesis were not detected. Long-term treatment with DEN slightly decreased the proportion of metabolism of antipyrine into norantipyrine, and increased the share of 4-hydroxyantipyrine; a decrease in the metabolism of disopyramide to N-deisopropyldisopyramide was also detected. It is concluded that the pattern of cytochrome P-450 isoenzymes is related to differences in individual susceptibility to nitrosamine-induced carcinogenesis. The relationship was most marked at low dose levels, which are the levels at which nitrosamine exposures of humans are known to occur.

Enantioselective steady-state kinetics of unbound disopyramide and its dealkylated metabolite in man

Disopyramide is provided as a racemic mixture of R and S enantiomers, which have different pharmacodynamic and pharmacokinetic characteristics. Five volunteers were given racemic disopyramide 100 mg and 200 mg t.d.s. in a cross-over design. Plasma and urine concentrations of disopyramide and its active metabolite monodesisopropyl-disopyramide (MND) were determined at steady state by an enantioselective HPLC method. Unbound drug in plasma was measured after ultrafiltration. There was enantioselective clearance of unbound disopyramide (0.39 l.h-1.kg-1 for R-disopyramide and 0.58 l.h-1.kg-1 for S-disopyramide after 100 mg t.d.s.). The enantioselectivity was due to differences in the metabolism of disopyramide to MND and in further non-renal clearance, and the renal clearance of disopyramide was not enantioselective. The in vivo protein binding of disopyramide, which was saturable for both enantiomers, was also enantioselective. The difference in binding of the two enantiomers was explained by a difference in apparent binding capacity rather than in apparent binding affinity. The renal clearance of S-MND was significantly higher than R-MND (0.29 and 0.19 l.h-1.kg-1, respectively, after 100 mg t.d.s.). The renal clearance of MND also showed a tendency to saturation at higher concentrations.

Effect of phenytoin on the clinical pharmacokinetics of amiodarone

Five healthy male volunteers were given oral amiodarone hydrochloride, 200 mg per day for 6 1/2 weeks, to determine its effects on the pharmacokinetics of both intravenous and oral phenytoin. Predose amiodarone and N-desethylamiodarone serum concentrations were obtained weekly during weeks 2-6. Amiodarone serum concentrations (ASC) increased during weeks 2-4 and then decreased sharply during weeks 5-6 when oral phenytoin, 2-4 mg/kg/day, was co-administered. In addition, N-desethylamiodarone serum concentrations (DEASC) exceeded corresponding ASC during weeks 5-6 whereas during weeks 2-4, DEASC were less than ASC. Because of the long elimination half-life for amiodarone previously reported in healthy volunteers after single doses of amiodarone and the frequent administration of amiodarone associated with this half-life, a modified equation for a continuous infusion was used to describe each subject's ASC versus time data. Pre-phenytoin ASC were fitted to an appropriate function to predict ASC during weeks 5-6 assuming no interaction. Observed versus predicted ASC were compared for weeks 5 and 6. Observed ASC during weeks 5 and 6 were (mean +/- SD) 0.25 +/- 0.09 micrograms/mL and 0.19 +/- 0.07 micrograms/mL, respectively. Corresponding predicted ASC were 0.36 +/- 0.12 micrograms/mL (P = .011) and 0.38 +/- 0.13 micrograms/mL (P = .004). These represented percent differences of 32.2 +/- 12.5% and 49.3 +/- 5.6% for weeks 5 and 6, respectively. Assuming there were no changes in the bioavailability of amiodarone during continuous administration, these findings strongly suggest induction of amiodarone metabolism by phenytoin. The clinical significance of this interaction remains to be determined.

Enzyme induction: rifampin-disopyramide interaction

A 62-year-old woman with a history of supraventricular tachycardia, paroxysmal atrial tachycardia, and premature ventricular contractions was admitted with palpitations and anxiety. Previous therapy with antiarrhythmics had resulted in intolerable adverse effects or no effect on the arrhythmia. She had been taking rifampin prior to admission for acid-fast bacilli. Disopyramide was started on admission for supraventricular tachycardia. Subtherapeutic disopyramide blood concentrations were noted with concomitant administration of rifampin and disopyramide at normal doses. This case report demonstrates a possible drug interaction between these two drugs and the importance of careful monitoring. It was found that it takes three to five days after rifampin is discontinued before the enzyme induction of disopyramide disappears and disopyramide concentrations return to normal.

Elimination kinetics and urinary excretion of disopyramide in human healthy volunteers

Elimination kinetics and the renal handling of disopyramide was examined in 8 healthy volunteers. Approximately 50% of the administered disopyramide undergoes hepatic metabolism (metabolic clearance = 116.1 +/- 42.2 ml/min.), while the rest is excreted by the kidneys (renal clearance = 101.9 +/- 21.6 ml/min.). Total renal excretion rate of disopyramide was 0.676 +/- 0.188 mumol/min. and 0.258 +/- 0.029 mumol/min. was excreted by glomerular filtration leaving a net tubular secretion of 60% of the total renal elimination. A significant positive correlation was observed between total serum concentrations and renal clearance values of disopyramide while no significant correlation could be obtained between serum concentrations of the unbound drug and renal clearance values of disopyramide, implying a constant value of unbound renal clearance. Hepatic blood flow was significantly (P less than 0.005) decreased following disopyramide infusion.

Overview of the clinical pharmacology of antiarrhythmic drugs

Antiarrhythmic drugs have been recognized to possess 1 or more classes of antiarrhythmic action. This classification scheme is useful, but has major limitations because the available drugs and their metabolites have multiple actions. This report presents an overview of the distinguishing features of the most frequently used agents having class I or III actions. Agents with class I actions are local anesthetic agents that depress the fast inward depolarizing sodium current and thereby slow the rate of the rise of the action potential (phase 0). This category is further divided into classes IA, IB, and IC according to the degree of potency as sodium channel inhibitors, and the individual effects of the drug on action potential, conduction velocity and repolarization. Included in the spectrum of agents with class I action are quinidine, procainamide, disopyramide, lidocaine, tocainide, mexiletine, flecainide, amiodarone, encainide and lorcainide. The antiarrhythmic drugs that exert class III action lengthen repolarization and refractoriness; included in this category are amiodarone, quinidine, bretylium and sotalol. Because of the broad range of effects that antiarrhythmic agents may exert, safe and effective therapy requires a thorough familiarity with the pharmacologic profile of each drug administered and a careful evaluation of the presenting condition and the patient history. In some cases, a multiple drug regimen may be most appropriate. Various combinations such as class IA and IB agents, have been shown to slow conduction synergistically and increase refractoriness while keeping adverse effects to a minimum.

Disopyramide. A reappraisal of its pharmacodynamic and pharmacokinetic properties, and therapeutic use in cardiac arrhythmias

Disopyramide is a widely used class IA antiarrhythmic drug with a pharmacological profile of action similar to that of quinidine and procainamide. Over the past 10 years disopyramide has demonstrated its efficacy in ventricular and atrial arrhythmias. In therapeutic trials, usually involving small numbers of patients, the efficacy of disopyramide was comparable with that of mexiletine, perhexiline, tocainide, propafenone or prajmalium. Recent comparisons with quinidine have confirmed the similar efficacy and better tolerability of disopyramide. The suggestion from initial studies that disopyramide may be less effective than amiodarone or flecainide requires further investigation. In addition, studies have failed to demonstrate that the early administration of disopyramide after acute myocardial infarction decreases important arrhythmias or early mortality. Thus, disopyramide is now well established as an effective antiarrhythmic drug in ventricular and supraventricular arrhythmias although its role in therapy relative to that of recently introduced antiarrhythmic agents is not clear.

Clinical pharmacokinetics of disopyramide

Disopyramide is an antiarrhythmic agent with proven efficacy in the management of atrial and ventricular arrhythmias. The drug is well absorbed and undergoes virtually no first-pass metabolism. Peak concentrations are achieved approximately 0.5 to 3.0 hours after a dose. Absorption is reduced and slightly slowed in patients with acute myocardial infarction. Disopyramide is excreted as unchanged drug (two-thirds) or as the metabolite mono-N-desisopropyldisopyramide, with elimination via both renal and biliary routes. Elimination half-life is approximately 7 hours in normal subjects and patients, but is prolonged in patients with renal insufficiency (creatinine clearance less than 60 ml/min). Disopyramide exhibits complex protein binding. It is bound to alpha 1-acid glycoprotein (AAG), an acute phase reactant, and binds in a concentration-dependent (saturable) manner. The unbound fraction is reduced in the presence of elevated concentrations of AAG, as are found in acute myocardial infarction and in some chronic haemodialysis patients and renal transplant recipients. Free disopyramide concentrations are low relative to total concentration in these patients. Because the pharmacological effects of disopyramide are determined by unbound drug, changes in the unbound fraction could make total disopyramide concentrations misleading as a guide to therapy. Changes in protein binding do not, however, alter free disopyramide or metabolite concentrations, both of which are dependent only on dosage and intrinsic clearance. Free drug concentration measurement could potentially improve therapeutic monitoring, but is as yet of unproven clinical value. Disopyramide is cleared more rapidly in children than in adults, and therefore children require higher dosages to attain therapeutic concentrations.

The pharmacokinetics and protein binding of disopyramide in pigs

The pharmacokinetics of disopyramide were investigated in 5 pigs. Disopyramide was administered as intravenous bolus injections at different doses and as intravenous infusions at different rates. By measuring the free and total concentrations in plasma of parent compound and desisopropyldisopyramide (the main metabolite in humans) and the amounts excreted in urine it was found that disopyramide had a concentration dependent protein binding and that free and total concentrations could be described by a two-compartment model with a t1/2, beta of approximately 2 hours. The free concentrations were found to be more valuable for estimating the kinetic parameters. The total clearance of disopyramide in the pig was found to approximately 3 ml/min./kg which is about 3 times greater than in man. In contrast to humans the free clearance in the pig increased with declining concentration. Apart from this the kinetics of disopyramide in the pig were very similar to those in man. In conclusion the pig could be a relevant model for studying the pharmacokinetics in humans.

Antiarrhythmic therapy: clinical pharmacology update

Currently available antiarrhythmic agents are limited by side effects and the potential for increasing arrhythmias. In addition, drug interactions, altered disposition of drug as a result of changes in protein binding or concomitant disease processes, active metabolites, and poorly defined therapeutic ranges with great interpatient variability are some of the factors which complicate therapy. An awareness of the possible contribution of these factors in the use of antiarrhythmics is invaluable in both the choice of agent and the establishment of an optimum benefit-to-risk ratio for the patient.

The interaction of mexiletine with other cardiovascular drugs

Drug-drug interactions can be adverse or beneficial and can be classified as pharmacokinetic or pharmacodynamic. Several adverse pharmacokinetic drug interactions have been described for mexiletine. Because it is a weak base, mexiletine undergoes several pH-dependent drug interactions in the gastrointestinal tract and kidney. Since mexiletine is metabolized by hepatic mixed-function oxidases, its metabolic rate can be altered by drugs that induce or inhibit this drug metabolizing system. Phenytoin and rifampin have been shown to increase mexiletine clearance and decrease its plasma concentration. Striking examples of beneficial pharmacodynamic interactions occur with mexiletine. Combining mexiletine with either beta-adrenergic blocking drugs or with quinidine markedly increases antiarrhythmic efficacy and substantially decreases the incidence of adverse effects. These beneficial interactions will have a major impact on the clinical use of mexiletine.

Disopyramide: six years' experience

Since its approval by the FDA six years ago, oral disopyramide has earned a recognized role in the treatment of ventricular arrhythmias. During this time, clinical experience has refined our knowledge of this agent, allowing revision of dosing guidelines and better selection of patients. This review explores the recent therapeutic experience with disopyramide.

Plasma concentration of disopyramide given as capsules and controlled release tablets

Steady state plasma levels and clinical effects of disopyramide have been compared following administration of standard capsules and controlled release (CR) tablets. Nineteen patients (29-70 years) with atrial or ventricular arrhythmias were treated for two weeks with disopyramide capsules 200 mg t.i.d. and then with CR tablets 300 mg b.i.d. for 14 weeks. After treatment either with capsules or CR tablets, plasma concentrations of disopyramide and its metabolite N-deisopropyldisopyramide were similar within 1 dosage interval. Maximum and minimum concentrations of the parent drug were 10.1 +/- 0.9 mumol/l (mean +/- SEM) and 5.7 +/- 0.5 mumol/l with CR tablets, and 10.2 +/- 0.5 mumol/l and 5.6 +/- 0.5 mumol/l with standard capsules. The bioavailability of disopyramide was the same after capsules and CR tablets. Disopyramide, independent of the formulation, produced good antiarrhythmic effects. The side-effects reported on questioning were mainly of the anti-cholinergic type and there was no significant difference between the formulations with respect to their incidence, type or severity. Of 16 patients who stated a preference for one of the dosage forms, 11 prefered the CR tablets. The study confirms the good antiarrhythmic effect of disopyramide and shows that the CR preparation permits twice daily administration of disopyramide.

Effect of ethanol intake on disopyramide elimination by healthy volunteers

The effect of ethanol intake on disopyramide elimination was examined in an open crossover study in six healthy volunteers. No effect of ethanol on the elimination half-life or total body clearance of disopyramide was found, although it did decrease the percentage of mono-N-dealkylated disopyramide excreted in the urine (p less than 0.05) as well as the relative metabolic clearance of disopyramide (p less than 0.05). The renal clearance of disopyramide was increased by 19 +/- 16% (p less than 0.05) in subjects in whom ethanol caused a diuresis.

Enhanced metabolism of mexiletine after phenytoin administration

1 Unexpectedly low plasma concentrations of mexiletine were observed in three patients treated with mexiletine and concurrently taking phenytoin. 2 Six healthy volunteers were given a single oral dose of mexiletine (400 mg), before and after 1 week of phenytoin administration (300 mg/day). 3 The mean +/- s.d. area under the plasma mexiletine concentration-time curve decreased from 17.67 +/- 6.21 to 8.01 +/- 3.64 micrograms ml-1 h (P less than 0.003). 4 The mean +/- s.d. half-life of elimination of mexiletine decreased from 17.2 +/- 5.26 to 8.4 +/- 4.17 h (P less than 0.02) 5 The suggested mechanism of the interaction is hepatic mixed-function oxidase enzyme induction by phenytoin. 6 The interaction is likely to be clinically significant.

Pharmacokinetic interactions with antiepileptic drugs

A large number of pharmacokinetic interactions with antiepileptic drugs have been reported in recent years. Among the interactions affecting the disposition of anticonvulsants, the most important are probably those resulting in inhibition of the metabolism of phenytoin, phenobarbitone and carbamazepine. Drugs which have been shown to inhibit the metabolism of these anticonvulsants and to precipitate clinical signs of intoxication in epileptic patients include sulthiame, valproic acid, chloramphenicol, certain sulphonamides, phenylbutazone, isoniazid and propoxyphene. Interactions affecting the plasma protein binding of antiepileptic drugs are less likely to cause long-lasting alterations in response, but they are important because they change the relationship between serum drug concentrations and clinical effect. Anticonvulsant agents may induce important alterations in the pharmacokinetics of other drugs. Phenytoin and phenobarbitone may decrease the gastrointestinal absorption of frusemide and griseofulvin, respectively. Many of the drugs used in the treatment of the adult epilepsies, including phenytoin, phenobarbitone, primidone and carbamazepine, are potent inducers of the hepatic microsomal enzymes. This results in an increased rate of metabolism and decreased clinical efficacy of a number of drugs, including dicoumarol, steroid oral contraceptives, metyrapone, glucocorticoid agents, doxycycline, quinidine and vitamin D.