Absorption of mexiletine after treatment with gastric antacids

Clinical pharmacokinetics of mexiletine

Mexiletine, a class Ib antiarrhythmic agent, is rapidly and completely absorbed following oral administration with a bioavailability of about 90%. Peak plasma concentrations following oral administration occur within 1 to 4 hours and a linear relationship between dose and plasma concentration is observed in the dose range of 100 to 600 mg. Mexiletine is weakly bound to plasma proteins (70%). Its volume of distribution is large and varies from 5 to 9 L/kg in healthy individuals. Mexiletine is eliminated slowly in humans (with an elimination half-life of 10 hours). It undergoes stereoselective disposition caused by extensive metabolism. Eleven metabolites of mexiletine are presently known, but none of these metabolites possesses any pharmacological activity. The major metabolites are hydroxymethyl-mexiletine, p-hydroxy-mexiletine, m-hydroxy-mexiletine and N-hydroxy-mexiletine. Formation of hydroxymethyl-mexiletine, p-hydroxy-mexiletine and m-hydroxy-mexiletine is genetically determined and cosegregates with polymorphic debrisoquine 4-hydroxylase [cytochrome P450 (CYP) 2D6] activity. On the other hand, CYP1A2 seems to be implicated in the N-oxidation of mexiletine. Various physiological, pathological, pharmacological and environmental factors influence the disposition of mexiletine. Myocardial infarction, opioid analgesics, atropine and antacids slow the rate of absorption, whereas metoclopramide enhances it. Rifampicin (rifampin), phenytoin and cigarette smoking significantly enhance the rate of elimination of mexiletine, whereas ciprofloxacin, propafenone and liver cirrhosis decrease it. Cimetidine, ranitidine, fluconazole and omeprazole do not modify the disposition of mexiletine. Conversely, mexiletine is known to alter the disposition of other drugs, such as caffeine and theophylline. Factors affecting the elimination of mexiletine may be clinically important and dosage adjustments are often necessary.

Mexiletine: pharmacology and therapeutic use

Mexiletine is a Class IB antiarrhythmic which has basic and clinical electrophysiologic properties similar to lidocaine. Like other Class I antiarrhythmic agents, mexiletine blocks the rapid inward sodium current responsible for phase 0 of the action potential. It has been noted in the clinical electrophysiology laboratory to have minimal effect on sinus node function and AV nodal and His-Purkinje system conduction. Pharmacokinetic studies have shown that oral absorption is rapid with bioavailability of 80-90%. Mexiletine is predominantly metabolized by the liver with elimination half-life of 9 to 12 hours. The antiarrhythmic effects of the primary drug's metabolites remain to be defined. Hemodynamic studies have shown mexiletine to have a lesser negative inotropic effect than procainamide or disopyramide. Although mexiletine as a single agent successfully suppresses 60 to 80% of spontaneous ventricular arrhythmias, it has lower efficacy in suppression of induced ventricular arrhythmias. Multiple studies have shown that as monotherapy mexiletine is effective in preventing the induction of ventricular tachycardia in approximately 20% of patients. When used in combination with a Class IA antiarrhythmic drug for suppression of induced ventricular arrhythmias, multiple investigators have reported greater efficacy. Neurological side effects (tremor, dizziness, memory loss) occur in approximately 10% of patients while gastrointestinal side effects (nausea, anorexia, gastric irritation) occur in up to 40% of patients. Proarrhythmia or other serious toxicity from the drug is uncommon.

Advances in oral anti-arrhythmic therapy: implications for the anaesthetist

Surgical patients often are receiving antiarrhythmic therapy. Thus, because anaesthetic agents can affect cardiac function and may interact with concurrent antiarrhythmic medications, the anaesthetist should be aware of the electrophysiology associated with dysrhythmias and their management. Tocainide, flecainide, mexiletine, encainide and amiodarone have been introduced recently and each has an unique pattern of bioavailability, metabolism and toxicity. Patients treated with these drugs need special concern as they have abnormal cardiovascular systems and may be at increased risk for perioperative morbidity. In addition, unexpected untoward reactions and toxicity can result from interactions of anaesthetic agents and these drugs. This review discusses normal cardiac electrophysiology, common dysrhythmias and the electrophysiological effects of the newer oral antiarrhythmic drugs.

Drug-antacid interactions: assessment of clinical importance

Antacids and adsorbents are commonly used preparations that are generally considered to be pharmacologically inert and free from adverse effects. They may, however, interact with a diverse range of primary drugs and the sequelae can be disadvantageous to the efficacy of the primary medication. Many such reports in the literature are based on animal experiments, or on single-dose studies in healthy subjects. Some reports are anecdotal and are unconfirmed; others are based solely on in vitro evidence. Potentially important interactions have been suggested for a relatively small group of drugs: tetracyclines, phenytoin, digoxin, chloroquine, cimetidine, quinidine, nonsteroidal antiinflammatory drugs, and beta-blocking agents. The evidence for these has been critically evaluated, as well as for antacid-anticoagulant and antacid-nitrofurantoin interactions that have been wrongly emphasized in the literature. The majority of literature reports on interactions with antacids have been overemphasized; only ferrous sulfate-, isoniazid-, and tetracycline-antacid interactions fall into a category I importance (scale I-III of descending importance). This category is for those interactions with good evidence of actual or potential importance in patients or in relevant studies on normal subjects.

Mexiletine: a new type I antiarrhythmic agent

Mexiletine is a type I antiarrhythmic drug that is structurally similar to lidocaine. Mexiletine has considerable potential for causing neurologic, cardiac, or gastrointestinal side effects. However, mexiletine does not undergo clinically significant first-pass metabolism and, thus, has good oral bioavailability. Mexiletine has a large and variable volume of distribution and an elimination half-life ranging from 6 to 12 hours. Mexiletine disposition is probably altered in patients with heart failure, liver disease, and severe renal dysfunction. Efficacy and toxicity are not well correlated with mexiletine serum concentrations. Mexiletine is as effective as traditional antiarrhythmics in the treatment of premature ventricular contractions. However, in patients with drug-refractory inducible ventricular tachycardia, mexiletine is usually ineffective when used alone. When mexiletine is combined with other antiarrhythmic agents, a significantly higher percentage of patients with this difficult arrhythmia have a good response. Mexiletine is a potentially important addition to the existing antiarrhythmic drugs currently available, but its place in the clinical setting and in therapeutic drug monitoring is not well defined at this time.

Pharmacology and clinical use of mexiletine

Mexiletine is an antiarrhythmic agent with structural and electrophysiologic properties similar to those of lidocaine. Mexiletine decreases ventricular automaticity while shortening both action potential duration and effective refractory period. The drug may be administered orally or intravenously. Hepatic metabolism is the major route of elimination. The elimination half-life is approximately 10 hours, but longer in patients with acute myocardial infarction, chronic congestive heart failure or hepatic insufficiency. Mexiletine suppresses ventricular ectopy in the acute phase of myocardial infarction. The drug is effective for some patients in whom lidocaine has failed. It suppresses chronic ventricular ectopy and is well tolerated in approximately two-thirds of stable outpatients treated with this agent. In that population, mexiletine is comparable in efficacy to quinidine, procainamide and disopyramide. It is effective in 30-50% of patients with ventricular arrhythmias refractory to other antiarrhythmic drugs. In patients with refractory arrhythmias, the efficacy of mexiletine may be enhanced by combination with propranolol, quinidine or amiodarone. Adverse reactions limit use of mexiletine in approximately 20% of patients. Gastrointestinal and central nervous system side effects are the most common. Mexiletine does not depress myocardial function. Aggravation of arrhythmias is uncommonly observed. The usual intravenous dose of mexiletine is 150-250 mg over at least 10 minutes. Long-term oral dosages are usually 200-300 mg 3 or 4 times daily.

Usefulness and safety of cimetidine in patients receiving mexiletine for ventricular arrhythmia

Cimetidine, a commonly used H2 receptor antagonist, was found to adversely interact with many drugs metabolized by the liver, including class I antiarrhythmic agents, lidocaine and quinidine. Mexiletine is a new class I antiarrhythmic agent similar to lidocaine which when used orally may have significant gastric side effects. Since some patients with peptic ulcer disease or gastric hyperacidity on mexiletine may benefit from the addition of cimetidine, it was important to rule out any significant adverse interaction between the two drugs in such patients. Eleven patients currently receiving long-term oral mexiletine for the treatment of complex ventricular arrhythmia underwent a double-blind crossover trial where they were maintained on their usual dose of mexiletine, and cimetidine, 300 mg orally every 6 hours, or placebo were added for a 1-week period each. Peak and trough mexiletine blood levels were not significantly altered by cimetidine. Similarly, there was no significant change in the frequency and severity of ventricular arrhythmia when cimetidine was added to mexiletine. Cimetidine reduced gastric side effects of mexiletine in 50% of patients who had complained of such symptoms on mexiletine alone or on mexiletine and placebo. We conclude that cimetidine can effectively reduce gastric side effects of mexiletine in many patients without adversely affecting the plasma concentration or the efficacy of the drug.

Clinical pharmacokinetics of the newer antiarrhythmic agents

This article reviews clinical pharmacokinetic data on 8 new antiarrhythmic agents. Some of these drugs have been studied extensively while others are relatively new, with incomplete data due to limited evaluation. Amiodarone is a class III antiarrhythmic drug which is effective in treating many atrial and ventricular arrhythmias that are refractory to other drugs. Amiodarone accumulates extensively in tissues and its disposition characteristics are best described by models with 3 and 4 compartments. Its apparent volume of distribution is very large (1300 to 11,000L) and its elimination half-life very long (53 days). A delay of up to 28 days from of treatment to onset of antiarrhythmic effect may be observed, and the antiarrhythmic effect may persist for weeks to months following cessation of therapy. Clinically significant drug interactions have been observed with amiodarone and warfarin, digoxin, quinidine and procainamide. Encainide is a class Ic antiarrhythmic drug. Although it has a short elimination half-life (1 to 3h), 2 major metabolites with antiarrhythmic effects accumulate in the plasma of patients during long term therapy. Plasma concentrations of O-demethyl encainide appear to correlate with the antiarrhythmic effect. Flecainide, another class Ic antiarrhythmic agent, has an elimination half-life of 14 hours which makes it suitable for twice daily dosing. Flecainide elimination is prolonged in patients with low output heart failure. Significant drug interactions with digoxin and cimetidine have been reported. Lorcainide is also a class Ic antiarrhythmic drug, the bioavailability of which is nonlinear. Clearance of the drug is reduced during long term therapy. A major active metabolite, norlorcainide, accumulates in the plasma of patients during long term therapy and its concentration exceeds that of lorcainide by a factor of 2. The elimination half-lives of lorcainide (9h) and norlorcainide (28h) allow for once or twice daily dosing. Mexiletine, a class Ib antiarrhythmic drug, is structurally similar to lignocaine (lidocaine). A sustained release formulation provides effective plasma concentrations when administered twice daily. The apparent volume of distribution of mexiletine is 5.0 to 6.6 L/kg, and the elimination half-life varies from 6 to 12 hours in normal subjects and from 11 to 17 hours in cardiac patients. Mexilitine is extensively metabolised but the metabolites are not pharmacologically active. Renal elimination of mexiletine is pH dependent. Drugs which induce hepatic metabolism significantly alter the pharmacokinetics of mexiletine.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Pharmacology, electrophysiology, and pharmacokinetics of mexiletine

Mexiletine is a class I antiarrhythmic agent that is active after both oral and intravenous administration and similar in structure and activity to lidocaine. It decreases phase O maximal rate of depolarization (Vmax) by fast sodium channel blockade. The marked rate dependence of Vmax depression may explain mexiletine's lack of effect on normal conduction and its efficacy against ventricular tachyarrhythmias. Mexiletine significantly decreases the relative refractory period in His-Purkinje fibers without changing the sinus rate or atrioventricular and His-Purkinje conduction times. Action potential duration is usually shortened. Mexiletine may aggravate preexisting impairment of impulse generation and conduction. Uptake and distribution of mexiletine are rapid, systemic bioavailability is about 90%, and tissue distribution is extensive. Mexiletine is primarily metabolized in the liver; 10% to 15% is excreted unchanged in the urine. Elimination half-life is 9 to 11 hours after intravenous or oral administration. Microsomal enzyme induction shortens mexiletine's elimination half-life, whereas hepatic disease and acute myocardial infarction prolong it. Renal disease has little effect, although hemodialysis increases mexiletine clearance. Plasma concentrations from 0.75 to 2.0 mg/L are usually associated with a desirable therapeutic response.