The interaction of mexiletine with other cardiovascular drugs

Evaluating the safety and efficacy of dextromethorphan/quinidine in the treatment of pseudobulbar affect

Pseudobulbar affect (PBA) is a common manifestation of brain pathology associated with many neurological diseases, including amyotrophic lateral sclerosis, Alzheimer's disease, stroke, multiple sclerosis, Parkinson's disease, and traumatic brain injury. PBA is defined by involuntary and uncontrollable expressed emotion that is exaggerated and inappropriate, and also incongruent with the underlying emotional state. Dextromethorphan/quinidine (DM/Q) is a combination product indicated for the treatment of PBA. The quinidine component of DM/Q inhibits the cytochrome P450 2D6-mediated metabolic conversion of dextromethorphan to its active metabolite dextrorphan, thereby increasing dextromethorphan systemic bioavailability and driving the pharmacology toward that of the parent drug and away from adverse effects of the dextrorphan metabolite. Three published efficacy and safety studies support the use of DM/Q in the treatment of PBA; significant effects were seen on the primary end point, the Center for Neurologic Study-Lability Scale, as well as secondary efficacy end points and quality of life. While concentration-effect relationships appear relatively weak for efficacy parameters, concentrations of DM/Q may have an impact on safety. Some special safety concerns exist with DM/Q, primarily because of the drug interaction and QT prolongation potential of the quinidine component. However, because concentrations of dextrorphan (which is responsible for many of the parent drug's side effects) and quinidine are lower than those observed in clinical practice with these drugs administered alone, some of the perceived safety issues may not be as relevant with this low dose combination product. However, since patients with PBA have a variety of other medical problems and are on numerous other medications, they may not tolerate DM/Q adverse effects, or may be at risk for drug interactions. Some caution is warranted when initiating DM/Q treatment, particularly in patients with underlying risk factors for torsade de pointes and in those receiving medications that may interact with DM/Q.

Mechanism of interaction between theophylline and mexiletine

The mechanism of an interaction between theophylline and mexiletine hydrochloride was investigated in 6 male inpatients coadministered both drugs and 16 inpatients (13 men, 3 women) administered theophylline only. Serum theophylline and mexiletine concentrations and urinary concentrations of theophylline and its metabolites were monitored. Theophylline clearance was 0.0278 +/- 0.0047 L/kg/h (mean +/- SD) in patients coadministered theophylline and mexiletine and 0.0441 +/- 0.0096 in patients administered theophylline only (p less than 0.05). The fractional urine contents of 1-methyluric acid and 3-methylxanthine were 18.7 +/- 2.5 and 12.6 +/- 2.1 percent in the former group and 26.5 +/- 6.0 and 17.1 +/- 2.0 percent in the latter group, respectively (p less than 0.05). The fractional urine content of 1,3-dimethyluric acid was 51.8 +/- 3.2 in the former and 44.7 +/- 4.1 percent in the latter, respectively (p less than 0.05). An inverse correlation was obtained between serum mexiletine concentrations and total fractional urine content of 1-methyluric acid and 3-methylxanthine (r = 0.704). These results suggest that the mechanism of an interaction between theophylline and mexiletine is an inhibition of demethylation of theophylline by mexiletine.

Survival following severe overdose with mexiletene, nifedipine, and nitroglycerine

Survival following attempted suicide in a 50-year-old man by ingestion of 12.4 g of mexiletine, 620 mg of nifedipine and 50 to 100 tablets of sublingual nitroglycerine 1:150 is reported. Initial presentation was that of mental obtundation, vomiting, tonic-clonic seizure, high-degree atrioventricular block, profound vasodilation and cardiovascular collapse. Treatment consisted of intravenous calcium gluconate and aggressive fluid management. Maintenance of cardiovascular stability required continuous infusion phenylephrine, dopamine and epinephrine. The patient made a full recovery and was medically discharged on the fourth hospital day. This case represents the largest overdose of mexiletine to date to end in patient survival.

Mexiletine. A review of its pharmacodynamic and pharmacokinetic properties, and therapeutic use in the treatment of arrhythmias

As a member of the class Ib antiarrhythmic drugs mexiletine's primary mechanism of action is blocking fast sodium channels, reducing the phase 0 maximal upstroke velocity of the action potential. It increases the ratio of effective refractory period to action potential duration, but has little effect on conductivity. Unlike quinidine it does not prolong QRS and QT (QTc) intervals. In the dosage range 600 to 900 mg daily mexiletine effectively suppresses premature ventricular contractions (PVCs) in 25% to 79% of patients, with or without underlying cardiac disease. In comparative studies the response rate was comparable to that with quinidine or disopyramide. However, the use of antiarrhythmic therapy in patients with asymptomatic arrhythmias is controversial. More importantly, mexiletine abolishes spontaneous or inducible ventricular tachycardia or fibrillation in the short term in 20% to 50% of patients with refractory arrhythmias. Arrhythmia suppression is maintained in 57% to over 80% of these early therapeutic successes in the long term, with mexiletine alone or in combination with another antiarrhythmic drug. As with other antiarrhythmic drugs, there is no substantial evidence that administration of mexiletine after acute myocardial infarction improves long term prognosis. Although the incidence of adverse effects associated with mexiletine is high, the majority are minor gastrointestinal or neurological effects which can be adequately controlled through dosage adjustment. Furthermore, mexiletine has minimal effects on haemodynamic variables, or on cardiac function in patients with or without pre-existing deterioration of left ventricular function, and it appears to have a low proarrhythmic potential. Thus, while the therapeutic efficacy of mexiletine for the prevention or suppression of symptomatic ventricular arrhythmias may be no greater than that of other antiarrhythmic drugs, and less than that of some (e.g. amiodarone), it is effective in a significant proportion of patients refractory to other treatments and can be administered without causing adverse haemodynamic effects to patients with complicating factors such as acute myocardial infarction or congestive heart failure.

Interaction between theophylline and mexiletine

Drugs influencing hepatic microsomal enzyme systems, such as mexiletine, may affect the elimination pattern of theophylline. The three patients reported here had a history of asthma and premature ventricular contractions, and were receiving theophylline therapy. A few days after starting the coadministration of mexiletine and theophylline, theophylline serum concentrations increased about twofold over concentrations during theophylline therapy. In one case, theophylline serum concentrations increased by 2.6 fold, and the patient developed nausea and anorexia. Mexiletine serum concentrations did not change. It seems that with mexiletine therapy, lower doses of theophylline may be required and careful monitoring of serum concentrations is 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.

Poisoning due to class 1B antiarrhythmic drugs. Lignocaine, mexiletine and tocainide

Since most of the toxicity associated with class 1B antiarrhythmic drugs is dose-related, this review examines adverse effects seen in both therapeutic practice and accidental or premeditated overdose. Toxicity is very common with these agents and can be life-threatening. A high percentage of patients must discontinue therapy because of adverse effects. Mexiletine and tocainide are structural analogues of lignocaine (lidocaine) and toxicity is similar with all 3 drugs. With gradual intoxication (the most common form) central nervous system effects such as lightheadedness, dizziness, drowsiness and confusion are seen first. Seizures and respiratory arrest can occur. Cardiovascular toxicity is manifested by progressive heart block, reduced cardiac contraction, hypotension and asystole. Both mexiletine and tocainide may have proarrhythmic effects. Gastrointestinal toxicity is also common. Shock, hypotension, cardiac failure and beta-blocker therapy reduce lignocaine clearance and enhance the risk of intoxication during routine therapy. Both lignocaine and mexiletine elimination is impaired in severe liver disease while tocainide clearance is reduced in renal failure. Management of toxicity is largely supportive and symptomatic. Lignocaine infusion must be discontinued and decontamination of the gut in the case of oral preparations is recommended. Serious intoxication requires intensive care unit admission. Haemodialysis or haemoperfusion may be helpful in serious lignocaine and tocainide poisoning. In institutions where extracorporeal circulatory assistance is available, massive lignocaine poisoning has been successfully treated with this intervention. In the therapeutic setting serious toxicity can be prevented by close clinical surveillance and appropriate dose reduction in patients with reduced drug clearance. Because of the large interindividual variation in lignocaine pharmacokinetic parameters, therapeutic drug monitoring is recommended if results can be reported quickly. Mexiletine and tocainide have stereoselective metabolism and assays do not distinguish the more active isomers. Therapeutic drug monitoring is less useful in this situation.

Comparative efficacy and safety of oral mexiletine and quinidine in benign or potentially lethal ventricular arrhythmias

The antiarrhythmic efficacy and safety of oral mexiletine hydrochloride and quinidine sulfate were compared at 29 clinical centers in a double-blind, parallel-group trial involving 491 patients with benign or potentially lethal ventricular arrhythmias. Responders were defined as those who had at least a 70% reduction in the frequency of ventricular premature complexes (VPCs) that persisted for 12 weeks, and who experienced no intolerable side effects that required discontinuation of therapy. Of the patients available for analysis, 71 of 232 (31%) in the mexiletine and 73 of 225 (32%) in the quinidine group met these criteria. The dose range used for mexiletine was 200 to 400 mg every 8 hours, and that for quinidine 200 to 400 mg every 6 hours. More than half of the patients in each group were successfully treated with the smallest dose (200 mg every 8 hours mexiletine vs 200 mg every 6 hours for quinidine). Quinidine significantly prolonged the QT interval, whereas mexiletine did not. Proarrhythmic reactions were recorded in 18 of 221 (9%) patients taking quinidine and 10 of 217 (5%) patients taking mexiletine. There was no difference in the incidence of adverse reactions between the 2 groups; in both, the most common side effects were related to the gastrointestinal and central nervous systems. Mexiletine thus represents an alternative to quinidine for the treatment of patients with ventricular arrhythmias.

New antiarrhythmic drugs: tocainide, mexiletine, flecainide, encainide, and amiodarone

Tocainide, mexiletine, flecainide, encainide, and amiodarone are antiarrhythmic agents that have recently been approved by the Food and Drug Administration for general use in the treatment of ventricular arrhythmias. All five agents are effective in the treatment of patients with ventricular arrhythmias, whereas encainide, flecainide, and amiodarone are also useful in patients with supraventricular arrhythmias and the Wolff-Parkinson-White syndrome (although not yet approved for these indications). Tocainide and mexiletine are similar to lidocaine and are as effective as quinidine in patients with ventricular arrhythmias. Encainide and flecainide are superior to quinidine for the control of ventricular ectopic beats and as effective as quinidine for patients with ventricular tachycardia. Amiodarone is the most effective agent available for treating patients with ventricular tachycardia, but it is also the most toxic antiarrhythmic agent and should be used only when other antiarrhythmic drugs have not been effective or tolerated.

Pharmacokinetics and pharmacodynamics of antiarrhythmic agents in patients with congestive heart failure

Changes in the pharmacokinetics of antiarrhythmic agents may be anticipated in patients with congestive heart failure (CHF), although the magnitude or direction of change is not always predictable. Factors complicating antiarrhythmic therapy in patients with CHF include both physiologic changes resulting from the disease state and unwanted effects of drug therapy for CHF. The volume of distribution is often significantly decreased (by as much as 50%) and loading doses should be reduced proportionately. Decreased blood flow to the liver and kidneys and decreased hepatic drug-metabolizing activity serve to diminish drug clearance. In some cases, simultaneous decreases in volume of distribution and clearance may result in little, if any, change in elimination half-life, despite higher plasma concentrations. Conversely, the elimination half-life of antiarrhythmic agents may be doubled in patients with CHF, necessitating a reduction in dosage. In the latter case, the time needed to reach steady state is lengthened, so that premature escalation of dosage may lead to excessive drug accumulation. In terms of their pharmacodynamics, most antiarrhythmic agents have a degree of negative inotropic effect at some concentration, and patients with reduced myocardial reserve are especially vulnerable to these effects. Some of the newer agents (such as tocainide, mexiletine, and encainide) appear to cause only minimal myocardial depression. Potential complications during therapy with all antiarrhythmic agents that are of special concern in patients with CHF include diuretic-induced hypokalemia, proarrhythmia, and possible interactions with cardiac glycosides and other drugs. Therapy for patients with CHF should be initiated with low doses of the agent selected, and the dosage carefully titrated while the patient is monitored, to confirm both efficacy and the absence of adverse effects. During subsequent outpatient therapy the patient should be carefully observed for sign of unexpected reactions, toxicity, or electrolyte imbalance.

Ventricular arrhythmia: management strategy

The management of ventricular arrhythmia continues to be one of the most difficult therapeutic problems in medicine today. Both invasive and noninvasive techniques have demonstrated success in management of patients at high risk for sudden cardiac death. High-risk subgroups include patients who have experienced sudden cardiac death and have been resuscitated successfully, patients with high-grade ventricular ectopy associated with left ventricular dysfunction, and patients who have had recent myocardial infarction. Traditional and experimental antiarrhythmic agents are available to the clinician, and in some patients combination therapy may prove more useful than application of a single agent alone. In individuals in whom pharmacologic intervention fails, map-guided surgical excision may be beneficial. The application of the automatic implantable defibrillator appears to have promise in truly refractory situations.

The role of beta blocking agents as adjunct therapy to membrane stabilizing drugs in malignant ventricular arrhythmia

Antiarrhythmic drugs are often either partially or totally ineffective for the suppression of ventricular arrhythmias in a given patient. Drug combinations afford an additional therapeutic option. We report the role of beta-blocking agents as adjunct therapy to membrane stabilizing drugs in the management of patients with malignant ventricular arrhythmias. The study group included 54 patients who were evaluated by 24-hour ambulatory monitoring and symptom-limited exercise testing. Patients underwent control studies without antiarrhythmic drugs, were evaluated on membrane stabilizing drugs and beta blocking agents separately, and were then tested on combination therapy. The combination of a beta-blocking agent and a membrane stabilizing drug abolished ventricular tachycardia and couplets in 83% and 86% of exercise tests in patients with this arrhythmia present during therapy with membrane drugs alone (p less than 0.01). The addition of a beta blocker to a membrane drug, as evaluated by ambulatory monitoring, resulted in an abolition of ventricular tachycardia and couplets in 43% and 20% of studies (p less than 0.05). Ventricular premature beat frequency was reduced by more than 50% in 65% of exercise tests and in 52% of monitoring studies (p less than 0.05). In this population, beta-blocking agents failed to reduce ventricular arrhythmias when used alone. Thus the addition of a beta blocker to a membrane stabilizing drug significantly enhances the suppression of ventricular arrhythmia, especially when assessed by exercise testing. This results from synergistic drug effects of the combination rather than from the effect of the individual drugs.

Relationship between ease of inducibility of arrhythmia with electrophysiologic testing and response to antiarrhythmic therapy

Electrophysiologic studies are an important tool for guiding the selection of an effective antiarrhythmic drug. The purpose of this study was to determine if there was a relationship between the number of extrastimuli necessary to induce arrhythmia and the response to antiarrhythmic drugs. A group of 56 patients with sustained ventricular tachycardia or ventricular fibrillation who were inducible underwent 235 single-drug studies (4.2 per patient). During the control study, one extrastimulus provoked an end point in 12 patients (group 1) and in only two patients (17%) was at least one drug effective. Of the 18 patients requiring two extrastimuli for induction during baseline (group 2), at least one drug was effective in 11 patients (61%). At least one drug was effective in 20 of 26 patients (77%) who required three extrastimuli (group 3). There were no significant differences among the three groups with respect to presenting arrhythmia, presence of coronary artery disease, or left ventricular ejection fraction. When single drugs or a combination of drugs were used, 58% of group 1, 72% of group 2, and 85% of group 3 were rendered noninducible. During a follow-up of 28 to 32 months, yearly recurrence of arrhythmia was 9.4%, 4.6%, and 1.7%, for the three groups, respectively.