Mexiletine: Clinical Overview
Mexiletine is a Class IB antiarrhythmic sodium channel blocker, structurally similar to lidocaine, primarily indicated as second- or third-line therapy for refractory ventricular arrhythmias in combination with amiodarone, and as genotype-specific first-line treatment for Long QT Syndrome Type 3 (LQT3). 1, 2
Primary Indications
Cardiac Arrhythmias
Refractory Ventricular Tachycardia/Fibrillation (VT/VF) with Structural Heart Disease:
- Use mexiletine only in combination with amiodarone (>300 mg daily) after failure of first-line therapies (sotalol, amiodarone alone, or catheter ablation) 1
- Mexiletine has limited efficacy as monotherapy and should not be used alone 1
- Critical warning: A pre-ICD era randomized trial showed increased mortality when mexiletine was used in patients with heart failure 1
- Use with extreme caution in patients with structural heart disease and heart failure 1
Long QT Syndrome Type 3 (LQT3):
- Mexiletine is recommended for patients with LQT3 and torsades de pointes 3
- First-line genotype-specific treatment that shortens QTc interval 2, 4
- Can be used to reduce recurrent arrhythmias and ICD shocks in LQT3 patients 2
- Also effective with ranolazine or flecainide in this population 2
Non-Cardiac Indications
Myotonic Disorders:
- Effective for symptomatic treatment of myotonia in non-dystrophic myotonia 5
- Significantly improves stiffness scores (SMD = -1.19) and reduces hand grip myotonia by 1.36 seconds compared to placebo 5
- Can be used off-label for myotonic dystrophy, but requires enhanced cardiac monitoring due to conduction system disease risk 6
Neuropathic Pain (Limited Role):
- Second- or third-line treatment for painful diabetic neuropathy at doses of 225-675 mg/day 7, 8
- Requires regular ECG monitoring, limiting long-term use 7
- Provides only modest analgesic effect 7
- Used for erythromelalgia pain, particularly in SCN9A-mediated cases 9
Dosing Regimen
Standard Dosing
- Oral: 150-300 mg every 8-12 hours 10, 11
- Typical range: 600-900 mg/day divided into 2-3 doses 12
- Maximum: Up to 1200 mg/day in some cases 13
- Titration: Start at lower doses and increase based on response and tolerability
Special Populations
- Hepatic impairment: Dosage reduction required; mexiletine clearance is impaired 14
- Renal impairment: No dosage adjustment needed 14
- CYP2D6 poor metabolizers: May require lower doses due to prolonged half-life (10-32 hours vs 2-10 hours in extensive metabolizers) 14
Intravenous Dosing (Limited Use)
- Bolus: 1-2 mg/kg or fixed dose 50-100 mg 9
- Infusion: 1 mg/kg per hour 9
- Note: IV lidocaine infusions are not currently recommended due to unclear safety profile and neurologic effects 9
Contraindications
Absolute Contraindications 13
- Cardiogenic shock
- Pre-existing second- or third-degree AV block (without pacemaker)
Relative Contraindications/Cautions
- Structural heart disease with heart failure (increased mortality risk) 1
- Severe hepatic impairment 14
- Concurrent use with CYP2D6 inhibitors in extensive metabolizers 14
Precautions and Monitoring
Cardiac Monitoring
- Baseline ECG required before initiation 7, 6
- Regular ECG monitoring throughout treatment 7
- Enhanced monitoring in myotonic dystrophy patients due to conduction system disease 6
- Monitor for QRS prolongation (though mexiletine typically does not prolong QRS or QT intervals) 12
Drug Interactions
- CYP2D6 inhibitors (e.g., quinidine): Increase mexiletine levels in extensive metabolizers 14
- Mexiletine inhibits: CYP2D6 (affects metoprolol) and CYP1A2 (affects theophylline) 14
- Phenytoin and rifampin: Induce mexiletine metabolism, reducing levels 14
Clinical Pitfalls
- Never use as monotherapy for ventricular arrhythmias—always combine with amiodarone 1
- Avoid in heart failure patients with structural heart disease unless absolutely necessary 1
- Do not use for asymptomatic arrhythmias—no evidence of improved long-term prognosis 12
- Combination with high-dose amiodarone (>300 mg/day) is inferior to catheter ablation for refractory VT/VF 1
Adverse Effects
Common (Dose-Related and Reversible) 13
Gastrointestinal (Most Common):
- Nausea/vomiting/heartburn: 39-41% 13
- Diarrhea: 5.2% 13
- Constipation: 4% 13
- Management: Take with food or antacid, reduce dose 13
- In myotonic patients: 3.7-fold increased risk of GI symptoms vs placebo 5
Central Nervous System:
- Dizziness/lightheadedness: 10.5-26.4% 13
- Tremor: 12.6-13.2% 13
- Coordination difficulties: 9.7-10.2% 13
- Sleep disturbances: 7.1-7.5% 13
- Nervousness: 5-11.3% 13
Cardiovascular:
- Palpitations: 4.3-7.5% 13
- Chest pain: 2.6-7.5% 13
- Minimal hemodynamic effects even in patients with left ventricular dysfunction 12
- Low proarrhythmic potential compared to other Class I agents 12
Serious but Rare
- Increased ventricular arrhythmias/PVCs: 1-2.7% 13
- Syncope, heart failure, AV block: <1% 13
- No serious cardiac arrhythmias reported in diabetic neuropathy trials 14
Discontinuation Rate
- 40% discontinuation in controlled trials due to adverse effects 13
- Most discontinuations due to GI or CNS effects, manageable with dose reduction 13
Pharmacokinetics 10, 14, 12
- Half-life: 10-14 hours (extensive metabolizers); 10-32 hours (poor metabolizers)
- Metabolism: Hepatic (CYP2D6, CYP1A2)
- Excretion: Urine
- Bioavailability: Well absorbed orally
- Steady state: Achieved in 2-3 days with regular dosing
Clinical Efficacy
Ventricular Arrhythmias
- Suppresses PVCs in 25-79% of patients at 600-900 mg/day 12
- Abolishes spontaneous/inducible VT/VF in 20-50% of refractory cases short-term 12
- Maintains arrhythmia suppression in 57-80% of responders long-term 12
- Comparable efficacy to quinidine and disopyramide 12
Myotonia
- Significant improvement in stiffness and hand grip myotonia 5
- Improves SF-36 Physical and Mental Component Scores in non-dystrophic myotonia 5