Toxic Dose of Dofetilide
There is no clearly defined toxic dose of dofetilide in humans, but a single case report documented significant toxicity at approximately 90 times the therapeutic dose (estimated ~5 mg total ingestion), while unintentional small overdoses (typically one extra capsule = 125-500 mcg) rarely produce clinically significant effects.
Evidence from Overdose Cases
The limited human overdose data comes primarily from poison center reports and case series:
The most severe documented case involved a 33-year-old man who ingested approximately 5 mg of dofetilide (roughly 10 times a maximum daily dose of 1000 mcg/day) in a suicide attempt 1. He developed a QTc of 570 msec and was successfully managed with activated charcoal, aggressive potassium and magnesium supplementation, and telemetry monitoring. Another case series reported a patient who took 90 times his usual dose and experienced an 8-beat run of non-sustained ventricular tachycardia, frequent multifocal PVCs, and ventricular bigeminy, but recovered with electrolyte supplementation 2.
Small unintentional overdoses (one extra capsule) in a series of 27 cases did not result in significant clinical effects and were often managed at home 2. However, this finding must be interpreted cautiously, as even therapeutic doses can cause torsades de pointes—the incidence ranges from 0.8-3.3% depending on the route and population 3.
Therapeutic Dosing Context
Understanding toxicity requires context of therapeutic dosing:
- Maximum therapeutic dose: 500 mcg twice daily (1000 mcg/day total) for patients with creatinine clearance >60 mL/min 4
- Dose-adjusted for renal function: 250 mcg BID for CrCl 40-60 mL/min, 125 mcg BID for CrCl 20-40 mL/min 5
- Contraindicated: CrCl <20 mL/min 4
Primary Toxicity: QT Prolongation and Torsades de Pointes
The main toxic effect is excessive QT prolongation leading to torsades de pointes, which can occur even at therapeutic doses. The risk is dose-dependent but also influenced by multiple patient factors:
High-Risk Features for Toxicity (Even at Therapeutic Doses):
- Female sex (3x higher risk than males) 6
- Baseline QTc >440 ms (>500 ms with ventricular conduction delay) 3
- Renal dysfunction (drug is 80% renally excreted) 7
- Hypokalemia or hypomagnesemia 4
- Concomitant QT-prolonging drugs 4
- Bradycardia 8
Critical QTc Thresholds:
- Discontinue if QTc >500 ms (>550 ms with ventricular conduction abnormalities) at any time after the second dose 4
- Reduce dose by 50% if QTc increases >15% from baseline 4
Management of Suspected Overdose
Based on the limited case reports and pharmacologic principles:
- Immediate cardiac monitoring with continuous telemetry
- Serial ECGs every 2-3 hours to monitor QTc progression
- Activated charcoal if presenting within 1-2 hours of ingestion 1
- Aggressive electrolyte repletion: Maintain potassium >4.5 mEq/L and magnesium >2.0 mg/dL 1
- Treat torsades de pointes per ACLS guidelines: IV magnesium sulfate 1-2 g over 15 minutes, temporary overdrive pacing if refractory 9
- Monitor for at least 72 hours given the drug's half-life of approximately 10 hours
Clinical Pitfalls
- Do not assume small overdoses are benign: Even a single extra capsule theoretically increases torsades risk, though the poison center data suggests this is uncommon in practice 2
- Women are at substantially higher risk: The 3-fold increased risk in females means extra vigilance is warranted 6
- Drug interactions matter: Concomitant use of cimetidine, verapamil, ketoconazole, trimethoprim, or hydrochlorothiazide is contraindicated as these increase dofetilide levels 4, 6
- Renal function changes rapidly: Acute kidney injury dramatically increases drug levels since 80% is renally excreted 7
The absence of a well-defined toxic dose reflects the drug's narrow therapeutic index and the fact that toxicity is heavily influenced by individual patient factors rather than dose alone. In clinical practice, any dose exceeding the maximum therapeutic dose (1000 mcg/day) or any dose in the presence of contraindications should be considered potentially toxic and warrant intensive monitoring.