Potassium Channel Blocker (Neuromuscular Junction Facilitator)

Amifampridine

Brand names: Firdapse, Zenas

Adult dose

Dose: Initially 15 mg/day in 3–4 divided doses; usual range 30–80 mg/day; max 80 mg/day

Route: Oral

Frequency: 3–4 times daily

Clinical pearls

Indicated for Lambert-Eaton myasthenic syndrome (LEMS) — rare paraneoplastic or autoimmune neuromuscular junction disorder
Mechanism: blocks voltage-gated potassium channels → prolongs nerve action potential → more acetylcholine released
NICE TA605 (2020): amifampridine recommended for symptomatic treatment of LEMS in adults
Perioral tingling is the expected pharmacodynamic effect and usually transient
3,4-diaminopyridine phosphate (3,4-DAP phosphate = Firdapse) is the licensed form; plain 3,4-DAP is unlicensed but used historically
Treat underlying malignancy (especially SCLC) if paraneoplastic origin

Contraindications

Epilepsy (risk of seizures)
Uncontrolled asthma
Concomitant use with drugs that significantly prolong QT interval

Side effects

Perioral and peripheral paraesthesia (very common — class effect, often dose-limiting)
Nausea
Abdominal pain
Seizures (rare — especially at high doses or with predisposing factors)
Elevated liver enzymes
Bronchospasm (rare)

Interactions

QT-prolonging drugs — avoid combination (additive QT risk)
Drugs that lower seizure threshold — caution
Anticholinesterases (pyridostigmine) — sometimes combined for Lambert-Eaton syndrome; monitor carefully

Monitoring

Seizure history and ECG before starting
Liver function tests periodically
Muscle strength and functional assessment (Quantitative Myasthenia Gravis score)
QTc interval (ECG)

Reference: BNF; NICE TA605 (Amifampridine for Lambert-Eaton myasthenic syndrome, 2020); Orphanet / European Neuromuscular Centre guidelines; https://bnf.nice.org.uk/drugs/amifampridine/. Verify against your local formulary and the latest BNF before prescribing.

Curated clinical cross-links plus same-class fallbacks.

Calculators

Pathways