Amifampridine Phosphate (Firdapse) Is Effective in a Confirmatory Phase 3 Clinical Trial in LEMS

OBJECTIVE: To assess tolerability and efficacy of amifampridine phosphate versus placebo for symptomatic treatment of Lambert–Eaton Myasthenic Syndrome (LEMS). METHODS: This phase 3 randomized, double-blind, placebo-controlled withdrawal trial in 26 adults with LEMS compared efficacy of amifampridine phosphate versus placebo over a 4-day period. The primary endpoints were quantitative myasthenia gravis score (QMG) and subject global impression, and the secondary endpoint was Clinical Global Impression–Improvement. The exploratory endpoints were 3TUG (timed up and go) test and QMG limb domain score. All participants had been receiving amifampridine phosphate (30–80 mg/d divided into 3 or 4 doses daily) in an expanded access protocol and had been titrated to the optimal dose and frequency for at least 1 week before randomization into the current study. After completion of assessments after 4 days of double-blind treatment, patients had the option to return to open-label amifampridine phosphate. The efficacy endpoints were mean changes from baseline in the various evaluation parameters. RESULTS: Amifampridine phosphate (n = 13) demonstrated significant benefit in QMG and subject global impression compared with placebo (n = 13) at 4 days. Other measures of efficacy, including Clinical Global Impression–Improvement, 3TUG, and QMG limb domain score were also improved. The most common “adverse events” in the placebo group were muscle weakness (n = 5) and fatigue (n = 4), as expected from withdrawal of amifampridine phosphate, whereas only back pain (n = 1), pain in extremity (n = 1), and headache (n = 1) were reported in amifampridine phosphate group. CONCLUSIONS: This phase 3 randomized, double-blind, placebo-controlled withdrawal trial in adults with LEMS provided class I evidence of efficacy of amifampridine phosphate as symptomatic treatment in LEMS.