Long-term Efficacy and Safety of Amifampridine Phosphate (Firdapse®) in Japanese Patients with Lambert-Eaton Myasthenic Syndrome (LMS-005 Study)

Objective To evaluate the efficacy and safety of amifampridine (3,4-diaminopyridine) phosphate in Japanese adults with Lambert-Eaton myasthenic syndrome (LEMS). Methods The LMS-005 study was an uncontrolled, single-blind (patient blinded), multicenter, one-year phase 3 clinical study. The administration of amifampridine phosphate was started at 15 mg/day, and the dose was increased every 3 to 4 days to determine the optimal dose for each patient. After 7 days of treatment with the optimal dose, efficacy was assessed by evaluating quantitative myasthenia gravis (QMG), subject global impression (SGI), and Clinical Global Impression-Improvement scale (CGI-I) scores. Patients Adult patients with LEMS were analyzed for safety [n=12, mean age±standard deviation (SD) of 61.1±14.6 years old] and efficacy (n=10, mean age±SD of 60.7±15.9 years old). Results In the efficacy population, the mean±SD [median (interquartile range)] QMG score was 13.2±3.1 [13.5 (11.0, 15.0)] at baseline and 8.0±2.7 [8.0 (6.0, 9.0)] at the end of the treatment period, with a mean±SD [median (interquartile range)] change of -5.2±2.8 [-5.5 (-7.0, -3.0)]. All patients showed a decrease in the QMG score from baseline and experienced improvement in their LEMS symptoms. The SGI/CGI-I scores also improved. Efficacy was maintained until the end of the study. Five patients in the safety population experienced adverse drug reactions, the most common of which was dysesthesia (n=2). Conclusion This study revealed the long-term efficacy and tolerability of amifampridine phosphate in Japanese adults with LEMS.