An Open-Label, Randomized, Cross-Over Study to Investigate the Efficacy and Safety of Mexiletine PR Compared to Mexiletine IR

This is a multicenter, open-label, randomized, cross-over study intended to evaluate the efficacy and the safety of mexiletine PR (QD) vs mexiletine IR (TID) in patients with non-dystrophic myotonias including myotonia congenita (MC), paramyotonia congenita (PC) and sodium channel myotonia (SCM). The study will consist of a 4-week screening period followed by two 12-week treatment periods. Eligible patients will be randomized to receive mexiletine PR or mexiletine IR for 12 weeks. After a wash out period of at least 7 days the patients will receive the opposite treatment for 12 weeks.

A total of 24 patients are planned to be enrolled (with a target enrollment of 12 naïve to previous mexiletine treatment and 12 previously treated with mexiletine).

Safety assessments include patient- and physician-reported adverse event reporting, electrocardiogram (ECG), standard clinical laboratory evaluations, physical examinations, and vital signs. Efficacy assessments include patient-reported outcomes (PROs) and functional capacity outcome measures.