Children with rare inherited myotonic disorders in Europe may soon gain access to a treatment already used in adults. The European Medicines Agency’s (EMA’s) Committee for Medicinal Products for Human Use (CHMP) has issued a positive recommendation, supporting authorization changes for Namuscla (mexiletine, Lupin Europe GmbH) to include children aged 6-11 years weighing at least 20 kg and adolescents aged 12-17 years with nondystrophic myotonic disorders. 

The CHMP also endorsed two new hard-capsule strengths, 62 mg and 83 mg, in addition to the previously authorized 167-mg formulation. 

Nondystrophic myotonias are a subset of rare inherited neuromuscular disorders with a prevalence of 1 in 100,000 individuals. These conditions arise from mutations within ion channels in the sarcolemma membrane of skeletal muscles, causing both sodium and chloride channelopathies that modify membrane excitability. Myotonia, the primary symptom, appears as an inability to relax muscle contractions after movements such as shaking a person’s hand, blinking, walking, or climbing stairs.

For patients with nondystrophic myotonias, the myotonia typically starts in childhood and continues throughout life, substantially affecting daily activities. Patients describe symptoms differently as rigidity, cramps, pain, difficulty releasing a fist, or trouble with swallowing and eating. These varied presentations often lead to considerable diagnostic and treatment delays, resulting in reduced quality of life and potential disability.

Mechanism of Action and Clinical Evidence 

Namuscla exerts its antimyotonic effect by selectively stabilizing hyperexcitable skeletal muscle membranes through use-dependent blockade of voltage-gated sodium channels. This action reduces repetitive muscle firing and myotonia independently of the underlying channelopathy, improving muscle relaxation and associated functional outcomes.

Namuscla was initially designated as an orphan medicine.

The CHMP’s recommendation relied on findings from the MEX-NM-301 (EudraCT 2019‑003757‑28) study, evaluating steady-state pharmacokinetics, safety, and efficacy of mexiletine in pediatric patients aged 6 to younger than 18 years with myotonic disorders.

The 56-day study enrolled 12 patients who received weight-based, titrated oral mexiletine. All participants completed treatment. Reductions in muscle stiffness were observed across age groups, with adolescents showing larger mean improvements on the visual analog scale (-53.7 mm) compared with younger children (-21.7 mm). Measurable improvements in hand-grip myotonia were also seen. 

Safety Profile and Next Steps 

The pediatric study showed that mexiletine was generally well tolerated in younger patients. Cardiac safety was supported by the absence of clinically significant ECG changes.

Common side effects include gastrointestinal discomfort, insomnia, somnolence, headache, paresthesia, blurred vision, vertigo, tachycardia, hypotension, nausea, musculoskeletal pain, fatigue, chest discomfort, and malaise.

Serious adverse reactions can occur, including Stevens-Johnson syndrome, DRESS (drug reaction with eosinophilia and systemic symptoms), severe hypersensitivity, and cardiac arrhythmias such as atrioventricular block or ventricular fibrillation. 

Detailed recommendations for use of Namuscla will be described in the updated Summary of Product Characteristics, which will be published on the EMA website in all official European Union languages after the European Commission grants the marketing authorization change.