Myotonia Trials in Disease

Study on Long-term Safety and Efficacy of Mexiletine for Children with Myotonic Disorders

This trial is designed to evaluate how safe and effective mexiletine is when used over an extended period in children with myotonic disorders. These disorders include nondystrophic myotonias and myotonic dystrophies, which cause muscle stiffness and difficulty relaxing muscles after they have been used.

Main inclusion criteria:

• Children who have completed a previous study called MEX-NM-301 and tolerated mexiletine well during that study
• Willingness and ability to participate, with written consent from a parent or legal guardian
• Continued fulfillment of the requirements from the previous MEX-NM-301 study
• Both male and female patients are eligible

Main exclusion criteria:

The trial specifically includes children who have already participated in and tolerated mexiletine in previous research. Those who did not complete the earlier study or experienced intolerance to the medication would not be eligible.

Focus and goal:

The study aims to gather comprehensive information about the long-term safety and effectiveness of mexiletine in reducing muscle stiffness in children. Participants take the medication orally and are monitored regularly for any side effects. The trial assesses changes in muscle stiffness every three months using simple scales, including a Visual Analogue Scale or Faces score. Every six months, handgrip myotonia is measured using a grip dynamometer, and clinical assessments for eyelid and flexor myotonia are conducted. The study also evaluates the overall quality of life of participants using the Paediatric Quality of Life score. Additional monitoring includes tracking vital signs and clinical laboratory values every three months. The study is estimated to conclude by June 2026.

Investigational drug:

The medication being tested is mexiletine, administered in capsule form at different doses: 62 mg, 83 mg, and 167 mg (marketed as Namuscla). Mexiletine works by blocking sodium channels in muscle cells, which helps reduce the excessive electrical activity that causes muscle stiffness. It is taken by mouth and is being studied specifically for its ability to treat symptoms of muscle stiffness in children with myotonic disorders.