An Open-Label Extension Study to Evaluate the Long-Term Safety and Efficacy of Once Daily Mexiletine PR in Patients With Myotonic Dystrophy Type 1 and Type 2 Who Have Completed MEX-DM-302 Study.

Enrolling by invitation · Phase 3 · Has a placebo group

Conditions studied: Myotonic Dystrophy

In brief

This is an open-label extension study intended to evaluate the long-term safety and efficacy of mexiletine PR in patients with myotonic dystrophy type 1 and type 2 (DM1 and DM2) who have completed the parent study MEX-DM-302.

Key facts

Study ID

NCT06549400

Run by

Lupin Ltd.

People needed

176

Starts

2025-09-12

Expected to finish

2028-07-06

Last updated by the study team

2026-01-26

Who can join

Age: 16 and older. Sex: any. Healthy volunteers: not accepted.

You may qualify if…

• DM1 or DM2 diagnosis confirmed genetically;
• Ability to comprehend and willingness to sign an informed consent (ICF) or ICF of the parent(s)/legal guardian and written assent from the patient (if patient < 18 years of age);
• Ability to understand the study requirements including intention to stay in the study until the end-of-study visit at 26 weeks of treatment;
• Male or non-pregnant female ≥16 years of age;
• Body Mass Index (BMI) of 18.5 kg/m2 to 30 kg/m2, and weight ≥45 kg;
• Female patients of childbearing potential must be using a highly effective form of birth control for the duration of the study and for at least 7 days after last dose of study drug;
• No significant cardiac abnormalities as determined by a cardiologist's assessment;
• Have sufficient finger flexor strength to grasp the handle of the dynamometer used to measure myotonia;
• DM1 patients only - Muscular impairment rating scale (MIRS) score of 2, 3 or 4.

You may not qualify if…

• Are pregnant or lactating;
• Have any one of the following medical conditions: uncontrolled diabetes mellitus, cancer other than skin cancer less than five years previously (e.g., basal-cell carcinoma (BCC) and squamous-cell carcinoma (SCC) of skin allowed), multiple sclerosis, seizure disorders, or other serious medical illness;
• Severe renal impairment (glomerular filtration rate (GFR) < 30 mL/min);
• Medical conditions which could interfere with muscle function such as infections, trauma, fractures, or planned surgery;
• Medical conditions that could affect hand functioning including but not limited to rheumatoid arthritis, Dupuytren's contracture, hand deformity, etc.;
• Severe arthritis or medical condition (other thanDM1/DM2) that would significantly impact ambulation;
• High incidence of falls or fall-associated fractures (>5 falls during the past 12 months);
• Preexisting elevated liver function tests > 3 times the upper limit of normal (ULN) on Day 1 (alanine transaminase (ALT)/aspartate transaminase (AST), gamma-glutamyl transferase (GGT)) and/or any abnormal chemistry, hematology or urine lab considered clinically significant by investigator;
• Serum potassium values < 3.5 mmol/L or > 5.0 mmol/L or serum magnesium values < 1.7 mg/dL. Electrolytic imbalance such as hypokalaemia, hyperkalaemia or hypomagnesaemia may increase the proarrhythmic effects of mexiletine. Electrolyte imbalances need to be corrected before administering mexiletine and will be monitored throughout treatment.
• Intake of any anti-myotonic treatment within 4 weeks prior to baseline (Day 1) or 5 half-lives, whichever is longer such as metformin, such as propafenone, flecainide, lamotrigine, carbamazepine or any other channel-blocker/ anticonvulsive drugs;
• Use of any concomitant medications that could increase the cardiac risk;
• Known allergy to mexiletine or any local anesthetics;
• Participation in another interventional clinical study during the last 3 months or 5 half-lives of the investigational medicinal product, whichever is longer (with the exception of participation in the previous MEX-DM-302 study);
• Wheelchair-bound or bed-ridden;
• Any cardiac safety associated condition including any of the following criteria detected by cardiac evaluations including 24-hr Holter monitor, echocardiogram and clinical evaluations:
• PR interval ≥240 ms or QRS duration ≥120 ms on resting ECG
• Personal history of 3rd degree or 2nd degree type 2 atrioventricular block or sinus node dysfunction with pauses ≥3 seconds, complete bundle branch block, bifascicular and trifascicular block or any heart block susceptible to evolve to complete heart block
• Personal history of sustained atrial fibrillation, flutter or tachycardia (duration >30 seconds)
• Personal history of non-sustained (ventricular triplets or more) or sustained ventricular tachycardia
• Myocardial infarction (acute or past) or coronary artery stenosis >50%, presence of abnormal Q waves
• New York Heart Association (NYHA) Class II to IV heart failure
• Left ventricular systolic dysfunction with ejection fraction <50%
• Sinus node dysfunction (including ECG sinus rate <50 beats per minute (BPM))
• Co-administration of antiarrhythmics inducing torsades de pointes (class Ia: quinidine, procainamide, disopyramide, ajmaline; class Ic: encainide, flecainide, propafenone, moricizine; class III: amiodarone, sotalol, ibutilide, dofetilide, dronedarone, vernakalant)
• Co-administration of other classes of antiarrhythmics (class Ib: lidocaine, phenytoin, tocainide; class II: propranolol, esmolol, timolol, metoprolol, atenolol, carvedilol, bisoprolol, nebivolol; class IV: verapamil, diltiazem)

Where it is running

• Laboratory for Muscle Diseases and Neuropathies — Leuven, Belgium
• Aarhus University Hospital — Aarhus, Denmark
• Ludug-Maximilians University — München, Germany
• Azienda Ospedaliera Universitaria Policlinico Tor Vergata — Rome, Italy
• University Hospital of Madrid — Madrid, Spain
• University College Hospital — London, United Kingdom

Full record on ClinicalTrials.gov

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