• 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;

• Presence of clinical handgrip myotonia (delayed relaxation of grip of ≥ 3 seconds after maximum voluntary contraction) at screening using VHOT;

⁃ Be able to walk independently 10 meters (cane, walker, orthoses allowed);

⁃ DM1 patients only - Muscular impairment rating scale (MIRS) score of 2, 3 or 4.-