Inclusion Criteria:

• Confirmed diagnosis of DM1, as defined as having a repeat sequence in the DMPK gene with at least 100 CTG repeats
• Medical Research Council (MRC) score of ≥ Grade 4- in bilateral tibialis anterior (TA) muscles (the ability to move through full range of motion and hold against at least moderate pressure from the examiner)
• Presence of myotonia
• Body Mass Index (BMI) of < 32.0 kg/m^2

Exclusion Criteria:

• Congenital DM1
• Known history or presence of any clinically significant conditions that may interfere with study safety assessments
• Abnormal laboratory tests at screening considered clinically significant by the Investigator
• Medications specific for the treatment of myotonia within 2 weeks prior to screening
• Percent predicted forced vital capacity (FVC) <40%
• Use of an investigational drug, device, or product within 30 days of 5 half-lives of the study drug (whichever is longer) prior to Screening

Note: Other inclusion and exclusion criteria may apply.