Inclusion Criteria:
- Diagnosis of DM1 with trinucleotide repeat size >100.
- Age of onset of DM1 muscle symptoms ≥12 years.
- Clinically apparent myotonia equivalent to hand opening time of at least 2 seconds in the opinion of the Investigator.
- Hand grip strength and ankle dorsiflexion strength.
- Able to complete 10-MWRT, stair ascend/descend (MAD cohorts only), and 5×STS at screening without the use of assistive devices such as canes, walkers, or orthoses.
Exclusion Criteria:
- History of major surgical procedure within 12 weeks prior to the start of investigative product administration or an expectation of a major surgical procedure (eg, implantation of cardiac defibrillator) during the study.
- History of anaphylaxis.
- Medical condition other than DM1 that would significantly impact ambulation or participation in functional assessments.
- Treatment with medications that can improve myotonia within a period of 5 half-lives of the medication prior to performing screening assessments.
- Electrocardiogram (ECG) with the corrected QT interval by Fridericia's Formula (QTcF) ≥450 milliseconds (ms) in men and QTcF ≥460 ms in women, PR ≥240 ms, left bundle-branch block, or a conduction defect, which is clinically significant in the opinion of the Investigator.
- Percent predicted forced vital capacity (FVC) <50%.
- History of tibialis anterior biopsy within 3 months of Day 1 or planning to undergo tibialis anterior biopsies during study period for reasons unrelated to the study.
- Participant has a history of suicide attempt, suicidal behavior, or has any suicidal ideation within 6 months prior to Screening that meets criteria at a level of 4 or 5 of the Columbia Suicide Severity Rating Scale (C-SSRS) or who, in the opinion of the Investigator, is at significant risk to commit suicide.
- Use of glucagon-like peptide 1 (GLP-1) agonist medications including semaglutide, dulaglutide, liraglutide, exenatide, or tirzepatide within a period of 5 half-lives of the medication prior to performing screening assessments.
- Significant weight loss during study participation may impact weight-based dosing, performance on muscle function assessments, and pharmacodynamic (PD) biomarkers.
Note: Other inclusion and exclusion criteria may apply.