Safety, Tolerability, Pharmacodynamic, Efficacy, and Pharmacokinetic Study of DYNE-101 in Participants With Myotonic Dystrophy Type 1

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 secondsin 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 atscreening 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 ofinvestigative 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 orparticipation in functional assessments.

• Treatment with medications that can improve myotonia within a period of 5 half-livesof 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, leftbundle-branch block, or a conduction defect, which is clinically significant in theopinion of the Investigator.

• Percent predicted forced vital capacity (FVC) <50%.

• History of tibialis anterior biopsy within 3 months of Day 1 or planning to undergotibialis anterior biopsies during study period for reasons unrelated to the study.

• Participant has a history of suicide attempt, suicidal behavior, or has any suicidalideation within 6 months prior to Screening that meets criteria at a level of 4 or 5of the Columbia Suicide Severity Rating Scale (C-SSRS) or who, in the opinion of theInvestigator, 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-livesof 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.