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

The primary purpose of the study is to evaluate the safety and tolerability of multiple intravenous (IV) doses of DYNE-101 administered to participants with Myotonic Dystrophy Type 1 (DM1). The study consists of 4 periods: A Screening Period (up to 8 weeks), a Placebo-Controlled Period (24 weeks), a Treatment Period (24 weeks) and a Long-Term Extension (LTE) Period (208 weeks) in both multiple-ascending dose (MAD) and dose expansion cohorts.

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) \