A Clinical Study of PGN-EDODM1 in People With Myotonic Dystrophy Type 1

The purpose of this study is to learn about the effects of an investigational medicine, PGN-EDODM1, to see how safe and tolerable multiple administrations of PGN-EDODM1 are for people with myotonic dystrophy type 1 (DM1) compared to placebo.

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.