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 * Presence of myotonia * Have sufficient muscle mass in bilateral tibialis anterior (TA) muscles that a needle biopsy can safely be performed * Body Mass Index (BMI) of \< 35.0 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) \