The primary purpose of the study is to evaluate the safety and tolerability of single intravenous (IV) doses of PGN-EDODM1 administered to participants with Myotonic Dystrophy Type 1 (DM1).

The study consists of 2 periods: A Screening Period (up to 30 days) and a Treatment and Observation Period (16 weeks).

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

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
• Medications specific for the treatment of myotonia within 2 weeks prior to screening
• Percent predicted forced vital capacity (FVC) <40%

Note: Other inclusion and exclusion criteria may apply.