**Study Purpose:** This study is testing a new medicine, PGN-EDODM1, for people with Myotonic Dystrophy Type 1 (*DM1*), which is a muscle disease caused by a problem in the *DMPK* gene. The study checks if the medicine is safe and how well people can handle it when given through a vein (*IV*).

**Study Phases:** The study has two parts: a Screening Period that can last up to 30 days, where doctors check if you can join the study, and a Treatment and Observation Period that lasts 16 weeks, where you get the medicine and doctors see how you respond.

**Who Can Join:** You can join if you have DM1 with at least 100 repeats in the *DMPK* gene, a good muscle strength score in certain muscles, and have myotonia, which is a muscle stiffness condition. You should not join if you have congenital DM1, other serious health problems, abnormal lab tests, or are on certain medications recently.

• The study lasts for about 16 weeks after screening.
• Participants receive a single dose of the treatment through IV.
• Safety and tolerability are the main focus of the study.