This study focuses on people with Myotonic Dystrophy Type 1, a genetic condition that causes progressive muscle weakness and difficulty relaxing muscles after use. The study will test a new medication called DYNE-101, which is given through intravenous administration. Some participants will receive DYNE-101 while others will receive a placebo solution.

The purpose of this research is to evaluate how safe DYNE-101 is and how well patients tolerate it when given multiple doses. The study will also examine how the medication affects muscle tissue in people with Myotonic Dystrophy Type 1. The medication being tested is a specially designed antibody that targets specific proteins in the body.

During the study, participants will receive multiple doses of either DYNE-101 or placebo through an intravenous line. The study will involve various assessments of muscle strength and function, including tests of hand grip strength and walking ability. Participants will also undergo muscle tissue sampling to evaluate how the medication affects their muscle cells.