This is a study of the investigational medicine ENTR-601-44 in participants who have Duchenne muscular dystrophy (DMD), a rare genetic condition. The researchers want to: Test how safe ENTR-601-44 is, learn about any side effects, and look at the potential positive effects of ENTR-601-44, compared to placebo. Placebo looks like the investigational medicine but does not contain any active ingredient. In this summary ENTR-601-44 and placebo are both called study treatments. The study has 2 parts: Part A: to evaluate if ENTR-601-44 is safe and to determine the best dose of ENTR-601-44 for Part B. Part B: to further evaluate the effect and safety of ENTR-601-44 at the dose determined in Part A. Participants will: * Receive study treatment in the form of multiple intravenous (IV) infusions (slow injection) into a vein over the course of several weeks in Part A and in Part B * Visit the clinic regularly for checkups and tests such as: blood and urine tests, physical examinations, questionnaires, and exercise tests. Participants will have a muscle biopsy at the beginning of their participation and after their last dose to allow researchers to compare whether there have been changes in the muscle as a result of the study drug. Participants are allowed to continue receiving their standard of care therapy for DMD during the study, as long as their health remains stable.