This is a study of investigational medicines ENTR-601-44 and ENTR-601-45 designed to evaluate the long-term safety and tolerability of study drugs in participants with Duchenne muscular dystrophy (DMD).

The investigational medicines are currently being investigated in multiple ascending dose parent studies. After participants complete their respective parent study, there is a need to understand the effects of long-term administration of ENTR-601-44 and ENTR-601-45.

Participants enrolling in this study will begin this long-term extension (LTE) study at the dose level they received upon completion of the parent study with possible dose escalation in the LTE study based on emerging safety and efficacy data from the parent studies.

Participants will:

• Receive study treatment in the form of multiple intravenous (IV) infusions (slow injections) into a vein over the course of several weeks
• Visit the clinic regularly for checkups and tests such as: blood and urine tests, physical examinations, questionnaires, and excersice 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 results 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.

Inclusion Criteria:

• Willing and able to provide consent (if at age of majority) or assent (if a minor)
• Participant completed clinical study ENTR-601-44-201 or ENTR-601-45-201
• Males who are sexually active with a female partner of childbearing potential must agree to use condoms during sexual intercourse

Exclusion Criteria:

• Any change from the applicable parent study eligibility criteria, including safety events during the parent study, that in the opinion of the investigator in consultation with the medical monitor and/or sponsor designee precludes safe use of study drug
• Participant has a condition or circumstance that in the view of the investigator places the subject at high risk of poor treatment compliance or for not completing the study

The datasets generated during and/or analyzed during the current study are not expected to be made available due to the data´s high commercial sensitivity.