This is a Phase 3, multi-center, open-label extension study in ambulant boys with DMD who have completed the 48-week treatment period of either viltolarsen or placebo in Study NS-065/NCNP-01-301.

This Phase 3 study is a multi-center, open-label extension study in ambulant boys with DMD who have completed the 48-week treatment period of either viltolarsen or placebo in Study NS-065/NCNP-01-301. Patients will receive viltolarsen administered IV at weekly doses of 80 mg/kg.

Study NS-065/NCNP-01-302 will be comprised of a 96-week treatment period.

Inclusion Criteria:

1. Patient has completed the NS-065/NCNP-01-301 study;
2. Patient's parent(s) or legal guardian(s) has (have) provided written informed consent and Health Insurance Portability and Accountability Act authorization, where applicable, prior to any study-related procedures; patients will be asked to give written or verbal assent according to local requirements;
3. Patient and parent(s)/guardian(s) are willing and able to comply with scheduled visits, investigational product (IP) administration plan, and study procedures.

Exclusion Criteria:

1. Patient had an adverse event in Study NS-065/NCNP-01-301 that, in the opinion of the investigator and/or the sponsor, precludes safe use of viltolarsen for the patient in this study;
2. Patient had a treatment which was made for the purpose of dystrophin or dystrophin-related protein induction after completion of Study NS-065/NCNP-01-301;
3. Patient took any other investigational drug(s) during or after completion of Study NS-065/NCNP-01-301;
4. Patient is judged by the investigator and/or the sponsor not to be appropriate to participate in the extension study for any reason.