An Open-Label Extension Study to Evaluate the Long-term Safety and Efficacy of BMN 351 in Participants With Duchenne Muscular Dystrophy

An Open-Label Extension Study to Evaluate the Long-term Safety and Efficacy of Weekly Intravenous Infusions of BMN 351 in Participants With Duchenne Muscular Dystrophy Amenable to Exon 51 Skipping

This open-label extension study aims to evaluate the long-term safety and tolerability of weekly BMN 351 infusions, as well as to assess the effect of BMN 351 on physical function, in participants with DMD who participated in the 351-201 study.

Study Overview

• Status: Enrolling by invitation
• Conditions: Duchenne Muscular Dystrophy (DMD)
• Intervention / Treatment: Drug: BMN 351

Detailed Description

This Phase 2, multi-center, open-label extension study is designed to assess the long-term safety, tolerability, and functional efficacy of weekly intravenous doses of BMN 351 administered to participants with Duchenne muscular dystrophy (DMD) amenable to exon 51 skipping in the Phase 1/2 study, 351-201. Up to 18 participants ages 4 through 23 at baseline will enroll in the trial after completing 351-201.

The first visit for this study is the same as the final visit of 351-201. To be eligible for this study, potential participants must satisfy the eligibility criteria described in the protocol.

• Study Type: Interventional
• Enrollment (Estimated): 18
• Phase: Phase 2

Contacts and Locations

Study Locations

• Italy
  • Milan, Italy
    • Fondazione Serena ETS - Centro Clinico NeMO Milano
  • Rome, Italy
    • UOC Fase I - Fondazione Policlinico Universitario A. Gemelli IRCCS - Universita Cattolica del Sacro Cuore
• Netherlands
  • Leiden, Netherlands, 2333 ZA
    • Leids Universitair Medisch Centrum
• Spain
  • Barcelona, Spain, 08950
    • Hospital Sant Joan de Déu
  • Seville, Spain, 41013
    • Hospital Viamed Santa Angela De la Cruz
• Turkey (Türkiye)
  • Turkey
    • Istanbul, Turkey, Turkey (Türkiye)
      • Yeditepe University Kosuyolu Hospital
• United Kingdom
  • London, United Kingdom, WC1N 3JH
    • Great Ormond Street Hospital NHS Foundation Trust

Participation Criteria

Eligibility Criteria

• Ages Eligible for Study: Child; Adult; Older Adult
• Accepts Healthy Volunteers: No

Description

Inclusion Criteria:

• Participants must have completed 351-201 without permanent discontinuation of the investigational medicinal product (IMP) or withdrawal from the study

• Currently receiving treatment with oral corticosteroids, on a stable dose regimen during 351-201, and must remain on a consistent dose regimen throughout 351-202 or 351-203 except for modifications to accommodate changes in weight

  • Transition to the equivalent dose of vamorolone is permitted in 351-202 where approved in participating countries.
• Willing and able to adhere to the study visit schedule and other protocol requirements

• Willing to use contraception (sexually mature males) throughout the study and for 90 days after the final dose, if sexually active

  • Contraceptive use by males should be consistent with local regulations regarding the methods of contraception for those participating in clinical studies.
• Willing and able to provide written assent (if required by local regulations or the IRB/IEC) after the nature of the study has been explained and prior to performance of any research-related procedure
• Willing and able to provide written, signed informed consent as parent or guardian after the nature of the study has been explained and prior to performance of any research-related procedure

Exclusion Criteria:

• Have known coagulation disorder

• Are taking any prohibited medications

  • any approved exon skipping therapy within 12 weeks prior to baseline or with any gene therapy for the treatment of DMD at any time
  • anti-coagulants, anti-thrombotics, or anti-platelet agents
  • immunosuppressants

Study Plan

How is the study designed?

Design Details

• Primary Purpose: Treatment
• Allocation: N/A
• Interventional Model: Single Group Assignment
• Masking: None (Open Label)

Number of Arms

1

Arms and Interventions

Participant Group / Arm

Intervention / Treatment

Experimental: BMN 351; Participants from 351-201 enrolling in 351-202 will initially receive BMN 351 at the dose level at which they completed 351-201. Once all 351-201 participants have completed their Week 25 visit in 351-202, available safety and PD data will be analyzed and a single optimal dose level will be selected for 351-202. All participants from 351-201 will transition to that dose level at their next applicable visit.

Drug: BMN 351; Anti-sense Oligonucleotide BMN 351 will be administered intravenously

What is the study measuring?

Primary Outcome Measures

Outcome Measure	Measure Description	Time Frame
To assess the long-term safety and tolerability of BMN 351 in participants with DMD	The safety and tolerability of BMN 351 will be assessed based on the incidence of adverse and serious adverse events.	Through study completion, at least 1 year

Secondary Outcome Measures

Outcome Measure	Measure Description	Time Frame
To evaluate the effect of BMN 351 on physical function	North Star Ambulatory Assessment (NSAA) will be assessed at the specific visits and compared to individual baseline and external contraols	Change from baseline and subsequent 24-week incremental visits
To evaluate the effect of BMN 351 on physical function	Timed 4 stair climb (4SC) will be assessed at the specified visits and compared to individual baseline and external controls	Change from baseline and subsequent 24-week incremental visits
To evaluate the effect of BMN 351 on physical function	SV95C will be assessed at the specified visits and compared to individual baseline and external controls	Change from baseline and subsequent 24-week incremental visits
To evaluate the effect of BMN 351 on physical function	PUL 2.0 will be assessed at the specified visits and compared to individual baseline and external controls	Change from baseline and subsequent 24-week incremental visits

Other Outcome Measures

Outcome Measure	Measure Description	Time Frame
To evaluate the trough plasma concentration of BMN 351	Serial predose measurements of trough plasma PK	Baseline, Week 13, Week 25, Week 37, Week 49
To evaluate the immune response to BMN 351	Anti-BMN 351 antibodies and Anti-dystrophin antibodies	Change from baseline and subsequent 24-week incremental visits
To evaluate lung function longitudinally for participants ages 7 and above	Forced Expiratory Volume in 1 second (FEV1) for participants ages 7 and above	Change from baseline to subsequent 24-week incremental visits
To evaluate lung function longitudinally for participants ages 7 and above	Forced Vital Capacity (FVC) for participants ages 7 and above	Change from baseline to subsequent 24-week incremental visits
To evaluate lung function longitudinally for participants ages 7 and above	Peak Cough Flow (PCF) for participants ages 7 and above	Change from baseline to subsequent 24-week incremental visits
To evaluate lung function longitudinally for participants ages 7 and above	Maximal Inspiratory Pressure (MIP) for participants ages 7 and above	Change from baseline to subsequent 24-week incremental visits
To evaluate lung function longitudinally for participants ages 7 and above	Maximal Expiratory Pressure (MEP) for participants ages 7 and above	Change from baseline to subsequent 24-week incremental visits

Collaborators and Investigators

• Sponsor: BioMarin Pharmaceutical

Study record dates

Study Major Dates

• Study Start (Estimated): April 1, 2026
• Primary Completion (Estimated): December 1, 2031
• Study Completion (Estimated): December 1, 2031

Study Registration Dates

• First Submitted: April 17, 2026
• First Submitted That Met QC Criteria: May 1, 2026
• First Posted (Actual): May 7, 2026

Study Record Updates

• Last Update Posted (Actual): May 14, 2026
• Last Update Submitted That Met QC Criteria: May 12, 2026
• Last Verified: May 1, 2026

More Information

Terms related to this study

• Additional Relevant MeSH Terms: Musculoskeletal Diseases; Nervous System Diseases; Muscular Diseases; Neuromuscular Diseases; Genetic Diseases, Inborn; Genetic Diseases, X-Linked; Muscular Disorders, Atrophic; Muscular Dystrophies; Congenital, Hereditary, and Neonatal Diseases and Abnormalities; Muscular Dystrophy, Duchenne
• Other Study ID Numbers: 351-202; 2024-511656-41-00 (Ctis)

Drug and device information, study documents

• Studies a U.S. FDA-regulated drug product: No
• Studies a U.S. FDA-regulated device product: No