Evaluation of Safety, Tolerability, and Efficacy of DYNE-251 in Duchenne Muscular Dystrophy Patients Amenable to Exon 51 Skipping

The primary objective of this study is to evaluate the **safety** and tolerability of multiple intravenous doses of DYNE-251 in participants with **Duchenne muscular dystrophy** (DMD) amenable to exon 51 skipping. Additionally, the study aims to assess dystrophin protein levels in muscle tissue following these doses. This is clinically relevant as it addresses the potential therapeutic impact of DYNE-251 on muscle integrity and function, which are critical factors in the management of DMD.

Secondary objectives include:

• Evaluating the effects on muscle tissue exon skipping, percent dystrophin-positive fibers, and blood creatine kinase following multiple IV doses of DYNE-251.
• Assessing muscle function after administration of the drug.
• Evaluating plasma and muscle tissue pharmacokinetics (PK) following the treatment.
• Assessing the immunogenicity of the drug.

The clinical trial involves a total of **59 participants** diagnosed with **Duchenne muscular dystrophy** (DMD). The study population consists exclusively of **male** subjects, aged between **4 to 16 years**. Participants were selected based on specific criteria, including a confirmed diagnosis of DMD with a mutation in the dystrophin gene amenable to exon 51 skipping. The trial includes both ambulatory and non-ambulatory individuals, with the latter having been non-ambulatory for less than two years prior to enrollment. All participants are required to have been on a stable dosage of glucocorticoids for at least 12 weeks before the commencement of the study drug administration. Additionally, participants must have a left ventricular ejection fraction of at least 50% by echocardiogram or 55% by cardiac MRI. The trial does not include female subjects and involves a vulnerable population due to the age and health condition of the participants. Lifestyle considerations such as diet and physical activity are not specified in the available data.

The clinical trial is designed as a **randomized, double-blind, placebo-controlled** study to evaluate the safety, tolerability, pharmacodynamics, efficacy, and pharmacokinetics of DYNE-251 in participants with **Duchenne muscular dystrophy** amenable to exon 51 skipping. The trial involves multiple ascending doses of DYNE-251, administered via **intravenous infusion**. The study is structured to include a placebo group, utilizing a commercially available 5% dextrose solution for intravenous use. The trial is expected to span from January 30, 2023, to December 4, 2027, with participant involvement potentially lasting up to 145 weeks, depending on cohort assignment and dosing schedule.

Participants will undergo a series of study visits, beginning with an inclusion (screening) visit to confirm eligibility based on criteria such as age, genetic mutation, and health status. Following successful screening, participants will be randomized to receive either DYNE-251 or placebo. Study visits will include regular assessments to monitor safety and efficacy, with primary endpoints focusing on the number and proportion of participants experiencing treatment-emergent adverse events and changes in dystrophin protein levels in muscle tissue. Secondary endpoints will assess various biomarkers and functional outcomes over the course of the study.

Follow-up visits will occur at specified intervals, with key assessments at Week 25 and Week 49, depending on the dosing schedule. These visits will include muscle biopsies and other evaluations to measure changes from baseline in dystrophin protein levels, exon 51 skipping levels, and other relevant biomarkers. The end-of-study visit will conclude the participant's involvement, with final assessments to ensure safety and collect comprehensive data on the study's endpoints.

Participants may be withdrawn from the study early if they experience significant adverse events, fail to comply with study procedures, or if the investigator deems it necessary for their safety. The trial's design ensures rigorous monitoring and data collection to achieve its objectives while maintaining participant safety throughout the study duration.

The clinical trial involves the administration of **DYNE-251**, an investigational medicinal product designed for the treatment of **Duchenne Muscular Dystrophy** (DMD) amenable to exon 51 skipping. DYNE-251 is a **biological/biotechnological** product of non-Advanced Therapy Investigational Medicinal Product (ATIMP) origin. The active substance in DYNE-251 is a **fragment antibody targeting human TfR1** conjugated to a **phosphorodiamidate morpholino oligomer**. This product is administered via **intravenous infusion**. The dosing schedule involves multiple ascending doses, although specific dosage amounts and frequency are not detailed in the provided data. The pharmaceutical form of DYNE-251 is an infusion, and it is not formulated specifically for pediatric use. Compliance with the administration schedule will be monitored throughout the trial to ensure adherence to the protocol.

The study also includes a **placebo** control, which is a commercially available 5% dextrose solution intended for intravenous use. The placebo is used to maintain the double-blind nature of the trial, ensuring that neither the participants nor the investigators are aware of the treatment assignments. The placebo is administered in the same manner as DYNE-251, via intravenous infusion, to provide a consistent comparison between the experimental treatment and the control group. The use of a placebo is critical in assessing the safety, tolerability, and efficacy of DYNE-251 in the study population.

The efficacy of DYNE-251 in participants with **Duchenne Muscular Dystrophy** (DMD) amenable to exon 51 skipping will be assessed through a series of primary and secondary endpoints. The primary efficacy endpoint involves measuring the change from baseline in dystrophin protein levels in muscle tissue, determined by Western blot analysis at Week 25. This endpoint is critical for evaluating the pharmacodynamic effect of DYNE-251 on muscle tissue.

Secondary efficacy endpoints include various muscle tissue, functional, and plasma parameters. Muscle tissue endpoints will assess changes from baseline at Week 25 and Week 49 in exon 51 skipping levels, phosphorodiamidate morpholino oligomer (PMO) concentration, and dystrophin protein levels. Functional endpoints will evaluate changes from baseline up to Week 145 in the Performance of the Upper Limb (PUL) scale V 2.0 score, percentage predicted forced vital capacity (%pFVC), North Star Ambulatory Assessment (NSAA) total score, time to rise (TTR), 10-meter run/walk (10MRW), and stride velocity 95th centile (SV95C) in ambulatory participants. Plasma endpoints will include pharmacokinetic parameters such as Cmax, tmax, AUCtlast, AUC∞, λZ, t½, clearance (CL), and volume of distribution (Vz and Vss, if appropriate). Additionally, the incidence of anti-drug antibodies (ADAs) will be monitored as an immunogenicity endpoint.

These efficacy parameters will be collected and analyzed at specified timepoints throughout the trial, with muscle biopsies conducted at Weeks 25 and 49 for certain cohorts. The comprehensive assessment of these endpoints will provide valuable insights into the therapeutic potential of DYNE-251 in modifying disease progression in DMD patients.