Global Study of Del-desiran for the Treatment of DM1

The study consists of a Screening Period of up to 6 weeks and 54-week Treatment Period. The anticipated duration is approximately 60 weeks.

Participants will be randomized to receive an intravenous infusion of either del-desiran or placebo at the clinical study site every 8 weeks for a total of 7 doses. The final dose will occur at Week 48, followed by a final assessment at Week 54.

After completion of Week 54 assessments, eligible participants will have the option to enroll into an open label extension (OLE) study, pending regulatory approval.

An Independent Data Monitoring Committee (IDMC) comprised of members independent and external to the Sponsor will review safety, tolerability, and efficacy (as needed) data of this study at regular intervals.

Key Inclusion Criteria:

• Clinical and genetic diagnosis (CTG repeat ≥ 100) of DM1
• Ability to walk independently (orthoses and ankle braces allowed) for at least 10 meters at screening

Key Exclusion Criteria:

• Breastfeeding, pregnancy, or intent to become pregnant during the study
• Unwilling or unable to comply with contraceptive requirements
• Abnormal lab values, conditions or diseases that would make the participant unsuitable for the study
• Diabetes that is not adequately controlled
• History of decompensated heart failure within 3 months of screening. Participants with preexisting pacemaker/ICD are not excluded.
• Body Mass Index > 35 kg/m2 at Screening
• Recently treated with an investigational drug or biological agent
• Treatment with anti-myotonic medication within 5 half-lives or 14 days of baseline, whichever is longer, prior to baseline.

Note: Additional protocol defined Inclusion and Exclusion criteria apply