The goal of this clinical trial is to test ATX-01 in participants with myotonic dystrophy type 1 (DM1). The main question it aims to answer is if ATX-01 is safe and well tolerated. The trial will compare the safety and tolerability of ATX-01 and a matching placebo.

There will be a single-ascending dose part of the trial and a multiple-ascending dose part. In the single-ascending dose, participants will receive one dose of ATX-01 or placebo. In the multiple-ascending dose part, participants will receive three doses of ATX-01 or placebo.

ATX-01 is a novel anti-miR (synthetic single stranded oligonucleotide) that inhibits a microRNA called miR-23b.

Key Inclusion Criteria:

• Participants with a documented clinical diagnosis of DM1 (CTG expansion of >150 repeats in DMPK gene measured in peripheral blood mononuclear cells)
• Ambulatory, defined as able to complete a 10-meter walk/run test at screening without the use of assistive devices such as canes, walkers, or orthoses, except for ankle-foot orthoses
• Presence for >3 seconds of grip myotonia as confirmed by a central reader

Key Exclusion Criteria:

• Participants with congenital DM1
• Medical Research Council Muscle Scale score of less than 4 on ankle dorsiflexion or significant tibialis anterior atrophy that prevents a muscle biopsy
• Use of mexiletine or other agent for myotonia within 21 days or 5 half-lives, whichever is longer, prior to screening