This is a Phase 1/2, open-label, multiple-dose, dose-escalating study to evaluate the safety, tolerability, and plasma and CSF concentrations of GTX-102 in pediatric patients with AS.

The study includes screening, baseline, treatment, and safety follow-up periods. Eligible patients who meet all of the inclusion criteria and none of the exclusion criteria and who successfully complete all screening and baseline assessments will be assigned to one of 5 sequential dose cohorts. GTX-102 will be administered by intrathecal (IT) injection via lumbar puncture (LP) at Baseline followed by 3 subsequent IT injections on study Days 30, 58 and 86. The procedure for IT administration of GTX-102 will performed by a clinician with experience in LP and anesthetic care will be directed by experienced anesthesiologists with a focus on patient safety and comfort. The total duration of study for each patient is anticipated to be approximately 6 months. An open-label extension (OLE) study will be conducted under a separate protocol where patients can receive continued treatment with GTX-102.