A Phase 1/2, Multicenter, Open-Label Study to Evaluate the Safety, Tolerability, Pharmacokinetics, and Pharmacodynamics of DNL126 in Pediatric Participants with Mucopolysaccharidosis Type IIIA (Sanfilippo Syndrome Type A)

Key Inclusion Criteria:

• Confirmed diagnosis of MPS IIIA

Key Exclusion Criteria:

• Have unstable or poorly controlled medical condition(s) or significant medical or psychological comorbidity or comorbidities that, in the opinion of the investigator, would interfere with safe participation in the trial or interpretation of study assessments
• Have lost the ability to walk independently, in the opinion of the investigator
• Are unable to take the majority of nutrition via mouth, in the opinion of the investigator
• Have used any CNS-targeted MPS IIIA enzyme replacement therapy (ERT) (eg, intrathecal SGSH or TfR-mediated SGSH delivery to CNS) within 3 months before Day 1
• Have a prior history of hematopoietic stem cell transplantation
• Have a prior history of gene therapy
• Have used genistein within 30 days of screening or intended use of genistein during the study
• Have a documented likely pathogenic mutation sufficient to cause disease (eg, taking into account zygosity) of other genes that are known to be associated with developmental delay, seizures, or other significant CNS disorders
• Have clinically significant thrombocytopenia, other clinically significant coagulation abnormality, significant active bleeding, or require treatment with an anticoagulant or more than two antiplatelet agents
• Contraindication for lumbar punctures
• Contraindication for MRI scan
• Have a clinically significant history of stroke, status epilepticus, head trauma with loss of consciousness, or any clinically significant CNS disease that is not MPS IIIA-related within 3 months of screening
• Have had a ventriculoperitoneal (VP) shunt placed or a clinically significant VP shunt malfunction within 30 days of screening
• Have any clinically significant CNS trauma or disorder, including severe untreated intracranial hypertension or brain surgery, that, in the opinion of the investigator, may interfere with assessment of study endpoints or make participation in the study unsafe