ATTUNE: A Study to Evaluate the Safety, Tolerability, Pharmacokinetics, and Pharmacodynamics of Intrathecally-Administered ION440 in Participants With Methyl CpG Binding Protein 2 (MECP2) Duplication Syndrome (MDS)

1. Documented diagnosis of severe MECP2 duplications including terminal duplication and/or translocation or MECP2 triplication OR clinical features associated with severe variant structure including (a) onset of seizures prior to age 5 (for those aged 5 and above at signing of ICF), (b) oxygen dependence, (c) microcephaly, IF MECP2 genetic structure information is unavailable.

2. Clinically significant vital sign or ECG abnormality at Screening

3. Known brain or spinal disease that would interfere with the LP procedure, or CSF circulation or presence of other factors would affect the safety of the LP procedure.

4. Has any concomitant disease or condition or circumstance, or any finding at Screening that, in the opinion of the Investigator, makes the participant unsuitable for enrollment or that could interfere with the conduct of the study or that would pose an unacceptable risk to the participant in this study.

5. Treatment with an investigational drug, biological agent, or device within 30 days of Screening, or 5 half-lives of investigational agent, whichever is longer.

6. Previous treatment with an oligonucleotide (including siRNA) within 4 months of Screening if single dose received, or within 12 months of Screening if multiple doses received (this exclusion does not apply to vaccines - both mRNA and viral vector vaccines are allowed including COVID-19). For centrally administered ASOs, a minimum of 12 months washout is required irrespective of the number of doses received.

7. Currently enrolled in a clinical trial of an investigational agent or device or has used any investigational agent or device within 5 half-lives of investigational agent, whichever is longer.

8. Has a history of gene therapy or cell transplantation or any other experimental brain surgery.

9. Active infection requiring systemic antiviral or antimicrobial therapy that will not be completed prior to Baseline (Day 1).

10. Has experienced Status Epilepticus in the past 6 months.

1\. Has developed any concomitant disease (e.g., gastrointestinal, renal, hepatic, endocrine, respiratory, or cardiovascular system disease) or condition or circumstance, or any finding during Part 1/MAD that, in the opinion of the Investigator, makes the participant unsuitable for continued treatment (e.g., could interfere with the conduct of the study or that would pose an unacceptable risk to the participant in this study).

What this study is about