The primary objectives are to evaluate the safety and tolerability of multiple doses of ION464 administered via intrathecal (IT) injection (Part 1) and to evaluate the long-term safety and tolerability of ION464 (Part 2) in participants with multiple system atrophy (MSA).

The secondary objectives are to evaluate the pharmacodynamic (PD) effect of ION464 on the level of a potential biomarker of target engagement (Parts 1 and 2) and to evaluate the pharmacokinetic (PK) profile of ION464 in serum (Part 1).

This is a first-in-human, randomized, blinded, placebo-controlled, multiple-ascending-dose (MAD) study (Part 1) to evaluate the safety, tolerability, PK, and PD of ION464 in adult participants diagnosed with MSA with a long-term extension (LTE) (Part 2). The study will include up to approximately 40 participants. Part 1 of the study consists of a Screening Period of up to 6 weeks, a Treatment Period of 12 weeks, and a Follow-up Period of 24 weeks. The study duration for each participant in Part 2 will be approximately 96 weeks, which consists of a 72-week Treatment Period and a 24-week Follow-up Period.

Key Inclusion Criteria:

• Screening single-photon emission computed tomography (SPECT) with DaTscan™ (ioflupane I123 injection) results demonstrating loss (whether symmetric or asymmetric) of dopamine nerve terminals in the striatum consistent with neurodegenerative parkinsonism, as assessed with qualitative, visual read.
• Diagnosed with probable or possible MSA, either parkinsonian-type (MSA-P) or cerebellar-type (MSA-C).
• Must be able to walk unassisted for at least 10 meters (approximately 30 feet)

Key Exclusion Criteria:

• Presence of cognitive dysfunction (defined as Montreal Cognitive Assessment (MoCA) score <25)
• Family history of ataxia or parkinsonism and known genetic cause of ataxia or parkinsonism.

NOTE: Other protocol defined Inclusion/ Exclusion criteria may apply.

Ionis may share anonymized individual participant data, aggregated clinical data, and other types of data that support the results in this study. Data requests from qualified researchers will be considered once all three of the following criteria are met: (1) 12 months from marketing approval of the study drug in both the United States and European Union; (2) 18 months from conclusion of the study; and (3) 6 months from publication of study article. Access would be via a secure environment and is contingent upon approval of a research proposal and entry into an appropriate data use agreement. Requests to access data can be submitted via the website https://vivli.org/ourmember/ionis/.