Study to Evaluate the Safety, Tolerability, and Pharmacokinetics of ION464 Administered to Adults With Multiple System Atrophy (HORIZON)

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.