The purpose of this study is to evaluate the safety, tolerability, pharmacokinetics and pharmacodynamics of intrathecal (IT) delivery of ION717.

This is a first-in-human, multi-center study in participants with prion disease. The study will consist of a screening period of up to 6 weeks, a 30-week treatment period, a 142-week open-label extension period and a 32-week post-treatment period. Multiple dose levels will be tested.

The trial consists of three Regimens. Regimens 1 and 2 are fully enrolled. Participants in Regimens 1 and 2 received multiple doses of study drug (ION717 and placebo) during the 30-week double-blind treatment period; the order of doses (i.e. whether a given dose was ION717 or placebo) was blinded. The trial sites listed below are actively recruiting eligible participants for Regimen 3. Regimen 3 is open label.

Key Inclusion Criteria

• A confirmed diagnosis of probable or definite prion disease.
• Early-stage prion disease at the time of Screening.
• Willing to meet all study requirements, including travel to Study Center, procedures, measurements and visits.
• Patients must have a caregiver who is ≥ 18 years old and who is able and willing to facilitate the patient's involvement, to the best of their ability, for the duration of the trial; caregivers must also be able and willing to provide information about themselves and the patient for the duration of the trial.
• Aged ≥ 18 at the time of informed consent.

Key Exclusion Criteria

• Clinically significant abnormalities in medical history, laboratory tests or physical examination that would render a patient unsuitable for inclusion.
• Any contraindication or unwillingness to undergo an MRI.
• Obstructive hydrocephalus, presence of a functional ventriculoperitoneal shunt for the drainage of cerebrospinal fluid (CSF) or an implanted central nervous system (CNS) catheter.
• Known brain or spinal disease that would interfere with the LP process, CSF circulation or safety assessment.
• Have any other condition, which, in the opinion of the Investigator would make the patient unsuitable for inclusion or could interfere with the patient participating in or completing the study.

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/.