PrProfile: A Study to Assess the Safety, Tolerability, Pharmacokinetics and Pharmacodynamics of ION717

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

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.