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