Intravenous Brincidofovir as an Antiviral for Treatment of Progressive Multifocal Leukoencephalopathy: A Pilot Study

* INCLUSION CRITERIA: * Able to provide informed consent or have a designated legally authorized representative (LAR) to provide consent * Stated willingness to comply with study procedures and to participate for the duration of the study including follow-up * Actively progressing, clinically definite or probable PML (2013 AAN Consensus Diagnostic Criteria) * Positive PCR for JCPyV in CSF * Age 18 or older * Medically stable and able to tolerate travel to NIH * Participants of childbearing or child-fathering potential must be willing to use a medically acceptable form of birth control, which includes abstinence, while being treated on this study EXCLUSION CRITERIA: * ALT or AST \> 5 x the ULN, total bilirubin \> 3 mg/dL (SI: \>51 micromol/L), or spontaneous prothrombin time-international normalized ratio (PT-INR) \> 2 x ULN within 7 days prior to Day 1 * An estimated glomerular filtration rate of \< 30 mL/min within 7 days prior to Day 1 * Hypersensitivity to CDV or to BCV or its formulation excipients, or prior intolerance to these agents that, in the opinion of the investigator, would pose an unacceptable safety risk. * Active CNS disease other than PML that, in the opinion of the investigator, would confound study assessments or pose an unacceptable safety risk. * Contraindication to MRI (including cardiac pacemakers and some infusion pumps, other metallic implants, metallic foreign objects) * Medical contraindication to LP * Positive pregnancy test or nursing