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

Study Description: This pilot study will test safety and tolerability of IV BCV as an antiviral treatment strategy for participants with PML, and will collect preliminary data on biological and clinical impact on PML disease course. Eighteen adults with PML from all causes will complete this study. Following a standardized baseline evaluation and confirmation of PML diagnosis with positive JCPyV DNA detection in CSF, participants will receive IV BCV 20mg (or 0.4mg/kg if participant weighs\<50kg) twice weekly in 4-week Infusion Cycles for up to 12 weeks total (3 Infusion Cycles). At completion of each Infusion Cycle, participants will be evaluated monthly for 3 months to determine if they meet criteria for 1) redosing with additional 4-weeks of IV BCV, 2) initiation/continuation of Clinical Monitoring or 3) definition of Treatment Failure (leading to optional withdrawal from study and pursuit of rescue treatments). As long as less than 12 weeks of cumulative dosing have been pursued, redosing may be offered. Upon completion of treatment, participants will be monitored for up to 12 months. Objectives: Primary Objective: -To describe the safety and tolerability of IV BCV 20mg (or 0.4mg/kg if participant weighs\<50kg) dosed twice weekly for a cumulative total of up to 12 weeks. Secondary Objectives: * To investigate kinetics and magnitude of antiviral effect of IV BCV on JCPyV load in CSF in participants with PML. * To investigate effect of IV BCV on clinical and radiological PML disease course. Exploratory Objectives: -To investigate pharmacodynamics and effect of IV BCV in urine, blood and CSF. Endpoints: Primary Endpoint: -Number of treatment-related adverse events (AEs) of Grade 3 severity or higher as determined by Common Terminology Criteria for Adverse Events (CTCAE 6.0) Secondary Endpoints: * Number of treatment-related AEs, irrespective of grade * Change from baseline in JCPyV load in CSF at completion of Infusion Cycles 1, 2 and 3 * Proportion of participants with 0.25 log decline or greater in JCPyV load in CSF at completion of Infusion Cycles 1, 2 and 3 * Number of Infusion Cycles * Time to increase in JCPyV load in CSF during monitoring phase * Number of participants meeting criteria for Treatment Failure * Change in performance measures and standardized disability rating scales from baseline to Month 6, 9 and 12 (or last assessment) * Change in PML lesion burden by MRI from baseline to Month 3, 6, 9 and 12 (or last assessment) * Survival at 3, 6, 9 and 12 months Exploratory Endpoints: -Exploratory measures in urine, blood and CSF to investigate pharmacodynamics of IV BCV, including virological and immunological changes in response to study intervention.