Study to Confirm of the Safety and Tolerability of Brincidofovir in Subjects With BK Virus Infection (Viremia) After Kidney Transplantation

Inclusion Criteria: * Male or female, at least 18 years of age at the time of signing the informed consent at screening. * Kidney transplant recipient. "BK viral load increase and ≥ 3.6 log IU/mL" at 2 weeks post immunosuppression reduction or "BK viral load does not decrease by ≥ 0.3 log IU/mL" at 4 weeks post immunosuppression reduction during prescreening. (Note: Immunosuppressant reduction needs to be continued during the screening period). * eGFR ≥ 30 mL/min. * Subjects under immunosuppression with tacrolimus, MMF/Myfortic, and/or corticosteroid. Exclusion Criteria: * Subjects who weigh ≥ 120 kg. * National Institutes of Health/NCI CTCAE Grade 2 or higher diarrhea (ie, increase of ≥ 4 stools per day over usual pretransplant stool output) within 7 days before Day 1. * Poor clinical prognosis, including active malignancy or use of vasopressors other than low dose (eg, ≤ 5 μg/kg/min) dopamine for renal perfusion within 7 days before Day 1. * Use of renal replacement therapy within 7 days before Day 1. * History of intolerance to cidofovir or related compounds (ie, other nucleotide derivatives \[adefovir or tenofovir\])