The purpose of this study is to determine the safety and tolerability of intravenous (IV) brincidofovir (BCV; SyB V-1901) 0.2 mg/kg, 0.3 mg/kg or 0.4 mg/kg dosed twice weekly (BIW) or 0.4 mg/kg dosed once weekly (QW) for 4 weeks in subjects with AdV, and IV BCV in subjects with CMV

This is a Phase IIa, open-label, multiple ascending dose confirmation, multicenter study to evaluate the safety and tolerability of intravenous Brincidofovir (BCV, SyB V-1901) 0.2 mg/kg, 0.3 mg/kg, or 0.4 mg/kg dosed BIW or 0.4 mg/kg dosed QW (Cohorts 1 to 4) in adult and pediatric subjects with AdV viremia, and IV BCV in subjects with CMV.

Inclusion Criteria:

• Male or female, aged 2 months and older at the time of informed consent.
• AdV DNA viremia >10,000 copies/mL from a single sample, or 2 samples greater than 48 hours apart with the second result higher than the first and both greater than 1000 copies/mL, from the data obtained from the designated central virology laboratory of the local laboratory using the blood sample(s) collected informed consent has been obtained and within 7 days prior to Day 1 (AdV DNA viremia results collected within the 7 day window, but prior to consent may be used if the Informed Consent Form (ICF) signed by the subject provides approval) . CMV viremia with or without evidence of tissue invasive CMV disease. For laboratory results that are generated in units other than copies/mL or IU/mL, please refer to the testing laboratory for guidance on the appropriate conversion calculation.
• Either (a) have disseminated AdV disease or (b) have an underlying immunocompromised state, and have asymptomatic AdV infection or localized AdV disease.
• In the judgment of the investigator, be in a serious condition to be treated with intravenous cidofovir for AdV.

Exclusion Criteria:

• Subjects who weigh ≥120 kg.
• NIH/NCI CTCAE (United States [US] National Institutes of Health [NIH]/National Cancer Institute) Grade 2 or higher diarrhea (i.e., increase of ≥ 4 stools per day over usual pre-transplant stool output) within 7 days prior to Day 1.
• NIH Stage 4 acute GVHD of the skin (i.e., generalized erythroderma with bullous formation) within 7 days prior to Day 1.
• NIH Stage 2 or higher acute GVHD of the liver function (i.e., bilirubin >3 mg/dL [SI: >51 μmol/L]) within 7 days prior to Day 1.
• NIH Stage 2 or higher acute GVHD of the gut (i.e., diarrhea >556 mL/m2/day for pediatric subjects [or >1000 mL/day for young adults as applicable, at centers in the United States only], or severe abdominal pain with or without ileus) within 7 days prior to Day 1.