Abstract Background: Hematopoietic cell transplant (HCT) patients are at risk for serious infections by Herpes simplex1/2 (HSV) and Varicella zoster(VZV). Current guidelines recommend prophylaxis (PPX) with acyclovir (ACV) during the peri-transplant period and continuing throughout the period of immunosuppression. Brincidofovir (BCV, CMX001) is a broad-spectrum nucleotide analog with in vitro activity against double stranded DNA (dsDNA) viruses including HSV and VZV. BCV is in Phase 3 development for the prevention of cytomegalovirus (CMV) infection post-HCT. At present, there is no clinical data to support the efficacy of BCV PPX for HSV/VZV post-HCT. At Memorial Sloan-Kettering Cancer Center (MSK) from 2010 to June 2015, approximately 70 alloHCT recipients were treated with BCV for various indications. We examined rates of breakthrough HSV and VZV infections during administration of BCV. Methods: Retrospective review of alloHCT patients (pts) treated for ≥14 days with BCV, on protocols or emergency investigational new drug application (EIND) at MSK. Administration of concomitant (CON) ACV for HSV PPX during BCV treatment was at clinician discretion. CMX001-350 was an expanded access study of BCV for treatment of subjects with dsDNA viruses and limited or no therapeutic options. CMX001-202 was a placebo-controlled study of BCV as preemption of adenovirus (ADV) post-HCT. CMX001-304 is a currently ongoing open label study of BCV for preemption of ADV disease. After June 2011, all pts received BCV 200mg total weekly dose [100mg twice weekly (BIW) or 200mg weekly (QW)] for adult/adolescent pts and 4mg/kg total weekly dose (2mg/kg BIW or 4mg/kg QW) for pediatric pts. Patients enrolled on CMX-304 through June 16, 2015 are included in this analysis. Based on CON ACV PPX during BCV treatment, 3 patient groups were identified: 1) No CON ACV PPX (group A); 2) Partial CON ACV PPX (group B); and 3) CON ACV PPX (group 3). Diagnosis of HSV or VZV infection was based on viral culture or PCR and clinical symptoms. Data was extracted from medical, pharmacy, and microbiology records. Results: Of 55 alloHCT pts who received BCV ≥14 days, 27 (49%) were treated on CMX001-350, 1 (2%) on CMX001-202, 12 (22%) on CMX001-304 and 15 (27%) under EIND. The median age was 53 years (range, 3-69). Thirty nine (71%) pts received T-cell depleted allografts, 9 (16%) cord blood, and 7 (13%) conventional allografts. BCV started a median of 85 days [Interquartile range (IQR), 59.5-209.5] post-HCT. The primary indication for BCV was: ACV resistant HSV in 2 (4%) pts, disseminated VZV in 1 (2%) pts, CMV in 28 (51%) pts, ADV in 21 (38%) pts and other dsDNA viruses (BK polyoma virus, Human herpesvirus 6, John Cunningham virus) in 3 (5%) pts. Thirty one (56%) pts received BCV 100mg BIW. Among 35 pts in group A, duration of BCV was a median of 39 days (IQR, 29.5-78). One pt in group A received concomitant anti-CMV antivirals for 12 days. Among 13 pts in group B, the median duration of BCV monotherapy was 23 days (IQR, 13-40). Two pts in group B had brief concomitant exposure to anti-CMV antivirals. Among 7 patients in group C, the median duration of BCV treatment was 71 days (IQR, 40-132) (Table 1). Two pts developed HSV infections during BCV treatment. For one pt, the primary indication for BCV was ACV resistant HSV. The pt initially responded to BCV with resolution of oral lesions. During his treatment with BCV, he had intermittent oral ulcers and oral cultures positive for HSV. The second pt developed breakthrough HSV infection during interruption of BCV for severe diarrhea related to graft versus-host disease. There were no breakthrough VZV infections. For one pt, the primary indication for BCV was disseminated VZV. The pt had complete resolution of VZV and no further VZV reactivation during BCV treatment. Conclusions: 1) During 2,806 patient-days of BCV monotherapy in highly immunosuppressed HCT pts, there were no VZV infections and only 2 breakthrough HSV infections. 2) BCV treatment interruptions and impaired gastrointestinal absorption likely explain the breakthrough HSV infections. 3) Our retrospective analysis in this heterogeneous cohort of sick alloHCT recipients supports that BCV administration at doses currently studied for CMV prevention is likely effective for prophylaxis of HSV/VZV in HCT. Disclosures Off Label Use: Brincidofovir (CMX001) for the efficacy of prophylaxis for Herpes simples 1/2 and Varicella zoster..