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Background: Cytomegalovirus (CMV) is associated with morbidity and mortality following allogeneic hematopoietic cell transplantation (alloHCT). Letermovir is a novel antiviral agent that prevents CMV reactivation in alloHCT patients, with limited data regarding influence on post‐alloHCT outcomes. Methods: We retrospectively examined 273 alloHCT recipients, 158 in the non‐letermovir cohort (NLC), and 115 in the cohort using letermovir prophylaxis (LC). Patients that received letermovir were CMV‐seropositive and met criteria for high risk of CMV reactivation. Results: Median start of letermovir was 21 days post‐alloHCT, median duration of prophylaxis was 86 days. Letermovir prophylaxis demonstrated a statistically significant reduction in first CMV reactivation (at 200 days post 63.9% in the NLC vs. 35.7% in the LC; p <.001). On univariate analysis at 1 year, overall survival (OS) for NLC was 79.6% and 79.5% for LC (p =.54). Non relapse mortality (NRM) at 1 year for NLC was 12% and 12.3% for LC (p =.69). Cumulative incidence of relapse (CIR) at 1 year was 13.9% for NLC versus 17.1 for the LC (p =.27). On multivariable analysis, there was no significant difference between the two cohorts for OS, NRM, and CIR. Conclusions: Letermovir prophylaxis started at day +21 post‐alloHCT reduced CMV reactivation, with no impact on posttransplant outcomes. [ABSTRACT FROM AUTHOR]

Letermovir, a novel anti‐cytomegalovirus (CMV) agent acts by inhibiting the viral terminase complex and is approved for primary prophylaxis in CMV seropositive patients post allogeneic hematopoietic cell transplantation (HCT). The favorable efficacy and safety profile make it an attractive option for use as secondary prophylaxis in patients at high‐risk for CMV reactivation. In this study, we report the efficacy and safety of letermovir secondary prophylaxis after at least one treated episode of CMV reactivation in a cohort of 39 high‐risk patients. Thirty two (82%) patients received anti‐thymocyte globulin (ATG), 27 (69%) received a combination of ATG and post‐transplant cyclophosphamide for graft‐versus‐host disease (GVHD) prophylaxis. Twenty one patients (54%) received CMV seronegative grafts. In addition, 18 (46%) patients had HLA mismatched unrelated or haploidentical donors while 18 (46%) had active GVHD requiring immunosuppression at the time of commencing secondary prophylaxis. Letermovir was initiated at a median of 47 days (range, 41–56) after HCT and was administered for a median duration of 77 days (range, 46–90). A single breakthrough CMV reactivation was noted in this high‐risk cohort. Four additional episodes of CMV reactivation occurred at a median of 28 days (range, 23–59 days) after discontinuation of secondary prophylaxis. The drug was well tolerated and 77% of the cohort completed the planned duration of secondary prophylaxis. None of the patients discontinued treatment due to treatment‐related adverse effects. In conclusion, letermovir is effective and well tolerated and may be considered for secondary prophylaxis in patients at high risk for CMV reactivation. Prospective studies are required to validate these findings. [ABSTRACT FROM AUTHOR]