PRIMARY OBJECTIVE:~I. To evaluate the efficacy of letermovir compared to no prophylaxis in the prevention of clinically significant cytomegalovirus (CMV) infection through Week 14 (~100 days) post-transplant in children and adolescents receiving allogeneic hematopoietic cell transplant (allo-HCT).~SECONDARY OBJECTIVES:~I. To evaluate the efficacy of letermovir compared to no prophylaxis as assessed by time to onset of any CMV DNAemia through week 14 (~100 days) post transplant.~II. To evaluate CMV-free survival through 24 weeks post transplant in pediatric patients who receive letermovir compared to those who do not.~EXPLORATORY OBJECTIVES:~I. To evaluate the efficacy of letermovir as assessed by time to onset of clinically significant CMV infection through weeks 24 (~6 months) and 48 (~1 year) post-transplant.~II. To evaluate overall survival through weeks 24 and 48 post-transplant in patients who receive letermovir versus those who do not.~III. To compare time to engraftment and describe the cumulative incidence of non-engraftment among patients who receive letermovir and those who do not.~IV. To examine the rates of several clinically significant adverse events among patients exposed to letermovir versus those who are not including: the total duration of neutropenia through week 14 (~100 days) post-transplant, the cumulative incidence of acute kidney injury and chronic kidney disease by 52 weeks post-transplant, and total inpatient hospital days by 14 weeks (~100 days) and 52 weeks post-transplant.~V. Describe patterns of anti-viral resistance at the onset of CMV DNAemia after allo-HCT.~VI. To compare immune reconstitution and CMV-specific immunity in pediatric patients treated with letermovir prophylaxis versus no prophylaxis.~OUTLINE: Patients are randomized to 1 of 2 arms.~ARM A: Patients receive letermovir orally (PO) or intravenously (IV) over 60 minutes once daily (QD) starting on day +1 post-transplant for 14 weeks. Patients undergo collection of blood samples for CMV polymerase chain reaction (PCR) analysis weekly for 14 weeks, every 2 weeks until week 24, week 32, week 40 and week 52.~ARM B: Patients undergo collection of blood samples for CMV PCR analysis weekly for 14 weeks, every 2 weeks until week 24, week 32, week 40 and week 52.
This phase III trial determines whether taking prophylactic letermovir will reduce the likelihood of infection with cytomegalovirus (CMV) in children and adolescents after stem cell transplant. The treatments used to prepare for HCT reduce the body's natural infection-fighting ability and increase the likelihood of an infection with a virus called cytomegalovirus. Prophylaxis means to take a drug to prevent a disease or side effect. Letermovir is an antiviral drug that stops cytomegalovirus from multiplying and may prevent cytomegalovirus infection and make the disease less severe.