Extension of Letermovir (LET) From Day 100 to Day 200 Post-transplant for the Prevention of Cytomegalovirus (CMV) Infection in Hematopoietic Stem Cell Transplant (HSCT) Participants (MK-8228-040)

Inclusion Criteria

• have documented positive CMV serostatus (CMV immunoglobulin G [IgG] seropositive) for recipient (R+) at the time of transplant
• has a history of allogeneic HSCT (bone marrow, peripheral blood stem cell, or cord blood transplant) within ~100 days prior to randomization
• has undetectable CMV deoxyribonucleic acid (DNA) or detectable/not quantifiable CMV DNA from a plasma sample collected within 14 days prior to randomization
• has received LET as primary prophylaxis that started within 28 days of HSCT and continued through Week 14 post-transplant (± 1 week) prior to randomization
• is at high risk of CMV disease, defined as meeting one or more of the following criteria:
• has a related donor with at least 1 mismatch at 1 of the specified 3 human leukocyte antigen (HLA) gene loci (HLA-A, B, or DR)
• has an unrelated donor with at least one mismatch at one of the specified four HLA gene loci (HLA-A, B, C, and DRB1)
• has a haploidentical donor
• has umbilical cord blood as the stem-cell source
• has ex-vivo T-cell-depleted grafts
• has received anti-thymocyte globulin
• has received alemtuzumab
• has graft versus host disease (GVHD) or other conditions, requiring the use of systemic prednisone (or equivalent) at a dose of ≥1 mg/kg of body weight per day within 6 weeks of randomization
• for female participants, is not pregnant or breastfeeding, and is either not a woman of childbearing potential (WOCBP) or is a WOBCP who agrees to use acceptable contraception during the treatment period and for ≥28 days after the last dose of study drug.

Exclusion Criteria

• has a history of CMV end-organ disease or preemptive treatment therapy for CMV after HSCT prior to randomization
• has a history of >14 days total of LET interruption during the first 100 days post-transplant prior to randomization
• has suspected or known hypersensitivity to active or inactive ingredients of LET formulations
• has severe hepatic insufficiency defined as Child-Pugh Class C within 14 days prior to randomization.
• has serum aspartate aminotransferase (AST) or alanine aminotransferase (ALT) >5× the upper limit of normal (ULN) within 14 days prior to randomization
• has end-stage renal impairment with a creatinine clearance less than 10 mL/min, as calculated by the Cockcroft-Gault equation using serum creatinine within 14 days prior to randomization
• has both moderate hepatic insufficiency AND moderate-to-severe renal insufficiency
• has an uncontrolled infection on the day of enrollment
• requires mechanical ventilation or is hemodynamically unstable at the time of enrollment
• has a documented positive result for a human immunodeficiency virus antibody (HIV-Ab) test at any time prior to screening, or for hepatitis C virus antibody (HCV-Ab) with detectable HCV ribonucleic acid (RNA), or hepatitis B surface antigen (HBsAg) within 6 months prior to screening.
• has active solid tumor malignancies with the exception of localized basal cell or squamous cell skin cancer or the condition under treatment (eg, lymphomas)
• has received cidofovir or CMV immunoglobulin with 30 days prior to screening
• is currently participating or has participated in a study with an unapproved investigational compound, monoclonal antibody, or device within 28 days or 5× half-life of the investigational compound or monoclonal antibody, whichever is longer, of initial dosing in this study
• has previously participated in this study or any other study involving LET, or is currently participating in any study involving administration of a CMV vaccine or another CMV investigational agent, or is planning to participate in a study of a CMV vaccine or another CMV investigational agent during the course of this study
• is pregnant or expecting to conceive, is breastfeeding, or plans to breastfeed from the time of consent through 28 days after the last dose of study therapy
• is expecting to donate eggs starting from the time of consent through 28 days after the last d