Merck has announced positive results from its Phase 3 trial of letermovir, an investigational antiviral medicine for the prevention of clinically-significant cytomegalovirus (CMV) infections in adult CMV-seropositive recipients of allogeneic hematopoietic stem cell transplant (HSCT). Letermovir is a member of a new class of non-nucleoside CMV inhibitors (3,4 dihydro-quinazolines) and inhibits viral replication by specifically targeting the viral terminase complex. 

A total of 495 CMV seropositive HSCT recipients took part in the study. They were randomized 2:1 to receive either letermovir (at 480 mg/day, or 240 mg/day if the patient was on the immunosuppressant medication cyclosporine), or placebo administered once daily, either in oral tablet or intravenous formulation, for 14 weeks following the bone marrow transplant. Patients were assessed weekly through Week 14 and biweekly through Week 24 for the primary efficacy endpoint of clinical significant CMV infection.

The primary efficacy endpoint was met with significantly fewer patients developing clinically significant CMV infections in the letermovir arm (37.5%) compared to the placebo arm (60.6%) through Week 24 post-HSCT. 

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Additionally, treatment with letermovir was tied to lower all-cause mortality through Week 24 post transplant. “In this study, letermovir was associated with lower all-cause mortality. Based on these findings, letermovir as primary prophylaxis of CMV infection represents a potential new strategy for the prevention of CMV in this high-risk patient population,” said Dr. Francisco M. Marty of Harvard Medical School and attending physician in transplant and oncology infectious diseases at Dana-Farber Cancer Institute and Brigham and Women’s Hospital, who presented the data.

Cytomegalovirus is the most common clinically significant viral infection following HSCT. Based on the results from this trial Merck plans to submit regulatory applications for the approval of letermovir sometime in 2017.

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