Surveillance of CMV Terminase Gene Mutations in Allogeneic Hematopoietic Stem Cell Transplant Recipients Receiving Letermovir Prophylaxis.

Letermovir (LMV), a CMV-specific terminase inhibitor, is recommended for cytomegalovirus (CMV) prophylaxis in CMV-seropositive allogeneic hematopoietic stem cell transplant (alloHSCT) recipients. The association between low-level CMV DNAemia during prophylaxis and the emergence of mutations in CMV terminase complex genes remains unclear. We retrospectively analyzed 127 alloHSCT recipients at Saint-Louis Hospital, Paris, receiving LMV prophylaxis (108 primary, 24 secondary, and 5 both). Weekly CMV DNAemia monitoring was performed, and samples with viral loads ≥ 3 log IU/mL underwent whole-genome next-generation sequencing to detect mutations in terminase complex genes. During LMV prophylaxis, CMV DNAemia was detected in 31.5% of primary and 62.5% of secondary prophylaxis patients; however, viral loads ≥ 3 log IU/mL remained low in both groups (8.3%). The frequency of polymorphisms in terminase complex genes increased 2.5-fold during prophylaxis compared with pre-prophylaxis, and then decreased 3.0-fold after prophylaxis withdrawal. Confirmed LMV resistance was found in two patients, representing 1.6% of all patients and 18.2% of those with CMV DNAemia ≥ 3 log IU/mL. Two additional patients harbored mutations at resistance-associated positions in UL56 and UL89, totaling 36% of patients with viral loads ≥ 3 log IU/mL displaying mutations at known resistance sites. Clinically significant CMV infections were successfully managed with preemptive therapies. This study confirms the efficacy of LMV prophylaxis and demonstrates that LMV exerts selective pressure on the terminase complex, increasing the risk of resistance when CMV DNA levels exceed 3 log IU/mL, underscoring the importance of timely consideration of alternative antiviral therapies when CMV replication persists.