Characterization of Cytomegalovirus Breakthrough Events in a Phase 2 Prophylaxis Trial of Letermovir (AIC246, MK 8228)

The efficacy of different letermovir (AIC246, MK8228) doses (60, 120, and 240 mg/day) against human cytomegalovirus (HCMV) was evaluated in a recent phase 2b dose-range-finding prophylaxis study in stem-cell transplant recipients. Here we report the genotypic and phenotypic characterization of 15 viral breakthroughs considered to be virological failures. Direct sequencing of an HCMV open reading frame UL56 region that included amino acids 230-370 and thus encompassed all known letermovir resistance mutations was followed by marker-transfer experiments to assess the impact of the identified sequence polymorphisms on viral fitness and susceptibility to letermovir. UL56 genotyping was successful for 12 of 15 patients. Six amino acid substitutions were detected in 5 patients. In 1 subject from the 60-mg-dose group, the known letermovir resistance mutation V236M was identified subsequent to a wild-type viremic episode. The remaining 5 sequence variants (L134V, S227I, Q228H, R410G, and D414N) were shown to be inert with regard to letermovir susceptibility, thus representing natural polymorphisms. Our findings represent the first case of a letermovir resistance mutation emerging in the clinic, apparently because of a suboptimal prophylactic dose (60 mg/day). This is in agreement with the trial's efficacy analyses, findings of which suggest that letermovir doses of 60 mg/day and 120 mg/day are suboptimal for prophylaxis whereas a dose of 240 mg/day appears to achieve complete suppression of viremia.