Phenotypic characterization of two naturally occurring human Cytomegalovirus sequence polymorphisms located in a distinct region of ORF UL56 known to be involved in in vitro resistance to letermovir

Letermovir is a new drug in Phase 3 clinical development for the prevention of human Cytomegalovirus (HCMV) infections in hematopoietic-stem-cell transplant recipients (HSCT). In contrast to marketed anti-HCMV drugs which all target the viral DNA polymerase, letermovir’s novel mode of action targets the UL56 subunit of the viral terminase complex. Consistently letermovir resistance has mapped in vitro to a distinct region within ORF UL56 (amino acid 230–370). Here we used marker transfer to demonstrate that two naturally occurring UL56 sequence variants within this region, located directly adjacent to sites known to mediate letermovir resistance in vitro (D242G and A327V) represent normal interstrain polymorphisms unrelated to drug-resistance.