Letermovir (AIC246 or MK-8228), a 3,4-dihydro-quinazoline- 4-yl-acetic acid derivative, is the prototype viral terminase complex inhibitor that is most advanced in its clinical development. The novel compound was initially developed by AiCuris. In April 2011, the drug was granted orphan drug designation for prevention of CMV disease by the European Commission. In August 2011, the US Food and Drug Administration granted it a fast track designation. In 2012, the results of Phase IIb clinical trials using letermovir in bone marrow transplant patients were presented at various international meetings, and the data were subsequently published in 2014.42 It`s continued clinical development is currently undertaken in agreement with Merck. Letermovir is highly potent in vitro and in vivo against cytomegalovirus. Because of a distinct mechanism of action, it does not exhibit cross-resistance with other antiviral drugs. It is predicted to be active against strains that are resistant to ganciclovir, foscarnet, and cidofovir. To date, early-phase clinical trials suggest a very low incidence of adverse effects. It targets the UL56 subunit of the viral terminase complex. Letermovir is currently in Phase III development.