In vitro activity: Letermovir (formerly known as MK-8828 and AIC-246; trade name: Prevymis) is a new potent anticytomegalovirus drug in clinical development. On 11/8/2017, Letermovir was approved by FDA to prevent infection after bone marrow transplant. Despite modern prevention and treatment strategies, human cytomegalovirus (HCMV) remains a common opportunistic pathogen associated with serious morbidity and mortality in immunocompromised individuals, such as transplant recipients and AIDS patients. All drugs currently licensed for the treatment of HCMV infection target the viral DNA polymerase and are associated with severe toxicity issues and the emergence of drug resistance.

Kinase Assay: AIC246 has consistent antiviral efficacy, and there is remarkable selectivity of AIC246 for human cytomegaloviruses. AD169 mutant strains and designated rAIC246-1 and rAIC246-2 are highly resistant to Letermovir (AIC246), with EC50s of 5.6 nM, 1.24 μM, 0.37 μM, respectively. Letermovir inhibits HCMV replication through a specific antiviral mechanism that involves the viral gene product UL56. Letermovir inhibits HCMV replication in cell culture by interfering with the proper cleavage/packaging of HCMV progeny DNA[2]. Letermovir inhibits the current gold standard GCV by more than 400-fold with respect to EC50s (mean, 4.5 nM versus 2 μM) and by more than 2,000-fold with respect to EC90 values (mean, 6.1 nM versus 14.5 μM)[3]. Letermovir in conbination with anti-HCMV drugs causes additive antiviral effects, but there is no interaction between letermovir and anti-HIV drugs.

Cell Assay: Briefly, 5×103 AD169-infected NHDF cells/well are seeded into the wells of 30 96-well microtiter plates. The infection is allowed to proceed under the exposure of 50 nM AIC246 (10×EC50) until a CPE developed in one or more of the compound-treated wells (indicative of resistant virus breakthrough). Noninfected and nontreated cells serve as controls on each plate. Mutant virus amplification is accomplwashed after cultures achieved maximum CPE by the passage of cell-free supernatant virus in the presence of 50 nM AIC246. The resultant AIC246-resistant progeny virus mutants are plaque purified three times by limiting dilutions in the presence of AIC246. The stability of resistance is tested by serially passaging plaque-purified viruses without selective pressure (8 to 10 times).