The Novel Anticytomegalovirus Compound AIC246 (Letermovir) Inhibits Human Cytomegalovirus Replication through a Specific Antiviral Mechanism That Involves the Viral Terminase

Abstract

Human cytomegalovirus (HCMV) remains the leading viral cause of birth defects and life-threatening disease in transplant recipients. All approved antiviral drugs target the viral DNA polymerase and are associated with severe toxicity issues and the emergence of drug resistance. Attempts to discover improved anti-HCMV drugs led to the identification of the small-molecular-weight compound AIC246 (Letermovir). AIC246 exhibits outstanding anti-HCMV activity in vitro and in vivo and currently is undergoing a clinical phase IIb trial. The initial mode-of-action studies suggested that the drug acts late in the HCMV replication cycle via a mechanism distinct from that of polymerase inhibitors. Here, we extend our mode-of-action analyses and report that AIC246 blocks viral replication without inhibiting the synthesis of progeny HCMV DNA or viral proteins. The genotyping of mutant viruses that escaped AIC246 inhibition uncovered distinct point mutations in the UL56 subunit of the viral terminase complex. Marker transfer analyses confirmed that these mutations were sufficient to mediate AIC246 resistance. The mapping of drug resistance to open reading frame UL56 suggests that viral DNA processing and/or packaging is targeted by AIC246. In line with this, we demonstrate that AIC246 affects the formation of proper unit-length genomes from viral DNA concatemers and interferes with virion maturation. However, since AIC246-resistant viruses do not exhibit cross-resistance to previously published terminase inhibitors, our data suggest that AIC246 interferes with HCMV DNA cleavage/ packaging via a molecular mechanism that is distinct from that of other compound classes known to target the viral terminase.

RhoB is a component of the human cytomegalovirus assembly complex and is required for efficient viral production

“…Moreover, viral protein pp65 has been reported to initially localize at the nucleus of the infected cells and at later times of infection both at the nucleus and the cytoplasm, at the viral AC compartment. 11,50,52 Upon GCV treatment, residual pUL32 and pp65 proteins that are expressed, 62,63 are restricted to their nuclear localization (Fig. 5, g,i and p,r).…”

Limits and patterns of cytomegalovirus genomic diversity in humans

“…Additional therapeutics have been developed, including the terminase inhibitor letermovir and at least one neutralizing monoclonal antibody against gH. However, resistance against these compounds has been generated in laboratory experiments (5,53), reinforcing the need to design therapeutics with reduced probability of resistance. The current study has revealed the existence of small, highly conserved, and thus likely highly constrained, sites across the HCMV genome, thereby highlighting potential therapeutic targets.…”

Treatment of CMV infection after allogeneic hematopoietic stem cell transplantation

“…Member of the new antiviral class of quinazolines, it acts after viral DNA synthesis by inhibiting the subunit protein pUL56 that, together with pUL89, is a key element of the enzyme complex named terminase, [72,73] directly involved in the cleavage and package of viral DNA chains in the virionic capside [74,75]. Because of its distinct mechanism of action, it does not show cross-resistance with other antiviral drugs and there are reports of clinically relevant activity against GCV-, FOS-and CDF-resistant CMV strains [76,77].…”