Since the introduction of the nucleoside analogs decades ago, treatment of herpes simplex virus (HSV) infections has not seen much innovation, except for the development of their respective prodrugs. The inhibitors of the helicase-primase complex of HSV represent a very innovative approach to the treatment of herpesvirus disease, and this article considers the development of some representatives of this class of therapeutics. The molecular and biochemical features of the helicase-primase complex are considered and the development of three inhibitors of helicase-primase, BILS 179 BS, AIC316 and ASP2151, is described. The clinical development of AIC316 is at an advanced stage and displays general safety as well as favorable, long-lasting exposures in healthy volunteers. The first efficacy data from a Phase II trial with more than 150 HSV-2-positive subjects demonstrated dose-dependent antiviral activity.

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.

Human cytomegalovirus (HCMV) remains a serious threat for immunocompromised individuals, including transplant recipients and newborns. To date, all drugs licensed for the treatment of HCMV infection and disease target the viral DNA polymerase. Although these drugs are effective, several drawbacks are associated with their use, including toxicity and emergence of drug resistance. Hence, new and improved antivirals with novel molecular targets are urgently needed. Here we report on the antiviral properties of AIC246, a representative of a novel class of low-molecular-weight compounds that is currently undergoing clinical phase II studies. The anti-HCMV activity of AIC246 was evaluated in vitro and in vivo using various cell culture assays and an engineered mouse xenograft model. In addition, antiviral properties of the drug were characterized in comparison to the current gold standard ganciclovir. We demonstrate that AIC246 exhibits excellent in vitro inhibitory activity against HCMV laboratory strains and clinical isolates, retains activity against ganciclovir-resistant viruses, is well tolerated in different cell types (median selectivity index, 18,000), and exerts a potent in vivo efficacy in a mouse xenograft model. Moreover, we show that the antiviral block induced by AIC246 is reversible and the efficacy of the drug is not significantly affected by cell culture variations such as cell type or multiplicity of infection. Finally, initial mode-of-action analyses reveal that AIC246 targets a process in the viral replication cycle that occurs later than DNA synthesis. Thus, AIC246 acts via a mode of action that differs from that of polymerase inhibitors like ganciclovir.

Antiviral drugs against DNA viruses are widely used for the management of diseases caused by infections with the Herpes viruses and have recently been introduced for Hepatitis B. There are also several emerging treatments (i.e. those that are in clinical development) and novel treatments that are still in the preclinical phase. Although the majority of emerging drugs are nucleoside analogues, there is a trend towards the development of non-nucleosidic drugs with unique mechanisms of action, in the hope that efficacy will be maximised and drug resistance and viral rebound minimised.

The vast majority of the world population is infected with at least one member of the human herpesvirus family. Herpes simplex virus (HSV) infections are the cause of cold sores and genital herpes as well as life-threatening or sight-impairing disease mainly in immunocompromized patients, pregnant women and newborns. Since the milestone development in the late 1970s of acyclovir (Zovirax), a nucleosidic inhibitor of the herpes DNA polymerase, no new non-nucleosidic anti-herpes drugs have been introduced. Here we report new inhibitors of the HSV helicase-primase with potent in vitro anti-herpes activity, a novel mechanism of action, a low resistance rate and superior efficacy against HSV in animal models. BAY 57-1293 (N-[5-(aminosulfonyl)-4-methyl-1,3-thiazol-2-yl]-N-methyl-2-[4- (2-pyridinyl)phenyl]acetamide), a well-tolerated member of this class of compounds, significantly reduces time to healing, prevents rebound of disease after cessation of treatment and, most importantly, reduces frequency and severity of recurrent disease. Thus, this class of drugs has significant potential for the treatment of HSV disease in humans, including those resistant to current medications.