Susceptibility of herpes simplex virus isolated from genital herpes lesions to ASP2151, a novel helicase-primase inhibitor

Recent advance in management of herpes simplex in Japan

Herpes simplex, a common infection caused by the herpes simplex virus (HSV), is transmitted through contact of the skin/mucous membrane and establishes latency in the sensory ganglia for the rest of the life of the host. HSV occasionally reactivates and forms blisters around the lips or genitalia in some patients. Repeated overt symptoms, and, much more frequent, subclinical reactivation in the mucosa, make the host retain anti-HSV immunity continuously, resulting in maintaining steadily elevated antibody titer at any point after infection. Clinical symptoms differ in primary infection and recurrence. Primary infections sometimes manifest as severe symptoms such as fever and lymphadenopathy in addition to blisters/erosions of the skin, gingiva, lips, and oral mucosa, while recurrent herpes is generally mild. Diagnosing typical herpes simplex is not difficult, but when the course and manifestations are typical, definitive tests to identify HSV infection are limited since serology is not useful except with primary infection. For treatment, safe and effective oral antiviral drugs are available. Patient-initiated therapy is a new method of administration labeled in Japan. Amenamevir, an inhibitor of viral helicase primase, is available in Japan and labeled in addition to herpes zoster. These new diagnostic and therapeutic tools should be used for better management of herpes simplex.

Coronavirus helicases: attractive and unique targets of antiviral drug-development and therapeutic patents

Introduction: Coronaviruses encode a helicase that is essential for viral replication and represents an excellent antiviral target. However, only a few coronavirus helicase inhibitors have been patented. These patents include drug-like compound SSYA10-001, aryl diketo acids (ADK), and dihydroxychromones. Additionally, adamantane-derived bananins, natural flavonoids, one acrylamide derivative [(E)-3-(furan-2-yl)-N-(4-sulfamoylphenyl)acrylamide], a purine derivative (7-ethyl-8-mercapto-3-methyl-3,7-dihydro-1 H-purine-2,6-dione), and a few bismuth complexes. The IC₅₀ of patented inhibitors ranges between 0.82 μM and 8.95 μM, depending upon the assays used. Considering the urgency of clinical interventions against Coronavirus Disease-19 (COVID-19), it is important to consider developing antiviral portfolios consisting of small molecules.

Areas covered: This review examines coronavirus helicases as antiviral targets, and the potential of previously patented and experimental compounds to inhibit the Severe Acute Respiratory Syndrome Coronavirus-2 (SARS-CoV-2) helicase.

Expert opinion: Small molecule coronavirus helicase inhibitors represent attractive pharmacological modalities for the treatment of coronaviruses such as SARS-CoV and SARS-CoV-2. Rightfully so, the current emphasis is focused upon the development of vaccines. However, vaccines may not work for everyone and broad-based adoption of vaccinations is an increasingly challenging societal endeavor. Therefore, it is important to develop additional pharmacological antivirals against the highly conserved coronavirus helicases to broadly protect against this and subsequent coronavirus epidemics.

Antiviral Drugs Against Herpesviruses

The discovery of the nucleoside analogue, acyclovir, represented a milestone in the management of infections caused by herpes simplex virus and varicella-zoster virus. Ganciclovir, another nucleoside analogue, was then used for the management of systemic and organ-specific human cytomegalovirus diseases. The pyrophosphate analogue, foscarnet, and the nucleotide analogue, cidofovir, have been approved subsequently and constitute the second-line antiviral drugs. However, the viral DNA polymerase is the ultimate target of all these antiviral agents with a possible emergence of cross-resistance between these drugs. Recently, letermovir that targets the viral terminase complex was approved for the prophylaxis of human cytomegalovirus infections in hematopoietic stem cell transplant recipients. Other viral targets such as the protein kinase and the helicase-primase complex are also evaluated for the development of novel potent inhibitors against herpesviruses.

Herbal Gel Formulation Developed for Anti-Human Immunodeficiency Virus (HIV)-1 Activity Also Inhibits In Vitro HSV-2 Infection

Herpes simplex virus-2 (HSV-2) infection is the most common cause of genital ulcers. The impact of ulcers also demonstrates a strong link to the human immunodeficiency virus (HIV) infection. Complications, drug resistance, and side-effects of anti-viral drugs make the treatment of HSV-2 infection challenging. Herbal medicines have shown potential against HSV-2 and HIV infections. In this context, polyherbal gel formulation comprising 50% ethanolic extracts from Acacia catechu, Lagerstroemia speciosa, Terminalia chebula and Phyllanthus emblica has been developed. The gel formulation significantly exhibited virucidal activity against both HIV-1 and HSV-2 infections with IC₅₀, 55.93 ± 5.30 µg/mL and 27.26 ± 4.87 µg/mL, respectively. It also inhibited HSV-2 attachment and penetration to the Vero cells with an IC₅₀ = 46.55 ± 1.25 µg/mL and 54.94 ± 2.52 µg/mL respectively, which were significantly lower than acyclovir. However, acyclovir is more potent in post-infection assay with an IC₅₀ = 0.065 ± 0.01 µg/mL whereas gel formulation showed an IC₅₀ = 469.05 ± 16.65 µg/mL under similar conditions. Gel formulation showed no inhibitory effect on the viability of lactobacilli, human vaginal keratinocyte cells (Vk2/E6E7), and the integrity of the Caco-2 cells monolayer. Gel formulation did not lead to any significant increase in the secretion of pro-inflammatory cytokines and mutagenic index. The proposed gel formulation may be a promising candidate microbicide for the prevention of sexually transmitted HIV-1 and HSV-2.

New Insights Into DNA Helicases as Druggable Targets for Cancer Therapy

Small molecules that deter the functions of DNA damage response machinery are postulated to be useful for enhancing the DNA damaging effects of chemotherapy or ionizing radiation treatments to combat cancer by impairing the proliferative capacity of rapidly dividing cells that accumulate replicative lesions. Chemically induced or genetic synthetic lethality is a promising area in personalized medicine, but it remains to be optimized. A new target in cancer therapy is DNA unwinding enzymes known as helicases. Helicases play critical roles in all aspects of nucleic acid metabolism. We and others have investigated small molecule targeted inhibition of helicase function by compound screens using biochemical and cell-based approaches. Small molecule-induced trapping of DNA helicases may represent a generalized mechanism exemplified by certain topoisomerase and PARP inhibitors that exert poisonous consequences, especially in rapidly dividing cancer cells. Taking the lead from the broader field of DNA repair inhibitors and new information gleaned from structural and biochemical studies of DNA helicases, we predict that an emerging strategy to identify useful helicase-interacting compounds will be structure-based molecular docking interfaced with a computational approach. Potency, specificity, drug resistance, and bioavailability of helicase inhibitor drugs and targeting such compounds to subcellular compartments where the respective helicases operate must be addressed. Beyond cancer therapy, continued and new developments in this area may lead to the discovery of helicase-interacting compounds that chemically rescue clinically relevant helicase missense mutant proteins or activate the catalytic function of wild-type DNA helicases, which may have novel therapeutic application.

Amenamevir, a novel helicase-primase inhibitor, for treatment of herpes zoster: A randomized, double-blind, valaciclovir-controlled phase 3 study

Amenamevir is a potent helicase-primase inhibitor and a novel class of antiviral agent other than nucleoside compounds, such as aciclovir, valaciclovir and famciclovir. This study is the first randomized, double-blind, valaciclovir-controlled phase 3 study to evaluate the efficacy and safety of amenamevir in Japanese patients with herpes zoster when treated within 72 h after onset of rash. A total of 751 patients were randomly assigned to receive either amenamevir 400 mg or 200 mg p.o. once daily or valaciclovir 1000 mg three times daily (daily dose, 3000 mg) for 7 days. The primary efficacy end-point was the proportion of cessation of new lesion formation by day 4 ("day 4 cessation proportion"). The day 4 cessation proportions for amenamevir 400 and 200 mg and valaciclovir were 81.1% (197/243), 69.6% (172/247) and 75.1% (184/245), respectively. Non-inferiority of amenamevir 400 mg to valaciclovir was confirmed by a closed testing procedure. Days to cessation of new lesion formation, complete crusting, healing, pain resolution and virus disappearance were evaluated as secondary end-points. No significant differences were observed in any of the treatment groups. Amenamevir 400 and 200 mg were well tolerated as well as valaciclovir. The proportions of patients who experienced drug-related adverse events were 10.0% (25/249), 10.7% (27/252) and 12.0% (30/249) with amenamevir 400 and 200 mg and valaciclovir, respectively. In conclusion, amenamevir 400 mg appears to be effective and well tolerated for treatment of herpes zoster in immunocompetent Japanese patients.

Distribution and effects of amino acid changes in drug-resistant α and β herpesviruses DNA polymerase

Emergence of drug-resistance to all FDA-approved antiherpesvirus agents is an increasing concern in immunocompromised patients. Herpesvirus DNA polymerase (DNApol) is currently the target of nucleos(t)ide analogue-based therapy. Mutations in DNApol that confer resistance arose in immunocompromised patients infected with herpes simplex virus 1 (HSV-1) and human cytomegalovirus (HCMV), and to lesser extent in herpes simplex virus 2 (HSV-2), varicella zoster virus (VZV) and human herpesvirus 6 (HHV-6). In this review, we present distinct drug-resistant mutational profiles of herpesvirus DNApol. The impact of specific DNApol amino acid changes on drug-resistance is discussed. The pattern of genetic variability related to drug-resistance differs among the herpesviruses. Two mutational profiles appeared: one favoring amino acid changes in the Palm and Finger domains of DNApol (in α-herpesviruses HSV-1, HSV-2 and VZV), and another with mutations preferentially in the 3′-5′ exonuclease domain (in β-herpesvirus HCMV and HHV-6). The mutational profile was also related to the class of compound to which drug-resistance emerged.

New strategies against drug resistance to herpes simplex virus

Herpes simplex virus (HSV), a member of the Herpesviridae family, is a significant human pathogen that results in mucocutaneous lesions in the oral cavity or genital infections. Acyclovir (ACV) and related nucleoside analogues can successfully treat HSV infections, but the emergence of drug resistance to ACV has created a barrier for the treatment of HSV infections, especially in immunocompromised patients. There is an urgent need to explore new and effective tactics to circumvent drug resistance to HSV. This review summarises the current strategies in the development of new targets (the DNA helicase/primase (H/P) complex), new types of molecules (nature products) and new antiviral mechanisms (lethal mutagenesis of Janus-type nucleosides) to fight the drug resistance of HSV.

Regulated bioanalysis of conformers - A case study with ASP2151 in dog plasma and urine

We developed and validated bioanalytical methods for a potent helicase-primase inhibitor ASP2151 that has two conformers. These conformers elute as unseparated broad peaks under ordinary high-performance liquid chromatographic conditions, indicating discernable differences in hydrophobicity. We observed that column temperature and mobile phase pH have no effect on these peaks and that conformers form a single symmetrical peak when tetrahydrofuran is added to the mobile phase. In addition, we needed to develop semi-automated methods where inter-conversion of the conformers is unlikely to cause sample-to-sample extraction variability. Briefly, following the addition of deuterium-labeled ASP2151 as an internal standard (IS), dog plasma samples or acetonitrile-added urine samples were filtrated. The filtrates were then injected into a column-switching liquid chromatography-tandem mass spectrometry (LC-MS/MS) system and trapped onto an extraction column. Extracts were back-flushed onto an analytical C18 column (4.6×50mm, 3μm) with a mobile phase consisting of methanol, tetrahydrofuran, and 20mmol/L ammonium acetate (45:5:50, v/v/v). The eluent was monitored in the negative atmospheric pressure chemical ionization mode. The calibration curve was linear over a range of 5-1000ng/mL for plasma and 0.5-100μg/mL for urine. Validation data met the acceptance criteria in accordance with regulatory guidance and demonstrated that these methods were selective, accurate, and reproducible. In addition, the present methods were successfully applied to a pharmacokinetic study in dogs.

Benzothiazole and Pyrrolone Flavivirus Inhibitors Targeting the Viral Helicase

The flavivirus nonstructural protein 3 (NS3) is a protease and helicase, and on the basis of its similarity to its homologue encoded by the hepatitis C virus (HCV), the flavivirus NS3 might be a promising drug target. Few flavivirus helicase inhibitors have been reported, in part, because few specific inhibitors have been identified when nucleic acid unwinding assays have been used to screen for helicase inhibitors. To explore the possibility that compounds inhibiting NS3-catalyzed ATP hydrolysis might function as antivirals even if they do not inhibit RNA unwinding in vitro, we designed a robust dengue virus (DENV) NS3 ATPase assay suitable for high-throughput screening. Members of two classes of inhibitory compounds were further tested in DENV helicase-catalyzed RNA unwinding assays, assays monitoring HCV helicase action, subgenomic DENV replicon assays, and cell viability assays and for their ability to inhibit West Nile virus (Kunjin subtype) replication in cells. The first class contained analogues of NIH molecular probe ML283, a benzothiazole oligomer derived from the dye primuline, and they also inhibited HCV helicase and DENV NS3-catalyzed RNA unwinding. The most intriguing ML283 analogue inhibited DENV NS3 with an IC₅₀ value of 500 nM and was active against the DENV replicon. The second class contained specific DENV ATPase inhibitors that did not inhibit DENV RNA unwinding or reactions catalyzed by HCV helicase. Members of this class contained a 4-hydroxy-3-(5-methylfuran-2-carbonyl)-2H-pyrrol-5-one scaffold, and about 20 μM of the most potent pyrrolone inhibited both DENV replicons and West Nile virus replication in cells by 50%.

Discovering new medicines targeting helicases: challenges and recent progress

Helicases are ubiquitous motor proteins that separate and/or rearrange nucleic acid duplexes in reactions fueled by adenosine triphosphate (ATP) hydrolysis. Helicases encoded by bacteria, viruses, and human cells are widely studied targets for new antiviral, antibiotic, and anticancer drugs. This review summarizes the biochemistry of frequently targeted helicases. These proteins include viral enzymes from herpes simplex virus, papillomaviruses, polyomaviruses, coronaviruses, the hepatitis C virus, and various flaviviruses. Bacterial targets examined include DnaB-like and RecBCD-like helicases. The human DEAD-box protein DDX3 is the cellular antiviral target discussed, and cellular anticancer drug targets discussed are the human RecQ-like helicases and eIF4A. We also review assays used for helicase inhibitor discovery and the most promising and common helicase inhibitor chemotypes, such as nucleotide analogues, polyphenyls, metal ion chelators, flavones, polycyclic aromatic polymers, coumarins, and various DNA binding pharmacophores. Also discussed are common complications encountered while searching for potent helicase inhibitors and possible solutions for these problems.

Pharmacokinetics and pharmacodynamics of ASP2151, a helicase-primase inhibitor, in a murine model of herpes simplex virus infection

ASP2151 (amenamevir) is a helicase-primase inhibitor against herpes simplex virus 1 (HSV-1), HSV-2, and varicella zoster virus. Here, to determine and analyze the correlation between the pharmacodynamic (PD) and pharmacokinetic (PK) parameters of ASP2151, we examined the PD profile of ASP2151 using in vitro plaque reduction assay and a murine model of HSV-1 infection. ASP2151 inhibited the in vitro replication of HSV-1 with a mean 50% effective concentration (EC(50)) of 14 ng/ml. In the cutaneously HSV-1-infected mouse model, ASP2151 dose dependently suppressed intradermal HSV-1 growth, with the effect reaching a plateau at a dose of 30 mg/kg of body weight/day. The dose fractionation study showed that intradermal HSV-1 titers were below the detection limit in mice treated with ASP2151 at 100 mg/kg/day divided into two daily doses and at 30 or 100 mg/kg/day divided into three daily doses. The intradermal HSV-1 titer correlated with the maximum concentration of drug in serum (C(max)), the area under the concentration-time curve over 24 h (AUC(24h)), and the time during which the concentration of ASP2151 in plasma was above 100 ng/ml (T(>100)). The continuous infusion of ASP2151 effectively decreased intradermal HSV-1 titers below the limit of detection in mice in which the ASP2151 concentration in plasma reached 79 to 145 ng/ml. Our findings suggest that the antiviral efficacy of ASP2151 is most closely associated with the PK parameter T(>100) in HSV-1-infected mice. Based on these results, we propose that a plasma ASP2151 concentration exceeding 100 ng/ml for 21 to 24 h per day provides the maximum efficacy in HSV-1-infected mice.