Antiviral drug resistance and helicase-primase inhibitors of herpes simplex virus

Efficacy and Safety of Amenamevir, a Helicase-Primase Inhibitor for the Treatment of Acyclovir-Resistant Herpes Simplex Virus 1 Keratitis

PURPOSE

The purpose of this study was to describe the efficacy and tolerance of amenamevir (AMNV), an inhibitor of the viral helicase-primase, for the treatment of recalcitrant herpes simplex keratitis (HSK) caused by acyclovir-resistant (ACVR) herpes simplex virus 1 strains.

METHODS

In this retrospective case series, 6 consecutive patients with HSK caused by an ACVR herpes simplex virus 1 strain with a failure of conventional antiviral therapy were included after having been treated with AMNV (there was no control group of comparable patients for whom previous treatment would have been continued despite its inefficacy). Medical files were assessed for clinical data including reason(s) for AMNV introduction (frequent recurrences despite appropriate preventive antiviral treatment and/or clinical resistance to suppressive antiviral treatment of an ongoing clinical relapse), genotypical resistance to herpes simplex virus 1 documentation, immune status, clinical types and number of HSK episodes before and during AMNV treatment, adverse effects observed during AMNV treatment, and best corrected visual acuity.

RESULTS

Of 6 patients, 4 (66%) did not experience a single recurrence during AMNV therapy while 2 others had recurrences (1 over 24 months of treatment and 2 over 23 months, ie two-fold less frequently than with conventional preventive treatment). On the overall history of these 6 patients, AMNV appeared to be associated with a reduction in HSK recurrences, with a mean of only 0.02 ± 0.04 episodes/month during follow-up under AMNV as compared to 0.14 ± 0.04 episodes/month in the year preceding AMNV introduction (P = 0.03). Improvement in vision acuity was also observed (mean best corrected visual acuity 0.17 ± 0.12 logarithm of the minimum angle of resolution at the end of follow-up vs. 0.30 ± 0.35 before AMNV onset), albeit nonsignificant probably due to the limited number of patients (P = 0.38). Neither clinical nor biological adverse effects were observed while under AMNV during the follow-up (16.5 ± 5.8 months).

CONCLUSIONS

Although there was no control group, AMNV may be a valuable option to reduce ACVR HSK recurrences.

Salvage Treatment of Refractory HSV Oral Lesions with Pritelivir in Allogeneic Hematopoietic Cell Transplant Recipients

We present two allogeneic hematopoietic cell transplantation recipients (HCTr) treated with pritelivir for acyclovir-resistant/refractory (r/r) HSV infection based on the expanded access program of the pritelivir manufacturer. Outpatient treatment with pritelivir was administered, with partial response by week 1 of treatment and complete response by week 4 of treatment in both patients. No adverse events were noted. Pritelivir appears to be an effective and safe option for the management of acyclovir-r/r HSV infections in highly immunocompromised patients in an outpatient setting.

Resistant herpes simplex virus infections - who, when, and what's new?

PURPOSE OF REVIEW

This review summarizes the literature on acyclovir resistant herpes infections and the most recent data pertinent to diagnosis and treatment in the immunocompromised patient population.

RECENT FINDINGS

Although fairly rare, acyclovir resistant herpes infections can be challenging to diagnose. Clinicians should be aware of this entity when facing refractory herpes infections. With updated diagnostics, the diagnosis is usually made through viral culture and sequencing. Therapeutic choices depend on the extent of disease. Topical therapy may be appropriate for mucocutaneous disease. Intravenous antiviral therapies such as foscarnet and cidofovir may be necessary for disseminated, ophthalmologic, central nervous system, or visceral disease. Experimental therapies such as pritelivir are in clinical trials.

SUMMARY

Immunosuppressed patients are at risk for developing acyclovir-resistant herpes, which can be challenging to diagnose and treat, although emerging therapeutic options look promising.

Discovery, Chemistry, and Preclinical Development of Pritelivir, a Novel Treatment Option for Acyclovir-Resistant Herpes Simplex Virus Infections

When the nucleoside analogue acyclovir was introduced in the early 1980s, it presented a game-changing treatment modality for herpes simplex virus infections. Since then, work has been ongoing to improve the weaknesses that have now been identified: a narrow time window for therapeutic success, resistance in immunocompromised patients, little influence on frequency of recurrences, relatively fast elimination, and poor bioavailability. The present Drug Annotation focuses on the helicase–primase inhibitor pritelivir currently in development for the treatment of acyclovir-resistant HSV infections and describes how a change of the molecular target (from viral DNA polymerase to the HSV helicase–primase complex) afforded improvement of the shortcomings of nucleoside analogs. Details are presented for the discovery process leading to the final drug candidate, the pivotal preclinical studies on mechanism of action and efficacy, and on how ongoing clinical research has been able to translate preclinical promises into clinical use.

Pseudorabies Virus Infection Triggers NF-κB Activation via the DNA Damage Response but Actively Inhibits NF-κB-Dependent Gene Expression

The nuclear factor kappa B (NF-κB) pathway is known to integrate signaling associated with very diverse intra- and extracellular stressors, including virus infections, and triggers a powerful (proinflammatory) response through the expression of NF-κB-regulated genes. Typically, the NF-κB pathway collects and transduces threatening signals at the cell surface or in the cytoplasm leading to nuclear import of activated NF-κB transcription factors. In the current work, we demonstrate that the swine alphaherpesvirus pseudorabies virus (PRV) induces a peculiar mode of NF-κB activation known as "inside-out" NF-κB activation. We show that PRV triggers the DNA damage response (DDR) and that this DDR response drives NF-κB activation since inhibition of the nuclear ataxia telangiectasia-mutated (ATM) kinase, a chief controller of DDR, abolished PRV-induced NF-κB activation. Initiation of the DDR-NF-κB signaling axis requires viral protein synthesis but occurs before active viral genome replication. In addition, the initiation of the DDR-NF-κB signaling axis is followed by a virus-induced complete shutoff of NF-κB-dependent gene expression that depends on viral DNA replication. In summary, the results presented in this study reveal that PRV infection triggers a noncanonical DDR-NF-κB activation signaling axis and that the virus actively inhibits the (potentially antiviral) consequences of this pathway, by inhibiting NF-κB-dependent gene expression. IMPORTANCE The NF-κB signaling pathway plays a critical role in coordination of innate immune responses that are of vital importance in the control of infections. The current report generates new insights into the interaction of the alphaherpesvirus pseudorabies virus (PRV) with the NF-κB pathway, as they reveal that (i) PRV infection leads to NF-κB activation via a peculiar "inside-out" nucleus-to-cytoplasm signal that is triggered via the DNA damage response (DDR), (ii) the DDR-NF-κB signaling axis requires expression of viral proteins but is initiated before active PRV replication, and (iii) late viral factor(s) allow PRV to actively and efficiently inhibit NF-κB-dependent (proinflammatory) gene expression. These data suggest that activation of the DDR-NF-κB during PRV infection is host driven and that its potential antiviral consequences are actively inhibited by the virus.

A helicase-primase drug candidate with sufficient target tissue exposure affects latent neural herpes simplex virus infections

More than 50% of the world population is chronically infected with herpesviruses. Herpes simplex virus (HSV) infections are the cause of herpes labialis (cold sores), genital herpes, and sight-impairing keratitis. Less frequently, life-threatening disseminated disease (encephalitis and generalized viremia) can also occur, mainly in immunocompromised patients and newborns. After primary infection, HSV persists for life in a latent state in trigeminal or sacral ganglia and, triggered by diverse stimuli, disease recurs in more than 30% of patients up to several times a year. Current therapy with nucleoside analogs targeting the viral polymerase is somewhat effective but limited by poor exposure in the nervous system, and latent infections are not affected by therapy. Here, we report on an inhibitor of HSV helicase-primase with potent in vitro anti-herpes activity, a different mechanism of action, a low frequency of HSV resistance, and a favorable pharmacokinetic and safety profile. Improved target tissue exposure results in superior efficacy in preventing and treating HSV infection and disease in animal models as compared to standard of care. Therapy of primary HSV infections with drug candidate IM-250 {(S)-2-(2',5'-difluoro-[1,1'-biphenyl]-4-yl)-N-methyl-N-(4-methyl-5-(S-methylsulfon-imidoyl)thiazol-2-yl)acetamide} not only reduces the duration of disease symptoms or time to healing but also prevents recurrent disease in guinea pigs. Treatment of recurrent infections reduces the frequency of recurrences and viral shedding, and, unlike nucleosidic drugs, IM-250 remains effective for a time after cessation of treatment. Hence, IM-250 has advantages over standard-of-care therapies and represents a promising therapeutic for chronic HSV infection, including nucleoside-resistant HSV.

Current Drugs to Treat Infections with Herpes Simplex Viruses-1 and -2

Herpes simplex viruses-1 and -2 (HSV-1 and -2) are two of the three human alphaherpesviruses that cause infections worldwide. Since both viruses can be acquired in the absence of visible signs and symptoms, yet still result in lifelong infection, it is imperative that we provide interventions to keep them at bay, especially in immunocompromised patients. While numerous experimental vaccines are under consideration, current intervention consists solely of antiviral chemotherapeutic agents. This review explores all of the clinically approved drugs used to prevent the worst sequelae of recurrent outbreaks by these viruses.

Identification of Amino Acids Essential for Viral Replication in the HCMV Helicase-Primase Complex

Promising new inhibitors that target the viral helicase-primase complex have been reported to block replication of herpes simplex and varicella-zoster viruses, but they have no activity against human cytomegalovirus (HCMV), another herpesvirus. The HCMV helicase-primase complex (pUL105-pUL102-pUL70) is essential for viral DNA replication and could thus be a relevant antiviral target. The roles of the individual subunits composing this complex remain to be defined. By using sequence alignment of herpesviruses homologs, we identified conserved amino acids in the putative pUL105 ATP binding site and in the putative pUL70 zinc finger pattern. Mutational analysis of several of these amino acids both in pUL105 and pUL70, proved that they are crucial for viral replication. We also constructed, by homology modeling, a theoretical structure of the pUL105 N-terminal domain which indicates that the mutated conserved amino acids in this domain could be involved in ATP hydrolysis.

Antiviral Therapies for Herpesviruses: Current Agents and New Directions

PURPOSE

The objective of this review was to summarize the recent literature describing the current burden of disease due to herpesviruses in the antiviral and transplant era; describe mechanisms of action of antiviral agents and the development of resistance; summarize the literature of recent antiviral agents brought to market as well as agents under development; and to present literature on future strategies for herpesvirus therapeutics.

METHODS

An extensive search of the medical literature related to antiherpesviral therapy was conducted to compose this narrative review. Literature searches were performed via PubMed and ultimately 137 articles were included as most relevant to the scope of this article.

FINDINGS

Herpesviruses are a family of DNA viruses that are ubiquitous throughout human populations and share the feature of establishing lifelong infections in a latent phase with the potential of periodic reactivation. With the exception of herpes simplex virus, varicella zoster virus, and Epstein-Barr virus, which have a significant disease burden in individuals with normal immune function, the morbidity and mortality of the remaining viruses are primarily associated with the immunocompromised host. Over the last half-century, several agents have been tested in large randomized, placebo-controlled trials that have resulted in safe and effective antiviral agents for the treatment of many of these infections.

IMPLICATIONS

With increasing use of antiherpesviral agents for extended periods, particularly in immunocompromised hosts, the emergence of resistant viruses has necessitated the development of newer agents with novel targets and better side-effect profiles.

No Evidence of Pritelivir Resistance Among Herpes Simplex Virus Type 2 Isolates After 4 Weeks of Daily Therapy

BACKGROUND

Pritelivir is a novel helicase-primase inhibitor in clinical development for treatment of herpes simplex virus type 2 (HSV-2) infections. In preclinical work, resistance-mediating mutations were identified in the HSV-2 genome at 3 loci in the UL5 gene and 1 locus in UL52.

METHODS

To evaluate whether daily pritelivir treatment results in emergence of resistance-mediating mutations, we analyzed HSV-2 strains detected in genital swab specimens from trial participants who were randomly assigned to receive different dosages of pritelivir. We sequenced resistance regions from 87 participants' samples, the UL5 gene in 73 samples from 44 participants, and the UL52 gene in 71 samples from 43 participants.

RESULTS

We found no evidence that pritelivir induced known resistance-mediating mutations or for amino acid variation at other loci. In one participant's HSV-2 isolate, we found a previously unidentified mutation close to the putative resistance-mediating region in UL5 and subsequently determined in vitro susceptibility to pritelivir. We characterized mutations from 32 cultivated HSV-2 isolates previously found to be susceptible to pritelivir in vitro and identified several novel mutations that most likely reflect preexisting variation in circulating HSV-2.

CONCLUSIONS

This study demonstrates evidence of retained susceptibility of HSV-2 to pritelivir in immunocompetent persons following daily therapy for up to 28 days.

Antiherpetic Plants: A Review of Active Extracts, Isolated Compounds, and Bioassays

Herpes simplex is a disease that is widely distributed throughout the world. It is caused by herpes simplex virus type 1 (HSV-1) and simplex virus type 2 (HSV-2). The drugs of choice for treatment are acyclovir (ACV), Penciclovir (PCV) and other guanine analogues, which have the same mechanism of action. However, due to the constant increase of ACV-resistant strains in immunocompromised patients, it is necessary to find new treatment alternatives. It has been shown that natural products are a good alternative for the treatment of these diseases as well as being an excellent source of compounds with anti-herpetic activity, which may be useful for the development of new drugs and act through a mechanism of action different from ACV and PCV. This paper compiles reports on extracts and compounds isolated from plants that have anti-herpetic activity. We present an analysis of the solvents most widely used for extraction from plants as well as cells and commonly used methods for evaluating cytotoxic and anti-herpetic activity. Families that have a higher number of plants with anti-herpetic activity are evaluated, and we also highlight the importance of studies of mechanisms of action of extracts and compounds with anti-herpetic activity.

High conservation of herpes simplex virus UL5/UL52 helicase-primase complex in the era of new antiviral therapies

The emergence of herpes simplex virus (HSV) resistance to current antiviral drugs, that all target the viral DNA polymerase, constitutes a major obstacle to antiviral treatment effectiveness of HSV infections, especially in immunocompromised patients. A novel and promising class of inhibitors of the HSV UL5/UL52 helicase-primase (HP) complex has been reported to hinder viral replication with a high potency. In this study, we describe the low natural polymorphism (interstrain identity >99.1% at both nucleotide and amino acid levels) of HSV HP complex subunits pUL5 and pUL52 among 64 HSV (32 HSV-1 and 32 HSV-2) clinical isolates, and we show that the HSV resistance profile to the first-line antiviral drug acyclovir (ACV) does not impact on the natural polymorphism of HSV HP complex. Genotypic tools and polymorphism data concerning HSV HP complex provided herein will be useful to detect drug resistance mutations in a relevant time frame when HP inhibitors (HPIs), i.e., amenamevir and pritelivir, will be available in medical practice.

Identification of inhibitors of Plasmodium falciparum RuvB1 helicase using biochemical assays

Human malaria is a major parasitic infection, and the situation has worsened mainly due to the emergence of resistant malaria parasites to several anti-malarial drugs. Thus, an urgent need to find suitable drug targets has led to the development of newer classes of anti-malarial drugs. Helicases have been targeted to develop therapeutics for viral, bacterial, and other microorganism infections. Recently, Plasmodium falciparum RuvB ATPases/helicases have been characterized and proposed as a suitable antimalarial drug target. In the present study, the screening of various compounds was done and the results suggest that PfRuvB1 ATPase activity is inhibited considerably by the novobiocin and partially by cisplatin and ciprofloxacin. Helicase assay of PfRuvB1 in the presence of various compounds suggest novobiocin, actinomycin, and ethidium bromide as potent inhibitors. Novobiocin inhibits the helicase activity of PfRuvB1 possibly by blocking the ATPase activity of PfRuvB1. This study is unique in respect to the identification of novobiocin as inhibitor of PfRuvB1, partially by competing with ATP binding at its active site and provides evidence for PfRuvB1 as target of novobiocin after DNA gyrase-B and HSP90. These studies will certainly help the pharmacologist to design and develop some novel inhibitor specific to PfRuvB1, which may serve as suitable chemotherapeutics to target malaria.

Copper(ii) complexes of terminally free alloferon mutants containing two histidyl binding sites inside peptide chain structure and stability

The characterization of Cu(ii) complexes with alloferon 1 mutants H1A/H12A, H1A/H9A and H1A/H6A by potentiometry, CD, UV-Vis and EPR spectroscopic techniques, and ESI-MS spectrometry is reported.

Helicase-primase as a target of new therapies for herpes simplex virus infections

The seminal discovery of acyclovir 40 years ago heralded the modern era of truly selective antiviral therapies and this drug remains the therapy of choice for herpes simplex virus infections. Yet by modern standards, its antiviral activity is modest and new drugs against novel molecular targets such as the helicase-primase have the potential to improve clinical outcome, particularly in high-risk patients. A brief synopsis of current therapies for these infections and clinical need is provided to help provide an initial perspective. The function of the helicase-primase complex is then summarized and the development of new inhibitors of the helicase-primase complex, such as pritelivir and amenamevir, is discussed. We review their mechanism of action, propensity for drug resistance, and pharmacokinetic characteristics and discuss their potential to advance current therapeutic options. Strategies that include combinations of these inhibitors with acyclovir are also considered, as they will likely maximize clinical efficacy.

Current and future therapies for herpes simplex virus infections: mechanism of action and drug resistance

Forty years after the discovery of acyclovir (ACV), it remains the mainstay of therapy for herpes simplex virus (HSV) infections. Since then, other antiviral agents have also been added to the armamentarium for these infections but ACV remains the therapy of choice. As the efficacy of ACV is reassessed, however, it is apparent that a therapy with increased efficacy, reduced potential for resistance, and improved pharmacokinetics would improve clinical outcome, particularly in high risk patients. Inhibitors of viral targets other than the DNA polymerase, such as the helicase primase complex, are of particular interest and will be valuable as new therapeutic approaches are conceived. This review focuses on currently approved HSV therapies as well as new systemic therapies in development.

Herpes simplex virus drug-resistance: new mutations and insights

PURPOSE OF REVIEW

Acyclovir (ACV) is the first-line treatment for the management of herpes simplex virus 1 (HSV-1) and 2 (HSV-2) diseases. Long-term administration of the drug for the treatment of chronic infections in the immunocompromised host can lead to the development of ACV-resistance. This review provides an update of the mutations linked to drug-resistance and issues to be considered in the management of HSV infections refractory to antiviral therapy.

RECENT FINDINGS

Recent data have shown that HSV drug-resistance should be taken into account not only in immunocompromised individuals but also in immunocompetent persons when HSV infections involve 'immune-privileged sites'. Thus, drug-resistance typing is recommended in cases of ACV unresponsive herpetic keratitis and herpes simplex encephalitis. Several issues regarding HSV drug-resistance were highlighted by recent studies. Phenotypic and genotypic antiviral resistance may vary not only from different compartments but also over time, highlighting the importance of characterizing longitudinal HSV isolates from all sites. Combination therapy should be considered when viruses with distinct phenotype/genotype are identified at one or at distinct body sites.

SUMMARY

Surveillance of HSV drug-resistance is highly recommended in immunocompromised patients and in immunocompetent individuals with infections implicating 'immune-privileged sites' to rationally adapt antiviral treatment.

Plasmodium falciparum UvrD activities are downregulated by DNA-interacting compounds and its dsRNA inhibits malaria parasite growth

Background

Human malaria parasite infection and its control is a global challenge which is responsible for ~0.65 million deaths every year globally. The emergence of drug resistant malaria parasite is another challenge to fight with malaria. Enormous efforts are being made to identify suitable drug targets in order to develop newer classes of drug. Helicases play crucial roles in DNA metabolism and have been proposed as therapeutic targets for cancer therapy as well as viral and parasitic infections. Genome wide analysis revealed that Plasmodium falciparum possesses UvrD helicase, which is absent in the human host.

Results

Recently the biochemical characterization of P. falciparum UvrD helicase revealed that N-terminal UvrD (PfUDN) hydrolyses ATP, translocates in 3’ to 5’ direction and interacts with MLH to modulate each other’s activity. In this follow up study, further characterization of P. falciparum UvrD helicase is presented. Here, we screened the effect of various DNA interacting compounds on the ATPase and helicase activity of PfUDN. This study resulted into the identification of daunorubicin (daunomycin), netropsin, nogalamycin, and ethidium bromide as the potential inhibitor molecules for the biochemical activities of PfUDN with IC₅₀ values ranging from ~3.0 to ~5.0 μM. Interestingly etoposide did not inhibit the ATPase activity but considerable inhibition of unwinding activity was observed at 20 μM. Further study for analyzing the importance of PfUvrD enzyme in parasite growth revealed that PfUvrD is crucial/important for its growth ex-vivo.

Conclusions

As PfUvrD is absent in human hence on the basis of this study we propose PfUvrD as suitable drug target to control malaria. Some of the PfUvrD inhibitors identified in the present study can be utilized to further design novel and specific inhibitor molecules.

Helicase-primase inhibitor pritelivir for HSV-2 infection

BACKGROUND

Pritelivir, an inhibitor of the viral helicase-primase complex, exhibits antiviral activity in vitro and in animal models of herpes simplex virus (HSV) infection. We tested the efficacy and safety of pritelivir in otherwise healthy persons with genital HSV-2 infection.

METHODS

We randomly assigned 156 HSV-2-positive persons with a history of genital herpes to receive one of four doses of oral pritelivir (5, 25, or 75 mg daily, or 400 mg weekly) or placebo for 28 days. Participants obtained daily swabs from the genital area for HSV-2 testing, which was performed with a polymerase-chain-reaction assay. Participants also maintained a diary of genital signs and symptoms. The primary end point was the rate of genital HSV shedding.

RESULTS

HSV shedding among placebo recipients was detected on 16.6% of days; shedding among pritelivir recipients was detected on 18.2% of days among those receiving 5 mg daily, 9.3% of days among those receiving 25 mg daily, 2.1% of days among those receiving 75 mg daily, and 5.3% of days among those receiving 400 mg weekly. The relative risk of viral shedding with pritelivir, as compared with placebo, was 1.11 (95% confidence interval [CI], 0.65 to 1.87) with the 5-mg daily dose, 0.57 (95% CI, 0.31 to 1.03) with the 25-mg daily dose, 0.13 (95% CI, 0.04 to 0.38) with the 75-mg daily dose, and 0.32 (95% CI, 0.17 to 0.59) with the 400-mg weekly dose. The percentage of days with genital lesions was also significantly reduced, from 9.0% in the placebo group to 1.2% in both the group receiving 75 mg of pritelivir daily (relative risk, 0.13; 95% CI, 0.02 to 0.70) and the group receiving 400 mg weekly (relative risk, 0.13; 95% CI, 0.03 to 0.52). The rate of adverse events was similar in all groups.

CONCLUSIONS

Pritelivir reduced the rates of genital HSV shedding and days with lesions in a dose-dependent manner in otherwise healthy men and women with genital herpes. (Funded by AiCuris; ClinicalTrials.gov number, NCT01047540.).

Helicase–primase inhibitors for herpes simplex virus: looking to the future of non-nucleoside inhibitors for treating herpes virus infections

Helicase–primase inhibitors (HPIs) are the first new family of potent herpes virus (herpes simplex and varicella-zoster virus) inhibitors to go beyond the preliminary stages of investigation since the emergence of the original nucleoside analog inhibitors. To consider the clinical future of HPIs, this review puts the exciting new findings with two HPIs, amenamevir and pritelivir, into the historical context of antiviral development for the prevention and treatment of herpes simplex virus over the last century and, on this basis, the authors speculate on the potential evolution of these and other non-nucleoside inhibitors in the future.

The DNA helicase-primase complex as a target for herpes viral infection

INTRODUCTION

The Herpesviridae are responsible for debilitating acute and chronic infections, and some members of this family are associated with human cancers. Conventional anti-herpesviral therapy targets the viral DNA polymerase and has been extremely successful; however, the emergence of drug-resistant virus strains, especially in neonates and immunocompromised patients, underscores the need for continued development of anti-herpes drugs. In this article, we explore an alternative target for antiviral therapy, the HSV helicase/primase complex.

AREAS COVERED

This review addresses the current state of knowledge of HSV DNA replication and the important roles played by the herpesvirus helicase- primase complex. In the last 10 years several helicase/primase inhibitors (HPIs) have been described, and in this article, we discuss and contrast these new agents with established inhibitors.

EXPERT OPINION

The outstanding safety profile of existing nucleoside analogues for α-herpesvirus infection make the development of new therapeutic agents a challenge. Currently used nucleoside analogues exhibit few side effects and have low occurrence of clinically relevant resistance. For HCMV, however, existing drugs have significant toxicity issues and the frequency of drug resistance is high, and no antiviral therapies are available for EBV and KSHV. The development of new anti-herpesvirus drugs is thus well worth pursuing especially for immunocompromised patients and those who develop drug-resistant infections. Although the HPIs are promising, limitations to their development into a successful drug strategy remain.

A herpes simplex virus scaffold peptide that binds the portal vertex inhibits early steps in viral replication

Previous experiments identified a 12-amino-acid (aa) peptide that was sufficient to interact with the herpes simplex virus 1 (HSV-1) portal protein and was necessary to incorporate the portal into capsids. In the present study, cells were treated at various times postinfection with peptides consisting of a portion of the Drosophila antennapedia protein, previously shown to enter cells efficiently, fused to either wild-type HSV-1 scaffold peptide (YPYYPGEARGAP) or a control peptide that contained changes at positions 4 and 5. These 4-tyrosine and 5-proline residues are highly conserved in herpesvirus scaffold proteins and were previously shown to be critical for the portal interaction. Treatment early in infection with subtoxic levels of wild-type peptide reduced viral infectivity by over 1,000-fold, while the mutant peptide had little effect on viral yields. In cells infected for 3 h in the presence of wild-type peptide, capsids were observed to transit to the nuclear rim normally, as viewed by fluorescence microscopy. However, observation by electron microscopy in thin sections revealed an aberrant and significant increase of DNA-containing capsids compared to infected cells treated with the mutant peptide. Early treatment with peptide also prevented formation of viral DNA replication compartments. These data suggest that the antiviral peptide stabilizes capsids early in infection, causing retention of DNA within them, and that this activity correlates with peptide binding to the portal protein. The data are consistent with the hypothesis that the portal vertex is the conduit through which DNA is ejected to initiate infection.

Selective anti-herpesvirus agents

This review article focuses on the anti-herpesvirus agents effective against herpes simplex virus, varicella-zoster virus and cytomegalovirus, which have either been licensed for clinical use (idoxuridine, trifluridine, brivudin, acyclovir, valaciclovir, valganciclovir, famciclovir and foscarnet) or are under clinical development (CMX001 [the hexadecyloxypropyl prodrug of cidofovir], the helicase-primase inhibitor BAY 57-1293 [now referred to as AIC316], FV-100 [the valine ester of Cf 1743] and the terminase inhibitor letermovir [AIC246]).

Antiviral agents for herpes simplex virus

This review starts with a brief description of herpes simplex virus types 1 and 2 (HSV-1 and HSV-2), the clinical diseases they cause, and the continuing clinical need for antiviral chemotherapy. A historical overview describes the progress from the early, rather toxic antivirals to acyclovir (ACV) which led the way for its prodrug, valacyclovir, to penciclovir and its prodrug, famciclovir (FCV). These compounds have been the mainstay of HSV therapy for two decades and have established a remarkable safety record. This review focuses on these compounds, the preclinical studies which reveal potentially important differences, the clinical trials, and the clinical experience through two decades. Some possible areas for further investigation are suggested. The focus shifts to new approaches and novel compounds, in particular, the combination of ACV with hydrocortisone, known as ME609 or zovirax duo, an HSV helicase-primase inhibitor, pritelivir (AIC316), and CMX001, the cidofovir prodrug for treating resistant HSV infection in immunocompromised patients. Letermovir has established that the human cytomegalovirus terminase enzyme is a valid target and that similar compounds could be sought for HSV. We discuss the difficulties facing the progression of new compounds. In our concluding remarks, we summarize the present situation including a discussion on the reclassification of FCV from prescription-only to pharmacist-controlled for herpes labialis in New Zealand in 2010; should this be repeated more widely? We conclude that HSV research is emerging from a quiescent phase.

Recent developments in anti-herpesvirus drugs

Background Herpesviruses notably establish lifelong infections, with latency and reactivation. Many of the known human herpesviruses infect large proportions of the population worldwide. Treatment or prevention of herpes infections and recurrent disease still pose a challenge in the 21st century. Sources of data Original papers and review articles, meeting abstracts, a book (Clinical Virology; DD Richman, RJ Whitley & FG Hayden eds) and company web sites. Areas of agreement For herpes simplex types 1 and 2 and for varicella zoster, acyclovir (ACV; now increasingly replaced by its prodrug valacyclovir, VACV) and famciclovir (FCV) have greatly reduced the burden of disease and have established a remarkable safety record. Drug-resistance, in the otherwise healthy population, has remained below 0.5% after more that 20 years of antiviral use. In immunocompromised patients, drug resistance is more common and alternative drugs with good safety profiles are desirable. For human cytomegalovirus disease, which occurs in immunocompromised patients, ganciclovir and increasingly its prodrug valganciclovir are the drugs of choice. However, alternative drugs, with better safety, are much needed. Areas of controversy Various questions are highlighted. Should the new 1-day therapies for recurrent herpes labialis and genital herpes replace the current standard multi-day therapies? The marked differences between VACV and FCV (e.g. triphosphate stability, effect on latency) may not yet be fully exploited? Do current antivirals reduce post-herpetic neuralgia (PHN)? For immunocompromised patients with varicella zoster virus (VZV) disease, should the first-line treatment be FCV, not ACV or VACV? Should there be more support to explore new avenues for current antivirals, for example in possibly reducing herpes latency or Alzheimer's disease (AD)? Should primary Epstein-Barr virus (EBV) disease in adolescents be treated with antivirals? How can new compounds be progressed when the perceived market need is small but the medical need is great. FCV was reclassified from prescription-only to pharmacist-controlled for herpes labialis in New Zealand in 2010; should this be repeated more widely? This article reviews new drugs in clinical trials and highlights some of the problems hindering their progress.

The helicase-primase complex as a target for effective herpesvirus antivirals

Herpes simplex virus and varicella-zoster virus have been treated for more that half a century using nucleoside analogues. However, there is still an unmet clinical need for improved herpes antivirals. The successful compounds, acyclovir; penciclovir and their orally bioavailable prodrugs valaciclovir and famciclovir, ultimately block virus replication by inhibiting virus-specific DNA-polymerase. The helicase-primase (HP) complex offers a distinctly different target for specific inhibition of virus DNA synthesis. This review describes the synthetic programmes that have already led to two HP-inhibitors (HPI) that have commenced clinical trials in man. One of these (known as AIC 316) continues in clinical development to date. The specificity of HPI is reflected by the ability to select drug-resistant mutants. The role of HP-antiviral resistance will be considered and how the study of cross--resistance among mutants already shows subtle differences between compounds in this respect. The impact of resistance on the drug development in the clinic will also be considered. Finally, herpesvirus latency remains as the most important barrier to a therapeutic cure. Whether or not helicase primase inhibitors alone or in combination with nucleoside analogues can impact on this elusive goal remains to be seen.

Synergistic activity of amenamevir (ASP2151) with nucleoside analogs against herpes simplex virus types 1 and 2 and varicella-zoster virus

ASP2151 (amenamevir) is a helicase-primase complex inhibitor with antiviral activity against herpes simplex virus HSV-1, HSV-2, and varicella-zoster virus (VZV). To assess combination therapy of ASP2151 with existing antiherpes agents against HSV-1, HSV-2, and VZV, we conducted in vitro and in vivo studies of two-drug combinations. The combination activity effect of ASP2151 with nucleoside analogs acyclovir (ACV), penciclovir (PCV), or vidarabine (VDB) was tested via plaque-reduction assay and MTS assay, and the data were analyzed using isobolograms and response surface modeling. In vivo combination therapy of ASP2151 with valaciclovir (VACV) was studied in an HSV-1-infected zosteriform spread mouse model. The antiviral activity of ASP2151 combined with ACV and PCV against ACV-susceptible HSV-1, HSV-2, and VZV showed a statistically significant synergistic effect (P<0.05). ASP2151 with VDB was observed to have additive effects against ACV-susceptible HSV-2 and synergistic effects against VZV. In the mouse model of zosteriform spread, the inhibition of disease progression via combination therapy was more potent than that of either drugs as monotherapy (P<0.05). These results indicate that the combination therapies of ASP2151 with ACV and PCV have synergistic antiherpes effects against HSV and VZV infections and may be feasible in case of severe disease, such as herpes encephalitis or in patients with immunosuppression.

New alloferon analogues: synthesis and antiviral properties

We have extended our study on structure/activity relationship studies of insect peptide alloferon (H-His-Gly-Val-Ser-Gly-His-Gly-Gln-His-Gly-Val-His-Gly-OH) by evaluating the antiviral effects of new alloferon analogues. We synthesized 18 alloferon analogues: 12 peptides with sequences shortened from N- or C-terminus and 6 N-terminally modified analogues H-X(1)-Gly-Val-Ser-Gly-His-Gly-Gln-His-Gly-Val-His-Gly-OH, where X(1) = Phe (13), Tyr (14), Trp (15), Phg (16), Phe(p-Cl) (17), and Phe(p-OMe) (18). We found that most of the evaluated peptides inhibit the replication of Human Herpesviruses or Coxsackievirus B2 in Vero, HEp-2 and LLC-MK(2) cells. Our results indicate that the compound [3-13]-alloferon (1) exhibits the strongest antiviral activity (IC(50) = 38 μM) among the analyzed compound. Moreover, no cytotoxic activity against the investigated cell lines was observed for all studied peptides at concentration 165 μM or higher.

Characterization of virus strains resistant to the herpes virus helicase-primase inhibitor ASP2151 (Amenamevir)

ASP2151 is an antiherpes agent targeting the helicase-primase complex of herpes simplex virus (HSV)-1, HSV-2, and varicella-zoster virus (VZV). We characterized the ASP2151-resistant HSV-1 and HSV-2 variants or mutants based on findings from sequencing analysis, growth, pathogenicity, and susceptibility testing, identifying several single base-pair substitutions resulting in amino acid changes in the helicase and primase subunit of ASP2151-resistant mutants. Amino acid alterations in the helicase subunit were clustered near helicase motif IV in the UL5 helicase gene of both HSV-1 and HSV-2, while the primase subunit substitution associated with reduced susceptibility, R367H, was found in ASP2151-resistant HSV-1 mutants. However, while susceptibility in the ASP2151-resistant HSV mutants to existing antiherpes agents was equivalent to that in wild-type HSV strains, ASP2151-resistant HSV mutants showed attenuated in vitro growth capability and in vivo pathogenicity compared with the parent strains. Taken together, our present findings demonstrated that important amino acid substitutions associated with reduced susceptibilities of HSV-1 and HSV-2 to ASP2151 exist in both the helicase and primase subunits of the helicase-primase complex, and that mutations in this complex against ASP2151 might confer defects in viral replication and pathogenicity.

Progress in the development of new therapies for herpesvirus infections

Resurgent interest in antiviral drugs for the treatment of herpesvirus has led to the development of new compounds that are progressing through clinical trials. This is important because there are few therapeutic options for resistant infections and some viruses such as human cytomegalovirus remain underserved. New compounds include conventional DNA polymerase inhibitors such as valomaciclovir and cyclopropavir, as well as CMX001 that has a broad spectrum of antiviral activity that includes all the herpesviruses. It also includes compounds with new molecular targets such as maribavir (MBV), FV-100, AIC361, and AIC246. Recent advances with each of these compounds will be reviewed including their virus specificity, mechanism of action, and stage of development. The potential of these new compounds to improve clinical outcome will also be discussed.

Helicase–primase inhibitors as the potential next generation of highly active drugs against herpes simplex viruses

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.

Lavomax - new potential of the complex therapy of genital herpes

The goal of this study was to assess clinical efficacy and safety of Lavomax and to provide pathogenetic grounds for its use in the complex therapy of patients suffering from genital herpes. The study subjects were 64 patients suffering from genital herpes. Their diagnosis was verified on the basis of ELISA and PCR results. Immunoassays revealed various disorders of immune homeostasis and natural resistance. These results confirm that there are pathogenetic grounds to use Lavomax in the complex therapy of genital herpes. Complex treatment with the use of Lavomax normalizes immune homeostasis and quickly reduces clinical manifestations of herpes significantly reducing the frequency and duration of recurrences of the disease.