Mode of action of 9-(4-hydroxy-3-hydroxymethylbut-1-yl)guanine (BRL 39123) against herpes simplex virus in MRC-5 cells

Antiviral Chemotherapy in Avian Medicine-A Review

This review article describes the current knowledge about the use of antiviral chemotherapeutics in avian species, such as farm poultry and companion birds. Specific therapeutics are described in alphabetical order including classic antiviral drugs, such as acyclovir, abacavir, adefovir, amantadine, didanosine, entecavir, ganciclovir, interferon, lamivudine, penciclovir, famciclovir, oseltamivir, ribavirin, and zidovudine, repurposed drugs, such as ivermectin and nitazoxanide, which were originally used as antiparasitic drugs, and some others substances showing antiviral activity, such as ampligen, azo derivates, docosanol, fluoroarabinosylpyrimidine nucleosides, and novel peptides. Most of them have only been used for research purposes and are not widely used in clinical practice because of a lack of essential pharmacokinetic and safety data. Suggested future research directions are also highlighted.

Effectiveness comparisons of antiviral treatments for Bell palsy: a systematic review and network meta-analysis

OBJECTIVES

Antiviral treatments for Bell palsy have been widely used, but there is no definite conclusion of which is the most effective antiviral drug. We conducted a systematic review of randomized controlled trials (RCTs) including network meta-analysis to investigate the comparative effectiveness of antiviral treatments for Bell palsy.

DATA

RCTs comparing effectiveness between antiviral treatments and placebo were included. Risk of bias within and across studies was assessed with the Cochrane tool and the GRADE approach, respectively. Random-effects pairwise meta-analyses were conducted, followed by network meta-analysis.

SOURCES

Three electronic databases were searched from inception to May 18, 2020.

STUDY SELECTION

11 trials and 3393 patients with four arms and eleven contrasts were included.

RESULTS

Significant differences were observed between placebo and famciclovir with respect to overall recovery and no statistically significant differences were found from other comparisons. Treatment ranking based on the evidence network indicated that famciclovir shared the best results, followed by valacyclovir, acyclovir, and finally placebo. Adverse events of famciclovir were too rare and slight to be analyzed. Our confidence in pairwise comparisons was moderate to low, due to study limitations, inconsistency, and imprecision; our confidence in ranking was moderate, due to study limitations. Inconsistency is not deemed to exist by a loop-specific approach and node-splitting procedure. Results of exploring publication bias are satisfying.

CONCLUSIONS

According to pairwise and network comparisons, famciclovir could be better than placebo and the effectiveness of other antiviral treatments are similar. For clinical efficacy, famciclovir obtains the best recovery rate of facial function for Bell palsy. Acyclovir has the lowest rate of synkinesis, though, it is not adequately recommended and more superior trails are needed in the future.

Evaluation of drug repositioning by molecular docking of pharmaceutical resources available in the Brazilian healthcare system against SARS-CoV-2

In 2020 SARS-CoV-2 reached pandemic status, reaching Brazil in mid-February. As of now, no specific drugs for treating the disease are available. In this work, the possibility of interaction between SARS-CoV-2 viral proteins (open and closed spike protein, isolate spike protein RBD, NSP 10, NSP 16, main protease, and RdRp polymerase) and multiple molecules is addressed through the repositioning of drugs available for the treatment of other diseases that are approved by the FDA and covered by SUS, the Brazilian Public Health System. Three different docking software were used, followed by a unification of the results by independent evaluation. Afterwards, the chemical interactions of the compounds with the targets were inspected via molecular dynamics and analyzed. The results point to a potential effectiveness of Penciclovir, Ribavirin, and Zanamivir, from a set of 48 potential candidates. They may also be multi-target drugs, showing high affinity with more than one viral protein. Further in vitro and in vivo validation is required to assess the suitability of repositioning the proposed drugs for COVID-19.

Current Antivirals and Novel Botanical Molecules Interfering With Herpes Simplex Virus Infection

Herpes simplex viruses type 1 (HSV-1) and type 2 (HSV-2) are highly prevalent within the human population and are characterized by lifelong infections and sporadic recurrences due to latent neuron infection. Upon reactivations, HSVs may manifest either, symptomatically or asymptomatically and be shed onto others through mucosae body fluids. Although, HSVs can produce severe disease in humans, such as life-threatening encephalitis and blindness, the most common symptoms are skin and mucosal lesions in the oro-facial and the genital areas. Nucleoside analogs with antiviral activity can prevent severe HSV infection, yet they are not very effective for treating skin manifestations produced by these viruses, as they only reduce in a few days at most the duration of lesions. Additionally, HSV variants that are resistant to these antivirals may arise, especially in immunosuppressed individuals. Thus, new antivirals that can reduce the severity and duration of these cutaneous manifestations would certainly be welcome. Here, we review currently available anti-herpetic therapies, novel molecules being assessed in clinical trials and new botanical compounds reported in the last 20 years with antiviral activities against HSVs that might represent future treatments against these viruses.

IL-36γ induces a transient HSV-2 resistant environment that protects against genital disease and pathogenesis

Herpes simplex virus 2 (HSV-2) causes a persistent, lifelong infection that increases risk for sexually transmitted infection acquisition. Both the lack of a vaccine and the need for chronic suppressive therapies to control infection presents the need to further understand immune mechanisms in response to acute HSV-2 infection. The IL-36 cytokines are recently identified members of the IL-1 family and function as inflammatory mediators at epithelial sites. Here, we first used a well-characterized three-dimensional (3-D) human vaginal epithelial cell (VEC) model to understand the role of IL-36γ in the context of HSV-2 infection. In 3-D VEC, IL-36γ is induced by HSV-2 infection, and pretreatment with exogenous IL-36γ significantly reduced HSV-2 replication. To assess the impact of IL-36γ treatment on HSV-2 disease pathogenesis, we employed a lethal genital infection model. We showed that IL-36γ treatment in mice prior to lethal intravaginal challenge significantly limited vaginal viral replication, delayed disease onset, decreased disease severity, and significantly increased survival. We demonstrated that IL-36γ treatment transiently induced pro-inflammatory cytokines, chemokines, and antimicrobial peptides in murine lower female reproductive tract (FRT) tissue and vaginal lavages. Induction of the chemokines CCL20 and KC in IL-36γ treated mice also corresponded with increased polymorphonuclear (PMN) leukocyte infiltration observed in vaginal smears. Altogether, these studies demonstrate that IL-36γ drives the transient production of immune mediators and promotes PMN recruitment in the vaginal microenvironment that increases resistance to HSV-2 infection and disease. Our data indicate that IL-36γ may participate as a key player in host defense mechanisms against invading pathogens in the FRT.

Comparison of Acyclovir and Famciclovir for Ramsay Hunt Syndrome

OBJECTIVE

Although antiviral agents are widely used to treat Ramsay hunt syndrome (RHS), their relative effectiveness has not been assessed. This study retrospectively compared clinical outcomes in patients with RHS treated with the antiviral agents acyclovir and famciclovir.

PATIENTS AND METHODS

This study involved 227 patients diagnosed with RHS from 2003 to 2015. Patients were treated with prednisolone plus acyclovir (n = 102) or famciclovir (n = 125). Patient outcomes were measured using the House-Brackmann scale according to age, initial severity of disease, electroneurography, and underlying disease.

RESULTS

Based on complications (p = 0.019) and disease severity (p = 0.013), the overall complete recovery rate was significantly higher with famciclovir than with acyclovir, whereas rates of recovery in patients with severe (p = 0.111) and initially moderate (grades III-IV; p = 0.070) facial palsy were similar. Electroneurography also showed no difference in remission rate between the two groups (p = 0.692). Complete recovery rates in patients with hypertension and/or diabetes mellitus were similar in the two groups. However, the complete recovery rate of patients without hypertension and diabetes was significantly higher in patients treated with famciclovir than acyclovir (p = 0.018).

CONCLUSION

Recovery rates in patients with RHS were higher following treatment with steroid plus famciclovir than with steroid plus acyclovir, especially in patients without hypertension and diabetes mellitus.

Antiherpetic Drugs in Equine Medicine

Since vaccination may not prevent disease, antiherpetic drugs have been investigated for the therapy of several equine herpesviruses. Drug efficacy has been assessed in horses with disease, but most evidence is in vitro, in other species, or empirical. Oral valacyclovir is most often administered in the therapy of equine herpesvirus type-1 (EHV-1) to protect adult horses from equine herpesvirus myeloencephalopathy, while oral acyclovir is frequently administered for EHV-5 infection in the therapy of equine multinodular pulmonary fibrosis. Other antiherpetic drugs are promising but require further investigation. Several topical drugs are also empirically used in the therapy of equine viral keratitis.

The Influence of Cyclosporin Immunosuppression on the Efficacy of Famciclovir or Valaciclovir Chemotherapy Studied in a Murine Herpes Simplex Virus Type 1 Infection Model

Mice with or without immunosuppression by cyclosporin (Cy) were inoculated with herpes simplex virus type 1 in the ear pinna. Without immunosuppression, 20% of the mice died; clinical signs resolved in survivors and infectious virus was cleared by 7 to 10 days post-inoculation (p.i.). With Cy, mortality was 50%, clinical signs increased and infectious virus persisted. Mice were treated with either valaciclovir (VACV) or famciclovir (FCV) from days 1–5 or 5–10 p.i. and both compounds moderated the disease, but only FCV led to rapid restoration of body weight and complete protection from mortality. Resolution of clinical signs was more marked with immunosuppression. On cessation of VACV therapy, infectious virus recurred on individual days. Without immunosuppression, recurrence was detected in neural tissues only, but with Cy, infectious virus also recurred in skin. No recurrences of infectious virus were observed in any FCV-treated mice.

Famciclovir and Penciclovir. The Mode of Action of Famciclovir Including Its Conversion to Penciclovir

Famciclovir is the oral form of penciclovir, a highly selective antiherpesvirus agent. Both famciclovir and penciclovir are being evaluated in clinical studies. This review covers the conversion of famciclovir to penciclovir, the formation and stability of penciclovir-triphosphate, its effect on herpesvirus DNA polymerases and the inhibition of viral DNA synthesis in cell cultures.

The conversion of famciclovir into penciclovir is described, emphasising work with human tissues and including the elucidation of the structures of its chiral monoacetylated derivatives. To reflect its metabolism during oral absorption, famciclovir was incubated in duodenal contents, extracts of intestinal wall and liver, and in blood. The major metabolic route was by de-acetylation followed by oxidation of the purine to form penciclovir. This work with human tissues was predictive of the major metabolic pathway in humans.

Penciclovir is phosphorylated much more efficiently than acyclovir in herpesvirus-infected cells yet the host cell kinases phosphorylate the two compounds to a small but comparable extent. This highly preferential metabolism in herpesvirus infected cells is a major factor in its selective antiviral activity. The ( S) enantiomer of penciclovir-triphosphate is the major metabolite and it has good stability in HSV-1, HSV-2, and VZV infected cells. At the high concentrations found in infected cells, penciclovir-triphosphate was an effective inhibitor of viral DNA polymerases and of viral DNA synthesis. The efficient entrapment of the active moiety can account for the markedly better antiviral activity of penciclovir than acyclovir when infected cell cultures were treated for a short time. Such assays reflect more closely the clinical situation after oral dosing than do standard assays in which the test compounds are present continuously. The role of these studies in compound selection and development is discussed.

The Effects of Delayed-Onset Chemotherapy using Famciclovir or Valaciclovir in a Murine Immunosuppression Model for HSV-1

BALB/c mice were immunosuppressed using a dosing regimen of cyclosporin-A previously shown to interfere with T-cell function. HSV-1 was then inoculated into the skin of the ear pinna and the infection allowed to progress for 5 days. In untreated mice, virus replication was observed in both ear tissue and the central nervous system (brainstem). Virus replication continued at high titre in both sites for 15 days, the duration of the experiment. This model was used to investigate the efficacy of famciclovir (FCV) and vaiaciclovir (VACV), which are the oral prodrugs of penciclovir and acyclovir, respectively, on an established HSV-1 infection. Mice were treated orally with either FCV or VACV at 50 mg kg−1, from day 5 to day 10 post-infection. Within 2 days of the onset of therapy, virus titres in both skin and neural tissues were reduced significantly, and by day 10 post-infection less than 101 p.f.u. per tissue was detectable compared with approximately 104 p.f.u. in the untreated control mice. However, when VACV therapy was stopped, there was a recurrence of infectious virus replication within the ear pinna and the brainstem 2 days later. No recurrence of virus replication was observed in mice that had been treated with FCV. In addition, FCV treatment was superior to VACV at reducing the incidence of clinical signs and disease. Furthermore, mice receiving FCV gained weight more quickly than infected controls and reached uninoculated control weights by the end of the study period. In contrast, VACV had no such beneficial effects. These results confirm that VACV and FCV have different effects on the pathogenesis of HSV. The reasons for this appear to result from critical differences in the mechanism of action of the two compounds at a cellular level. We suggest that these data have important implications for the treatment of severe herpesvirus infections in man.

Progress in the Clinical Management of Herpesvirus Infections

Antiviral drug discovery has produced a series of drugs active against herpesviruses in vitro. Several of these are now licensed and/or have been used in clinical practice. This article reviews the mechanisms of action of acyclovir, ganciclovir, penciclovir, sorivudine and foscarnet, the development of resistance to these drugs and their pharmacokinetic and cellular toxicities. Based upon the natural histories of HSV, VZV and CMV, treatment objectives for each virus are discussed and the performance of each drug matched against these objectives. Overall, it is concluded that the perfect drug for treating herpesviruses does not exist, but that significant progress has been made towards controlling several herpesvirus diseases. It is suggested that further progress will require not just improved drug discovery programmes, but also an understanding of different pathogeneses and an appreciation by practising physicians that antiviral drugs must be given early in the infectious process to achieve the best results.

Inhibitory Effect of Penciclovir-Triphosphate on Duck Hepatitis B Virus Reverse Transcription

The aim of this study was to investigate the mechanism of inhibition of hepatitis B virus replication by penciclovir-triphosphate, the active metabolite of famciclovir. A recently developed in vitro translation assay for the expression of an enzymatically active duck hepatitis B virus (DHBV) reverse transcriptase was used to assess the inhibitory activity of penciclovir-triphosphate (PCV-TP) in comparison with other guanosine analogue triphosphates. Acyclovir-triphosphate (ACV-TP), the chiral triphosphates of penciclovir (PCV), ( R)-PCV-TP and ( S)-PCV-TP, and carbocyclic 2′-deoxyguanosine-TP (CDG-TP) did inhibit reproducibly minus strand DNA synthesis to different extents. CDG-TP was the most potent inhibitor of dGTP incorporation. The inhibitory effect of these compounds against the incorporation of the first nucleotide of minus strand DNA, dGMP, was similar to that observed with DNA chain elongation. 2′,3′-dideoxyguanosine-TP (ddG-TP), ACV-TP and both ( R) and ( S)-PCV-TP inhibited the incorporation of the next nucleotides in the short DNA primer, whereas CDG-TP did not. These results demonstrate that PCV-TP inhibits hepadnavirus reverse transcription by inhibiting the synthesis of the short DNA primer. The data obtained with the inhibition of the enzymatic activity of the DHBV polymerase provides a new insight into the mechanism of action of penciclovir-triphosphate on HBV replication.

Inhibition of Varicella-Zoster Virus by Penciclovir in Cell Culture and Mechanism of Action

The effect of penciclovir (BRL 39123) on the replication of varicella-zoster virus (VZV) in human embryonic lung fibroblasts (MRC-5 cells) was similar to aciclovir when the compounds were present continuously. However, when the compounds were withdrawn the antiviral activity of penciclovir was maintained more effectively than that of aciclovir. In the plaque reduction assay, median 50% effective concentrations (EC50s) were 3.8 μg ml−1 for penciclovir and 4.2 μg ml−1 for aciclovir ( n = 29 clinical isolates). Similarly, penciclovir and aciclovir were equally effective in reducing the numbers of VZV-infected MRC-5 cells and in reducing VZV DNA synthesis within infected cells following continuous treatment. Within VZV-infected cells (S)-penciclovir-triphosphate was formed from penciclovir with >95% enantiomeric purity, and the concentration of penciclovir-triphosphate was 360-fold greater than aciclovir-triphosphate immediately after treatment. This phosphorylation ratio compensates for the lower affinity of VZV DNA polymerase for penciclovir-triphosphate compared with aciclovir-triphosphate (Kis = 7.5 μM and 0.2 μM, respectively). When VZV-infected cultures were treated for 3 days, followed by withdrawal of the compound, inhibition of viral DNA synthesis by penciclovir was maintained for 24 h, whereas viral DNA synthesis resumed more readily after removal of aciclovir. Furthermore, following 8 h daily pulse treatment for 5 days, penciclovir was significantly more active than aciclovir in reducing VZV DNA synthesis ( p = 0.006, n = 10 clinical isolates). The long intracellular half-life of penciclovir-triphosphate (9.1 h) compared with that of aciclovir-triphosphate (0.8 h) accounts for the sustained inhibition of virus replication by penciclovir. This property may contribute to the clinical efficacy of famciclovir, the oral form of penciclovir.

Effects of Penciclovir and Famciclovir in a Murine Model of Encephalitis Induced by Intranasal Inoculation of Herpes Simplex Virus Type 1

Mice inoculated intranasally with a potentially lethal dose of HSV-1 (strain SC16) were used to evaluate the efficacy of penciclovir (PCV) and its oral form, famciclovir (FCV) in the treatment of acute herpes encephalitis. In a comparative experiment, orally administered PCV was less effective than ganciclovir (GCV) in reducing mortality of infected mice, but was of similar effectivenss to acyclovir (ACV), Late deaths from ongoing infections were observed following cessation of treatment with both ACV and FCV. Delaying the onset of treatment by more than 1 day post infection markedly reduced the ability of orally administered ACV, PCV, and FCV to prevent death. Treatment from day 1 with FCV administered ad libitum in the drinking water resulted in rapid clearance of infectious virus from all parts of the brain, but latent virus was detected by Southern blot hybridization in the peripheral and central nervous system of survivors 1 month post infection. In untreated mice, infectious virus was also detected in the epithelium of the turbinate bones and in the lungs from 1 day after intranasal inoculation, and acute inflammatory changes were seen in the lungs. FCV was more effective in clearing virus from the lungs than from the turbinate bones in these mice. These data, concerning the importance of virus replication in the respiratory tract with regard to subsequent encephalitis, are considered to be a novel aspect of this study.

Penciclovir Pharmacokinetics and Distribution to the Brain and Muscle of Rats, Studied by Microdialysis

Famciclovir, the oral form of penciclovir, is a potent, highly selective antiherpesvirus agent licenced for the treatment of herpes zoster (shingles). Some herpesviruses are prone to infect the central nervous system. To obtain guidance for the possible treatment of herpes encephalitis it is important to study the extent of transport of antiviral agents into the brain. We have used microdialysis to sample the unbound extracellular concentration of penciclovir in the gastrocnemic muscle (which corresponds directly to plasma free concentrations) and in the brain of rats under halothane anaesthesia. Penciclovir (50 mg kg−1) was given intravenously (i.v.) and samples were taken for 5 h after administration. The AUC (area under the time versus concentration curve) (0–5 h) of penciclovir in the brain was 0.096±0.018 (mean±SEM) of the AUC in muscle while the mean ratio of brain to muscle concentration 5 h post-injection was 0.1 80±0.084. Famciclovir given per os to rat at a dose of 1 20 mg kg−1 resulted in a concentration ratio for penciclovir between brain and muscle of 0.415±0.078 at 5 h after administration, while the AUC ratio (0–5 h) was 0.143±0.012. Both of these are higher than after i.v. injection of penciclovir. Penciclovir and famciclovir were also administrated by i.v. infusion (60 and 80 mg kg−1 h−1 respectively). Famciclovir administration (AUC 0.075±0.025 mmol h L−1) did not increase penciclovir transport to the brain compared with penciclovir administration (AUC 0.163±0.018 mmol h L−1).

Antiviral Activities and Pharmacokinetics of Penciclovir and Famciclovir in Pekin Ducks Chronically Infected with Duck Hepatitis B Virus

Famciclovir (FCV) is the oral form of the potent and selective antiherpesvirus agent penciclovir (PCV). In order to provide more information on the spectrum of activity of PCV, the activities of FCV and PCV against duck hepatitis B virus (DHBV) in chronically infected ducks were examined. As part of this investigation, the oral pharmacokinetics of FCV and PCV were determined in ducks. Oral treatment of DHBV-infected ducks (twice daily for 21 days) with either PCV (20 mg kg−1 or 100 mg kg−1) or FCV (5 mg kg−1 or 25 mg kg−1) suppressed virus replication, as measured by both plasma viral DNA and DNA polymerase levels. Both markers were reduced to undetectable levels within 4 days of the start of treatment, and after the cessation of treatment there was a delay of 2 to 8 days before plasma DHBV DNA and DNA polymerase levels began to increase, indicating that virus replication had resumed. The demonstration of efficacy of PCV and its oral form FCV against DHBV suggested that the two compounds may have clinical benefits. against human hepatitis B virus. Clinical trials are currently ongoing.

Further Characterization of the Potent and Prolonged Inhibition of Herpes Simplex Virus Replication in Human Cell Lines by Penciclovir

The replication of herpes simplex virus type 1 (HSV-1) or HSV-2 in MRC-5 cells infected at 0.01 pfu cell−1 treated continuously for 72 h, was inhibited more efficiently by penciclovir than aciclovir ( p = 0.0001). However, multiple cycles of replication were required in order to distinguish the compounds. Virus from cultures treated for 72 h with either compound, at 3 or 10 μg ml−1 was resistant to penciclovir and aciclovir (50% effective concentrations > 10 μg ml−1), but infectivity titres of supernatants from these aciclovirtreated cultures were higher than for penciclovir. Increased production of resistant virus in aciclovirtreated cultures may be the consequence of the less potent inhibition of virus replication by aciclovir. Penciclovir caused prolonged inhibition of HSV-1 and HSV-2 replication in three human cell lines infected at 1 pfu cell−1 following treatment for 18 h, whereas virus replication resumed rapidly after withdrawal of aciclovir. Neither compound showed prolonged activity after 18 h treatment, when the multiplicity of infection was reduced to 0.01 pfu cell−1. This surprising observation prompted experiments testing the effect of repeated pulse treatment in cultures infected at low multiplicity. Penciclovir inhibited HSV-1 replication significantly more effectively than aciclovir in MRC-5 cells infected at 10−4 pfu cell−1 treated daily for 6 h ( p < 0.001, n = 5) but only a trend was observed for HSV-2 ( p = 0.06, n = 6).

Penciclovir-Resistance Mutations in the Herpes Simplex Virus DNA Polymerase Gene

Penciclovir is the active form of the orally available prodrug famciclovir, which is entering clinical use for herpesvirus infections. Like aciclovir, penciclovir is an acyclic guanosine analogue that is phosphorylated by viral thymidine kinase and whose triphosphate can inhibit viral DNA polymerase. We tested several well-characterized herpes simplex virus mutants with aciclovir-resistance mutations in the viral DNA polymerase gene for altered sensitivity to penciclovir. The mutants varied in their susceptibilities to penciclovir with one exhibiting 2-fold hypersensitivity, one marginal resistance and three about 3-fold resistance. Marker rescue and DNA sequencing analyses mapped the penciclovir-resistance mutation of one mutant, AraA r7, to a single base change that alters a glycine to a cysteine at residue 841 within conserved region III of α-like DNA polymerases. The results have implications for the mechanism of selective action of penciclovir, for the potential for development of resistance in the clinic, and for the substrate recognition properties of herpes simplex virus DNA polymerase.

A case series using famciclovir in stem cell transplant recipients with valacyclovir hypersensitivity reactions

Background

Herpes simplex virus and varicella zoster virus reactivation can occur in up to 32% and 40% of patients, respectively in the first year post-transplant without prophylaxis. Antiviral therapy consisting of acyclovir or valacyclovir is recommended for at least 1 year post stem cell transplant per evidence-based guidelines.

Objective

In the event of a contraindication or hypersensitivity reaction to either drug, an alternative is essential based on the proven efficacy in reducing clinically significant herpes simplex virus and varicella zoster virus reactivations. We report two cases of successful initiation of famciclovir in stem cell transplant recipients experiencing hypersensitivity to valacyclovir or acyclovir.

Discussion

In both cases, there were no hypersensitivity reactions or breakthrough viral infections after famciclovir initiation but this observation is limited by a small patient population.

Conclusion

Due to the limited data available, famciclovir appears to be a reasonable option in immunocompromised patients with a mild valacyclovir or acyclovir hypersensitivity reaction.

A designed equine herpes thymidine kinase (EHV4 TK) variant improves ganciclovir-induced cell-killing

The limitations of the ganciclovir (GCV)/herpes simplex virus thymidine kinase (HSV1 TK: EC 2.7.1.21) system as a suicide gene therapy approach have been extensively studied over the years. In our study, we focused on improving the cytotoxic profile of the GCV/equine herpes virus-4 thymidine kinase (EHV4 TK: EC 2.7.1.21) system. Our approach involved the structure-guided mutagenesis of EHV4 TK in order to switch its ability to preferentially phosphorylate the natural substrate deoxythymidine (dT) to that of GCV. We performed steady-state kinetic analysis, genetic complementation in a thymidine kinase-deficient Escherichia coli strain, isothermal titration calorimetry, and analysis of GCV-induced cell killing through generation of HEK 293 stable cell-lines expressing EHV4 TK mutants and wild-type EHV4 TK. We found that the EHV4 TK S144H-GFP mutant preferentially phosphorylates GCV and confers increased GCV-induced cytotoxicity compared to wild-type EHV4 TK.

Review of antiviral therapy for herpes labialis, genital herpes and herpes zoster

Acyclovir (Zovirax) was approved for the treatment of herpesvirus infections almost two decades ago. It was the first agent in a novel group of antiviral medications that now include valacyclovir (Valtrex), penciclovir (Denavir and famciclovir (Famvir). These agents have made a dramatic impact on the morbidity associated with herpes simplex virus infections and herpes zoster. Topical and oral antiviral use have shown modest but statistically significant efficacy in treating herpes labialis with most studies demonstrating a significant reduction in episode length and/or healing time. Oral acyclovir, valacyclovir and famciclovir are efficacious and safe for the treatment of the first episode and recurrent genital herpes and are useful as suppressive therapy for individuals with frequent genital herpes recurrences. In addition, high doses of oral acyclovir, valacyclovir and famciclovir have been shown to speed the healing of herpes zoster, and data suggests that these agents also decrease associated acute and chronic pain in people of 50 years of age or older. Further research is required to clarify the safety of these agents in pregnant women with genital herpes, the role of antiviral therapy in decreasing the sexual transmission of genital herpes, and the efficacy and cost-effectiveness of these agents in treating herpes zoster in people below the age of 50 years.

C-5 hydroxyethyl and hydroxypropyl acyclonucleosides as substrates for thymidine kinase of herpes simplex virus type 1 (HSV-1 TK): syntheses and biological evaluation

The efficient syntheses of 5-(2-hydroxyethyl)- and 5-(3-hydroxypropyl)-substituted pyrimidine derivatives bearing 2,3-dihydroxypropyl, acyclovir-, ganciclovir- and penciclovir-like side chains are reported. A synthetic approach that included the alkylation of an N-anionic-5-substituted pyrimidine intermediate (method A) provided the target acyclonucleosides in significantly higher overall yields in comparison to those obtained by method B using sylilation reaction. The phosphorylation assays of novel compounds as potential substrates for thymidine kinase of herpes simplex virus type 1 (HSV-1 TK) showed that solely pyrimidine 5-substituted acyclonucleosides with a penciclovir-like side chain acted as a fraudulent substrates of HSV-1 TK. Moreover, the uracil derivative with penciclovir-like side chain with less bulky 2-hydroxyethyl substituent at C-5 proved to be a better substrate than the corresponding one with a 3-hydroxypropyl substituent. Therefore, this acyclonucleoside was selected as a lead compound for the development of a positron emission tomography HSV-1 TK activity imaging agent.

Intraocular penetration of penciclovir after oral administration of famciclovir: a population pharmacokinetic model

OBJECTIVES

We developed a population model that describes the ocular penetration and pharmacokinetics of penciclovir in human aqueous humour and plasma after oral administration of famciclovir.

METHODS

Fifty-three patients undergoing cataract surgery received a single oral dose of 500 mg of famciclovir prior to surgery. Concentrations of penciclovir in both plasma and aqueous humour were measured by HPLC with fluorescence detection. Concentrations in plasma and aqueous humour were fitted using a two-compartment model (NONMEM software). Inter-individual and intra-individual variabilities were quantified and the influence of demographics and physiopathological and environmental variables on penciclovir pharmacokinetics was explored.

RESULTS

Drug concentrations were fitted using a two-compartment, open model with first-order transfer rates between plasma and aqueous humour compartments. Among tested covariates, creatinine clearance, co-intake of angiotensin-converting enzyme inhibitors and body weight significantly influenced penciclovir pharmacokinetics. Plasma clearance was 22.8±9.1 L/h and clearance from the aqueous humour was 8.2×10(-5) L/h. AUCs were 25.4±10.2 and 6.6±1.8 μg·h/mL in plasma and aqueous humour, respectively, yielding a penetration ratio of 0.28±0.06. Simulated concentrations in the aqueous humour after administration of 500 mg of famciclovir three times daily were in the range of values required for 50% growth inhibition of non-resistant strains of the herpes zoster virus family.

CONCLUSIONS

Plasma and aqueous penciclovir concentrations showed significant variability that could only be partially explained by renal function, body weight and comedication. Concentrations in the aqueous humour were much lower than in plasma, suggesting that factors in the blood-aqueous humour barrier might prevent its ocular penetration or that redistribution occurs in other ocular compartments.

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.

Famciclovir, a new oral antiherpes drug: results of the first controlled clinical study demonstrating its efficacy and safety in the treatment of uncomplicated herpes zoster in immunocompetent patients

This multicentre, double-blind, double-dummy, randomised study was undertaken to compare the efficacy and tolerability of famciclovir administered at 250 mg, 500 mg and 750 mg three times daily with acyclovir 800 mg five times daily for the treatment of acute uncomplicated herpes zoster in immunocompetent adults. A total of 545 patients participated in this trial. Treatment was initiated within 72 h of the onset of the zoster rash and was continued for seven days. When treatment was initiated within 72 h, famciclovir was found to be as effective as acyclovir at all dose levels for cutaneous lesion healing as demonstrated by the median times to full crusting, cessation of new lesion formation, loss of vesicles and loss of crusts; time to loss of acute pain was comparable in patients receiving famciclovir and acyclovir. Time to resolution of zoster-associated pain, however, occured at a significantly faster rate in patients treated with famciclovir within 48 h of rash onset compared with acyclovir treatment. Famciclovir was well tolerated with a safety profile comparable to that of acyclovir. Gastrointestinal disturbances and headache were the most common adverse experiences in all treatment groups. In conclusion, famciclovir, administered less frequently and at lower unit doses than acyclovir, is an effective treatment for patients with uncomplicated herpes zoster.

Single-day treatment for orolabial and genital herpes: a brief review of pathogenesis and pharmacology

Herpes simplex virus (HSV) infection is a highly prevalent condition responsible for significant morbidity and occasional mortality each year. Approximately half of all patients infected by HSV will experience at least one recurrence in their lifetime. For these recurrences, traditional therapy has included both suppressive and episodic treatment with nucleoside analogs. In regards to episodic treatment, 2- to 5-day oral regimens are best studied and most commonly reported. As with any medical condition having a well-understood mechanism of action and targeted treatment, therapeutic intervention is only as effective as allowed by patient compliance. Based on these concerns, recent studies have focused on shorter, less complicated, and more affordable options. This review delineates the evidence for single-day treatments of orolabial and genital herpes. Randomized, double-blind studies of both valacyclovir and famciclovir as single-day episodic therapy for HSV have been reported in the literature. Although no head-to-head studies between the drugs have been performed, both regimens produced significant improvement in healing time and symptom resolution over placebo. Single-day therapy for HSV infection is appealing for multiple reasons. First, it simplifies the regimen, increasing likelihood of patient compliance. Additionally, it allows complete delivery of the medication at the onset of symptoms, when viral replication is highest and intervention has greatest effect. Lastly, the reduced number of pills necessary for single versus multiple day therapy decreases the overall cost of treatment per episode, an important factor in modern-day healthcare.

Novel 1,2,4-triazole and purine acyclic cyclopropane nucleoside analogues: synthesis, antiviral and cytostatic activity evaluations

BACKGROUND

Several published studies indicate that the acyclic guanine nucleoside analogues possessing bis(1,2-hydroxymethyl) substituted cyclopropane rings mimicking the sugar moiety are potent inhibitors of replication of several herpes viruses.

METHODS

Established synthetic methods and antiviral and cytostatic activity assays were used for the evaluation of new 1,2,4-triazole and purine acyclic nucleoside analogues.

RESULTS

The synthesis of new types of acyclic nucleoside analogues which incorporate 1,2,4-triazole or purine moiety bound via flexible methylenic spacer to the bis(1,2-hydroxymethyl) cyclopropane ring. None of the new compounds showed pronounced antiviral activities at subtoxic concentrations on a broad panel of DNA and RNA viruses. Evaluation of their affinity for herpes simplex type 1 (HSV-1) and varicella-zoster virus-encoded thymidine kinases (VZV TK) also showed that none of the compounds was able to significantly inhibit 1 μM deoxythymidine phosphorylation by HSV-1 and VZV TK at 500 μM concentrations. The in vitro cytostatic activity evaluation results indicated a weak antiproliferative activity for all tested compounds. Only 6-pyrrolylpurine derivative bearing a carboxylic group substituted cyclopropane ring produced a rather slight inhibitory effect at higher micromolar concentrations on a breast carcinoma cell line (MCF-7) and no cytotoxic effect on human normal fibroblasts (WI 38).

CONCLUSIONS

The lack of antiherpetic activity may be due to poor, if any, recognition of the compounds by virus-induced nucleoside kinases as an alternative substrate to become metabolically activated.

Famciclovir, from the bench to the patient--a comprehensive review of preclinical data

Famciclovir is converted rapidly and efficiently after oral administration to the selective antiviral compound, penciclovir. In cell culture, penciclovir is a potent inhibitor of herpes simplex virus (HSV) types 1 and 2, varicella-zoster virus (VZV), Epstein-Barr virus (EBV) and hepatitis B virus (HBV). Phosphorylation of penciclovir and aciclovir in uninfected cells is limited, and penciclovir, like aciclovir, has minimal effect on replicating cells in culture as expected for a selective antiviral agent. Mode of action studies with VZV and HSV have shown that the phosphorylation of penciclovir in infected cells is far more efficient than for aciclovir. This compensates for differences observed between penciclovir triphosphate and aciclovir triphosphate in the inhibition of HSV and VZV DNA polymerases. Because HBV is not known to encode a thymidine kinase, a different rationale for the selective inhibition of this virus by penciclovir is required. Recent data indicate that the DNA polymerase of HBV is far more sensitive to inhibition by penciclovir triphosphate than cellular DNA polymerases, suggesting that for this virus, selectivity operates at the DNA polymerase. Penciclovir triphosphate is more stable within infected cells than aciclovir triphosphate, and consequently penciclovir has more prolonged antiviral activity than aciclovir. Similarly, famciclovir is more effective than aciclovir or valaciclovir in suppressing HSV replication when given at a lower dosing frequency in certain animal models. These preclinical properties have helped to provide the foundation for the famciclovir clinical programme.

Antiviral prodrugs - the development of successful prodrug strategies for antiviral chemotherapy

Following the discovery of the first effective antiviral compound (idoxuridine) in 1959, nucleoside analogues, especially acyclovir (ACV) for the treatment of herpesvirus infections, have dominated antiviral therapy for several decades. However, ACV and similar acyclic nucleosides suffer from low aqueous solubility and low bioavailability following oral administration. Derivatives of acyclic nucleosides, typically esters, were developed to overcome this problem and valaciclovir, the valine ester of ACV, was among the first of a new series of compounds that were readily metabolized upon oral administration to produce the antiviral nucleoside in vivo, thus increasing the bioavailility by several fold. Concurrently, famciclovir was developed as an oral formulation of penciclovir. These antiviral 'prodrugs' thus established a principle that has led to many successful drugs including both nucleoside and nucleotide analogues for the control of several virus infections, notably those caused by herpes-, retro- and hepatitisviruses. This review will chart the origins and development of the most important of the antiviral prodrugs to date.

Famciclovir for the treatment of recurrent genital herpes: a clinical and pharmacological perspective

Herpes simplex virus is one of the most common causes of genital ulcer disease worldwide. Herpes simplex virus-2 is the more common cause of genital herpes, a chronic infection that is characterised by periodic reactivation, with the capacity to produce recurrent symptomatic disease in the host (e.g., vesicular eruption), as well as intermittent asymptomatic shedding. Relapsing episodes may be physically and psychologically distressing. Shedding accounts for the majority of cases of transmission of genital herpes to sexual partners. Pregnant women who are shedding may transmit the virus at the time of delivery, with severe and potentially fatal consequences to the baby. Famciclovir, a synthetic acyclic guanine derivative, is the prodrug of penciclovir, which demonstrates in vitro antiviral activity against various types of herpes virus, including herpes simplex virus-2. Its pharmacokinetics allow for administration in a convenient dosing regimen compared with acyclovir, which may improve compliance. Clinical studies have demonstrated its efficacy in the episodic treatment of relapses, with the most recent report demonstrating its efficacy and tolerance as a single-day treatment. It is also efficacious and well tolerated for the suppression of frequently recurring episodes. These results have been demonstrated in various patient populations, including immunocompetent patients and those infected with HIV. Famciclovir is well tolerated, with an adverse events profile similar to placebo.

New antiviral agents

During the last three decades, a better understanding of viral replication and disease states caused by viral infections have led to the development of newer antiviral agents with enhanced activity and better tolerability. This review focuses on newer systemic and topical antiviral agents that are used in treatment of herpes viruses including herpes simplex type-1 (HSV-1) and type-2 (HSV-2), varicella-zoster virus (VZV) and cytomegalovirus CMV) as well as the human papilloma virus (HPV). Included in this article are the agents famciclovir, penciclovir, valganciclovir, imiquimod, docosanole and brivudin.

Achievements and challenges in antiviral drug discovery

The last 40 years have seen the development of several antiviral drugs with therapeutic value in treating life-threatening or debilitating diseases such as those caused by HIV, hepatitis B virus, herpesviruses (such as herpes simplex virus and varicella zoster virus) and influenza virus. These relatively recent advances have been due to technical breakthroughs in the cultivation of viruses in the laboratory, identification of viral enzymes and, more recently, their molecular biology. We describe here the antecedence of several of the existing antivirals and their strengths and weaknesses. We indicate where the major challenges lie for future improvements of current therapies and possible new indications, such as hepatitis C virus and papillomavirus. We also describe how current antiviral therapies are restricted to a rather limited number of viral diseases of sufficient interest to the pharmaceutical industry. Finally we describe the potential threat of emerging viruses and bio-weapons and the challenges that they present to therapy.

Resistance of Herpes Simplex Virus Infections to Nucleoside Analogues in HIV‐Infected Patients

Antiviral treatment of herpes simplex virus (HSV) infections with nucleoside analogues has been well established for >2 decades, but isolation of drug-resistant HSV from immunocompetent patients has remained infrequent (0.1%-0.7% of isolates) during this period. Even when drug-resistant HSV is isolated from an immunocompetent patient, this virus, with rare exceptions, is cleared normally without adverse clinical outcome. Although drug-resistant HSV is more commonly isolated from immunocompromised patients (4%-7% of isolates) and is more likely to be clinically significant, the prevalence of drug-resistant HSV even among these patients, has been stable over the past 2 decades. Despite this stable prevalence, disease due to drug-resistant HSV remains an important problem for many immunocompromised patients, including those with HIV infection. This article reviews the prevalence, pathogenesis, and implications of drug-resistant HSV infections in HIV-infected patients.

Current and potential therapies for the treatment of herpes-virus infections

Human herpesviruses are found worldwide and are among the most frequent causes of viral infections in immunocompetent as well as in immunocompromised patients. During the past decade and a half a better understanding of the replication and disease-causing state of herpes simplex virus types 1 and 2 (HSV-1 and HSV-2), varicella zoster virus (VZV), and human cytomegalovirus (HCMV) has been achieved due in part to the development of potent antiviral compounds that target these viruses. While some of these antiviral therapies are considered safe and efficacious (acyclovir, penciclovir), some have toxicities associated with them (ganciclovir and foscarnet). In addition, the increased and prolonged use of these compounds in the clinical setting, especially for the treatment of immunocompromised patients, has led to the emergence of viral resistance against most of these drugs. While resistance is not a serious issue for immunocompetent individuals, it is a real concern for immunocompromised patients, especially those with AIDS and the ones that have undergone organ transplantation. All the currently approved treatments target the viral DNA polymerase. It is clear that new drugs that are more efficacious than the present ones, are not toxic, and target a different viral function would be of great use especially for immunocompromised patients. Here, an overview is provided of the diseases caused by the herpesviruses as well as the replication strategy of the better studied members of this family for which treatments are available. We also discuss the various drugs that have been approved for the treatment of some herpesviruses in terms of structure, mechanism of action, and development of resistance. Finally, we present a discussion of viral targets other than the DNA polymerase, for which new antiviral compounds are being considered.

Differential mutation patterns in thymidine kinase and DNA polymerase genes of herpes simplex virus type 1 clones passaged in the presence of acyclovir or penciclovir

A total of 21 clones of acyclovir (ACV)-resistant (ACV(r)) herpes simplex virus type 1 (HSV-1) and 23 clones of penciclovir (PCV)-resistant (PCV(r)) HSV-1, emerging during serial passages in the presence of ACV or PCV, were isolated under conditions excluding contamination of resistant mutants in the starting virus culture, and their mutations in the thymidine kinase (TK) and DNA polymerase (DNA Pol) genes were analyzed comparatively. Mutations in the TK genes from ACV(r) mutants consisted of 50% single nucleotide substitutions and 50% frameshift mutations, while the corresponding figures for the PCV(r) mutants were 4 and 96%, respectively (P < 0.001). Eight of the 21 ACV(r) clones, but none of the 23 PCV(r) clones, had mutations in DNA Pol. Only nucleotide substitution(s) could be detected in the DNA Pol gene, as the gene is essential for virus replication. Therefore, the results for the DNA Pol mutants are concordant with those for the TK mutants in that a single nucleotide substitution was commonly observed in the ACV(r), but not in the PCV(r), mutants. These results clearly point to differential mutation patterns between ACV(r) and PCV(r) HSV-1 clones.

Herpes simplex virus resistance to acyclovir and penciclovir after two decades of antiviral therapy

Acyclovir, penciclovir, and their prodrugs have been widely used during the past two decades for the treatment of herpesvirus infections. In spite of the distribution of over 2.3 x 10(6) kg of these nucleoside analogues, the prevalence of acyclovir resistance in herpes simplex virus isolates from immunocompetent hosts has remained stable at approximately 0.3%. In immuncompromised patients, in whom the risk for developing resistance is much greater, the prevalence of resistant virus has also remained stable but at a higher level, typically 4 to 7%. These observations are examined in the light of characteristics of the virus, the drugs, and host factors.

Susceptibilities of herpes simplex viruses to penciclovir and acyclovir in eight cell lines

The commonly used antiviral drugs acyclovir (ACV) and penciclovir (PCV) possess similarly potent antiviral activities in vivo against herpes simplex virus (HSV). Assay methods for sensitivity to ACV are not necessarily transferable to PCV, even though the two drugs have similar in vivo potencies and mechanisms of action. We determined by plaque reduction assay the relative activities of ACV and PCV against five laboratory-adapted strains of HSV types 1 and 2 (including sensitive and resistant strains) in seven human cell lines and one nonhuman primate cell line. Seven characteristics were used to evaluate the cell lines. All cell lines were similar in their plating efficiencies and abilities to discriminate between sensitive and resistant HSV isolates. Vero and MRC-5 cells yielded the most discordant 50% inhibitory concentrations (IC50s) for the two HSV types, while Vero and WI-38 VA-13 cells yielded large differences in the IC50s of ACV and PCV. The limited life spans and poor plaque morphologies of the fibroblast lines were undesirable characteristics. Among the transformed cell lines producing well-defined plaques, A549 cells provided the best concordance between IC50s for the two HSV types and two antiherpes drugs. Comparison experiments with a yield reduction format indicated that the use of assays of this type might allow some of the cell-specific properties observed in plaque reduction assays to be avoided.