Mode of action of (R)-9-[4-hydroxy-2-(hydroxymethyl)butyl]guanine against herpesviruses

Small Molecule Drugs Targeting Viral Polymerases

Small molecules that specifically target viral polymerases-crucial enzymes governing viral genome transcription and replication-play a pivotal role in combating viral infections. Presently, approved polymerase inhibitors cover nine human viruses, spanning both DNA and RNA viruses. This review provides a comprehensive analysis of these licensed drugs, encompassing nucleoside/nucleotide inhibitors (NIs), non-nucleoside inhibitors (NNIs), and mutagenic agents. For each compound, we describe the specific targeted virus and related polymerase enzyme, the mechanism of action, and the relevant bioactivity data. This wealth of information serves as a valuable resource for researchers actively engaged in antiviral drug discovery efforts, offering a complete overview of established strategies as well as insights for shaping the development of next-generation antiviral therapeutics.

Large Subunit of the Human Herpes Simplex Virus Terminase as a Promising Target in Design of Anti-Herpesvirus Agents

Herpes simplex virus type 1 (HSV-1) is an extremely widespread pathogen characterized by recurrent infections. HSV-1 most commonly causes painful blisters or sores around the mouth or on the genitals, but it can also cause keratitis or, rarely, encephalitis. First-line and second-line antiviral drugs used to treat HSV infections, acyclovir and related compounds, as well as foscarnet and cidofovir, selectively inhibit herpesvirus DNA polymerase (DNA-pol). It has been previously found that (S)-4-[6-(purin-6-yl)aminohexanoyl]-7,8-difluoro-3,4-dihydro-3-methyl-2H-[1,4]benzoxazine (compound 1) exhibits selective anti-herpesvirus activity against HSV-1 in cell culture, including acyclovir-resistant mutants, so we consider it as a lead compound. In this work, the selection of HSV-1 clones resistant to the lead compound was carried out. High-throughput sequencing of resistant clones and reference HSV-1/L2 parent strain was performed to identify the genetic determinants of the virus's resistance to the lead compound. We identified a candidate mutation presumably associated with resistance to the virus, namely the T321I mutation in the UL15 gene encoding the large terminase subunit. Molecular modeling was used to evaluate the affinity and dynamics of the lead compound binding to the putative terminase binding site. The results obtained suggest that the lead compound, by binding to pUL15, affects the terminase complex. pUL15, which is directly involved in the processing and packaging of viral DNA, is one of the crucial components of the HSV terminase complex. The loss of its functional activity leads to disruption of the formation of mature virions, so it represents a promising drug target. The discovery of anti-herpesvirus agents that affect biotargets other than DNA polymerase will expand our possibilities of targeting HSV infections, including those resistant to baseline drugs.

Advances and Perspectives in the Management of Varicella-Zoster Virus Infections

Varicella-zoster virus (VZV), a common and ubiquitous human-restricted pathogen, causes a primary infection (varicella or chickenpox) followed by establishment of latency in sensory ganglia. The virus can reactivate, causing herpes zoster (HZ, shingles) and leading to significant morbidity but rarely mortality, although in immunocompromised hosts, VZV can cause severe disseminated and occasionally fatal disease. We discuss VZV diseases and the decrease in their incidence due to the introduction of live-attenuated vaccines to prevent varicella or HZ. We also focus on acyclovir, valacyclovir, and famciclovir (FDA approved drugs to treat VZV infections), brivudine (used in some European countries) and amenamevir (a helicase-primase inhibitor, approved in Japan) that augur the beginning of a new era of anti-VZV therapy. Valnivudine hydrochloride (FV-100) and valomaciclovir stearate (in advanced stage of development) and several new molecules potentially good as anti-VZV candidates described during the last year are examined. We reflect on the role of antiviral agents in the treatment of VZV-associated diseases, as a large percentage of the at-risk population is not immunized, and on the limitations of currently FDA-approved anti-VZV drugs. Their low efficacy in controlling HZ pain and post-herpetic neuralgia development, and the need of multiple dosing regimens requiring daily dose adaptation for patients with renal failure urges the development of novel anti-VZV drugs.

MODERN ETHIOTROPIC CHEMOTHERAPY OF HERPESVIRUS INFECTIONS: ADVANCES, NEW TRENDS AND PERSPECTIVES. ALPHAHERPESVIRINAE (part I)

Modern therapy of infections caused by alpha-herpesviruses is based on drugs belonging to the class of modified nucleosides (acyclovir) and their metabolic progenitors - valine ester of acyclovir and famciclovir (prodrug of penciclovir). The biological activity of these compounds is determined by the similarity of their structure to natural nucleosides: modified nucleosides compete with natural nucleosides for binding to DNA-polymerase and, due to their structural features, inhibit its activity. However, the emergence of variants of viruses resistant to the antiviral drugs available in the arsenal of modern medicine necessitates the search for new compounds able of effectively inhibiting the reproduction of viruses. These compounds should be harmless to the macroorganisms, convenient to use, and overcoming the drug resistance barrier in viruses. The search for literature in international databases (PubMed, MedLine, RINC, etc.) in order to obtain information on promising developments that open new possibilities for treating herpesvirus infection and subsequent analysis of the collected data made it possible to determine not only the main trends in the search for new antiviral agents, but also to provide information on the compounds most promising for the development of anti-herpesvirus drugs.

Thymidine kinase and protein kinase in drug-resistant herpesviruses: Heads of a Lernaean Hydra

Herpesviruses thymidine kinase (TK) and protein kinase (PK) allow the activation of nucleoside analogues used in anti-herpesvirus treatments. Mutations emerging in these two genes often lead to emergence of drug-resistant strains responsible for life-threatening diseases in immunocompromised populations. In this review, we analyze the binding of different nucleoside analogues to the TK active site of the three α-herpesviruses [Herpes Simplex Virus 1 and 2 (HSV-1 and HSV-2) and Varicella-Zoster Virus (VZV)] and present the impact of known mutations on the structure of the viral TKs. Furthermore, models of β-herpesviruses [Human cytomegalovirus (HCMV) and human herpesvirus-6 (HHV-6)] PKs allow to link amino acid changes with resistance to ganciclovir and/or maribavir, an investigational chemotherapeutic used in patients with multidrug-resistant HCMV. Finally, we set the basis for the understanding of drug-resistance in γ-herpesviruses [Epstein-Barr virus (EBV) and Kaposi's sarcoma associated herpesvirus (KSHV)] TK and PK through the use of animal surrogate models.

Herpes simplex virus and varicella zoster virus: recent advances in therapy

PURPOSE OF REVIEW

The mainstay of antiviral therapy for the alpha-herpesviruses [herpes simplex virus (HSV)-1, HSV-2, and varicella zoster virus (VZV)] over the past 40 years has been the nucleoside analogues such as aciclovir. Although conventional antiviral therapy has reduced mortality in severe disease, novel agents are needed to address the emergence of resistance and toxicity associated with current second-line therapy. Treatment and prophylaxis of VZV and HSV reactivations remains a challenge.

RECENT FINDINGS

A number of compounds have recently been evaluated in human clinical trials, amongst them brincidofovir, an intracellularly acting derivative of cidofovir currently undergoing phase III trials. The helicase-primase inhibitors are a new class of antiviral agent and may circumvent resistance to existing agents. Amenamevir and pritelivir are two examples of these agents that have been evaluated clinically along with novel nucleoside analogues such as valomaciclovir and FV-100. Tenofovir, an agent used in HIV and hepatitis B therapy, may also have a role in the prevention of HSV-2 acquisition and reduce viral shedding.

SUMMARY

Although several novel antiviral agents have undergone clinical trials in recent years, all are yet to gain licensure. Brincidofovir appears to be the candidate with most promise for adoption into routine practice in the near future.

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.

Effect of Herpesvirus Inhibition on Primary SIV Infection in Cynomolgus Monkeys

Foscarnet and (-)9-[4-hydroxy-2-(hydroxymethyl)butyl] guanine (H2G) have already been shown to inhibit herpesviruses in vitro and also to inhibit viral antigen production in primary SIV infection in monkeys. Attempts have been made to determine if these invivo effects on SIV were due to a direct effect on SIV or were mediated through inhibition of endogenous transactivating herpesviruses. The possible involvement of herpesviruses in primary SIVsm infection in monkeys was studied by the use of various inhibitors of herpesvirus replication. Subcutaneous injections of 3 × 5 mg kg−1 day−1 of aciclovir, 3 × 5 mg kg−1 day−1 of ganciclovir and 3 × 28 mg kg−1 day−1 of phosphonoacetic acid had no effect on primary SIVsm infection in cynomolgus monkeys. These doses of aciclovir, ganciclovir and phosphonoacetic acid are inhibitory to several herpesviruses. The results suggest that the effects of foscarnet and H2G on primary SIVsm infection in monkeys are direct and not mediated through inhibition of a replicating herpesvirus.

Spectrum of activity and mechanisms of resistance of various nucleoside derivatives against gammaherpesviruses

The susceptibilities of gammaherpesviruses, including Epstein-Barr virus (EBV), Kaposi's sarcoma-associated herpesvirus (KSHV), and animal rhadinoviruses, to various nucleoside analogs was investigated in this work. Besides examining the antiviral activities and modes of action of antivirals currently marketed for the treatment of alpha- and/or betaherpesvirus infections (including acyclovir, ganciclovir, penciclovir, foscarnet, and brivudin), we also investigated the structure-activity relationship of various 5-substituted uridine and cytidine molecules. The antiviral efficacy of nucleoside derivatives bearing substitutions at the 5 position was decreased if the bromovinyl was replaced by chlorovinyl. 1-β-D-Arabinofuranosyl-(E)-5-(2-bromovinyl)uracil (BVaraU), a nucleoside with an arabinose configuration of the sugar ring, exhibited no inhibitory effect against rhadinoviruses but was active against EBV. On the other hand, the fluoroarabinose cytidine analog 2'-fluoro-5-iodo-aracytosine (FIAC) showed high selectivity indices against gammaherpesviruses that were comparable to those of brivudin. Additionally, we selected brivudin- and acyclovir-resistant rhadinoviruses in vitro and characterized them by phenotypic and genotypic (i.e., sequencing of the viral thymidine kinase, protein kinase, and DNA polymerase) analysis. Here, we reveal key amino acids in these enzymes that play an important role in substrate recognition. Our data on drug susceptibility profiles of the different animal gammaherpesvirus mutants highlighted cross-resistance patterns and indicated that pyrimidine nucleoside derivatives are phosphorylated by the viral thymidine kinase and purine nucleosides are preferentially activated by the gammaherpesvirus protein kinase.

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.

Anti-herpesvirus agents: a patent and literature review (2003 to present)

INTRODUCTION

The standard therapy used to treat herpesvirus infections is based on the application of DNA polymerase inhibitors such as ganciclovir or aciclovir. Unfortunately, all of these compounds exhibit relatively high toxicity and the mutation of herpesviruses results in the appearance of new drug-resistant strains. Consequently, there is a great need for the development of new, effective and safe anti-herpesvirus agents that employ different patterns of therapeutic action at various stages of the virus life cycle.

AREAS COVERED

Patents and patent applications concerning the development of anti-herpesvirus agents displaying different mechanisms of action that have been published since 2003 are reviewed. In addition, major discoveries in this field that have been published in academic papers have also been included.

EXPERT OPINION

Among all the anti-herpesvirus agents described in this article, the inhibitors of viral serine protease seem to present one of the most effective/promising therapeutics. Unfortunately, the practical application of these antiviral agents has not yet been proven in any clinical trials. Nevertheless, the dynamic and extensive work on this subject gives hope that a new class of anti-herpesvirus agents aimed at the enzymatic activity of herpesvirus serine protease may be developed.

Advances in the treatment of varicella-zoster virus infections

Varicella-zoster virus (VZV) causes two distinct diseases, varicella (chickenpox) and shingles (herpes zoster). Chickenpox occurs subsequent to primary infection, while herpes zoster (usually associated with aging and immunosuppression) appears as a consequence of reactivation of latent virus. The major complication of shingles is postherpetic neuralgia. Vaccination strategies to prevent varicella or shingles and the current status of antivirals against VZV will be discussed in this chapter. Varivax®, a live-attenuated vaccine, is available for pediatric varicella. Zostavax® is used to boost VZV-specific cell-mediated immunity in adults older than 50 years, which results in a decrease in the burden of herpes zoster and pain related to postherpetic neuralgia. Regardless of the availability of a vaccine, new antiviral agents are necessary for treatment of VZV infections. Current drugs approved for therapy of VZV infections include nucleoside analogues that target the viral DNA polymerase and depend on the viral thymidine kinase for their activation. Novel anti-VZV drugs have recently been evaluated in clinical trials, including the bicyclic nucleoside analogue FV-100, the helicase-primase inhibitor ASP2151, and valomaciclovir (prodrug of the acyclic guanosine derivative H2G). Different candidate VZV drugs have been described in recent years. New anti-VZV drugs should be as safe as and more effective than current gold standards for the treatment of VZV, that is, acyclovir and its prodrug valacyclovir.

Valomaciclovir versus valacyclovir for the treatment of acute herpes zoster in immunocompetent adults: a randomized, double-blind, active-controlled trial

Herpes zoster is a common infectious disease that can result in significant acute and chronic morbidity. The safety and efficacy of once-daily oral valomaciclovir (EPB-348) was evaluated for non-inferiority to 3-times daily valacyclovir, an approved therapy. In this study, 373 immunocompetent adults with onset of a herpes zoster rash within the preceding 72 hr were randomly assigned to receive one of four treatments for 7 days: (1) EPB-348 1,000 mg once-daily; (2) EPB-348 2,000 mg once-daily; (3) EPB-348 3,000 mg once-daily; or (4) valacyclovir 1,000 mg 3-times daily. A 20% margin was the reference for non-inferiority assessment. For the primary efficacy measure of time to complete crusting of the zoster rash by Day 28, non-inferiority criteria were met for once-daily EPB-348 2,000 mg and once-daily EPB-348 3,000 mg compared to 3-times daily valacyclovir. Additionally, EPB-348 3,000 mg significantly shortened the time to complete rash crusting by Day 28 compared to valacyclovir. For secondary efficacy measures, non-inferiority was achieved for the EPB-348 1,000 and 2,000 mg groups compared to the valacyclovir group for time to rash resolution by Day 28. No EPB-348 group was non-inferior to valacyclovir for time to cessation of new lesion formation or time to cessation of pain by Day 120, though no significant differences occurred between treatment groups. Nausea, headache, and vomiting were the most common adverse events. Based on these results, additional studies are warranted to define further EPB-348's potential as an effective and safe therapy for acute herpes zoster.

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.

Emerging drugs for varicella-zoster virus infections

INTRODUCTION

Varicella-zoster virus (VZV) is the etiological agent of two distinct diseases, varicella (chickenpox) and shingles (herpes zoster). Chickenpox occurs following primary infection, while herpes zoster (usually associated with ageing and immunosuppression) is the consequence of reactivation of the latent virus. Post-herpetic neuralgia is the major complication of shingles.

AREAS COVERED

This review will discuss vaccination strategies and the current status of antivirals against VZV. A live attenuated vaccine, Varivax, is available for pediatric varicella while Zostavax was developed to boost VZV-specific cell-mediated immunity in adults older than 60 years and, via this mechanism, to decrease the burden of herpes zoster and pain associated with post-herpetic neuralgia. Despite the availability of a vaccine, there is a need for new antiviral agents. Current drugs approved for the treatment of VZV infections include nucleoside analogs that target the viral DNA polymerase and depend on the viral thymidine kinase. Novel anti-VZV drugs have recently been evaluated in clinical trials, including the bicyclic nucleoside analog FV-100, the helicase-primase inhibitor ASP2151 and valomaciclovir (prodrug of the acyclic guanosine derivative H2G).

EXPERT OPINION

New anti-VZV drugs should be as safe as and more effective than acyclovir and its prodrug valacyclovir (current gold standard for the treatment of VZV).

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.

Simultaneous determination of acyclovir, ganciclovir, and (R)-9-[4-hydroxy-2-(hydroxymethyl)butyl]guanine in human plasma using high-performance liquid chromatography

Acyclovir, ganciclovir and (R)-9-[4-hydroxy-2-(hydroxymethyl)butyl]guanine are active in vitro against the Epstein-Barr virus (EBV) but their in vivo anti-EBV activity is not well understood. We developed a novel, sensitive high-performance liquid chromatography assay with ultraviolet detection for measuring acyclovir, ganciclovir and (R)-9-[4-hydroxy-2-(hydroxymethyl)butyl]guanine in human plasma to identify quantitative relationships between in vitro anti-EBV activity and therapeutic response. Characteristics of the assay include a low plasma volume (200 microL), perchloric acid protein precipitation, use of penciclovir as the internal standard, run times less than 8 min and a 50 ng/mL lower limit of quantification. The within- and between-assay variability is 0.7-4.8 and 1.0-7.9%, respectively. Accuracy for all three drugs ranges from 89.5 to 106.4% for four quality controls (50, 100, 1000 and 10,000 ng/mL). This assay supports pharmacokinetic and pharmacodynamic studies of candidate anti-EBV drugs in children and adults with EBV infections.

Effect of (r)-9-[4-hydroxy-2-(hydroxymethyl)butyl]guanine (H2G) and AZT-lipid-PFA on human herpesvirus-6B infected cells

BACKGROUND

Human herpesvirus-6 (HHV-6) has been associated with a wide spectrum of diseases. (r)-9-[4-Hydroxy-2-(hydroxymethyl)butyl]guanine (H2G) is an acyclic guanosine analogue that is structurally similar to acyclovir and is in clinical development for treatment of herpesvirus infections. H2G has been found to have activity against HSV type 1, HSV type 2, and HHV-6 in lymphoblast cell lines. A new anti-viral duplex drug, 3'-azido-3'-deoxythymidylyl-(5'-->2-O)-3-O-octadecyl-sn-glycerol (AZT-lipid-PFA), linking zidovudine (AZT) and foscarnet (PFA) via a lipophilic octadecylglycerol residue (lipid) also exhibits anti-viral activities against HIV, HSV type 1 and HCMV.

OBJECTIVE

To assess the efficacy of H2G and AZT-lipid-PFA conjugate against HHV-6.

STUDY DESIGN

Drug-associated toxicity and proliferative response were evaluated. We conducted in vitro experiments to determine the efficacy of H2G and an AZT-lipid-PFA conjugate in interfering with expression HHV-6 viral transcript in primary human peripheral blood mononuclear cells (PBMC).

RESULTS

Both H2G and AZT-lipid-PFA were effective at inhibiting expression of HHV-6 gene transcript at comparable concentrations. Additionally, while AZT-lipid-PFA treatment was toxic to cells at concentrations above 5microM, H2G treatment was associated with minimal cytotoxicity.

CONCLUSION

These data suggest the potential application of these anti-viral compounds in controlling HHV-6 infection.

Antimicrobial strategies: inhibition of viral polymerases by 3'-hydroxyl nucleosides

Over the past 20 years, nucleoside analogues have constituted an arsenal of choice in the fight against HIV, hepatitis B and C viruses, and herpesviruses. Classical antiviral nucleosides such as zidovudine act as obligate chain terminators. Once incorporated as monophosphates into the viral nucleic acid, they immediately block the progression of the polymerase as a result of their lack of a reactive 3'-hydroxyl (3'-OH) group. This review explores beyond the paradigm of obligate chain termination, from a structural and a mechanistic perspective, the strategy of inhibiting viral polymerases (RNA- and DNA-dependant) with nucleoside analogues containing a 3'-OH group. Depending on their mechanism of action, these molecules typically fall into the following three categories: (i) delayed chain terminators; (ii) pseudo-obligate chain terminators; or (iii) mutagenic nucleosides. Delayed chain terminators (i.e. penciclovir, cidofovir and entecavir) block the polymerase at an internal position within the viral nucleic acid, whereas R7128 and the 4'C substituted nucleosides do not permit subsequent incorporation events. Ribavirin, 5-hydroxydeoxycytidine and KP1461 are not chain terminators. Instead, they inhibit viral replication after mispairing with the template base, resulting in random mutations that are often lethal. Finally, brivudine, clevudine and other L-nucleosides have unique or yet to be defined mechanisms of inhibition.

Prodrugs of nucleoside analogues for improved oral absorption and tissue targeting

Nucleoside analogues are widely used for the treatment of antiviral infections and anticancer chemotherapy. However, many nucleoside analogues suffer from poor oral bioavailability due to their high polarity and low intestinal permeability. In order to improve oral absorption of these polar drugs, prodrugs have been employed to increase lipophilicity by chemical modification of the parent. Alternatively, prodrugs targeting transporters present in the intestine have been exploited to facilitate the transport of the nucleoside analogues. Valacyclovir and valganciclovir are two successful valine ester prodrugs transported by the PepT1 transporter. Recently, research efforts have focused on design of prodrugs for tissue specific delivery to improve efficacy and safety. This review presents advances of prodrug approaches for improved oral absorption of nucleoside analogues and recent developments in tissue targeting.

Main adult herpes virus infections of the CNS

Herpes viruses are widely involved in human infectious diseases, and some are life threatening, such as CNS infections. These manifestations vary according to the type of virus involved and the immune status of the patient. This article will review the clinical manifestations (encephalitis, myelitis, meningitis and postinfectious encephalomyelitis), the diagnostic strategies and the presently used drugs (acyclovir, valacyclovir, ganciclovir, valgancyclovir, foscarnet and cidofovir). The review will also discuss drugs that are currently in the pipeline and that could be used in the future.

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.

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.

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.

Selection and characterization of varicella-zoster virus variants resistant to (R)-9-[4-hydroxy-2-(hydroxymethy)butyl]guanine

(R)-9-[4-Hydroxy-2-(hydroxymethy)butyl]guanine (H2G) is a potent and selective inhibitor of herpesvirus replication. It is a nucleoside analog, and its triphosphate derivative (H2G-TP) is a competitive inhibitor of herpesvirus DNA polymerases. In this study, the antiviral activities of H2G and acyclovir (ACV) and the development of viral resistance to these agents were compared in varicella-zoster virus (VZV)-infected cells. In plaque reduction assays, the 50% effective concentration of H2G for VZV was 60- to 400-fold lower than that of ACV, depending on the virus strain and the cell line tested. The enhanced efficacy of H2G against VZV can be accounted for in part by the fact that the intaracellular H2G-TP level (>170 pmol/10(6) cells) is higher than the intracellular ACV-TP level (<1 pmol/10(6) cells). In addition, H2G-TP has extended half-lives of 3.9 and 8.6 h in VZV-infected MRC-5 and MeWo cells, respectively. To assess the emergence of H2G-resistant VZV in vitro, VZV was passaged in the presence of increasing concentrations of H2G. Earlier in the passage, when the concentration of H2G was relatively low, the predominant variant had the (A)76 deletion in the viral thymidine kinase (TK) gene. This mutant was identical to an ACV-resistant mutant generated in parallel experiments. However, higher concentrations of H2G appeared to favor a novel mutant, which had deletions of two consecutive nucleotides at positions 805 and 806 of the TK gene. All of these changes introduced frameshift mutations in the TK gene resulting in the expression of truncated polypeptides. H2G-resistant viruses were cross-resistant to ACV, and vice versa.

Hydroxyurea potentiates the antiherpesvirus activities of purine and pyrimidine nucleoside and nucleoside phosphonate analogs

Hydroxyurea has been shown to potentiate the anti-human immunodeficiency virus activities of 2',3'-dideoxynucleoside analogs such as didanosine. We have now evaluated in vitro the effect of hydroxyurea on the antiherpesvirus activities of several nucleoside analogs (acyclovir [ACV], ganciclovir [GCV], penciclovir [PCV], lobucavir [LBV], (R)-9-[4-hydroxy-2-(hydroxymethyl)butyl]guanine [H2G], and brivudin and nucleoside phosphonate analogs (cidofovir [CDV] and adefovir [ADV]). When evaluated in cytopathic effect (CPE) reduction assays, hydroxyurea by itself had little effect on CPE progression and potentiated in a subsynergistic (herpes simplex virus type 1 [HSV-1]) to synergistic (HSV-2) fashion the antiviral activities of ACV, GCV, PCV, LBV, H2G, ADV, and CDV. Hydroxyurea also caused marked increases in the activities of ACV, GCV, PCV, LBV, and H2G (compounds that depend for their activation on a virus-encoded thymidine kinase [TK]) against TK-deficient (TK(-)) HSV-1. In fact, in combination with hydroxyurea the 50% effective concentrations of these compounds for inhibition of TK(-) HSV-1-induced CPE decreased from values of 20 to > or = 100 microg/ml (in the absence of hydroxyurea) to values of 1 to 5 microg/ml (in the presence of hydroxyurea at 25 to 100 microg/ml). When evaluated in a single-cycle virus yield reduction assay, hydroxyurea at a concentration of 100 microg/ml inhibited progeny virus production by 60 to 90% but had little effect on virus yield at a concentration of 25 microg/ml. Under these assay conditions hydroxyurea still elicited a marked potentiating effect on the antiherpesvirus activities of GCV and CDV, but this effect was less pronounced than that in the CPE reduction assay. It is conceivable that the potentiating effect of hydroxyurea stems from a depletion of the intracellular deoxynucleoside triphosphate pools, thus favoring the triphosphates of the nucleoside analogues (or the diphosphates of the nucleoside phosphonate analogues) in their competition with the natural nucleotides at the viral DNA polymerase level. The possible clinical implications of these findings are discussed.

The antiherpesvirus activity of H2G [(R)-9-[4-hydroxy-2-(hydroxymethyl)butyl]guanine] is markedly enhanced by the novel immunosuppressive agent mycophenolate mofetil

Mycophenolate mofetil (MMF) has been approved as an immunosuppressive agent in kidney transplant recipients and may thus be used concomitantly with antiherpetic agents, which are used for the treatment of intercurrent herpesvirus infections. We have recently demonstrated that MMF and its parent compound mycophenolic acid (MPA), which is a potent inhibitor of IMP dehydrogenase, potentiate the antiherpesvirus activity of acyclovir, ganciclovir, and penciclovir. We have now evaluated the antiviral efficacy of the combination of MPA and the novel antiherpesvirus agent H2G [(R)-9-[4-hydroxy-2-(hydroxymethyl)butyl]guanine]. When combined with H2G, MPA (at concentrations ranging from 0.25 to 10 microgram/ml, which are readily attainable in human plasma) markedly potentiated the antiviral efficacy of H2G against herpes simplex virus type 1 (HSV-1) and type 2 (HSV-2), as reflected by a 10- to 150-fold decrease in the 50% effective concentration. Moreover, the activity of H2G against a thymidine kinase-deficient strain of HSV-1 (TK- HSV-1) was increased more than 2,500-fold when combined with MPA. MPA by itself had little or no effect on the replication of these viruses. Similar observations were made for varicella-zoster virus. Also, ribavirin (another inhibitor of IMP dehydrogenase) caused a marked enhancement of the activity of H2G against HSV-1 (10-fold), HSV-2 (10-fold), and TK- HSV-1 (>185-fold). Exogenously added guanosine reversed the potentiating effects of MPA on the antiviral activity of H2G, indicating that this potentiating effect resulted from a depletion of the endogenous dGTP pools, thus favoring the inhibitory action of the H2G triphosphate on the viral DNA polymerase.

Mode of action of (1'S,2'R)-9-[[1',2'-bis(hydroxymethyl) cycloprop-1'-yl]methyl]guanine (A-5021) against herpes simplex virus type 1 and type 2 and varicella-zoster virus

The mode of action of (1'S,2'R)-9-([1', 2'-bis(hydroxymethyl)cycloprop-1'-yl]methyl)guanine (A-5021) against herpes simplex virus type 1 (HSV-1), HSV-2, and varicella-zoster virus (VZV) was studied. A-5021 was monophosphorylated at the 2' site by viral thymidine kinases (TKs). The 50% inhibitory values for thymidine phosphorylation of A-5021 by HSV-1 TK and HSV-2 TK were comparable to those for penciclovir (PCV) and lower than those for acyclovir (ACV). Of these three agents, A-5021 inhibited VZV TK most efficiently. A-5021 was phosphorylated to a mono-, di-, and triphosphate in MRC-5 cells infected with HSV-1, HSV-2, and VZV. A-5021 triphosphate accumulated more than ACV triphosphate but less than PCV triphosphate in MRC-5 cells infected with HSV-1 or VZV, whereas HSV-2-infected MRC-5 cells had comparable levels of A-5021 and ACV triphosphates. The intracellular half-life of A-5021 triphosphate was considerably longer than that of ACV triphosphate and shorter than that of PCV triphosphate. A-5021 triphosphate competitively inhibited HSV DNA polymerases with respect to dGTP. Inhibition was strongest with ACV triphosphate, followed by A-5021 triphosphate and then (R,S)-PCV triphosphate. A DNA chain elongation experiment revealed that A-5021 triphosphate was incorporated into DNA instead of dGTP and terminated elongation, although limited chain extension was observed. Thus, the strong antiviral activity of A-5021 appears to depend on a more rapid and stable accumulation of its triphosphate in infected cells than that of ACV and on stronger inhibition of viral DNA polymerase by its triphosphate than that of PCV.

Penciclovir and pathogenesis phenotypes of drug-resistant Herpes simplex virus mutants

We compared the penciclovir susceptibilities and pathogenesis phenotypes of mutants of Herpes simplex virus type 1 that are resistant to acyclovir and/or foscarnet. The mutants, which were derived from laboratory strain KOS, included six DNA polymerase mutants, a thymidine kinase negative mutant, a thymidine kinase partial mutant, and a double mutant. Two of four polymerase mutants not previously examined for penciclovir susceptibility exhibited modest resistance to this drug. A thymidine kinase negative mutant exhibited approximately 20-fold resistance while a thymidine kinase partial mutant was penciclovir-sensitive. Following intracerebral inoculation of 7-week old CD1 mice, the mutants ranged from exhibiting near wild-type neurovirulence (thymidine kinase partial) to modest attenuation (e.g. thymidine kinase negative) to more severe attenuation. Following corneal inoculation, three polymerase mutants exhibited modest deficits (relative to those of thymidine kinase negative mutants) in their abilities to replicate acutely in the ganglion and reactivate from latency. For mutant AraA(r)13, the deficit in ganglionic replication was shown to be due to its polymerase mutation by analysis of recombinant viruses derived by marker rescue. These results may have implications for issues of penciclovir action and resistance, for drug resistance in the clinic, and for the interactions of herpes viruses with the peripheral and central nervous systems.

Unique intracellular activation of the potent anti-human immunodeficiency virus agent 1592U89

The anabolism of 1592U89, (-)-(1S,4R)-4-[2-amino-6-(cyclopropylamino)-9H-purin-9-yl]-2-cyclo pentene-1-methanol, a selective inhibitor of human immunodeficiency virus (HIV), was characterized in human T-lymphoblastoid CD4+ CEM cells. 1592U89 was ultimately anabolized to the triphosphate (TP) of the guanine analog (-)-carbovir (CBV), a potent inhibitor of HIV reverse transcriptase. However, less than 2% of intracellular 1592U89 was converted to CBV, an amount insufficient to account for the CBV-TP levels observed. 1592U89 was anabolized to its 5'-monophosphate (MP) by the recently characterized enzyme adenosine phosphotransferase, but neither its diphosphate (DP) nor its TP was detected. The MP, DP, and TP of CBV were found in cells incubated with either 1592U89 or CBV, with CBV-TP being the major phosphorylated species. We confirmed that CBV is phosphorylated by 5'-nucleotidase and that mycophenolic acid increased the formation of CBV-TP from CBV 75-fold. However, mycophenolic acid did not stimulate 1592U89 anabolism to CBV-TP. The adenosine deaminase inhibitor erythro-9-(2-hydroxy-3-nonyl)adenine (EHNA) did not inhibit CBV-TP formation from CBV or 1592U89, whereas the adenylate deaminase inhibitor 2'-deoxycoformycin selectively inhibited 1592U89 anabolism to CBV-TP and reversed the antiviral activity of 1592U89. 1592U89-MP was not a substrate for adenylate deaminase but was a substrate for a distinct cytosolic deaminase that was inhibited by 2'-deoxycoformycin-5'-MP. Thus, 1592U89 is phosphorylated by adenosine phosphotransferase to 1592U89-MP, which is converted by a novel cytosolic enzyme to CBV-MP. CBV-MP is then further phosphorylated to CBV-TP by cellular kinases. This unique activation pathway enables 1592U89 to overcome the pharmacokinetic and toxicological deficiencies of CBV while maintaining potent and selective anti-HIV activity.