Inhibition of herpesvirus replication by a series of 4-oxo-dihydroquinolines with viral polymerase activity

Pd-Catalyzed C-7 Arylation of Indolines with Aryltriazenes under Mild Conditions

A palladium-catalyzed direct C-H arylation of indolines at C-7 position has been achieved at near-ambient temperature. The reaction was carried out with aryltriazene as a stable aryl source and electron shuttle to sustainably release aryl radical in situ under the action of promoter, and pyrimidine as a detachable directing group for the synthesis of 7-arylindolines under oxidant- and ligand-free conditions. Notably, this catalytic system can also be applied to the direct and site-selective arylation of tetrahydroquinolines (C-8) and carbazoles (C-1).

Inhibition of the hERG potassium ion channel by different non-nucleoside human cytomegalovirus polymerase antiviral inhibitor series and the exploration of variations on a pyrroloquinoline core to reduce cardiotoxicity potential

Many non-nucleoside human cytomegalovirus (HCMV) inhibitors have been reported in patent and scientific literature, however, none have reached commercialization despite the urgent need for new HCMV treatments. Herein we report select compounds from different templates that all had low micromolar human ether-à-go-go (hERG) ion channel IC₅₀ values. We also describe a series of pyrroloquinoline derivatives that were designed and synthesized to understand the effect of various substitution on human cytomegalovirus (HCMV) polymerase activity, antiviral activity, and hERG inhibition. These results demonstrated that hERG inhibition can be significantly altered based on the substitution on this template. An HCMV inhibitor with low hERG inhibition and reduced cytotoxicity is also described. The results suggest substitution can be fine tuned for the non-nucleoside polymerase inhibitors to reduce hERG inhibition and maintain HCMV antiviral potency.

Synthesis and Photophysical Characterizations of Pyrroloquinolone Photosensitizers for Singlet Oxygen Production

A series of pyrroloquinolone photosensitizers bearing different halogen substituents (Cl, Br, I) on the heterocyclic framework was studied. These structures were readily prepared through a multi-step synthetic sequence involving an oxidative protocol as an important step to access the quinolone framework. Spectroscopic characterizations and computational investigations were carried out to study the dyes before and after the oxidative step. Interestingly, the fluorescence emission was significantly reduced upon oxidation. In spite of a low photostability under UV light, the pyrroloquinolone photosensitizers proved effective to produce singlet oxygen. Higher singlet oxygen quantum yields were obtained with photosensitizers bearing halogen atoms with a higher atomic number.

Advances in the Development of Therapeutics for Cytomegalovirus Infections

The development of therapeutics for cytomegalovirus (CMV) infections, while progressing, has not matched the pace of new treatments of human immunodeficiency virus (HIV) infections; nevertheless, recent developments in the treatment of CMV infections have resulted in improved human health and perhaps will encourage the development of new therapeutic approaches. First, the deployment of ganciclovir and valganciclovir for both the prevention and treatment of CMV infections and disease in transplant recipients has been further improved with the licensure of the efficacious and less toxic letermovir. Regardless, late-onset CMV disease, specifically pneumonia, remains problematic. Second, the treatment of congenital CMV infections with valganciclovir has beneficially improved both hearing and neurologic outcomes, both fundamental advances for these children. In these pediatric studies, viral load was decreased but not eliminated. Thus, an important lesson learned from studies in both populations is the need for new antiviral agents and the necessity for combination therapies as has been shown to be beneficial in the treatment of HIV infections, among others. The development of monoclonal antibodies, sirtuins, and cyclopropovir may provide new treatment options.

Palladium-Catalyzed Direct and Specific C-7 Acylation of Indolines with 1,2-Diketones

The indole scaffold is a ubiquitous and useful substructure, and extensive investigations have been conducted to construct the indole framework and/or realize indole modification. Nevertheless, the direct selective functionalization on the benzenoid core must overcome the high activity of the C-3 position and still remains highly challenging. Herein, a palladium-catalyzed direct and specific C-7 acylation of indolines in the presence of an easily removed directing group was developed. This strategy usually is considered as a practical strategy for the preparation of acylated indoles because indoline can be easily converted to indole under oxidation conditions. In particular, our strategy greatly improved the alkacylation yield of indolines for which only an unsatisfactory yield could be achieved in the previous studies. Furthermore, the reaction can be scaled up to gram level in the standard reaction conditions with a much lower palladium loading (1 mol %).

In vitro comparison of currently available and investigational antiviral agents against pathogenic human double-stranded DNA viruses: A systematic literature review

BACKGROUND

Double-stranded (ds) DNA virus infections often occur concomitantly in immunocompromised patients. We performed a systematic search of published in vitro activity for nine approved and investigational antivirals to understand the spectrum of in vitro activity against dsDNA viruses.

METHODS

A literature search was performed (PubMed and the WoS Core Collection) using keywords related to: 1) targeted approved/developmental antivirals (acyclovir, artesunate, brincidofovir, cidofovir, cyclopropavir (filociclovir), foscarnet, ganciclovir, letermovir, and maribavir); 2) pathogenic dsDNA viruses; 3) in vitro activity. We summarized data from 210 publications.

RESULTS

Activity against ≤3 viruses was documented for maribavir (cytomegalovirus, Epstein-Barr virus), and letermovir, while activity against > 3 viruses was shown for ganciclovir, cidofovir, acyclovir, foscarnet, cyclopropavir, artesunate, and brincidofovir. The EC₅₀ values of brincidofovir were the lowest, ranging from 0.001 to 0.27 μM, for all viruses except papillomaviruses. The next most potent agents included cidofovir, ganciclovir, foscarnet, and acyclovir with EC₅₀ values between 0.1 μM and >10 μM for cytomegalovirus, herpes simplex virus, and adenovirus.

CONCLUSION

Most of the identified antivirals had in vitro activity against more than one dsDNA virus. Brincidofovir and cidofovir have broad-spectrum activity, and brincidofovir has the lowest EC₅₀ values. These findings could assist clinical practice and developmental research.

New therapies for human cytomegalovirus infections

The recent approval of letermovir marks a new era of therapy for human cytomegalovirus (HCMV) infections, particularly for the prevention of HCMV disease in hematopoietic stem cell transplant recipients. For almost 30 years ganciclovir has been the therapy of choice for these infections and by today's standards this drug exhibits only modest antiviral activity that is often insufficient to completely suppress viral replication, and drives the selection of drug-resistant variants that continue to replicate and contribute to disease. While ganciclovir remains the therapy of choice, additional drugs that inhibit novel molecular targets, such as letermovir, will be required as highly effective combination therapies are developed not only for the treatment of immunocompromised hosts, but also for congenitally infected infants. Sustained efforts, largely in the biotech industry and academia, have identified additional highly active lead compounds that have progressed into clinical studies with varying levels of success and at least two have the potential to be approved in the near future. Some of the new drugs in the pipeline inhibit new molecular targets, remain effective against isolates that have developed resistance to existing therapies, and promise to augment existing therapeutic regimens. Here, we will describe some of the unique features of HCMV biology and discuss their effect on therapeutic needs. Existing drugs will also be discussed and some of the more promising candidates will be reviewed with an emphasis on those progressing through clinical studies. The in vitro and in vivo antiviral activity, spectrum of antiviral activity, and mechanism of action of new compounds will be reviewed to provide an update on potential new therapies for HCMV infections that have progressed significantly in recent years.

Identification of an indol-based derivative as potent and selective varicella zoster virus (VZV) inhibitor

We report the synthesis and antiviral activity of a new family of non-nucleoside antivirals, derived from the indole nucleus. Modifications of this template through Mannich and Friedel-Crafts reactions, coupled with nucleophilic displacement and reductive aminations led to 23 final derivatives, which were pharmacologically tested. Tryptamine derivative 17a was found to have a selective inhibitory activity against human varicella zoster virus (VZV) replication in vitro, being inactive against a variety of other DNA and RNA viruses. A structure-activity relationship (SAR) study showed that the presence of a biphenyl ethyl moiety and the acetylation at the amino group of tryptamine are a prerequisite for anti-VZV activity. The novel compound shows the same activity against thymidine kinase (TK)-competent (TK⁺) and TK-deficient (TK⁻) VZV strains, pointing to a novel mechanism of antiviral action.

•

A library of indole-based derivatives has been designed and synthesized as potential antiviral agents.

•

Friedel-Crafts and Mannich reactions were used for the synthesis of different indole derivatives.

•

Tryptamine derivative 17a displays significant inhibitory activity against VZV replication.

•

A mechanism of action independent from the virus-encoded thymidine kinase is suggested.

Ru-Catalyzed selective C–H oxidative olefination with N-heteroarenes directed by pivaloyl amide

Ruthenium-catalysed C7-position alkynylation for indoline derivatives with various olefins was reported.

A Novel Pd-Catalysed Annulation Reaction for the Syntheses of Pyrroloindoles and Pyrroloquinolines

Pd-catalysed annulation reactions between indole derivatives and internal alkyne esters leading to various pyrrolo[1,2-a]indoles and pyrroloquinolines have been developed. The strategy involves an intermolecular addition of the indole nitrogen on to the internal alkyne ester followed by an intramolecular insertion of a vinyl-palladium complex into the carbonyl group. This method offers a facile and practical approach to pyrrolo[1,2-a]indoles and pyrroloquinolines.

Crystal structure of (E)-N-[(2-chloro-6-methoxyquinolin-3-yl)methylidene]-9-ethyl-9H-carbazol-3-amine

In the title compound, C₂₅H₂₀ClN₃O, the dihedral between the carbazole and quinoline ring systems is 50.2 (1)°. The crystal packing features C—H⋯π and π—π inter­actions, which generate a three-dimensional network.

In the title compound, C₂₅H₂₀ClN₃O, the C=N bond of the central imine group adopts an E conformation. The mean planes through the essentially planar carbazole [maximum deviation = 0.052 (2)Å] and quinoline [maximum deviation = 0.050 (2) Å] ring systems form a dihedral angle of 50.2 (1)°. In the crystal, mol­ecules are linked by C—H⋯π and π—π inter­actions [centroid–centroid distances ranging from 3.635 (2) to 3.739 (2) Å], forming a three-dimensional supra­molecular network.

KSHV targeted therapy: an update on inhibitors of viral lytic replication

Kaposi’s sarcoma-associated herpesvirus (KSHV) is the causative agent of Kaposi’s sarcoma, primary effusion lymphoma and multicentric Castleman’s disease. Since the discovery of KSHV 20 years ago, there is still no standard treatment and the management of virus-associated malignancies remains toxic and incompletely efficacious. As the majority of tumor cells are latently infected with KSHV, currently marketed antivirals that target the virus lytic cycle have shown inconsistent results in clinic. Nevertheless, lytic replication plays a major role in disease progression and virus dissemination. Case reports and retrospective studies have pointed out the benefit of antiviral therapy in the treatment and prevention of KSHV-associated diseases. As a consequence, potent and selective antivirals are needed. This review focuses on the anti-KSHV activity, mode of action and current status of antiviral drugs targeting KSHV lytic cycle. Among these drugs, different subclasses of viral DNA polymerase inhibitors and compounds that do not target the viral DNA polymerase are being discussed. We also cover molecules that target cellular kinases, as well as the potential of new drug targets and animal models for antiviral testing.

Recent advances in the understanding of infectious mononucleosis: are prospects improved for treatment or control?

Symptomatic primary Epstein-Barr virus infection is known more commonly as infectious mononucleosis, an illness known for afflicting adolescents and younger adults as a febrile illness accompanied by pharyngitis and lymphadenopathy. Historically believed to be generally benign, infectious mononucleosis has been linked more recently to increased risks of developing Hodgkin's lymphoma and multiple sclerosis. Advances in the understanding of host immune responses to Epstein-Barr virus have begun to elucidate the reasons why younger children typically experience subclinical infection whereas older individuals develop infectious mononucleosis. This review will highlight recent advances in the understanding of primary Epstein-Barr virus infection, and whether prospective treatments or vaccine strategies may affect native infection and its sequelae.

Development of a high-throughput human cytomegalovirus quantitative PCR cell-based assay

This report describes the development and optimization of a quantitative real-time PCR assay for evaluating human cytomegalovirus (CMV) replication in vitro and susceptibility to antiviral drugs. This assay measures the level of intracellular CMV DNA in both 96- and 384-well microplate formats. Normalization of CMV levels using mitochondrial DNA enhanced the robustness of the assay and minimized variability. The assay throughput was further enhanced by eliminating several wash steps and by lysing the cells directly in the presence of cell culture media, both of which had no impact on the assay metrics. The assay was validated using several known CMV antiviral compounds. The CMV quantitative PCR (qPCR) assay represents a rapid, reliable and reproducible method that can be used with both CMV laboratory strains and clinical isolates.

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.

Identification of a varicella-zoster virus replication inhibitor that blocks capsid assembly by interacting with the floor domain of the major capsid protein

A novel anti-varicella-zoster virus compound, a derivative of pyrazolo[1,5-c]1,3,5-triazin-4-one (coded as 35B2), was identified from a library of 9,600 random compounds. This compound inhibited both acyclovir (ACV)-resistant and -sensitive strains. In a plaque reduction assay under conditions in which the 50% effective concentration of ACV against the vaccine Oka strain (V-Oka) in human fibroblasts was 4.25 μM, the 50% effective concentration of 35B2 was 0.75 μM. The selective index of the compound was more than 200. Treatment with 35B2 inhibited neither immediate-early gene expression nor viral DNA synthesis. Twenty-four virus clones resistant to 35B2 were isolated, all of which had a mutation(s) in the amino acid sequence of open reading frame 40 (ORF40), which encodes the major capsid protein (MCP). Most of the mutations were located in the regions corresponding to the "floor" domain of the MCP of herpes simplex virus 1. Treatment with 35B2 changed the localization of MCP in the fibroblasts infected with V-Oka but not in the fibroblasts infected with the resistant clones, although it did not affect steady-state levels of MCP. Overexpression of the scaffold proteins restored the normal MCP localization in the 35B2-treated infected cells. The compound did not inhibit the scaffold protein-mediated translocation of MCP from the cytoplasm to the nucleus. Electron microscopic analysis demonstrated the lack of capsid formation in the 35B2-treated infected cells. These data indicate the feasibility of developing a new class of antivirals that target the herpesvirus MCPs and inhibit normal capsid formation by a mechanism that differs from those of the known protease and encapsidation inhibitors. Further biochemical studies are required to clarify the precise antiviral mechanism.

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).

Development of drugs for Epstein-Barr virus using high-throughput in silico virtual screening

IMPORTANCE OF THE FIELD

Epstein-Barr virus (EBV) is a ubiquitous human herpesvirus that is causally associated with endemic forms of Burkitt's lymphoma, nasopharyngeal carcinoma and lymphoproliferative disease in immunosuppressed individuals. On a global scale, EBV infects > 90% of the adult population and is responsible for ∼ 1% of all human cancers. To date, there is no efficacious drug or therapy for the treatment of EBV infection and EBV-related diseases.

AREAS COVERED IN THIS REVIEW

In this review, we discuss the existing anti-EBV inhibitors and those under development. We discuss the value of different molecular targets, including EBV lytic DNA replication enzymes as well as proteins that are expressed exclusively during latent infection, such as EBV nuclear antigen 1 (EBNA-1) and latent membrane protein 1. As the atomic structure of the EBNA-1 DNA binding domain has been described, it is an attractive target for in silico methods of drug design and small molecule screening. We discuss the use of computational methods that can greatly facilitate the development of novel inhibitors and how in silico screening methods can be applied to target proteins with known structures, such as EBNA-1, to treat EBV infection and disease.

WHAT THE READER WILL GAIN

The reader is familiarized with the problems in targeting of EBV for inhibition by small molecules and how computational methods can greatly facilitate this process.

TAKE HOME MESSAGE

Despite the impressive efficacy of nucleoside analogs for the treatment of herpesvirus lytic infection, there remain few effective treatments for latent infections. As EBV latent infection persists within and contributes to the formation of EBV-associated cancers, targeting EBV latent proteins is an unmet medical need. High-throughput in silico screening can accelerate the process of drug discovery for novel and selective agents that inhibit EBV latent infection and associated disease.

Least-Squares Support Vector Machine Approach to Viral Replication Origin Prediction

Replication of their DNA genomes is a central step in the reproduction of many viruses. Procedures to find replication origins, which are initiation sites of the DNA replication process, are therefore of great importance for controlling the growth and spread of such viruses. Existing computational methods for viral replication origin prediction have mostly been tested within the family of herpesviruses. This paper proposes a new approach by least-squares support vector machines (LS-SVMs) and tests its performance not only on the herpes family but also on a collection of caudoviruses coming from three viral families under the order of caudovirales. The LS-SVM approach provides sensitivities and positive predictive values superior or comparable to those given by the previous methods. When suitably combined with previous methods, the LS-SVM approach further improves the prediction accuracy for the herpesvirus replication origins. Furthermore, by recursive feature elimination, the LS-SVM has also helped find the most significant features of the data sets. The results suggest that the LS-SVMs will be a highly useful addition to the set of computational tools for viral replication origin prediction and illustrate the value of optimization-based computing techniques in biomedical applications.

Inhibition of herpesvirus replication by 5-substituted 4'-thiopyrimidine nucleosides

A series of 4'-thionucleosides were synthesized and evaluated for activities against orthopoxviruses and herpesviruses. We reported previously that one analog, 5-iodo-4'-thio-2'-deoxyuridine (4'-thioIDU), exhibits good activity both in vitro and in vivo against two orthopoxviruses. This compound also has good activity in cell culture against many of the herpesviruses. It inhibited the replication of herpes simplex virus type 1 (HSV-1), HSV-2, and varicella-zoster virus with 50% effective concentrations (EC(50)s) of 0.1, 0.5, and 2 microM, respectively. It also inhibited the replication of human cytomegalovirus (HCMV) with an EC(50) of 5.9 microM but did not selectively inhibit Epstein-Barr virus, human herpesvirus 6, or human herpesvirus 8. While acyclovir-resistant strains of HSV-1 and HSV-2 were comparatively resistant to 4'-thioIDU, it retained modest activity (EC(50)s of 4 to 12 microM) against these strains. Some ganciclovir-resistant strains of HCMV also exhibited reduced susceptibilities to the compound, which appeared to be related to the specific mutations in the DNA polymerase, consistent with the observed incorporation of the compound into viral DNA. The activity of 4'-thioIDU was also evaluated using mice infected intranasally with the MS strain of HSV-2. Although there was no decrease in final mortality rates, the mean length of survival after inoculation increased significantly (P < 0.05) for all animals receiving 4'-thioIDU. The findings from the studies presented here suggest that 4'-thioIDU is a good inhibitor of some herpesviruses, as well as orthopoxviruses, and this class of compounds warrants further study as a therapy for infections with these viruses.

3-[2-(9-Ethyl-9H-carbazol-3-yl)-6-methyl-3-quinol-yl]propan-1-ol

In the title compound, C₂₇H₂₆N₂O, the mean planes through the carbazole and quinoline ring systems form a dihedral angle of 67.23 (5)°. Mol­ecules are linked into cyclic centrosymmetric dimers by O—H⋯N hydrogen bonds, and C—H⋯π inter­actions with the pyridine ring of the quinoline ring system as an acceptor. The dimers are linked through C—H⋯O hydrogen bonds.

Human cytomegalovirus DNA replication: antiviral targets and drugs

Human cytomegalovirus (HCMV) infection is associated with severe morbidity and mortality in immunocompromised individuals, in particular transplant recipients and AIDS patients, and is the most frequent congenital viral infection in humans. There are currently five drugs approved for HCMV treatment: ganciclovir and its prodrug valganciclovir, foscarnet, cidofovir and fomivirsen. These drugs have provided a major advance in HCMV disease management, but they suffer from poor bioavailability, significant toxicity and limited effectiveness, mainly due to the development of drug resistance. Fortunately, there are several novel and potentially very effective new compounds which are under pre-clinical and clinical evaluation and may address these limitations. This review focuses on HCMV proteins that are directly or indirectly involved in viral DNA replication and represent already established or potential novel antiviral targets, and describes both currently available drugs and new compounds against such protein targets.

2-(4,5-Dihydro-1,3-oxazol-2-yl)quinoline

The title compound, C(12)H(10)N(2)O, is approximately planar. The angle between the quinoline and 4,5-dihydro-oxazole ring systems is 11.91 (12)°. The mol-ecules pack into a herringbone array with no significant π-π inter-actions. The dihydro-oxazole N and O atoms are disordered over two positions, with almost equal site occupancy factors.

7-Oxo-4,7-dihydrothieno[3,2-b]pyridine-6-carboxamides: Synthesis and biological activity of a new class of highly potent inhibitors of human cytomegalovirus DNA polymerase

We report a new class of non-nucleoside antivirals, the 7-oxo-4,7-dihydrothieno[3,2-b]pyridine-6-carboxamides, some of which possess remarkable potency versus a broad spectrum of herpesvirus DNA polymerases and excellent selectivity compared to human DNA polymerases. A critical factor in the level of activity is hypothesized to be conformational restriction of the key 2-aryl-2-hydroxyethylamine sidechain by an adjacent methyl group.

Selective phosphorylation of antiviral drugs by vaccinia virus thymidine kinase

The antiviral activity of a new series of thymidine analogs was determined against vaccinia virus (VV), cowpox virus (CV), herpes simplex virus, and varicella-zoster virus. Several compounds were identified that had good activity against each of the viruses, including a set of novel 5-substituted deoxyuridine analogs. To investigate the possibility that these drugs might be phosphorylated preferentially by the viral thymidine kinase (TK) homologs, the antiviral activities of these compounds were also assessed using TK-deficient strains of some of these viruses. Some of these compounds were shown to be much less effective in the absence of a functional TK gene in CV, which was unexpected given the high degree of amino acid identity between this enzyme and its cellular homolog. This unanticipated result suggested that the CV TK was important in the mechanism of action of these compounds and also that it might phosphorylate a wider variety of substrates than other type II enzymes. To confirm these data, we expressed the VV TK and human TK1 in bacteria and isolated the purified enzymes. Enzymatic assays demonstrated that the viral TK could efficiently phosphorylate many of these compounds, whereas most of the compounds were very poor substrates for the cellular kinase, TK1. Thus, the specific phosphorylation of these compounds by the viral kinase may be sufficient to explain the TK dependence. This unexpected result suggests that selective phosphorylation by the viral kinase may be a promising new approach in the discovery of highly selective inhibitors of orthopoxvirus replication.

Activity and mechanism of action of N-methanocarbathymidine against herpesvirus and orthopoxvirus infections

N-Methanocarbathymidine [(N)-MCT] is a conformationally locked nucleoside analog that is active against some herpesviruses and orthopoxviruses in vitro. The antiviral activity of this molecule is dependent on the type I thymidine kinase (TK) in herpes simplex virus and also appears to be dependent on the type II TK expressed by cowpox and vaccinia viruses, suggesting that it is a substrate for both of these divergent forms of the enzyme. The drug is also a good inhibitor of viral DNA synthesis in both viruses and is consistent with inhibition of the viral DNA polymerase once it is activated by the viral TK homologs. This mechanism of action explains the rather unusual spectrum of activity, which is limited to orthopoxviruses, alphaherpesviruses, and Epstein-Barr virus, since these viruses express molecules with TK activity that can phosphorylate and thus activate the drug. The compound is also effective in vivo and reduces the mortality of mice infected with orthopoxviruses, as well as those infected with herpes simplex virus type 1 when treatment is initiated 24 h after infection. These results indicate that (N)-MCT is active in vitro and in vivo, and its mechanism of action suggests that the molecule may be an effective therapeutic for orthopoxvirus and herpesvirus infections, thus warranting further development.

Distinct thymidine kinases encoded by cowpox virus and herpes simplex virus contribute significantly to the differential antiviral activity of nucleoside analogs

Orthopoxviruses and herpesviruses are both large enveloped DNA viruses, yet these virus families exhibit very different susceptibilities to antiviral drugs. We investigated the activation of nucleoside analogs by the types I and II thymidine kinase (TK) homologs expressed by herpes simplex virus type 1 (HSV-1) and cowpox virus (CV). Antiviral activity against TK(-) and TK(+) strains of HSV-1 and CV was determined, and the ratio of the EC(50) values was used as a measurement of TK dependence. As to HSV-1, most of the selected compounds were markedly less effective against the TK(-) strains, suggesting that this enzyme was required for the activation of these nucleoside analogs. This differs from the results for CV where only idoxuridine and bromodeoxyuridine appeared to be activated, putatively by the type II TK expressed by this virus. These data confirm that the type II TK encoded by CV exhibits a more limited substrate specificity than the type I TK encoded by HSV-1. These data suggest that the inefficient activation of nucleoside analogs by the orthopoxvirus TK significantly limits their activity. Additional screening against orthopoxviruses will be required to identify nucleoside analogs that are efficiently activated by their type II TK.

Scoring schemes of palindrome clusters for more sensitive prediction of replication origins in herpesviruses

Many empirical studies show that there are unusual clusters of palindromes, closely spaced direct and inverted repeats around the replication origins of herpesviruses. In this paper, we introduce two new scoring schemes to quantify the spatial abundance of palindromes in a genomic sequence. Based on these scoring schemes, a computational method to predict the locations of replication origins is developed. When our predictions are compared with 39 known or annotated replication origins in 19 herpesviruses, close to 80% of the replication origins are located within 2% of the genome length. A list of predicted locations of replication origins in all the known herpesviruses with complete genome sequences is reported.

Epstein-Barr virus infections: prospects for treatment

Epstein-Barr virus (EBV) causes infectious mononucleosis and oral hairy leucoplakia, and is associated with a number of malignancies. There are, however, no regulatory agency-approved treatments for EBV-related diseases. Several antiviral drugs inhibit replication of EBV in cell culture including acyclic nucleoside and nucleotide analogues and pyrophosphate analogues, all of which inhibit the EBV DNA polymerase. Despite their potency in vitro, these drugs have limited use in vivo for treatment of acute primary EBV infection as well as EBV-associated malignancies for several reasons. Here we discuss novel anti-EBV compounds, including maribavir, potentially useful for the treatment of acute EBV infections. A number of experimental approaches for treatment of EBV-related malignancies that are not susceptible to conventional antiviral drug treatment are also discussed.