THERAPEUTIC ANTIVIRAL ACTION OF 5-TRIFLUOROMETHYL-2'-DEOXYURIDINE IN HERPES SIMPLEX KERATITIS

Synthesis of Fluorinated Nucleosides/Nucleotides and Their Antiviral Properties

The FDA has approved several drugs based on the fluorinated nucleoside pharmacophore, and numerous drugs are currently in clinical trials. Fluorine-containing nucleos(t)ides offer significant antiviral and anticancer activity. The insertion of a fluorine atom, either in the base or sugar of nucleos(t)ides, alters its electronic and steric parameters and transforms the lipophilicity, pharmacodynamic, and pharmacokinetic properties of these moieties. The fluorine atom restricts the oxidative metabolism of drugs and provides enzymatic metabolic stability towards the glycosidic bond of the nucleos(t)ide. The incorporation of fluorine also demonstrates additional hydrogen bonding interactions in receptors with enhanced biological profiles. The present article discusses the synthetic methodology and antiviral activities of FDA-approved drugs and ongoing fluoro-containing nucleos(t)ide drug candidates in clinical trials.

Structural and Synthetic Aspects of Small Ring Oxa- and Aza-Heterocyclic Ring Systems as Antiviral Activities

Antiviral properties of different oxa- and aza-heterocycles are identified and properly correlated with their structural features and discussed in this review article. The primary objective is to explore the activity of such ring systems as antiviral agents, as well as their synthetic routes and biological significance. Eventually, the structure-activity relationship (SAR) of the heterocyclic compounds, along with their salient characteristics are exhibited to build a suitable platform for medicinal chemists and biotechnologists. The synergistic conclusions are extremely important for the introduction of a newer tool for the future drug discovery program.

Hydrogen Bonding (Base Pairing) in Antiviral Activity

Base pairing based on hydrogen bonding has, since its inception, been crucial in the antiviral activity of arabinosyladenine, 2'-deoxyuridines (i.e., IDU, TFT, BVDU), acyclic nucleoside analogues (i.e., acyclovir) and nucleoside reverse transcriptase inhibitors (NRTIs). Base pairing based on hydrogen bonding also plays a key role in the mechanism of action of various acyclic nucleoside phosphonates (ANPs) such as adefovir, tenofovir, cidofovir and O-DAPYs, thus explaining their activity against a wide array of DNA viruses (human hepatitis B virus (HBV), human immunodeficiency (HIV) and human herpes viruses (i.e., human cytomegalovirus)). Hydrogen bonding (base pairing) also seems to be involved in the inhibitory activity of Cf1743 (and its prodrug FV-100) against varicella-zoster virus (VZV) and in the activity of sofosbuvir against hepatitis C virus and that of remdesivir against SARS-CoV-2 (COVID-19). Hydrogen bonding (base pairing) may also explain the broad-spectrum antiviral effects of ribavirin and favipiravir. This may lead to lethal mutagenesis (error catastrophe), as has been demonstrated with molnutegravir in its activity against SARS-CoV-2.

Exploring Nucleobase Modifications in Oligonucleotide Analogues for Use as Environmentally Responsive Fluorophores and Beyond

Over the past two decades, it has become abundantly clear that nucleic acid biochemistry, especially with respect to RNA, is more convoluted and complex than previously appreciated. Indeed, the application and exploitation of nucleic acids beyond their predestined role as the medium for storage and transmission of genetic information to the treatment and study of diseases has been achieved. In other areas of endeavor, utilization of nucleic acids as a probe molecule requires that they possess a reporter group. The reporter group of choice is often a luminophore because fluorescence spectroscopy has emerged as an indispensable tool to probe the structural and functional properties of modified nucleic acids. The scope of this review spans research done in the Hudson lab at The University of Western Ontario and is focused on modified pyrimidine nucleobases and their applications as environmentally sensitive fluorophores, base discriminating fluorophores, and in service of antisense applications as well as tantalizing new results as G-quadruplex destabilizing agents. While this review is a focused personal account, particularly influential work of colleagues in the chemistry community will be highlighted. The intention is not to make a comprehensive review, citations to the existing excellent reviews are given, any omission of the wonderful and impactful work being done by others globally is not intentional. Thus, this review will briefly introduce the context of our work, summarize what has been accomplished and finish with the prospects of future developments.

Trends and strategies to combat viral infections: A review on FDA approved antiviral drugs

The infectious microscopic viruses invade living cells to reproduce themselves, and causes chronic infections such as HIV/AIDS, hepatitis B and C, flu, etc. in humans which may lead to death if not treated. Different strategies have been utilized to develop new and superior antiviral drugs to counter the viral infections. The FDA approval of HIV nucleoside reverse transcriptase inhibitor, zidovudine in 1987 boosted the development of antiviral agents against different viruses. Currently, there are a number of combination drugs developed against various viral infections to arrest the activity of same or different viral macromolecules at multiple stages of its life cycle; among which majority are targeted to interfere with the replication of viral genome. Besides these, other type of antiviral molecules includes entry inhibitors, integrase inhibitors, protease inhibitors, interferons, immunomodulators, etc. The antiviral drugs can be toxic to human cells, particularly in case of administration of combination drugs, and on the other hand viruses can grow resistant to the antiviral drugs. Furthermore, emergence of new viruses like Ebola, coronaviruses (SARS-CoV, SARS-CoV-2) emphasizes the need for more innovative strategies to develop better antiviral drugs to fight the existing and the emerging viral infections. Hence, we reviewed the strategic enhancements in developing antiviral drugs for the treatment of different viral infections over the years.

Broad-Spectrum Antiviral Activity of 3'-Deoxy-3'-Fluoroadenosine against Emerging Flaviviruses

Emerging flaviviruses are causative agents of severe and life-threatening diseases, against which no approved therapies are available. Among the nucleoside analogues, which represent a promising group of potentially therapeutic compounds, fluorine-substituted nucleosides are characterized by unique structural and functional properties. Despite having first been synthesized almost 5 decades ago, they still offer new therapeutic opportunities as inhibitors of essential viral or cellular enzymes active in nucleic acid replication/transcription or nucleoside/nucleotide metabolism. Here, we report evaluation of the antiflaviviral activity of 28 nucleoside analogues, each modified with a fluoro substituent at different positions of the ribose ring and/or heterocyclic nucleobase. Our antiviral screening revealed that 3'-deoxy-3'-fluoroadenosine exerted a low-micromolar antiviral effect against tick-borne encephalitis virus (TBEV), Zika virus, and West Nile virus (WNV) (EC₅₀ values from 1.1 ± 0.1 μM to 4.7 ± 1.5 μM), which was manifested in host cell lines of neural and extraneural origin. The compound did not display any measurable cytotoxicity up to concentrations of 25 μM but had an observable cytostatic effect, resulting in suppression of cell proliferation at concentrations of >12.5 μM. Novel approaches based on quantitative phase imaging using holographic microscopy were developed for advanced characterization of antiviral and cytotoxic profiles of 3'-deoxy-3'-fluoroadenosine in vitro In addition to its antiviral activity in cell cultures, 3'-deoxy-3'-fluoroadenosine was active in vivo in mouse models of TBEV and WNV infection. Our results demonstrate that fluoro-modified nucleosides represent a group of bioactive molecules with excellent potential to serve as prospective broad-spectrum antivirals in antiviral research and drug development.

Cation Radicals of Hachimoji Nucleobases P and Z: Generation in the Gas Phase and Characterization by UV-Vis Photodissociation Action Spectroscopy and Theory

We report the generation of gas-phase cation radicals of unusual nucleobases 5-aza-7-deazaguanine (P) and 6-amino-5-nitro-(1H)pyrid-2-one (Z) that have been used as building blocks of base-expanded (hachimoji) DNA. The cation radicals were generated by collision-induced intramolecular electron transfer and dissociation of ternary copper-terpyridine complexes. The cation radicals were characterized by deuterium labeling and tandem mass spectrometry including MS³ collision-induced dissociation, UV-vis photodissociation, and action spectroscopy. Vibronic absorption UV-vis spectra were calculated by time-dependent density functional theory (TD-DFT) and compared with the action spectra to unequivocally assign the most closely matching structures for the gas-phase cation radicals. Ab initio calculations up to the coupled clusters-complete basis set (CCSD(T)/CBS) level of theory were used to rank by energy the P and Z neutral molecules and cation-radical isomers and provided transition-state and dissociation energies. The 5-aza-7-deazaguanine cation radicals were determined to have the canonical N-1-H, 6-oxo structure (P1^+•) that was the global energy minimum within this group of isomers. The Z cation radicals were found to have the 1H-pyrid-2-one structure (Z1^+•). The formation of P1^+• and Z1^+• was shown to be controlled by the solution thermodynamics of the Cu-terpyridine complexes and the kinetics of their dissociations. We also report and compare CCSD(T)/CBS-calculated adiabatic recombination energies of cation radicals for the entire hachimoji set of eight nucleobases, P^+• (7.92 eV), Z^+• (8.51 eV), S^+• (8.51 eV), B^+• (7.76 eV), T^+• (8.98 eV), C^+• (8.62 eV), A^+• (8.32 eV), and G^+• (7.97 eV), to assess the thermodynamics of base-to-base electron transfer following random ionization.

The evolution of nucleoside analogue antivirals: A review for chemists and non-chemists. Part 1: Early structural modifications to the nucleoside scaffold

This is the first of two invited articles reviewing the development of nucleoside-analogue antiviral drugs, written for a target audience of virologists and other non-chemists, as well as chemists who may not be familiar with the field. Rather than providing a simple chronological account, we have examined and attempted to explain the thought processes, advances in synthetic chemistry and lessons learned from antiviral testing that led to a few molecules being moved forward to eventual approval for human therapies, while others were discarded. The present paper focuses on early, relatively simplistic changes made to the nucleoside scaffold, beginning with modifications of the nucleoside sugars of Ara-C and other arabinose-derived nucleoside analogues in the 1960's. A future paper will review more recent developments, focusing especially on more complex modifications, particularly those involving multiple changes to the nucleoside scaffold. We hope that these articles will help virologists and others outside the field of medicinal chemistry to understand why certain drugs were successfully developed, while the majority of candidate compounds encountered barriers due to low-yielding synthetic routes, toxicity or other problems that led to their abandonment.

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This is the first of two invited articles reviewing the development of nucleoside-analogue antiviral drugs.

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It is written for a target audience of virologists and other non-chemists, and for chemists unfamiliar with the field.

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Numerous modifications have been made to the nucleoside scaffold in order to impart therapeutic benefits.

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Nucleoside modifications led to the development of potent antivirals such as acyclovir, entecavir, and tenofovir.

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We examine thought processes, progress in synthetic chemistry and results of antiviral testing that led to approved drugs.

Phosphoramidates and phosphonamidates (ProTides) with antiviral activity

Following the first report on the nucleoside phosphoramidate (ProTide) prodrug approach in 1990 by Chris McGuigan, the extensive investigation of ProTide technology has begun in many laboratories. Designed with aim to overcome limitations and the key resistance mechanisms associated with nucleoside analogues used in the clinic (poor cellular uptake, poor conversion to the 5'-monophosphate form), the ProTide approach has been successfully applied to a vast number of nucleoside analogues with antiviral and anticancer activity. ProTides consist of a 5'-nucleoside monophosphate in which the two hydroxyl groups are masked with an amino acid ester and an aryloxy component which once in the cell is enzymatically metabolized to deliver free 5'-monophosphate, which is further transformed to the active 5'-triphosphate form of the nucleoside analogue. In this review, the seminal contribution of Chris McGuigan's research to this field is presented. His technology proved to be extremely successful in drug discovery and has led to two Food and Drug Administration-approved antiviral agents.

The one-carbon metabolism pathway highlights therapeutic targets for gastrointestinal cancer (Review)

After the initial use of anti-folates for treatment of malignancies, folate metabolism has emerged as a rational diagnostic and therapeutic target in gastrointestinal cancer. The one-carbon metabolic pathway, which comprises three critical reactions (i.e., folate and methionine cycles), underlies this effect in conjunction with the trans-sulfuration pathway. Understanding of the one-carbon metabolism pathway has served to unravel the link between the causes and effects of cancer phenotypes leading to several seminal discoveries such as that of diadenosine tri-phosphate hydrolase, microRNAs, 5-FU and, more recently, trifluridine. In the folate cycle, glycine and serine fuel the mitochondrial enzymes SHMT2, MTHFD2 and ALDH1L2, which play critical roles in the cancer survival and proliferation presumably through purine production. In the methionine cycle, S-adenocyl methionine serves hydrocarbons and polyamines that are critical for the epigenetic controls. The trans-sulfuration pathway is a critical component in the synthesis of glutathione, which is involved in the production of reactive oxygen species in cancer stem cells. Therefore, characterization of one-carbon metabolism is indispensable to the development of precision medicine in the context of cancer diagnostics and therapeutics. In the present study, we review the historical issues associated with one-carbon metabolism and highlight the recent advances in cancer research.

Strategies for Antiviral Drug Discovery

The need for antiviral drugs is growing rapidly as more viral diseases are recognized. The methods used to discover these drugs have evolved considerably over the past 40 years and the overall process of discovery can be broken down into sub-processes which include lead generation, lead optimization and lead development. Various methods are now employed to ensure these processes are carried out efficiently. For lead generation, screening methodologies have developed to the extent where hundreds of thousands of compounds can be screened against a particular target. An alternative approach is to use the structures of enzyme substrates as a starting point for drug discovery. Much use is now made of X-ray crystallographic data of target–inhibitor complexes for the optimization of lead structures, and methods for preparing libraries of compounds to assist both generation and optimization of leads are welldeveloped. The methods used to predict and improve the pharmacokinetic properties of compounds are also changing rapidly. Finally, novel approaches to antiviral therapy using oligonucleotide-based compounds or modulating the host immune response are also being explored. This review discusses these approaches, provides examples of where their application has been successful and sets them against a historical background.

Approved Antiviral Drugs over the Past 50 Years

Since the first antiviral drug, idoxuridine, was approved in 1963, 90 antiviral drugs categorized into 13 functional groups have been formally approved for the treatment of the following 9 human infectious diseases: (i) HIV infections (protease inhibitors, integrase inhibitors, entry inhibitors, nucleoside reverse transcriptase inhibitors, nonnucleoside reverse transcriptase inhibitors, and acyclic nucleoside phosphonate analogues), (ii) hepatitis B virus (HBV) infections (lamivudine, interferons, nucleoside analogues, and acyclic nucleoside phosphonate analogues), (iii) hepatitis C virus (HCV) infections (ribavirin, interferons, NS3/4A protease inhibitors, NS5A inhibitors, and NS5B polymerase inhibitors), (iv) herpesvirus infections (5-substituted 2'-deoxyuridine analogues, entry inhibitors, nucleoside analogues, pyrophosphate analogues, and acyclic guanosine analogues), (v) influenza virus infections (ribavirin, matrix 2 protein inhibitors, RNA polymerase inhibitors, and neuraminidase inhibitors), (vi) human cytomegalovirus infections (acyclic guanosine analogues, acyclic nucleoside phosphonate analogues, pyrophosphate analogues, and oligonucleotides), (vii) varicella-zoster virus infections (acyclic guanosine analogues, nucleoside analogues, 5-substituted 2'-deoxyuridine analogues, and antibodies), (viii) respiratory syncytial virus infections (ribavirin and antibodies), and (ix) external anogenital warts caused by human papillomavirus infections (imiquimod, sinecatechins, and podofilox). Here, we present for the first time a comprehensive overview of antiviral drugs approved over the past 50 years, shedding light on the development of effective antiviral treatments against current and emerging infectious diseases worldwide.

Antiviral Treatment: From Concept to Reality

The initial development of antiviral compounds was slow, with the first clinical antiviral agent not available until the 1960s. Early development was hindered by the lack of understanding of virus life-cycles and the absence of an assay system for antiviral activity. The appearance of the first assay system, the fertilized egg yolk sac, led to the identification of methisazone. This was the first antiviral agent to be used clinically. The improvement in antiviral assay systems, and the start of directed research programmes led to the development of the first antiherpes agents idoxuridine, trifluorothymidine (TFT) and vidarabine. However, all of these agents were associated with significant adverse effects. Antiherpes therapy took a major step forward with the development of the acyclic nucleoside analogue, aciclovir. Aciclovir is much more potent than previous antiherpesvirus agents. Its mode of action results in selectivity for herpesvirus-infected cells, thus significantly reducing the side-effects seen with earlier agents. Because of this, it became the standard therapy for herpes simplex virus type 1 (HSV-1), HSV-2 and varicella zoster virus infections. However, the bioavailability of oral aciclovir is poor, requiring high and frequent doses. This led to the search for better absorbed agents and to the development of two prodrugs, famciclovir and valaciclovir. Both famciclovir and valaciclovir offer much improved bioavailability of the nucleosides penciclovir and aciclovir, compared with aciclovir. Once in the body the conversion of valaciclovir to aciclovir means that the two agents have similar pharmacokinetic properties. Similarly, famciclovir is converted to penciclovir in the body. Penciclovir, when phosphorylated in virus-infected cells, persists within the cell for a much longer period than aciclovir triphosphate. Over the last 10 years we have seen an acceleration in the development of antiviral agents and some major advances in antiviral therapy. Many challenges, however, still lie ahead.

Helicase Primase: Targeting the Achilles Heel of Herpes Simplex Viruses

The majority of the population is infected by several herpesviruses. Once these infections are established the viruses persist for life. Therefore, current therapy may at best reduce symptoms but does not cure the infection. Moreover, the only classes of compounds licensed for systemic treatment of disease are nucleoside, nucleotide and pyrophosphate analogues; all of these ultimately target the herpesvirus DNA polymerase. A vaccine against varicella zoster virus (VZV) is available, but so far no effective vaccines against other human herpesviruses have been launched. At the same time, rising resistance to current medication, especially in the immunocompromised patient population, is a concern. For these reasons, there is an urgent need for new treatment options. Recently, some promising new drugs have been discovered; one of these compounds, developed at Bayer HealthCare under the name BAY 57–1293, is a potent HSV helicase primase inhibitor.

A novel antimetabolite: TAS-102 for metastatic colorectal cancer

TAS-102 is a new oral anti-tumor drug, composed of a thymidine-based nucleoside analog (trifluridine: FTD) and a thymidine phosphorylase inhibitor (tipiracil hydrochloride: TPI). TAS-102 has been shown to significantly improve overall survival and progression-free survival in patients with refractory metastatic colorectal cancer (mCRC) in placebo-controlled randomized phase II and III trials. The current review summarizes mechanisms of action, pharmacokinetics/dynamics and preclinical and clinical data of TAS-102 in colorectal cancer. TAS-102 is a new salvage-line treatment option for patients with mCRC. TAS-102 is well tolerated and has great potential in future clinical drug combination therapies.

Antiviral agents against equid alphaherpesviruses: Current status and perspectives

Equid herpesvirus infections cause respiratory, neurological and reproductive syndromes. Despite preventive and control measures and the availability of vaccines and immunostimulants, herpesvirus infections still constitute a major threat to equine health and for the equine industry worldwide. Antiviral drugs, particularly nucleoside analogues and foscarnet, are successfully used for the treatment of human alphaherpesvirus infections. In equine medicine, the use of antiviral medications in alphaherpesvirus infections would decrease the excretion of virus and diminish the risk of contagion and the convalescent time in affected horses, and would also improve the clinical outcome of equine herpesvirus myeloencephalopathy. The combined use of antiviral compounds, along with vaccines, immune modulators, and effective preventive and control measures, might be beneficial in diminishing the negative impact of alphaherpesvirus infections in horses. The purpose of this review is to analyse the available information regarding the use of antiviral agents against alphaherpesviruses, with particular emphasis on equine alphaherpesvirus infections.

Enzymatic synthesis of 2'-deoxyuridine by whole cell catalyst co-expressing uridine phosphorylase and thymidine phosphorylase through auto-induction system

Genes encoding uridine phosphorylase (UP) and thymidine phosphorylase (TP) from Escherichia coli K12 were cloned and recombined respectively into plasmids pET-21a(+) and pET-28a(+). The recombinant plasmids BL21/pET21a-UP and BL21/pET28a-TP were co-transformed into E. coli BL21(DE3) to construct highly effective BTU strain (BL21/pET28a-TP, pET21a-UP) overexpressing both of UP and TP. BTU was cultivated in ZYM-Fe-5052 medium for 10 h and used as catalyst to synthesize 2'-deoxyuridine (dUR). It was found to increase the productivity of dUR by 8-9 fold when compared to wild E. coli K12 and E. coli BL21(DE3) strains. A series of experiments were carried out to find out the optimal conditions for synthesis of dUR. At 50°C, with 0.25‰ dry wt./v to catalyze the reaction of 2:1 β-thymidine: uracil (60 mM β-thymidine, 30 mM uracil), the conversion rate of dUR would reach 61.6% at 1 h, which was much higher than the rates obtained by BTU strain cultured in LB medium and induced by IPTG. This result proved co-expression and auto-induction were efficient methods in enhancing the expression quantity and activity of nucleoside phosphorylases, and they also had significant implications for large-scale industrial production of dUR and synthesis of other nucleoside derivatives.

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

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

Dancing with chemical formulae of antivirals: a personal account

A chemical structure is a joy forever, and this is how I perceived the chemical structures of a number of antiviral compounds with which I have been personally acquainted over the past 3 decades: (1) amino acid esters of acyclovir (i.e. valaciclovir); (2) 5-substituted 2'-deoxyuridines (i.e. brivudin); (3) 2',3'-dideoxynucleoside analogues (i.e. stavudine); (4) acyclic nucleoside phosphonates (ANPs) (i.e. cidofovir, adefovir); (5) tenofovir disoproxil fumarate (TDF) and drug combinations therewith; (6) tenofovir alafenamide (TAF, GS-7340), a new phosphonoamidate prodrug of tenofovir; (7) pro-prodrugs of PMEG (i.e. GS-9191 and GS-9219); (8) new ANPs: O-DAPy and 5-aza-C phosphonates; (9) non-nucleoside reverse transcriptase inhibitors (NNRTIs): HEPT and TIBO derivatives; and (10) bicyclam derivatives (i.e. AMD3100).

Selective anti-herpesvirus agents

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

Rabbit and mouse models of HSV-1 latency, reactivation, and recurrent eye diseases

The exact mechanisms of HSV-1 establishment, maintenance, latency, reactivation, and also the courses of recurrent ocular infections remain a mystery. Comprehensive understanding of the HSV-1 disease process could lead to prevention of HSV-1 acute infection, reactivation, and more effective treatments of recurrent ocular disease. Animal models have been used for over sixty years to investigate our concepts and hypotheses of HSV-1 diseases. In this paper we present descriptions and examples of rabbit and mouse eye models of HSV-1 latency, reactivation, and recurrent diseases. We summarize studies in animal models of spontaneous and induced HSV-1 reactivation and recurrent disease. Numerous stimuli that induce reactivation in mice and rabbits are described, as well as factors that inhibit viral reactivation from latency. The key features, advantages, and disadvantages of the mouse and rabbit models in relation to the study of ocular HSV-1 are discussed. This paper is pertinent but not intended to be all inclusive. We will give examples of key papers that have reported novel discoveries related to the review topics.

Acute Herpetic Keratitis: What is the Role for Ganciclovir Ophthalmic Gel?

Herpes simplex keratitis (HSK) is a major cause of corneal blindness in the world. Following the primary infection, the virus enters into a latent phase. Recurrent infectious or immune keratitis cause structural damage to the cornea, scarring, and may lead to blindness. Several commercially available topical and oral antiviral drugs for HSK are currently available. However, toxicity and low patient compliance hamper their use in HSK. Further, oral antiviral drugs alone are not always effective in HSK. Thus, there had been a need for safe and effective topical antiviral agents against HSK. Systemic ganciclovir has been in use for the treatment of cytomegalovirus infections. Recently, topical ganciclovir has become available for use in patients with HSK. Ganciclovir 0.15% ophthalmic gel has been shown to be both safe and effective against viruses of the herpes family. Topical ganciclovir ophthalmic gel is well tolerated and does not cause significant toxic effects on the ocular surface. Several multicenter studies have revealed the potential role of ganciclovir ophthalmic gel in the treatment and prophylaxis of epithelial HSK. In this paper, we have reviewed the pharmacology, efficacy, side effects, and the role of ganciclovir ophthalmic gel 0.15% in the treatment of acute herpetic keratitis.

Another ten stories in antiviral drug discovery (part C): "Old" and "new" antivirals, strategies, and perspectives

The ten stories told here deal with (i) ribavirin as an inhibitor of IMP dehydrogenase and (ii) ribavirin, in combination with pegylated interferon, as the present "standard of care" for hepatitis C; (iii) S-adenosylhomocysteine hydrolase inhibitors as antiviral agents; (iv) new adamantadine derivatives for the treatment of influenza A virus infections; (v) 5-substituted 2'-deoxyuridines (i.e. IDU, TFT) for the treatment of herpes simplex virus (HSV) infections; (vi) acyclic guanosine analogues (e.g. acyclovir) for the treatment of HSV infections; (vii) OMP decarboxylase inhibitors (i.e. pyrazofurin) and CTP synthetase inhibitors (i.e. cyclopentenylcytosine) as possible antiviral agents; (viii) the future of cidofovir (and alkoxyalkyl esters thereof) and ST-246 as potential antipoxvirus agents; (ix) the two decade journey from tivirapine to rilpivirine in the ultimate therapy of HIV infections; and (x) the extension of the therapeutic application of tenofovir disoproxil fumarate (Viread) to the treatment of hepatitis B virus infection, in addition to HIV infection.

Trifluorthymidine in the treatment of herpes simplex corneal ulcers. A clinical evaluation

The treatment of herpetic corneal ulcerations with trifluorthymidine (F3T) has been evaluated in a clinical trial. F3T was found to be an effective antiviral agent for herpetic keratitis and a valuable alternative to idoxuridine (IDU)-therapy. Clinically IDU-resistent herpetic ulcers responded favourably to treatment with F3T.

A human apolipoprotein E mimetic peptide effectively inhibits HSV-1 TK-positive and TK-negative acute epithelial keratitis in rabbits

PURPOSE

To compare the antiviral effect of a peptide derived from human apolipoprotein E (1% apoEdp) with 1% trifluorothymidine against HSV-1 thymidine kinase (TK)-positive (HSV-1 McKrae) and with 3% foscarnet against HSV-1 TK-negative (KOS background) in the rabbit eye model of acute HSV-1 epithelial keratitis.

MATERIALS AND METHODS

Topical treatment began three days post-infection and continued for five days. Rabbits were treated 5 times per day with 1% apoEdp, 1% trifluorothymidine, 3% foscarnet, or PBS. Slit lamp examination was performed on a masked basis from post-infection days 3-10 to determine the severity of epithelial keratitis.

RESULTS

Topical treatment with 1% apoEdp was as effective as 1% trifluorothymidine against TK-positive and 3% foscarnet against TK-negative HSV-1 epithelial keratitis.

CONCLUSIONS

ApoEdp 1% has efficacy for the topical treatment of TK-positive and TK-negative ocular herpes in the rabbit eye model. Thus, apoEdp has the potential to be used for treatment of keratitis caused by TK mutants.

Antiviral Chemistry & Chemotherapy's Current Antiviral Agents FactFile (2nd Edition): DNA Viruses

Although most of the recent attempts to develop new antiviral agents have been focussed on RNA viruses (in particular, HIV and hepatitis C virus), a few new compounds are now awaiting their entry into the field of DNA viruses, particularly poxviruses, such as variola virus, because of the bioterrorist context, and herpesviruses, such as herpes simplex virus and cytomegalovirus, where the market scene has for many years been dominated by acyclovir, penciclovir and ganciclovir and their respective orally bioavailable prodrugs: valaciclovir, famciclovir and valganciclovir. Here, we review the current ‘state of the art’ with old compounds ready to rotate off and new compounds eagerly awaiting to appear on the continuously evolving scene of antiviral drug development.

Agents and strategies in development for improved management of herpes simplex virus infection and disease

The quiet pandemic of herpes simplex virus (HSV) infections has plagued humanity since ancient times, causing mucocutaneous infection such as herpes labialis and herpes genitalis. Disease symptoms often interfere with every-day activities and occasionally HSV infections are the cause of life-threatening or sight-impairing disease, especially in neonates and the immuno-compromised patient population. After infection the virus persists for life in neurons of the host in a latent form, periodically reactivating and often resulting in significant psychosocial distress for the patient. Currently no cure is available. So far, vaccines, ILs, IFNs, therapeutic proteins, antibodies, immunomodulators and small-molecule drugs with specific or non-specific modes of action lacked either efficacy or the required safety profile to replace the nucleosidic drugs acyclovir, valacyclovir, penciclovir and famciclovir as the first choice of treatment. The recently discovered inhibitors of the HSV helicase-primase are the most potent development candidates today. These antiviral agents act by a novel mechanism of action and display low resistance rates in vitro and superior efficacy in animal models. This review summarises the current therapeutic options, discusses the potential of preclinical or investigational drugs and provides an up-to-date interpretation of the challenge to establish novel treatments for herpes simplex disease.

[Advances in antiviral chemotherapy]

Establishment of selective antiviral chemotherapy has achieved dramatic improvement of the prognosis of several viral infections. It has been considered for a long time that, unlike bacterial infections, viral diseases cannot be successfully treated with chemotherapeutic agents, since viral replication mostly depends on the host-cellular machinery. In fact, some compounds were reported to inhibit viral replication even in the 1950s and 1960s, yet they were also quite toxic to the host cells. The first antiviral compound that strongly inhibits viral replication without affecting the uninfected cells is the anti-herpes agent acyclovir (ACV), which was discovered in the 1970s. Furthermore, in the 1980s, the world-wide epidemic of AIDS caused by human immunodeficiency virus type 1 (HIV-1) infection has dramatically accelerated the development of new antiviral agents. At present, most of the effective antivirals are targeted at virus-specific enzymes, such as ACV for herpes virus thymidine kinase, zidovudine for HIV-1 reverse transcriptase, squinavir for HIV-1 protease, and oseltamivir for neuraminidase of influenza virus. These agents can be administered systemically without serious side effects. However, several drawbacks, including delayed toxicity and drug-resistance, are associated with long-term treatment with several antiviral agents mostly in highly active antiretroviral therapy for HIV-1 infection. Thus, it seems still mandatory to continue the search for more effective and less toxic compounds against various viral infections.

A new synthetic analogue of thymidine, 7-(3-bromo-phenoxy)-thymidine, inhibits the proliferation of tumor cells

Modified thymidine analogues have been highlighted as useful agents for the treatment of cancer and viral diseases due to their potent biological activities. In the present study, we synthesized a new thymidine analogue, 7-(3-bromo-phenoxy)-thymidine (4a), as a potential lead for anti-tumor agent. Compound 4a potently inhibits HeLa cell proliferation with an IC(50) of 15 microM.

Novel agents and strategies to treat herpes simplex virus infections

The quiet pandemic of herpes simplex virus (HSV) infection has plagued humanity since ancient times, causing mucocutaneous infection, such as herpes labialis and herpes genitalis. Disease symptoms often interfere with everyday activities and occasionally HSV infections are the cause of life-threatening or sight-impairing disease, especially in neonates and the immunocompromised patient population. After primary or initial infection the virus persists for life in a latent form in neurons of the host, periodically reactivating and often resulting in significant psychosocial distress for the patient. Currently, no cure is available. In the mid-1950s the first antiviral, idoxuridine, was developed for topical treatment of herpes disease and, in 1978, vidarabine was licensed for systemic use to treat HSV encephalitis. Acyclovir (Zovirax), a potent, specific and tolerable nucleosidic inhibitor of the herpes DNA polymerase, was a milestone in the development of antiviral drugs in the late 1970s. In the mid-1990s, when acyclovir became a generic drug, valacyclovir (Valtrex) and famciclovir (Famvir), prodrugs of the gold standard and penciclovir (Denavir), Vectavir), a close analogue, were launched. Though numerous approaches and strategies were tested and considerable effort was expended in the search of the next generation of an antiherpetic therapy, it proved difficult to outperform acyclovir. Notable in this regard was the award of a Nobel Prize in 1988 for the elucidation of mechanistic principles which resulted in the development of new drugs such as acyclovir. Vaccines, interleukins, interferons, therapeutic proteins, antibodies, immunomodulators and small-molecule drugs with specific or nonspecific modes of action lacked either efficacy or the required safety profile to replace the nucleosidic drugs acyclovir, valacyclovir, penciclovir and famciclovir as the first choice of treatment. Recently though, new inhibitors of the HSV helicase-primase with potent in vitro antiherpes activity, novel mechanisms of action, low resistance rates and superior efficacy against HSV in animal models have been discovered. This review summarises the current therapeutic options, discusses the potential of preclinical or investigational drugs and provides an up-to-date interpretation of the challenge to establish novel treatments for herpes simplex disease.

Treatment of viral diseases of the cornea and external eye

Ocular virus infections remain an important cause of corneal and external disease. Herpes simplex, the most important, is easily treated when it is confined to the epithelium. New studies indicate that herpetic stromal disease and iritis are effectively treated with a combination of corticosteroid and antiviral without additional risk. Recurrences of ocular herpetic disease can be reduced with acyclovir given orally; the benefit seems to be greatest in patients who have had at least one episode of stromal keratitis. Herpes zoster can be treated with either acyclovir or famciclovir, but to be effective, treatment must be initiated within 72 hours of onset. Early treatment reduces the risk of post-herpetic neuralgia and may reduce the risk of ocular complications. Adenovirus infection (epidemic keratoconjunctivitis) is often spread by the ophthalmologist. New medications such as cidofovir appear to be effective against the adenoviruses in non-human systems and may have some effect in man, although previously, drugs that appeared to have an effect in vitro have proven to be ineffective in the clinical setting.

Virus infections of the eye

In reviewing the clinical features, diagnostic evaluations and therapies of the most common ocular viral infections we attempt to whet your appetite for attacking the numerous challenges in diagnosis and treatment of viral eye disease. The herpes viruses, HSV, VZV and CMV are the cause of significant ocular morbidity. HSV most commonly affects the cornea producing keratitis that can be recurrent and may lead to corneal clouding and neovascularisation. Manifestations can be purely infectious or immunological and treatment options must be tailored to the underlying pathophysiology. Herpes zoster ophthalmicus, caused by VZV infection of the first branch of the trigeminal nerve, produces a characteristic rash and can progress to keratitis and uveitis. HSV and VZV can cause retinitis in both immunocompetent and immunocompromised individuals. There has been a significant increase in the incidence of CMV retinitis since the beginning of the AIDS epidemic. We review the numerous new treatments, diagnostic tests and treatment strategies which have been developed in response to this potentially blinding retinal infection. Adenovirus produces an epidemic conjunctivitis and epidemic keratoconjunctivitis which are severe and extremely contagious conjunctival infections. HIV, molluscum contagiosum, EBV and rubeola also cause ocular diseases which are described.Copyright 1998 John Wiley & Sons, Ltd.

In search of a selective antiviral chemotherapy

This article describes several approaches to a selective therapy of virus infections: (E)-5-(2-bromovinyl)-2'-deoxyuridine (BVDU [brivudin]) for the therapy of herpes simplex virus type 1 and varicella-zoster virus infections: (S)-9-(3-hydroxy-2-phosphonylmethoxypropyl)cytosine (HPMPC [cidofovir]) for the therapy of various DNA virus (i.e., herpesvirus, adenovirus, papillomavirus, polyomavirus, and poxvirus) infections; 9-(2-phosphonylmethoxyethyl)adenine (PMEA [adefovir]) for the therapy of retrovirus, hepadnavirus, and herpesvirus infections; (R)-9-(2-phosphonylmethoxypropyl)adenine (PMPA) for the therapy and prophylaxis of retrovirus and hepadnavirus infections; and nonnucleoside reverse transcriptase inhibitors (NNRTIs), such as tetrahydroimidazo[4,5,1-jk][1,4]-benzodiazepin-2(IH)-one and -thione (TIBO), 1-[(2-hydroxyethoxy)methyl]-6-(phenylthio)thymine (HEPT), alpha-anilinophenylacetamide (alpha-APA), and 2',5'bis-O-(tert-butyldimethylsilyl)-3'-spiro-5"-(4"-amino-1",2"-oxat hiole- 2",2"-dioxide)pyrimidine (TSAO) derivatives, and thiocarboxanilides for the treatment of human immunodeficiency virus type 1 (HIV-1) infections. For the clinical use of NNRTIs, some guidelines have been elaborated, such as starting treatment with combinations of different compounds at sufficiently high concentrations to effect a pronounced and sustained suppression of the virus. Despite the diversity of the compounds described here and the different viruses at which they are targeted, they have a number of characteristics in common. As they interact with specific viral proteins, the compounds achieve a selective inhibition of the replication of the virus, which, in turn, should be able to develop resistance to the compounds. However, as has been established for the NNRTIs, the problem of viral resistance may be overcome if the compounds are used from the start at sufficiently high doses, which could be reduced if different compounds are combined. For HIV infections, drug treatment regimens should be aimed at reducing the viral load to such an extent that the risk for progression to AIDS will be minimized, if not avoided entirely. This may result in a real "cure" of the disease but not necessarily of the virus infection, and in this sense, HIV disease may be reduced to a dormant infection, reminiscent of the latent herpesvirus infections. Should virus replication resume after a certain time, the armamentarium of effective anti-HIV and anti-herpesvirus compounds now available, if applied at the appropriate dosage regimens, should make the virus return to its dormant state before it has any chance to damage the host. It is unlikely that this strategy would eradicate the virus and thus "cure" the viral infection, but it definitely qualifies as a cure of the disease.

Evaluation of a peptidomimetic ribonucleotide reductase inhibitor with a murine model of herpes simplex virus type 1 ocular disease

The ribonucleotide reductase (RR) of herpes simplex virus type 1 (HSV-1) is an important virulence factor, being required for neurovirulence, ocular virulence, and reactivation from latency. The RR activity requires the association of two distinct homodimeric subunits, and the association of the subunits is inhibited in the presence of a peptide homologous to the carboxy terminus of the small subunit. A structural analog of the inhibitory peptide (BILD 1263) has been shown to inhibit the replication of HSV-1 at micromolar concentrations in vitro. We used a mouse model of HSV-1 ocular infection to determine the in vivo efficacy of topical BILD 1263. Treatment of HSV-1 KOS-infected mice resulted in significant reductions in the severity and incidence of stromal keratitis and corneal neovascularization. At higher concentrations (5%) BILD 1263 reduced the severity but not the incidence of blepharitis. Treatment with 5% BILD 1263 also reduced viral shedding from the cornea by 10- to 14-fold (P < 0.001). In uninfected mice treated with 5% BILD 1263, we found no evidence of corneal epithelial damage, conjunctivitis, or blepharitis, and histopathological studies revealed no changes in the corneas of these mice. These results show that the peptidomimetic RR inhibitor BILD 1263 is effective in preventing disease, has an antiviral effect in vivo, and has little or no toxicity.

Structure-activity relationships and conformational features of antiherpetic pyrimidine and purine nucleoside analogues. A review

A rational approach to the design of antiherpetic nucleoside analogues is based in part on the broad specificity of virus-coded thymidine kinases. Herpes virus thymidine kinase 'activates' many 5-substituted 2'-deoxyuridines, analogues of thymidine (e.g., idoxuridine, trifluridine, edoxudine, brivudine), 5-substituted arabinofuranosyluracil derivatives (e.g., 5-Et-Ara-U, BV-Ara-U, Cl-Ara-U), acyclonucleosides of guanine (e.g., aciclovir, ganciclovir, penciclovir), and purine nucleosides with the pentafuranosyl ring replaced by a cyclobutane ring (e.g., cyclobut-G, cyclobut-A). Activation involves selective, and frequently regiospecific, phosphorylation of these analogues to the 5'-monophosphates. These are further phosphorylated by cellular enzymes to the 5'-triphosphates, which are usually competitive inhibitors of the viral-coded DNA polymerases. Some analogues are also incorporated into viral, and to a lesser extent cellular, DNA. A recent, unusual, exception is human cytomegalovirus, which does not code for a thymidine kinase, but for a protein with the sequence characteristics of protein kinase and which phosphorylates ganciclovir to its 5'-monophosphate. The interaction of the analogues with cellular catabolic enzymes such as uridine and thymidine nucleoside phosphorylases is also discussed, as is the relationship between physicochemical properties (configuration, conformation, electronic and hydrophobic parameters) and antiviral activities, with particular reference to those drugs that are licensed, or under consideration, for clinical use.

Detection of new metabolites of trifluridine (F3TdR) using 19F NMR spectroscopy

The metabolism of 5-trifluoromethyl-2'-deoxyuridine (trifluridine, F3TdR) in male BALB/C mice has been studied by 19F NMR spectroscopy. Contrary to previous reports, a number of fluorinated metabolites were observed in urine, whole livers and blood samples taken from mice after i.p. injection of F3TdR. The present study describes the identification of two new metabolites in mouse urine using the 19F NMR technique. The NMR of crude urine showed the presence of F3TdR 5-trifluorothymine (F3T), the newly-identified metabolites, 5-trifluoromethyl-5,6-dihydrouracil (DHF3T) and 5-trifluoromethyl-5,6-dihydroxyracil (DOHF3T), and several new, as yet unidentified fluorinate metabolites. These two new metabolites were characterized by comparison to authentic compounds prepared synthetically from F3T.

A murine model of herpes simplex virus-induced ocular disease for antiviral drug testing

Previous studies to determine the efficacy of new antiviral compounds in treating HSV ocular infections have used rabbit models. However, rabbits are expensive to purchase and maintain, and require the use of substantial amounts of test compounds. We have used the currently licensed antiviral trifluorothymidine in a murine model of HSV-induced ocular infection to demonstrate that the less expensive murine model can be used for the in vivo evaluation of potentially useful antiviral compounds. Treatment with TFT reduced the severity of blepharitis, vascularization of the cornea, stromal keratitis, and the percentage of animals developing symptoms. TFT treatment did not reduce the peak titers of infectious virus in the eyes of the infected animals, but did enhance clearance of virus from the tissues in a dose-dependent manner. Treatment with 1.0% TFT prevented the establishment of reactivatable latent infections. However, treatment with 0.01% or 0.1% TFT did not affect latency. The ED50 values for blepharitis, vascularization, and stromal keratitis ranged between 0.007% and 0.023%. These results are very similar to results obtained in rabbits and establish baseline data for comparing rabbit and murine models. This murine model provides a potentially less expensive alternative for in vivo drug testing.