2'-fluoro-5-iodo-aracytosine, a potent and selective anti-herpesvirus agent

Antiviral Chemotherapy in Avian Medicine-A Review

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

Anti-Herpes Virus Activity of 5-Methoxymethyl-2′-Deoxycytidine in Combination with Deaminase Inhibitors

5-Methoxymethyl-2′-deoxycytidine (MMdCyd) is an anti-metabolite with selective anti-herpes activity and low cytotoxicity. MMdCyd is dependent upon initial activation by the viral-induced deoxythymidine-deoxycytidine (dThd/dCyd) kinase for its activity against herpes simplex virus (HSV). Antiviral activity of MMdCyd is cell-dependent and is influenced by the deaminase content of the cell line used for assays. The antiviral potency against HSV-1 in this study was higher in RK-13 cells (ED50 3–5 μm) than in Vero and HEP-2 cells (ED50 14–26 μm). The potency of MMdCyd increased approximately 20-fold against HSV-1 and twofold against HSV-2 in the presence of tetrahydrodeoxyuridine (H4dUrd; which inhibits both dCyd deaminase and dCMP deaminase) in Vero cells. MdCyd in combination with H4dUrd was effective in preventing the cytopathogenic effect of HSV-1 and decreasing the production of infectious virus particles. The IC99 (concentration required to reduce the yield of infectious virus obtained 72 h after infection by 99% relative to control cultures) was 1.6 μm. In combination with tetrahydrouridine (H4Urd; an inhibitor of Cyd/dCyd deaminase) the potency of MMdCyd was only slightly enhanced (ED50 7–8 μm). Dihydrodeoxyuridine and deoxyuridine reversed the antiviral activity of MMdCyd. The minimum cytotoxic concentration for rapidly dividing cells (RK-13, HEP-2 and Vero) for MMdCyd was greater than 3 mm. H4Urd and H4dUrd were devoid of cytotoxicity and antiviral activity up to 2.12 mm (the highest concentration tested). Diacetyl-MMdCyd (pro-drug form) was approximatewly 20 times less potent than MMdCyd.

New carbocyclic N(6)-substituted adenine and pyrimidine nucleoside analogues with a bicyclo[2.2.1]heptane fragment as sugar moiety; synthesis, antiviral, anticancer activity and X-ray crystallography

New nucleoside analogues with an optically active bicyclo[2.2.1]heptane skeleton as sugar moiety and 6-substituted adenine were synthesized by alkylation of 6-chloropurine intermediate. Thymine and uracil analogs were synthesized by building the pyrimidine ring on amine 1. X-ray crystallography confirmed an exo-coupling of the thymine to the ring and an L configuration of the nucleoside analogue. The library of compounds was tested for their inhibitory activity against influenza virus A∖California/07/09 (H1N1)pdm09 and coxsackievirus B4 in cell culture. Compounds 13a and 13d are the most promising for their antiviral activity against influenza, and compound 3c against coxsackievirus B4. Compounds 3b and 3g were tested for anticancer activity.

An investigation on stereospecific fluorination at the 2'-arabino-position of a pyrimidine nucleoside: radiosynthesis of 2'-deoxy-2'-[(18)F]fluoro-5-methyl-1-β-D-arabinofuranosyluracil

Direct fluorination at the 2'-arabino-position of a pyrimidine nucleoside has been a long-standing challenge, yet we recently reported such a stereospecific fluorination for the first time in the synthesis of [(18)F]FMAU, albeit in low yields. Herein we report the results of an investigation on stereospecific fluorination on a variety of precursors for synthesis of [(18)F]FMAU. Several precursors were synthesized in multiple steps and fluorination was performed at the 2'-arabino position using K[(18)F]/kryptofix 2.2.2. All precursors produced [(18)F]FMAU in low yields.

Competitive inhibition of human poly(A)-specific ribonuclease (PARN) by synthetic fluoro-pyranosyl nucleosides

Poly(A)-specific ribonuclease (PARN) is a cap-interacting deadenylase that mediates, together with other exonucleases, the eukaryotic mRNA turnover and thus is actively involved in the regulation of gene expression. Aminoglycosides and natural nucleotides are the only reported modulators of human PARN activity, so far. In the present study, we show that synthetic nucleoside analogues bearing a fluoro-glucopyranosyl sugar moiety and benzoyl-modified cytosine or adenine as a base can effectively inhibit human PARN. Such nucleoside analogues exhibited substantial inhibitory effects, when tested against various cancer cell lines, as has been previously reported. Kinetic analysis showed that the inhibition of PARN is competitive and could not be released by altering Mg(II) concentration. Moreover, substitution of the 2', 4', or 6'-OH of the sugar moiety with acetyl and/or trityl groups was crucial for inhibitory efficacy. To understand how the nucleosides fit into the active site of PARN, we performed molecular docking experiments followed by molecular dynamics simulations. The in silico analysis showed that these compounds can efficiently dock into the active site of PARN. Our results support the idea that the sugar moiety mediates the stabilization of the nucleoside into the active site through interactions with catalytic amino acid residues. Taken together, our in vitro and in silico data suggest that human PARN is among the molecular targets of these compounds and could act therapeutically by lowering the mRNA turnover rate, thus explaining their known in vivo inhibitory effect at the molecular level.

Radiosynthesis of F-18 labeled cytidine analog 2'-fluoro-5-iodo-l-beta-d-arabinofuranosylcytosine ([(18)F]FIAC)

We reported the synthesis of 2'-deoxy-2'-[(18)F]fluoro-5-iodo-1-beta-d-arabinofuranosyl-5-iodo-cytosine ([(18)F]FIAC) with 15-20% radiochemical yield (decay corrected) in 3.5h. 2-deoxy-2-[(18)F]fluoro-1,3,5-tri-O-benzoyl-alpha-d-arabinofuranose was prepared following literature procedures with some modifications (yield>70%). The (18)F-fluorosugar was converted to 1-bromo-(18)F-fluorosugar, and then coupled with 5-iodocytocine silyl ether. A mixture of acetonitrile (ACN) and 1,2-dichloroethane (DCE) were employed to achieve optimum radiochemical yield and acceptable beta-anomer selectivity (alpha/beta=1/3). After hydrolyzed with sodium methoxide, the crude product was purified using HPLC to afford the beta-[(18)F]FIAC with high radiochemical purity (>or=98%).

2′F-Arabinonucleic acids (2′F-ANA) — History, properties, and new frontiers

The development of arabinonucleosides and oligoarabinonucleotides is described, focusing especially on 2′-deoxy-2′-fluoroarabinonucleosides (araF-N) and -oligonucleotides (2'F-ANA). In addition to their chemical and enzymatic synthesis, we discuss various properties of 2′F-ANA: hydrolytic stability (to nucleases, acids, and bases), binding affinity to complementary strands, structure and conformation, and optimization of RNase H activity. We also discuss the use of 2′F-ANA in gene-silencing approaches (antisense, siRNA), and in the stabilization of higher-order structures (such as triplexes and quadruplexes) including aptamers. Finally, we examine several other oligonucleotide derivatives based on 2′F-ANA and look ahead to the future of 2′-fluoroarabinonucleosides and -oligonucleotides.Key words: arabinonucleic acids, 2′F-ANA, antisense oligonucleotides, siRNA, modified oligonucleotides.

Chapter 3 Antiviral drugs: general considerations

The development of an antiviral drug as well as of other drugs is a long process. In most programmes the screening and evaluation start using inhibition of virus multiplication in cell cultures, but in some instances the screening starts in animal models of different viral diseases. In these cases, the mechanism of action has to be analyzed after the in vivo effect has been found. It is not possible to specify precisely the time and resources required in a newly started project to find a compound active against a virus infection but 5-10 years is a reasonable estimation. For some viruses such as herpesviruses, where a number of active inhibitors are already known, the task is simpler than it is to find inhibitors of a virus such as influenza against which only a few active inhibitors have been reported. Evaluation of clinical efficacy in humans is a large and difficult part of the development of an antiviral drug. The number of uncontrolled clinical studies claiming efficacy of different drugs against viral diseases is depressingly large. It is essential to perform double-blind, placebo-controlled and statistically well evaluated studies to be able to judge the clinical efficacy of an antiviral drug. As the knowledge of the detailed natural history and molecular biology of viral diseases and viruses themselves increases, one will obviously have better opportunities to find new drugs. Methods such as X-ray diffraction measurement and NMR determinations will probably lead to a detailed understanding of the structures and interactions taking place at the active site of viral enzymes and their cellular counterparts.

Cisplatin Derivatives as Antiviral Agents

The use of polymeric derivatives of cisplatin as antiviral drugs is reviewed. Some of these drugs inhibit a wide variety of both RNA and DNA viruses including those responsible for herpes, common colds, chickenpox, and smallpox. The desirability of polymeric drugs is described as is the mode(s) of action of cisplatin itself. A description of viruses and methods of combating viruses is presented. Included is a review of current antiviral agents as well as modes of action of these antiviral agents.

(E)-5-(2-bromovinyl)-2?-deoxyuridine (BVDU)

(E)-5-(2-bromovinyl)-2'-deoxyuridine (BVDU, Brivudin, Zostex, Zerpex, Zonavir), now more than 20 years after its discovery, still stands out as a highly potent and selective inhibitor of herpes simplex virus type 1 (HSV-1) and varicella-zoster virus (VZV) infections. It has been used in the topical treatment of herpetic keratitis and recurrent herpes labialis and the systemic (oral) treatment of herpes zoster (zona, shingles). The high selectivity of BVDU towards HSV-1 and VZV depends primarily on a specific phosphorylation of BVDU to its 5'-diphosphate (DP) by the virus-encoded thymidine kinase (TK). After further phosphorylation (by cellular enzymes), to the 5'-triphosphate (TP), the compound interferes as a competitive inhibitor/alternate substrate with the viral DNA polymerase. The specific phosphorylation by the HSV- and VZV-induced TK also explains the marked cytostatic activity of BVDU against tumor cells that have been transduced by the viral TK genes. This finding offers considerable potential in a combined gene therapy/chemotherapy approach for cancer. To the extent that BVDU or its analogues (i.e., BVaraU) are degraded (by thymidine phosphorylase) to (E)-5-(2-bromovinyl)uracil (BVU), they may potentiate the anticancer potency, as well as toxicity, of 5-fluorouracil. This ensues from the direct inactivating effect of BVU on dihydropyrimidine dehydrogenase, the enzyme that initiates the degradative pathway of 5-fluorouracil. The prime determinant in the unique behavior of BVDU is its (E)-5-(2-bromovinyl) substituent. Numerous BVDU analogues have been described that, when equipped with this particular pharmacophore, demonstrate an activity spectrum characteristic of BVDU, including selective anti-VZV activity.

Discovery and development of BVDU (brivudin) as a therapeutic for the treatment of herpes zoster

This Commentary is dedicated to the memory of Dr. Jacques Gielen, the late Editor of Biochemical Pharmacology, whom I have known as both an author and reviewer for the Journal for about 25 years. This is, quite incidentally, about the time it took for bringing brivudin (BVDU) [(E)-5-(2-bromovinyl)-2'-deoxyuridine] from its original description as an antiviral agent to the market place (in a number of European countries, including Germany and Italy) for the treatment of herpes zoster in immunocompetent persons. BVDU is exquisitely active and selective against varicella-zoster virus (VZV) and herpes simplex virus type 1 (HSV-1). BVDU owes this high selectivity and activity profile to a specific phosphorylation by the virus-encoded thymidine kinase, followed by a potent interaction with the viral DNA polymerase. The (E)-5-(2-bromovinyl)-substituent can be considered as the hallmark for the activity of BVDU against VZV and HSV-1. Extensive clinical studies have indicated that BVDU as a single (oral) daily dose of 125 mg (for no more than 7 days) is effective in the treatment of herpes zoster, as regards both short-term (suppression of new lesion formation) and long-term effects (prevention of post-herpetic neuralgia). In this sense, BVDU is as efficient and/or convenient, if not more so, than the other drugs (acyclovir, valaciclovir, famciclovir) that have been licensed for the treatment of herpes zoster. There is one caveat; however, BVDU should not be given to patients under 5-fluorouracil therapy, as the degradation product of BVDU, namely (E)-5-(2-bromovinyl)uracil (BVU), may potentiate the toxicity of 5-fluorouracil, due to inhibition of dihydropyrimidine dehydrogenase, the enzyme involved in the catabolism of 5-fluorouracil.

Antitumor activity of sugar-modified cytosine nucleosides

Nucleoside analogues which show antimetabolic activity in cells have been successfully used in the treatment of various tumors. Nucleosides such as 1-beta-D-arabinofuranosylcytosine (araC), 6-mercaptopurine, fludarabine and cladribine play an important role in the treatment of leukemias, while gemcitabine, 5-fluorouracil and its prodrugs are used extensively in the treatment of many types of solid tumors. All of these compounds are metabolized similarly to endogenous nucleosides and nucleotides. Active metabolites interfere with the de novo synthesis of nucleosides and nucleotides or inhibit the DNA chain elongation after being incorporated into the DNA strand as terminators. Furthermore, nucleoside antimetabolites incorporated into the DNA strand induce strand-breaks and finally cause apoptosis. Nucleoside antimetabolites target one or more specific enzyme(s). The mode of inhibitory action on the target enzyme is not always similar even among nucleoside antimetabolites which have the same nucleoside base, such as araC and gemcitabine. Although both nucleosides are phosphorylated by deoxycytidine kinase and are also good substrates of cytidine deaminase, only gemcitabine shows antitumor activity against solid tumors. This suggests that differences in the pharmacological activity of these nucleoside antimetabolites may reflect different modes of action on target molecules. The design, in vitro cytotoxicity, in vivo antitumor activity, metabolism and mechanism of action of sugar-modified cytosine nucleosides, such as (2'S)-2'-deoxy-2'-C-methylcytidine (SMDC), 1-(2-deoxy-2-methylene-beta-D-erythro-pentofuranosyl)cytosine (DMDC), 1-(2-C-cyano-2-deoxy-1-beta-D-arabino-pentofuranosyl)cytosine (CNDAC) and 1-(3-C-ethynyl-beta-D-ribo-pentofura-nosyl)cytosine (ECyd), developed by our groups, are discussed here.

Inducible liver injury in the transgenic rat by expressing liver-specific suicide gene

Suicide gene expression in specific tissue of transgenic animals has been used for cell-specific ablation. To examine the influence of hepatocyte removal, we produced the herpes simplex virus thymidine kinase (HSVtk) transgenic rat, whose gene was regulated by an albumin enhancer promoter. The liver presence of HSVtk was demonstrated in one line of the transgenic rats. We injected ganciclovir (GCV, 50mg/kg) into the rat on alternate days. After 28 days of GCV administration, liver tissues, and blood of the rats were collected. The histological investigation revealed infiltration of T cells, macrophages, granulocytes/neutrophils, and hepatocyte cell death. The biochemistry analysis demonstrated elevated levels of AST, ALT, and total bilirubin in transgenic rat. In conclusion, the transgenic rat with expressed albumin-specific HSVtk developed experimental hepatitis with administration of GCV, and will be a useful model to facilitate the evaluation of drug effects for clinical control of liver disease.

Drug discovery and development of antiviral agents for the treatment of chronic hepatitis B virus infection

A safe and effective vaccine for hepatitis B virus (HBV) has been available for nearly twenty years and currently campaigns to provide universal vaccination in developing countries are underway. Nevertheless, chronic HBV infection remains a leading cause of chronic hepatitis worldwide and there is a strong need for safe and effective antiviral therapies. Attempts to identify and develop antiviral agents to treat chronic HBV infection remains focused on nucleoside analogs such as 3TC (lamivudine), adefovir dipivoxil, (bis-POMPMEA), and others. However, advances in our understanding of the molecular biology of HBV and the development of new assays for HBV polymerase activity, such as the reconstitution of active HBV polymerase in vitro, should facilitate large screening efforts for non-nucleoside reverse transcriptase inhibitors. Recent advances have furthered our understanding of clinical resistance to lamivudine, have provided new approaches to treatment, and have offered new perspectives on the major challenges to the identification and development of antiviral agents for chronic HBV infection. Here, in an update to our previous review article that appeared in this series [59a], we focus on recent advances that have occurred in the areas of virus structure and replication, in vitro viral polymerase assays, cell culture systems, and animal models.

The identification and development of antiviral agents for the treatment of chronic hepatitis B virus infection

Hepatitis B virus (HBV) is the leading cause of chronic hepatitis throughout the world. Notwithstanding the availability of a safe and effective vaccine, the world prevalence of HBV has not declined significantly, thus resulting in the need for a selective antiviral agent. HBV is a small, partially double-stranded DNA virus which replicates through an RNA intermediate. Most efforts to develop anti-HBV agents have been targeted to the viral DNA polymerase which possesses reverse transcriptase activity. Currently, the most promising anti-HBV agents are nucleoside analogs which interfere with viral DNA replication. Although earlier nucleoside analogs such as vidarabine (ara-A) and fialuridine (FIAU) have displayed unacceptable toxicities, newer analogs such as lamivudine (3TC), bis-POM PMEA (GS-840), lobucavir, and BMS-200,475 have demonstrated clinical utility. In particular, the use of lamivudine has generated considerable interest in the development of other L-enantiomeric nucleoside analogs for use against HBV. Here, we provide an overview of HBV structure and replication strategy and discuss the use of cell culture systems, in vitro viral polymerase systems, and animal models to identify and evaluate anti-HBV agents. We also discuss the various classes of nucleoside analogs in terms of structure, mechanism of action, status in clinical development, ability to select for resistant HBV variants, and use in combination therapies. Finally, we present a discussion of novel antiviral approaches, including antisense and gene therapy, and address the various challenges to successful anti-HBV chemotherapeutic intervention.

Current status of anti-HBV chemotherapy

In the past decade, significant progress has been achieved in the battle against hepatitis B virus. In addition to the immunomodulating agents such as interferon-alpha and thymosin, many novel antiviral agents have been discovered, among which nucleoside analogues are the mainstay. New-generation compounds such as 3TC and famciclovir have shown promise in the treatment of patients chronically infected by this virus, and are on the line for approval. However, viral rebound after cessation of therapy still remains a major problem. Additionally, the reports on the drug resistance to these antiviral agents suggest that combination therapy will be the eventual strategy (Bartholomew et al., 1997; Tipples et al., 1996). Therefore, developments of safe and effective antiviral agents which do not cross-resist with currently available antiviral drugs are still much needed.

The FIAU Derivative (2′S)-2′-Deoxy-2′-C-Methyl-5-Iodouridine (SMIU) is a Novel, Less Cytotoxic and Potent anti-HSV and anti-VZV Agent

In this study, the anti-herpetic activities of novel 2′-methyl nucleoside analogues which were substituted at the 5 position of the pyrimidine with a halogen were investigated. The 2′-fluoro-5-iodo-aracytosine (FIAC) congeners (2′S)-2′-deoxy-2′- C-methylcytidine which were substituted with Br or I at the 5 position (SMBC or SMIC); and 2′-fluoro-5-iodo-arauridine (FIAU) congeners (2′S)-2′-deoxy-2′-C-methyluridine which were substituted with Br or I at the 5 position (SMBU or SMIU), proved to have potent antiviral activities against herpes simplex virus type-1 (HSV-1) and varicella-zoster virus (VZV) but not against herpes simplex virus type-2 (HSV-2). SMIU has a higher selective index against HSV-1 than FIAU, and both SMIC and SMIU showed higher inhibitory effects against VZV replication than aciclovir. The four effective compounds were not inhibitory to a thymidine kinase (TK)-negative HSV-1 strain, and this result indicates that phosphorylation of the compounds by HSV or VZV-TK is necessary for the activation of these compounds.

Mechanisms for the anti-hepatitis B virus activity and mitochondrial toxicity of fialuridine (FIAU)

Fialuridine (FIAU) is a thymidine nucleoside analog with activity against various herpesviruses and hepatitis B virus (HBV) in vitro and in vivo. In a clinical evaluation for its use as a treatment for chronic HBV infection, long term (HBV) in vitro and in vivo. In a clinical evaluation for its term oral administration of FIAU resulted in severe multi-organ toxicity characterized by a delayed onset and refractory lactic acidosis. These clinical manifestations led to the hypothesis that the toxicity of FIAU was mediated through mitochondrial dysfunction, possibly as a result of the inhibition of mitochondrial DNA polymerase gamma and/or incorporation of FIAU into mitochondrial DNA. In addition to describing the anti-HBV activity of FIAU, this review discusses results from in vitro experiments carried out by various laboratories in an effort to evaluate and understand more fully the mitochondrial toxicity of FIAU.

Therapy for chronic hepatitis B infection

Interferon-alpha is currently the only therapy approved for treatment of chronic HBV in the United States and Europe. Interferon-alpha therapy causes loss of HBeAg and HBV DNA in approximately a third of treated patients, and the loss of these markers of active viral replication is associated with improvements in hepatic histology and ALT levels. However, the long-term effects of interferon-alpha on morbidity and mortality, and especially on the incidence of the complications of chronic HBV infection, remain to be defined. The currently available treatment for chronic HBV is far from perfect. Interferon therapy is usually associated with significant side effects and requires subcutaneous administration. Additionally, there are a large number of patients who either fail to meet criteria for treatment, or who, with therapy, fail to respond (at least 60% of all patients). Moreover, interferon treatment is expensive (approximately $5,000 for a 16 week course of 5MU daily). Hence the search continues for effective, orally-available and cost-efficacious therapy. Of the agents available, the nucleoside analogues appear to have the greatest promise. The availability of cell culture systems and animal models for studying potential anti-HBV drugs will aid in the future development of these agents. Therapeutic vaccines, and combination therapies (given either concurrently or sequentially) may also play a future role in the management of chronic HBV infection. While prevention of disease must be a primary goal in the war against this common infection, a continued focus must be maintained on the treatment of the approximately 300 million individuals world-wide with established chronic HBV infection.

Different transmission rates of herpesvirus thymidine kinase reporter transgenes from founder male parents and male parents of subsequent generations

Previously we demonstrated that lines of transgenic mice carrying the herpes simplex type 1 virus thymidine kinase (HSV1-tk) reporter gene are male-sterile. Ectopic transcription of the HSV1-tk reporter in the testis was initiated downstream of the normal translation initiation codon and truncated proteins consistent with translational initiation at the second and third ATG codons were synthesized. Here we describe the effects on fertility 1) of converting the second and third ATG codons of the HSV1-tk reporter to CTG codons and 2) of utilizing the HSV type 2 thymidine kinase (HSV2-tk) reporter gene, in which the second ATG codon is located downstream of the ATP-binding pocket of the enzyme. Both reporters were coupled to the bovine thyroglobulin promoter (bTG-tk1 alpha and bTG-tk2 transgenes). The level of ectopic expression of these transgenes in the testis, relative to expression in the thyroid, was one to two orders of magnitude less than that of bTG-tk1. Sixty percent of male founders carrying the bTG-tk1 alpha and bTG-tk2 transgenes were fertile but did not transmit the transgene. In contrast, most males from subsequent generations were fertile and transmitted the transgenes at the expected frequency. This difference between founder males and male descendants is also observed with certain constructs in which the HSV1-tk reporter is coupled to other promoters. We attribute the effect to mosaicism among male founders, leading to competition between transgenic and nontransgenic spermatozoa and/or spermatogenic precursor cells and resulting in a lack of fertilization by transgenic sperm that would successfully fertilize eggs in the absence of competition.

Sensitive and specific radioimmunoassay for fialuridine: initial assessment of pharmacokinetics after single oral doses to healthy volunteers

Fialuridine (FIAU) is a halogen-substituted analog of thymidine that was undergoing clinical investigation as a drug for the treatment of chronic hepatitis B viral infection. However, clinical trials of FIAU were terminated after adverse events occurred following chronic oral administration. Prior to the termination of clinical trials, a sensitive assay was needed for the measurement of FIAU because of the anticipated low dose administered to patients. We therefore undertook the development of a radioimmunoassay (RIA). A specific antiserum was raised in rabbits following immunization with a 5'-O-hemisuccinate analog of FIAU coupled to keyhole limpet hemocyanin. Radiolabeled FIAU was synthesized by a destannylation procedure by using sodium [125I]iodide. We developed a competitive-binding procedure and used precipitation with polyethylene glycol as the method for separating the bound and free forms of FIAU. The RIA is sensitive (0.2 ng/ml), specific (negligible interference from known metabolites and endogenous nucleosides), and reproducible (interassay coefficients of variation range from 5 to 19.7% for serum controls). We used the RIA to assess the pharmacokinetics of FIAU in healthy adult volunteers following administration of a single 5-mg oral dose. The sensitivity of the RIA permitted the detection of a prolonged elimination phase for FIAU in healthy volunteers and dogs, with mean elimination half-lives of 29.3 and 35.3 h, respectively. We conclude the RIA is a valid method for the quantification of FIAU in biological fluids.

The in vitro anti-hepatitis B virus activity of FIAU [1-(2'-deoxy-2'-fluoro-1-beta-D-arabinofuranosyl-5-iodo)uracil] is selective, reversible, and determined, at least in part, by the host cell

A human hepatoblastoma cell line was stably transfected with a head-to-tail dimer of the Hepatitis B virus (HBV), subtype adw, genome to generate a cell line which produces HBV. FIAU [1-(2'-deoxy-2'-fluoro-1-beta-D-arabinofuranosyl-5-iodo)uracil] inhibited viral replication in these cells with an IC50 of 0.90 microM, as determined by PCR analysis of extracellular Dane particle DNA, and displayed a 50% cytotoxic concentration (TC50) of 344.3 microM, as determined using the MTT assay. The selectivity index of FIAU (TC50/IC50) was 382.6. In cells incubated for 10 days with FIAU (100 microM) and then incubated with drug-free media with daily media changes for 7 days, viral DNA replication was markedly inhibited but resumed within 24 h after drug removal, demonstrating that the in vitro anti-HBV activity of FIAU is reversible. Both the antiviral activity and cytotoxicity of FIAU were reversed by the addition of equimolar to 10-fold excess molar concentrations of thymidine. The de-iodinated metabolite of FIAU, FAU, had only marginal anti-HBV activity at 100 microM, indicating that this metabolite does not contribute significantly to the activity of FIAU. The examination of intracellular viral DNA replicative intermediates revealed that FIAU was 2000-fold more active against duck HBV DNA replication in human hepatoma cells (IC50 = 0.075 microM) than against this same virus in chicken liver cells (IC50 = 156 microM). FIAU was anabolized to a 25-fold greater extent in human hepatoma cells than in chicken cells, indicating that the anti-HBV activity of this nucleoside analog is dependent, in part, on its phosphorylation by the host cell.

Relationship between Conformation and Antiviral Activity-II. 5-Methoxymethyl-2′-deoxycytidine and 5-methoxymethyl-N4-methyl-2′-deoxycytidine

5-methoxymethyl-N4-methyl-2′-deoxycytidine (N4-Me-MMdCyd) and 5-methoxymethyl-N4-methyl-2′-deoxycytidine-5′-monophosphate (N4-Me-MMdCMP) were synthesized to confer resistance to deamination by deaminating enzymes. N4-Me-MMdCyd and N4-Me-MMdCMP were inactive against Herpes simplex virus type 1 (HSV-1) and also nontoxic to VERO cells up to 1796 μM (highest concentration tested). 5-methoxymethyl-2′-deoxycytidine-5′-monophosphate (MMdCMP) was more potent than the nucleoside against HSV-1 in VERO cells. In HSV-infected VERO cells (10 PFU/cell), N4-Me-MMdCyd caused only slight perturbations of deoxyribonucleoside triphosphate pools. 5-methoxymethyl-N4-methyl-2′-deoxycytidine-5′-triphosphate (N4-Me-MMdCTP) was synthesized and the nature of interaction of N4-Me-MMdCTP and dCTP with DNA polymerase of Escherichia coli, HSV-1 and human α was investigated. N4-Me-MMdCTP was neither an effective substrate nor a strong inhibitor of Escherichia coli, HSV-1 or human α DNA polymerase.

The relationship between molecular conformation and antiviral activity for MMdCyd and N4-Me-MMdCyd is discussed. The conformation of the deoxyribofuranose ring in MMdCyd and N4-Me-MMdCyd are different. In N4-Me-MMdCyd, the exocyciic C(5′) side chain has the t conformation whereas MMdCyd has the g+rotomer conformation. The orientation of the N4-methyl group may also impede binding to the HSV-induced kinase by steric hindrance and/or by hindering hydrogen bonding between the enzyme and the lone pair of electrons at N(3). The results suggest that attempts to render resistance to deamination by alkylation at the N(4) position of the cytosine moiety is not likely to yield compounds with activity against HSV.

Evaluation of 1-(2'-deoxy-2'-fluoro-beta-D-arabinofuranosyl)-5-ethyluracil in a rabbit model of herpetic keratitis

The nucleoside analog 1-(2'-deoxy-2'-fluoro-beta-D-arabinofuranosyl)-5- ethyluracil (FEAU) was tested in a rabbit model of acute herpetic keratitis and its effectiveness compared with that of acyclovir (ACV). FEAU or ACV was applied topically 3 times daily, beginning 3 days post-HSV-1 inoculation and continued for a period of 7 days. FEAU at a concentration of 1% (w/v) or 3% ACV resulted in significant lessening of the severity of corneal lesions, conjunctivitis, iritis, and corneal clouding at 24 to 48 h after beginning chemotherapy. No toxic reaction was observed in any rabbit eyes treated with either FEAU or ACV. The duration of virus shedding into tear film and colonization of the trigeminal ganglia, however, were not reduced by either FEAU or ACV treatment begun 3 days post-inoculation. Fifty percent effective dose (ED50) of FEAU determinations performed on isolates from tear film and on the virus inoculum in secondary rabbit kidney cultures yielded a range of 4.6-7 microM, with two in vitro resistant isolates having ED50S of greater than or equal to 1500 microM of FEAU. Fifty percent cell growth inhibition for FEAU was 3000 microM at 72 h.

Structure-activity relationship between (E)-5-(2-bromovinyl)- and 5-vinyl-1-beta-D-arabinofuranosyluracil (BV-araU, V-araU) in inhibition of Epstein-Barr virus replication

The structure-activity relationship between (E)-5-(2-bromovinyl)- and 5-vinyl-1-beta-D-arabinofuranosyluracil (BV-araU and V-araU) in inhibition of Epstein-Barr virus (EBV) was evaluated. Both V-araU and BV-araU effectively inhibited EBV replication in virus-producer P3HR-1(LS) cells, as determined by DNA-DNA hybridization. The 50% effective doses (ED50) for viral DNA replication were 0.005 and 0.3 microM for V-araU and BV-araU, respectively. The in vitro therapeutic index was 4000 for V-araU and 1300 for BV-araU. Synthesis of EBV-induced polypeptides with molecular weights of 145,000 (145, 140, 130, and 110 kDa) was significantly inhibited by both drugs. Only V-araU inhibited the synthesis of 85-, 55-, and 32-kDa polypeptides by approx. 50%. Kinetic analysis of inhibition and reversibility of EBV DNA replication after removal of the drugs indicated that BV-araU has a more prolonged inhibitory effect than V-araU. These results indicate that the substitution of H by Br in the 5-vinyl group results in marked reduction in anti-EBV activity while prolonging the drug effect and diminishing cytotoxicity.

Incorporation and metabolism of 2'-fluoro-5-substituted arabinosyl pyrimidines and their selective inhibition of viral DNA synthesis in herpes simplex virus type 1 (HSV-1)-infected and mock-infected Vero cells

The incorporation and metabolism of 2'-fluoro-5-substituted arabinosyl pyrimidine analogs, and their selective inhibition of viral DNA synthesis in herpes simplex virus type 1 (HSV-1)-infected and mock-infected Vero cells were studied by HPLC and CsCl isopycnic density gradient analysis of isolated DNAs. The amounts of radiolabeled analogs incorporated as parent compound following 10 microM exposure for 4 h were 10-fold higher in HSV-1-infected vs mock-infected cells for 2'-fluoro-5-difluoromethyl-Ara-U (F2FMAU); 4.3-fold higher for 5-ethyl deoxyuridine (EdU); 2.6-fold higher for 2'-fluoro-5-methyl-Ara-U (FMAU) and 1.7-fold higher for dThd. For 2'-fluoro-5-ethyl-Ara-U (FEAU), 3.0 pmole of unchanged moiety was incorporated per 10(6) HSV-1-infected cells but no incorporation was detected in mock-infected cells. HPLC profiles showed that the percentages of radiolabeled analogs incorporated as parent compound in the DNA extracted from HSV-1-infected cells were 31.0% for F2FMAU, 99.6% for EdU, 83.5% for FEAU and 98.3% for FMAU; from mock-infected cells, they were 63.6% for F2FMAU, 96.7% for EdU, 97.3% for FMAU and no incorporation into DNA for FEAU was detected. CsCl density gradient analyses of isolated DNA showed that viral DNA synthesis was inhibited 98% by 10 microM FEAU, 92% by 10 microM F2FMAU, 90% by 2 microM FMAU and 80% by 50 microM EdU, whereas cellular DNA synthesis was inhibited by 53, 44, 61, 66 and 54%, respectively. We conclude that: (a) FEAU incorporation into host-cell DNA was not detectable but FEAU was selectively incorporated into HSV-infected cells; (b) FMAU and FEAU were metabolically stable; however, F2FMAU was extensively metabolized; (c) FEAU and F2FMAU were among the most selective inhibitors of HSV-1 DNA synthesis while allowing cellular DNA synthesis to continue.

Nucleotide sequence changes in thymidine kinase gene of herpes simplex virus type 2 clones from an isolate of a patient treated with acyclovir

To identify the nucleotide changes that occur in drug-induced thymidine kinase (TK) mutants of herpes simplex virus type 2 (HSV-2), we compared the nucleotide sequences of the tk genes of two mutant HSV-2 clones isolated from a patient who had been treated with acyclovir [9-(2-hydroxyethoxymethyl)guanine; ACV] with the nucleotide sequence of the parental TK+ HSV-2(8703) strain isolated from the same patient. One of the mutants, TK-altered (TKA) HSV-2(9637), was ACV resistant but induced the incorporation of [14C]thymidine into the DNA of infected rabbit skin cells. The nucleotide sequence of the tk gene of mutant TKA HSV-2(9637) had a single change (G to A) at nucleotide 668, which would cause an arginine-to-histidine substitution at amino acid residue 223 of the TK polypeptide. The second ACV-resistant mutant, TK- HSV-2(8710), did not induce detectable incorporation of [14C]thymidine into the DNA of infected rabbit skin cells. This mutant exhibited a deletion of a single base at nucleotide 217 of its nucleotide sequence. This deletion would cause a frameshift mutation at amino acid residue 73 and chain termination at amino acid residue 86 of the TK polypeptide. The nucleotide sequence of TK+ HSV-2(8703) was the same as that of the laboratory strain, TK+ HSV-2(333). The nucleotide sequence of a bromodeoxyuridine-resistant TK- HSV-2(333) mutant of TK+ HSV-2(333) also exhibited a single-base deletion, but at nucleotide 439. This deletion would cause a frameshift mutation at amino acid residue 147 and chain termination at amino acid residue 182. The frameshift mutations of TK- HSV(8710) and TK- HSV-2(333), respectively, occurred in sequences in which C was repeated three times and G was repeated seven times. The results raise the possibility that TK- frameshift mutations of HSV-2 may be common.

In vitro and in vivo antiviral activity of 2'-fluorinated arabinosides of 5-(2-haloalkyl)uracil

5-(2-Fluoroethyl)-2'-deoxyuridine (FEDU), its 2'-fluoroarabinofuranosyl analog (FEFAU) and the 2'-fluoroarabinofuranosyl analog (CEFAU) of the potent anti-herpesvirus compound 5-(2-chloroethyl)-2'-deoxyuridine (CEDU) were evaluated for activity against herpes simplex virus type 1 (HSV-1) and HSV-2 in vitro and in vivo. FEDU, FEFAU and CEFAU proved to be potent and selective anti-herpesvirus agents in vitro. Their potency is evident from their low minimum inhibitory concentrations for HSV-1 and HSV-2, and their selectivity is attested by the marginal inhibition of cell proliferation at relatively high concentrations, and by the high concentrations at which DNA-, RNA- or protein synthesis in normal uninfected host cells is inhibited. Their activity spectrum is broader than that of CEDU: in addition to being highly effective against HSV-1 replication, these derivatives, in particular FEFAU, inhibit HSV-2 replication at concentrations comparable to acyclovir (ACV). In the systemic and cutaneous HSV-1 infection models in mice, FEDU, FEFAU and CEFAU were markedly less potent than CEDU in suppressing the development of lesions and in reducing the mortality rate. In HSV-2 infections in mice and in guinea pigs FEDU, FEFAU and CEFAU were virtually ineffective. CEDU, however, exerted a protective effect in these animal models, albeit at relatively high concentrations.

Relationship between structure and antiviral activity of 5-methoxymethyl-2'-deoxyuridine and 5-methoxymethyl-1-(2'-deoxy-beta-D-lyxofuranosyl)uracil

5-Methoxymethyl-1-(2'-deoxy-beta-D-lyxofuranosyl)uracil (MMdLU) was not active against the herpes simplex viruses. The relationship between molecular conformation and antiviral activity for the two epimers, 5-methoxymethyl-2'-deoxyuridine (MMdUrd) and MMdLU, is discussed. MMdUrd was phosphorylated by the virus-induced deoxythymidine kinase. In contrast, MMdLU did not serve as a substrate for the kinase. The geometry and distance between the 5'-CH2OH and 3'-OH groups of the furanose ring appear to be key factors in determining the efficiency of phosphorylation by the virus-induced deoxythymidine kinase, and hence antiviral activity.

Comparative efficacy of three 2'-fluoropyrimidine nucleosides and 9-(1,3-dihydroxy-2-propoxymethyl)guanine (BW B759U) against pseudorabies and equine rhinopneumonitis virus infection in vitro and in laboratory animals

The three 2'-fluoropyrimidine nucleosides 1-(2-deoxy-2-fluoro-beta-D-arabinofuranosyl)-5-iodocytosine (FIAC), 1-(2-deoxy-2-fluoro-beta-D-arabinofuranosyl)-5-iodouracil (FIAU), and 1-(2-deoxy-2-fluoro-beta-D-arabinofuranosyl)-5-methyluracil (FMAU), showed high activity in RK13 monolayers against equine rhinopneumonitis virus, (EHV-1, IC50 range 0.02-0.18 microM), Aujeszky's disease virus (SHV-1, pseudorabies, IC50 range 0.25-7 microM) and infectious bovine rhinotracheitis virus (1BR, BHV-1, IC50 range 0.1-3 microM). The activity of these compounds was compared with 9-(1,3-dihydroxy-2-propoxymethyl)guanine (BW B759U, DHPG) in two laboratory animal disease models: EHV-1-induced hepatitis in hamsters and SHV-1-induced encephalitis in mice. All the compounds, provided from 3 to 5 h pre-infection for 5 days, were effective in preventing EHV-1 mortality (at 3-5 mg/kg per day) and in significantly reducing SHV-1 mortality (at 60 mg/kg per day). While FIAU had the greatest activity in vitro, FMAU tended to be more potent in vivo. The reasons for these differences between relative in vitro and in vivo activities are briefly discussed.

Enzymology and pathogenicity in mice of a herpes simplex virus type 1 mutant resistant to 2'-fluoro-2'-deoxy-1-beta-D-arabinofuranosyl-5-iodocytosine

The deoxypyrimidine nucleoside analog 2'-fluoro-2'-deoxy-1-beta-D-arabinofuranosyl-5-iodocytosine (FIAC) is a potent and selective inhibitor of herpes simplex virus type 1 in vitro (C. Lopez, K. A. Watanabe, and J. J. Fox, Antimicrob. Agents Chemother. 17:803-806, 1980). Isopycnographic analysis demonstrated that 1 microM FIAC inhibited herpes simplex virus DNA replication by more than 95% but inhibited cellular DNA replication by only 32%. Mutant herpes simplex virus type 1 selected resistant to FIAC displayed linked resistance to other nucleoside analogs, including arabinosylthymine and acyclovir. Lysates derived from Vero cells infected with FIAC-resistant virus showed markedly lower levels of thymidine kinase activity and were unable to phosphorylate selectively arabinosylthymine or FIAC, in contrast to lysates from cells infected with wild-type herpes simplex virus type 1. Finally, drug-resistant virus displayed a 6,000-fold decrease in pathogenicity when inoculated intraperitoneally into genetically susceptible A/J mice. These results indicate that resistance to deoxypyrimidine nucleoside analogs is due, at least in part, to alterations in viral thymidine kinase and is accompanied by decreased pathogenicity in vivo.

Mutagenic potential of anti-herpes agents

A number of anti-herpes agents which are either licensed for clinical use (acyclovir) or subject of clinical studies (bromovinyldeoxyuridine, fluoroiodoaracytidine, dihydroxypropoxymethylguanine) or under preclinical investigation (i.e., fluoroiodoarauridine), fluoromethylarauridine, dihydroxybutylguanine, bromovinyldeoxycytidine, bromovinylarauridine and carbocyclic bromovinyldeoxyuridine) were evaluated for their ability to induce sister chromatid exchange (SCE), an indicator of mutagenesis. SCE was scored on metaphase chromosomes of human lymphocytes which had been exposed to 5-bromo-2'-deoxyuridine and varying concentrations of the test compounds. The antiviral assays were based on the inhibition of the cytopathogenicity of herpes simplex virus for human diploid fibroblasts. Most compounds, i.e. acyclovir, bromovinyldeoxyuridine or carbocyclic bromovinyldeoxyuridine, did either not induce SCE or only so at concentrations far above their minimum antiviral concentrations. However, fluoroiodoaracytidine and dihydroxypropoxymethylguanine were found to affect the SCE rate at a concentration (greater than or equal to 4.5 micrograms/ml) that is readily achievable in blood following intravenous injection.