Incorporation of metabolites of 2'-fluoro-5-iodo-1-beta-D-arabinofuranosylcytosine into deoxyribonucleic acid of neoplastic and normal mammalian tissues

Alkyl-fluorinated thymidine derivatives for imaging cell proliferation

Derivatives of 2'-deoxyuridine that contain fluoroalkyl groups at the C5 position and derivatives of thymidine that contain fluoroalkyl groups at the N3 position were synthesized and examined in three in vitro assays designed to evaluate their potential as radiopharmaceuticals for imaging cellular proliferation. Three of the former nucleosides and five of the latter were synthesized. The three assays were as follows: (a) phosphoryl transfer assay, which showed that all three of the former nucleosides and four of the latter ones were phosphorylated by recombinant human thymidine kinase 1 (TK1) and that N(3)-(2-fluoroethyl)-thymidine (NFT202) was the most potent substrate of the eight nucleosides studied; (b) transport assay, which indicated that all eight nucleosides had good affinity for an 6-[(4-nitrobenzyl)thio]-9-beta-d-ribofuranosylpurine-sensitive mouse erythrocyte nucleoside transporter, with inhibition constants in the range of 0.02-0.55 mM; and (c) degradation assay, which showed that all but one of the former nucleosides and none of the latter were degraded by recombinant Escherichia coli thymidine phosphorylase (an enzyme that catalyzes the glycosidic bond of thymidine and 2'-deoxyuridine derivatives). From these in vitro screening assays, we selected NFT202 as a candidate for subsequent in vivo evaluation because this compound met the three minimum requirements of the in vitro screening assays and had the most potent phosphorylation activity as a substrate for recombinant human TK1.

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.

Antiviral activity and toxicity of fialuridine in the woodchuck model of hepatitis B virus infection

Woodchucks were used to study the antiviral activity and toxicity of fialuridine (FIAU; 1,-2'deoxy-2'fluoro-1-beta-D-arabinofuranosyl-5-iodo-uracil). In an initial experiment, groups of six chronic woodchuck hepatitis virus (WHV) carrier woodchucks received daily doses of FIAU by intraperitoneal injection for 4 weeks. At 0.3 mg/kg/d, the antiviral effect was equivocal, but at 1.5 mg/kg/d, FIAU had significant antiviral activity. No evidence of drug toxicity was observed during the 4-week period of treatment or during posttreatment follow-up. In a second experiment, groups of nine WHV carriers or uninfected woodchucks were given 1.5 mg/kg/d of FIAU orally for 12 weeks, and the results compared with placebo-treated controls. After 4 weeks, the serum WHV-DNA concentration in the FIAU-treated carrier group was two to three logs lower than that in the placebo-treated group. After 12 weeks of FIAU treatment, serum WHV DNA was not detectable by conventional dot-blot analysis, hepatic WHV-DNA replicative intermediates (RI) had decreased 100-fold, and hepatic expression of WHV core antigen was remarkably decreased. No evidence of toxicity was observed after 4 weeks, but, after 6 to 7 weeks, food intake decreased and, after 8 weeks, the mean body weights of woodchucks treated with FIAU were significantly lower than controls. Anorexia, weight loss, muscle wasting, and lethargy became progressively severe, and all FIAU-treated woodchucks died or were euthanized 78 to 111 days after treatment began. Hepatic insufficiency (hyperbilirubinemia, decreased serum fibrinogen, elevated prothrombin time), lactic acidosis, and hepatic steatosis were characteristic findings in the final stages of FIAU toxicity in woodchucks. The syndrome of delayed toxicity in woodchucks was similar to that observed previously in humans treated with FIAU, suggesting that the woodchuck should be valuable in future investigations of the molecular mechanisms of FIAU toxicity in vivo and for preclinical toxicological evaluation of other nucleoside analogs before use in patients.

Preparation of 5-[131I]iodo- and 5-[211At]astato-1-(2-deoxy-2-fluoro-beta-D-arabinofuranosyl) uracil by a halodestannylation reaction

To circumvent the in vivo instability of 5-iodo-2'-deoxyuridine (IUdR), a 2'-fluorine-substituted analogue, 5-iodo-1-(2-deoxy-2-fluoro-beta-D-arabinofuranosyl)uracil (FIAU) recently has been introduced. To facilitate the preparation of radioiodinated FIAU as well as its astatinated analogue, a tin precursor, 5-trimethylstannyl-1-(2-deoxy-2-fluoro-beta-D-arabinofuranosyl)ura cil (FTAU) was synthesized. Both [125/131I]FIAU and 5-[211At]astato-1-(2-deoxy-2-fluoro-beta-D-arabinofuranosyl)uracil (FAAU) were prepared from FTAU in more than 85% radiochemical yield under mild conditions. The in vitro serum stability of both fluorine-substituted derivatives was higher than that of the corresponding unsubstituted parents. The enhanced stability of fluorinated derivatives was even more apparent in whole blood. The uptake of [125I]FIAU in D-247 MG human glioma cells in vitro was 20-fold higher than that of [125I]IUdR over an activity concentration range of 5-100 kBq/mL; the uptake of FAAU was not significantly different from that of 5-[211At]astato-2'-deoxyuridine (AUdR). Accumulation of radioiodine in mouse thyroid in vivo with [131I]FIAU was fivefold lower than [125I]IUdR, indicating that the former was less susceptible to deiodination. The tissue uptake of FAAU was similar to that reported for AUdR.

Differential effects of the incorporation of 1-(2-deoxy-2-fluoro-beta-D-arabinofuranosyl)-5-iodouracil (FIAU) on the binding of the transcription factors, AP-1 and TFIID, to their cognate target DNA sequences

The thymidine analog, 1-(2-deoxy-2-fluoro-beta-D-arabino-furanosyl)-5-iodouracil (FIAU), is incorporated into DNA in cell culture and in vivo. To investigate the effect of incorporation of FIAU into DNA on the binding of transcription factors, oligonucleotide duplexes which bind specifically to activator protein 1 (AP-1) or to TFIID were synthesized and binding of these oligonucleotides to their respective proteins was studied using gel-shift analysis. When thymidine at position -3, -1, 1 or 7 (relative to the first thymidine of the core binding sequence) was replaced with FIAU, binding to AP-1 was approximately 82, 28, 86 and 51%, respectively, of the binding to the non-substituted oligonucleotide to AP-1. When thymidine at position 3 or 5 (each adjacent to the center of dyad symmetry) was replaced with FIAU, binding to AP-1 was abrogated. Oligonucleotides containing FIAU at positions -1, 3 or 5, were much less able to compete with radiolabeled wild-type oligonucleotides for binding to AP-1. In contrast, the presence of FIAU, depending on its location, resulted in the increased binding of TFIID to its consensus target DNA sequence. These results indicate that incorporation of FIAU into DNA may induce local conformational changes resulting in the altered ability of transcriptional factors to bind to their cognate DNA sequences. Additional studies demonstrated that the presence of FIAU at a position 5' to the cleavage site in the consensus sequence T*TAA (where * is the cleavage site) inhibited restriction of the oligomeric duplex by MseI.

Fialuridine and its metabolites inhibit DNA polymerase gamma at sites of multiple adjacent analog incorporation, decrease mtDNA abundance, and cause mitochondrial structural defects in cultured hepatoblasts

The thymidine analog fialuridine deoxy-2-fluoro-beta-D-arabinofuranosyl)-5-iodouracil (FIAU) was toxic in trials for chronic hepatitis B infection. One mechanism postulated that defective mtDNA replication was mediated through inhibition of DNA polymerase-gamma (DNA pol-gamma), by FIAU triphosphate (FIALTP) or by triphosphates of FIAU metabolites. Inhibition kinetics and primer-extension analyses determined biochemical mechanisms of FIAU, 1-(2-deoxy-2-fluoro-beta-D-arabinofuranosyl) -5-methyluracil (FAU), 1-(2-deoxy-2-fluoro-beta-D-arabinofuranosyl)uracil triphosphate (TP) inhibition of DNA pol-gamma. dTMP incorporation by DNA pol-gamma was inhibited competitively by FIAUTP, FMAUTP, and FAUTP (K1=0.015, 0.03, and 1.0 microM, respectively). By using oliginucleotide template-primers. DNA pol-gamma incorporated each analog into DNA opposite a single adenosine efficiently without effects on DNA chain elongation. Incorporation of multiple adjacent analogs at positions of consecutive adenosines dramatically impaired chain elongation by DNA pol-gamma. Effects of FIAU, FMAU, and FAU on HepG2 cell mmtDNA abundance and ultrastructure were determined. After 14 days, mtDNA decreased by 30% with 20 microM FIAU or 20 microM FMAU and decreased less than 10% with 100 microM FAU. FIAU and FMAU disrupted mitochondria and caused accumulation of intracytoplasmic lipid droplets. Biochemical and cell biological findings suggest that FIAU and its metabolites inhibit mtDNA replication, most likely at positions of adenosine tracts, leading to decreased mtDNA and mitochondrial ultrastructural defects.

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.

Synthesis of 2'-fluoro-5-[11C]-methyl-1-beta-D-arabinofuranosyluracil ([11C]-FMAU): a potential nucleoside analog for in vivo study of cellular proliferation with PET

Rapid in vivo catabolism limits the use of currently available radiotracers used in tumor proliferation studies with PET. This is manifested by the need to develop complex mathematical models to interpret kinetic and metabolite data obtained from imaging studies with agents such as carbon-11 labeled thymidine. A potential carbon-11 labeled radiotracer for cellular proliferation, 2'-fluoro-5-([11C]-methyl)-1-beta-D-arabinofuranosyluracil (FMAU), has been prepared using a previously described method for preparation of [11C]methyl-thymidine where selective alkylation of a pyrimidyl dianion is accomplished with [11C]methyl iodide at the 5-position of the pyrimidine ring. FMAU shares many in vivo characteristics of thymidine, including cellular transport, phosphorylation by mammalian kinase, and incorporation into DNA. Most importantly, in vivo catabolism of FMAU is limited, potentially yielding simplified kinetic models for determination of cellular proliferation with positron emission tomography.

Cellular and molecular events leading to mitochondrial toxicity of 1-(2-deoxy-2-fluoro-1-beta-D-arabinofuranosyl)-5-iodouracil in human liver cells

We have explored the mechanism(s) related to FIAU-induced liver toxicity, particularly focusing on its effect on mitochondrial function in a human hepatoma cell line-HepG2. The potential role of FMAU and FAU, metabolites detected in FIAU-treated patients were also ascertained. FIAU and FMAU inhibited cell growth and were effectively phosphorylated. A substantial increase in lactic acid production in medium of cells incubated with 1-10 microM FIAU or FMAU was consistent with mitochondrial dysfunction. Slot blot analysis demonstrated that a two week exposure to 10 microM FIAU or FMAU was not associated with a decrease in total mitochondrial (mt) DNA content. However, FIAU and FMAU were incorporated into nuclear and mtDNA and relative values suggest that both compounds incorporate at a much higher rate into mtDNA. Electron micrographs of cells incubated with 10 microM FIAU or FMAU revealed the presence of enlarged mitochondria with higher cristae density and lipid vesicles. In conclusion, these data suggest that despite the lack of inhibition of mtDNA content, incorporation of FIAU and FMAU into mtDNA of HepG2 cells leads to marked mitochondrial dysfunction as evidenced by disturbance in cellular energy metabolism and detection of micro- and macrovesicular steatosis.

Fialuridine accumulates in DNA of dogs, monkeys, and rats following long-term oral administration

Accumulation of the antiviral nucleoside analogue fialuridine (FIAU; 1-(2'-deoxy-2'-fluoro-beta-D-arab-inofuranosyl-5-iodouracil) in genomic DNA was examined with a modified version of a recently developed RIA for FIAU. DNA was obtained from tissues of dogs administered FIAU at 0, 1, 2, or 3 mg/kg of body weight per day for 90 days, monkeys administered FIAU at 0 or 25 mg/kg per day for 30 days, and rats administered FIAU at 0, 255, or 510 mg/kg per day for 70 days. FIAU incorporation was observed in all species. In the rat, FIAU was incorporated into DNA of all tissues examined, with highest concentrations in the liver followed by jejunum, spleen, and heart. FIAU was also incorporated into sperm DNA. Incorporation rates were as high as 11,000 pmol of FIAU per mumol of thymidine or 1 FIAU molecule per 90 thymidine molecules. In dogs and rats, the extent of incorporation was dose-dependent. Across species, FIAU concentrations in DNA were not singly dependent on the total dose administered but also may have been dependent on the duration of exposure. These studies show that FIAU accumulates to high concentrations in genomic DNA of liver as well as other tissues during chronic oral administration and suggest that net accumulation of FIAU in DNA may be a critical step in FIAU-induced toxicity.

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.

Effect of fialuridine on replication of mitochondrial DNA in CEM cells and in human hepatoblastoma cells in culture

Fialuridine (FIAU) is a nucleoside analog with potent activity against hepatitis B virus in vitro and in vivo. In this report, the effect of FIAU on mitochondrial DNA (mtDNA) replication in vitro was investigated. CEM cells, a cell line derived from human T cells, were incubated for 6 days in up to 20 microM FIAU. Total cellular DNA was isolated, normalized for the number of cells, and slot hybridized to a probe specific for mtDNA sequences. Treatment of CEM cells with FIAU did not result in a dose-dependent decrease in the amount of mtDNA. In contrast, dideoxycytidine (ddC) inhibited mtDNA replication by 50% at a concentration of approximately 0.1 microM. After 6 days of incubation, both compounds displayed a 50% toxic dose at a concentration of approximately 2 microM in CEM cells and approximately 34 microM in human hepatoblastoma cells (HepG2). In further experiments, CEM cells were incubated for 15 days in up to 2.5 microM FIAU, and again, no inhibition of mtDNA was observed. Over a 6-day incubation, FIAU, at concentrations of up to 200 microM, also failed to inhibit mtDNA replication in either HepG2 or HepG2 cells which constitutively replicate duck hepatitis B virus. In contrast, ddC inhibited mtDNA replication in these cells with a 50% inhibitory concentration of approximately 0.2 microM over a 6-day incubation. Treatment of cells with either FIAU or ddC resulted in a dose-dependent increase in lactate levels in the cell medium, indicating that any effect of FIAU on mitochondrial function may not be related to inhibition of mtDNA replication on the basis of the in vitro data. Alternative explanations for mitochondrial toxicity are considered.

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.

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.

Targeting of an inducible toxic phenotype in animal cells

We have developed a toxic, or suicide, vector whose action is based on the targeted expression of the herpes simplex virus 1 thymidine kinase gene product in cultured cells or transgenic animals. This protein is able to convert nucleoside analogs such as acyclovir and 1-(2-deoxy-2-fluoro-beta-D-arabino-furanosyl)-5-iodouracil (FIAU) to toxic intermediates. The activation of these compounds disrupts cellular DNA replication, leading to rapid cell death. Neither acyclovir, FIAU, nor the herpes thymidine kinase alone is harmful to cells. This approach is simple and should have widespread applicability in studying lineage formation in cultured cells and transgenic animals.

Synthesis and biological effects of 2'-fluoro-5-ethyl-1-beta-D-arabinofuranosyluracil

2'-Fluoro-5-ethyl-1-beta-D-arabinofuranosyluracil (FEAU) was synthesized, and its biological activities were compared with those of 2'-fluoro-5-methyl-1-beta-D-arabinofuranosyluracil (FMAU). Earlier studies indicated that both compounds showed potent anti-herpes simplex virus activity, with a 50% effective dose (ED50) of less than 0.25 microM. In the present study the cell growth inhibitory activity of FEAU (ED50, 200 to 2,060 microM) was found to be about 100-fold less than that of FMAU. With an ED50 ranging from 630 to 3,700 microM, FEAU only weakly inhibited thymidine incorporation into DNA, as compared with FMAU with an ED50 of 9 to 28 microM. Following exposure to [2-14C]FEAU (100 microM), 0.48 pmol/10(6) cells per h was incorporated into the DNA of herpes simplex virus type 1-infected Vero cells, whereas no detectable incorporation was found in uninfected Vero cells or L1210 cells. The Ki of FEAU for thymidine kinase purified from human leukemic cells was greater than 150 microM. For herpes simplex virus type 1- and 2-encoded thymidine kinases, the Kis were 0.6 and 0.74 microM, respectively. Both FEAU and FMAU were relatively nontoxic for mice, with a 50% lethal dose of greater than 800 mg/kg per day (four intraperitoneal doses). However, the lethal dose of FEAU for dogs was 100 mg/kg per day (10 intravenous doses), a dose which is 40- to 80-fold greater than the toxic dose of FMAU. These results suggest that FEAU is a worthy candidate for further development as an antiherpetic agent.

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.

Quantitative determination of antiviral nucleoside analog in DNA

A technique for the analysis of the amount of an antiviral nucleoside analog incorporated into DNA, utilizing enzymatic digestion of DNA, followed by high-performance liquid chromatography is described. The cells or tissue samples were treated with perchloric acid to inactivate the nucleases, then digested with pronase in the presence of EDTA. DNA was purified by CsCl centrifugation followed by Sephadex chromatography and treatment with deoxyribonuclease 1 and venom phosphodiesterase. The deoxyribonucleoside monophosphates and the monophosphate of the nucleoside analog liberated from DNA were separated and quantitated by HPLC analysis and measurement of radioactivity. This assay is more sensitive, specific, and precise than the determination of DNA density shift. It is also applicable for nucleoside analogs which do not change the density of DNA either because of their structure or their very small degree of incorporation.

Activities of 1-(2-deoxy-2-fluoro-beta-D-arabinofuranosyl)-5-iodocytosine and its metabolites against herpes simplex virus types 1 and 2 in cell culture and in mice infected intracerebrally with herpes simplex virus type 2

As measured by plaque and yield reduction assays, several metabolites of 1-(2-deoxy-2-fluoro-beta-D-arabinofuranosyl)-5-iodocytosine (FIAC) were highly active against herpes simplex virus types 1 and 2. These metabolites included the 2'-deoxy-2'-fluoroarabinosyl derivatives of 5-iodouracil (FIAU), cytosine (FAC), uracil (FAU), and thymine (FMAU). In mice inoculated intracerebrally with herpes simplex virus type 2, the relative order of potency of these compounds and licensed antiviral drugs was as follows: FMAU much greater than FIAC approximately equal to FIAU greater than acyclovir approximately equal to vidarabine much greater than FAC approximately equal to FAU. One of the main metabolites of FMAU, 2'-fluoro-5-hydroxymethyl-arabinosyluracil, was essentially inactive in vivo. FIAC-, FIAU-, FMAU-, FAC-, and FAU-resistant herpes simplex virus variants prepared in cell culture were found to be (i) devoid of viral thymidine kinase, (ii) cross-resistant to one another and resistant to drugs requiring viral thymidine kinase for activation, and (iii) sensitive to vidarabine or phosphonoformate. These results indicate that FIAC, FIAU, and FMAU require the virally encoded thymidine kinase for activation and suggest that the antiviral activity of FAU and FAC in cell cultures is also mediated by this enzyme. The interaction of the fluoroarabinosyl pyrimidine nucleosides with herpes simplex virus thymidine kinase in a cell-free system is also described.

Comparative anti-herpesvirus activities of 9-(1,3-dihydroxy-2-propoxymethyl)guanine, acyclovir, and two 2'-fluoropyrimidine nucleosides

9-(1,3-Dihydroxy-2-propoxymethyl)guanine (DHPG), was evaluated in cell culture and in animals for its inhibitory effect on herpes simplex viruses. Compounds run for comparison included acyclovir, 2'-fluoro-2'-deoxy-5-iodo-arabinofuranosylcytosine (FIAC), and 2'-fluoro-2'-deoxy-5-methyl-arabinofuranosyluracil (FMAU). In plaque reduction assays DHPG, acyclovir, FIAC, and FMAU were inhibitory to six herpes types 1 and 2 virus strains at concentrations of 0.2-2.4 microM. These concentrations were much lower than those required to inhibit Vero cell proliferation. In guinea pig vaginal infections, DHPG provided significantly greater inhibition of herpetic lesions than did acyclovir. In a herpes type 2 infection model in mice, DHPG, and FMAU were active at 5 mg/kg, whereas acyclovir and FIAC showed no statistically significant effect at 80 mg/kg. In a herpes type 1 encephalitis model, DHPG and FMAU were active at doses less than 10 mg/kg, with FMAU being about 4 times more potent than DHPG in that model.

Biotransformation and elimination of [2-14C]-1-(2-deoxy-2'-fluoro-beta-D -arabinofuranosyl)-5-iodocytosine in immunosuppressed patients with herpesvirus infections

The metabolism of the drug [2-14C]-1-(2'-deoxy-2'-fluoro-beta-D -arabinofuranosyl)-5-iodocytosine (FIAC), a potent inhibitor of herpesvirus replication, was studied in immunosuppressed patients with herpesvirus infections. FIAC was administered intravenously by 15-min infusion and by mouth 24 h later to four patients at doses of 50 or 100 mg/m2. FIAC was cleared from the plasma primarily by biotransformation in liver, kidney, and peripheral blood, with a terminal-phase half-life of 0.92 to 1.80 h (mean, 1.36 h) after intravenous administration. The area under the concentration-time curve from zero to infinity (AUC0-infinity) for FIAC was 1.6 to 4.7% (mean, 3.4%) of the AUC0-infinity for total radioactivity. 1-(2'-Deoxy-2'-fluoro-beta-D-arabinofuranosyl)-5-iodouracil (FIAU) was the major metabolite; the AUC0-infinity for FIAU was 54.3 to 72.5% (mean, 63.4%) of the AUC0-infinity for total radioactivity. The terminal-phase half-life for FIAU was 3.32 to 4.49 h (mean, 3.91 h); FIAU was cleared from plasma by renal elimination and further biotransformation. lesser amounts of 1-(2'-deoxy-2'-fluoro-beta-D-arabinofuranosyl)uracil, 1-(2'-deoxy-2'-fluoro-beta-D-arabinofuranosyl)cytosine, the glucuronide conjugates of these metabolites, and the glucuronide conjugates of FIAC and FIAU were also formed. A comparison of the AUC0-infinity for total radioactivity after intravenous and oral administration suggested that nearly all of the oral dose was absorbed. Plasma levels of FIAU, also a potent inhibitor of herpesvirus replication in vitro, exceeded the 50% effective dose for herpes simplex virus and varicella-zoster virus as late as 12 h after administration of FIAC.

Kinetics of the interaction of monophosphates of the antiviral nucleosides 2'-fluoro-1-beta-D-arabinofuranosylpyrimidine and (E)-5-(2-bromovinyl)-2'-deoxyuridine with thymidylate kinases from Vero cells and herpes simplex virus types 1 and 2

The affinities of the monophosphates of 2'-fluoro-5-iodo-1-beta-D-arabinofuranosyluracil and its 5-methyl analog for cellular thymidylate kinase were two or more orders of magnitude greater than for the thymidine-thymidylate kinases from herpes simplex virus types 1 and 2. In contrast, the monophosphate of (E)-5-(2-bromovinyl)-2'-deoxyuridine was found to have a higher affinity for the viral enzymes than for the cellular enzyme.

The inhibition of ultraviolet radiation-induced DNA repair in human diploid fibroblasts by arabinofuranosyl nucleosides

The antiviral compounds 9-beta-D-arabinofuranosyladenine (ara-A), 9-beta-D-arabinofuranosyl-2-fluoroadenine (FAA), 9-beta-D-arabinofuranosylhypoxanthine (ara-Hx), 9-beta-D-arabinofuranosylguanine (ara-G), 1-beta-D-arabinofuranosylthymine (ara-T), 1-beta-D-arabinofuranosyl-2'-fluorocytosine (FAC), 1-beta-D-arabinofuranosyl-2'-fluoro-5-iodocytosine (FIAC) and 1-beta-D-arabinofuranosyl-2'-fluoro-5-methyluracil (FMAU) were compared to 1-beta-D-arabinofuranosyl cytosine (ara-C) in their ability to inhibit ultraviolet (UV) light-induced DNA repair in log phase and confluent human diploid fibroblasts. Inhibition of the polymerization or ligation steps of DNA excision repair manifests itself in the form of DNA single-strand breaks which may be quantitated through velocity sedimentation analysis in alkaline sucrose gradients. In UV-irradiated quiescent, confluent human fibroblast cultures, treatment with any of the aranucleosides leads to accumulation of single-strand breaks but the effective dose for this inhibition varies greatly. The order of their effectiveness in confluent cultures was ara-C and its derivatives greater than ara-A, FAA, ara-G, ara-Hx greater than ara-T. In rapidly cycling cells on the other hand, sensitivity to repair inhibition was exhibited only in response to ara-C and FAC. If 2 mM hydroxyurea (HU) was administered with ara-A, FAA or FMAU, however, DNA strand breaks were seen. HU also increased the efficiencies of ara-C and FAC. No significant strand breaks were observed in UV-irradiated log phase cells treated with FIAC, ara-Hx, ara-G or ara-T even in the presence of HU. The efficiencies of inhibition of unscheduled DNA synthesis (UDS) and semiconservative DNA synthesis by the aranucleosides is consistent with their relative efficiencies at producing strand breaks. The ability of the aranucleosides to inhibit DNA repair is discussed with respect to their hypothesized effects on DNA metabolic processes in eucaryotic cells.