Inhibitory effects of 2'-fluorinated arabinosyl-pyrimidine nucleosides on woodchuck hepatitis virus replication in chronically infected woodchucks

Pharmacokinetics of (-)-β-D-2,6-Diaminopurine Dioxolane and its Metabolite, Dioxolane Guanosine, in Woodchucks (Marmota Monax)

The woodchuck ( Marmota monax) is a useful animal model for evaluating the in-vivo efficacy of antiviral agents against hepatitis B viral infection (HBV). The pharmacokinetics of a newly synthesized antiviral agent (-)-β-D-2,6-diaminopurine dioxolane (DAPD) in woodchucks is reported. DAPD is a nucleoside analogue, having potent and selective activity against human immunodeficiency virus and HBV in vitro. DAPD is susceptible to deamination in vivo by the ubiquitously present enzyme adenosine deaminase yielding the active metabolite dioxolane guanosine (DXG). The pharmacokinetics of DAPD and DXG were characterized following intravenous (i.v.) and oral (p.o.) administration of 20 mg kg−1 of DAPD to woodchucks. Plasma and urine samples were collected, and nucleoside concentrations were determined by HPLC. Following intravenous administration, the half-life of DAPD averaged 6.7 ± 4.3 h, and that of DXG averaged 17.6 ± 14.5 h. The mean total clearance and steady state volume of distribution of DAPD were 0.33 ± 0.14 L h kg−1 and 1.76 ± 0.65 L kg−1, respectively. The oral bioavailability of DAPD ranged from 3.7-8.2%; however, the apparent availability of DXG following oral administration of DAPD was 10.5-53%.

Profound antiviral effect of oral administration of MIV-210 on chronic hepadnaviral infection in a woodchuck model of hepatitis B

MIV-210 is a prodrug of 3'-fluoro-2',3'-dideoxyguanosine with high oral bioavailability in humans and potent activity against hepatitis B virus (HBV). Woodchucks infected with woodchuck hepatitis virus (WHV) represent an accurate model of HBV infection that is utilized for evaluation of the efficacy and safety of novel anti-HBV agents. Oral administration of MIV-210 at 20 or 60 mg/kg of body weight/day induced a rapid virological response in chronically infected woodchucks, reducing serum WHV DNA levels by 4.75 log10 and 5.72 log10, respectively, in 2 weeks. A progressive decline in WHV viremia occurred throughout the 10-week therapy, giving final reductions of 7.23 log10 and 7.68 log10 in the 20- and 60-mg/kg/day groups, respectively. Further, a daily dose of 10 mg/kg decreased the serum WHV load 400-fold after 4 weeks of treatment, and a dose of 5 mg/kg/day was sufficient to maintain this antiviral effect during the following 6-week period. MIV-210 at 20 or 60 mg/kg/day reduced the liver WHV DNA load 200- to 2,500-fold from pretreatment levels and, importantly, led to a 2.0 log10 drop in the hepatic content of WHV covalently closed circular DNA. The treatment with 60 mg/kg/day was well tolerated. Liver biopsy specimens obtained after the 10-week treatment with 20 or 60 mg/kg/day and after the 10-week follow-up showed hepatocyte and mitochondrial ultrastructures comparable to those in the placebo-treated group. It was concluded that MIV-210 is highly effective against chronic WHV infection. These findings, together with the previously demonstrated inhibitory activity of MIV-210 against lamivudine-, adefovir-, and entecavir-resistant HBV variants, make MIV-210 a highly valuable candidate for further testing as an agent against chronic hepatitis B.

The woodchuck as an animal model for pathogenesis and therapy of chronic hepatitis B virus infection

This review describes the woodchuck and the woodchuck hepatitis virus (WHV) as an animal model for pathogenesis and therapy of chronic hepatitis B virus (HBV) infection and disease in humans. The establishment of woodchuck breeding colonies, and use of laboratory-reared woodchucks infected with defined WHV inocula, have enhanced our understanding of the virology and immunology of HBV infection and disease pathogenesis, including major sequelae like chronic hepatitis and hepatocellular carcinoma. The role of persistent WHV infection and of viral load on the natural history of infection and disease progression has been firmly established along the way. More recently, the model has shed new light on the role of host immune responses in these natural processes, and on how the immune system of the chronic carrier can be manipulated therapeutically to reduce or delay serious disease sequelae through induction of the recovery phenotype. The woodchuck is an outbred species and is not well defined immunologically due to a limitation of available host markers. However, the recent development of several key host response assays for woodchucks provides experimental opportunities for further mechanistic studies of outcome predictors in neonatal- and adult-acquired infections. Understanding the virological and immunological mechanisms responsible for resolution of self-limited infection, and for the onset and maintenance of chronic infection, will greatly facilitate the development of successful strategies for the therapeutic eradication of established chronic HBV infection. Likewise, the results of drug efficacy and toxicity studies in the chronic carrier woodchucks are predictive for responses of patients chronically infected with HBV. Therefore, chronic WHV carrier woodchucks provide a well-characterized mammalian model for preclinical evaluation of the safety and efficacy of drug candidates, experimental therapeutic vaccines, and immunomodulators for the treatment and prevention of HBV disease sequelae.

Mitochondrial dysfunction and nucleoside reverse transcriptase inhibitor therapy: experimental clarifications and persistent clinical questions

Nucleoside reverse transcriptase inhibitors (NRTIs) in combination with other antiretrovirals (HAART) are critical in current AIDS therapy, but mitochondrial side effects have come to light with the increased use of these compounds. Clinical experience, pharmacological, cell and molecular biological evidence links altered mitochondrial (mt-) DNA replication to the toxicity of NRTIs in many tissues, and conversely, mtDNA replication defects and mtDNA depletion in specific target tissues are observed. The shared features of mtDNA depletion and energy depletion became key observations and related the clinical and in vivo experimental findings to inhibition of mtDNA replication by NRTI triphosphates in vitro. Subsequent to those findings, other observations suggested that mitochondrial energy deprivation is concomitant with or the result of mitochondrial oxidative stress in AIDS (from HIV, for example) or from NRTI therapy itself. With increased use of NRTIs, mtDNA mutations may become increasingly important pathophysiologically. One important future goal is to prevent or attenuate the side effects so that improved efficacy is achieved.

Mitochondrial DNA replication, nucleoside reverse-transcriptase inhibitors, and AIDS cardiomyopathy

Nucleoside reverse-transcriptase inhibitors (NRTIs) in combination with other antiretrovirals (HAART) are the cornerstones of current AIDS therapy, but extensive use brought mitochondrial side effects to light. Clinical experience, pharmacological, cell, and molecular biological evidence links altered mitochondrial (mt-) DNA replication to the toxicity of NRTIs in many tissues, and conversely, mtDNA replication defects and mtDNA depletion in target tissues are observed. Organ-specific pathological changes or diverse systemic effects result from and are frequently attributed to HAART in which NRTIs are included. The shared features of mtDNA depletion and energy depletion became key observations and related the clinical and in vivo experimental findings to inhibition of mtDNA replication by NRTI triphosphates in vitro. Subsequent to those findings, other observations suggested that mitochondrial energy deprivation is concomitant with or the result of mitochondrial oxidative stress in AIDS (from HIV, for example) or from NRTI therapy itself.

Design and synthesis of novel 5-substituted acyclic pyrimidine nucleosides as potent and selective inhibitors of hepatitis B virus

A novel class of 5-substituted acyclic pyrimidine nucleosides, 1-[(2-hydroxyethoxy)methyl]-5-(1-azidovinyl)uracil (9a), 1-[(2-hydroxy-1-(hydroxymethyl)ethoxy)methyl]-5-(1-azidovinyl)uracil (9b), and 1-[4-hydroxy-3-(hydroxymethyl)-1-butyl]-5-(1-azidovinyl)uracil (9c), were synthesized by regiospecific addition of bromine azide to the 5-vinyl substituent of the respective 5-vinyluracils (2a-c) followed by treatment of the obtained 5-(1-azido-2-bromoethyl) compounds (3a-c) with t-BuOK, to affect the base-catalyzed elimination of HBr. Thermal decomposition of 9b and 9c at 110 degrees C in dioxane yielded corresponding 5-[2-(1-azirinyl)]uracil analogues (10b,c). The 5-(1-azidovinyl)uracil derivatives 9a-c were found to exhibit potent and selective in vitro anti-HBV activity against duck hepatitis B virus (DHBV) infected primary duck hepatocytes at low concentrations (EC(50) = 0.01-0.1 microg/mL range). The most active anti-DHBV agent (9c), possessing a [4-hydroxy-3-(hydroxymethyl)-1-butyl] substituent at N-1, exhibited an activity (EC(50) of 0.01-0.05 microg/mL) comparable to that of reference compound (-)-beta-L-2',3'-dideoxy-3'-thiacytidine (3-TC) (EC(50) = 0.01-0.05 microg/mL). In contrast, related 5-[2-(1-azirinyl)]uracil analogues (10b,c) were devoid of anti-DHBV activity, indicating that an acyclic side chain at C-5 position of the pyrimidine ring is essential for anti-HBV activity. The pyrimidine nucleosides (9a-c, 10b,c) exhibited no cytotoxic activity against a panel of 60 human cancer cell lines. All of the compounds investigated did not show any detectable toxicity to several stationary and proliferating host cell lines or to mitogen stimulated proliferating human T lymphocytes, up to the highest concentration tested.

Novel 5-vinyl pyrimidine nucleosides with potent anti-hepatitis B virus activity

Synthesis and antiviral activities of novel N-1 alkyl substituted pyrimidines, 1-[(2-hydroxyethoxy)methyl]-5-vinyluracil (5), 1-[(2-hydroxy-1-(hydroxymethyl)ethoxy)methyl]-5-vinyluracil (6), and 1-[4-hydroxy-3-(hydroxymethyl)-1-butyl]-5-vinyluracil (7) are reported. Compounds 6 and 7 were potent inhibitors of DHBV in cell culture, in contrast, all of the compounds described were devoid of activity against TK(+) HSV-1 and TK(-) HSV-1.

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.

Antiviral efficacy of lobucavir (BMS-180194), a cyclobutyl-guanosine nucleoside analogue, in the woodchuck (Marmota monax) model of chronic hepatitis B virus (HBV) infection

Lobucavir (BMS-180194), a cyclobutyl-guanosine nucleoside analogue, effectively reduced WHV-viremia in chronically infected carrier woodchucks (Marmota monax) by daily per os treatment. WHV-viremia in the animals was measured by the serum content of hybridizable WHV-genomic DNA. Lobucavir, given at daily doses of 10 and 20 mg/kg body weight, reduced WHV-viremia by a 10- to 200-fold range during therapy. Lobucavir, given at 5 mg/kg, suppressed WHV-viremia by a 10- to 30-fold range, whereas a 0.5 mg/kg dose had no significant effect. WHV-viremia was also measured by hepadnaviral endogenous polymerase activity (EPA) in sera of animals treated for 6 weeks at 5 and 0.5 mg/kg. Changes in EPA in sera of lobucavir treated animals were comparable to changes in WHV DNA levels. Viremia in treated carriers recrudesced to pretreatment levels by 2 weeks of therapy cessation. These results indicated that the minimally effective antiviral daily per os dose of lobucavir in WHV-carrier woodchucks was approximately 5 mg/kg.

Antiviral activities of oral 1-O-hexadecylpropanediol-3-phosphoacyclovir and acyclovir in woodchucks with chronic woodchuck hepatitis virus infection

Acyclovir triphosphate is a potent inhibitor of hepatitis B virus DNA polymerase, but acyclovir treatment provides no benefit in patients with hepatitis B virus infection. This is due in part to the fact that hepatitis B virus, unlike herpes simplex virus, does not code for a viral thymidine kinase which catalyzes the initial phosphorylation of acyclovir. We synthesized 1-O-octadecyl-sn-glycero-3-phospho (3-P)-acyclovir and found that it was highly active in reducing hepatitis B virus replication in 2.2. 15 cells, while acyclovir was inactive. The greater antiviral activity of 1-O-octadecyl-sn-glycero-3-P-acyclovir appeared to be due to liver cell metabolism of the compound to acyclovir monophosphate (K. Y. Hostetler et al., Biochem. Pharmacol. 53:1815-1822, 1997). However, a closely related compound without a hydroxyl group at the sn-2 position of glycerol, 1-O-hexadecylpropanediol-3-P-acyclovir, was more active and selective in 2.2.15 cells in vitro. In this study, we treated woodchucks chronically infected with woodchuck hepatitis virus with increasing oral doses of 1-O-hexadecylpropanediol-3-P-acyclovir and assessed the response to therapy versus acyclovir or a placebo. At a dosage of 10 mg/kg of body weight twice a day, the test compound significantly inhibited viral replication in vivo, as indicated by a 95% reduction in serum woodchuck hepatitis virus DNA levels and by a 54% reduction in levels of woodchuck hepatitis virus replicative intermediates in the liver. Higher doses were somewhat less effective. In contrast, 20 mg of acyclovir/kg twice daily, a 5. 3-fold-higher molar dosage, had no demonstrable activity against woodchuck hepatitis virus. Oral 1-O-hexadecylpropanediol-3-P-acyclovir appeared to be safe and effective in chronic woodchuck hepatitis virus infection.

Enhanced Antiviral Benefit of Combination Therapy with Lamivudine and Alpha Interferon against WHV Replication in Chronic Carrier Woodchucks

Cell culture studies in our laboratory previously demonstrated synergistic antiviral activity for the combinations of lamivudine and a novel recombinant hybrid human alpha B/D interferon (rHuαB/D IFN) against hepatitis B virus (HBV) replication. Based on these results, a study was designed to determine if an enhanced antiviral effect with this drug combination could be demonstrated in vivo using the woodchuck hepatitis virus (WHV)/woodchuck experimental model of chronic HBV infection. Both antiviral agents have been shown to be effective against WHV replication in WHV chronic carriers during previous studies by our laboratories. Two combination treatment regimens were compared to matched monotherapies in a placebo-controlled trial. The first used simultaneous administration of rHuαB/D IFN and lamivudine for 24 weeks. The other combination treatment regimen used a staggered dosing schedule of 12 weeks of administration of lamivudine alone, followed by 12 weeks of simultaneous dosing with both drugs, followed by 12 weeks of therapy with rHuαB/D IFN alone. Both treatment regimens with combinations of lamivudine and rHuαB/D IFN were more effective at reducing WHV replication in chronically infected wood-chucks than the corresponding monotherapies. Both combination treatments produced antiviral effects that were at least equal to that expected for additive activity based on estimations generated by Bliss Independence calculations. The staggered treatment regimen reduced viraemia and intrahepatic WHV replication significantly more than that expected for additive interactions, indicating synergistic antiviral effects. These studies demonstrate that combination therapy of chronic WHV infection has enhanced antiviral benefit over corresponding monotherapies and indicate that combination treatment of chronic HBV infection can be superior to therapies using a single antiviral agent.

Enhanced antiviral benefit of combination therapy with lamivudine and famciclovir against WHV replication in chronic WHV carrier woodchucks

Cell culture studies in our laboratory and others have previously demonstrated synergistic antiviral activity for combinations of 3TC (lamivudine) and penciclovir against Hepatitis B Virus (HBV) replication and the Duck Hepatitis B Virus (DHBV). Based on these results, a study was designed to determine if an enhanced antiviral effect with combinations of 3TC and famciclovir (FCV, oral prodrug of penciclovir) could be demonstrated in vivo using the Woodchuck Hepatitis Virus (WHV)/woodchuck experimental model of chronic HBV infection. Both antiviral agents have been shown to be effective against WHV replication in WHV chronic carriers in previous studies by our laboratories. The antiviral effects of four different combinations of lamivudine and FCV were found to be greater than those observed for the corresponding monotherapies. All four combination treatments produced antiviral effects that were at least equal to that expected for additive activity based on estimations generated by Bliss Independence calculations. Two of the combination treatments produced antiviral effects that were significantly greater than that expected for additive effects, indicative of synergistic antiviral interactions. These studies demonstrate that combination therapy of chronic WHV infection has enhanced antiviral benefit over corresponding monotherapies and indicate that combination treatment of chronic HBV infection can be superior to therapies using a single antiviral agent.

An evaluation of the toxicities of 2'-fluorouridine and 2'-fluorocytidine-HCl in F344 rats and woodchucks (Marmota monax)

The toxicities of 2'-fluorouridine (2'-FU) and 2'-fluorocytidine-HCl (2'-FC) were separately evaluated in 2 species, male Fischer 344 (F334) rats and woodchucks. Particular attention was focused on the ability of these nucleosides to induce toxicities similar to those induced by the antiviral drug fialuridine (FIAU). 2'-FU or 2'-FC was administered to F344 male rats by intravenous injection at doses of 5, 50, and 500 mg/kg/day for 90 consecutive days and to male and female woodchucks at doses of 0.75 and 7.5 mg/kg/day for 90 consecutive days. Clinical chemistry, hematology, and urinalysis (woodchuck only) profiles were assessed during and at the termination of the study. At necropsy, organs were weighed and tissues collected for routine histologic analysis. Cytochrome c oxidase activity, citrate synthase activity, and mitochondrial DNA content were measured, and micronucleus formation in the bone marrow (rats only) was evaluated. No adverse clinical effects were observed in either species. Rats treated with high doses of either 2'-FU or 2'-FC had body weights that were 90% of those of controls. 2'-FU and 2'-FC both induced a moderate decrease in the median lymphocyte count, and 2'-FC and 2'-FU induced a mild increase in mean corpuscular hemoglobin and mean corpuscular volume. Both compounds caused slight to moderate, reversible, histologic changes in the spleen and thymus. In the woodchuck, 2'-FC caused a slight increase in mean absolute lymphocytes, and 2'-FC and 2'-FU slightly increased hepatic periportal vacuolation and/or mononuclear cell infiltration. In summary, neither compound showed evidence of the toxicity induced by fialuridine in either species. Although compound effects were observed, none of these effects were considered to be adverse, and the no-observed adverse effect level was determined to be 500 mg/kg/day for both compounds in the male F344 rat and 7.5 mg/kg/day in the woodchuck.

Effect of 1-(2-deoxy-2-fluoro-beta-D-arabinofuranosyl)-5-ethyluracil on mitochondrial functions in HepG2 cells

The effects of 1-(2-deoxy-2-fluoro-beta-D-arabinofuranosyl)-5-ethyluracil (D-FEAU) on mitochondrial functions were examined in HepG2 cells. D-FEAU between 0.1 and 10 microM had no apparent inhibitory effect on cell proliferation for 2-week period; however, D-FEAU caused a decrease in mitochondrial DNA (mtDNA) content in a dose-dependent manner with an IC50 value of 2.7 microM. A 20.9% of increase in lactic acid production was observed after the cells were incubated with 10 microM of D-FEAU for 4 days without substantial effect being detected at 0.1 and 1 microM. In addition, no significant changes on mitochondrial morphology were observed in the cells treated with 10 microM of D-FEAU for 14 days under the electron microscope.

Cell-based and animal models for hepatitis B and C viruses

Reliable cell-based assays and animal models have been developed for evaluating agents against hepatitis B virus. Although much progress has been made, in vitro and in vivo assays for hepatitis C virus are still on the horizon. Advances towards establishing inexpensive and reliable experimental models have accelerated the development of therapeutic modalities for these life-threatening viral infections. The characterization of well-defined viral targets coupled with improved molecular diagnostic technologies have illuminated this field.

Efficacy of the carbocyclic 2'-deoxyguanosine nucleoside BMS-200475 in the woodchuck model of hepatitis B virus infection

Daily oral treatment with the cyclopentyl 2'-deoxyguanosine nucleoside BMS-200475 at doses ranging from 0.02 to 0.5 mg/kg of body weight for 1 to 3 months effectively reduced the level of woodchuck hepatitis virus (WHV) viremia in chronically infected woodchucks as measured by reductions in serum WHV DNA levels and endogenous hepadnaviral polymerase activity. Within 4 weeks of daily therapy with 0.5 or 0.1 mg of BMS-200475 per kg, endogenous viral polymerase levels in serum were reduced about 1,000-fold compared to pretreatment levels. Serum WHV DNA levels determined by a dot blot hybridization technique were comparably decreased in these treated animals. In the 3-month study, the sera of animals that had undetectable levels of WHV DNA by the dot blot technique were further analyzed by a highly sensitive semiquantitative PCR assay. The results indicate that BMS-200475 therapy reduced mean WHV titers by 10(7)- to 10(8)-fold, down to levels as low as 10(2) to 10(3) virions/ml of serum. Southern blot hybridization analysis of liver biopsy samples taken from animals during and after BMS-200475 treatment showed remarkable reductions in the levels of WHV DNA replicative intermediates and in the levels of covalently closed circular viral DNA. WHV viremia in BMS-200475-treated WHV carriers eventually returned to pretreatment levels after therapy was stopped. These results indicate that BMS-200475 should be evaluated in clinical trials for the therapy of chronic human hepatitis B virus infections.

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.

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.

Inhibitory effect of 2'-fluoro-5-methyl-beta-L-arabinofuranosyl-uracil on duck hepatitis B virus replication

The antiviral activity of 2'-fluoro-5-methyl-beta-L-arabinofuranosyluracil (L-FMAU), a novel L-nucleoside analog of thymidine known to be an inhibitor of hepatitis B virus (HBV) replication in hepatoma cells (2.2.1.5 cell line), was evaluated in the duck HBV (DHBV) model. Short-term oral administration (5 days) of L-FMAU (40 mg/kg of body weight/day) to experimentally infected ducklings induced a significant decrease in the level of viremia. This antiviral effect was sustained in animals when therapy was prolonged for 8 days. The histological study showed no evidence of liver toxicity in the L-FMAU-treated group. By contrast, microvesicular steatosis was found in the livers of dideoxycytidine-treated animals. L-FMAU administration in primary duck hepatocyte cultures infected with DHBV induced a dose-dependent inhibition of both virion release in culture supernatants and intracellular viral DNA synthesis, without clearance of viral covalently closed circular DNA. By using a cell-free system for the expression of an enzymatically active DHBV reverse transcriptase, it was shown that L-FMAU triphosphate exhibits an inhibitory effect on the incorporation of dAMP in the viral DNA primer. Thus, our data demonstrate that L-FMAU inhibits DHBV replication in vitro and in vivo. Long-term administration of L-FMAU for the eradication of viral infection in animal models of HBV infection should be evaluated.

Inhibition of human and duck hepatitis B virus by 2',3'-dideoxy-3'-fluoroguanosine in vitro

The fluorinated guanosine analog 2',3'-dideoxy-3'-fluoroguanosine (FLG) has been shown to have an effect on duck hepatitis B virus (DHBV) in vivo and in vitro. In this study the inhibitory effect of FLG on DHBV and human hepatitis B virus (HBV) was evaluated in vitro. Cell lines transfected either with DHBV or HBV DNA and primary duck hepatocyte cell cultures were used. Virus production was analysed by PCR and a quantitative PCR was established for DHBV for determination of the inhibitory concentrations of the drug. 50% inhibition was achieved with an FLG concentration of 0.2 microg/ml (0.7 microM) and 90% inhibition was observed with an FLG concentration of 1.0 microg/ml (3.7 microM) using the DHBV transfected cell line. FLG showed an effect on DHBV production in primary duck hepatocyte cell cultures at concentrations down to 0.1 microg/ml (0.4 microM). However, the DHBV production returned to pre-treatment levels within a few days after cessation of treatment. HBV production in transfected cell lines was also inhibited by FLG. Both DHBV and HBV DNA-polymerases were inhibited by FLG triphosphate and 50% inhibition was observed at a concentration of 0.05 microg/ml (0.1 microM) for DHBV and 0.03 microg/ml (0.05 microM) for HBV. FLG is an efficient inhibitor of DHBV replication both in vivo and in vitro and of HBV in vitro which makes it a good candidate for treatment of HBV infections. However, it does not completely eliminate the virus since a relapse in virus production was observed when treatment was withdrawn. Therefore it would be interesting to evaluate FLG in combination with other types of anti-HBV drugs.

In vivo antiviral activity and pharmacokinetics of (-)-cis-5-fluoro-1-[2-(hydroxymethyl)-1,3-oxathiolan-5-yl]cytosine in woodchuck hepatitis virus-infected woodchucks

The (-) enantiomer of cis-5-fluoro-1l-[2-(hydroxymethyl)-1,3-oxathiolan-5-yl]cytosine [(-)-FTC)], a substituted oxathiolane compound with anti-hepatitis B virus activity in vitro, was assessed for its efficacy in woodchucks with naturally acquired woodchuck hepatitis virus (WHV) infection. Pharmacokinetics and in vitro anabolism were also determined. (-)-FTC was anabolized to the 5'-triphosphate in a dose-related fashion, reaching a maximum concentration at about 24 h in cultured woodchuck hepatocytes. Following administration of a dose of 10 mg/kg of body weight intraperitoneally (i.p.), the clearance of (-)-FTC from plasma was monoexponential, the terminal half-life was 3.76 +/- 1.4 h, and the systemic clearance was 0.12 +/- 0.06 liters/h/kg. The antiviral efficacy of (-)-FTC in the woodchuck model was assessed by quantitation of serum WHV DNA levels and by WHV particle-associated DNA polymerase activity at two dosages, 30 and 20 mg/kg given i.p. twice daily (b.i.d.), respectively. The level of WHV DNA in serum was reduced 20- to 150-fold (average, 56-fold) in the 30-mg/kg-b.i.d. treatment group and 6- to 49-fold (average, 27-fold) in the 20-mg/kg-b.i.d. treatment group. Viral DNA polymerase levels diminished accordingly. One week after treatment was discontinued, WHV levels returned to pretreatment levels in both studies. These animals were biopsied before and following treatment with 30 mg of (-)-FTC per kg. Their livers were characterized by a mild increase in cytoplasmic lipid levels, but this change was not associated with altered liver enzyme levels. Serum chemistry and hematology results were within the normal ranges for all treated animals. We conclude that (-)-FTC is a potent antihepadnaviral agent and that it has no detectable toxic effects in woodchucks when given for up to 25 days. Further development of (-)-FTC as an anti-hepatitis B virus therapy for patients is warranted.

Mitochondrial injury. Lessons from the fialuridine trial

Fialuridine is an antiviral agent with potent activity against hepatitis B virus replication in vitro and in vivo. In a phase II study, 7 of 15 patients experienced severe toxicity due to the drug after 9 to 13 weeks of treatment. Adverse effects included nausea, vomiting and painful paraesthesia; subsequently, hepatic failure, pancreatitis, neuropathy, myopathy and lactic acidosis developed, probably due to multisystem mitochondrial toxicity. Possible mechanisms of fialuridine toxicity include mitochondrial injury and pyruvate oxidation inhibition. While other nucleoside analogues have shown evidence of inducing mitochondrial injury (zidovudine, didanosine, zalcitabine), others to date have not (lamivudine, famciclovir). Specific recommendations for future study of existing and new nucleoside analogues include testing for toxicity after prolonged incubation, specific investigations to measure mitochondrial function, toxicological tests and well designed clinical trials with appropriate testing to monitor for any adverse effects on mitochondrial integrity and function.

Treatment of woodchuck hepatitis virus infection in vivo with 2', -3'-dideoxycytidine (ddC) and 2',-3'-dideoxycytidine monophosphate coupled to lactosaminated human serum albumin (L-HSA ddCMP)

Dideoxycytidine (ddC) is a nucleoside analogue active against human immunodeficiency virus and with in vitro activity against human hepatitis B virus. We investigated the ability of ddC to inhibit one of the Hepadnaviridae, the woodchuck hepatitis virus and compared the results with the effect obtained by a conjugate of lactosaminated human serum albumin 2',-3'-dideoxycytidine monophosphate (L-HSA ddCMP). This compound specifically enters the hepatocyte via the asialoglycoprotein receptor. We treated five chronic woodchuck hepatitis virus carriers with intravenous injections of 0.5 mg/kg body weight of ddC for 5 consecutive days, and under the same protocol five woodchucks with 10.4 mg/ kg L-HSA ddCMP, a dose equivalent to 0.25 mg/kg of free ddC. A reduction of serum woodchuck hepatitis virus DNA (5-125 fold) was observed during therapy in three out of five animals receiving ddC and in two of the five animals treated with L-HSA ddCMP. In responding woodchucks, virus DNA levels rebounded immediately after stopping therapy. No signs of toxicity were observed during or after the course of therapy. These preliminary results of short-term treatment indicate that ddC has anti-viral activity against woodchuck hepatitis virus. When the dose was reduced by 50%, L-HSA ddCMP showed anti-viral activity to an even lesser degree.

Pharmacokinetics of (-)-2'-3'-dideoxy-3'-thiacytidine in woodchucks

The woodchuck (Marmota monax) has proven to be a suitable animal model for studying hepatitis B virus (HBV) infection owing to similarities in the course of infection between woodchuck hepatitis virus (WHV) in woodchucks and HBV in humans. (-)-beta-L-2',3'-Dideoxy-3'-thiacytidine (3TC; lamivudine) is a nucleoside analog which has demonstrated antiviral activity against HBV as well as human immunodeficiency virus (HIV). The purpose of the present investigation was to characterize the pharmacokinetics of 3TC following intravenous and oral administration of 20 mg of 3TC per kg of body weight to woodchucks. Following intravenous administration, the concentrations of 3TC in plasma declined, with a terminal half-life of 2.84 +/- 0.85 h (mean +/- standard deviation). The systemic clearance and steady-state volume of distribution of 3TC were 0.22 +/- 0.078 liters/h/kg and 0.75 +/- 0.13 liters/kg, respectively. The renal clearance of the nucleoside analog was 0.063 +/- 0.016 liters/h/kg. The oral bioavailability of 3TC ranged from 18 to 54%. Allometric relationships between pharmacokinetic parameters and body weight developed by Hussey et al. (E.K. Hussey, K.H. Donn, M.J. Daniel, S.T. Hall, A.J. Harker, and G.L. Evans, J. Clin. Pharmacol. 34:975-977, 1994) were augmented by including data from woodchucks, monkeys (S.M. Blaney, M.J. Daniel, A.J. Harker, K. Godwin, and F.M. Balis, Antimicrob. Agents Chemother. 39:2779-2782, 1995), and additional data from rats (P. Rajagopalan, L. Moore, C.K. Chu, R.F. Schinazi, and F.D. Boudinot, submitted for publication). Interspecies scaling of the pharmacokinetic parameters of 3TC demonstrated a good correlation between clearance (0.74 . W0.76 [where W is body weight]; r = 0.93; P < 0.025), apparent volume of distribution (1.62 . W0.81; r = 0.98; P < 0.005), and steady-state volume of distribution (1.09 . W0.94; r = 0.99; P < 0.05) and species body weight. The allometric relationships for clearance and volume of distribution at steady state predicted the observed pharmacokinetic parameters in humans quite well; however, the apparent volume of distribution was underestimated in humans. Thus, the pharmacokinetic data obtained with the woodchuck HBV animal model should be useful for designing clinical trials.

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.

Inhibition of episomal hepatitis B virus DNA in vitro by 2,4-diamino-7- (2-deoxy-2-fluoro-beta-D-arabinofuranosyl)-pyrrolo[2,3-d]pyrimidine

The nucleoside analog 2,4-diamino-7-(2-deoxy-2-fluoro-beta-D- arabinofuranosyl)pyrrolo[2,3-d]pyrimidine (T70080) and several related compounds were evaluated for anti-hepatitis B virus (HBV) activity by using cultured 2.2.15 cells. T70080 reduced episomal viral replication in these cells by 50% at a concentration of 0.7 microgram/ml. At the same time, T70080 reduced cellular proliferation by 50% at a concentration in excess of 100 micrograms/ml, yielding a therapeutic index of > 143. In cells cultured for 12 days in the presence of 10 or 50 micrograms of T70080 per ml and then with drug-free medium, for an additional 12 days, viral DNA replication was completely inhibited initially but resumed between 6 and 12 days post-drug removal. In view of the potent anti-HBV activity shown, T70080 is a good candidate for further evaluation as a treatment of human HBV infection.

Hepatic purine and pyrimidine metabolism: implications for antiviral chemotherapy of viral hepatitis

The use of nucleoside analogues as antiviral agents is expanding. For most nucleoside analogues, intracellular phosphorylation is the major prerequisite for activity. Antiviral activity may be limited by poor uptake, absence of appropriate activating enzymes, catabolism, and competition from endogenous nucleotides. Appreciation of these factors, which are species-, tissue- and cell-specific is important in the understanding of the pharmacology and toxicology of nucleoside analogues. The use of nucleoside analogues against the agents of viral hepatitis is inherently problematic for many reasons including active hepatic nucleoside catabolism, probable absence of virus-specific activating enzymes, competition from endogenous nucleotides synthesised de novo or derived from RNA turnover, and factors related to mitochondrial toxicity. Despite these drawbacks, some nucleoside analogues have been found efficacious against hepatitis B virus and it is likely that as knowledge of their mechanism of action accumulates, their efficacy can be improved both by rational drug design and by use in combination with other drugs, including interferon.

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.

Effects of antiviral nucleoside analogs on human DNA polymerases and mitochondrial DNA synthesis

Inhibition constants were determined for 16 nucleoside analog triphosphates against human DNA polymerases alpha, beta, gamma, and epsilon, and 7 nucleoside analogs were examined as inhibitors of mitochondrial DNA synthesis in human Molt-4 cells in culture. The results demonstrate no clear quantitative or qualitative correlation between inhibition of DNA polymerases, particularly mitochondrial DNA polymerase gamma, and the inhibition of mitochondrial DNA synthesis in Molt-4 cell culture. Furthermore, the data indicate that inhibition of isolated DNA polymerases may not be predictive of in vitro or in vivo toxicity. Finally, it is not clear whether inhibition of mitochondrial DNA synthesis will be an accurate predictor of the potential in vivo toxicity of antiviral nucleoside analogs.

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.

Use of a standardized cell culture assay to assess activities of nucleoside analogs against hepatitis B virus replication

A cell culture system for the evaluation of compounds which inhibit HBV replication (Korba and Milman, Antiviral Res. 15:217, 1991) has been developed into a standardized assay. Toxicity of test compounds was assessed by the uptake of neutral red dye under culture and treatment conditions which were identical to those used for the antiviral assays. A total of 667 separate cultures of 2.2.15 cells were evaluated for this study. In 86 untreated cell cultures, representing 15 experiments over a 24-month period, the levels of extracellular HBV virion DNA and intracellular HBV DNA forms were found to vary by less than 2.5-fold overall. Virion DNA in serum and intracellular viral DNA replication intermediates [RI] are the two most reliable and commonly followed markers of hepadnavirus replication in patients and experimental animals. In these assays, levels of extracellular HBV virion DNA and intracellular HBV RI were well correlated in 2.2.15 cells. Less correlation was observed between the levels of HBV virion DNA and the 3.2-kb episomal HBV genomes present in the cells. A threshold level of 22-37 intracellular replicating HBV genomes appeared to be required before virions were detected in the culture medium. The activities of several 2'-substituted and 3'-substituted deoxynucleoside analogs against HBV replication were compared using this standardized assay. Dideoxycytosine [ddC] and dideoxyguanosine [ddG] were the most selective 2',3'-dideoxynucleosides against HBV in 2.2.15 cells. Substitution of fluorine at the 2' position abolished the antiviral activity of ddC, but enhanced the selective antiviral activities of dideoxythymidine and dideoxyuracil. Several 2'-fluorinated pyrimidine arabinosyl furanosides, reported to be potent (but toxic) inhibitors of hepadnaviruses in vivo, demonstrated relatively low selective antiviral activities in 2.2.15 cells. The current data base allows for validation of any given set of test evaluations through statistical analysis of both the positive and the negative treatment controls present in each experiment; thus, relevant comparisons of the selectivity of anti-HBV activities for different compounds examined in future experiments can be made.

Effects of 2'-fluorinated arabinosyl-pyrimidine nucleosides on duck hepatitis B virus DNA level in serum and liver of chronically infected ducks

The 2'-fluorinated arabinosyl-pyrimidine nucleosides, 1-(2'-deoxy-2'-fluoro-beta-D-arabinofuranosyl)-5-iodocytosine (FIAC) and 1-(2'-deoxy-2'-fluoro-beta-D-arabinofuranosyl)-5-methyluracil (FMAU), are new antiviral compounds with in vitro inhibitory activity against the DNA polymerase of hepadnaviruses. Those compounds also induced permanent inhibition of viral replication in woodchucks chronically infected by woodchuck hepatitis virus. The effects of these antiviral compounds were assessed in ducks chronically infected by duck hepatitis B virus (DHBV). Following intraperitoneal administration for 5 days, FMAU (2 mg/kg/day) and FIAC (10 mg/kg/day) induced a transient decrease in DHBV replication, as shown by the decrease in both the serum and liver DHBV DNA level. After stopping therapy, DHBV replication rebounded immediately to the pretreatment level. The supercoiled form of liver viral DNA was found to be less affected by the therapy. By contrast, no obvious antiviral effect was observed with vidarabine monophosphate (ara-AMP) (80 mg/kg/day) therapy. No sign of toxicity was observed during the course of the treatment. These preliminary results confirmed in the DHBV model the higher efficacy of FIAC and FMAU as compared to ara-AMP. Pharmacokinetic studies are needed to explain the differences observed in viral replication in these 2 models of HBV infection.

Cytidine diphosphate diglyceride analogs of antiretroviral dideoxynucleosides: evidence for release of dideoxynucleoside-monophosphates by phospholipid biosynthetic enzymes in rat liver subcellular fractions

We recently synthesized phospholipid analogs with antiviral nucleosides in the polar headgroup and demonstrated their antiretroviral activity in vitro in human immunodeficiency virus-infected cells (Hostetler, K.Y., Stuhmiller, L.M., Lenting, H.B.M., van den Bosch, H. and Richman, D.D. (1990) J. Biol. Chem. 265, 6112-6117). Dideoxynucleoside analogs of cytidine diphosphate diglyceride (CDP-DG) represent one class of such phospholipid prodrugs from which the antiviral active principle may be released through established pathways of cellular phospholipid metabolism. We now demonstrate that the liponucleotides of dideoxycytidine, 3'-deoxythymidine and 3'-azido-3'-deoxythymidine (AZT, Zidovudine) can substitute to varying extents for CDP-DG in the biosynthesis of phosphatidylinositol, phosphatidylglycerol or diphosphatidylglycerol by rat liver subcellular fractions. In all three biosynthetic pathways dideoxycytidine diphosphate diglyceride was the most active donor of the phosphatidyl unit. The nearly stoichiometric formation of dideoxycytidine-5'-monophosphate during phosphatidylinositol biosynthesis supports the rationale that the antiretroviral liponucleotides may provide cells with a depot form from which the antiviral drug can be released in 5'-monophosphorylated form, thus bypassing the initial phosphorylation of free dideoxynucleosides.

Comparative inhibition of hepatitis B virus DNA polymerase and cellular DNA polymerases by triphosphates of sugar-modified 5-methyldeoxycytidines and of other nucleoside analogs

Of a series of 14 nucleoside 5'-triphosphates, those of 2',3'-dideoxy-3'-fluoro-5-methylcytidine, 3'-azido-2',3'-dideoxy-5-methylcytidine, 2',3'-dideoxy-3'-fluoroguanosine, 2',3'-didehydro-2',3'-dideoxy-5-methylcytidine, 2',3'-dideoxy-3'-fluoro-5-ethylcytidine, and 2',3'-dideoxy-3'-fluoroadenosine emerged as the most potent inhibitors of hepatitis B virus DNA polymerase (50% inhibitory dose, 0.03 to 0.35 microM). In contrast, cellular DNA polymerases proved to be resistant to (alpha) or partially affected by (beta) these analogs. These compounds are among the most effective and selective inhibitors of endogenous hepatitis B virus DNA polymerase recognized to date.