Perspectives for the treatment of hepatitis B virus infections

Pharmacokinetics of anti-HBV polyoxometalate in rats

Polyoxometalates are non-nucleoside analogs that have been proven to exhibit broad-spectrum antiviral activity. In particular, Cs₂K₄Na[SiW₉Nb₃O₄₀].H₂O 1 shows low toxicity and high activity against HBV. The preclinical pharmacokinetics of Compound 1 in rats were characterized by establishing and applying inductively coupled plasma-mass spectrometry method to determine the concentration of W in plasma, urine, feces, bile and organ samples. The quantitative ICP-MS method demonstrated good sensitivity and application in the pharmacokinetics study of polyoxometalates. The pharmacokinetic behavior of Compound 1 after intravenous or oral administration fit a two-compartment model. T_max ranges from 0.1 h to 3 h and the T_1/2 of Compound 1 is between 20 h and 30 h. The absolute bioavailability of Compound 1 at 45, 180 and 720 mg/kg groups were 23.68%, 14.67% and 11.93%, respectively. The rates of plasma protein binding of Compound 1 at 9, 18 and 36 mg/ml of Compound 1 are 62.13±9.41%, 71.20±24.98% and 49.00±25.59%, respectively. Compound 1 was widely distributed throughout the body, and high levels of compound 1 were found in the kidney and liver. The level of Compound 1 in excretion was lower: 30% for urine, 0.28% for feces and 0.42% for bile, respectively. For elaborate pharmacokinetic characteristics to be fully understood, the metabolism of Compound 1 needs to be studied further.

Treatment of HBeAg positive chronic hepatitis B: interferon or nucleoside analogues

Interferon alpha has restricted efficacy in as much as only a proportion of patients show a response. However, in appropriately selected HBeAg-positive and HBeAg-negative patients, sustained suppression of viral replication can be achieved, and HBeAg or even HBsAg seroconversion can be attained. Thus, finite course of interferon alpha can be successful, and offer an advantage to patient. Interferon (IFN) remains a benchmark therapy for chronic hepatitis B. The main advantages of IFN-α over nucleoside analogues are the absence of resistance and the possibility of immune-mediated clearance of hepatitis B. Unfortunately, side effects preclude the use of interferon alpha in substantial proportions of patients, and prolonged maintenance therapy to suppress hepatitis B virus (HBV) is not feasible. Nucleoside analogues are given by mouth, once per day, and the safety, potency and efficacy have improved and facilitated treatment. However, maintenance of long-term suppression is required for the majority of patients. In general, treatment of chronic hepatitis B should target patients with active disease and viral replication, preferably before the signs and symptoms of cirrhosis or significant injury has occurred. Current EASL guidelines suggest that treatment be based on the evaluation of three criteria: Serum aminotransferase levels, serum HBV DNA levels and histological grade and stage. Many questions remain unanswered on the optimal treatment of patients with chronic hepatitis B with a nucleoside vs interferon alpha. Both forms of treatment have benefits and the choice should be selected and tailored. Stopping or futility rules can be implemented in patients who fail interferon. Recent data suggest the safety and efficacy of nucleoside analogues in the third trimester of pregnancy to reduce the risk of transmission from mothers to their children.

In vitro inhibition of HBV replication by a novel compound, GLS4, and its efficacy against adefovir-dipivoxil-resistant HBV mutations

BACKGROUND

HBV infection continues to be an important worldwide cause of morbidity and mortality. Patients with chronic hepatitis B can be successfully treated using nucleoside/nucleotide analogues. However, drug-resistant HBV mutants frequently arise, leading to treatment failure and progression to liver disease. Here, we report the effects of GLS4, a non-nucleosidic inhibitor that exhibits a novel and highly specific anti-HBV activity.

METHODS

The median inhibitory concentrations (IC(50)s) of GLS4 on HBV were measured by Southern blotting. HBV capsid and core protein levels were detected by immunoblotting. To determine the antiviral activity of GLS4 against adefovir dipivoxil (ADV)-resistant HBV mutants, HepG2 cells transiently transfected with PUC-HBV1.2 plasmids that contained one of three major ADV-resistant mutations (rtA181T, rtA181V and rtN236T) were treated with GLS4. Intracellular HBV replicative intermediates were detected by Southern blotting. The effect on the in vitro assembly of HBV capsid protein was examined using dynamic light scattering and electron microscopy.

RESULTS

The IC(50) of GLS4 was 0.012 μM, which is significantly lower than that of lamivudine (0.325 μM). Immunoblot analysis of HepG2.2.15 cells and transiently transfected HepG2 cells indicated that GLS4 treatment interfered with the formation of core particles (assembly). The ADV-resistant HBV mutant strains were also sensitive to GLS4. Upon examining the in vitro assembly of HBV core protein 149 by electron microscopy, increased aberrant particles were observed after GLS4 treatment.

CONCLUSIONS

GLS4 is a new and unique potential anti-HBV agent that possesses a different mechanism of action than existing therapeutic drugs.

Molecular epidemical characteristics of Lamivudine resistance mutations of HBV in southern China

BACKGROUND

Lamivudine (LMV), as the preferred oral drug for use in treatment of HBV, always results in development of resistance mutations after long-term treatment. In this study we investigated chronic hepatitis B (CHB) patients in southern China to determine whether different HBV genotypes affect the incidence of LMV resistance mutations.

MATERIAL/METHODS

The study recruited 185 CHB patients living in southern China. Enzyme-linked immunosorbent assay was used to test for HBV serological markers, and HBV DNA was quantified by real-time PCR. Sequencing was performed to detect HBV genotypes and mutations.

RESULTS

There were 49.19% (91/185) CHB patients with HBV resistant to LMV. Only 2 genotypes were found: B and C; 62.16% (115/185) of patients were infected with genotype B HBV and 37.84% (70/185) of patients were infected with genotype C HBV. The incidence rate of LMV resistance was not significantly different between genotype B and C (49.57% vs. 48.57%, P>0.05). For the mean age and sex ratio, no significant difference was found. The pattern of rtM204I alone was predominantly observed (36.26%, 33/91), followed by rtM204V+rtL180M (23.08%, 21/91). The overall incidence rate of rtM204I mutation in genotype B (45.61%, 26/57) was more frequent than that in genotype C (20.59%, 7/34) (45.61% vs. 20.59%, P<0.05), but the incidence rate of other mutation patterns was not significantly different between genotypes B and C.

CONCLUSIONS

Our results emphasize that a LMV resistance test before treatment is of great importance in rational and optimal CHB therapy.

In vitro anti-hepatitis B and SARS virus activities of a titanium-substituted-heteropolytungstate

A structural determined heteropolytungstate, [K₄(H₂O)₈Cl][K₄(H₂O)₄PTi₂W₁₀O₄₀]·NH₂OH 1, has been synthesized and evaluated for in vitro antiviral activities against hepatitis B (HBV) and SARS virus. The identity and high purity of compound 1 were confirmed by elemental analysis, NMR, IR analysis and single-crystal X-ray diffraction. The compound 1, evaluated in HepG 2.2.15 cells expressing permanently HBV, significantly reduced the levels of HBV antigens and HBV DNA in a dose-dependent and time-dependent manner. EC₅₀ values were determined to be 54 μM for HBeAg, 61 μM for HBsAg and 2.66 μM for supernatant HBV DNA, as compared to 1671, 1570, 169 μM, respectively, for the commercially-available hepatitis B drug adefovir dipivoxil (ADV). Intracellular cccDNA, pgRNA and HBcAg were also found to be decreased by compound 1 in a concentration-dependent manner. Cytotoxicity results showed that compound 1 has low toxicity in HepG 2 cells with CC₅₀ value of 515.20 μM. The results indicate that compound 1 can efficiently inhibit HBV replication in HepG 2.2.15 cells line in vitro. Additionally, compound 1 also shows high anti-SARS activity at an EC₅₀ of 7.08 μM and toxicity with a CC₅₀ of 118.6 μM against MDCK cells.

Computational model of hepatitis B virus DNA polymerase: molecular dynamics and docking to understand resistant mutations

Hepatitis B virus (HBV) DNA polymerase (HDP) is a pharmacological target of intense interest. Of the seven agents approved in USA for the treatment of HBV infections, five are HDP inhibitors. However, resistance development against HDP inhibitors, such as lamivudine and adefovir, has severely hurt their efficacy to treat HBV. As a step toward understanding the mechanism of resistance development and for gaining detailed insights about the active site of the enzyme, we have built a homology model of HDP which is an advance over previously reported ones. Validation using various techniques, including PROSTAT, PROCHECK, and Verify-3D profile, proved the model to be stereochemically significant. The stability of the model was studied using a 5 ns molecular dynamics simulation. The model was found to be sufficiently stable after the initial 2.5 ns with overall root mean squared deviation (RMSD) of 4.13 A. The homology model matched the results of experimental mutation studies of HDP reported in the literature, including those of antiviral-resistant mutations. Our model suggests the significant role of conserved residues, such as rtLys32, in binding of the inhibitors, contrary to previous studies. The model provides an explanation for the inactivity of some anti-HIV molecules which are inactive against HDP. Conformational changes which occurred in certain binding pocket amino acids helped to explain the better binding of some of the inhibitors in comparison to the substrates.

LB80380: a promising new drug for the treatment of chronic hepatitis B

Hepatitis B virus is a significant cause of liver cirrhosis and hepatocellular carcinoma in patients with chronic infection. Higher levels of viral load are associated with increased risk of developing liver-related complications. The current available oral therapies suppress viral replication through their action on the hepatitis B virus polymerase. As treatment with oral nucleoside/nucleotide analogues is associated with the development of drug-resistant mutations, there is continuing research for newer and more potent antiviral agents to reduce the chance of drug resistance. LB80380, a prodrug, is an oral nucleotide analogue that inhibits viral replication by incorporation into the viral DNA. Antiviral activity against wild-type virus and virus with drug-resistant mutations was demonstrated in Phase II trials, with significant reduction of viral load in patients treated with LB80380. LB80380 was also shown to be safe and well tolerated.

Dietary amino acid taurine ameliorates liver injury in chronic hepatitis patients

The effect of dietary amino acid taurine on the liver function of chronic hepatitis patients was investigated. The 24 chronic hepatitis patients with 2-5 times over normal activities of alanine aminotransferase (ALT) or aspartate aminotransferase (AST) were selected and equally divided into taurine treatment and control groups. In taurine treatment group, each patient took 2 g taurine 3 times a day for three months, and then stopped treatment for 1 month. Patients taking placebo without taurine for 4 months served as a control group. ALT and AST activities and levels of cholesterol, triglyceride and thiobarbituric acid relative substances of serum plasma in the taurine group were all decreased at the end of three month treatment. The study suggested that dietary amino acid taurine may ameliorate liver injury for chronic hepatitis patients.

Racemic synthesis and antiviral evaluation of 4'(alpha)-hydroxymethyl and 6'(alpha)-methyl substituted apiosyl nucleosides

This article reports the novel synthesis of substituted apiosyl nucleosides. The key apiosyl intermediate 9 was constructed by sequential ozonolysis, reductions, and acetylation from the ester derivative 6. The nucleosides of uracil, thymine, cytosine, and adenine were synthesized using the glycosyl condensation procedure (silyated base and TMSOTf). The antiviral activities of the synthesized compounds against the HIV-1, HSV-1, HSV-2, and HCMV viruses were evaluated. The adenine derivative 26 showed weak anti-HIV activity (EC(50) = 10.1 microg/ml) without exhibiting any cytotoxicity up to a concentration of 100 microM.

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.

Effect of Various Pyrimidines Possessing the 1-[(2-Hydroxy-1-(hydroxymethyl)ethoxy)methyl] Moiety, Able To Mimic Natural 2‘-Deoxyribose, on Wild-type and Mutant Hepatitis B Virus Replication

Hepatitis B virus (HBV) is the most common cause of chronic liver disease worldwide. Development of drug resistance against clinical anti-HBV drug lamivudine due to long-term use and rebound of viral DNA after cessation of treatment has been a major setback of the current therapy. We have synthesized a series of pyrimidine nucleosides possessing a variety of substituents at the C-5 position, and a 1-[(2-hydroxy-1-(hydroxymethyl)ethoxy)methyl] flexible acyclic glycosyl moiety at the N-1 position, that have the ability to mimic the natural 2'-deoxyribosyl moiety. Some of these potential antiviral compounds included variations at both C-5 and C-6 positions of the uracil base. Other variations of the uracil derivatives were the 6-aza congeners. 4-Amino and 4-methoxy pyrimidine derivatives were also made. Compounds in which the base moiety was substituted by 5-chloro- (25), 5-(2-bromovinyl)- (32), or 5-bromo-6-methyl- (37) groups possess significant activity against duck-HBV, wild-type human HBV (2.2.15 cells), and lamivudine-resistant HBV containing single and double mutations. No cytotoxicity was seen in host HepG2 and Vero cells, up to the highest concentration tested. The anti-HBV activity exhibited by compounds 25, 32, and 37 was superior for human HBV and comparable for DHBV to that of the corresponding purine nucleoside, ganciclovir. Further, they were only 10-15-fold less inhibitory against human HBV in 2.2.15 cells than the reference drug, lamivudine. Other compounds in the series were moderately inhibitory against DHBV and wild-type human HBV. The size of the halogen and the electronegativity of the substituents at the 5- and 6-positions are important for antiviral activity toward HBV. These compounds were also evaluated for their antiviral activity for West Nile virus, respiratory syncytial virus, SARS-coronavirus, and hepatitis C virus. They were generally inactive in these antiviral assay systems (at concentrations up to 100 microg/mL). 1-[(2-Hydroxy-1-(hydroxymethyl) ethoxy)methyl]-5-fluorocytosine (34) showed some inhibitory activity against hepatitis C virus. Taken together, these data support our previous observations that the 5-substituted pyrimidine nucleosides containing acyclic glycosyl moieties have potential to serve as a new generation of potent, selective, and nontoxic anti-HBV agents for wild-type and lamivudine-resistant mutant HBV.

Synthesis and anti-HIV activity of D- and L-thietanose nucleosides

Various D- and L-thietanose nucleosides were synthesized from D- and L-xylose. The four-membered thietane ring was efficiently synthesized by the cyclization of 1-thioacetyl-3-mesylate (4/38) under basic conditions. Condensation with various heterocyclic bases was conducted via Pummerer-type rearrangement to afford various nucleoside derivatives. Among the synthesized nucleosides, D-uridine (23), D-cytidine (24), D-5-fluorocytidine (25), and L-cytidine (52) analogues showed moderate anti-HIV activity, with EC50 = 6.9, 1.3, 5.8, and 14.1 microM, respectively. However, these four nucleoside analogues are cytotoxic in peripheral blood mononuclear and CEM cells. The other nucleosides are neither active nor cytotoxic. Interestingly, the oxetanocin A analogue 33 was not active. Comparison of the minimized reverse transcriptases (RTs) complexed with the corresponding triphosphates of the cytidine analogue 24 and the adenosine analogue 33 by molecular modeling studies showed that there is no difference in the binding mode of the triphosphate of the cytidine analogue 24 to the active site of HIV-1 RT from that of the triphosphate of the adenosine analogue 33. Modeling studies on the initial monophosphorylation step by deoxycytidine kinase showed that the catalytic efficiency of phosphorylation through a nucleophilic attack of the 4'-hydroxyl group of thietanose on the gamma-phosphate of ATP is diminished in the case of L-cytidine analogue (52) due to the increased distance between the 4'-hydroxyl group and the gamma-phosphate.

New targets and inhibitors of HBV replication to combat drug resistance

All the approved chemotherapeutic drugs for the treatment of HBV hepatitis are nucleoside analogs targeting on HBV DNA polymerase. Drugs targeting on other viral unique targets are needed. A new class of chemicals with novel action against HBV replication was discovered. A brief description of their mode of action is given.

Inhibition of hepatitis B virus gene expression and replication by helioxanthin and its derivative

Chronic hepatitis B virus (HBV) infection continues to be an important worldwide cause of morbidity and mortality. All the currently approved therapeutic drugs have their limitations: interferon-alpha (IFN-alpha) has limited efficacy and a high incidence of adverse effects; nucleoside analogues are very efficient HBV DNA inhibitors, but resistance occurs eventually. Therefore, it is important to develop new non-nucleoside/nucleotide agents with different modes of action that can be used for antiviral combination therapy. Here, we report on a novel class of compounds, helioxanthin and its derivative 5-4-2, which had potent anti-HBV activities in HepG2.2.15 cells, with the EC50s of 1 and 0.08 microM, respectively. The lamivudine-resistant HBV, L526M/M550V double mutant strain, was also sensitive to helioxanthin and 5-4-2. This class of compounds not only inhibited HBV DNA, but also decreased HBV mRNA and HBV protein expression. The EC50 of HBV DNA inhibition was consistent with the EC50 of HBV 3.5 Kb transcript inhibition, which was 1 and 0.09 microM for helioxanthin and 5-4-2 respectively. Western blot analysis of cell lysate from HepG2.2.15 cells showed that the core protein expression decreased in a dose-dependent manner after drug treatment. In conclusion, helioxanthin and 5-4-2 are potentially unique new anti-HBV agents, which possess a different mechanism of action from existing therapeutic drugs. Both compounds inhibited HBV RNA and protein expression in addition to inhibiting HBV DNA.

Behavior of thymidylate kinase toward monophosphate metabolites and its role in the metabolism of 1-(2'-deoxy-2'-fluoro-beta-L-arabinofuranosyl)-5-methyluracil (Clevudine) and 2',3'-didehydro-2',3'-dideoxythymidine in cells

L-nucleoside analogs are a new class of antiviral and anticancer agents, several of which are currently used in the clinic. The phosphorylation of these agents to the triphosphate form is thought to be important for exertion of their pharmacological activities. 1-(2'-deoxy-2'-fluoro-beta-L-arabinofuranosyl)-5-methyluracil (L-FMAU; Clevudine) is a thymidine analog that is currently under phase III clinical trials as an anti-human hepatitis B virus agent. We examined the behavior of its monophosphate metabolite with human recombinant thymidylate kinase (TMPK) and showed that L-FMAU monophosphate (L-FMAUMP) is a poorer substrate than its D-configuration anomer (D-FMAUMP). The phosphorylation efficiency of l-FMAUMP is similar to that of the monophosphate of 2',3'-didehydro-2',3'-dideoxythymidine (d4T), an anti-human immunodeficiency virus analog, both of which are approximately 1% TMP. To clarify the role of human TMPK in the phosphorylation of L-FMAUMP to the diphosphate metabolite in cells, a Tet-On inducible human TMPK cell line system was established. In this system, the expression of TMPK is closely regulated in response to various concentrations of doxycycline. When the cells were treated with L-FMAU or d4T, the amounts of the diphosphate and triphosphate metabolites of these analogs were increased, in accordance with an increase in human TMPK activity in cells. In conclusion, this is the first demonstration of the behavior of TMPK toward L-FMAUMP. This study indicates that human TMPK can phosphorylate L-FMAUMP and play a critical role in L-FMAU metabolism in cells.

The potential anti-HBV effect of amantadine in combination with ursodeoxycholic acid and biphenyl dimethyl dicarboxylate in HepG2 2.2.15 cells

Experimental studies have demonstrated that the triple combination of amantadine (A)/ ursodeoxycholic acid (UDCA, U)/ biphenyl dimethyl dicarboxylate (DDB, D) might have a preferential antiviral effect compared with that observed in interferon-induced antiviral signal pathways, such as those of STAT1alpha and the 6-16 genes. To confirm the result, this study examined whether the signal transduction for the antiviral activity in HepG2 2.2.15 was induced dependently or independently of interferon. To accomplish this, the correlation between the STAT1alpha and 6-16 genes, and nitric oxide, for the mediation of the antiviral activity was assessed. The increase in nitric oxide in the UDCA groups suggests that the inhibition of viral gene replication was enhanced by the amantadine combinations (AU and AUD), and might be more effective if incubated for longer periods. It was found that STAT1alpha was activated by the amantadine combination, although to a lesser extent than that of interferon-alpha, and the primary endpoints examined for the inhibition of gene expression (HBsAg and HBcAg) were remarkably well regulated. This suggests that the amantadine triple, or at least the double, combination had better clinical benefits than those of IFN-alpha and the nucleoside analogue single treatment. This demonstrates that the amantadine combination might be a substitute for the existing HBV therapy if the results of in vivo and in vitro studies concur.

Increased DNA polymerase fidelity of the Lamivudine resistant variants of human hepatitis B virus DNA polymerase

Although efficient antiviral lamivudine is used for HBV-infected patients, a prolonged treatment with nucleoside analogs often results in lamivudine-resistant variants. In this study, we evaluated the fidelity of the lamivudine-resistant variants. The FLAG-tagged wild-type (FPolE) and Met550 variants (FPolE/M550A, M550V, and M550I) of HBV DNA polymerases were expressed in insect cells, then purified. Like many other reverse transcriptases, no 3' --> 5' exonuclease activity was detected in the HBV DNA polymerase. Since there is no proofreading activity, then the use of the site-specific nucleotide misincorporation method is beneficial. From the f(ins) value analysis, it is evident that M550I and M550V exhibit higher fidelity values than the wild-type HBV DNA polymerase, while M550A exhibits similar fidelity values. It is therefore suggested that lamivudine resistance comes from the stringency to dNTP binding and the discrimination of dCTP and lamivudine in M550V and M550I.

A novel class of phosphonate nucleosides. 9-[(1-phosphonomethoxycyclopropyl)methyl]guanine as a potent and selective anti-HBV agent

9-[1-(Phosphonomethoxycyclopropyl)methyl]guanine (PMCG, 1), representative of a novel class of phosphonate nucleosides, blocks HBV replication with excellent potency (EC(50) = 0.5 microM) in a primary culture of HepG2 2.2.15 cells. It exhibits no significant cytotoxicity in several human cell lines up to 1.0 mM. It does not inhibit replication of human immunodeficiency virus (HIV-1) or herpes simplex virus (HSV-1) at 30 microM. Many purine base analogues of 1 also exhibit inhibitory activity against HBV, but at 30 microM, pyrimidine analogues do not. 1 is 4 times more potent than 9-[2-(phosphonomethoxy)ethyl]adenine (PMEA), which was used as a positive control (EC(50) = 2.0 microM). The characteristic cyclopropyl moiety at the 2'-position of 1 was prepared by titanium-mediated Kulinkovich cyclopropanation. 1 was modified to give the orally available drug candidate, PMCDG Dipivoxil (2). Compound 2 exhibited excellent efficacy when administered at 5 mg per kg per day in a study with woodchucks infected with woodchuck hepatitis B virus (WHBV). Drug candidate 2 has successfully completed phase I clinical trials and is currently undergoing phase II clinical studies for evaluation of efficacy.

Novel role of 3-phosphoglycerate kinase, a glycolytic enzyme, in the activation of L-nucleoside analogs, a new class of anticancer and antiviral agents

l-Nucleoside analogs are a new class of clinically active antiviral and anticancer agents. The phosphorylation of these analogs from diphosphate to triphosphate metabolites is crucial for their biological action. We studied the role of 3-phosphoglycerate kinase, a glycolytic enzyme, in the metabolism of l-nucleoside analogs, using small interfering RNAs to down-regulate the amount of this enzyme in HelaS3 and 2.2.15 cells, chosen as models for studying the impact of the enzyme on the anticancer and antihepatitis B virus activities of these analogs. Decrease in the expression of 3-phosphoglycerate kinase led to a corresponding decrease in the formation of the triphosphate metabolites of l-nucleoside analogs (but not d-nucleoside analogs), resulting in detrimental effects on their activity. The enzyme is important for generating as well as maintaining the steady state levels of l-nucleotides in the cells, thereby playing a key role in the activity of l-nucleoside analogs against human immunodeficiency virus, hepatitis B virus, and cancer. This study also indicates a structure-based distinction in the metabolism of l- and d-nucleoside analogs, disputing the classic notion that nucleoside diphosphate kinases are responsible for the phosphorylation of all classes of nucleoside analog diphosphates.

Fidelity of hepatitis B virus polymerase

Although efficient vaccines are available, chronic hepatitis B (HBV) infection poses a major health problem worldwide, and prolonged treatment of chronically infected HBV patients with nucleoside analogs often results in drug-resistant HBV variants. Therefore, it is critical to evaluate the contribution of the HBV polymerase to mutations. FLAG-tagged wild-type (FPolE) and mutant (FPolE/D551A) HBV polymerases have been expressed in insect cells and purified. The purified FPolE showed DNA polymerase activity, but FPolE/D551A did not, implying that the activity was derived from FPolE. No 3'-->5'exonuclease activity was detected in FPolE. The fidelity of FPolE was investigated and compared with that of HIV-1 RT, which is highly error-prone. The fidelity of HBV polymerase seems to be achieved by increasing the Km for the dNTP being misinserted. The nucleotide misinsertion efficiency of FPolE and HIV-1 RT ranged from 3.59 x 10-4 (C : T) to 1.51 x 10-3 (G : T) and from 1.75 x 10-4 (C : T) to 1.62 x 10-3 (G : T), respectively, and the overall misinsertion efficiency of HIV-1 RT was just 1.04-fold higher than that of FPolE, implying that HBV polymerase is fairly error-prone. Though HBV genetic mutation rate in replication is thought to be between those in RNA and DNA viruses, our data shows that the rate of mutation by HBV polymerase is higher than the rate of genetic mutation in vivo. This may be a result from more overlapping HBV genes in the HBV genome than that of other retroviruses.

An open-label study of tenofovir in HIV-1 and Hepatitis B virus co-infected individuals

BACKGROUND

Tenofovir is a novel nucleotide analogue recommended for use in HIV-1 infected treatment-experienced patients. Recent data suggest an effect on Hepatitis B virus (HBV) replication. We therefore investigated the use of tenofovir in HIV-1 and HBV co-infected individuals.

METHODS

Twenty HIV-1/HBV co-infected patients with a median of 108 weeks lamivudine experience (range, 0-270 weeks) received tenofovir 245 mg daily in addition to or as part of their combination antiretroviral therapy. Their immunologic parameters and HIV-1 RNA and HBV DNA viral loads were followed over a period of 52 weeks. In addition, their HBV DNA polymerase was sequenced at baseline to measure the frequency of YMDD mutations that are associated with lamivudine resistance.

FINDINGS

A significant decrease in HBV DNA viral load (4 x log ) and alanine aminotransferase levels was observed. There were no significant overall differences between the lamivudine-experienced (n = 15) and -naive (n = 5) individuals and tenofovir was well tolerated. Five patients (25%) underwent HBe antigen seroconversion during the study period. Out of the 15 lamivudine-experienced individuals, 10 had YMDD mutations and one had YIDD mutations in HBV DNA.

INTERPRETATION

These results indicate that 52 weeks of tenofovir in addition to antiretroviral therapy is active against HBV, and it appears to overcome lamivudine resistance.

Characterizing antiviral activity of adefovir dipivoxil in transgenic mice expressing hepatitis B virus

Oral adefovir dipivoxil (ADV) reduced viral load in transgenic mice expressing hepatitis B virus (HBV). Liver HBV DNA was reduced to <0.1 pg of viral DNA per microg of total DNA (pg/microg) following oral ADV therapy at a dosage of 100 mg/kg/day twice daily for 10 days as compared to a mean of 3.0 pg/microg for the placebo control group. Oral ADV treatment also reduced serum HBV DNA to 3.5 log10 genomic equivalents (ge)/ml compared to 5.3 log10 ge/ml for the placebo control group. With once daily treatments, ADV antiviral activity reached near maximum viral reduction by day 10 in the liver and reached an endpoint of liver virus inhibition at 1.0 mg/kg/day. The minimum effective dose was less than 0.1 mg/kg/day using inhibition of serum virus. Lamivudine (3TC) given orally at 500 mg/kg/day using the same treatment schedule marginally reduced the serum HBV DNA by 4-fold, but did not significantly reduce HBV liver DNA. Serum titer reduction was also identified in untreated or placebo-treated animals, which may have been caused by the stress of pre-treatment bleeding and multiple oral gavage treatments. This trauma/placebo-effect may have masked the extent of viral reduction in the serum in ADV- and 3TC-treated animals. Liver HBV RNA was not reduced by oral ADV treatments. The lack of RNA reduction was expected, because the HBV transgene is stably integrated into the chromosome and ADV inhibits polymerase activity after transcription of pregenomic RNA. ADV was identified to have potent anti-HBV activity in this HBV transgenic mouse model and could serve as a suitable positive control for future drug discovery experiments.

Management of viral hepatitis B

Hepatitis B virus (HBV) infection is a major global health concern and is the most common cause of chronic liver disease worldwide. The natural history and clinical outcomes of chronic HBV infection are determined by the viral replication cycle and the host immune responses. Treatment of chronic hepatitis B is directed at interrupting the natural history by suppressing HBV replication before development of any significant irreversible liver cell damage. Effective antiviral therapies should be followed by sustained suppression of HBV-DNA, normalization of transaminases levels and a stable stage of HBeAg seroconversion with persistence of circulating anti-HBeAg antibodies. Two major classes of antiviral therapeutic agents that have been approved for treatment of chronic hepatitis B are immunomodulating agents (i.e. interferon) and the nucleoside analogs (i.e. lamivudine). A 4-6 month course of interferon-alpha has resulted in improvement of survival in 20%-30% of patients with chronic hepatitis B who had elevated serum ALT levels without hepatic decompensation. Interferon-alpha therapy is associated with HBeAg seroconversion; normalization of ALT levels, reduced hepatic inflammation, and possibly reduced disease progression to cirrhosis and hepatocellular carcinoma. Interferon can also be used with caution in patients with early compensated cirrhosis. A 12-month course of lamivudine has been shown to be well tolerated and effective. Lamivudine can be used in decompensated cirrhosis and immunosuppressed patients and for prevention of recurrent HBV infection after liver transplantation. The response rates after 3 years of lamivudine therapy account for 40-65%. A major problem of antiviral treatment is the emergence of drug resistance conferred by mutations in the YMDD motif of HBV reverse transcriptase. The prevalence of YMDD mutations increases with longer durations of antiviral therapies and this has been detected in 20% of immunocompetent patients receiving lamivudine per year. Contentious issues remain when to stop the treatment if HBeAg seroconversion does not occur. Many new immunomodulatory therapies and antiviral agents are in various stages of clinical development and have shown some promise. Among newer HBV antivirals, adefovir dipivoxil, entecavir, emtricitabine, DAPD and clevudine appear to be at least as potent as lamivudine in suppressing HBV replication. In vitro studies have shown that YMDD mutations confer cross-resistance between lamivudine and emtricitabine. However, adefovir, dipivoxil, lobucavir, DAPD and possibly clevudine suppress replications of both YMDD mutants and wild types of HBV. Immunomodulatory approaches for treatment of chronic hepatitis B are conceptually attractive, but newer agents used to date (thymosin-alpha, interleukin-12, therapeutic vaccines) have not demonstrated sufficient efficacy for widespread use. Combinations of an immunomodulatory agent and nucleoside analog may improve the therapeutic efficacy and reduce the emergence of drug resistance. Nevertheless, combinations of interferon and lamivudine therapies do not confer such additional benefits. The next challenge for HBV treatment is to use antivirals in combination and/or in cyclical therapy to minimize the emergence of drug resistance and increase efficacy, particularly to achieve sustainable post-treatment suppression of HBV. Randomized prospective control trials of combined antiviral therapies given simultaneously or sequentially are needed to establish safe and effective combined regimens that can be recommended for future treatment strategies.

Potent efficacy of entecavir (BMS-200475) in a duck model of hepatitis B virus replication

The ability of entecavir (ETV) to inhibit Duck hepatitis B virus (DHBV) infection in duck hepatocytes and ducklings was examined using lamivudine (3TC) as a comparator drug. ETV exhibited antiviral activity (50% effective concentration [EC(50)], 0.13 nM) in DHBV-infected duck hepatocytes that was >1,000-fold more potent than that of 3TC (EC(50), 138 nM). A 21-day treatment of ducklings with 1 mg of ETV per kg of body weight per day by oral gavage resulted in a mean reduction of log(10) 3.1 in serum DHBV DNA levels. Daily treatment with 0.1 mg of ETV/kg was nearly as effective, achieving an average viral DNA level decrease of log(10) 2.1. Reducing the daily dose of ETV to only 0.01 mg/kg resulted in an average viral DNA level decrease of log(10) 0.97. Daily treatment with 25 mg of 3TC/kg resulted in an average viral DNA level decrease of log(10) 0.66, compared to the log(10) 0.20 drop seen for ducklings given the vehicle alone. ETV was also more effective in decreasing the DHBV DNA levels in duck livers after 21 days of treatment, causing average drops of log(10) 1.41, log(10) 0.76, and log(10) 0.26 for dose levels of 1.0, 0.1, and 0.01 mg/kg, respectively, compared to a decrease of log(10) 0.06 for 3TC at a dose level of 25 mg/kg. Levels of viral covalently closed circular DNA in the treatment group receiving 1 mg of ETV/kg were reduced compared to those in the vehicle-treated group. ETV and 3TC were both well tolerated in all treated animals. These results show that ETV is a highly potent and effective antiviral in the DHBV duck model.

Acyclic nucleoside/nucleotide analogues with an imidazole ring skeleton

Syntheses of a few acyclic nucleoside and acyclic nucleoside phosphonate analogues containing an imidazole ring have been reported. These analogues include methyl 1-(2-hydroxyethoxymethyl)imidazole-4, 5-dicarbo-xylate (1), 4,5-dicarbamoyl-1-(2-hydroxyethoxymethyl)imidazole (2), 4,5-dicyano-1-(2-hydroxyethoxymethyl)imidazole (4), Methyl 1-(2-bromoethoxymethyl)imidazole-4,5-dicarboxylate (7), 4,5-dicyano-(2-bromoethoxymethyl)imidazole (8), and Methyl 1-(2-phosphonomethoxyethyl)imidazole (10). Also reported are a few potential prodrugs of the above compounds, including the acetyl derivatives 5 and 6 (of 1 and 4, respectively), and the diethyl phosphonate ester 9 (of 10). In addition, the corresponding benzyl-protected precursors 11 and 12 (of 1 and 4, respectively), along with their common hydrolysis product, 1-(2-benzyloxy-ethoxymethyl)-4,5-imidazoledicarboxylic acid (3), are reported. Another potential prodrug included in the list is 1-(2-acetoxyethyl)-4,5-dicyanoimidazole (15). The compounds were screened for in vitro antiviral activity against a wide variety of herpes and respiratory viruses. The most active compound was the phosphonate analogue 9 which exhibited an anti-measles virus activity with an EC50 of <2.5 microg/mL and an SI value of > 176.

Molecular modeling and biochemical characterization reveal the mechanism of hepatitis B virus polymerase resistance to lamivudine (3TC) and emtricitabine (FTC)

Success in treating hepatitis B virus (HBV) infection with nucleoside analog drugs like lamivudine is limited by the emergence of drug-resistant viral strains upon prolonged therapy. The predominant lamivudine resistance mutations in HBV-infected patients are Met552IIe and Met552Val (Met552Ile/Val), frequently in association with a second mutation, Leu528Met. The effects of Leu528Met, Met552Ile, and Met552Val mutations on the binding of HBV polymerase inhibitors and the natural substrate dCTP were evaluated using an in vitro HBV polymerase assay. Susceptibility to lamivudine triphosphate (3TCTP), emtricitabine triphosphate (FTCTP), adefovir diphosphate, penciclovir triphosphate, and lobucavir triphosphate was assessed by determination of inhibition constants (K(i)). Recognition of the natural substrate, dCTP, was assessed by determination of Km values. The results from the in vitro studies were as follows: (i) dCTP substrate binding was largely unaffected by the mutations, with Km changing moderately, only in a range of 0.6 to 2.6-fold; (ii) K(i)s for 3TCTP and FTCTP against Met552Ile/Val mutant HBV polymerases were increased 8- to 30-fold; and (iii) the Leu528Met mutation had a modest effect on direct binding of these beta-L-oxathiolane ring-containing nucleotide analogs. A three-dimensional homology model of the catalytic core of HBV polymerase was constructed via extrapolation from retroviral reverse transcriptase structures. Molecular modeling studies using the HBV polymerase homology model suggested that steric hindrance between the mutant amino acid side chain and lamivudine or emtricitabine could account for the resistance phenotype. Specifically, steric conflict between the Cgamma2-methyl group of Ile or Val at position 552 in HBV polymerase and the sulfur atom in the oxathiolane ring (common to both beta-L-nucleoside analogs lamivudine and emtricitabine) is proposed to account for the resistance observed upon Met552Ile/Val mutation. The effects of the Leu528Met mutation, which also occurs near the HBV polymerase active site, appeared to be less direct, potentially involving rearrangement of the deoxynucleoside triphosphate-binding pocket residues. These modeling results suggest that nucleotide analogs that are beta-D-enantiomers, that have the sulfur replaced by a smaller atom, or that have modified or acyclic ring systems may retain activity against lamivudine-resistant mutants, consistent with the observed susceptibility of these mutants to adefovir, lobucavir, and penciclovir in vitro and adefovir in vivo.

New bis(SATE) prodrug of AZT 5'-monophosphate: in vitro anti-HIV activity, stability, and potential oral absorption

The in vitro anti-HIV activity, stability, and potential for oral absorption of a phosphotriester derivative of AZT (zidovudine; 3'-azido-2',3'-deoxythymidine) bearing a new esterase-labile S-acyl-2-thioethyl (SATE) group as transient phosphate protection are reported. The biolabile protection is characterized by the presence of a hydroxyl function in the acyl chain. In accordance with previously reported data in the bis(SATE) prodrug series, the present results demonstrate that the studied bis(hydroxytBuSATE)phosphotriester exerts its biological effects via intracellular delivery of the 5'-monophosphate of AZT. The hydroxyl function confers a high resistance against esterase hydrolysis, and the studied prodrug is able to cross the Caco-2 cell monolayers in intact form, suggesting that its further development as a possible anti-HIV pronucleotide candidate is warranted.

Characterization of novel human hepatoma cell lines with stable hepatitis B virus secretion for evaluating new compounds against lamivudine- and penciclovir-resistant virus

L-Nucleoside analogs are new therapeutic agents for treatment of chronic hepatitis B. However, their clinical application was limited by the emergence of viral resistance. It is important to develop a new system to evaluate drug cross-resistance and to test new agents that may overcome resistant virus. In this report, three cell lines HepG2-WT10, HepG2-SM1, and HepG2-DM2 are presented; these cell lines were established by transfection of HepG2 cells with unique fully functional 1.1x hepatitis B virus (HBV) genomes: wild-type HBV-adr and its L526M and L526MM550V variants, respectively. We have demonstrated that these genomes have different susceptibilities to lamivudine [L(-)SddC] and penciclovir (PCV). By examining HBV RNA transcription, antigen expression, progeny DNA replication, and viral susceptibilities to L(-)SddC, PCV, and other nucleoside analogs, it is concluded that the cell lines are able to stably produce L(-)SddC- and PCV-sensitive and -resistant HBV virions. In addition, the relative susceptibilities of the wild-type and mutant HBV produced from the stably transfected cell lines to several anti-HBV nucleoside analogs were also examined and found to be about the same as those found by using a transient infection system. PMEA [9-(2-phosphonylmethoxytehyl)-adenine] and QYL685 are able to suppress L(-)SddC- and PCV-resistant HBV. In conclusion, this cell culture system is a novel and useful tool for evaluating anti-HBV compounds and biologics.

Viral hepatitis. From prevention to antivirals

The challenge of viral hepatitis has been acknowledged and confronted in the last decade. Significant progress in prevention of infection with HAV and HBV may eradicate these serious infections from the United States and other parts of the world in the coming decades. Application of prophylactic strategies to children will be a major mechanism in accomplishing this task. The quest for potent antiviral medications continues. The next critically important development will be ways to prevent new HCV infections and to treat the millions of already infected individuals at risk for the serious consequences of this disease. For pediatricians, realizing these goals requires a greater understanding of perinatal HCV transmission, use of vaccines for prevention of viral hepatitis, and identification of HCV-infected children who are likely to benefit from new therapeutic strategies as they become available.

Ribavirin and mycophenolic acid potentiate the activity of guanine- and diaminopurine-based nucleoside analogues against hepatitis B virus

Mycophenolic acid [the active metabolite of the immunosuppressive agent mycophenolate mofetil (MMF)] and ribavirin were found to potentiate the anti-HBV activity of the guanine-based nucleoside analogues penciclovir (PCV), lobucavir (LBV) and 3'-fluorodideoxyguanosine (FLG) and diaminopurine dioxolane (DAPD). Ribavirin and mycophenolic acid are both inhibitors of inosine 5'-monophosphate dehydrogenase and cause a depletion of intracellular dGTP levels. It may be assumed that the 5'-triphosphorylated derivatives of the guanine-based nucleoside analogues, in the presence of reduced levels of dGTP, inhibit more efficiently the priming reaction as well as the reverse transcription and DNA-dependent DNA polymerase activity of the HBV polymerase. This assumption is corroborated by the observation that exogenously added guanosine reversed the potentiating effect of ribavirin and mycophenolic acid on the anti-HBV activity of the guanosine analogues. Our observations may have implications for those (liver) transplant recipients that receive MMF as (part of their) immunosuppressive regimen and that, because of de novo or persistent infection with HBV, need specific anti-HBV therapy.

Clinical potential of emerging new agents in hepatitis B

Treatment of chronic hepatitis B is directed at interrupting the natural history and clinical outcomes of the disease. It needs to take into account the virology and replication cycle of the hepatitis B virus (HBV), and the host immune response to HBV. Long term follow-up of patients treated with interferon supports the paradigm that a sustained, major suppression of HBV replication, particularly that associated with hepatitis B e antigen (HBeAg) seroconversion, interrupts the natural history of hepatitis B. The availability of potent but well tolerated and orally available HBV antivirals, of which lamivudine is the prototype, has allowed clearer treatment objectives to be formulated. These are: temporary or permanent reduction of hepatitis (necroinflammatory) activity, arrest of fibrotic progression, prevention of cirrhosis and liver failure, and prevention of recurrent HBV infection after liver transplantation. Lamivudine has good medium term efficacy in achieving each of these objectives. The only significant problem for the longer term is emergence of antiviral resistance conferred by mutations in the YMDD (tyrosine-methionine-aspartic acid-aspartic acid) motif of the HBV reverse transcriptase. As a result, contentious issues remain about defining when antiviral therapy is indicated, whether to treat for a defined interval or indefinitely, and when to stop treatment if HBeAg seroconversion is not achieved. Some personal views are expressed in this review. Among newer HBV antivirals in clinical studies, adefovir dipivoxil, entecavir and emtricitabine appear to be at least as potent as lamivudine in suppressing HBV replication. Famciclovir appears less potent. In vitro studies show that YMDD mutations confer cross-resistance between lamivudine, emtricitabine and beta-L-Fd4C (L-2',3'-didehydro-dideoxy-5-fluorocytidine). However, adefovir dipivoxil, lobucavir, entecavir, DAPD (beta-D-2,6-diaminopurine dioxolane) and possibly clevudine (L-FMAU) suppress replication of YMDD mutant HBV, as well as wildtype. Preliminary studies indicate clinical efficacy of adefovir dipivoxil once resistance to lamivudine has developed. Immunomodulatory approaches to treatment of chronic hepatitis B are conceptually attractive, but newer agents used to date (thymalfasin, interleukin-12, therapeutic vaccines) have not demonstrated sufficient efficacy for widespread use. The next challenge for HBV treatment is to use antivirals in combination and/or in cyclical therapy to reduce the emergence of drug resistance and increase efficacy, particularly to achieve sustainable post-treatment suppression of hepatitis B.

Combination chemotherapy for hepatitis B virus: the path forward?

Hepatitis B virus (HBV) was identified as a cause of viral hepatitis more than 30 years ago and hepatitis B vaccines have been available for almost 20 years, but HBV infection continues to be a global health problem, responsible for about 1.2 million deaths annually. By the end of this year, almost 400 million people--about 5% of the world's population and more than ten times the number infected with human immunodeficiency virus (HIV)--will be infected with HBV. Chemotherapy remains the only treatment option for controlling chronic HBV infection once acquired, but none of the many different chemotherapeutic strategies used in the past has proven consistently successful. Prospects for successful treatment of HBV have improved dramatically during the past decade due to the development of new, well tolerated and efficacious anti-HBV drugs, and to advances in our understanding of HBV replication and pathogenesis. The newer anti-HBV drugs are capable of reducing viral loads very rapidly, but the initial response is invariably followed by very much slower elimination of residual virus. As more effective anti-HBV drugs become available, the emergence of drug resistance during the slower phase of HBV elimination will probably become the most significant obstacle in the way of eventual control of HBV infection. Experience with HIV indicates that combination chemotherapy may suppress or eliminate drug resistance and methods for pre-clinical and clinical assessment of anti-HBV drug combinations are being developed. Basic research into mechanisms of drug action and interaction should assist in the design and optimisation of combination chemotherapy for HBV infection, for which additional new anti-HBV drugs will undoubtedly be required in future.

Evolution of lamivudine-resistant hepatitis B virus and HIV-1 in co-infected individuals: an analysis of the CAESAR study. CAESAR co-ordinating committee

OBJECTIVES

Lamivudine has potent activity against HIV-1 and hepatitis B virus (HBV). Co-infection with these two viruses is common, and this may therefore influence the choice of antiretroviral therapies. A cohort of co-infected patients treated with lamivudine were studied in order to evaluate the differential effects of lamivudine on the two viral populations within the same individual after 44-52 weeks of therapy.

DESIGN AND METHODS

Retrospective virological analysis of an HIV-1/HBV co-infected lamivudine cohort derived from a randomized, placebo-controlled study of lamivudine in HIV infection, the CAESAR study.

RESULTS

Five of thirteen patients with HBV viral load > 10,000 copies/ml after 44-52 weeks of lamivudine therapy had genotypic drug resistance. Four of these five had a rebound of viral replication over the period of study and in one case this was associated with an alanine transaminase serum elevation. Ten of the thirteen patients had a 44-52 week HIV viral load > 1000 copies/ml, all of whom also had HIV reverse transcriptase M184V or M184I mutations.

CONCLUSIONS

Extrapolating these results to the population yields an estimated 1-year incidence of drug-resistant HBV of at least 14% in lamivudine-treated HIV-1/HBV co-infected patients. The clinical and virological benefit of HBV lamivudine monotherapy in co-infected patients should be balanced against the potential for emergence of drug resistance. Further, these data suggest that the determinants of HIV and HBV drug resistance are different and that parallel evolution, rather than co-evolution of HBV and HIV-1 in co-infected individuals occurs.

Antiviral agents in the critically ill child

The treatment for most viral infections in children primarily is supportive. Severe viral illnesses and significant secondary complications that require treatment in the intensive care unit may occur in immunocompromised patients and also in infants and children who were previously healthy. Antiviral agents with specific activity against certain respiratory viruses, herpesviruses, and enteric viruses are available. New drugs are under development, and their use in pediatric patients is a subject of active research. The clinician's knowledge of the mechanisms of action, spectrum of activity, and side effects of these drugs is an important tool for their judicious use in the treatment of the critically ill child. Copyright © 2000 by W.B. Saunders Company