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

Active site polymerase inhibitor nucleotides (ASPINs): Potential agents for chronic HBV cure regimens

Chronic hepatitis B virus (HBV) infection affects 240 to 300 million people worldwide. In the nucleus of infected hepatocytes, the HBV genome is converted to covalently closed circular DNA (cccDNA), which persists and serves as a transcriptional template for viral progeny. Therefore, a long-term cure for chronic HBV infection will require elimination of cccDNA. Although currently available nucleos(t)ide analogues (eg, tenofovir disoproxil fumarate, tenofovir alafenamide, entecavir) effectively control HBV replication, they are seldom curative (functional cure rate ∼10%) and require lifelong treatment for most patients. As such, antiviral agents with novel mechanisms of action are needed. Active site polymerase inhibitor nucleotides (ASPINs) noncompetitively distort the HBV polymerase active site to completely inhibit all polymerase functions, unlike traditional chain-terminating nucleos(t)ide analogues, which only target select polymerase functions and are consumed in the process. Clevudine, a first-generation ASPIN, demonstrated potent and prolonged HBV suppression in phase 2 and 3 clinical studies, but long-term treatment was associated with reversible myopathy in a small number of patients. ATI-2173, a novel next-generation ASPIN, is structurally similar to clevudine but targets the liver and demonstrates potent anti-HBV activity on and off treatment, and may ultimately demonstrate an improved pharmacokinetic and safety profile by significantly reducing systemic clevudine exposure. Thus, ATI-2173 is currently in clinical development as an agent for HBV cure. Here, we review the mechanism of action and preclinical and clinical profiles of clevudine and ATI-2173 to support the role of ASPINs as part of curative regimens for chronic HBV infection.

Novel targets for hepatitis B virus therapy

Treatment with either pegylated interferon-alpha (pegIFN-α) or last generation nucleos(t)ide analogues (NAs) successfully leads to serum viral load suppression in most chronically infected hepatitis B (CHB) patients, but HBsAg loss is only achieved in 10% of the cases after a 5-year follow-up. Thus, therapy must be administered long-term and it will not completely eliminate infection because of the persistent hepatitis B virus (HBV) minichromosome in infected cells, and cannot completely abolish the risk of developing severe sequelae such as cirrhosis and hepatocellular carcinoma. Recent progress in the development of in vitro and in vivo models of HBV infection have helped renew interest in the investigation of the viral life cycle, as well as specific virus-host cell interactions to identify new targets for the development of new antiviral drugs. This includes either direct inhibition of viral replication by targeting fundamental steps such as entry, cccDNA formation/stability, viral transcripts, capsid assembly and secretion or the manipulation of the host immune system for better defence against infection. Multiple strategies are currently under investigation, including boosting endogenous innate responses and/or restoring adaptive immunity via engineering of HBV-specific T cells or via the use of inhibitors of negative regulators, as well as therapeutic vaccines. It is increasingly clear that multiple therapeutic strategies must be combined to reach a cure of HBV and that the definition of clinical, virological and immunological correlates for the management of treatment are urgently needed.

Antiviral activity of methyl helicterate isolated from Helicteres angustifolia (Sterculiaceae) against hepatitis B virus

The anti-HBV effect of methyl helicterate (MH), a triterpenoid isolated from the Chinese herb Helicteres angustifolia, was explored both in vitro and in vivo. In the HBV-transfected cell line HepG2.2.15, the secretion of HBsAg/HBeAg, the levels of HBV DNA and cccDNA, and the amount of viral RNA were significantly decreased after treatment with MH for 144h. In addition, MH had no inhibitory effect on the mitochondrial DNA content. In DHBV-infected ducklings, MH significantly reduced the serum DHBV DNA, liver total viral DNA, and cccDNA levels. Furthermore, analysis of the liver pathological changes confirmed the hepatoprotective effect of MH. These results indicate that MH efficiently inhibits HBV replication both in vitro and in vivo and that MH may be a major bioactive ingredient in H. angustifolia.

Update on new antivirals under development for the treatment of double-stranded DNA virus infections

All the currently available antiviral agents used in the treatment of double-stranded (ds) DNA viruses, with the exception of interferon-α, inhibit the same target, the viral DNA polymerase. With increasing reports of the development of resistance of herpes simplex virus (HSV), cytomegalovirus (CMV), and hepatitis B virus (HBV) to some of these drugs, new antiviral agents are needed to treat these infections. Additionally, no drugs have been approved to treat several DNA virus infections, including those caused by adenovirus, smallpox, molluscum contagiosum, and BK virus. We report the status of 10 new antiviral drugs for the treatment of dsDNA viruses. CMX-001 has broad activity against dsDNA viruses; 3 helicase-primase inhibitors, maribavir, and FV-100 have activity against certain herpesviruses; ST-246 inhibits poxviruses; GS-9191 inhibits papillomaviruses; and clevudine and emtricitabine are active against HBV. Most of these drugs have completed at least phase I trials in humans, and many are in additional clinical trials.

Chronic Hepatitis B Infection: Principles of Therapy

Chronic hepatitis B is a global health concern in many resource-limited settings due to perinatal or pediatric hepatitis B virus transmission. In the United States, pediatric infection has been virtually eliminated due to maternal screening during pregnancy and the availability of an effective vaccine. However, young adults remain an at-risk group for hepatitis B virus infection due to sexual transmission and injection drug use. The frequency of progression from acute hepatitis B virus infection to chronic hepatitis B infection depends on multiple factors, including host immune function and age at time of hepatitis B virus infection. Fortunately, there are 7 currently approved therapies for chronic hepatitis B infection, and several emerging therapies that show promise. Despite the availability of these agents, many clinical questions still surround chronic hepatitis B therapy including when to start therapy, which agent is ideal for first and second line therapy, the appropriate duration of therapy, and the role of combination antiviral therapy. This review focuses on agents available for chronic hepatitis B management, including pharmacology, safety and efficacy data, monitoring parameters, and the role for each in chronic hepatitis B therapy in adult patients.

[Treatment of hepatitis B: new perspectives]

Despite the development of new anitiviral agents, the treatment of chronic hepatitis B remains a major clinical challenge. Major achievements have been made with the rationale use of antivirals exhibiting a complementary cross resistance profile to prevent antiviral drug resistance. The current concept of modern antiviral therapy of chronic hepatitis B relies on a precise virologic monitoring and early treatment adaptation to prevent drug resistance. The difficulty of achieving viral clearance and the risk of drug resistance development are major arguments to continue research in the field of antivirals and to identify new targets for therapy. The development of true combination therapy is highly desirable to fulfil the objective of long-term viral suppression, clearance of viral cccDNA and infected cells and ultimately cure of the disease.

Development of a novel mouse model to evaluate drug candidates against hepatitis B virus

Woodchuck hepatitis virus (WHV)-infected woodchucks have been used for preclinical development of drugs against hepatitis B virus (HBV). However, there is no simple in vivo model to evaluate small amounts of compounds against HBV. To develop such a model, HepAD38 cells, in which HBV replication is regulated by tetracycline (tet), were grown as subcutaneous tumours in nude mice. Mice developing viraemia were then left untreated or given tet in the drinking water. In some of the mice given tet, it was removed and the mice were injected intraperitoneally with phosphate buffer saline (PBS), lamivudine (3TC), clevudine (CLV) or tenofovir dipivoxil fumarate (TDF). Virus DNA titres were measured by real-time PCR during and after drug treatment. In water-fed and PBS-injected mice, virus titres reached approximately 10(9) copies/ml serum within 35 days of HepAD38 injection, whereas in tet-treated mice, virus titres remained at 10(4)-10(5) copies/ml. HBV DNA levels were suppressed by 3TC, TDF and CLV, with the latter two drugs showing more sustained virus suppression compared with 3TC. Combination therapy with CLV plus TDF was much more effective than either drug alone in suppressing virus titre for at least 3 weeks after the end of treatment. There was no demonstrable toxicity to HepAD38 cells in drug-treated mice. Hence, a robust tet-controlled system for HBV replication in vivo was demonstrated, validated with monotherapies against HBV and shown to be useful in assessing combination therapy. This system will be useful for preclinical assessment of small amounts of single or multiple compounds against HBV in vivo.

Clevudine is Efficiently Phosphorylated to the Active Triphosphate form in Primary Human Hepatocytes

Background

Clevudine (CLV) is a nucleoside analogue of the unnatural l-configuration that has demonstrated potent activity against hepatitis B virus (HBV) in vitro and in Phase III clinical studies. Human hepatoma cell lines are the only liver-derived cells in which CLV metabolism has been investigated. Here, we examine CLV metabolism in primary human hepatocytes, which better represent CLV metabolism in the liver.

Methods

HPLC analysis of primary human hepatocyte extracts incubated with radiolabelled CLV was used to determine the time course of CLV phosphorylation. Effects of the exogenous cell concentration of CLV on phosphorylation were assessed and the half-life of the CLV phosphorylated forms was determined.

Results

The major CLV metabolite formed in human primary hepatocytes was 5‘-monophosphate, whereas in the hepatoma cell lines the major metabolite was 5’-triphosphate. The level of CLV 5’-triphosphate was similar in both cell types. In primary hepatocytes the conversion of CLV 5’-monophosphate to the corresponding 5’-diphosphate was the rate-limiting step in CLV phosphorylation; the level of CLV phosphorylation was dependent upon exogenous drug concentration and exposure time. CLV 5’-triphosphate accumulated rapidly with peak levels observed after ∼8 h. When cells were incubated with 1 μM CLV, the approximate maximal plasma concentration achieved in individuals receiving the 30 mg dose, the average intracellular concentration of CLV 5’-triphosphate was 41.3 ±8.4 pmols/106 cells (∼10 μM). The initial half-life of CLV triphosphate was ∼11 h.

Conclusions

These results are consistent with once daily CLV dosing currently being used in Phase III clinical studies.

Clevudine: a potent inhibitor of hepatitis B virus in vitro and in vivo

Clevudine (CLV) is a nucleoside analog of the unnatural L-configuration that has potent anti-hepatitis B virus (HBV) activity in vitro and in vivo with a favorable toxicity profile in all species tested. In cell culture, CLV is readily phosphorylated to the corresponding 5'-triphosphate form of the compound. The mechanism of action of CLV involves the inhibition of the HBV polymerase by CLV 5'-triphosphate. In vivo efficacy studies performed in the duck and woodchuck models showed marked, rapid inhibition of virus replication and no significant toxicity. In the woodchuck model, there was a dose-dependent delay in viral recrudescence and a reduction or loss of covalently closed circular DNA. In Phase II clinical studies, CLV was well tolerated and exhibited potent antiviral activity at all doses investigated. In Phase III studies in both hepatitis B e antigen (HBeAg)-positive and -negative patients, CLV 30 mg administered once daily demonstrated potent antiviral efficacy and significant biochemical improvement after only 24 weeks of therapy. These effects were sustained in a significant portion of the patients when therapy was stopped after 6 months with no viral rebound occurring in approximately 3 and 16% in HBeAg-positive and -negative patients, respectively. There have been no significant safety or tolerance issues associated with the drug in these studies. Future studies will investigate the safety and tolerance of CLV 30 mg given once daily over 48 weeks and longer.

The current status of antiviral therapy of chronic hepatitis B

In recent years, marked progress has been made in the treatment of chronic viral hepatitis. Recent studies suggest that pegylated interferons (PEG IFNs) are more effective than standard IFNs in the treatment of chronic hepatitis B. So far, the combination of PEG IFN with lamivudine, used simultaneously, is disappointing in terms of short-term efficacy. However, long-term efficacy needs to be addressed and different schedules of combination, for example sequential, need to be evaluated. A number of nucleoside analogues, with favourable toxicity profiles and a promise of increased effectiveness against HBV, are in various stages of clinical development. Entecavir has recently been approved in the USA. The future of chronic hepatitis B therapy seems to be in the combination of different drugs. Ideally, the optimal drugs to combine would meet the following criteria: they should have a potent antiviral effect, an excellent safety profile and the duration of therapy should be limited. Indeed, the concept of combination therapy has been recently developed in order to increase efficacy and to decrease the occurrence of viral resistance. However, so far there are few data available and no combination therapy demonstrated a clear benefit as compared with monotherapy. More potent drugs and new combinations together with the understanding of the mechanisms of resistance to therapy are challenges to improve the efficacy of treatment and decrease in the future the global burden related to chronic hepatitis B.

Animal models for the study of HBV replication and its variants

Enormous progresses in hepatitis B virus research have been made through the identification of avian and mammalian HBV related viruses, which offer ample opportunities for studies in naturally occurring hosts. However, none of these natural hosts belongs to the commonly used laboratory animals, and the development of various mouse strains carrying HBV transgenes offered unique opportunities to investigate some mechanisms of viral pathogenesis. Furthermore, the need to perform infection studies in a system harbouring HBV-permissive hepatocytes has lately led researchers to create new challenging human mouse chimera models of HBV infection. In this review, we will overview the type of animal models currently available in hepadnavirus research.

Treatment of chronic hepatitis B

In the last years, marked progress has been made in the treatment of chronic hepatitis B. The efficacy of lamivudine, the first nucleoside analogue available, is limited by the high incidence of resistance. Adefovir, which was recently approved has a comparable efficacy with a very low frequency of resistance. However, adefovir needs to be indefinitely administered as withdrawal of therapy is generally associated with reactivation and sustained response is uncommon. Recent large randomized controlled trials showed that PEG IFNs induce relatively high sustained response rates both in HBeAg positive and HBeAg negative chronic hepatitis B. So far, the combination of PEG IFN with lamivudine, used simultaneously, is disappointing in terms of short-term efficacy. However, long-term efficacy needs to be assessed and different schedules of combination (for example sequential) need to be evaluated. A number of nucleoside analogues, with favourable toxicity profiles and a promise of increased effectiveness against HBV, are at various stages of clinical development. Results of phase III trials of entecavir and emtricitabine confirmed their efficacy. However, while entecavir is associated with a low incidences of resistance, emtricitabine is associated with a relatively high incidence of resistance which limits its use as a monotherapy. The efficacy and safety of new and more potent drugs like telbivudine and clevudine need to be confirmed. The future of chronic hepatitis B therapy seems to be in the combination of different drugs. Ideally, the optimal drugs to combine would meet the following criteria: they should have different sites of action on HBV DNA replication, a potent antiviral effect, an excellent safety profile and they should induce a sustained response with a limited duration of therapy. Indeed, the concept of combination therapy has been recently developed in order to increase efficacy and to decrease the occurrence of viral resistance. However, so far few combinations have been evaluated. No combination therapy demonstrated a benefit as compared with monotherapy. More potent drugs and new combinations together with the understanding of the mechanisms of resistance to therapy are important challenges to improve the efficacy of treatment and decrease in the future the global burden related to chronic hepatitis B.

Review article: Nucleoside analogues for the treatment of chronic hepatitis B

Current accepted treatment for chronic hepatitis B uses either the immunomodulator interferon alpha or nucleoside analogues lamivudine or adefovir. Interferon has side effects which mean it is often poorly tolerated. Long-term use of lamivudine is associated with increasing viral resistance for each year it is taken and the rebound viraemia that can occur when the drug is stopped is also of concern to many. Adefovir appears to have less of the resistance issues of lamivudine but is still a relatively new drug and at present its use is principally limited to patients with lamivudine-resistant disease. A number of other nucleoside analogues are currently being developed with some now at the stage of early clinical trials. A proportion share the significant resistance problems of lamivudine but many appear to have more potent anti-viral effect than the drugs currently available. If some of these newer anti-viral agents are approved for use in chronic hepatitis B, the potential for prolonged suppression of hepatitis B virus replication with resultant stabilization or improvement in liver disease may be achieved.

Entecavir, FTC, L-FMAU, LdT and others

To summarize, the future of chronic hepatitis B therapy seems to be the combination of different drugs. Ideally, the optimal drugs to combine would meet the following criteria: they should be orally applicable, they should have an excellent safety profile and the duration of therapy should limited. Currently, the drugs most likely to fulfill these criteria are the nucleoside analogs.

Agents in clinical development for the treatment of chronic hepatitis B

Chronic hepatitis B remains a public health problem of global importance despite the availability of an effective vaccine. Between 350 and 400 million people, approximately 6% of the world's population, suffer from chronic hepatitis B and face a 30% likelihood of developing cirrhotic liver disease or hepatocellular carcinoma. Current treatment options include three monotherapies of subcutaneous interferon, oral nucleoside lamivudine and oral nucleotide adefovir dipivoxil. Unfortunately, these agents have not effectively and frequently been able to attain a 'cure' or complete eradication of the virus. Consequently, the expectation of current therapies is confined to the achievement of clinically beneficial and durable responses defined by lasting suppression of virus replication, histological improvement and increased survival for patients with decompensated liver diseases. Other disadvantages include the undesirable tolerability of interferon, the rapid resistance to lamivudine and the compromise between efficacy and toxicity that led to the development of the 10 mg dose of adefovir dipivoxil. Clearly, better therapeutics and treatment strategies are needed. Increased potency, activity against current treatment-refractory viruses, as well as efficacy in difficult-to-treat populations will be critical to meeting the therapeutic challenge of chronic hepatitis B.

Nucleoside analogues and other antivirals for treatment of hepatitis B in the peritransplant period

Chronic HBV infection is a common cause of advanced liver disease that is associated with substantial mortality. Furthermore, chronic hepatitis B was historically a controversial indication for liver transplantation because of a low post-transplant survival, with graft infection being the major contributor to adverse outcomes. The initial use of hepatitis B immune globulin as prophylaxis, followed later by combined therapy with lamivudine, markedly reduced viral recurrence and improved the survival of patients transplanted for acute or chronic hepatitis B with liver failure. Lamivudine alone can also be used for long-term prophylaxis against de novo HBV infection that can be transmitted by organs from donors positive for anti-HBc or anti-HBs. When used in patients with decompensated chronic hepatitis B with cirrhosis, lamivudine has been shown to improve clinical manifestations, prolong pretransplant survival, and defer, or even obviate, the need for transplantation. Despite prophylaxis, viral mutations with breakthrough reinfection may occur and lead to liver failure. The recently approved adefovir dipivoxil, which is active against lamivudine-resistant mutation, and other nucleoside analogs that are in various phases of development, offer hope as rescue therapy for viral recurrence. Other therapeutic alternatives in the future may include gene therapy and immune interventions.

Effects of pyrimidine and purine analog combinations in the duck hepatitis B virus infection model

To design new strategies of antiviral therapy for chronic hepatitis B, we have evaluated the antiviral activity of the combination of amdoxovir (DAPD), emtricitabine [(-)FTC], and clevudine (L-FMAU) in the duck hepatitis B virus (DHBV) model. Using their triphosphate (TP) derivatives in a cell-free system expressing a wild-type active DHBV reverse transcriptase (RT), the three dual combinations exhibited a greater additive inhibitory effect on viral minus-strand DNA synthesis than the single drugs, according to the Bliss independence model. Both dual combinations with DAPD TP were the most efficient while the triple combination increased the inhibitory effect on the DHBV RT activity in comparison with the dual association, however, without additive effect. Postinoculation treatment of experimentally infected primary duck hepatocytes showed that dual and triple combinations potently inhibited viral DNA synthesis during treatment but did not inhibit the reinitiation of viral DNA synthesis after treatment cessation. Preinoculation treatment with the same combinations exhibited antiviral effects on intracellular viral DNA replication, but it was unable to prevent the initial covalently closed circular DNA (cccDNA) formation. Short-term in vivo treatment in acutely infected ducklings showed that the dual combinations were more-potent inhibitors of virus production than the single treatments, with the L-FMAU and FTC combination being the most potent. A longer administration of L-FMAU and FTC for 4 weeks efficiently suppressed viremia and viral replication. However, no viral clearance from the liver was observed, suggesting that the enhanced antiviral effect of this combination was not sufficient for cccDNA suppression and HBV eradication from infected cells.

Comparison of anti-hepatitis B virus activities of lamivudine and clevudine by a quantitative assay

In this study, we used a quantitative assay to measure the concentration-dependent effects of antivirals on extracellular hepatitis B virus (HBV) DNA as well as on different cytoplasmic and nuclear forms of HBV DNA that participate in HBV replication. HBV recombinant baculovirus, which efficiently delivers the HBV genome to HepG2 cells, was used for this study because (i) antivirals can be administered prior to initiation of HBV infection or after HBV infection and (ii) sufficiently high HBV replication levels are achieved that HBV covalently closed circular (CCC) DNA can be easily detected and individual HBV DNA species can be quantitatively analyzed separately from total HBV DNA. The results showed that the levels of HBV replicative intermediate and extracellular DNA decreased in a concentration-dependent fashion following antiviral treatment. The 50% effective concentration (EC(50)) and EC(90) values and the Hill slopes differed for the different HBV DNA species analyzed. The data clearly indicated that (i) nuclear HBV DNAs are more resistant to antiviral therapy than cytoplasmic or extracellular HBV DNAs and (ii) nuclear HBV CCC DNA is more resistant than the nuclear relaxed circular form. This report presents the first in vitro comparison of the effects of two antivirals administered prior to initiation of HBV infection and the first thorough in vitro quantitative study of concentration-dependent antiviral effects on HBV CCC DNA.

Animal models for the study of HBV infection and the evaluation of new anti-HBV strategies

BACKGROUND

Our aim was to evaluate the anti-HBV activity of a novel L-nucleoside analog, 2',3'-dideoxy-2',3'-didehydro-beta-L-5-fluorocytidine (beta-L-Fd4C), in study models of HBV infection.

METHOD

Its mechanism of action was evaluated on the in vitro expressed duck HBV (DHBV) reverse transcriptase and in primary hepatocyte cultures of duck and human origin. The capacity of antiviral therapy to clear viral infection was analyzed in vivo in the duck and woodchuck models.

RESULTS

beta-L-Fd4C-TP exhibited a more potent inhibitory effect on the RT activity of the DHBV polymerase than other cytidine analogs (lamivudine-TP, ddC-TP, beta-L-FddC-TP). In primary duck hepatocyte cultures, beta-L-Fd4C exhibited a long-lasting inhibitory effect on viral DNA synthesis but could not clear viral cccDNA. In vivo treatment with beta-L-Fd4C in infected ducklings and woodchucks, induced a greater suppression of viremia and intrahepatic viral DNA synthesis than with lamivudine. However, covalently closed circular DNA persistence explained the relapse of viral replication after treatment withdrawal. Viral spread was strongly reduced in the case of early therapeutical intervention, but the number of infected cells did not decline when therapy was started during chronic infection. Liver histology analysis showed a decrease in the inflammatory activity of chronic hepatitis while no ultrastructural modification of liver cells was observed in electron microscopy studies. Furthermore, in human primary hepatocyte cultures, beta-L-Fd4C induced a significant inhibition of HBV DNA synthesis.

CONCLUSION

beta-L-Fd4C is a potent inhibitor of hepadnavirus RT and inhibits viral DNA synthesis in hepatocytes both in vitro and in vivo. These experimental studies allowed as to show that beta-L-Fd4C is a promising anti-HBV agent. Combination therapy should be evaluated to eradicate viral infection.

Inhibitory activity of dioxolane purine analogs on wild-type and lamivudine-resistant mutants of hepadnaviruses

To design combination strategies for chronic hepatitis B therapy, we evaluated in vitro the inhibitory activity of 4 nucleoside analogs, (-)FTC, L-FMAU, DXG, and DAPD, in comparison with lamivudine (3TC) and PMEA. In a cell-free assay for the expression of wild-type duck hepatitis B virus (DHBV) reverse transcriptase, DAPD-TP was found to be the most active on viral minus strand DNA synthesis, including the priming reaction, followed by 3TC-TP, (-)FTC-TP, and DXG-TP, whereas L-FMAU-TP was a weak inhibitor. In cell culture experiments, important differences in drug concentration allowing a 50% inhibition of viral replication or polymerase activity (IC50s) were observed depending on the cell type used, showing that antiviral effect of nucleoside analogs may depend on their intracellular metabolism. IC50s obtained for wild-type DHBV replication in primary duck hepatocytes were much lower than with DHBV transfected LMH cells. IC50s were also significantly lower in the 2.2.1.5 and HepG2 cells compared with HBV transfected HuH7 cells. Moreover, L-FMAU inhibited preferentially HBV plus strand DNA synthesis in these cell lines. The antiviral effect of these inhibitors was also evaluated against 3TC-resistant mutants of the DHBV and HBV polymerases. These mutants were found to be cross resistant to (-)FTC. By contrast, the double DHBV polymerase mutant was sensitive to DXG-TP and DAPD-TP. Moreover, both purine analogs remained active against DHBV and HBV 3TC-resistant mutants in transfected LMH and HepG2 cells, respectively. In conclusion, the unique mechanism of action of these new inhibitors warrants further evaluation in experimental models to determine their capacity to delay or prevent the selection of drug resistant mutants.

Rebound of hepatitis B virus replication in HepG2 cells after cessation of antiviral treatment

Treatment of patients with lamivudine (3TC) results in loss of detectable levels of hepatitis B virus (HBV) DNA from serum; however, the relapse rate, with regard to both reappearance of virus in the bloodstream and hepatic inflammation, is high when therapy is terminated. Although the rebound observed in patients has also been seen in animal hepadnavirus models, rebound has not been analyzed in an in vitro cell culture system. In this study, we used the HBV recombinant baculovirus/HepG2 system to measure the time course of antiviral agent-mediated loss of HBV replication as well as the time course and magnitude of HBV production after release from antiviral treatment. Because of the sensitivity of the system, it was possible to measure secreted virions, intracellular replicative intermediates, and nuclear non-protein-bound HBV DNA and separately analyze individual species of DNA, such as single-stranded HBV DNA compared to the double-stranded form and relaxed circular compared to covalently closed circular HBV DNA. We first determined that HBV replication in the HBV recombinant baculovirus/HepG2 system could proceed for at least 35 days, with a 30-day plateau level of replication, making it possible to study antiviral agent-mediated loss of HBV followed by rebound after cessation of drug treatment. All HBV DNA species decreased in a time-dependent fashion following antiviral treatment, but the magnitude of decline differed for each HBV DNA species, with the covalently closed circular form of HBV DNA being the most resistant to drug therapy. When drug treatment ceased, HBV DNA species reappeared with a pattern that recapitulated the initiation of replication, but with a different time course.

Inhibitory effect of adefovir on viral DNA synthesis and covalently closed circular DNA formation in duck hepatitis B virus-infected hepatocytes in vivo and in vitro

The elimination of viral covalently closed circular DNA (CCC DNA) from the nucleus of infected hepatocytes is an obstacle to achieving sustained viral clearance during antiviral therapy of chronic hepatitis B virus (HBV) infection. The aim of our study was to determine whether treatment with adefovir, a new acyclic nucleoside phosphonate, the prodrug of which, adefovir dipivoxil, is in clinical evaluation, is able to suppress viral CCC DNA both in vitro and in vivo using the duck HBV (DHBV) model. First, the effect of adefovir on viral CCC DNA synthesis was examined with primary cultures of DHBV-infected fetal hepatocytes. Adefovir was administered for six consecutive days starting one day before or four days after DHBV inoculation. Dose-dependent inhibition of both virion release in culture supernatants and synthesis of intracellular viral DNA was observed. Although CCC DNA amplification was inhibited by adefovir, CCC DNA was not eliminated by antiviral treatment and the de novo formation of CCC DNA was not prevented by pretreatment of the cells. Next, preventive treatment of experimentally infected ducklings with lamivudine or adefovir revealed that both efficiently suppressed viremia and intrahepatic DNA. However, persistence of viral DNA even when detectable only by PCR was associated with a recurrence of viral replication following drug withdrawal. Taken together, our results demonstrate that adefovir is a potent inhibitor of DHBV replication that inhibits CCC DNA amplification but does not effectively prevent the formation of CCC DNA from incoming viral genomes.

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.

Current Options for the Therapy of Chronic Hepatitis B Infection

Until recently the only available treatment for chronic hepatitis B was interferon-alpha. Over the past few years a new class of antiviral has become available, the reverse transcriptase inhibitors. Lamivudine is a nucleoside analogue reverse transcriptase inhibitor that was recently approved in many countries for the treatment of hepatitis B. Despite the potent action of lamivudine, the development of new antivirals and new strategies to treat hepatitis B are still the major goal. We review the latest options for therapy of chronic hepatitis B, including combination strategies that could be an approach to improving the response rate of antivirals.

The polymerase L528M mutation cooperates with nucleotide binding-site mutations, increasing hepatitis B virus replication and drug resistance

After receiving lamivudine for 3 years to treat chronic hepatitis B, 67-75% of patients develop B-domain L528M, C-domain M552I, or M552V mutations in the HBV polymerase that render hepatitis B virus (HBV) drug-resistant. The aim of this study was to evaluate the influence of these mutations on viral replication and resistance to antiviral agents. We investigated the replication fitness and susceptibility of the wild-type and five mutant HBVs (L528M, M552I, M552V, L528M/M552I, and L528M/M552V) to 11 compounds [lamivudine, adefovir, entecavir (BMS-200475) (+)-BCH-189 (+/-)-FTC (racivir) (-)-FTC (emtricitabine) (+)-FTC, L-D4FC, L-FMAU (clevudine), D-DAPD, and (-)-carbovir] by transfecting HBV DNA into hepatoma cells and monitoring viral products by Southern blotting. The replication competency of the single C-domain mutants M552I and M552V was markedly decreased compared with that of wild-type HBV. However, addition of the B-domain mutation L528M restored replication competence. Only adefovir and entecavir were effective against all five HBV mutants, and higher doses of these compounds were necessary to inhibit the double mutants compared with the single mutants. The B-domain mutation (L528M) of HBV polymerase not only restores the replication competence of C-domain mutants, but also increases resistance to nucleoside analogues.

Kinetics of hepadnavirus loss from the liver during inhibition of viral DNA synthesis

Hepadnaviruses replicate by reverse transcription, which takes place in the cytoplasm of the infected hepatocyte. Viral RNAs, including the pregenome, are transcribed from a covalently closed circular (ccc) viral DNA that is found in the nucleus. Inhibitors of the viral reverse transcriptase can block new DNA synthesis but have no direct effect on the up to 50 or more copies of cccDNA that maintain the infected state. Thus, during antiviral therapy, the rates of loss of cccDNA, infected hepatocytes (1 or more molecules of cccDNA), and replicating DNAs may be quite different. In the present study, we asked how these losses compared when woodchucks chronically infected with woodchuck hepatitis virus were treated with L-FMAU [1-(2-fluoro-5-methyl-beta-L-arabinofuranosyl) uracil], an inhibitor of viral DNA synthesis. Viremia was suppressed for at least 8 months, after which drug-resistant virus began replicating to high titers. In addition, replicating viral DNAs were virtually absent from the liver after 6 weeks of treatment. In contrast, cccDNA declined more slowly, consistent with a half-life of approximately 33 to 50 days. The loss of cccDNA was comparable to that expected from the estimated death rate of hepatocytes in these woodchucks, suggesting that death of infected cells was one of the major routes for elimination of cccDNA. However, the decline in the actual number of infected hepatocytes lagged behind the decline in cccDNA, so that the average cccDNA copy number in infected cells dropped during the early phase of therapy. This observation was consistent with the possibility that some fraction of cccDNA was distributed to daughter cells in those infected hepatocytes that passed through mitosis.

Duck hepatitis B virus polymerase gene mutants associated with resistance to lamivudine have a decreased replication capacity in vitro and in vivo

BACKGROUND/AIMS

Hepatitis B virus mutants of the polymerase gene are frequently selected during lamivudine therapy for chronic hepatitis B. To study the biology of these mutants, we analyzed their replication capacity in the duck hepatitis B virus (DHBV) infection.

METHODS

The B and C domain polymerase mutants corresponding to the clinical isolates were engineered by site directed mutagenesis in the DHBV genome in different expression vectors.

RESULTS

The study of the enzymatic activity of the mutated viral polymerase polypeptides analyzed in a cell free system demonstrated a lower priming activity and a decreased capacity of elongation of viral minus strand DNA that was consistent with the lower replication capacity of these mutants in transfected leghorn male hepatoma cells compared to wild type genome. These mutants had a lower replication capacity in primary hepatocytes and in in vivo transfected ducklings. Although resistant to lamivudine, these mutants remained sensitive to PMEA.

CONCLUSION

YMDD mutants of the DHBV reverse transcriptase have a decreased replication capacity both in vitro and in vivo, and are not cross-resistant to PMEA. These results may be important to design new antiviral strategies to combat the replication of the lamivudine resistant viral strains.

Unnatural enantiomers of 5-azacytidine analogues: syntheses and enzymatic properties

Although 2'-deoxy-beta-D-5-azacytidine (Decitabine) and beta-D-5-azacytidine display potent antileukemic properties, their therapeutic use is hampered by their sensitivity to nucleophiles and to deamination catalysed by cytidine deaminase. As shown earlier [Shafiee M., Griffon J.-F., Gosselin G., Cambi A., Vincenzetti S., Vita A., Erikson S., Imbach J.-L., Maury G., Biochem. Pharmacol. 56 (1998) 1237-1242], beta-L-enantiomers of cytidine derivatives are resistant to cytidine deaminase. We thus synthesized several 5-azacytosine beta-L-nucleoside analogues to evaluate their enzymatic and biological properties. 2'-Deoxy-beta-L-5-azacytidine (L-Decitabine), beta-L-5-azacytidine, 1-(beta-L-xylo-furanosyl)5-azacytosine, and 1-(2-deoxy-beta-L-threo-pentofuranosyl)5-azacytosine were stereospecifically prepared starting from L-ribose and L-xylose. D- and L-enantiomers of 2'-deoxy-beta-5-azacytidine were weak substrates of human recombinant deoxycytidine kinase (dCK) compared to beta-D-deoxycytidine, whereas both enantiomers of beta-5-azacytidine or the L-xylo-analogues were not substrates of the enzyme. As expected, none of the presently reported derivatives of beta-L-5-azacytidine was a substrate of human recombinant cytidine deaminase (CDA). The prepared compounds were tested for their activity against HIV and HBV and they did not show any significant activity or cytotoxicity. In the case of L-Decitabine, this suggests that the enantioselectivities of concerned enzymes other than dCK and CDA might not be favourable.

The enantioselectivity of enzymes involved in current antiviral therapy using nucleoside analogues: a new strategy?

This review is primarily intended for synthetic bio-organic chemists and enzymologists who are interested in new strategies in the design of virus inhibitors. It is an attempt to assess the importance of the enzymatic properties of L-nucleosides and their analogues, particularly those that are active against viruses such as human immunodeficiency virus (HIV), hepatitis B virus (HBV), herpes simplex virus (HSV), etc. Only data obtained with purified enzymes have been considered and discussed. The examined enzymes include nucleoside- or nucleotide-phosphorylating enzymes, catabolic enzymes, viral target enzymes and cellular polymerases. The enantioselectivities of these enzymes were determined from existing data and are significant only when a sufficient number of enantiomeric pairs of substrates could be examined. The reported data emphasize the weak enantioselectivities of cellular or viral nucleoside kinases and some viral DNA polymerases. Thus, cellular deoxycytidine kinase has a considerably relaxed enantioselectivity with respect to a large number of nucleosides or their analogues, and it occupies a strategic position in the intracellular activation of the compounds. Similarly, HIV-1 reverse transcriptase often has a relatively weak enantioselectivity and can be inhibited by the 5-triphosphates of a large series of L-nucleosides and analogues. In contrast, degradation enzymes, such as adenosine or cytidine deaminases, generally demonstrate strict enantioselectivities favouring D-enantiomers and are used by chemists in asymmetric syntheses. The weak enantioselectivities of some enzymes involved in nucleoside metabolism are more or less pronounced, and one enantiomer or the other is favoured depending on the substrate. This suggests that the low enantioselectivity is fortuitous and does not result from evolutionary pressure, since these enzymes do not create or modify asymmetric centres in substrates. The combined enantioselectivities of the enzymes examined in this review strongly suggest that the field of L-nucleosides and their analogues should be systematically explored in the search for new virus inhibitors.

Inhibition of the replication of the DNA polymerase M550V mutation variant of human hepatitis B virus by adefovir, tenofovir, L-FMAU, DAPD, penciclovir and lobucavir

Several nucleoside analogues (penciclovir, lobucavir, dioxalane guanine [DXG], 1-beta-2,6-diaminopurine dioxalane [DAPD], L-FMAU, lamivudine) and acyclic nucleoside phosphonate analogues (adefovir, tenofovir) that are in clinical use, in clinical trials or under preclinical development for the treatment of hepatitis B virus (HBV) infections, were evaluated for their inhibitory effect on the replication of a la- mivudine-resistant HBV variant containing the methionine --> valine substitution (M550V) in the polymerase nucleoside-binding domain. The antiviral activity was determined in the tetracycline-responsive HepAD38 and HepAD79 cells, which are stably transfected with either a cDNA copy of the wild-type pregenomic RNA or with cDNA containing the M550V mutation. As expected, lamivudine was much less ( approximately 200-fold) effective at inhibiting replication of the M550V mutant virus than the wild-type virus. In contrast, adefovir, tenofovir, lobucavir, L-FMAU, DXG and DAPD proved almost equally effective against both viruses. A second objective of this study was to directly compare the antiviral potency of the anti-HBV agents in HepG2 2.2.15 cells (which are routinely used for anti-HBV drug-screening purposes) with that in HepAD38 cells. HepAD38 cells produce much larger quantities of HBV than HepG2 2.2.15 cells, and thus allow drug screening in a multiwell plate format. All compounds were found to be almost equally effective at inhibiting HBV replication in HepAD38 cells (as in HepG2 2.2.15 cells), except for penciclovir, which was clearly less effective in HepAD38 cells.

Characterization of the antiviral effect of 2',3'-dideoxy-2', 3'-didehydro-beta-L-5-fluorocytidine in the duck hepatitis B virus infection model

A novel L-nucleoside analog of deoxycytidine, 2',3'-dideoxy-2', 3'-didehydro-beta-L-5-fluorocytidine (beta-L-Fd4C), was recently shown to strongly inhibit hepatitis B virus (HBV) replication in the 2.2.15 cell line. Therefore, its antiviral activity was evaluated in the duck HBV (DHBV) infection model. Using a cell-free system for the expression of the DHBV polymerase, beta-L-Fd4C-TP exhibited a concentration-dependent inhibition of dCTP incorporation into viral minus-strand DNA with a 50% inhibitory concentration of 0.2 microM which was lower than that of other tested deoxycytidine analogs, i.e. , lamivudine-TP, ddC-TP, and beta-L-FddC-TP. Further analysis showed that beta-L-Fd4C-TP is likely to be a competitive inhibitor of dCTP incorporation and to cause premature DNA chain termination. In primary duck hepatocyte cultures infected in vitro, beta-L-Fd4C administration exhibited a long-lasting inhibitory effect on viral DNA synthesis but could not clear viral covalently closed circular DNA (CCC DNA). Results of short-term antiviral treatment in experimentally infected ducklings showed that beta-L-Fd4C exhibited the most potent antiviral effect, followed by beta-L-FddC, lamivudine, and ddC. Longer administration of beta-L-Fd4C induced a sustained suppression of viremia (>95% of controls) and of viral DNA synthesis within the liver. However, the persistence of trace amounts of viral CCC DNA detected only by PCR was associated with a recurrence of viral replication after drug withdrawal. In parallel, beta-L-Fd4C treatment suppressed viral antigen expression within the liver and decreased intrahepatic inflammation and was not associated with any sign of toxicity. Our data, therefore, demonstrate that in the duck model of HBV infection, beta-L-Fd4C is a potent inhibitor of DHBV reverse transcriptase activity in vitro and suppresses viral replication in the liver in vivo.

Therapy of chronic hepatitis B virus infection: inhibition of the viral polymerase and other antiviral strategies

Chronic hepatitis B infection remains a major public health problem worldwide. The hepatitis B virus belongs to the family of hepadnaviruses that replicate their DNA genome via a reverse transcription pathway. The chronicity of infection in infected hepatocytes is maintained by the persistence of the viral covalently closed circular DNA. The main strategies to combat chronic HBV infection rely on the stimulation of the specific antiviral immune response and on the inhibition of viral replication. While the prolonged administration of reverse transcriptase inhibitors is most often associated with a control of viral replication rather than eradication, it may select for resistant mutants. The search for new viral targets is therefore mandatory to design combination strategies to prevent the emergence of resistant mutants and eventually clear viral infection.

Perspectives for the treatment of hepatitis B virus infections

Primarily resulting as a spin-off of the search for effective anti-HSV or anti-HIV agents, several compounds have been identified as effective and promising candidate anti-HBV drugs, i.e. famciclovir (penciclovir), BMS-200475, lamivudine (3TC), (-)FTC, L(-)Fd4C, L-FMAU, DAPD (DXG), bis(POM)-PMEA and bis(POC)-PMPA. They all inhibit HBV replication in Hep G2 2.2.15 at concentrations that are well below the cytotoxicity threshold. All these nucleoside analogues require three phosphorylation steps to be active, in their triphosphate form, as inhibitors of the HBV DNA polymerase, except for PMEA (adefovir) and PMPA (tenofovir), which need only two phosphorylation steps, to PMEApp and PMPApp, respectively, to interact as chain terminators with the HBV DNA polymerase reaction. Several of these compounds (for example, famciclovir, lamivudine and adefovir) have proven to be efficacious in the duck and/or woodchuck hepatitis models, and, accordingly, famciclovir, lamivudine and adefovir have also proven to be effective (i.e. in reducing HBV DNA levels) in patients with chronic HBV infection. Yet, famciclovir and lamivudine may lead to the emergence of resistance mutations (i.e. L528M and M552V/I) in the HBV DNA polymerase upon long-term treatment. These penciclovir- and lamivudine-resistant HBV mutants still retain susceptibility to adefovir, which, in turn, has so far not been found to engender resistance mutations in HBV. As has become obvious from the experience with the treatment of HIV infections, future HBV chemotherapy may reside in combination drug therapy so as to achieve the highest possible virus reduction, thereby minimizing the likelihood of drug resistance development.

Protective and therapeutic effect of DNA-based immunization against hepadnavirus large envelope protein

BACKGROUND & AIMS

Studies in the murine model suggest that injection of DNA encoding hepatitis B virus structural proteins is promising for the induction of a specific immune response. We used the duck hepatitis B virus (DHBV) model to study the protective and therapeutic effects of naked DNA immunization against hepadnaviral large envelope protein.

METHODS

A pCI-preS/S plasmid expressing the DHBV large protein was used for intramuscular immunization of ducks. The humoral response was tested by enzyme-linked immunosorbent assay, immunoblotting, neutralization, and in vivo protection tests. For DNA therapy, DHBV-carrier ducks received four injections of this plasmid. Viremia was monitored for 10 months; thereafter, liver biopsies were performed.

RESULTS

Immunization with pCI-preS/S plasmid induced a specific, long-lasting, neutralizing, and highly protective anti-preS humoral response in uninfected animals. After pCI-preS/S treatment, a significant and sustained decrease in serum and liver DHBV DNA was observed for carrier ducks compared with the controls.

CONCLUSIONS

DNA immunization against DHBV large protein results in a potent and protective anti-preS response in the duck model. The results of long-term follow-up of DNA-treated chronically infected ducks are promising and show the usefulness of this model for the study of genetic immunization in chronic hepatitis B therapy.

Lamivudine therapy of WHV-infected woodchucks

Hepatitis B viruses establish a chronic, productive, and noncytopathic infection of hepatocytes. Viral products are produced by transcription from multiple copies (5-50) of covalently closed circular (ccc) viral DNA. This cccDNA does not replicate, but can be replaced by DNA precursors that are synthesized in the cytoplasm. The present study was carried out to determine if long-term treatment with an inhibitor of viral DNA synthesis would lead to loss of virus products, including cccDNA, from the liver of woodchucks chronically infected with woodchuck hepatitis virus. Viral DNA synthesis was inhibited with the nucleoside analog, lamivudine (2'-deoxy-3'-thiacytidine). Lamivudine treatment produced a slow but progressive decline in viral titers in serum, to about 0.3% or less of the initial level. However, even after maintenance of drug therapy for 3-12 months, > 95% of the hepatocytes in most animals were still infected. Significant declines in the percentage of infected hepatocytes and of intrahepatic cccDNA levels were observed in only three woodchucks, two in the group receiving lamivudine and one in the placebo control group. Moreover, virus titers eventually rose in woodchucks receiving lamivudine, suggesting that drug-resistant viruses began to spread through the liver starting at least as early as 9-12 months of treatment. Three types of mutation that may be associated with drug resistance were found at this time, in a region upstream of the YMDD motif in the active site of the viral reverse transcriptase. The YMDD motif itself remained unchanged. Not unexpectedly, the lamivudine therapy did not have a impact on development of liver cancer.