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

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.

Developments in Cell-Penetrating Peptides as Antiviral Agents and as Vehicles for Delivery of Peptide Nucleic Acid Targeting Hepadnaviral Replication Pathway

Alternative therapeutic approaches against chronic hepatitis B virus (HBV) infection need to be urgently developed because current therapies are only virostatic. In this context, cell penetration peptides (CPPs) and their Peptide Nucleic Acids (PNAs) cargoes appear as a promising novel class of biologically active compounds. In this review we summarize different in vitro and in vivo studies, exploring the potential of CPPs as vehicles for intracellular delivery of PNAs targeting hepadnaviral replication. Thus, studies conducted in the duck HBV (DHBV) infection model showed that conjugation of (D-Arg)₈ CPP to PNA targeting viral epsilon (ε) were able to efficiently inhibit viral replication in vivo following intravenous administration to ducklings. Unexpectedly, some CPPs, (D-Arg)₈ and Decanoyl-(D-Arg)₈, alone displayed potent antiviral effect, altering late stages of DHBV and HBV morphogenesis. Such antiviral effects of CPPs may affect the sequence-specificity of CPP-PNA conjugates. By contrast, PNA conjugated to (D-Lys)₄ inhibited hepadnaviral replication without compromising sequence specificity. Interestingly, Lactose-modified CPP mediated the delivery of anti-HBV PNA to human hepatoma cells HepaRG, thus improving its antiviral activity. In light of these promising data, we believe that future studies will open new perspectives for translation of CPPs and CPP-PNA based technology to therapy of chronic hepatitis B.

Role of Cell-Penetrating Peptides in Intracellular Delivery of Peptide Nucleic Acids Targeting Hepadnaviral Replication

Peptide nucleic acids (PNAs) are potentially attractive antisense agents against hepatitis B virus (HBV), although poor cellular uptake limits their therapeutic application. In the duck HBV (DHBV) model, we evaluated different cell-penetrating peptides (CPPs) for delivery to hepatocytes of a PNA-targeting hepadnaviral encapsidation signal (ε). This anti-ε PNA exhibited sequence-specific inhibition of DHBV RT in a cell-free system. Investigation of the best in vivo route of delivery of PNA conjugated to (D-Arg)₈ (P1) showed that intraperitoneal injection to ducklings was ineffective, whereas intravenously (i.v.) injected fluorescein-P1-PNA reached the hepatocytes. Treatment of virus carriers with i.v.-administered P1-PNA resulted in a decrease in viral DNA compared to untreated controls. Surprisingly, a similar inhibition of viral replication was observed in vivo as well as in vitro in primary hepatocyte cultures for a control 2 nt mismatched PNA conjugated to P1. By contrast, the same PNA coupled to (D-Lys)₄ (P2) inhibited DHBV replication in a sequence-specific manner. Interestingly, only P1, but not P2, displayed anti-DHBV activity in the absence of PNA cargo. Hence, we provide new evidence that CPP-PNA conjugates inhibit DHBV replication following low-dose administration. Importantly, our results demonstrate the key role of CPPs used as vehicles in antiviral specificity of CPP-PNA conjugates.

Early Viral Kinetics with Telbivudine, Tenofovir or Combination of Both in Immunotolerant Patients with Hepatitis B e Antigen-Positive Chronic Hepatitis B

Introduction

Viral kinetics has proved useful in understanding antiviral potency, determining antiviral profiles and optimizing treatment strategy.

Methods

This was a randomized, open-label study comparing the viral kinetics in 46 hepatitis B e antigen-positive patients during 12-week treatment with telbivudine monotherapy, tenofovir monotherapy or the combination of telbivudine plus tenofovir. A standard biphasic mathematical model was used to compare hepatitis B virus (HBV) DNA decay parameters.

Results

Forty-six patients received telbivudine (n = 16), tenofovir (n = 14) or telbivudine plus tenofovir (n = 16). From baseline to Week 12, the mean (SD) reduction in HBV DNA levels was not significantly different between treatment groups: −3.9 (0.9) log₁₀ copies/mL in telbivudine group, −4.2 (0.7) log₁₀ copies/mL in tenofovir group, and −4.4 (1.0) log₁₀ copies/mL in combination group. No significant difference was observed among the three groups for viral clearance rate per day (0.97, 1.02, and 0.88, respectively) or for infected cell loss rate per day (0.04, 0.05, and 0.05, respectively). Antiviral efficiency in blocking viral production was similar in the monotherapy groups (median; 99.7% in telbivudine group and 99.4% in tenofovir group), but was slightly better and more homogeneous in the combination treatment group than in the monotherapy groups: mean (SD), 99.1% (0.8%) and 98.8% (1.6%), respectively (Wald–Wolfowitz test; P = 0.038). All treatments were well tolerated and no serious adverse event was reported during the study. Of the 46 patients in the safety population, 23 experienced adverse events. Most of the adverse events were not suspected to be related to the study drug by the investigators.

Conclusion

Monotherapy with telbivudine or tenofovir showed similar antiviral effectiveness in HBV DNA reduction and viral kinetics of HBV DNA decay. Efficiency in blocking viral production was slightly improved in the combination treatment group compared to the monotherapy groups.

Electronic supplementary material

The online version of this article (doi:10.1007/s40121-014-0039-5) contains supplementary material, which is available to authorized users.

Hepatitis B virus DNA quantification with the three-in-one (3io) method allows accurate single-step differentiation of total HBV DNA and cccDNA in biopsy-size liver samples

BACKGROUND

Hepatitis B virus (HBV) replicates via reverse transcription converting its partially double stranded genome into the covalently closed circular DNA (cccDNA). The long-lasting cccDNA serves as a replication intermediate in the nuclei of hepatocytes. It is an excellent, though evasive, parameter for monitoring the course of liver disease and treatment efficiency.

OBJECTIVE

To develop and test a new approach for HBV DNA quantification in serum and small-size liver samples.

STUDY DESIGN

The p3io plasmid contains an HBV fragment and human β-actin gene (hACTB) as a standard. Respective TaqMan probes were labeled with different fluorescent dyes. A triplex real-time PCR for simultaneous quantification of total HBV DNA, cccDNA and hACTB could be established.

RESULTS

Three-in-one method allows simultaneous analysis of 3 targets with a lower limit of quantification of 48 copies per 20 μl PCR reaction and a wide range of linearity (R(2)>0.99, p<0.0001) for all measured sequences. The method showed a pan-genotypic specificity among genotypes A-F with serum DNA samples from HBV infected patients. Total HBV DNA and cccDNA could be quantified in 32 and 22 of 33 FFPE preserved liver specimens, respectively. Total HBV DNA concentrations quantified by the 3io method remained comparable with Cobas TaqMan HBV Test v2.0.

CONCLUSIONS

The three-in-one protocol allows the single step quantification of viral DNA in samples from different sources. Therefore lower sample input, faster data acquisition, a lowered error and significantly lower costs are the advantages of the method.

Efficacy of adefovir-based combination therapy for patients with Lamivudine- and entecavir-resistant chronic hepatitis B virus infection. Antimicrob Agents Chemother. 2013;57:6325-6332. [PMID: 24100506 DOI: 10.1128/aac.01742-13aac.01742-13]

Treatment strategies for entecavir (ETV)-resistant chronic hepatitis B (CHB) patients are not yet well established. The aim of this study was to evaluate overall antiviral efficacy and to compare the efficacy of combination therapy with adefovir (ADV) plus nucleoside analogues (lamivudine [LAM], telbivudine [LdT], or ETV) in patients infected with LAM- and ETV-resistant hepatitis B virus (HBV) variants. Virologic, biochemical, and serologic responses during combination therapy with ADV plus nucleoside analogues were assessed. Propensity score analysis was used to select a matched group of patients for the comparison of rescue therapy regimens. A total of 67 consecutive patients were analyzed. Complete virologic suppression was achieved in 27 patients. The overall cumulative incidence of complete virologic suppression at month 24 was 47.4%: 44.3% in the LAM or LdT plus ADV group and 51.4% in the group given ETV and ADV. There was no significant difference between these two groups (P = 0.234). The cumulative incidences of complete virologic suppression were still comparable between the two groups selected and matched using the propensity score model (P = 0.419). Virologic breakthrough was observed in 9 patients, and rtA181V substitution was newly detected in one patient. Hepatitis B e antigen (HBeAg) negativity and lower baseline HBV DNA level were associated with complete virologic suppression in univariate analysis. In multivariate analysis, lower baseline HBV DNA level remained an independent predictor. In conclusion, combination therapy with ADV plus nucleoside analogues fails to show sufficient antiviral efficacy in CHB patients with resistance to both LAM and ETV. Further study is warranted to evaluate the efficacy of a more potent tenofovir-based regimen in such patients.

Efficacy of adefovir-based combination therapy for patients with Lamivudine- and entecavir-resistant chronic hepatitis B virus infection

Treatment strategies for entecavir (ETV)-resistant chronic hepatitis B (CHB) patients are not yet well established. The aim of this study was to evaluate overall antiviral efficacy and to compare the efficacy of combination therapy with adefovir (ADV) plus nucleoside analogues (lamivudine [LAM], telbivudine [LdT], or ETV) in patients infected with LAM- and ETV-resistant hepatitis B virus (HBV) variants. Virologic, biochemical, and serologic responses during combination therapy with ADV plus nucleoside analogues were assessed. Propensity score analysis was used to select a matched group of patients for the comparison of rescue therapy regimens. A total of 67 consecutive patients were analyzed. Complete virologic suppression was achieved in 27 patients. The overall cumulative incidence of complete virologic suppression at month 24 was 47.4%: 44.3% in the LAM or LdT plus ADV group and 51.4% in the group given ETV and ADV. There was no significant difference between these two groups (P = 0.234). The cumulative incidences of complete virologic suppression were still comparable between the two groups selected and matched using the propensity score model (P = 0.419). Virologic breakthrough was observed in 9 patients, and rtA181V substitution was newly detected in one patient. Hepatitis B e antigen (HBeAg) negativity and lower baseline HBV DNA level were associated with complete virologic suppression in univariate analysis. In multivariate analysis, lower baseline HBV DNA level remained an independent predictor. In conclusion, combination therapy with ADV plus nucleoside analogues fails to show sufficient antiviral efficacy in CHB patients with resistance to both LAM and ETV. Further study is warranted to evaluate the efficacy of a more potent tenofovir-based regimen in such patients.

Experimental models and therapeutic approaches for HBV

Liver disease associated to persistent infection with the hepatitis B virus (HBV) continues to be a major health problem of global impact. In spite of the existence of an effective vaccine, approximately 360 million people are chronically infected worldwide, who are at high risk of developing liver cirrhosis and hepatocellular carcinoma. Although current therapeutic regimens can efficiently suppress viral replication, the unique replication strategies employed by HBV permit the virus to persist within the infected hepatocytes. As a consequence, relapse of viral activity is commonly observed after cessation of treatment with polymerase inhibitors. The narrow host range of HBV has hindered progresses in understanding specific steps of HBV replication and the development of more effective therapeutic strategies aiming at achieving sustained viral control and, eventually, virus eradication. This review will focus on summarizing recent advances obtained with well-established and more innovative experimental models, giving emphasis on the strength of the different systems as tools for elucidating distinct aspects of HBV persistence and for the development of new therapeutic approaches.

Evaluation of the in vitro anti-HBV activity of clevudine in combination with other nucleoside/nucleotide inhibitors

BACKGROUND

To reduce the incidence of drug resistance and to maintain viral suppression, patients chronically infected with HBV might require combination therapy using two or more drugs with different resistance profiles. We investigated the activity of clevudine (CLV) in combination with other nucleoside/nucleotide analogues to determine if these combinations were compatible in vitro.

METHODS

Using the HepAD38 cell line, which expresses wild-type HBV, and a real-time PCR assay, we tested the anti-HBV activity of CLV in combination with entecavir, lamivudine, adefovir, tenofovir and telbivudine (TBV). We evaluated the uptake and phosphorylation of CLV in the presence of TBV, using HepAD38 cells and primary hepatocytes to determine the effect of TBV on the phosphorylation of CLV and vice versa. Phosphorylation of TBV and CLV to their corresponding monophosphate by deoxycytidine kinase, thymidine kinase-1 and thymidine kinase-2, and the phosphorylation of TBV monophosphate and CLV monophosphate by thymidylate kinase was evaluated and compared.

RESULTS

When CLV was combined with entecavir, lamivudine, adefovir or tenofovir, a synergistic antiviral effect was observed; however, the combination of CLV and TBV gave an antagonistic antiviral response. The results of in vitro metabolism and enzyme studies suggest that the antagonism observed with the CLV/TBV combination involves competition for uptake and phosphorylation.

CONCLUSIONS

The results of our studies demonstrate that combination treatments can provide enhanced antiviral activity and, when used in conjunction with appropriate metabolic investigations, provide a rational basis for the design and development of combination regimens for treating chronic HBV infection.

Effect of newer oral antiviral agents on future therapy of chronic hepatitis B

Long-term therapy with oral nucleoside/nucleotide analogues (NAs) is a favoured approach to the treatment of patients with chronic hepatitis B (CHB); however, all oral agents currently approved for the treatment of such patients are associated with some risk for drug resistance. This can lead to a rebound in HBV levels and, eventually, progressive liver disease. Combination therapy is one strategy that has the potential for enhanced antiviral effects and diminished or delayed resistance. The disadvantages of combination therapy include increased cost, the potential for drug interactions and increased toxicity. Additional therapeutic efficacy from combination therapy has not been demonstrated in clinical trials of HBV, and this approach might be less relevant now that potent NAs with excellent drug resistance profiles are available. However, it might be possible to identify subsets of patients (for example, those with extremely high viraemia or low baseline alanine aminotransferase levels) who derive added benefit from combination therapy. This review examines efficacy and resistance data for new low resistance oral NAs and clinical experience to date with de novo combination therapy in patients with CHB. The application of combination therapy in select populations of patients with CHB is also discussed.

Viral load in 1-day-old ducklings acutely infected with duck hepatitis B virus by different doses and routes of inoculation

In order to define clearly the conditions leading to the outcome of acute duck hepatitis B virus (DHBV) infection, 1-day-old Pekin ducklings were infected with DHBV by different routes and given different doses of inoculum. Groups of 24 ducklings were inoculated either intravenously via the vena cruralis, or intraperitoneally with pooled serum containing either 1.6 x 10(7) or 1.6 x 10(4) DHBV genomes. One control duck from each group was inoculated with an equal volume of normal duck serum. A sensitive and reproducible real-time polymerase chain reaction assay based on TaqMan technology was developed for the detection and quantitation of DHBV DNA in the serum and liver. DHBAg was observed in the hepatocytes by immunohistochemistry. Histological changes in the liver tissue were also observed. The results demonstrate that ducklings at each time point and in all groups developed detectable viraemia. In each group, DHBV DNA in the liver was at a lower level than in serum and the peak DNA titre was found in serum earlier than in the liver. In the low-dose groups it was always at a lower level than in the high-dose groups. The DHBV replication levels appeared to be directly related to the number of DHBAg-positive hepatocytes. The variation trends of DHBAg-positive hepatocytes were similar in the high-dose groups. Histological changes were associated with liver viral DNA levels. We suggest that this dose and route of inoculation can be used as a model to study acute DHBV infections.

Anti-hepatitis B virus activity in vitro of combinations of tenofovir with nucleoside/nucleotide analogues

BACKGROUND

Long-term management of some chronic hepatitis B patients might require combination therapy using drugs with distinct resistance profiles to sustain viral suppression and to reduce the resistance-associated failure. Tenofovir disoproxil fumarate (TDF), approved for hepatitis B virus (HBV) and HIV-1 treatment, is active against wildtype HBV and HBV containing YMDD mutations, which confer resistance to emtricitabine (FTC), lamivudine (3TC) and telbivudine (LdT) and contribute to entecavir (ETV) resistance. We therefore evaluated the in vitro anti-HBV activity of tenofovir (TFV), the active parent drug of TDF, combined with FTC, 3TC, ETV, LdT and adefovir (AFV).

METHODS

The anti-HBV activities of the compounds were tested using the AD38 cell line that expresses wild-type HBV from a tetracycline-controllable promoter. Intracellular HBV DNA levels were quantified using real-time PCR assay and cytotoxicities were assessed with XTT assays. The antiviral data of the drug combinations were evaluated using MacSynergy analyses on the basis of the Bliss independence model as well as isobologram analyses on the basis of the Loewe additivity theory.

RESULTS

All drug combinations tested, FTC+TFV, 3TC+TFV, ETV+TFV, LdT+TFV and AFV+TFV, showed additive antiviral interactions as analysed by MacSynergy. Isobologram analyses revealed that these combination pairs were additive, with the exception of FTC+TFV, which demonstrated slight synergistic activity. No cytotoxic or antagonistic effects were observed with any of the combinations tested.

CONCLUSIONS

The combination of TFV with FTC, 3TC, ETV, LdT or AFV had additive to slightly synergistic anti-HBV effects in vitro. These results support the use of TDF as a component in combination regimens with currently available anti-HBV nucleoside analogues.

Antiviral effects of lamivudine, emtricitabine, adefovir dipivoxil, and tenofovir disoproxil fumarate administered orally alone and in combination to woodchucks with chronic woodchuck hepatitis virus infection. Antimicrob Agents Chemother. 2008;52:3617-3632. [PMID: 18676881 DOI: 10.1128/aac.00654-08]

Adefovir dipivoxil (ADV) and tenofovir disoproxil fumarate (TDF) are nucleotide analogs that inhibit the replication of wild-type hepatitis B virus (HBV) and lamivudine (3TC)-resistant virus in HBV-infected patients, including those who are coinfected with human immunodeficiency virus. The combination of ADV or TDF with other nucleoside analogs is a proposed strategy for managing antiviral drug resistance during the treatment of chronic HBV infection. The antiviral effect of oral ADV or TDF, alone or in combination with 3TC or emtricitabine (FTC), against chronic woodchuck hepatitis virus (WHV) infection was evaluated in a placebo-controlled study in the woodchuck, an established and predictive model for antiviral therapy. Once-daily treatment for 48 weeks with ADV plus 3TC or TDF plus FTC significantly reduced serum WHV viremia levels from the pretreatment level by 6.2 log(10) and 6.1 log(10) genome equivalents/ml serum, respectively, followed by TDF plus 3TC (5.6 log(10) genome equivalents/ml), ADV alone (4.8 log(10) genome equivalents/ml), ADV plus FTC (one survivor) (4.4 log(10) genome equivalents/ml), TDF alone (2.9 log(10) genome equivalents/ml), 3TC alone (2.7 log(10) genome equivalents/ml), and FTC alone (2.0 log(10) genome equivalents/ml). Individual woodchucks across all treatment groups also demonstrated pronounced declines in serum WHV surface antigen, characteristically accompanied by declines in hepatic WHV replication and the hepatic expression of WHV antigens. Most woodchucks had prompt recrudescence of WHV replication after drug withdrawal, but individual woodchucks across treatment groups had sustained effects. No signs of toxicity were observed for any of the drugs or drug combinations administered. In conclusion, the oral administration of 3TC, FTC, ADV, and TDF alone and in combination was safe and effective in the woodchuck model of HBV infection.

Hepatitis B: reflections on the current approach to antiviral therapy

This article summarizes the current state of antiviral therapy of hepatitis B with special attention given to areas that remain controversial or poorly defined. Strict adherence to liver association practice guidelines may result in missed opportunities to treat patients with significant underlying liver disease. In particular, recommended ALT thresholds may not appropriately reflect disease activity or degree of fibrosis. There is growing evidence that an alternative treatment paradigm for preventing late-stage disease complications may be indicated in highly viremic patients with early life exposure to hepatitis B. Pegylated interferon therapy is often a better choice for young to middle-aged patients with genotype A and B because of the higher rate of HBeAg seroconversion and a greater chance for HBsAg seroconversion in both HBeAg-positive and -negative patients as compared to nucleoside analogs. Nucleoside analog monotherapy is the current standard of care for many patients. However, long-term monotherapy results in resistance to a variable degree and sequential monotherapy may result in multi-drug resistant virus. Which patients would specifically benefit from early combination therapy also remains poorly defined. The rapidity and robustness of the suppression of HBV DNA while on a nucleoside analog should be monitored relatively early during treatment because it affects treatment outcome and the rate of resistance. While great progress has been made in treating hepatitis B, many important issues require further study.

DNA vaccination in combination or not with lamivudine treatment breaks humoral immune tolerance and enhances cccDNA clearance in the duck model of chronic hepatitis B virus infection

This study used a duck hepatitis B virus (DHBV) model to evaluate whether a novel DNA vaccination protocol alone or associated with antiviral (lamivudine) treatment was able to clear the intrahepatic covalently closed, circular viral DNA (cccDNA) pool responsible for persistence of infection. DHBV carriers received DNA vaccine (on weeks 6, 10, 13, 14, 28 and 35) targeting the large envelope and/or core proteins alone or combined with lamivudine treatment (on weeks 1-8) or lamivudine monotherapy. After 10 months of follow-up, a dramatic decrease in viraemia and liver DHBV cccDNA (below 0.08 cccDNA copies per cell) was observed in 9/30 ducks (30 %) receiving DNA mono- or combination therapy, compared with 0/12 (0 %) from lamivudine monotherapy or the control groups, suggesting a significant antiviral effect of DNA immunization. However, association with the drug did not significantly improve DHBV DNA vaccine efficacy (33 % cccDNA clearance for the combination vs 27 % for DNA monotherapy), probably due to the low antiviral potency of lamivudine in the duck model. Seroconversion to anti-preS was observed in 6/9 (67 %) ducks showing cccDNA clearance, compared with 1/28 (3.6 %) without clearance, suggesting a significant correlation (P<0.001) between humoral response restoration and cccDNA elimination. Importantly, an early (weeks 10-12) drop in viraemia was observed in seroconverted animals, and virus replication did not rebound following the cessation of immunotherapy, indicating a sustained effect. This study provides the first evidence that therapeutic DNA vaccination is able to enhance hepadnaviral cccDNA clearance, which is tightly associated with a break in humoral immune tolerance. These results also highlight the importance of antiviral drug potency and an effective DNA immunization protocol for the design of therapeutic vaccines against chronic hepatitis B.

Chronic hepatitis B: preventing, detecting, and managing viral resistance

Licensed oral agents for antiviral therapy in patients with chronic hepatitis B virus (HBV) infection include lamivudine, adefovir, entecavir, and telbivudine. Emtricitabine, tenofovir, and the combination of tenofovir plus emtricitabine in 1 tablet, which are licensed for the treatment of human immunodeficiency virus infection, are additional off-label options for treating HBV infection. Preventing HBV antiviral drug resistance to nucleoside/nucleotide analogues and appropriate management when resistance occurs has become a major focus in the management of chronic hepatitis B. HBV antiviral drug resistance may be best prevented by using an agent or combination of agents with a high genetic barrier to resistance, and 2 potent nucleoside and nucleotide drugs with different resistance profiles may prove to be the optimal first-line treatment for chronic hepatitis B. Frequent assessment of quantitative serum HBV DNA remains the best approach to early detection of resistance, and antiviral therapy should be modified as soon as resistance is detected. Results from several clinical trials have shown that the addition or substitution of newer antiviral agents can restore suppression of viral replication, normalize alanine aminotransferase levels, and reverse histologic progression in patients with resistance to lamivudine, but little information exists regarding the long-term benefits of second-line treatment regimens. Despite the substantial advances in treatment made to date, new agents with novel viral targets will be needed for patients who ultimately may fail second- or third-line therapy.

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.

Antiviral effects of PNA in duck hepatitis B virus infection model

AIM

To study the efficacy of antiviral treatment with PNA for the duck model of HBV (DHBV)-infected ducks. PNA is a 2-amine-9-(2,3-dideoxy-2,3-dihydro-beta-D-arabinofuranosyl)-6-methoxy-9H-purine.

METHODS

The Sichuan Mallard ducklings in the hepatitis B virus model were treated with PNA, a new antiviral agent. DHBV DNA from the blood serum and liver tissues were measured at 0, 5, and 10 d during the treatment and at 3 d withdrawal by real-time PCR. The duck hepatitis B surface antigen (DHBsAg) in the liver cells was observed by Immunohistochemistry (IHC). Pathological changes in the liver tissues were also observed. Control group I was administered with distilled water and control group II was administered with 3-thiacytidine. Treatment group I was administered with PNA at a dose of 40 mg/kg and treatment group II was administered perorally (po) with PNA at a dose of 80 mg/kg. Treatment group III was administered with PNA at a dose of 20 mg/kg and treatment group IV was intravenously administered with PNA at a dose of 40 mg/kg. Each group contained 15 ducklings.

RESULTS

PNA can significantly lower the DHBV replication levels in serum and liver. Compared with control group II, there were no significant differences in inhibiting efficacy in treatment groups I and III (P>0.05) and there were significant differences in inhibiting efficacy in treatment groups II and IV (P<0.05). Interestingly, significant differences were observed at 3 d withdrawal. The DHBV replication levels in each group slightly increased at 3 d withdrawal, but rebounded slightly in the PNA treatment groups than in control group II (P<0.05). The DHBV replication levels in the treatment groups were lower than in control group I. The DHBV replication levels in sera had a positive relationship with that in the liver, but the DHBV replication levels in the liver was lower than that in sera. Pathological changes in the treatment groups were obviously improved and the changes were associated with liver viral DNA levels.

CONCLUSION

The results demonstrate that PNA is a strong inhibitor of DHBV replication in the DHBV-infected duck model.

Antiviral effect of orally administered (-)-beta-D-2-aminopurine dioxolane in woodchucks with chronic woodchuck hepatitis virus infection

(-)-beta-D-2-Aminopurine dioxolane (APD) is a nucleoside prodrug that is efficiently converted to 9-(beta-D-1,3-dioxolan-4-yl)guanine (DXG). DXG has antiviral activity in vitro against hepatitis B virus (HBV) but limited aqueous solubility, making it difficult to administer orally to HBV-infected individuals. APD is more water soluble than DXG and represents a promising prodrug for the delivery of DXG. A placebo-controlled, dose-ranging efficacy and pharmacokinetic study was conducted with woodchucks that were chronically infected with woodchuck hepatitis virus (WHV). APD was efficiently converted to DXG after oral and intravenous administrations of APD, with serum concentrations of DXG being higher following oral administration than following intravenous administration, suggestive of a considerable first-pass intestinal and/or hepatic metabolism. APD administered orally at 1, 3, 10, and 30 mg/kg of body weight per day for 4 weeks produced a dose-dependent antiviral response. Doses of 3 and 10 mg/kg/day reduced serum WHV viremia by 0.4 and 0.7 log(10) copies/ml, respectively. The 30-mg/kg/day dose resulted in a more pronounced, statistically significant decline in serum WHV viremia of 1.9 log(10) copies/ml and was associated with a 1.5-fold reduction in hepatic WHV DNA. Individual woodchucks within the highest APD dose group that had declines in serum WHV surface antigen levels, WHV viremia, and hepatic WHV DNA also had reductions in hepatic WHV RNA. There was a prompt recrudescence of WHV viremia following drug withdrawal. Therefore, oral administration of APD for 4 weeks was safe in the woodchuck model of chronic HBV infection, and the effect on serum WHV viremia was dose dependent.

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.

Rise in gamma interferon expression during resolution of duck hepatitis B virus infection

Gamma interferon (IFN-γ) expression plays a crucial role in the control of mammalian hepatitis B virus (HBV) infection. However, the role of duck INF-γ (DuIFN-γ) in the outcome of duck HBV (DHBV) infection, a reference model for hepadnavirus replication studies, has not yet been investigated. This work explored the dynamics of DuIFN-γ expression in liver and peripheral blood mononuclear cells (PBMCs) during resolution of DHBV infection in adolescent ducks in relation to serum and liver markers of virus replication, histological changes and humoral response induction. DHBV infection of 3-week-old ducks resulted in transient expression of intrahepatic preS protein (days 3–14) and mild histological changes. Low-level viraemia was detected only during the first 10 days of infection and was accompanied by early anti-preS antibody response induction. Importantly, a strong increase in intrahepatic DuIFN-γ RNA was detected by real-time RT-PCR at days 6–14, which coincided with a sharp decrease in both viral DNA and preS protein in the liver. Interestingly, liver DuIFN-γ expression remained augmented to the end of the follow-up period (day 66) and correlated with portal lymphocyte infiltration and persistence of trace quantities of intrahepatic DHBV DNA in animals that had apparently completely resolved the infection. Moreover, in infected ducks, a moderate increase was detected in the levels of DuIFN-γ in PBMCs (days 12–14), which coincided with the peak in liver DuIFN-γ RNA levels. These data reveal that increased DuIFN-γ expression in liver and PBMCs is concomitant with viral clearance, characterizing the resolution of infection, and provide new insights into the host–virus interactions that control DHBV infection.

Pharmacokinetics of telbivudine in healthy subjects and absence of drug interaction with lamivudine or adefovir dipivoxil

Two phase I studies were conducted to assess the plasma pharmacokinetics of telbivudine and potential drug-drug interactions between telbivudine (200 or 600 mg/day) and lamivudine (100 mg/day) or adefovir dipivoxil (10 mg/day) in healthy subjects. Study drugs were administered orally. The pharmacokinetics of telbivudine were characterized by rapid absorption with biphasic disposition. The maximum concentrations in plasma (Cmax) were reached at median times ranging from 2.5 to 3.0 h after dosing. Mean single-dose Cmax and area under the plasma concentration-time curve from time zero to infinity (AUC0-infinity) were 1.1 and 2.9 microg/ml and 7.4 and 21.8 microg . h/ml for the 200- and 600-mg telbivudine doses, respectively. Steady state was reached after daily dosing for 5 to 7 days. The mean steady-state Cmax and area under the plasma concentration-time curve over the dosing interval (AUCtau) were 1.2 and 3.4 microg/ml and 8.9 and 27.5 microg . h/ml for the 200- and 600-mg telbivudine repeat doses, respectively. The steady-state AUCtau of telbivudine was 23 to 57% higher than the single-dose values. Concomitant lamivudine or adefovir dipivoxil did not appear to significantly alter the steady-state plasma pharmacokinetics of telbivudine; the geometric mean ratios and associated 90% confidence interval (CI) for the AUCtau of telbivudine alone versus in combination were 106.3% (92.0 to 122.8%) and 98.6% (86.4 to 112.5%) when coadministered with lamivudine and adefovir dipivoxil, respectively. Similarly, the steady-state plasma pharmacokinetics of lamivudine or adefovir were not markedly affected by the coadministration of telbivudine; the geometric mean ratios and associated 90% CI, alone versus in combination with telbivudine, were 99.0% (87.1 to 112.4%) and 92.2% (84.0 to 101.1%), respectively, for the lamivudine and adefovir AUCtau values. Moreover, the combination regimens studied were well tolerated in all subjects. The results from these studies provide pharmacologic support for combination therapy or therapy switching involving telbivudine, lamivudine, and adefovir dipivoxil for the treatment of chronic hepatitis B virus infection.

Modeling infection with hepatitis B viruses in vivo

Hepatitis B virus (HBV) is a human-specific liver pathogen whose viral cycle and mechanisms of pathogenesis are not yet fully understood. Along with invaluable infection studies in chimpanzees, avian and mammalian HBV-related viruses continue to offer ample opportunities for studies in their natural hosts. Yet, none of these hosts are commonly used laboratory animals; the lack of reliable in vitro infection systems and convenient animal models has severely hampered progress in HBV research. The need to perform studies in HBV-permissive hepatocytes has led researchers to create new, challenging human–mouse chimera infection models. The types of animal models currently available to perform infection studies with HBV are presented and discussed in this review.

Hepatitis B virus drug resistance: mechanism and clinical implications for the prevention of treatment failure

Hepatitis B virus (HBV) infection remains a major public health problem worldwide. Recently, the research efforts to identify new inhibitors enabled the development of antiviral agents to treat patients chronically infected by HBV. In clinical practice, the use of nucleoside analogs, which inhibit viral polymerase activity, induces suppression of viral replication accompanied by an improvement in biochemical and histological conditions in most patients. However, many clinical studies revealed the emergence of drug-resistant mutants during extended treatment. This review focuses on the mechanism of HBV drug-resistant mutant selection and on the clinical implications of HBV drug resistance for the prevention of treatment failure.

Resistance of human hepatitis B virus to reverse transcriptase inhibitors: from genotypic to phenotypic testing

The treatment of HBV infected patients with analogues of nucleos(t)ides, including lamivudine and adefovir dipivoxil, has significantly increased the rate of anti-HBe seroconversion and therefore reduced the impact of chronic hepatitis B (CHB) on liver disease. Altogether, these antivirals have offered novel options for the treatment of patients who did not respond to previous therapy with interferon alpha, the only available treatment against CHB until 1998. However, therapies using analogues of nucleos(t)ides have been confronted with viral resistances which are often associated to with worsening of liver disease. Drug resistance is conferred by the appearance of one or several mutations within the HBV polymerase gene. These mutations confer to the mutant viral population a phenotypic advantage over the wild-type pretherapeutic viral quasispecies, as they induce a reduction of drug susceptibility of mutant strains in vivo. This reduction of drug susceptibility can be as well measured in vitro, i.e in cell culture, using phenotypic assays. The detection of these mutations has become of crucial importance to better adapt clinical option to the virological status of the patient. Genotypic and more recently phenotypic assays have been developed and both assays can be used for drug resistance testing. Genotypic assay gives information about already characterized mutations associated with viral resistance, while phenotypic testing measures the overall drug susceptibility of patient-derived viral strains in cell culture. These assays are described and their potential use in the clinical setting is discussed.

Clevudine for the treatment of chronic hepatitis B virus infection

Chronic hepatitis B virus (HBV) infection is a major health problem that is responsible for < or = 1 million deaths and 500,000 cases of hepatocellular carcinoma worldwide each year. Drugs that are currently approved by the FDA for the treatment of chronic HBV consist of two groups: the immunomodulators, such as conventional IFN-alpha and pegylated IFN-alpha2a; and nucleoside/nucleotide analogues, such as lamivudine, adefovir dipivoxil and entecavir. However, due to the limitations of these agents, newer agents with improved efficacy are currently being developed. One nucleoside/nucleotide analogue that is drawing a wide range of interest is clevudine, which is an analogue of the unnatural beta-L configuration. In the woodchuck hepatitis virus (WHV), clevudine 10 mg/kg has proven to be effective in suppressing viral replication with < or = 9 log10 decreases in WHV. At this dose, a significant reduction of intrahepatic WHV RNA and covalently closed circular WHV DNA levels can also be observed. Treatment with clevudine 10 mg/kg can confer additional antiviral benefit in the form of a more sustained reduction in WHV replication, serum woodchuck hepatitis surface antigen and intrahepatic woodchuck hepatitis core antigen expression following the withdrawal of clevudine. In humans, clevudine 10, 50, 100 or 200 mg/day for 28 days can reduce the median HBV DNA by -2.5, -2.7, -3 and -2.6 log10, respectively. More importantly, this suppression of antiviral activity is maintained at 12 and 24 weeks post treatment. Based on the early results of clevudine, more large-scale human studies with clevudine monotherapy or combination therapy is eagerly awaited.

Replication of hepatitis B virus in primary duck hepatocytes transfected with linear viral DNA

AIM: To explore the expression and replication of hepatitis B virus (HBV) DNA in primary duck hepatocytes (PDHs).

METHODS: Complete HBV genome was transfected into PDHs by electroporation (transfected group, 1.19×10¹² copies of linear HBV DNA/1×10⁷ PDHs). After 1-5 d of transfection, HBsAg and HBeAg in the supernatant and lysate of PDHs were measured with the IMX System. Meanwhile, replicative intermediates of HBV DNA were analyzed by Southern blotting and Dot blotting. PDHs electroporated were used as control group.

RESULTS: HBsAg in the hepatocyte lysates of transfected group was 15.24 (1 d), 14.55 (3 d) and 5.13 (5 d; P/N values, positive≥2.1) respectively. HBeAg was negative (<2.1). Both HBsAg and HBeAg were negative in the supernatant of transfected group. Dot blotting revealed that HBV DNA was strongly positive in the transfected group and negative in the control group. Southern blot analysis of intracellular total DNA indicated that there were relaxed circular (rc DNA), covalently closed circular (ccc DNA), and single-stranded (ss DNA) HBV DNA replicative intermediates in the transfected group, there was no integrated HBV DNA in the cellular genome. These parameters were negative in control group.

CONCLUSION: Expression and replication of HBV genes can occur in hepatocytes from non-mammalian species. HBV replication has no critical species-specificity, and yet hepatic-specific regulating factors in hepatocytes may be essential for viral replication.

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.

Antiviral therapy of chronic hepatitis B: can we clear the virus and prevent drug resistance?

Antiviral therapy of chronic HBV infection remains a clinical challenge. Once this infection has been-established, the viral genome persists for life, either as an integrated genome or as episomal covalently closed circular DNA (cccDNA). The latter is the source of renewed viral replication in case of immune depression or after antiviral drug withdrawal. The mechanisms of clearance of infected cells involve CD8+ cell-mediated cytolytic and non-cytolytic pathways. Antiviral therapy, using nucleoside analogues that inhibit the viral polymerase, induces a slow depletion of intrahepatic cccDNA. The persistence of low-grade viral replication under antiviral therapy may then lead to the selection of drug-resistant mutants. New assays have been developed to study the functional consequences of these polymerase mutations in terms of replication capacity and drug susceptibility. Together with the development of new HBV polymerase inhibitors and novel immunostimulatory approaches, this should lead to the design and evaluation of rational treatment combinations for a better control of viral replication and prevention of drug resistance.

Combination of nucleoside analogues in the treatment of chronic hepatitis B virus infection: lesson from experimental models

Owing to the persistence of hepatitis B virus (HBV) and the selection of drug-resistant mutants, a new concept of antiviral therapy for chronic hepatitis B relies on the combination of nucleoside analogues. In experimental models of HBV infection, several key points concerning these combinations were addressed. (i) Is it possible to achieve a synergic antiviral effect with polymerase inhibitors? (ii) Is it possible to impact on intracellular viral covalently closed circular DNA? (iii) What is the impact of the cross-resistance patterns of the different nucleoside analogues? (iv) What is the effect of viral load suppression on the restoration of specific antiviral cellular responses? The clinical impact of these key issues is discussed in the perspective of new clinical trials.

Combinations of adefovir with nucleoside analogs produce additive antiviral effects against hepatitis B virus in vitro

Combination therapies may be required for long-term management of some patients chronically infected with hepatitis B virus (HBV). Adefovir is a nucleotide analog that has similar activity against wild-type and lamivudine-resistant HBV. In contrast to lamivudine, clinical resistance to the prodrug adefovir dipivoxil emerges infrequently. Based on its clinical efficacy and low frequency of resistance, adefovir dipivoxil may form an important component of combination regimens. We therefore investigated the in vitro antiviral efficacy of combinations of adefovir with other nucleoside analogs (lamivudine, entecavir, emtricitabine [FTC],and telbivudine [L-dT]) and the nucleotide analog tenofovir. Using a novel stable cell line that expresses high levels of wild-type HBV, we assayed the antiviral activity of each drug alone and in combination with adefovir. All two-drug combinations resulted in greater antiviral effects than treatments with single agents and could be characterized as additive by the Bliss independence model. Analysis using the Loewe additivity model indicated that adefovir exerted additive antiviral effects when combined with lamivudine, FTC, or L-dT and moderately synergistic effects when combined with entecavir or tenofovir. There was no evidence of cytotoxicity with any of the drugs when used alone or in combination at the tested doses.

Establishment and assessment of two methods for quantitative detection of serum duck hepatitis B virus DNA

AIM: To establish and assess the methods for quantitative detection of serum duck hepatitis B virus (DHBV) DNA by quantitative membrane hybridization using DHBV DNA probe labeled directly with alkaline phosphatase and fluorescence quantitative PCR (qPCR).

METHODS: Probes of DHBV DNA labeled directly with alkaline phosphatase and chemiluminescent substrate CDP-star were used in this assay. DHBV DNA was detected by autoradiography, and then scanned by DNA dot-blot. In addition, three primers derived from DHBV DNA S gene were designed. Semi-nested primer was labeled by AmpliSensor. Standard curve of the positive standards of DHBV DNA was established after asymmetric preamplification, semi-nested amplification and on-line detection. Results from 100 samples detected separately by alkaline phosphatase direct-labeled DHBV DNA probe with dot-blot hybridization and digoxigenin-labeled DHBV DNA probe hybridization. Seventy samples of duck serum were tested by fluorescent qPCR and digoxigenin-labeled DHBV DNA probe in dot-blot hybridization assay and the correlation of results was analysed.

RESULTS: Sensitivity of alkaline phosphatase direct-labeled DHBV DNA probe was 10 pg. The coincidence was 100% compared with digoxigenin-labeled DHBV DNA probe assay. After 30 cycles, amplification products showed two bands of about 180 bp and 70 bp by 20 g/L agarose gel electrophoresis. Concentration of amplification products was in direct proportion to the initial concentration of positive standards. The detection index was in direct proportion to the quantity of amplification products accumulated in the current cycle. The initial concentration of positive standards was in inverse proportion to the number of cycles needed for enough quantities of amplification products. Correlation coefficient of the results was (0.97, P < 0.01) between fluorescent qPCR and dot-blot hybridization.

CONCLUSION: Alkaline phosphatase direct-labeled DHBV DNA probe in dot-blot hybridization and fluorescent qPCR can be used as valuable means to quantify DHBV DNA in serum.

Effect of a combination of clevudine and emtricitabine with adenovirus-mediated delivery of gamma interferon in the woodchuck model of hepatitis B virus infection

Our aim was to evaluate the antiviral effect of a combination of two nucleoside reverse transcriptase inhibitors, emtricitabine (FTC) and clevudine (L-FMAU), with the addition of an adenovirus-driven delivery of recombinant gamma interferon (IFN-gamma) in the woodchuck model of hepatitis B virus infection. Six woodchuck hepatitis virus (WHV)-infected woodchucks received L-FMAU (10 mg/kg) plus FTC (30 mg/kg) intraperitoneally for 8 weeks; six other animals received in addition an intravenous injection of a recombinant adenovirus vector expressing woodchuck IFN-gamma (Ad-IFN) at weeks 4 and 8. In the control group, two animals received Ad-IFN alone, two received adenovirus vector expressing the green fluorescent protein reporter gene, and one remained untreated. In less than 2 weeks, all woodchucks that received L-FMAU plus FTC showed a rapid and marked inhibition of viral replication, with a 4-log(10) drop in serum WHV DNA. In two animals, viremia remained suppressed for several months after the end of treatment. Similarly, a dramatic decrease in intrahepatic replicative intermediates of viral DNA was observed in the L-FMAU/FTC-treated groups. The additional administration of Ad-IFN led to increased inflammation in the liver but did not enhance the antiviral effect of the L-FMAU/FTC combination. In conclusion, therapies combining L-FMAU and FTC in WHV-infected woodchucks resulted in a potent and sustained antihepadnaviral effect both in the liver and in the blood circulation. However, no extra benefit of adding IFN-gamma gene transduction to the L-FMAU/FTC combination could be detected.

Adefovir dipivoxil: review of a novel acyclic nucleoside analogue

Adefovir dipivoxil (ADF) is a novel acyclic nucleoside analogue that has recently been approved for the treatment of chronic hepatitis B virus (HBV). Adefovir was initially assessed at higher doses for the treatment of human immunodeficiency virus (HIV) infection. However, in these studies, nephrotoxicity proved a dose-limiting side effect. Large randomised controlled studies have recently shown that ADF results in histological, virological and biochemical improvement in both hepatitis B e antigen (HBeAg)-positive and HBeAg-negative chronic HBV. While the rate of HBeAg seroconversion at 1 year (12%) was lower than both lamivudine and interferon, this increases with prolonged treatment. The clinical improvements occurred without serious side effects or the development of resistance at the dose of 10 mg daily, in treatment trials of up to 2 years, although resistance has now been observed. In addition, the drug is efficacious in HBV/HIV co-infection and hepatitis B-infected liver transplant recipients, particularly in those who have developed lamivudine resistance. ADF can be added as a treatment option to existing treatment options (interferon-alpha and lamivudine) and assumes a role in the ongoing management of chronic HBV. The optimal use of ADF as either a monotherapy or as part of combination therapy requires further assessment.

Antiviral therapy: nucleotide and nucleoside analogs

For the management of HBV infection, an increasing number of nucleotide and nucleoside analogs are active against wild-type HBV and some against HBV with YMDD and other compensatory mutations. Table 2 depicts the IC50 and susceptibilities of HBV to various antiviral agents. The dichotomy between in vitro and in vivo susceptibilities to YMDD mutants is due to a change in IC50 between wild-type and mutant virus. Thus a drug may have less activity in vitro but at doses used in vivo show activity against YMDD and other compensatory mutations. Some HBV drugs share activity against HIV, which may be useful in the co-infected patient. Other nucleoside analogs are in various stages of development, including MCC-478 and DAPD. In the future, clinicians will have a plethora of reagents to chose from, and combination therapies may be invoked.

Selection of a hepatitis B virus strain resistant to adefovir in a liver transplantation patient

BACKGROUND/AIMS

In contrast to lamivudine, adefovir dipivoxil (ADV) therapy is associated with delayed and infrequent selection of drug resistant hepatitis B virus (HBV).

METHODS

A 52 year-old man was treated with lamivudine for an HBV recurrence on his liver graft. A viral breakthrough was observed and the patient received ADV. Serum HBV DNA decreased rapidly and lamivudine was discontinued while ADV monotherapy was maintained. Serum HBV DNA levels remained suppressed until a second breakthrough was observed. Lamivudine was then reintroduced together with ADV, and serum HBV DNA became undetectable by polymerase chain reaction.

RESULTS

Sequence analyses of the HBV polymerase gene revealed a sequential selection of lamivudine resistance mutations L180M+M204V, followed by a reversion to wild-type, and subsequently the selection of a novel adefovir resistance mutation N236T. Phenotypic analyses in cell culture assays demonstrated that the HBV isolates at the time of ADV breakthrough had reduced susceptibility to ADV. This mutant remained sensitive to lamivudine, entecavir and emtricitabine in vitro.

CONCLUSIONS

We describe the first case of sequential selection of lamivudine and adefovir resistant strains of HBV in a liver transplantation patient. The selection of the N236T polymerase mutant was associated with resistance to ADV but remained sensitive to lamivudine in vitro and in vivo.

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.