Antiviral therapy of chronic hepatitis B

Hepcidin expression levels involve efficacy of pegylated interferon-α treatment in hepatitis B-infected liver

BACKGROUND

Hepcidin is the master iron regulator hormone produced by the liver. The association of serum hepcidin with pegylated interferon therapy in patients with chronic hepatitis C infection has been studied. However, the role of serum hepcidin level in predicting the effect of pegylated interferon treatment in patients with chronic hepatitis B (CHB) infection is yet to be elucidated. Our study aims to investigate the correlation between hepcidin expression levels and the curative effect of interferon-alpha therapy in patients with CHB.

METHODS

A total of 47 patients with CHB who accepted pegylated interferon-α (PEG-IFN- α) treatment were recruited. The serum level of hepcidin was estimated by ELISA. The alternation in the gene expression level of hepcidin was detected by RT-PCR, and immunofluorescence cell staining was performed to detect hepcidin peptide. The induction of antiviral proteins was analyzed by Western blotting. The predictive value of early on-treatment variation in serum hepcidin during treatment progress was assessed by receiver operating characteristic analysis.

RESULTS

High levels of early on-treatment serum hepcidin were observed in patients who achieved a decline in HBsAg > 1 log10 IU/mL or HBV DNA > 1 log10 IU/mL. In vitro, an elevation of the hepcidin expression in HepG2.2.15 cells induced by PEG-IFN-α treatment was noted. Furthermore, combined treatment with hepcidin and PEG-IFN-α increased the levels of antiviral proteins. The predictive cut-off value of hepcidin for HBsAg decline > 1 log10 IU/mL was 239 pg/mL, and the sensitivity and specificity were 72.73% and 70.97%, respectively. The predictive cut-off value of hepcidin for the decline in HBV DNA > 1 log10 IU/mL was 190.4 pg/mL, and the sensitivity and specificity were 72.73% and 61.11%, respectively. Early-on treatment changes in the hepcidin level signified the predictive value of the PEG-IFN-α curative effect.

CONCLUSIONS

A higher early-on treatment hepcidin level indicates a higher possibility of HBsAg and HBV DNA decline in patients with CHB during PEG-IFN-α treatment. A high early-on treatment serum hepcidin level is significant in predicting the PEG-IFN-α therapeutic effect in patients with CHB.

Pharmacokinetics studies of 4'-cyano-2'-deoxyguanosine, a potent inhibitor of the hepatitis B virus, in rats

OBJECTIVES

4'-cyano-2'-deoxyguanosine (CdG), a novel nucleoside analogue, has a high degree of antiviral activity against the chronic hepatitis B virus (HBV). The objective of this study was to develop an analytical method for quantitatively determining CdG levels in biological samples by liquid chromatography-mass spectrometry (LC/MS) and to investigate the pharmacokinetic properties of CdG in rats after intravenous and oral administration.

METHODS

An analytical method using a UPLC system interfaced with a TOF-MS system was developed and validated. The pharmacokinetic properties after the intravenous and oral administration of CdG to rats were evaluated. In vivo pharmacokinetic interactions between CdG and entecavir were also investigated.

KEY FINDINGS

A rapid, simple and selective method for the quantification of CdG in biological samples was established using LC/MS with solid-phase extraction. In vivo pharmacokinetic studies of CdG in rats demonstrated that CdG is highly bioavailable, is rapidly absorbed from the intestinal tract, is then distributed to the liver rather than kidney and is ultimately excreted via the urine in an unchanged form. The co-administration of CdG and entecavir led to pharmacokinetic interactions with each other.

CONCLUSIONS

The data generated in this study provide support for the clinical development of CdG for use in the treatment of HBV.

Development of a novel hepatitis B virus encapsidation detection assay by viral nucleocapsid-captured quantitative RT-PCR

After encapsidation, where pregenomic RNA (pgRNA) is packaged into viral nucleocapsids, hepatitis B virus (HBV) uses the pgRNA as a template to replicate its DNA genome by reverse transcription. To date, there are only two encapsidation detection methods for evaluating the amount of pgRNA packaged into nucleocapsids: (i) the RNase protection assay and (ii) the native agarose gel electrophoresis assay. However, these methods are complex and laborious because they require multiple pgRNA purification steps followed by detection via an isotope-labeled probe. Moreover, both assays are unsuitable for evaluating a large number of antiviral agents in a dose-dependent manner. To overcome these limitations, we devised a novel HBV encapsidation assay in a 96-well plate format using nucleocapsid capture plates coated with an anti-HBV core (HBc) antibody, usually employed in enzyme-linked immunosorbent assays, to immobilize viral nucleocapsids. Viral pgRNA is then detected by quantitative RT-PCR (RT-qPCR). This strategy allows fast, convenient, and quantitative analysis of multiple viral RNA samples to evaluate encapsidation inhibitors. Furthermore, our protocol is potentially suitable for high-throughput screening (HTS) of compounds targeting HBV pgRNA encapsidation.

The serum IL-23 level predicts the response to pegylated interferon therapy in patients with chronic hepatitis B

BACKGROUND & AIMS

This study investigated the possible use of interleukin (IL)-23 and IL-17 serum levels as indicators for anti-hepatitis B virus (HBV) therapy.

METHODS

A total of 127 patients with chronic hepatitis B (CHB) who received pegylated interferon (PegIFN) therapy, 20 chronic asymptomatic HBV carriers (AsCs) and 32 healthy controls were recruited. The serum levels of IL-23 and IL-17 were detected by ELISA. The predictive value of baseline and early on-treatment changes in the levels of IL-23 and IL-17 for therapeutic response were evaluated by receiver operating characteristic analysis. Multivariate logistic regression models were generated to identify independent factors that affect the clearance of hepatitis B e antigen (HBeAg) and the decline in hepatitis B surface antigen (HBsAg).

RESULTS

The baseline serum levels of IL-23 and IL-17 were higher in patients with CHB than in normal controls and in AsCs. High levels of pre-treatment IL-23 and IL-17 and more significant on-treatment reductions in IL-23 and IL-17 levels were observed in patients with CHB who achieved HBeAg clearance or a decline in HBsAg >1 log10  IU/ml compared with patients who were persistently HBeAg-positive or who experienced a decline in HBsAg <1 log10  IU/ml. The predictive cut-off value of IL-23 for HBeAg clearance was 135 pg/ml, and the specificity and sensitivity were 71.4% and 70% respectively. A high pre-treatment level of IL-23 was an independent factor for the prediction of the therapeutic response in patients with HBeAg-positive CHB. Early on-treatment changes of IL-23 and IL-17 showed no predictive value.

CONCLUSIONS

A high pre-treatment serum IL-23 level predicts the therapeutic response in HBeAg-positive CHB patients during PegIFN therapy.

Risk factors for hepatocellular carcinoma in patients with drug-resistant chronic hepatitis B

AIM: To investigate the risk factors and characteristics of hepatocellular carcinoma (HCC) in the patients with drug-resistant chronic hepatitis B (CHB).

METHODS: A total of 432 patients with drug-resistant CHB were analyzed retrospectively from January 2004 to December 2012. The patients were divided into two groups: the HCC group (n = 57) and the non-HCC group (n = 375). Two groups compared using logistic regression for various patients and viral characteristics in order to identify associated risk factors for HCC. Secondarily, patient and tumor characteristics of HCC patients with naïve CHB (N group, n = 117) were compared to the HCC group (R group, n = 57) to identify any difference in HCC characteristics between them.

RESULTS: A significant difference was found for age, platelet count, alpha-fetoprotein (AFP), positivity of HBeAg, seroconversion rate of HBeAg, virologic response, the Child-Pugh score, presence of rtM204I, and the duration of antiviral treatment in non-HCC and HCC group. Cirrhosis, age (> 50 years), HBeAg (+), virologic non-responder status, and rtM204I mutants were independent risk factors for the development of HCC. The R group had lower serum C-reactive protein (CRP) and AFP levels, earlier stage tumors, and a shorter mean tumor surveillance period than the N group. However, the total follow-up duration was not significantly different between the two groups.

CONCLUSION: 13.2% of patients with drug-resistant CHB developed HCC. Age, cirrhosis, YIDD status, HBeAg status, and virologic response are associated with risk of HCC. Patients with drug-resistant CHB and these clinical factors may benefit from closer HCC surveillance.

Substitution rtq267h of hepatitis B virus increases the weight of replication and Lamivudine resistance

BACKGROUND

Nucleus(t)ide analogs (NAs), containing Lamivudine (LMV), adefovir dipivoxil (ADV), endeavor (ETV), telbivudine (LdT), and tenofovir (TDF) are widely used for the treatment of chronic hepatitis B (CHB), but long term anti-Hepatitis B virus (HBV) therapy with NAs may give rise to the emergence of drug-resistant viral mutants.

OBJECTIVES

This study aimed to find and identify some new resistance mutations of HBV from the patients accepted anti-HBV therapy.

PATIENTS AND METHODS

The reverse transcriptase (RT) coding region of HBV was PCR-amplified using HBV DNA extracted from patients' blood samples and sequenced.

RESULTS

Nineteen substitution mutations were detected. Among them, rtQ267H was often observed in patients receiving LMV administration. This LMV therapy-related mutation was introduced into HBV replication-competent plasmids. The in vitro susceptibility of both wild-type (WT) and mutant-type (MT) HBV to NAs was analyzed by Southern blot, and/or quantitative real-time PCR (qRT-PCR). The rtQ267H substitution enhanced HBV replication not merely in single-site mutation, but also in multisite mutations. The in vitro susceptibility analysis showed that the existence of rtQ267H in WT and LMV-resistant (LMVr) HBV were responsible for the reduced susceptibility to LMV to varying degrees, and enhanced HBV replication capacity. However, HBV harbored this substitution retained normal susceptibility to ADV, LdT, ETV, and TDF.

CONCLUSIONS

The result suggested that rtQ267H is a potential adaptive mutation of HBV to LMV.

Ultrasensitive amplification refractory mutation system real-time PCR (ARMS RT-PCR) assay for detection of minority hepatitis B virus-resistant strains in the era of personalized medicine

Resistance to antiviral treatment for chronic hepatitis B virus (HBV) has been associated with mutations in the HBV polymerase region. This study aimed at developing an ultrasensitive method for quantifying viral populations with all major HBV resistance-associated mutations, combining the amplification refractory mutation system real-time PCR (ARMS RT-PCR) with a molecular beacon using a LightCycler. The discriminatory ability of this method, the ARMS RT-PCR with molecular beacon assay, was 0.01 to 0.25% for the different HBV resistance-associated mutations, as determined by laboratory-synthesized wild-type (WT) and mutant (Mut) target sequences. The assay showed 100% sensitivity for the detection of mutant variants A181V, T184A, and N236T in samples from 41 chronically HBV-infected patients under antiviral therapy who had developed resistance-associated mutations detected by direct PCR Sanger sequencing. The ratio of mutant to wild-type viral populations (the Mut/WT ratio) was >1% in 38 (63.3%) of 60 samples from chronically HBV-infected nucleos(t)ide analogue-naive patients; combinations of mutations were also detected in half of these samples. The ARMS RT-PCR with molecular beacon assay achieved high sensitivity and discriminatory ability compared to the gold standard of direct PCR Sanger sequencing in identifying resistant viral populations in chronically HBV-infected patients receiving antiviral therapy. Apart from the dominant clones, other quasispecies were also quantified. In samples from chronically HBV-infected nucleos(t)ide analogue-naive patients, the assay proved to be a useful tool in detecting minor variant populations before the initiation of the treatment. These observations need further evaluation with prospective studies before they can be implemented in daily practice.

Methyltransferase PRMT1 is a binding partner of HBx and a negative regulator of hepatitis B virus transcription

The hepatitis B virus X protein (HBx) is essential for virus replication and has been implicated in the development of liver cancer. HBx is recruited to viral and cellular promoters and activates transcription by interacting with transcription factors and coactivators. Here, we purified HBx-associated factors in nuclear extracts from HepG2 hepatoma cells and identified protein arginine methyltransferase 1 (PRMT1) as a novel HBx-interacting protein. We showed that PRMT1 overexpression reduced the transcription of hepatitis B virus (HBV), and this inhibition was dependent on the methyltransferase function of PRMT1. Conversely, depletion of PRMT1 correlated with increased HBV transcription. Using a quantitative chromatin immunoprecipitation assay, we found that PRMT1 is recruited to HBV DNA, suggesting a direct effect of PRMT1 on the regulation of HBV transcription. Finally, we showed that HBx expression inhibited PRMT1-mediated protein methylation. Downregulation of PRMT1 activity was further observed in HBV-replicating cells in an in vivo animal model. Altogether, our results support the notion that the binding of HBx to PRMT1 might benefit viral replication by relieving the inhibitory activity of PRMT1 on HBV transcription.

The hepatitis B virus ribonuclease H is sensitive to inhibitors of the human immunodeficiency virus ribonuclease H and integrase enzymes

Nucleos(t)ide analog therapy blocks DNA synthesis by the hepatitis B virus (HBV) reverse transcriptase and can control the infection, but treatment is life-long and has high costs and unpredictable long-term side effects. The profound suppression of HBV by the nucleos(t)ide analogs and their ability to cure some patients indicates that they can push HBV to the brink of extinction. Consequently, more patients could be cured by suppressing HBV replication further using a new drug in combination with the nucleos(t)ide analogs. The HBV ribonuclease H (RNAseH) is a logical drug target because it is the second of only two viral enzymes that are essential for viral replication, but it has not been exploited, primarily because it is very difficult to produce active enzyme. To address this difficulty, we expressed HBV genotype D and H RNAseHs in E. coli and enriched the enzymes by nickel-affinity chromatography. HBV RNAseH activity in the enriched lysates was characterized in preparation for drug screening. Twenty-one candidate HBV RNAseH inhibitors were identified using chemical structure-activity analyses based on inhibitors of the HIV RNAseH and integrase. Twelve anti-RNAseH and anti-integrase compounds inhibited the HBV RNAseH at 10 µM, the best compounds had low micromolar IC₅₀ values against the RNAseH, and one compound inhibited HBV replication in tissue culture at 10 µM. Recombinant HBV genotype D RNAseH was more sensitive to inhibition than genotype H. This study demonstrates that recombinant HBV RNAseH suitable for low-throughput antiviral drug screening has been produced. The high percentage of compounds developed against the HIV RNAseH and integrase that were active against the HBV RNAseH indicates that the extensive drug design efforts against these HIV enzymes can guide anti-HBV RNAseH drug discovery. Finally, differential inhibition of HBV genotype D and H RNAseHs indicates that viral genetic variability will be a factor during drug development.

Current therapy for HBV blocks DNA synthesis by the viral reverse transcriptase and can control the infection indefinitely, but treatment rarely cures patients. More patients could be cured by suppressing HBV replication further using a new drug in combination with the existing ones. The HBV RNAseH is a logical drug target because it is the second of only two viral enzymes that are essential for viral replication, but it has not been exploited, primarily because it is very difficult to produce active enzyme. We expressed active recombinant HBV RNAseHs and demonstrated that it was suitable for antiviral drug screening. Twenty-one candidate HBV RNAseH inhibitors were identified based on antagonists of the HIV RNAseH and integrase enzymes. Twelve of these compounds inhibited the HBV RNAseH in enzymatic assays, and one inhibited HBV replication in cell-based assays. The high percentage of compounds developed against the HIV RNAseH and integrase that were also active against the HBV RNAseH indicates that the extensive drug design efforts against these HIV enzymes can be used to guide anti-HBV RNAseH drug discovery.

Emergence of the rtA181T/sW172* mutant increased the risk of hepatoma occurrence in patients with lamivudine-resistant chronic hepatitis B

Background

Development of the hepatitis B virus (HBV) rtA181T/sW172* mutant could occur during prolonged lamivudine (LAM) therapy, conferring cross resistance to adefovir. Recent studies demonstrated an increased oncogenic potential of this mutant in NIH3T3 cells. In this study, we aimed to investigate the clinical significance of this finding.

Methods

Serum samples from 123 LAM-resistant chronic hepatitis B patients were submitted for virological assays. A highly sensitive amplification created restriction enzyme site (ACRES) method was devised to detect small amounts of the rtA181T mutant in the serum. Virological factors including HBV-DNA level, genotype, precore G1896A, BCP A1762T/G1764A, rtM204I/V, rtA181T and pre-S internal deletion mutations as well as clinical variables including subsequent use of rescue drugs were submitted for outcome analysis.

Results

By use of the highly sensitive ACRES method, the rtA181T mutant was detectable in 10 of the 123 LAM-resistant patients. During the mean follow-up period of 26.2 ± 16.4 months (range 2 to 108 months), 3 of the 10 (30.0%) rtA181T-positive patients and 2 of the 113 (1.8%) rtA181T-negative patients developed hepatocellular carcinoma (HCC). Kaplan-Meier analysis indicated that the presence of rtA181T mutation (P < 0.001), age > 50 years (P = 0.001), and liver cirrhosis (P < 0.001) were significantly associated with subsequent occurrence of HCC. All 5 HCC patients belonged to the older age and cirrhosis groups.

Conclusions

Emergence of the rtA181T/sW172* mutant in LAM-resistant patients increased the risk of HCC development in the subsequent courses of antiviral therapy.

Comparison of the 48-week efficacy between entecavir and adefovir in HBeAg-positive nucleos(t)ide-naïve Asian patients with chronic hepatitis B: a meta-analysis

Background

Although entacavir and adefovir were widely used in most Asian countries, there were few conclusions drawn from a meta-analysis for comparing the efficacy between entecavir and adefovir in nucleos(t)ide-naïve Asian patients with chronic hepatitis B (CHB). The aim of this study was to evaluate the 48-week efficacy between the two drugs in HBeAg-positive nucleos(t)ide-naïve Asian CHB patients with the method of Meta analysis, which was generally accepted by the international as the best evidence for evaluating the efficacy of drugs.

Methods

We searched all data documented in Pubmed, Embase, Wanfang Database and CNKI (China National Knowledge Infrastructure) before November 30, 2010. Heterogeneity was examined by Chi-square test, the relative risk calculated and forest plot drawn. Rates of undetected serum HBV DNA, serum alanine aminotransferase (ALT) normalization, HBeAg clearance and HBeAg seroconversion were analyzed. A total of 6 articles was included. Meta analysis showed that the rate of undetected serum HBV DNA (relative risk, 1.73; 95% confidence interval, 1.38-2.17; P < 0.00001) and that of serum ALT normalization (relative risk, 1.25; 95% confidence interval, 1.06-1.49; P = 0.009) in the entecavir group were higher than those in the adefovir group. However, no statistic significance existed between the two groups in the rate of HBeAg clearance (relative risk, 0.77; 95% confidence interval, 0.44-1.35; P = 0.36), or the rate of HBeAg seroconversion (relative risk, 0.74; 95% confidence interval, 0.28-1.94; P = 0.53).

Conclusions

Entecavir is superior to adefovir in decreasing serum HBV DNA and normalizing ALT but similar with adefovir in clearing HBeAg and encouraging HBeAg seroconversion for the HBeAg-positive nucleos(t)ide-naive Asian patients with chronic hepatitis B. Adefovir can be still used for first-line therapy in these patients.

Interactions between buprenorphine and antiretrovirals: nucleos(t)ide reverse transcriptase inhibitors (NRTI) didanosine, lamivudine, and tenofovir

To improve outcomes among injection drug users with HIV and/or chronic hepatitis B, it is important to identify drug interactions between antiretroviral and opiate therapies. We report the results of a study designed to examine the interaction between buprenorphine and the nucleos(t)ide reverse transcriptase inhibitors (NRTI) didanosine (ddI), lamivudine (3TC), and tenofovir (TDF). Opioid-dependent, buprenorphine/naloxone-maintained, HIV-negative volunteers (n = 27) participated in two 24-hour sessions to determine (1) pharmacokinetics of buprenorphine alone and (2) pharmacokinetics of both buprenorphine and either ddI, 3TC, or TDF. Among buprenorphine/naloxone-maintained study participants, no significant changes in buprenorphine pharmacokinetics were observed following ddI, 3TC, or TDF administration. Buprenorphine had no significant effect on NRTI concentrations. Concomitant use of buprenorphine with ddI, 3TC, or TDF results in neither a significant pharmacokinetic nor pharmacodynamic interaction.

Hepatitis B virus polymerase blocks pattern recognition receptor signaling via interaction with DDX3: implications for immune evasion

Viral infection leads to induction of pattern-recognition receptor signaling, which leads to interferon regulatory factor (IRF) activation and ultimately interferon (IFN) production. To establish infection, many viruses have strategies to evade the innate immunity. For the hepatitis B virus (HBV), which causes chronic infection in the liver, the evasion strategy remains uncertain. We now show that HBV polymerase (Pol) blocks IRF signaling, indicating that HBV Pol is the viral molecule that effectively counteracts host innate immune response. In particular, HBV Pol inhibits TANK-binding kinase 1 (TBK1)/IkappaB kinase-epsilon (IKKepsilon), the effector kinases of IRF signaling. Intriguingly, HBV Pol inhibits TBK1/IKKepsilon activity by disrupting the interaction between IKKepsilon and DDX3 DEAD box RNA helicase, which was recently shown to augment TBK1/IKKepsilon activity. This unexpected role of HBV Pol may explain how HBV evades innate immune response in the early phase of the infection. A therapeutic implication of this work is that a strategy to interfere with the HBV Pol-DDX3 interaction might lead to the resolution of life-long persistent infection.

Antiviral treatment of chronic hepatitis B virus (HBV) infections

While 25 compounds have been formally licensed for the treatment of HIV infection (AIDS), only seven licensed products are currently available for the treatment of chronic hepatitis B virus (HBV) infection: interferon-α, pegylated interferon-α, lamivudine, adefovir (dipivoxil), entecavir, telbivudine and tenofovir (disoproxil fumarate). In contrast to the treatment of HIV infections where the individual drugs are routinely used in combination, for the treatment of chronic HBV infection the individual drugs are generally used in monotherapy. In principle, combination drug therapy should allow reducing the likelihood of drug-resistant development.

Heat stress cognate 70 host protein as a potential drug target against drug resistance in hepatitis B virus

Heat stress cognate 70 (Hsc70) is a host protein associated with hepatitis B virus (HBV) replication. The goal of this study was to investigate whether Hsc70 could be an anti-HBV drug target. Our results showed that introducing Hsc70 increased HBV replication in HBV(+) human hepatocytes (HepG2.2.15 cells). The coiled-coil region on Hsc70 (nucleotides 1533 to 1608; amino acids 511 to 536) was the key sequence for HBV replication. Knockdown of Hsc70 expression by RNA interference (RNAi) largely inhibited HBV replication with no cytotoxicity to the host. Using an Hsc70 mRNA screening assay, the natural compound oxymatrine (OMTR) was found to be a selective inhibitor for Hsc70 expression. Then, OMTR was used to investigate the potential of Hsc70 as an anti-HBV drug target. OMTR inhibited Hsc70 mRNA expression by 80% and HBV DNA replication by over 60% without causing cytotoxicity. The anti-HBV effect of OMTR appeared to be mediated by destabilizing Hsc70 mRNA. The half-life (T(1/2)) of Hsc70 mRNA decreased by 50% in OMTR-treated hepatocytes. The Hsc70 mRNA 3'-untranslated-region (UTR) sequence was the element responsible for OMTR's destabilization activity. OMTR suppressed HBV de novo synthesis at the reverse transcription stage from pregenomic RNA (pgRNA) to DNA and was active against either wild-type HBV or strains resistant to lamivudine, adefovir, and entecavir. Therefore, host Hsc70 could be a novel drug target against HBV, and OMTR appears to inhibit HBV replication by destabilizing Hsc70 mRNA. As the target is not a viral protein, OMTR is active for either wild-type HBV or strains resistant to reverse transcriptase (RT) inhibitors.

The HepaRG cell line: biological properties and relevance as a tool for cell biology, drug metabolism, and virology studies

Liver progenitor cells may play an important role in carcinogenesis in vivo and represent therefore useful cellular materials for in vitro studies. The HepaRG cell line, which is a human bipotent progenitor cell line capable to differentiate toward two different cell phenotypes (i.e., biliary-like and hepatocyte-like cells), has been established from a liver tumor associated with chronic hepatitis C. This cell line represents a valuable alternative to ex vivo cultivated primary human hepatocytes (PHH), as HepaRG cells share some features and properties with adult hepatocytes. The cell line is particularly useful to evaluate drugs and perform drug metabolism studies, as many detoxifying enzymes are expressed and functional. It is also an interesting tool to study some aspect of progenitor biology (e.g., differentiation process), carcinogenesis, and the infection by some pathogens for which the cell line is permissive (e.g., HBV infection). Overall, this chapter gives a concise overview of the biological properties and potential applications of this cell line.

Cellular pharmacology of the anti-hepatitis B virus agent beta-L-2',3'-didehydro-2',3'-dideoxy-N4-hydroxycytidine: relevance for activation in HepG2 cells

Beta-l-2',3'-didehydro-2',3'-dideoxy-N(4)-hydroxycytidine (l-Hyd4C) was demonstrated to be an effective and highly selective inhibitor of hepatitis B virus (HBV) replication in HepG2.2.15 cells (50% effective dose [ED(50)] = 0.03 microM; 50% cytotoxic dose [CD(50)] = 2,500 microM). In the present study, we investigated the intracellular pharmacology of tritiated l-Hyd4C in HepG2 cells. l-[(3)H]Hyd4C was shown to be phosphorylated extensively and rapidly to the 5'-mono-, 5'-di-, and 5'-triphosphate derivatives. Other metabolites deriving from a reduction or removal of the NHOH group of l-Hyd4C could not be detected, although both reactions were described as the primary catabolic pathways of the stereoisomer ss-d-N(4)-hydroxycytidine in HepG2 cells. Also, the formation of liponucleotide metabolites, such as the 5'-diphosphocholine derivative of l-Hyd4C, as described for some l-deoxycytidine analogues, seems to be unlikely. After incubation of HepG2 cells with 10 microM l-[(3)H]Hyd4C for 24 h, the 5'-triphosphate accumulated to 19.4 +/- 2.7 pmol/10(6) cells. The predominant peak belonged to 5-diphosphate, with 43.5 +/- 4.3 pmol/10(6) cells. The intracellular half-life of the 5'-triphosphate was estimated to be 29.7 h. This extended half-life probably reflects a generally low affinity of 5'-phosphorylated l-deoxycytidine derivatives for phosphate-degrading enzymes but may additionally be caused by an efficient rephosphorylation of the 5'-diphosphate during a drug-free incubation. The high 5'-triphosphate level and its extended half-life in HepG2 cells are consistent with the potent antiviral activity of l-Hyd4C.

[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.

Four conserved cysteine residues of the hepatitis B virus polymerase are critical for RNA pregenome encapsidation

Hepadnaviruses replicate via reverse transcription of an RNA template, the pregenomic RNA (pgRNA). Although hepadnaviral polymerase (Pol) and retroviral reverse transcriptase are distantly related, some of their features are distinct. In particular, Pol contains two additional N-terminal subdomains, the terminal protein and spacer subdomains. Since much of the spacer subdomain can be deleted without detrimental effects to hepatitis B virus (HBV) replication, this subdomain was previously thought to serve only as a spacer that links the terminal protein and reverse transcriptase subdomains. Unexpectedly, we found that the C terminus of the spacer subdomain is indispensable for the encapsidation of pgRNA. Alanine-scanning mutagenesis revealed that four conserved cysteine residues, three at the C terminus of the spacer subdomain and one at the N terminus of the reverse transcriptase subdomain, are critical for encapsidation. The inability of the mutant Pol proteins to incorporate into nucleocapsid particles, together with other evidence, argued that the four conserved cysteine residues are critical for RNA binding. One implication is that these four cysteine residues might form a putative zinc finger motif. Based on these findings, we speculate that the RNA binding activity of HBV Pol may be mediated by this newly identified putative zinc finger motif.

Evolution of hepatitis B virus polymerase mutations in a patient with HBeAg-positive chronic hepatitis B virus treated with sequential monotherapy and add-on nucleoside/nucleotide analogues

BACKGROUND

Nucleoside/nucleotide analogues are a fundamental tool for the treatment of chronic hepatitis B virus (HBV). Sequential anti-HBV treatment might lead to the selection of mutations.

OBJECTIVE

This report aimed to analyze the genetic evolution of the reverse-transcriptase (RT) gene of viral quasispecies in a patient with hepatitis B e antigen (HBeAg)-positive chronic HBV who received, sequentially, lamivudine (LAM), adefovir dipivoxil (ADV), and ADV + telbivudine (LDT) combination treatment over a total of 108 weeks.

METHODS

A 20-year-old Chinese man presented to Huashan Hospital, Fudan University, Shanghai, People's Republic of China, with hepatitis B surface antigen-positive and HBeAg-positive chronic HBV and was sequentially treated with LAM 100 mg/d for 18weeks,ADV 10mg/d for 68weeks, and ADV 10mg/d + LDT 600 mg/d combination treatment for 22 weeks. Compliance was monitored every 4 weeks using a pill count. For genotypic analysis, the RT region of the polymerase gene from the serum of this patient was amplified, cloned, and sequenced. Fifty clones with HBV insert were selected for sequencing at weeks 0 (baseline), 18, 22, 60, 70, 86, and 108.

RESULTS

The rtM204V/L LAM-resistance mutation was detected in 4.4% (2/45) of clones prior to LAM treatment. At week 18 during LAM treatment, the rtM204I mutation became predominant, being present in 79.5% (35/44) of clones. The rtM204I mutation was associated with compensatory mutations (rtL180M and rtT184L). A total of 9.1% (4/44) of the clones harbored the rtL180M + rtT184L + rtM204I mutations. Two new mutations, rtL229V and rtV191I, were detected in 75.0% (33/44) and 11.4% (5/44) of clones, respectively. At week 22 during ADV treatment, LAM-resistance mutations (rtL180M, rtT184L, rtM204I, rtV191I, and rtL229V) were not detected. At week 86 during ADV therapy, the rtN236T ADV-resistance mutation was detected in 58.8% (20/34) of clones. A total of 20.6% (7/34) of the clones harbored the rtK212T + rtM250L mutation, and rtA181V was found in 2.9% (1/34) of the clones. At week 108, after the patient had been receiving ADV + LDT combination therapy for 22 weeks, rtS202G and rtI269T had emerged, representing 28.9% (13/45) and 8.9% (4/45), respectively, of the viral population during ADV + LDT combination treatment. We also detected several polymorphic sites,including rtF221Y, rtS223A, rtI224V, rtN238H, rtL267Q, and rtQ271M, during the sequential treatment. After 22 weeks of combination treatment, HBV DNA count was decreased to less than the lower limit of quantitation (<200 copies/mL).

CONCLUSIONS

This report identified HBV mutations that escaped the antiviral pressure of LAM, ADV, and ADV + LDT in this patient and provided insight into the process of mutation selection through genotypic analysis during antiviral treatment. Mutations selected under sequential treatments of LAM, ADV, and ADV + LDT can lead to a series of compensatory mutations, which partially restore the level of viral replication. ADV administered in combination with LDT appeared to be effective in this selected case with clinical or virologic resistance to sequential treatment with LAM and ADV.

Impact of hepatitis B virus genotypes and surface antigen variants on the performance of HBV real time PCR quantification

Quantitative PCR assays used to monitor hepatitis B virus (HBV) load differ in their ability to detect different HBV variants. This study evaluated the performance of the Abbott RT PCR assay for quantitating DNA from different HBV genotypes and from HBV variants bearing HBsAg gene mutations. The study was performed on a randomly-selected sample with a viral load >6logIU/mL for each genotype and on 25 HBsAg variants. Each sample was assayed using the Abbott RT assay and with the Roche Cobas AmpliPrep-Cobas TaqMan as a reference method. All HBV genotypes were detected with the Abbott RT assay with an equivalent dynamic range (1-8logIU/mL). For each genotype, the data suggest that the assay was linear over the entire dilution range (r(2): 0.985-0.995). For the 25 HBsAg variants, viral titres determined with the two assays correlated well (r(2): 0.929). The mean difference between the two methods was -0.295 (95% CI: -0.520 to -0.071). The difference was lower than 1log unit in all but two cases. In conclusion, the Abbott RT assay can detect and quantify DNA from different HBV variants with equivalent performance and is thus suitable for routine monitoring of patients with chronic HBV infections.

Use of the novel INNO-LiPA line probe assay for detection of hepatitis B virus variants that confer resistance to entecavir therapy

A line probe assay (INNO-LiPA DR, version 3) for the detection of hepatitis B virus mutations that confer resistance to entecavir therapy was evaluated. The INNO-LiPA DR assay is a highly sensitive assay that is easily applicable for the detection and monitoring of entecavir resistance-conferring mutations and is more sensitive than sequencing for the detection of mixed sequences.

Antiviral combination therapy for treatment of chronic hepatitis B, hepatitis C, and human immunodeficiency virus infection

This chapter reviews the main chemotherapeutic strategies used against human infections caused by agents responsible for the most important chronic viral illnesses, namely hepatitis B virus (HBV), hepatitis C virus (HCV), and human immunodeficiency virus (HIV). There is no doubt that most current knowledge about combination antiviral therapy has been developed in the battle against HIV. The availability of more than 20 antiretroviral drugs has permitted to explore their efficacy when given in combination, an opportunity that unfortunately has only been possible since recent years for chronic hepatitis C and still is in the early stages for chronic hepatitis B. However, new antiviral compounds targeting each of these viruses are developed rapidly and will provide further opportunities to explore the efficacy of combination antiviral therapy. While sufficient suppression of HIV RNA and HBV DNA can only be achieved by long-term administration of potent antiviral drugs, HCV RNA may be completely eradicated from the infected individual after a limited duration of treatment.

[Combination therapy in chronic hepatitis B]

The HBV genome variability is responsible for the complexitiy of the viral quasi-species and its evolution during the course of the infection. During antiviral therapy, the persistence of infected cells is another important determinant involved in the selection of drug resistant strains. The development of nucleoside analogs with complementary resistance profiles has provided the rationale for add-on strategies in case of virologic breakthrough. The current trend is to add antivirals earlier, before the rebound of viral load, especially in case of partial virologic response. Clinical trials are required to determine if a de novo combination of nucleoside analogs provides an added benefit compared to an early add-on strategy. However, de novo combination is recommended in patients whose liver functions cannot tolerate treatment failure, and in patients with a high risk of developing viral resistance because of a complex viral quasi-species prior to therapy.

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.

Vector design for liver-specific expression of multiple interfering RNAs that target hepatitis B virus transcripts

RNA interference (RNAi) is a process that can target intracellular RNAs for degradation in a highly sequence-specific manner, making it a powerful tool that is being pursued in both research and therapeutic applications. Hepatitis B virus (HBV) is a serious public health problem in need of better treatment options, and aspects of its life cycle make it an excellent target for RNAi-based therapeutics. We have designed a vector that expresses interfering RNAs that target HBV transcripts, including both viral RNA replicative intermediates and mRNAs encoding viral proteins. Our vector design incorporates many features of endogenous microRNA (miRNA) gene organization that are proving useful for the development of reagents for RNAi. In particular, our vector contains an RNA pol II driven gene cassette that leads to tissue-specific expression and efficient processing of multiple interfering RNAs from a single transcript, without the co-expression of any protein product. This vector shows potent silencing of HBV targets in cell culture models of HBV infection. The vector design will be applicable to silencing of additional cellular or disease-related genes.

Effects of two novel nucleoside analogues on different hepatitis B virus promoters

AIM: To explore the effects of the nucleoside analogues β-L-D4A and β-LPA on hepatitis B virus (HBV) promoters.

METHODS: Four HBV promoters were amplified by polymerase chain reaction (PCR) and subcloned into the expression vector pEGFP-1. The four recombinants controlled by HBV promoters were confirmed by restriction analysis and sequencing. Human hepatoma HepG2 cells transfected with the recombinant plasmids were treated with various concentrations of β-L-D4A and β-LPA. Then, enhanced green fluorescent protein (EGFP)-positive cells were detected by fluorescence microscopy and using a fluorescence activated cell sorter (FACS).

RESULTS: Four HBV promoters were separately obtained and successfully cloned into pEGFP-1. Expression of EGFP under the control of the surface promoter (Sp) and the X promoter (Xp) was inhibited by β-L-D4A in a dose-dependent manner, while expression of EGFP under the control of the core promoter (Cp) and Xp was inhibited by β-LPA in a dose-dependent manner.

CONCLUSION: The two novel nucleoside analogues investigated here can inhibit the activities of HBV promoters in a dose-dependent manner. These findings may explain the mechanisms of action by which these two novel compounds inhibit HBV DNA replication.

Hepatitis B viruses: reverse transcription a different way

Hepatitis B virus (HBV), the causative agent of B-type hepatitis in humans, is the type member of the Hepadnaviridae, hepatotropic DNA viruses that replicate via reverse transcription. Beyond long-established differences to retroviruses in gene expression and overall replication strategy newer work has uncovered additional distinctions in the mechanism of reverse transcription per se. These include protein-priming by the unique extra terminal protein domain of the reverse transcriptase (RT) utilizing an RNA hairpin for de novo initiation of first strand DNA synthesis, and the strict dependence of this process on cellular chaperones. Recent in vitro reconstitution systems enabled first biochemical insights into this multifactorial reaction, complemented by high resolution structural information on the RNA, though not yet the protein, level. Genetic approaches have revealed long-distance interactions in the nucleic acid templates as an important factor enabling the puzzling template switches required to produce the relaxed circular (RC) DNA found in infectious virions. Finally, the failure of even potent HBV RT inhibitors to eliminate nuclear covalently closed circular (ccc) DNA, the functional equivalent of integrated proviral DNA, has spurred a renewed interest in the mechanism of cccDNA generation. These new developments are in the focus of this review.

Antiviral treatment of chronic hepatitis B virus infections: the past, the present and the future

A decade ago, standard therapy against chronic hepatitis B virus infections only consisted of lamivudine or IFN-alpha. Treatment with lamivudine and IFN has been compounded by, respectively, the emergence of drug-resistant virus strains and the appearance of serious side effects. In the last 10 years, hepatitis B treatment has made much progress. Several treatments are now licensed for the treatment of patients with chronic hepatitis B and others are under development. Here, we provide an overview of the potential and mode of action of anti-HBV agents that are currently available, and/or may become available in the near future. Foremost among these newer compounds are adefovir dipivoxil, entecavir and telbivudine.

Inhibition of the replication of hepatitis B virus in vitro by a novel 2,6-diaminopurine analog, beta-LPA

Antiviral therapy of chronic hepatitis B remains a major clinical problem worldwide. Like lamivudine, nucleoside analogs have become the focus of investigation of anti-hepatitis B virus (anti-HBV) drugs. Here, beta-LPA is a novel 2,6-diaminopurine analog found to possess potent anti-HBV activity. In HepG2.2.15 cell line, beta-LPA had a 50% effective concentration (EC(50)) of 0.01 microM against HBV, as determined by analysis of secreted and intracellular episomal HBV DNA. Levels of HBV surface antigen (HBsAg) and e antigen (HBeAg) in drug-treated cultures revealed that beta-LPA had no significant inhibitory effects on HBsAg and HBeAg. beta-LPA didn't show any cytotoxicity up to 0.4 microM with a 50% cytotoxic concentration (CC(50)) of 50 microM. Furthermore, treatment with beta-LPA resulted in no apparent inhibitory effects on mitochondrial DNA content. Considering the potent inhibition of HBV DNA synthesis and no obvious toxicity of beta-LPA, this compound should be further explored for development as an anti-HBV drug.

Personalized therapy in chronic viral hepatitis

Chronic carriers of major hepatitis viruses (i.e., hepatitis B and C viruses, HBV and HCV) account for at least 600 millions people worldwide. About 50% of them are at risk for chronic hepatitis and 20-30% of patients with chronic hepatitis develop progressive liver disease and symptomatic life-threatening liver lesions. Therefore, the identification of the carrier at risk is mandatory to prevent progressive liver disease, avoiding non-appropriate treatments. The decision making has three major steps. The 1st is the identification of the patient who needs to be treated; the 2nd is the choice of the best therapeutic strategy and the most appropriate drugs and timing during the phase of infection and disease; the 3rd is the treatment optimization to reduce non effective therapy and avoid drug resistance virus mutants. This careful evaluation takes into account the individual variability, the host/virus interplays and the drug impact on viral replication with the risk of selection of resistant mutants. The complexity of the virus/host interactions, however, cannot be managed by simple mean of probabilistic statistics and/or step-wise algorithms based on population statistics. A better answer for personalized antiviral therapy may come from the combined use of molecular biology and bio-mathematical modeling that can help the medical doctor to follow the dynamic of viral infection during therapy, like the flight simulator helps the pilot. We provide a concise review of the potentials of this approach in clinical practice.

[Current data on the treatment of chronic hepatitis B]

Despite the development of new therapeutic options, treatment of chronic hepatitis B remains a clinical challenge because of the need for long-term treatment in most patients. Treatment with pegylated interferon is the only option that allows a defined duration of treatment. Nonetheless, approximately 70% of the patients treated do not have a prolonged treatment response. A variety of nucleoside analog viral polymerase (reverse transcriptase) inhibitors have been developed (lamivudine, adefovir dipivoxil, entecavir, telbivudine); they can be administered orally and are well tolerated. These antiviral agents effectively induce viral suppression, which is accompanied by an improvement in transaminases and hepatic histology. Nonetheless, the rates of HBe and HBs seroconversion remain low with nucleoside analogs, and the absence of these immunologic events necessitates prolonged antiretroviral treatment. Treatment with nucleoside analogs leads to selection of resistant mutant viruses. They therefore require close clinical and virologic follow-up to enable early screening for resistance and adaptation of treatment before the liver disease worsens. The development of these different treatment options has made possible very significant improvements in the management of patients with chronic hepatitis B, by preventing aggravation of liver disease in most of them.

Antiviral-resistant hepatitis B virus: can we prevent this monster from growing?

Despite the recent progress in antiviral therapy of chronic hepatitis B, clinical experience has shown that antiviral drug resistance is inevitable with the administration of nucleoside analog monotherapy. The long-term persistence of the viral genome in infected cells and the high rate of spontaneous mutation is the basis for the selection of HBV mutants that are resistant to polymerase inhibitors. Selection of antiviral-resistant mutations leads to a rise in viral load and progression of liver disease. The incidence of antiviral resistance depends on the potency and genetic barrier to resistance of the antiviral drug, highlighting the importance of the choice if first line therapy. The determination of cross-resistance profile of each drug has allowed the design of rescue therapy for patients with virologic breakthrough. Early diagnosis and treatment intervention allow the majority of patients to maintain in clinical remission despite the occurrence of drug resistance. Clinical studies are ongoing to determine the best strategy to prevent or delay antiviral drug resistance and of its impact on liver disease.

Antiviral therapy of chronic hepatitis B: prevention of drug resistance

The emergence of viral resistance during treatment is becoming an important clinical issue for hepatitis B virus (HBV) antiviral therapy. Considerable progress has been achieved in the efficacy of treatment, with the development of new drugs that allow a sustained suppression of HBV replication, or at least maintain the viral load below a clinically relevant threshold. Although most drugs currently registered for the treatment of chronic hepatitis B are effective in suppressing viral load, long-term therapy is required to avoid viral reactivation and progression of liver disease. Because of the variability of the HBV genome, such long-term treatments are associated with the emergence of resistant viral strains, which may compromise the initial clinical benefit of the treatment.

Emerging drugs for hepatitis B

Chronic hepatitis B remains a treatment challenge despite the availability of new nucleoside analogs. This is due to the persistence of viral infection during therapy, which exposes the patient to the risk of developing antiviral drug resistance. Therefore, new polymerase inhibitors are needed to manage resistance to existing drugs and new trials of combination therapy are required to delay drug resistance. In the future, antiviral agents targeting other steps of the viral life cycle will be needed to achieve antiviral synergy and prevent antiviral drug resistance. Immune modulators are also expected to enhance antiviral response and to achieve sustained response. Discovery of new antiviral drugs and design of new treatment strategies are, therefore, needed to manage this disease, which is still the main cause of cirrhosis and hepatocellular carcinoma worldwide.

Emergence of hepatitis B virus quasispecies with lower susceptibility to nucleos(t)ide analogues during lamivudine treatment

OBJECTIVES

We studied the impact of hepatitis B virus (HBV) polymerase/reverse transcriptase (Pol/Rt) heterogeneity on adefovir rescue therapy in 34 consecutive chronic hepatitis B patients with viral breakthrough during lamivudine monotherapy.

METHODS

The Pol/Rt A-F domains were directly sequenced in all patients at baseline, and 12 and 24 months. Response to therapy was evaluated at 3, 6, 12 and 24 months by quantitative HBV-DNA.

RESULTS

Primary treatment failures did not occur. At 6 months 24/34 (70.6%) patients had viraemia<10(4) copies/mL [initial viral response (IVR)]; at 12 and 24 months 23 (71.9%) and 26 (81.3%) of 32 had HBV-DNA<200 copies/mL [complete viral response (CVR)]. IVR or CVR patients did not show viral breakthroughs, which occurred in one of the six remaining patients. All but three patients had baseline rtM204I/V substitutions associated with rtL180M in 23, rtL80I/V in 14, rtV173L in 4, rtT184S in 3, rtQ215S in 2 and rtA181S in 2 cases. rtA181S without rtM204I/V was found in one patient. Four of the six patients (67%) without 24 month CVR showed rtA181S or rtT184S substitutions either alone or with typical lamivudine resistance profiles. Baseline HBV-DNA levels were negatively associated with IVR (univariate analysis, P=0.023). At least one of rtA181S and rtT184S substitutions correlated negatively with IVR and CVR (univariate analysis, P=0.001) and was independently associated with absence of CVR (P = 0.016).

CONCLUSIONS

Lamivudine monotherapy favours the emergence of viral quasispecies that influence the response rate to adefovir rescue therapy independently from baseline viraemia and lower the susceptibility to other nucleos(t)ide analogues.

In vitro activity of 2,4-diamino-6-[2-(phosphonomethoxy)ethoxy]-pyrimidine against multidrug-resistant hepatitis B virus mutants

The susceptibilities of drug-resistant hepatitis B virus (HBV) mutants to lamivudine, adefovir, tenofovir, entecavir, and 2,4-diamino-6-[2-(phosphonomethoxy)ethoxy]-pyrimidine (PMEO-DAPym), a novel acyclic pyrimidine analogue, were assessed in vitro. Most drug-resistant mutants, including multidrug-resistant strains, remained sensitive to tenofovir and PMEO-DAPym. Therefore, the latter molecule deserves further evaluation for the treatment of HBV infection.

Hepatitis B virus replication

Hepadnaviruses, including human hepatitis B virus (HBV), replicate through reverse transcription of an RNA intermediate, the pregenomic RNA (pgRNA). Despite this kinship to retroviruses, there are fundamental differences beyond the fact that hepadnavirions contain DNA instead of RNA. Most peculiar is the initiation of reverse transcription: it occurs by protein-priming, is strictly committed to using an RNA hairpin on the pgRNA, ε, as template, and depends on cellular chaperones; moreover, proper replication can apparently occur only in the specialized environment of intact nucleocapsids. This complexity has hampered an in-depth mechanistic understanding. The recent successful reconstitution in the test tube of active replication initiation complexes from purified components, for duck HBV (DHBV), now allows for the analysis of the biochemistry of hepadnaviral replication at the molecular level. Here we review the current state of knowledge at all steps of the hepadnaviral genome replication cycle, with emphasis on new insights that turned up by the use of such cell-free systems. At this time, they can, unfortunately, not be complemented by three-dimensional structural information on the involved components. However, at least for the ε RNA element such information is emerging, raising expectations that combining biophysics with biochemistry and genetics will soon provide a powerful integrated approach for solving the many outstanding questions. The ultimate, though most challenging goal, will be to visualize the hepadnaviral reverse transcriptase in the act of synthesizing DNA, which will also have strong implications for drug development.