Drug insight: Nucleoside and nucleotide analog inhibitors for hepatitis B

VHL-independent degradation of hepatitis B virus e antigen (HBeAg) by VHL-binding chimeric small molecules

Hepatitis B virus (HBV) is a leading cause of liver cancer worldwide, with current treatment options unable to provide lasting efficacy against chronic infection. A key viral protein, HBV e antigen (HBeAg), plays an important role in suppressing the cellular and humoral immune response during infection and its loss is a precursor to clearance of chronic HBV infection. Its structural similarity to capsid forming HBV core protein antigen (HBcAg) makes it an intriguing, yet understudied target for pharmaceutical intervention. Recently, targeted protein degradation has been successfully applied against several viral proteins. This work investigates the targeting of HBeAg using heterobifunctional degraders derived from reported HBcAg ligands known to interact with HBeAg. Multiple compounds designed to recruit the VHL E3 ligase were found to be capable of reducing recombinant HBeAg protein levels in a HiBiT reporter assay system. Surprisingly, this decrease was found to be independent of VHL recruitment but driven by structural motifs of the VHL recruiting ligand, VH032. Virological assessment of these compounds against wildtype virus revealed an equipotent capability to reduce secreted HBeAg compared to the parental inhibitor, however increased efficacy was observed against an inhibitor resistant strain. Together, this work provides an initial description of the feasibility of converting HBV capsid-targeting ligands into degraders and provides evidence that such degraders may harbour improved activity against mutated forms of target which are resistant to parental compounds.

The Loop-In Binding Mode of Dihydroorotase: Implications for Ligand Binding and Therapeutic Targeting

Dihydroorotase (DHOase; EC 3.5.2.3) is a zinc-dependent metalloenzyme that plays a key role in the de novo pyrimidine biosynthesis pathway, catalyzing the reversible cyclization of N-carbamoyl aspartate to dihydroorotate. This reaction is essential for the production of uridine monophosphate, the precursor of all pyrimidine nucleotides required for DNA and RNA synthesis. Despite its conserved enzymatic function, DHOase exhibits significant structural diversity across species, particularly in its oligomeric states, gene fusion patterns, and active site architecture. A crucial structural feature of DHOase is its flexible active site loop, which undergoes dynamic conformational changes during catalysis. Previously, the loop-in conformation was associated with substrate binding, whereas the loop-out conformation was linked to product release and non-substrate ligand binding. However, recent crystallographic studies challenge this paradigm, revealing that certain non-substrate ligands and inhibitors, including malate, 5-fluoroorotate, plumbagin, 5-aminouracil, and 5-fluorouracil, interact with DHOase via a loop-in binding mechanism rather than the previously assumed loop-out mode. These findings necessitate a reassessment of the catalytic mechanism of DHOase and underscore the active site loop as a potential target for drug development. This review revisits the structural and biochemical mechanisms of DHOase, with a focus on recent crystallographic insights that redefine the loop-in binding mode for ligand interaction. By leveraging the unique conformational dynamics of the active site loop, novel inhibitors may be developed to selectively target pyrimidine biosynthesis in cancer cells and microbial pathogens. These insights emphasize the crucial role of structural biology in therapeutic design and highlight DHOase as a promising drug target.

Crystal Structure of DNA Replication Protein SsbA Complexed with the Anticancer Drug 5-Fluorouracil

Single-stranded DNA-binding proteins (SSBs) play a crucial role in DNA metabolism by binding and stabilizing single-stranded DNA (ssDNA) intermediates. Through their multifaceted roles in DNA replication, recombination, repair, replication restart, and other cellular processes, SSB emerges as a central player in maintaining genomic integrity. These attributes collectively position SSBs as essential guardians of genomic integrity, establishing interactions with an array of distinct proteins. Unlike Escherichia coli, which contains only one type of SSB, some bacteria have two paralogous SSBs, referred to as SsbA and SsbB. In this study, we identified Staphylococcus aureus SsbA (SaSsbA) as a fresh addition to the roster of the anticancer drug 5-fluorouracil (5-FU) binding proteins, thereby expanding the ambit of the 5-FU interactome to encompass this DNA replication protein. To investigate the binding mode, we solved the complexed crystal structure with 5-FU at 2.3 Å (PDB ID 7YM1). The structure of glycerol-bound SaSsbA was also determined at 1.8 Å (PDB ID 8GW5). The interaction between 5-FU and SaSsbA was found to involve R18, P21, V52, F54, Q78, R80, E94, and V96. Based on the collective results from mutational and structural analyses, it became evident that SaSsbA's mode of binding with 5-FU diverges from that of SaSsbB. This complexed structure also holds the potential to furnish valuable comprehension regarding how 5-FU might bind to and impede analogous proteins in humans, particularly within cancer-related signaling pathways. Leveraging the information furnished by the glycerol and 5-FU binding sites, the complexed structures of SaSsbA bring to the forefront the potential viability of several interactive residues as potential targets for therapeutic interventions aimed at curtailing SaSsbA activity. Acknowledging the capacity of microbiota to influence the host's response to 5-FU, there emerges a pressing need for further research to revisit the roles that bacterial and human SSBs play in the realm of anticancer therapy.

Molecular Insights into How the Dimetal Center in Dihydropyrimidinase Can Bind the Thymine Antagonist 5-Aminouracil: A Different Binding Mode from the Anticancer Drug 5-Fluorouracil

Dihydropyrimidinase (DHPase) is a key enzyme for pyrimidine degradation. DHPase contains a binuclear metal center in which two Zn ions are bridged by a posttranslationally carbamylated lysine. DHPase catalyzes the hydrolysis of dihydrouracil to N-carbamoyl-β-alanine. Whether 5-aminouracil (5-AU), a thymine antagonist and an anticancer drug that can block DNA synthesis and induce replication stress, can interact with DHPase remains to be investigated. In this study, we determined the crystal structure of Pseudomonas aeruginosa DHPase (PaDHPase) complexed with 5-AU at 2.1 Å resolution (PDB entry 7E3U). This complexed structure revealed that 5-AU interacts with Znα (3.2 Å), Znβ (3.0 Å), the main chains of residues Ser289 (2.8 Å) and Asn337 (3.3 Å), and the side chain of residue Tyr155 (2.8 Å). These residues are also known as the substrate-binding sites of DHPase. Dynamic loop I (amino acid residues Pro65-Val70) in PaDHPase is not involved in the binding of 5-AU. The fluorescence quenching analysis and site-directed mutagenesis were used to confirm the binding mode revealed by the complexed crystal structure. The 5-AU binding mode of PaDHPase is, however, different from that of 5-fluorouracil, the best-known fluoropyrimidine used for anticancer therapy. These results provide molecular insights that may facilitate the development of new inhibitors targeting DHPase and constitute the 5-AU interactome.

Plumbagin, a Natural Product with Potent Anticancer Activities, Binds to and Inhibits Dihydroorotase, a Key Enzyme in Pyrimidine Biosynthesis

Dihydroorotase (DHOase) is the third enzyme in the de novo biosynthesis pathway for pyrimidine nucleotides, and an attractive target for potential anticancer chemotherapy. By screening plant extracts and performing GC-MS analysis, we identified and characterized that the potent anticancer drug plumbagin (PLU), isolated from the carnivorous plant Nepenthes miranda, was a competitive inhibitor of DHOase. We also solved the complexed crystal structure of yeast DHOase with PLU (PDB entry 7CA1), to determine the binding interactions and investigate the binding modes. Mutational and structural analyses indicated the binding of PLU to DHOase through loop-in mode, and this dynamic loop may serve as a drug target. PLU exhibited cytotoxicity on the survival, migration, and proliferation of 4T1 cells and induced apoptosis. These results provide structural insights that may facilitate the development of new inhibitors targeting DHOase, for further clinical anticancer chemotherapies.

Structural basis for the interaction modes of dihydroorotase with the anticancer drugs 5-fluorouracil and 5-aminouracil

Dihydroorotase (DHOase) is the third enzyme in the de novo biosynthesis pathway of pyrimidine nucleotides and considered an attractive target for potential antimalarial, anticancer, and antipathogen chemotherapy. Whether the FDA-approved clinical drug 5-fluorouracil (5-FU) that is used to target the enzyme thymidylate synthase for anticancer therapy can also bind to DHOase remains unknown. Here, we report the crystal structures of DHOase from Saccharomyces cerevisiae (ScDHOase) complexed with malate, 5-FU, and 5-aminouracil (5-AU). ScDHOase shares structural similarity with Escherichia coli DHOase. We also characterized the binding of 5-FU and 5-AU to ScDHOase by using the fluorescence quenching method. These complexed structures revealed that residues Arg18, Asn43, Thr106, and Ala275 of ScDHOase were involved in the 5-FU (PDB entry 6L0B) and 5-AU binding (PDB entry 6L0F). Overall, these results provide structural insights that may facilitate the development of new inhibitors targeting DHOase and constitute the 5-FU and 5-AU interactomes for further clinical chemotherapies.

New Approaches to the Treatment of Chronic Hepatitis B

The currently recommended treatment for chronic hepatitis B virus (HBV) infection achieves only viral suppression whilst on therapy, but rarely hepatitis B surface antigen (HBsAg) loss. The ultimate therapeutic endpoint is the combination of HBsAg loss, inhibition of new hepatocyte infection, elimination of the covalently closed circular DNA (cccDNA) pool, and restoration of immune function in order to achieve virus control. This review concentrates on new antiviral drugs that target different stages of the HBV life cycle (direct acting antivirals) and others that enhance both innate and adaptive immunity against HBV (immunotherapy). Drugs that block HBV hepatocyte entry, compounds that silence or deplete the cccDNA pool, others that affect core assembly, agents that degrade RNase-H, interfering RNA molecules, and nucleic acid polymers are likely interventions in the viral life cycle. In the immunotherapy category, molecules that activate the innate immune response such as Toll-like-receptors, Retinoic acid Inducible Gene-1 (RIG-1) and stimulator of interferon genes (STING) agonists or checkpoint inhibitors, and modulation of the adaptive immunity by therapeutic vaccines, vector-based vaccines, or adoptive transfer of genetically-engineered T cells aim towards the restoration of T cell function. Future therapeutic trends would likely be a combination of one or more of the aforementioned drugs that target the viral life cycle and at least one immunomodulator.

Current Progress on Equilibrative Nucleoside Transporter Function and Inhibitor Design

Physiological nucleosides are used for the synthesis of DNA, RNA, and ATP in the cell and serve as universal mammalian signaling molecules that regulate physiological processes such as vasodilation and platelet aggregation by engaging with cell surface receptors. The same pathways that allow uptake of physiological nucleosides mediate the cellular import of synthetic nucleoside analogs used against cancer, HIV, and other viral diseases. Physiological nucleosides and nucleoside drugs are imported by two families of nucleoside transporters: the SLC28 concentrative nucleoside transporters (CNTs) and SLC29 equilibrative nucleoside transporters (ENTs). The four human ENT paralogs are expressed in distinct tissues, localize to different subcellular sites, and transport a variety of different molecules. Here we provide an overview of the known structure-function relationships of the ENT family with a focus on ligand binding and transport in the context of a new hENT1 homology model. We provide a generic residue numbering system for the different ENTs to facilitate the interpretation of mutational data produced using different ENT homologs. The discovery of paralog-selective small-molecule modulators is highly relevant for the design of new therapies and for uncovering the functions of poorly characterized ENT family members. Here, we discuss recent developments in the discovery of new paralog-selective small-molecule ENT inhibitors, including new natural product-inspired compounds. Recent progress in the ability to heterologously produce functional ENTs will allow us to gain insight into the structure and functions of different ENT family members as well as the rational discovery of highly selective inhibitors.

Crystal structure of dihydropyrimidinase in complex with anticancer drug 5-fluorouracil

Dihydropyrimidinase (DHPase) catalyzes the reversible cyclization of dihydrouracil to N-carbamoyl-β-alanine in the second step of the pyrimidine degradation pathway. Whether 5-fluorouracil (5-FU), the best-known fluoropyrimidine that is used to target the enzyme thymidylate synthase for anticancer therapy, can bind to DHPase remains unknown. In this study, we found that 5-FU can form a stable complex with Pseudomonas aeruginosa DHPase (PaDHPase). The crystal structure of PaDHPase complexed with 5-FU was determined at 1.76 Å resolution (PDB entry 6KLK). Various interactions between 5-FU and PaDHPase were examined. Six residues, namely, His61, Tyr155, Asp316, Cys318, Ser289 and Asn337, of PaDHPase were involved in 5-FU binding. Except for Cys318, these residues are also known as the substrate-binding sites of DHPase. 5-FU interacts with the main chains of residues Ser289 (3.0 Å) and Asn337 (3.2 Å) and the side chains of residues Tyr155 (2.8 Å) and Cys318 (2.9 Å). Mutation at either Tyr155 or Cys318 of PaDHPase caused a low 5-FU binding activity of PaDHPase. This structure and the binding mode provided molecular insights into how the dimetal center in DHPase undergoes a conformational change during 5-FU binding. Further research can directly focus on revisiting the role of DHPase in anticancer therapy.

Management of Clevudine-Resistant Chronic Hepatitis B: A Multicenter Cohort Study

BACKGROUND/AIMS

Data are lacking regarding the management of chronic hepatitis B (CHB) with resistance to clevudine (CLV). This study evaluated the efficacy of different rescue therapies for CLV-resistant CHB.

METHODS

Patients with CLV-resistant CHB were enrolled in the cohort, and all patients developed virologic breakthrough during CLV therapy and had confirmed-genotypic resistance to CLV (rtM204I mutation) before enrollment.

RESULTS

Of the 107 patients, 12 received adefovir (ADV), 21 received a CLV plus ADV combination (CLV+ADV), 34 received a lamivudine plus ADV combination (LAM+ADV), and 40 received entecavir (ETV) therapy for 48 weeks. The CLV+ADV group had the lowest hepatitis B virus (HBV) DNA level (p<0.0001) and showed the greatest reduction of HBV DNA levels from baseline compared to all other groups (p=0.004) at week 48. HBV DNA was undetectable (<70 IU/mL) in 0%, 57.1%, 21.2%, and 27.5% (p=0.003) of the patients in each group, respectively, at week 48. At the end of the study, the mean alanine transaminase (ALT) level, rate of ALT normalization, and rate of hepatitis B envelope antigen loss or seroconversion did not differ between groups.

CONCLUSIONS

CLV+ADV combination therapy in patients with CLV-resistant CHB more effectively suppresses HBV replication than ETV, ADV, or LAM+ADV therapy.

Elevation in Serum Concentration of Bone-Specific Alkaline Phosphatase without Elevation in Serum Creatinine Concentration Secondary to Adefovir Dipivoxil Therapy in Chronic Hepatitis B Virus Infection

Of 168 patients with chronic hepatitis B virus (HBV) infection-related liver disease, 20 patients who had received 100 mg of lamivudine plus 10 mg/day of adefovir dipivoxil (ADV) (ADV group) and 124 patients who had received 0.5 mg/day of entecavir or 100 mg/day of lamivudine (non-ADV group) for >1 year were enrolled. For comparative analyses, 19 well-matched pairs were obtained from the groups by propensity scores. At the time of enrollment, serum creatinine and phosphate concentrations were similar between the ADV and non-ADV groups; however, urinary phosphate (P = 0.0424) and serum bone-specific alkaline phosphatase (BAP) (P = 0.0228) concentrations were significantly higher in the ADV group than in the non-ADV group. Serum BAP was significantly higher at the time of enrollment than before ADV administration in the ADV group (P = 0.0001), although there was no significant change in serum BAP concentration in the non-ADV group. There was a significant positive correlation between the period of ADV therapy and ΔBAP (R (2) = 0.2959, P = 0.0160). Serum BAP concentration increased before increase in serum creatinine concentration and was useful for early detection of adverse events and for developing adequate measures for continuing ADV for chronic HBV infection-related liver disease.

Discovery and development of anti-HBV agents and their resistance

Hepatitis B virus (HBV) infection is a prime cause of liver diseases such as hepatitis, cirrhosis and hepatocellular carcinoma. The current drugs clinically available are nucleot(s)ide analogues that inhibit viral reverse transcriptase activity. Most drugs of this class are reported to have viral resistance with breakthrough. Recent advances in methods for in silico virtual screening of chemical libraries, together with a better understanding of the resistance mechanisms of existing drugs have expedited the discovery and development of novel anti-viral drugs. This review summarizes the current status of knowledge about and viral resistance of HBV drugs, approaches for the development of novel drugs as well as new viral and host targets for future drugs.

Renal dysfunction in chronic hepatitis B patients treated with adefovir dipivoxil

Renal dysfunction has been reported in patients treated with adefovir dipivoxil (ADV); however, its incidence and clinical importance may be underappreciated given the lack of long-term follow-up and data outside of a clinical trial setting. Our goal was to examine the severity and incidence of renal dysfunction in a real-life setting for patients treated with ADV and whose baseline estimated glomerular filtration rate (eGFR) was >50 mL/minute. We performed a cohort study of 290 chronic hepatitis B patients: 145 patients treated with 10 mg ADV and 145 patients unexposed to ADV at two community clinics, who were matched for age (+/-10 years), sex, and baseline eGFR. The exposed and unexposed populations were well-matched with a similar mean age (46-47 years), proportion of male patients (76.5%), baseline serum creatinine (0.97-0.99 mg/dL), and baseline creatinine clearance (85.0-85.4 mL/minute). The incidence density for renal dysfunction defined by treatment termination and/or development of eGFR < or =50 mL/minute was five cases per 100 patient-years in the exposed group compared with 1.36 cases per 100 patient-years in the unexposed group (P = 0.02). The relative risk of exposed to unexposed was 3.68 (95% confidence interval 1.1-19.3). On Cox proportional hazard analysis also inclusive of sex, ADV was a significant predictor of significant renal dysfunction (hazard ratio [HR] 3.94, P = 0.03). There were also significant trends for age >50 years (HR 3.49, P = 0.087), mild renal impairment at baseline (HR 4.49, P = 0.073), and hypertension and/or diabetes mellitus (HR 2.36, P = 0.074).

CONCLUSION

ADV is an independent predictor for significant deterioration of renal function. Patients on ADV should be monitored, especially patients who are older, have baseline renal insufficiency, or have hypertension and/or diabetes mellitus.

New antiviral choice for chronic hepatitis B: tenofovir disoproxil fumarate

Tenofovir disoproxil fumarate (TDF) is an oral prodrug of tenofovir, a novel, acyclic nucleotide analogue with in vitro activity against HIV-1 and HIV-2. TDF is licensed by American Food and Drug Administration (FDA) in 2001 for the treatment of HIV infection. TDF is currently one of the most widely used nucleotide reverse transcriptase inhibitors (NRTIs) for the treatment of HIV infection. Its efficacy, favorable toxicity profile, and convenient dosing have made this drug one of the most popular first-line treatment. Numerous studies have demonstrated the use of TDF in the treatment of HIV infection. It also has been shown to be effective in HIV/HBV coinfected patients and in patients with wild-type and lamivudine-resistant strains. Accumulating evidence suggests that TDF is more potent in suppressing HBV replication. In this review, we summarize the study progress of TDF in treating HBV infection.

Telbivudine: a new treatment for chronic hepatitis B

Three hundred and fifty million people worldwide are estimated to be chronically infected with hepatitis B virus. 15%-40% of these subjects will develop cirrhosis, liver failure or hepatocellular carcinoma during their life. The treatment of chronic hepatitis B has improved dramatically over the last decade merits to the advent of nucleoside/nucleotide analogues and the use of pegylated interferons. Approved drugs for chronic hepatitis B treatment include: standard interferon-alpha 2b, pegylated interferon-alpha 2a, lamivudine, adefovir dipivoxil, and entecavir. Unfortunately, these agents are not effective in all patients and are associated with distinct side effects. Interferons have numerous side effects and nucleoside or nucleotide analogues, which are well tolerated, need to be used for prolonged periods, even indefinitely. However, prolonged treatment with nucleoside or nucleotide analogues is associated with a high rate of resistance. Telbivudine is a novel, orally administered nucleoside analogue for use in the treatment of chronic hepatitis B. In contrast to other nucleoside analogues, Telbivudine has not been associated with inhibition of mammalian DNA polymerase with mitochondrial toxicity. Telbivudine has demonstrated potent activity against hepatitis B with a significantly higher rate of response and superior viral suppression compared with lamivudine, the standard treatment. Telbivudine has been generally well tolerated, with a low adverse effect profile, and at its effective dose, no dose-limiting toxicity has been observed. Telbivudine is one of the most potent antiviral agents for chronic hepatitis B virus and was approved by the FDA in late 2006.

Telbivudine: a new treatment for chronic hepatitis B

Three hundred and fifty million people worldwide are estimated to be chronically infected with hepatitis B virus. 15%-40% of these subjects will develop cirrhosis, liver failure or hepatocellular carcinoma during their life. The treatment of chronic hepatitis B has improved dramatically over the last decade merits to the advent of nucleoside/nucleotide analogues and the use of pegylated interferons. Approved drugs for chronic hepatitis B treatment include: standard interferon-alpha 2b, pegylated interferon-alpha 2a, lamivudine, adefovir dipivoxil, and entecavir. Unfortunately, these agents are not effective in all patients and are associated with distinct side effects. Interferons have numerous side effects and nucleoside or nucleotide analogues, which are well tolerated, need to be used for prolonged periods, even indefinitely. However, prolonged treatment with nucleoside or nucleotide analogues is associated with a high rate of resistance. Telbivudine is a novel, orally administered nucleoside analogue for use in the treatment of chronic hepatitis B. In contrast to other nucleoside analogues, Telbivudine has not been associated with inhibition of mammalian DNA polymerase with mitochondrial toxicity. Telbivudine has demonstrated potent activity against hepatitis B with a significantly higher rate of response and superior viral suppression compared with lamivudine, the standard treatment. Telbivudine has been generally well tolerated, with a low adverse effect profile, and at its effective dose, no dose-limiting toxicity has been observed. Telbivudine is one of the most potent antiviral agents for chronic hepatitis B virus and was approved by the FDA in late 2006.

The emergence of YMDD mutants precedes biochemical flare by 19 weeks in lamivudine-treated chronic hepatitis B patients: an opportunity for therapy reevaluation

Given the loss of therapeutic efficacy associated with the development of resistance to lamivudine (LMV) and the availability of new alternative treatments for chronic hepatitis B patients, early detection of viral genotypic resistance could allow the clinician to consider therapy modification before viral breakthrough and biochemical relapse occur. To this end, 28 LMV-treated patients (44 +/- 12 years; 24 men), on their first therapy schedule, were monitored monthly at four Brazilian centers for the emergence of drug resistance using the reverse hybridization-based INNO-LiPA HBV DR assay and occasionally sequencing (two cases). Positive viral responses (HBV DNA clearance) after 6, 12, and 18 months of therapy were achieved by 57, 68, and 53% of patients, while biochemical responses (serum alanine aminotransferase normalization) were observed in 82, 82, and 53% of cases. All viral breakthrough cases (N = 8) were related to the emergence of YMDD variants observed in 7, 21, and 35% of patients at 6, 12, and 18 months, respectively. The emergence of these variants was not associated with viral genotype, HBeAg expression status, or pretreatment serum alanine aminotransferase levels. The detection of resistance-associated mutations was observed before the corresponding biochemical flare (41 +/- 14 and 60 +/- 15 weeks) in the same individuals. Then, if highly sensitive LMV drug resistance testing is carried out at frequent and regular intervals, the relatively long period (19 +/- 2 weeks) between the emergence of viral resistance and the onset of biochemical relapse can provide clinicians with ample time to re-evaluate drug therapy.

Entecavir: a new nucleoside analog for the treatment of chronic hepatitis B infection

BACKGROUND

Chronic hepatitis B infection carries considerable risk for the development of cirrhosis and hepatocellular carcinoma. Treatment options are increasing but are limited to interferon alfa-2b, pegylated interferon alfa-2a, lamivudine, adefovir dipivoxil, and entecavir. Entecavir, a nucleoside analog, is the newest oral antiviral approved in the United States for treatment of chronic hepatitis B.

OBJECTIVE

To review the available data for entecavir regarding its pharmacology, pharmacokinetics, safety, efficacy, and clinical use.

METHODS

A MEDLINE, EMBASE, and Cochrane search of the English-language literature from January 1997-May 2006 was performed. Preapproval studies provided by the manufacturer and abstracts from recent scientific meetings on infectious disease and hepatology were also reviewed.

RESULTS

Three phase III clinical trials representing more than 1600 subjects established the superior efficacy and equivalent safety of entecavir compared with lamivudine for treating patients who are hepatitis B early antigen (HBeAg) positive, HBeAg negative, or refractory to lamivudine. Entecavir resistance has not occurred in nucleoside-naïve patients but may develop in those who already possess lamivudine resistance mutations.

CONCLUSION

Trial results, along with previously published response rates for adefovir dipivoxil and interferon monotherapy, make entecavir the preferred first-line treatment option for patients with chronic hepatitis B who are nucleoside naïve, HBeAg positive or negative, and have compensated liver disease. Both entecavir and adefovir dipivoxil maintain activity against hepatitis B virus in patients with chronic hepatitis B who are refractory to lamivudine, and both agents are reasonable first-line treatment options. Longer trials involving nucleoside-naïve, lamivudine-refractory patients are needed to determine entecavir's optimal treatment duration, long-term safety, and durability of response, including rate of resistance.