Evolving therapies for the treatment of chronic hepatitis B virus infection

A new strategy for constructing in vitro replication-competent 1.3 copies of hepatitis B virus genome

In the absence of a robust infectable cell culture system, assays related to replication of clinical HBV isolates are based on the transfection of replication-competent HBV DNA into hepatoma cell lines that are able to replicate and secrete HBV virions. Current methods for constructing HBV 1.1 genomes work well for drug susceptibility assays, but are not very suitable for research on HBV replication capacity or regulation since a heterogeneous promoter is required to drive pgRNA transcription. A new strategy for constructing HBV 1.3 genomes that contain HBV intrinsic promoter necessary for pgRNA transcription is reported in this paper. Using this strategy, three HBV 1.3 genomes from isolates of three patients were constructed. When the three HBV 1.3 genomes were transfected into the HepG2 cell line, replicative intermediates were detectable by Southern blotting with digoxigenin-labeled DNA probe in two of the three constructs. Using overlap extension PCR and avoiding as much as possible the digestion-and-ligation process, this strategy could be applied to constructing longer-than-genome units for most genotypes of HBV strains.

Treatments for hepatitis B

New optimism surrounds treatments for chronic hepatitis B (CHB). Interferon- alpha , lamivudine, and adefovir dipivoxil are currently approved by the United States Food and Drug Administration for the treatment of CHB. All 3 treatments possess unique characteristics with respect to their side effect profiles, potencies, and treatment niches within the spectrum of CHB. New agents, which are in various stages of clinical development, represent potential improvements within existing, as well as novel, classes of antiviral therapy, and they offer significant promise of a cure for the many patients with chronic and progressive hepatitis B. However, there remain many challenges in understanding the implications of drug resistance, the role of combination therapy, and how to define the response to therapy within subsets of patients with hepatitis B.

Clinical emergence of entecavir-resistant hepatitis B virus requires additional substitutions in virus already resistant to Lamivudine

Entecavir (ETV) exhibits potent antiviral activity in patients chronically infected with wild-type or lamivudine (3TC)-resistant (3TC(r)) hepatitis B virus (HBV). Among the patients treated in phase II ETV clinical trials, two patients for whom previous therapies had failed exhibited virologic breakthrough while on ETV. Isolates from these patients (arbitrarily designated patients A and B) were analyzed genotypically for emergent substitutions in HBV reverse transcriptase (RT) and phenotypically for reduced susceptibility in cultures and in HBV polymerase assays. After 54 weeks of 3TC therapy, patient A (AI463901-A) received 0.5 mg of ETV for 52 weeks followed by a combination of ETV and 100 mg of 3TC for 89 weeks. Viral rebound occurred at 133 weeks after ETV was started. The 3TC(r) RT substitutions rtV173L, rtL180M, and rtM204V were present at study entry, and the additional substitutions rtI169T and rtM250V emerged during ETV-3TC combination treatment. Reduced ETV susceptibility in vitro required the rtM250V substitution in addition to the 3TC(r) substitutions. For liver transplant patient B (AI463015-B), previous famciclovir, ganciclovir, foscarnet, and 3TC therapies had failed, and RT changes rtS78S/T, rtV173L, rtL180M, rtT184S, and rtM204V were present at study entry. Viral rebound occurred after 76 weeks of therapy with ETV at 1.0 mg, with the emergence of rtT184G, rtI169T, and rtS202I substitutions within the preexisting 3TC(r) background. Reduced susceptibility in vitro was highest when both the rtT184G and the rtS202I changes were combined with the 3TC(r) substitutions. In summary, infrequent ETV resistance can emerge during prolonged therapy, with selection of additional RT substitutions within a 3TC(r) HBV background, leading to reduced ETV susceptibility and treatment failure.

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The hepatitis B virus polymerase mutation rtV173L is selected during lamivudine therapy and enhances viral replication in vitro

Therapy of chronic hepatitis B virus (HBV) infection with the polymerase inhibitor lamivudine frequently is associated with the emergence of viral resistance. Genotypic changes in the YMDD motif (reverse transcriptase [rt] mutations rtM204V/I) conferred resistance to lamivudine as well as reducing the in vitro replication efficiency of HBV. A second mutation, rtL180M, was previously reported to partially restore replication fitness as well as to augment drug resistance in vitro. Here we report the functional characterization of a third polymerase mutation (rtV173L) associated with resistance to lamivudine and famciclovir. rtV173L was observed at baseline in 9 to 22% of patients who entered clinical trials of adefovir dipivoxil for the treatment of lamivudine-resistant HBV. In these patients, rtV173L was invariably found as a third mutation in conjunction with rtL180M and rtM204V. In vitro analyses indicated that rtV173L did not alter the sensitivity of wild-type or lamivudine-resistant HBV to lamivudine, penciclovir, or adefovir but instead enhanced viral replication efficiency. A molecular model of HBV polymerase indicated that residue rtV173 is located beneath the template strand of HBV nucleic acid near the active site of the reverse transcriptase. Substitution of leucine for valine at this residue may enhance polymerization either by repositioning the template strand of nucleic acid or by affecting other residues involved in the polymerization reaction. Together, these results suggest that rtV173L is a compensatory mutation that is selected in lamivudine-resistant patients due to an enhanced replication phenotype.

Agents in clinical development for the treatment of chronic hepatitis B

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

Combination therapy in the treatment of chronic viral hepatitis and prevention of hepatocellular carcinoma

Treatment of chronic hepatitis B and C viruses (HBV and HCV) is still disappointing, and both are the major causes of liver cirrhosis and hepatocarcinoma. Interferon and lamivudine are the registered drugs for chronic HBV but are scarcely effective on HBeAg-negative patients, and resistance due to virus mutation is the rule with lamivudine. Interferon and ribavirine represent the standard treatment for chronic HCV but less than the half of the infected population is eligible for this treatment and less of the half of treated patients will experience a sustained response. No single new drug to date has shown the potential to overcome this dismal picture. Combined strategies are thus the currently most available approach to improve the response rate of chronic HBV and HCV infection, with a subsequent decrease in the number of patients developing hepatocellular carcinoma (HCC). Combination of thymosin alpha 1 with interferon or antiviral agents is currently the most promising option, but nontoxic immunomodulants, such as oral MIMP, should be explored. This review focuses on the difficulties with current therapy and the rationale for use of combination therapy with thymosin alpha 1 for both HBV and HCV therapies.

Generation of cytotoxic T cell against HBcAg using retrovirally transduced dendritic cells

AIM: Cytotoxic T lymphocytes (CTLs) play an important role in resolving HBV infection. In the present study, we attempted to evaluate the efficiency of bone marrow-derived dendritic cells (DCs) transduced with recombinant retroviral vector bearing hepatitis B virus (HBV) core gene and the capability of generating CTLs against HBcAg by genetically modified DCs in vivo.

METHODS: A retroviral vector containing HBV core gene was constructed. Replicating DC progenitor of C57BL/6 mice was transduced by retroviral vector and continually cultured in the presence of recombinant mouse granulocyte-macrophage colony-stimulating factor (rmGM-CSF) and interleukin-4(IL-4) for 6 d. LPS was added and cultured for additional two days. The efficiency of gene transfer was determined by PCR, Western blot and FACS. Transduced DCs immunized C57BL/6 mice subcutaneously 2 times at an one-week interval. Intracellular IFN-γ and IL-4 of immunized mice lymphocytes were analyzed. Generation of CTLs in lymphocytes stimulated with mitomycin C-treated EL4-C cell which stably expresses HBcAg was determined by LDH release assays.

RESULTS: Recombinant retroviral expression vector (pLCSN) was positively detected by PCR as well as enzyme digestion with EcoRI and BamH I. Retroviruses were generated by pLCSN transfection packing cell and the virus titer was 3 × 10⁵ CFU/ml. Indirect immunofluorescence and FACS showed that HBV core gene was expressed in murine fibroblasts. Transduced bone marrow cells had capability of differentiating into DCs in vitro in the presence of rmGM-CSF and rmIL-4. The result of PCR showed that HBV core gene was integrated into the genome of transduced DCs. Western blot analysis showed that HBV core gene was expressed in DCs. The transduction rate was 28% determined by FACS. Retroviral transduction had no influence on DCs expressions of CD80 and MHC class II. HBcAg specific CTLs and Th1 type immune responses could be generated in the mice by using transduced DCs as antigen presenting cells (APCs).

CONCLUSION: Retroviral transduction of myeloid DCs progenitors expresses efficiently HBcAg, and genetically modified DCs evoke a higher CTLs response than HBcAg in vivo.

Antiviral therapy for hepatitis B virus infections: new targets and technical challenges

There are presently only two licensed therapies for treating liver disease caused by infection with the hepatitis B virus (HBV). These are interferon-alpha and lamivudine. Neither agent was specifically developed as an antiviral compound for treating patients infected with HBV. Both therapies are limited in the clinic by a low response rate and in the case of lamivudine, selection of drug-resistant mutants, whilst troublesome side effects limit the use of interferon-alpha. Several promising nucleoside/nucleotide analogues are undergoing clinical trials, including adefovir dipivoxil and entecavir, both of which appear to be active against lamivudine- resistant HBV. In addition to these nucleoside/nucleotide analogues, it will be important to develop new agents with different modes of action, which can be added to the antiviral cocktails that will be required to adequately suppress and hopefully eliminate HBV replication.

Treatment of chronic hepatitis B: case selection and duration of therapy

Hepatitis B viral (HBV) infection is a major health burden in the Asia-Pacific region. The seriousness of chronic hepatitis B (CHB) is often realized at a late stage. The resultant morbidity and mortality from cirrhosis complications is considerable, with a high human cost. The most affected patients are men aged 40 years or older. Two decades ago, the prognosis for the 300 million "Australia antigen"-positive people (people with chronic HBV infection) was gloomy, with no effective intervention. Twenty years on, research and development have changed their outlook. Chronic hepatitis should now be diagnosed early, at the asymptomatic stage. Proper assessment and judicial introduction of therapy can suppress replication of HBV and resolve liver inflammation, thereby preventing the silent progression of chronic liver disease to end-stage cirrhosis. Interferon (IFN) monotherapy has been available for nearly 20 years, but various limitations restrict its general application. Injection-based therapies are inconvenient, the response rate is low (33% hepatitis B e antigen (HBeAg) seroconversion rate among optimal cases), side-effects are many, and some serious, and the cost is unaffordable for most people. However, in non-cirrhotic patients with mild to moderate disease activity, IFN is still a worthwhile option because the treatment course is shorter, mutation seems less of a problem and most responses are permanent and reduce or abolish late complications. Lamivudine, an oral nucleoside analog with potent antiviral effects, has been approved in many countries. Daily dosing of 100 mg reduces serum HBV-DNA to below detectable levels within 6 weeks. In HBeAg-positive patients, approximately 16% of treated patients seroconverted with the first year. This was associated with significant improvement in liver histology. Long-term treatment induces further HBeAg seroconversion, but overall clinical benefit is undermined by continuous emergence of drug-resistant YMDD mutants. In an Asian multicentre study, 58 patients on 5 years lamivudine therapy showed annual cumulative HBeAg seroconversion rates at 1, 2, 3, 4 and 5 years of 22, 29, 40, 47 and 50%, respectively. The best predictor of response is pretreatment alanine aminotransferase (ALT). Among patients with ALT > 2x the upper limit of normal (ULN), annual HBeAg seroconversion is increased to 38, 42, 65, 73 and 77%, respectively. However, emergence of YMDD mutants occurred at a cumulative rate of 15, 38, 55, 67 and 69%, respectively. The impact of this emergence on disease activity is unpredictable. Thus, while continued disease suppression, or even HBeAg seroconversion, still occurred in some patients, in others hepatitis may relapse and liver failure has been reported despite continuation of lamivudine. While the duration of lamivudine therapy is difficult to define, the best strategy may be to define only active CHB with major ALT elevation (par-ticularly ALT > 5x ULN) for a duration of 1 year or less. Lamivudine can be stopped in responders. The response is durable in approximately 80% of responders. Non-responders should be monitored closely for rebound off treatment. Therapy can be re-instituted if ALT is over 5x ULN. Management of patients with YMDD mutants can be challenging, but there is no clear evidence to recommend stopping or continuing lamivudine, or to add other possible effective agents, such as adefovir dipivoxil. More data are required to help draw up guidelines. Hepatitis B e antigen-negative CHB has been less well studied. Both IFN and lamivudine can suppress disease activity, but permanent responses are few. Without a distinct marker as an end-point for response, the duration of treatment is even more difficult to define. Quantitative polymerase chain reaction for low viral levels may give a clue, but definitive studies are required. Monotherapy is clearly not the answer for the majority of CHB patients with active disease. Combination therapy has the theoretical advantage of additional or synergistic efficacy. Preliminary results on IFN and lamivudine are promising and further clinical trials are ongoing. Emtricitabine (FTC), adefovir dipivoxil, entecavir, BL-thymidine (L-dT), DAPD, clevudine (l-FMAU), thymosin, therapeutic vaccines and various herbal medicines are potential candidates. Antiviral action in conjunction with immune modulation may have a better chance of eradicating HBV and its cccDNA in the hepatocytes as the basis for an eventual successful outcome. The key points are: (i) approved therapeutic agents for chronic hepatitis B (CHB) are IFN, lamivudine and thymosin (in a few countries only); (ii) indications for IFN therapy are viremia in compensated CHB patients with moderately raised ALT; (iii) lamivudine has broader therapeutic indications: it is effective in subgroups of CHB patients with compensated or decompensated liver diseases, but generally works better if patients have raised ALT; (iv) lamivudine has a potent suppressive action on HBV replication, including HBeAg-negative variants, but cannot eliminate cccDNA; this is the reason for the relapse of disease after discontinuing treatment, unless HBeAg seroconversion is obtained; (v) successful use of lamivudine aims at HBeAg seroconversion or profound suppression of HBV-DNA to serum levels of less than 100 000 viral copies/mL, in order to prevent emergence of drug-resistant YMDD mutants (which commences from 6 months onward); (vi) YMDD mutants may cause a flare of hepatitis, resulting in deterioration of liver histology and, occasionally, liver failure; (vii) combination therapy of lamivudine with IFN (standard or pegylated) or other nucleoside analogs should be the next advance. Preliminary data from IFN and lamivudine combination therapy show some promise, but there are conflicting results.