Gastroenterology
Volume 123, Issue 6 , Pages 2135-2140, December 2002

Chemotherapy for hepatitis B: New treatment options necessitate reappraisal of traditional endpoints☆☆☆

Victorian Infectious Diseases Reference Laboratory, North Melbourne, Victoria, Australia

Article Outline

Abstract 

GASTROENTEROLOGY 2002;123:2135-2140

 

See article on page 1831.

Despite the availability of safe and effective vaccines, hepatitis B remains a global health problem, responsible for about 1.2 million deaths annually.1 It has been estimated that more than 2 billion people either have been, or are currently infected, with hepatitis B virus (HBV) and that about 5% of the world's population are chronic carriers. This means that the global frequency of chronic infection with HBV is about 10 times higher than the frequency of infection with the human immunodeficiency virus (HIV). HBV infection is a complex and heterogeneous disease entity that may either resolve spontaneously or manifest itself in a variety of ways. Although HBV preferentially infects hepatocytes, infection of other cells including bile duct epithelium cells, mesangial cells of the kidney, pancreatic islet cells, and lymphoid cells have also been detected. Some or all of these extrahepatic sites may act as virus sanctuaries, protecting HBV from the antiviral activity of drugs and the immune system. HBV infection is rarely, if ever, directly cytopathic and it is believed that pathogenic sequelae of HBV infection are caused by deregulation of host cell gene expression and/or proinflammatory and cytotoxic effects secondary to abortive attempts by the host's own immune system to eliminate infected cells. These indirect effects, rather than viral integration or direct oncogenic potential, most likely account for the substantially increased risks for development of potentially fatal cirrhosis, liver failure, and/or primary hepatocellular carcinoma (HCC) that are associated with HBV infection. The heterogeneity of hepatitis B disease and its slow and variable progression to “real” endpoints (liver failure, cirrhosis, HCC, and death), together with limitations of available diagnostic technology, makes it difficult to define criteria by which the success of different therapeutic strategies can be judged. We can expect that a need to redefine treatment goals will become increasingly evident as the number of treatment options increases.

Detection of elevated levels of liver enzymes including aminotransferases (notably alanine aminotransferase [ALT]) in serum is regarded as a surrogate marker of liver damage and is often the first indicator of HBV infection. Diagnosis of hepatitis B is routinely based on interpretation of subsequent serological assays. During the course of infection, the viral core protein (HBcAg), surface antigens (HBsAg), and the secreted e antigen (HBeAg) all elicit immune responses in immunocompetent hosts, producing the corresponding antibodies (anti-HBc, anti-HBs, and anti-HBe). Assay for serum HBsAg, which is exposed on the surface of the virus envelope as a mosaic of glycoproteins, is a reliable and sensitive diagnostic test, because during infection an excess of HBsAg is secreted into the blood, where it circulates as 22-nm diameter spherical and filamentous particles. Its persistence for longer than 6 months is indicative of chronic infection and the presence of its corresponding antibody (anti-HBs) is recognized as a marker of immunity and disease resolution. The age at which infection is acquired is the main determinant of progression to chronicity. In adults, acute hepatitis B is characterized by active viral replication and very high levels of viremia (~1010 HBV copies per mL) may occur after de novo HBV infection, which usually resolves spontaneously. By contrast, chronic hepatitis B is the common outcome when infection occurs soon after birth or early in life, as it does in hyperendemic areas, where transmission from maternal carriers to their children is the most frequent route of infection. Although the presence of anti-HBs after recovery from HBV infection implies that HBV has been cleared form tissues, recent work shows that this is not always true. Individuals who are seropositive for HBsAg but have no evidence of liver disease often have low level viremia and may harbor occult infection.2, 3 HBeAg status distinguishes 2 subcategories of chronic HBV infection: HBeAg-positive hepatitis B, considered to be the classical form, is characterized by stable high level viremia (107–1010 HBV copies per mL). HBeAg negative hepatitis B is characterized by less stable, less severe viremia (typically <106 HBV copies per mL). The latter is caused by infection with HBV variant(s) that carry mutations in the basal core promoter and/or precore regions of the genome: these reduce or prevent synthesis of the HBe antigen.4

Although viral load is clearly an important determinant of the natural history of infection, virological assessment after serological diagnosis of HBV infection is usually restricted to quantification of viremia by signal or target amplification assays for viral DNA. Several different types of quantitative assays are now available, each using different technologies. They vary widely in detection limits, dynamic ranges, and in the units used to express results, which highlights the need for both assay standardization and for freely available reference standards. In general, signal amplification assays have a poor lower limit of detection, whereas target amplification assays are limited by having a restricted upper limit. The dynamic range restrictions may be overcome with real-time polymerase chain reaction (PCR) assays that are currently undergoing commercial development, but standardization of these assays will still be required.

Recovery from HBV infection is associated with resolution of viremia, HBeAg seroconversion, HBsAg seroconversion, and normalization of ALT. For HBeAg positive chronic hepatitis B, loss of HBeAg and normalization of ALT have been regarded as surrogate markers of successful treatment. For HBeAg-negative hepatitis B, the corresponding treatment endpoint is usually normalization of ALT and suppression of viremia to levels that are undetectable. Achievement of these endpoints does not guarantee long-term, durable improvement in underlying liver disease or favorable long-term prognoses. The introduction of more potent and effective drugs for treatment of HBV infection (see below) will make it necessary to develop more sophisticated monitoring methods to define more precise end-points.

While therapeutic vaccination remains in its infancy, chemotherapy is the only available means to control chronic hepatitis B, but none of the numerous chemotherapeutic strategies tested in the past has proven consistently successful. Indeed, HBV infection has remained a challenge since it was first identified as a major cause of serum hepatitis more than 30 years ago. For more than 20 years, interferon-alpha (IFN-α) has been the only drug licensed for use against chronic HBV infection in most countries. Treatment with IFN-α frequently causes undesirable side effects; its efficacy is limited, it is expensive, and it must be given by injection. Consequently, the need for more efficacious anti-HBV agents has long been evident. Development and introduction of new anti-HBV drugs, in particular nucleoside analogues, has been frustrating for a variety of reasons. Because the HBV genome does not encode virus-specific enzymes for nucleoside salvage, its replication depends on host cell enzymes for supply of DNA precursors.5, 6 In addition, its unusual strategy of replication via an RNA intermediate means that transcription and translation of the RNA intermediate can continue even when reverse transcription and duplication of the resulting DNA transcript are completely blocked by nucleoside analogues or other drugs targeted exclusively at the viral polymerase.7 Initial attempts in the mid-1970s to treat HBV using nucleoside analogues were aborted due to lack of efficacy and/or toxicity,5 a scenario which has now changed and which, it seems, will change even more dramatically in the near future. Two safe and efficacious new anti-HBV drugs: lamivudine (LMV: [-]-β-L-2'-3'-dideoxy-3'-thiacytidine)8 and adefovir dipivoxil, a prodrug for the deoxyadenosine monophosphate analogue adefovir (ADV; 9-[2-phosphonylmethoxyethyl]-adenine)9 are now approved by the United States Food and Drug Administration for treatment of chronic HBV infection. Several other drugs, including entecavir (ETV; [1S-(1,3,4)]-2-amino-1,9-dihydro-9[4-hydroxy-3-(hydroxymethyl)-2-methylenecyclopentyl]-6H-purin-6-one), clevudine (L-FMAU; 1-[2'-fluoro-5-methy-β-L-arabinofuranosyl]uracil), emtricitabine ([-]-β-L-2',3'-dideoxy-5-fluoro-3'-thiacytidine; FTC), LY582563 (2-amino-9-[2 (phosphonomethoxy)ethyl]-6-(4-methoxyphenylthio)purine bis[2,2,2-trifluoroethyl]ester), and β-L-thymidine have progressed to advanced phase II or phase III clinical trials. Although all the new drugs listed above are nucleoside or nucleotide analogues, development of a smaller number of nonnucleoside analogue inhibitors of HBV replication is also in progress.7

Treatment of HBV-infected individuals with either lamivudine or adefovir dipivoxil consistently produces rapid and dramatic decreases in viremia and a small proportion (12%–25%) show loss of HBeAg and normalization of serum ALT, frequently accompanied by improvements in liver histology. Preliminary reports indicate that although all the newer anti-HBV drugs listed above appear to be capable of producing roughly comparable effects, the dramatic initial response is always followed by a much slower elimination of residual virus,10, 11, 12, 13 and HBeAg seroconversion is relatively infrequent. Drug-resistant HBV appears almost invariably during this second phase during lamivudine treatment, but adefovir dipivoxil has been shown to retain activity against lamivudine-resistant HBV.14 Although it has yet to be reported, current paradigms of drug resistance predict that development of in vivo resistance to adefovir dipivoxil and other newer anti-HBV agents is probably inevitable.

Both lamivudine and adefovir were first licensed for use as anti-HIV agents and their anti-HBV activity was initially observed in individuals who were coinfected with HIV and HBV. By contrast, entecavir, a deoxyguanosine analogue, has not previously been used clinically and appears to be essentially inactive against viral pathogens other than HBV.15 Like adefovir dipivoxil, it is active against lamivudine-resistant HBV, at least in vitro.15 In this issue of GASTROENTEROLOGY, Lai et al.16 report the results of a 24-week, multicenter, double-blind randomized phase II clinical trial of entecavir, the safety and efficacy of which had already been demonstrated in an earlier short-term trial.17 They treated a large cohort of hepatitis B patients including both HBeAg negative and HBeAg positive cases, with entecavir, given orally at 1 of 3 different doses (0.01 mg/day, 0.1 mg/day, or 0.5 mg/day). Efficacy was compared with that of lamivudine at the usual dose of 100 mg/day, using end-of-treatment reduction in viremia as the primary endpoint. Entecavir was well tolerated and, at the 2 higher doses, was significantly more effective than lamivudine at reducing the viral load by the end of week 22. At this time, more patients in the entecavir-treated groups than in the lamivudine-treated group also showed normalization of serum ALT, although the difference was not statistically significant. On the basis of these results, entecavir is clearly an extremely potent anti-HBV drug, particularly considering that the doses required to produce comparable effects are 20–100-fold less than for adefovir dipivoxil and 200–1000-fold less than for lamivudine. Entecavir's remarkable potency may perhaps be explained by the preference of the HBV polymerase for entecavir triphosphate (ETV-TP) over the natural substrate deoxyguanosine triphosphate and by the ability of ETV-TP to inhibit priming of reverse transcription by the HBV polymerase, as well as its RNA- and DNA-dependent DNA polymerase activities.15 Long-term trials in woodchucks infected with the woodchuck hepatitis virus demonstrated that treatment with entecavir can produce a profound (~4 log10) and sustained suppression of viral replication. It also reduced the incidence of HCC and prolonged survival in this animal model, which is regarded as a good model for HBV-related human HCC.18 Interestingly, in both the duck and woodchuck models of HBV, the effects of entecavir on tissue viral DNA and the key replicative intermediate, viral covalently closed circular (ccc) DNA, have also been assessed. In the short-term treatment of ducks, substantially reduced levels of viral ccc DNA were observed compared with controls.19 In long-term treated woodchucks, the reduction of viral ccc DNA to below the level of detection was consistent with a sustained virological response in some of the animals.18 HBV ccc DNA is generated in the host cell nucleus, where it associates with cellular histones to form a viral minichromosome.20 Current opinion suggests that recovery from chronic infection will require complete elimination of viral minichromosomes, but their eradication by chemotherapy has proven to be exceptionally difficult. Immune system activity has been shown to be responsible for their removal during HBV infections which resolve, so it may be that the role of chemotherapy in chronic infection will be to reduce the viral burden sufficiently to allow the host's immune response to complete the task of elimination.

This raises the issue of how the efficacies of the new anti-HBV agents and therapeutic strategies can be assessed and compared, because by existing criteria, at least one index of success (decrease in serum HBV DNA to undetectable levels) is frequently achieved soon after initiating treatment. During an address at the recent EASL International Consensus Conference on Hepatitis B in Geneva, Dr. Jay Hoofnagle proposed the adoption of an hierarchical system of chronological end-points. According to the proposed scheme, responses that occur within 6 months of starting treatment could be arbitrarily defined as initial responses, which would distinguish them from end-of-treatment responses. Irrespective of treatment duration, the latter are often reported as evidence for antiviral efficacy in short- and medium-term trials of new agents or therapeutic strategies. Since even the most successful of the newly developed anti-HBV drugs are necessarily virustatic rather than virucidal (because resistance may develop during long-term treatment), end-of-treatment responses are not reliable predictors of long-term prognoses. Demonstration of the safety and efficacy of new antiviral agents should require production of evidence of a maintained response, which could be defined as suppression of markers of viral replication below critical levels beyond the duration of treatment. In longer-term studies, posttreatment responses maintained for 6, 8, or 12 months have been used to define a successful outcome: these could be defined as sustained responses. Because relapses can still occur more than 12 months after apparently successful anti-HBV therapy has been stopped, it seems desirable to distinguish sustained responses lasting for 12 or more months after treatment as durable responses.

Experience with HIV and hepatitis C virus (HCV) infections indicates that monitoring viral load will play an important role in quantifying responses. Pathogenesis of infection with HBV, HIV, and HCV is characterized by a dynamic equilibrium between virus replication and clearance, which is unbalanced by introduction of antiviral therapy.21 Mathematical models by which the outcome of antiviral treatment can be monitored or predicted have been developed for HIV, HCV, and more recently, for HBV.10, 21, 22 Each shows an initial (first phase) rapid decline in viremia after introduction of successful antiviral therapy, followed by a second much slower elimination phase.10, 11, 12, 13, 21, 22 Most investigators interpret the first phase as representing viral clearance resulting from the antiviral replication block and the second phase being correlated with death and clearance of the infected cell population. Many assumptions are implicit in these interpretations. In the simplest terms, the amplitude and rate of decline of viremia during the first phase provides a measure of the effectiveness of the antiviral agent(s). The amplitude and rate of the second, slower phase is presumably mainly determined by death of infected cells caused by aging and/or elimination by the host's immune system. In reality, the situation during HBV infection is undoubtedly more complex.10, 21, 22 The rate of virus production and turnover (and hence clearance) varies enormously depending on whether infection is acute or chronic and whether the phenotype is HBeAg-positive or HBeAg-negative. Not surprisingly, estimates of the half-lives of infected cells are similarly variable. Despite variation, it has become clear that first phase decreases in viral load to below 104 copies per mL during lamivudine treatment is significantly associated with subsequent HBeAg seroconversion.23 Moreover, low viral load has been shown to be significantly associated with longer survival in HCC patients in both HBeAg-positive and HBeAg-negative patients24 and with more favorable prognoses in HBsAg-positive transplant patients.25

How can the amplitude and rate of the first phase elimination be accelerated? Experience with anti-HIV therapy supplies clues. The rather optimistically named highly active anti-retroviral therapy (HAART) combination chemotherapy regimes for control of HIV replication, which were introduced in 1995 have resulted in significantly improved prognoses. The basis for the introduction of HAART was recognition of the improbability that any single drug would be capable of permanently controlling or eliminating chronic HIV (or HBV) infection and that development of viral resistance to new drugs is probably inevitable.26, 27 HAART regimes typically require concurrent treatment with at least 3 antiviral agents and are based on a “combinatorial ledge” rationale.27 This is a theoretical argument that the high viral loads that occur in HIV (and HBV) infected individuals will ensure that although viral quasispecies that are potentially resistant to 1 or 2 drugs will almost certainly already be present before treatment begins, pre-existence of viral quasispecies that are able to resist combinations of 3 or more drugs is very unlikely. As more anti-HBV drugs become available, there can be little doubt that introduction of combination therapy for treatment of chronic hepatitis B is a logical progression,6, 21 which raises yet more new issues. Which drug combinations will be optimal, and how can the relative efficacies of different combinations be compared? By extrapolation of results from animal models, a rational argument can be made for use of combinations of nucleoside/nucleotide analogues and immunomodulators.21 Extension of this reasoning suggests that because the antiviral effects of the latter are typically delayed, optimal responses may be achieved by withholding treatment with nucleoside analogue(s) until maximal response to immunomodulators is achieved.21 However, because only a minority of patients respond favorably to immunomodulators, it will probably be necessary to use combinations of 2 or more nucleoside/nucleotide analogues to treat most cases of chronic HBV infection. Continuing studies of viral dynamics using the new quantitative PCR techniques will be required to determine which combinations are optimal. Objective comparisons would be facilitated by the adoption of a system of chronological endpoints as suggested by Dr. Hoofnagle. Initial and end-of-treatment responses would be measured mainly in terms of rates and magnitudes of the first and second phase reduction in viremia respectively, and maintained and sustained responses would be measured in terms of the duration of antiviral effects including clearance of viral antigens. Criteria defining durable responses would include HBsAg (anti-HBs) seroconversion and clearance of viral ccc DNA from the liver. Eventually, assays for HBV DNA in the liver and other tissues will probably be used to monitor and predict treatment efficacy, but they are not yet sufficiently developed for routine use. Similarly, the possible use of intrahepatic viral ccc DNA measurements as “real” treatment endpoints bears serious consideration, although significant problems related to assay validation and standardization need to be overcome before this can become a reality.28 Assuming that future research will eventually produce solutions to the problem of eradication of intrahepatic HBV ccc DNA (presumably by optimizing combination therapies), would this lead to truly durable responses that provide improved quality of life and extended life expectancy, and could the achievement of this goal be regarded as a real “cure”? Only time will tell.

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☆ Address requests for reprints to: Stephen Locarnini, M. D., Victorian Infectious Diseases Reference Laboratory (VIDRL), Locked Bag 815, Carlton South, Victoria 3053 Australia. e-mail: stephenlocarnini@compuserve.com; fax: (61) 3 9342-2666.

☆☆ Supported by Eli Lilly, Medivir, Innogenetics, BMS, Gilead Sciences, Novirio (Idenix), Pharmasset, CSL, and Triangle Pharmaceuticals.

PII: S0016-5085(02)00483-3

doi:10.1053/gast.2002.37288

Refers to article:

  • Entecavir is superior to lamivudine in reducing hepatitis B virus DNA in patients with chronic hepatitis B infection

    Ching–Lung Lai, Mohamed Rosmawati, Judy Lao, Hans Van Vlierberghe, Frank H. Anderson, Neal Thomas, Deborah Dehertogh
    Gastroenterology December 2002 (Vol. 123, Issue 6, Pages 1831-1838)

Gastroenterology
Volume 123, Issue 6 , Pages 2135-2140, December 2002

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