ReviewMechanism of viral persistence and resistance to nucleoside and nucleotide analogs in chronic Hepatitis B virus infection
Introduction
Despite the availability of an efficient vaccine, chronic Hepatitis B virus (HBV) infection remains a major public health problem worlwide. Indeed, according to the World Health Organization, more than 400 million people are chronic carriers of the virus, and more than one billion have been in contact with HBV (Lee, 1997). Chronic carriers are exposed to the complications of the disease which include the development of chronic hepatitis, liver cirrhosis, and hepatocellular carcinoma (Ganem and Prince, 2004). The latter, usually develops after several decades of infection, but represents a major clinical problem in geographical areas where HBV is transmitted vertically.
To combat or prevent these complications, several strategies are available. With respect to prophylaxis, HBV vaccine is safe and efficient. In endemic areas, it was shown that mass vaccination targeting not only adults but also infants was able to decrease the prevalence of HBV infection and the incidence of hepatocellular carcinoma (Chang et al., 1997). In patients who are already chronic carriers, the goals of antiviral therapy are to control viral replication and decrease liver damage to prevent the development of liver cirrhosis and subsequent hepatocellular carcinoma. In clinical practice, treatment relies mainly on the use of IFN alpha, or nucleoside analogs (Hoofnagle and Di Bisceglie, 1997), such as lamivudine or adefovir dipivoxil. However, results of meta-analysis of IFN clinical trials showed that only a minority of patients are long-term responders (approximately 20%) (Wong et al., 1993). Its antiviral effect is also limited by the numerous side effects of this treatment. On the other hand, nucleoside analogs are well tolerated and exhibit an early and potent antiviral effect which is limited by the selection of resistant mutants during long-term therapy (Villeneuve et al., 2003, Zoulim, 2002). Therefore, antiviral therapy of chronic hepatitis B remains a clinical challenge.
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The disease and the study models
As HBV replication does not lead by itself to a cytopathic effect, chronic hepatitis B is the result of the ongoing liver injury mediated by the T cell immune response targeting hepatocytes supporting a persistent HBV replication (Bertoletti and Naoumov, 2003, Ganem and Prince, 2004). Our knowledge of the pathobiology of HBV infection has come from clinical studies and from experimental investigations performed in animal models. HBV belongs to the hepadnaviridae family that include animal
Viral replication and persistence
HBV mainly infects hepatocytes, but other cell types including biliary epithelial cells, pancreas, kidney, skin, spleen, bone marrow, peripheral blood mononuclear cells, have been shown to harbor viral genome sequences and to support HBV expression to some extents. Whether these cells represent an extrahepatic reservoir for infectious HBV particle production remains controversial. The HBV replication cycle has been reviewed in detail (for further references, see Ganem and Prince, 2004, Seeger
Mechanisms of spontaneous or antiviral induced viral clearance
The mechanisms of viral clearance have been studied in detail by clinical studies as well as by the use of experimental models. Viral clearance requires the coordinated action of several components of the immune response (Bertoletti and Naoumov, 2003): (1) a cytotoxic TH1 response by CD8 positive cells recognizing infected hepatocytes expressing viral antigens; (2) a non-cytolytic TH1 response whereby the CD8 positive cells produce TH1 cytokines such as IFN gamma, TNF alpha, Interleukin 12 that
Antiviral therapy with nucleoside analogs
The primary mechanism of action of nucleoside analogs is to inhibit viral polymerase activity. The structure of the compounds and their molecular mechanism of action have been reviewed recently (De Clercq, 1999, De Clercq, 2001). Fig. 3 shows their site of action during the viral genome synthesis process.
Lamivudine inhibits viral reverse transcriptase activity, i.e. elongation of viral minus strand DNA, as it is a competitive inhibitor of its natural substrate, dCTP. Furthermore, after its
Mechanism of viral drug resistance
The selection of drug resistant mutants depends on several factors (Fig. 2). Viral persistence is the result of both the long half-life of hepatocytes because of a defective immune response against infected cells, and the persistence of viral cccDNA in infected cells. As the viral polymerase is subjected to a spontaneous error rate, viral mutants are generated and accumulate during the natural history of the disease. The more viable variants in the context of the environmental pressure
Phenotypic analysis of HBV clinical isolates
In the case of HIV therapy, drug resistance testing is now recommended to guide the choice of new drug regimens after the first or multiple treatment failures. In addition to genotypic assays, several phenotypic assays have been developed for HIV and are currently used in clinical practice to monitor drug resistance.
Until recently, no phenotypic drug susceptibility assay had been developed for HBV because of the small number of drug used in clinic, that rendered such assays unnecessary yet and
Prevention of and combating drug resistance
The rationale for combination therapy relies on the following evidence that are based on experimental findings in the hepadnavirus models and on HIV therapy experience (Richman, 2000): (1) simple mutants pre-exist as shown by longitudinal studies of viral polymerase gene sequence, (2) genetic variants harboring multiple mutations have less chance to pre-exist or to occur, (3) re-treatment leads to a rapid re-emergence of resistant mutants although wild type virus re-emerges after first
Perspective
The better knowledge of viral replication should allow the development of new therapeutic concepts (Fig. 1). In this view, novel inhibitors of viral replication are being evaluated in experimental systems. These includes myristilated pre-S1 peptides to compete with the interaction of the virion envelope proteins and the hepatocyte receptor (Urban and Gripon, 2002), peptides or recombinant core proteins exhibiting a trans dominant negative effect on nucleocapsid assembly (von Weizsacker et al.,
Acknowledgement
This work was supported by fundings from the European Community (contract QLRT2001-00977) and was presented in part in the William Prusoff award lecture, at the ICAR, Tucson, May 2004.
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