ReviewHBV drug resistance: Mechanisms, detection and interpretation
Introduction
Besides interferon, only two drugs—lamivudine and adefovir dipivoxil—are approved as first-line therapy for chronic hepatitis B (CHB) in the EU [1], [2]. Each is a nucleoside or nucleotide analogue that acts mainly as a specific inhibitor of the viral polymerase/reverse transcriptase (for convenience, the acronym Nucleos(t)ide Reverse Transcriptase Inhibitor (NRTI) will be used here to refer all nucleoside and nucleotide derivatives that act by inhibiting functions of the HBV polymerase). Several other NRTIs besides lamivudine and adefovir dipivoxil have been found to be effective against HBV [1]. They include entecavir, which is licensed in the USA [3], and tenofovir, which is used to treat HIV-1 infection [4], [5]. In most cases, treatment of CHB with any single NRTI produces rapid suppression of HBV replication in the short-term, an effect that is not often sustainable due to the emergence of drug-resistant HBV strains [1], [6]. Although a variety of other factors including adverse short-term side effects, long-term toxicity, previous sub-optimal treatment regimes, inadequate drug exposure (due to pharmacological properties of particular drugs, poor patient compliance with prescribed treatment, or host genetic polymorphisms) influence the efficacy of treatments for chronic HBV, drug resistance is emerging as the single most significant factor in treatment failure [7]. Failure of NRTI treatment presents a significant clinical challenge, because remaining treatment options are limited [2], [8]. Here, we attempt to summarise the current state of knowledge of HBV resistance to NRTI, briefly describe available methods for detecting and quantifying drug resistance and discuss the interpretation of drug resistance and its clinical applications.
Resistance can be defined in technical, virological or clinical terms. Technically, resistance is defined by either (1) a statistically significant change in the test parameter (increase in EC50, for example) or (2) the minimum change in a parameter that is required for statistical significance (for example, a single log10 change). Technical definitions only describe the attributes of specific tests and have no biological or clinical significance.
Virological resistance is now commonly described as high-, intermediate- or low-level based on fold-changes in EC50 in vitro. Such ranking, unfortunately, does not relate directly to clinical observations. For example, lamivudine failure is associated with >100 fold increase in EC50 in vitro, but HBV isolates responsible for adefovir resistance exhibit only low-level (<10-fold increase in) resistance in vitro.
Clinical resistance is most easily defined in terms of viral load fluctuations (based on results of assays for serum HBV DNA), which are currently the best available indicators of HBV replication in vivo. The following definitions have been proposed as a basis for further development [8]. An antiviral effect is defined as a minimum reduction in serum HBV DNA of 1 log10 IU/mL from the pre-treatment baseline within the first 3 months. Failure to achieve this decrease constitutes primary treatment failure. A confirmed increase in serum HBV DNA of 1 log10 IU/mL from the nadir following initially effective treatment constitutes secondary treatment failure. Genotyping and/or phenotyping of clinical isolates are required to confirm that treatment failure is due to resistant virus (see Fig. 1, Fig. 2).
Section snippets
Resistance to NRTI
Resistance and cross-resistance to NRTIs is more or less structure-specific. Anti-HBV NRTIs can be separated into three main structural groups (see Fig. 3 and Table 1).
- (1)
l-Nucleoside analogues, including lamivudine, its 5-fluoro-derivative emtricitabine, telbivudine (l-thymidine), torcitabine (l-deoxycytidine) and clevudine (l-FMAU).
- (2)
Acyclic nucleoside phosphonates, represented by the dAMP analogues adefovir and tenofovir, together with others that have reached late stage clinical trials.
- (3)
Molecular modelling identifies mechanisms for resistance to NRTI
Laboratory studies of mechanisms for HBV resistance to NRTI have been frustrated by the inability to obtain sufficient quantities of purified polymerase. However, it has been possible to create three-dimensional models of the reverse transcriptase (rt) region of HBV polymerase have based on its homology with related polymerases, including HIV-1 rt. Using these models, the amino acid changes resulting from mutations that confer antiviral resistance can be mapped to functional regions to provide
Detection of drug resistance
A variety of different methods for detection, and in some cases, quantification, of drug resistance in HBV are now available, but they need to be improved and standardized, as interpretation and comparison of drug resistance data obtained by different methods is currently difficult and contentious. Drug resistant HBV can be detected by genotyping and/or phenotyping. Only the latter provides direct measurements of in vitro resistance to specific drugs, but each approach has inherent advantages
Interpretation of genotypes and phenotypes
Prediction of phenotypes from genotypes is likely to be difficult when multiple resistance and compensatory mutations are present, a situation that is common in HBV isolates from individuals who have been treated for long periods with a variety of antiviral drugs. Rather than being dichotomous, antiviral resistance and susceptibility are extremes of a continuous variable that is modified by, and modifies, replication fitness. The major advantage of phenotyping is that it provides quantitative
Resistance testing for treatment optimisation
As more drugs become available and databases such as SeqHepB continue to expand, resistance testing is likely to guide therapeutic decision-making for all treatment-experienced CHB patients [36]. The probability of primary antiviral resistance occurring by transmission, although low, is not negligible, so suppressing apparently ‘benign’ replication-deficient multi-drug resistant mutants in experienced patients may be necessary. Designing new regimes that have the greatest chance of achieving
Preventing drug resistance and management of NRTI-resistant infection
There are three main ways to minimise development and spread of drug resistance: (1) avoiding unnecessary therapy, (2) careful choice therapy and (3) continuous surveillance for drug resistance.
Conclusions
Ideally, treatment for HBV infection should begin at diagnosis, but this is not yet feasible because of limitations of existing drugs. Ongoing clinical trials and concurrent improvements in diagnostic technology guarantee that treatment options and opinions on patient management will continue to evolve. Problems caused by cross-resistance may eventually be solved by the introduction of antiviral drugs that block stages of the viral life cycle distinct from those inhibited by NRTIs, but such
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