Novel patterns of amino acid mutations in the hepatitis B virus polymerase in association with resistance to lamivudine therapy in Japanese patients with chronic hepatitis B

Coevolution analysis of amino-acids reveals diversified drug-resistance solutions in viral sequences: a case study of hepatitis B virus

The study of mutational landscapes of viral proteins is fundamental for the understanding of the mechanisms of cross-resistance to drugs and the design of effective therapeutic strategies based on several drugs. Antiviral therapy with nucleos(t)ide analogues targeting the hepatitis B virus (HBV) polymerase protein (Pol) can inhibit disease progression by suppression of HBV replication and makes it an important case study. In HBV, treatment may fail due to the emergence of drug-resistant mutants. Primary and compensatory mutations have been associated with lamivudine resistance, whereas more complex mutational patterns are responsible for resistance to other HBV antiviral drugs. So far, all known drug-resistance mutations are located in one of the four Pol domains, called reverse transcriptase. We demonstrate that sequence covariation identifies drug-resistance mutations in viral sequences. A new algorithmic strategy, BIS2TreeAnalyzer, is designed to apply the coevolution analysis method BIS2, successfully used in the past on small sets of conserved sequences, to large sets of evolutionary related sequences. When applied to HBV, BIS2TreeAnalyzer highlights diversified viral solutions by discovering thirty-seven positions coevolving with residues known to be associated with drug resistance and located on the four Pol domains. These results suggest a sequential mechanism of emergence for some mutational patterns. They reveal complex combinations of positions involved in HBV drug resistance and contribute with new information to the landscape of HBV evolutionary solutions. The computational approach is general and can be applied to other viral sequences when compensatory mutations are presumed.

Risk factors for resistance development against lamivudine during long-term treatment of chronic hepatitis B virus infections

BACKGROUND/AIM

The use of lamivudine for the treatment of chronic hepatitis B (CHB) is limited by high rates of lamivudine resistance. However, it is still in use in many regions. Factors associated with lamivudine resistance development have been studied in only a few European cohorts. The aim of our study was to assess the rate and risk factors for lamivudine resistance in a large real-life European cohort.

PATIENTS AND METHODS

We retrospectively analyzed patients with CHB treated in three German University centers over up to 12 years. Lamivudine resistance was defined as virologic breakthrough and presence of genotypic lamivudine resistance. The probability of resistance was estimated by Kaplan-Meier analysis and resistance predictors by Cox regression.

RESULTS

A total of 227 patients were included into the analysis (hepatitis B envelope antigen positive or negative). Rates of lamivudine resistance by years 1-7 were 7, 26, 35, 41, 46, 53, and 55%, respectively. Interestingly, two hepatitis B envelope antigen-negative patients developed resistance during the year 12 of treatment. Independent risk factors for resistance development were hepatitis B virus DNA levels of at least 10 copies/ml before and detectable hepatitis B virus DNA by month 6 of treatment.

CONCLUSION

Even after long-term response to lamivudine more than 10 years, resistance may still develop. Our findings further discourage the use of lamivudine for the treatment of CHB.

Inhibitory effect of Phyllanthus urinaria L. extract on the replication of lamivudine-resistant hepatitis B virus in vitro

BACKGROUND

Long-term treatment of chronic hepatitis B (CHB) with nucleos(t)ide analogs results in the emergence of drug-resistant hepatitis B virus (HBV) harboring mutations in the polymerase (P) gene. The Phyllanthus extract has anti-HBV activity; however, its antiviral activity against lamivudine (LMV)-resistant mutants has not been examined.

METHODS

HBV harboring LMV-resistant mutations (rtM204I, rtM204V, and rtM204S) in the P gene at the YMDD ((203)tyrosine-methionine-aspartate-aspartate(206)) reverse transcriptase (RT) active site were generated and their sensitivity to Phyllanthus urinaria koreanis extract examined. Southern blotting and real-time PCR were used to determine the concentration of plant extract required to inhibit HBV DNA synthesis by 50 and 90% (EC50 and EC90, respectively). An enzyme-linked immunosorbent assay was used to measure the EC50 of HBV surface antigen (HBsAg) and HBV core antigen (HBcAg) secretion, and the 50% cytotoxic concentration of the extract was measured in a 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay. Real-time RT-PCR was used to measure mRNA expression levels.

RESULTS

The expression of intracellular HBV DNAs in HBV WT- or mutant-transfected HepG2 cells decreased upon treatment with Phyllanthus extract. The secretion of HBsAg and HBcAg also fell in a dose-dependent manner. Phyllanthus extract induced interferon-beta (IFN-β), cyclooxygenase-2 (COX-2), and interleukin-6 (IL-6) mRNA expression in HBV WT-transfected HepG2 cells, possibly via activation of extracellular signal-regulated kinases and c-jun N-terminal kinases and the induction of retinoic acid inducible gene-I, toll-like receptor 3, myeloid differentiation primary response gene 88, and/or tumor necrosis factor receptor-associated factor 6 gene expression. HBV transfection in the absence of extract or exposure of cells to extract alone did not trigger these signaling cascades.

CONCLUSIONS

Phyllanthus extract inhibited HBV DNA synthesis and HBsAg and HBcAg secretion by replicating cells harboring HBV wild-type and LMV-resistant mutants, likely by inducing the expression of IFN-β, COX-2, and IL-6. These data indicate that Phyllanthus extract may be useful as an alternative therapeutic agent for the treatment of drug-resistant CHB patients.

Prevelance of common YMDD motif mutations in long term treated chronic HBV infections in a Turkish population

In the current study we aimed to show the common YMDD motif mutations in viral polymerase gene in chronic hepatitis B patients during lamivudine and adefovir therapy. Forty-one serum samples obtained from chronic hepatitis B patients (24 male, 17 female; age range: 34-68 years) were included in the study. HBV-DNA was extracted from the peripheral blood of the patients using an extraction kit (Invisorb, Instant Spin DNA/ RNA Virus Mini Kit, Germany). A line probe assay and direct sequencing analyses (INNO-LIPA HBV DR v2; INNOGENETICS N.V, Ghent, Belgium) were applied to determine target mutations of the viral polymerase gene in positive HBV-DNA samples. A total of 41 mutations located in 21 different codons were detected in the current results. In 17 (41.5%) patients various point mutations were detected leading to lamivudin, adefovir and/ or combined drug resistance. Wild polymerase gene profiles were detected in 24 (58.5%) HBV positive patients of the current cohort. Eight of the 17 samples (19.5%) having rtM204V/I/A missense transition and/or transversion point mutations and resistance to lamivudin. Six of the the mutated samples (14.6%) having rtL180M missense transversion mutation and resistance to combined adefovir and lamivudin. Three of the mutated samples (7.5%) having rtG215H by the double base substituation and resistance to adefovir. Three of the mutated samples (7.5%) having codon rtL181W due to the missense transversion point mutations and showed resistance to combined adefovir and lamivudin. Unreported novel point mutations were detected in the different codons of polymerase gene region in the current HBV positive cohort fromTurkish population. The current results provide evidence that rtL180M and rtM204V/I/A mutations of HBV-DNA may be associated with a poor antiviral response and HBV chronicity during conventional therapy in Turkish patients.

Optimal management of chronic hepatitis B patients with treatment failure and antiviral drug resistance

The management of treatment failure in patients with chronic hepatitis B, remains a clinical concern. Incomplete viral suppression and the emergence of drug resistance are key determinants of treatment failure. The correct choice of a potent first-line therapy to achieve sustained long-term suppression of viral replication provides the best chance of preventing treatment failure and drug resistance. Clinical studies have demonstrated that drugs with a high barrier to resistance have significantly lower rates of resistance compared with those with a low barrier to resistance. Management of treatment failure requires precise clinical and virological monitoring as well as early treatment intervention with appropriate noncross-resistant antivirals. Long-term surveillance of treatment efficacy and possible emergence of drug resistance is necessary in patients who have been sequentially treated with multiple antivirals. The identification of novel treatment targets remains a major research goal to improve the efficacy of current antiviral therapy through combination therapy regimens.

Drug-related mutational patterns in hepatitis B virus (HBV) reverse transcriptase proteins from Iranian treatment-naïve chronic HBV patients

BACKGROUND

Immunomodulators and Nucleotide analogues have been used globally for the dealing of chronic hepatitis B virus (HBV) infection. However, the development of drug resistance is a major limitation to their long-term effectiveness.

OBJECTIVES

The aim of this study was to characterize the hepatitis B virus reverse transcriptase (RT) protein variations among Iranian chronic HBV carriers who did not receive any antiviral treatments.

MATERIALS AND METHODS

Hepatitis B virus partial RT genes from 325 chronic in active carrier patients were amplified and directly sequenced. Nucleotide/amino acid substitutions were identified compared to the sequences obtained from the database.

RESULTS

All strains belonging to genotype D.365 amino-acid substitutions were found. Mutations related to lamivudine, adefovir, telbivudine, and entecavir occurred in (YMDD) 4% (n = 13), (SVQ) 17.23% (n = 56), (M204I/V + L180M) 2.45% (n = 8) and (M204I) 2.76% (n = 9) of patients, respectively.

CONCLUSIONS

RT mutants do occur naturally and could be found in HBV carriers who have never received antiviral therapy. However, mutations related to drug resistance in Iranian treatment-naïve chronic HBV patients were found to be higher than other studies published formerly. Chronic HBV patients should be monitored closely prior the commencement of therapy to achieve the best regimen option.

Management of treatment failure in chronic hepatitis B

Antiviral therapy of chronic hepatitis B remains a clinical challenge. The primary goal of therapy is to prevent liver disease progression. Because of the mechanism of viral persistence in infected hepatocytes, long-term antiviral therapy is needed in the majority of patients. Incomplete viral suppression and emergence of drug resistance is a major concern. The correct choice of a first-line potent therapy to achieve sustained long-term suppression of viral replication provides the best chance of preventing treatment failure and drug resistance. Clinical studies have demonstrated that drugs with a high barrier to resistance, such as entecavir and tenofovir, have significantly lower rates of resistance when compared with those with a low barrier to resistance such as lamivudine, adefovir, or telbivudine. Management of treatment failure requires a precise clinical and accurate virologic monitoring as well as an early treatment intervention with appropriate complementary drugs with respect to their cross-resistance profile. Long-term surveillance for treatment efficacy and possible emergence of drug resistance is necessary for those patients who have been sequentially treated with multiple antivirals. Finally, the identification of novel treatment targets remains a major research challenge to improve the efficacy of current antiviral therapy.

Antiviral therapies: focus on hepatitis B reverse transcriptase

Hepatitis B virus (HBV) is the etiologic agent of mankind's most serious liver disease. While the availability of a vaccine has reduced the number of new HBV infections, the vaccine does not benefit the approximately 350 million people already chronically infected by the virus. Most of the drugs approved by the FDA for the treatment of hepatitis B target the reverse transcriptase (RT or P gene product) and are nucleoside RT inhibitors (NRTIs) that suppress viral replication. However, prolonged monotherapies directed against a single target result in the emergence of viral resistance. HBV genotypic differences affect NRTI resistance, and because the reading frames of the S (surface antigen) and P genes partially overlap, genomic differences that affect the surface of the virus may also alter the viral polymerase sequence, function and drug susceptibility. The scope of this review is to assess the effects of HBV genotypic variation on the development of drug resistance to NRTIs. Some RT residues that vary among different genotypes are in the vicinity of residues that mutate and give rise to NRTI resistance. Interactions between these amino acids can help explain the effect of HBV genotype on the development of NRTI resistance during antiviral therapies, and might help in the design of improved therapeutic strategies.

The influence of YMDD mutation patterns on clinical outcomes in patients with adefovir add-on lamivudine combination treatment

BACKGROUND/AIM

The aim of this study was to assess the patterns of lamivudine (LAM)-resistant mutations and the influence on biochemical and virological responses to adefovir (ADV) add-on LAM combination therapy in patients with LAM-resistant chronic hepatitis B (CHB).

METHODS

Seventy-eight CHB patients with confirmed genotypic resistance to LAM, who initiated ADV add-on LAM combination treatment, were enrolled at our institution between April 2007 and April 2009.

RESULTS

The baseline tyrosine-methionine-aspartate-aspartate (YMDD) mutation patterns were as follows: rtM204I 45 (57.7%); and rtM204V + rtM204I/V 33 (42.3%). The decrease in the mean ± standard deviation (SD) serum log(10) HBV-DNA level did not differ between the patients carrying the rtM204I vs. rtM204IV +rtM204I/V mutations at 3, 6 and 12 months after the initiation of ADV add-on LAM combination treatment. The proportion of patients who achieved ALT normalization (<40 IU/L) 12 months after the initiation of ADV add-on LAM combination treatment were significantly higher in patients with a rtM204I mutation than rtM204V+ rtM204I/V mutations (39 [86.7%] vs. 22 [66.7%], P = 0.05). The proportion of patients in whom the log(10) HBV-DNA decreased <2 log(10) copies/ml, 6 months after the initiation of ADV add-on LAM combination treatment (non-responders), was significantly higher in patients with a rtM204V + rtM204I/V mutations than rtM204I mutation (7 [21.2%] vs. 2 [4.4%], P = 0.032).

CONCLUSION

Biochemical response at 12 months from baseline was better in patients with a rtM204I mutation than rtM204V+ rtM204I/V mutations. In addition, early treatment failure was more common in patients with rtM204V+ rtM204I/V mutations than a rtM204I mutation.

rtL180M mutation of hepatitis B virus is closely associated with frequent virological resistance to adefovir dipivoxil therapy

BACKGROUND AND AIM

We intended to investigate the effects of pre-existing mutations at reverse transcriptase region of hepatitis B virus (HBV) on the occurrence of virological breakthrough (VB) to adefovir dipivoxil (ADV) in patients with lamivudine (LAM)-resistant chronic hepatitis B (CHB).

METHODS

Ninety-seven patients with LAM-resistant CHB were treated with ADV at a dose of 10 mg daily, and were followed for a median period of 13 months. Just before the initiation of ADV therapy, the whole length of reverse transcriptase region of serum HBV-DNA was sequenced using direct sequencing.

RESULTS

All patients had genotype C HBV and mutations in the YMDD motif, specifically, YIDD (65%), YVDD (28%), or both (7%). The rtL180M and rtL80V/I mutations were identified in 68% and 69%, respectively. The cumulative probability of VB was 19% and 27% at 1 and 2 years, respectively. There was no difference in the occurrence of VB with regard to types of YMDD mutation or rtL80V/I. However, interestingly, patients carrying rtL180M experienced VB during ADV monotherapy more frequently than those not carrying rtL180M (2-year cumulative probability of VB: 37% vs 3% at 2 years, P < 0.01). On multivariate Cox proportional hazards analysis, rtL180M (hazard ratio [HR]: 8.62, 95% confidence interval: 1.08-69.09, P = 0.042) and decrease in HBV-DNA for 1 year of treatment (HR: 0.69, 95% CI: 0.51-0.95, P = 0.024) are independently associated with VB.

CONCLUSIONS

The rtL180M mutation of HBV, as well as a small decrease in HBV-DNA after 1 year of treatment might be closely associated with frequent occurrence of virological resistance to ADV in patients with LAM-resistant CHB.

Hepatitis B virus drug resistance to current nucleos(t)ide analogs: Mechanisms and mutation sites

Nucleos(t)ide analogs (NAs) have become the mainstream drugs for the treatment of chronic hepatitis B virus infection. Drug resistance to NAs, however, has posed a major obstacle in obtaining sustained viral suppression. Standardized definitions of terms and nomenclature in discussing NAs resistance have been proposed. Drug resistance to NAs is produced by a combination of viral, host and antiviral drug factors. A detailed understanding of the mechanisms and effects of mutation sites that cause resistance to NAs is important for the design of rational treatment and management of patients with existing drug resistance.

Development and validation of a hepatitis B virus DNA sequencing assay for assessment of antiviral resistance, viral genotype and surface antigen mutation status

The objective of this study was to develop a DNA sequencing assay that examines sensitively and reliably all conserved domains of the reverse transcriptase-encoding region of the HBV genome for antiviral resistance-associated mutations while simultaneously producing ample information for precise genotyping and determination of HBsAg mutation. This assay was used to examine 1000 de-identified HBV DNA positive samples with known viral loads from a broad-based, unselected patient population from across the United States. Of these, 946 were assayed successfully. Antiviral resistance-associated mutations were identified in 104 samples. The escape mutation sG145R in the surface antigen was identified in 0.8% of patient samples. Infections with genotypes A, B, C, D, E, F, G and H were observed in 36.6%, 19.6%, 21.7%, 13.5%, 3.6%, 0.7%, 2.2%, and 0.5% of patient samples respectively. Fifteen samples (1.6%) appeared to harbor infections with multiple genotypes as shown by the presence of double peaks throughout sequence electropherograms. The limit of detection of this assay was approximately 150IU/mL.

Molecular genetics of HBV infection

HBV has evolved a unique life cycle that results in the production of enormous viral loads during active replication without actually killing the infected cells directly. Two of the key events in the viral life cycle of HBV involve firstly the generation of a covalently closed circular (ccc)DNA transcriptional template, either from input genomic DNA or newly replicated capsid-associated DNA, and secondly, reverse transcription of the viral pregenomic (pg)RNA to form progeny HBV DNA genomes. New data are emerging regarding the epigenetic control of cccDNA, which might represent another key factor involved in the pathogenesis and natural history of the disease. Because HBV uses reverse transcription to copy its genome, mutant viral genomes emerge frequently. Particular selection pressures, both endogenous (host immune clearance) and exogenous (vaccines and antiviral drugs), readily select out these escape mutants. The particular viral mutations or combination of mutations that directly affect the clinical outcome of infection are not known; however, four major 'pathways' of antiviral drug resistance-associated substitutions have now been identified. Further studies are clearly needed to identify the pathogenetic basis and clinical sequelae arising from the selection of these particular mutants. In the clinical context of antiviral drug resistance, treating physicians need to adopt therapeutic strategies that effectively control viral replication. Finally, the role of host genetics in influencing the outcome of HBV disease in the context of natural history and therapy is beginning to aid understanding in pathogenesis and, when this knowledge is linked to pathogen-specific databases, this should translate into more individualized patient care.

Hepatitis B virus reverse transcriptase sequence variant database for sequence analysis and mutation discovery

Drug resistance resulting from reverse transcriptase (RT) mutations is one of the main obstacles to successful hepatitis B virus (HBV) therapy. Indeed, HBV treatment guidelines recommend HBV genotypic resistance testing for patients receiving nucleos(t)ide RT inhibitors (N(t)RTIs) who develop virological failure. N(t)RTI-resistance mutations at 10 RT positions have been well characterized in phenotypic studies, however, data are lacking on the relative frequency of these mutations in N(t)RTI-treated and untreated individuals. There are also few published data on the extent of amino acid variation at most of the 344 positions of HBV RT and the extent to which this variation is influenced by N(t)RTI treatment. We retrieved 23,871 HBV RT sequences from GenBank and reviewed the published reports of these sequences to ascertain the number of individuals from whom the sequences were obtained, the N(t)RTI treatments of these individuals, and the year and region of virus sampling. We then used these data to populate a relational database we named HBVrtDB. As of July 2010, HBVrtDB contained 6811 sequences from 3869 individuals reported in 281 references. Among these 3869 individuals, 73% were N(t)RTI-naïve and 27% received one or more N(t)RTIs. Among the 10 well-characterized N(t)RTI-resistance mutations, L80I/V, V173L, L180M, A181T, T184S, S202G and M204I/V were significantly associated with treatment with lamivudine, an l-nucleoside analog, and A181S/T/V and N236T were significantly associated with treatment with adefovir, an acyclic nucleoside phosphonate. A similar analysis of ten additional less well-characterized resistance mutations demonstrated a significant association with N(t)RTI treatment for four of the mutations: L82M, S85A, A200V, and Q215S. We also created an interactive program, HBVseq, to enable users to identify mutations in submitted sequences and retrieve the prevalence of these mutations in HBVrtDB according to genotype and N(t)RTI treatment. HBVrtDB and HBVseq are available at http://hivdb.stanford.edu/HBV/releaseNotes/.

Drug resistance in antiviral therapy

The introduction of nucleos(t)ide analog therapy has seen the emergence of antiviral drug resistance, which has become the main factor limiting the long-term application of these antiviral agents for patients with chronic hepatitis B. The prevention of resistance requires the adoption of strategies that effectively control virus replication and exploit an understanding of the mechanisms and processes that drive the emergence of drug resistance, namely high replication rates, low fidelity of the hepatitis B virus rt/polymerase, selective pressure of the nucleos(t)ide analog, role of replication space (liver turnover), fitness of the mutant, and genetic barrier to the drug.

Hepatitis B virus resistance to nucleos(t)ide analogues

Patients with chronic hepatitis B (CHB) can be successfully treated using nucleos(t)ide analogs (NA), but drug-resistant hepatitis B virus (HBV) mutants frequently arise, leading to treatment failure and progression to liver disease. There has been much research into the mechanisms of resistance to NA and selection of these mutants. Five NA have been approved by the US Food and Drug Administration for treatment of CHB; it is unlikely that any more NA will be developed in the near future, so it is important to better understand mechanisms of cross-resistance (when a mutation that mediates resistance to one NA also confers resistance to another) and design more effective therapeutic strategies for these 5 agents. The genes that encode the polymerase and envelope proteins of HBV overlap, so resistance mutations in polymerase usually affect the hepatitis B surface antigen; these alterations affect infectivity, vaccine efficacy, pathogenesis of liver disease, and transmission throughout the population. Associations between HBV genotype and resistance phenotype have allowed cross-resistance profiles to be determined for many commonly detected mutants, so genotyping assays can be used to adapt therapy. Patients that experience virologic breakthrough or partial response to their primary therapy can often be successfully treated with a second NA, if this drug is given at early stages of these events. However, best strategies for preventing NA resistance include first-line use of the most potent antivirals with a high barrier to resistance. It is important to continue basic research into HBV replication and pathogenic mechanisms to identify new therapeutic targets, develop novel antiviral agents, design combination therapies that prevent drug resistance, and decrease the incidence of complications of CHB.

Hepatitis B virus with the rtL80V/I mutation is associated with a poor response to adefovir dipivoxil therapy

BACKGROUND/AIMS

Lamivudine (LAM) resistance is frequently associated with various types of genomic changes in hepatitis B virus (HBV)-DNA including YMDD mutations (rtM204V/I). We intended to examine the effects of these genotypic variants on the antiviral efficacy of adefovir dipivoxil (ADV) therapy.

METHODS

A total of 97 chronic hepatitis B (CHB) patients with YMDD mutants who had been treated with ADV for >12 months were analysed. Mutations of the entire polymerase domain of HBV were determined by direct sequencing.

RESULTS

All the 97 patients had genotype C HBV associated with rtM204V/I mutations; 63 (65%) rtM204I, 27 (28%) rtM204V and seven (7%) both. The rtL80V/I and rtL180M variants were identified in 66 (68%) and 67 (69%) patients respectively. The rtM204I and rtM204V variants were strongly associated with rtL80V/I and rtL180M respectively (P<0.01). There was no difference in antiviral response at 12 months after ADV therapy between patients in relation to the type of YMDD mutation or the presence of rtL180M. However, interestingly, after ADV therapy for 12 months, patients with rtL80V/I achieved a much smaller reduction in serum HBV-DNA titre than those without it (mean, -3.43 vs. -4.43 log(10) copies/ml; P=0.018). In addition, patients with rtL80V/I had lower rates of undetectable HBV-DNA (20 vs. 26%), alanine aminotransferase normalization (70 vs. 81%) and HBeAg loss (16 vs. 26%) than those without it, although none of these differences was statistically significant.

CONCLUSIONS

These results provide evidence that rtL80V/I variants of HBV may be associated with a poor antiviral response to ADV in CHB patients with YMDD mutants.

Association of lamivudine-resistant mutational patterns with the antiviral effect of adefovir in patients with chronic hepatitis B

Adefovir has a potent antiviral activity as a rescue treatment against lamivudine-resistant strains. The aim of this study was to assess the patterns of lamivudine-resistant mutations and their influence on the virologic response to adefovir rescue therapy in patients with lamivudine-resistant chronic hepatitis B. Sixty-seven patients with lamivudine-resistant chronic hepatitis B were treated with adefovir monotherapy. Baseline blood samples were analyzed for lamivudine-resistant mutations via restriction fragment mass polymorphism. Virologic responses, ALT normalization and loss of HBeAg were assessed. Serum HBV DNA levels were measured using real-time PCR at baseline and 24 weeks of adefovir therapy. Of the 67 patients with chronic hepatitis B, 65 patients (97%) had lamivudine-resistant mutations in the YMDD motif [27 (41%) rtM204I, 22 (34%) rtM204V, and 16 (25%) rtM204I/V]. In addition to the YMDD mutations, the rtL180M, rtL80I, and rtV173L mutations were also present in 78%, 43%, and 11% of patients, respectively. The rtM204V mutation always accompanied rtL180M, and rtL80I was always observed in conjunction with rtM204I. Decrease in mean serum HBV did not differ between patients carrying the rtM204I versus rtM204V mutant at week 24 (-3.3 vs. -3.3 log(10) copies/ml, respectively; P = 0.303). The presence of the rtL180M, rtL80I, and rtV173L did not significantly affect viral load reduction during adefovir administration. These results demonstrate that the rtL80I mutant is co-selected with rtM204I as a compensatory mutation in the same manner as rtL180M with rtM204V, and that adefovir shows similar antiviral efficacy against all of the evaluated patterns of lamivudine-resistant HBV mutations.

Selection of an entecavir-resistant mutant despite prolonged hepatitis B virus DNA suppression, in a chronic hepatitis B patient with preexistent lamivudine resistance: successful rescue therapy with tenofovir

BACKGROUND AND OBJECTIVE

Entecavir has potent activity against hepatitis B virus. Drug resistance has not been reported in nucleoside-naïve patients and is low in lamivudine-refractory patients.

METHODS AND RESULTS

A 43-year-old man was treated with lamivudine for hepatitis B e antigen-positive chronic hepatitis B. A viral breakthrough owing to a drug-resistant mutant was observed and entecavir 1 mg daily was added. After the viral load had been near the lower detection range of the PCR assay for 30 weeks, lamivudine was discontinued. The serum hepatitis B virus DNA remained low until a second viral breakthrough was observed after 45 weeks of entecavir monotherapy. Treatment was switched from entecavir to tenofovir disoproxil 245 mg daily, which resulted in a decline below 1000 copies/ml. Sequence analysis revealed the presence of rtL180M and rtM204V lamivudine-resistant-associated mutations at the start of entecavir treatment. During entecavir treatment, the rtS202G mutation was selected. Retrospective analysis revealed that during lamivudine treatment three other mutations had been selected as well, namely rtE1D, rtV207L, and rtI220L.

CONCLUSION

We describe the first case of entecavir resistance in a lamivudine-resistant patient with good initial suppression of viral replication for 70 weeks. On the basis of the data from cross-resistance and sensitivity testing in vitro and treatment outcomes, tenofovir proves to be a good treatment option for entecavir-resistant patients.

Genotyping and genomic sequencing in clinical practice

The global prevalence of chronic hepatitis B and its associated serious sequelae demand technologically advanced techniques of management. Nucleic acid testing (NAT) plays a key role in the diagnosis, surveillance, and treatment of chronic hepatitis B. NAT includes quantitative PCR-based HBV DNA assays, HBV genotyping, tests for mutations associated with resistance to antiviral medications, and assays to detect precore and core promoter mutations. This article reviews the uses of NAT in the diagnosis and management of chronic hepatitis B.

Liver histology of children with chronic hepatitis treated with interferon-alpha alone or in combination with lamivudine

AIM

To evaluate histological changes with interferon monotherapy or interferon plus lamivudine combination therapy in children with hepatitis B e antigen (HBeAg)-positive chronic hepatitis B.

PATIENTS AND METHODS

31 children aged 2-13 years were randomly treated with interferon (IFN) (group 1, n = 16) or IFN plus simultaneously started lamivudine (group 2, n = 15). IFN-alpha 2a was given 9 MU/m2 3 times per week for 6 months in each group; lamivudine was given 4 mg x kg(-1) x day(-1) for 24 months. Liver biopsy specimens were evaluated according to the Knodell score before therapy and after 24 months of therapy. Histological response was defined as a decrease in the histological activity index (HAI) score by at least 2 points. Efficacy of therapy was evaluated at 24 months of therapy in all children.

RESULTS

Alanine aminotransferase normalization, HbeAg, and hepatitis B virus DNA clearance were not different. Complete response and histological response were 37.5%/62.5% and 40%/46.7% in groups 1 and 2, respectively (P = NS). At baseline and at 24 months of therapy, total HAI and components of HAI were not different in the 2 groups. In comparison with baseline, a significant decrease in scores of periportal +/- bridging necrosis was observed in group 1 (P = 0.01); periportal +/- bridging necrosis, intralobular degeneration, focal necrosis, and necroinflammation scores significantly decreased in group 2 (P = 0.04 and P = 0.02) at 24 months of therapy.

CONCLUSIONS

The addition of lamivudine to IFN-alpha did not increase the effectiveness of the treatment in terms of complete and histological responses. Both therapies seemed to be effective in the regression of periportal +/- bridging necrosis. In addition, combination therapy was also effective in the regression of intralobular degeneration, focal necrosis, and necroinflammatory activity index.

Characteristics of lamivudine-resistant hepatitis B virus (HBV) strains with and without breakthrough hepatitis in patients with chronic hepatitis B evaluated by serial HBV full-genome sequences

Lamivudine therapy often causes breakthrough of hepatitis B virus (HBV) DNA and breakthrough hepatitis. The aim of this study was to determine the viral factors that relate to HBV-DNA breakthrough with and without breakthrough hepatitis. Among 82 patients with chronic hepatitis B (CHB) who received lamivudine at a dose of 100 mg daily for more than 24 months, 23 patients had HBV-DNA breakthrough induced by a lamivudine-resistant mutant. Of these 23 patients, 16 had breakthrough hepatitis and 7 had only HBV-DNA breakthrough. Serial HBV-DNA full-genome sequences during therapy were examined in 10 (7 had breakthrough hepatitis and 3 did not) of these 23 patients by direct sequencing. Mutations in the S region were examined by cloning in representative patients. There were no significant differences in the baseline clinical backgrounds and virus marker between patients with and without breakthrough hepatitis. The HBV amino acid substitutions at breakthrough hepatitis were identical to those at HBV-DNA breakthrough. Cloning analysis revealed that monoclonal mutational strain appeared at breakthrough and no such mutations existed at baseline. Regarding HBV amino acid substitutions in the polymerase region, S region, X region, and precore-core region with breakthrough compared to baseline, there was no significant differences of the numbers of amino acid substitution between breakthrough hepatitis and non-breakthrough hepatitis. There were no common amino acid changes in patients with breakthrough hepatitis. Although monoclonal lamivudine-resistant strain emerged at HBV-DNA breakthrough in patients with CHB, there were no common amino acid changes, suggesting viral factor may have insignificant role in breakthrough hepatitis.

Antiviral drug-resistant HBV: standardization of nomenclature and assays and recommendations for management

Substantial advances have been made in the treatment of chronic hepatitis B in the past decade. Approved treatments for chronic hepatitis B include 2 formulations of interferon and 4 nucleos(t)ide analogues (NAs). Sustained viral suppression is rarely achieved after withdrawal of a 48-week course of NA therapy, necessitating long, and in many cases, indefinite treatment with increasing risk of development of drug resistance. Antiviral resistance and poor adherence are the most important factors in treatment failure of hepatitis B. Thus, there is a need to standardize nomenclature relating to hepatitis B antiviral resistance, and to define genotypic, phenotypic, and clinical resistance to NA therapy.

The L80I substitution in the reverse transcriptase domain of the hepatitis B virus polymerase is associated with lamivudine resistance and enhanced viral replication in vitro

Long-term lamivudine (LMV) treatment of chronic hepatitis B almost inevitably engenders viral resistance. Mutations that result in the replacement of the methionine at position 204 of the deoxynucleoside triphosphate-binding site of the hepatitis B virus (HBV) reverse transcriptase (rt) by isoleucine, valine, or (rarely) serine (rtM204I/V/S) confer high-level resistance to LMV but reduce replication efficiency. The subsequent selection or coselection of secondary mutations that partially restore replication efficiency is common and may influence drug resistance. Genotyping has shown that LMV treatment can select for HBV rtL80V/I mutants, but their prevalence and phenotype have not been documented. Analysis of a large sequence database revealed that rtL80V/I occurred almost exclusively in association with LMV resistance, and 85% of these isolates encoded rtL80I. Coselection of rtL80V/I occurred in 46% of isolates in which LMV resistance was attributable to rtM204I but only 9% of those in which resistance was attributable to rtM204V. Moreover, rtL80V/I did not occur in HBV genotype A isolates but occurred at similar frequencies in genotype B, C, and D isolates. In vitro phenotyping showed that although the rtL80I mutant by itself replicated less efficiently and was hypersensitive to LMV compared to the replication efficiency and sensitivity of its wild-type parent, the presence of rtL80I enhanced the replication efficiency of rt204I/V mutants without significantly affecting LMV resistance. Molecular modeling revealed that rt80 does not interact directly with the enzyme's substrates. Collectively, these results suggest that coselection of rtL80V/I and rtM204I/V occurs because the former compensates for the loss of replication efficiency associated with the acquisition of LMV resistance, particularly in the case of rtM204I.

Major Causes of Antiviral drug Resistance and Implications for Treatment of Hepatitis B virus Monoinfection and Coinfection with HIV

The two key events in the life-cycle of the hepatitis B virus (HBV) involve (1) the generation from viral genomic DNA of the covalently closed circular DNA transcriptional template, and (2) the reverse transcription of the viral pregenomic RNA to form the HBV DNA genome. Diversity in the HBV genome is ensured by the low fidelity of the viral reverse transcriptase (rt). Particular selection pressures such as antiviral therapy readily select out escape mutants from this pre-existing quasispecies pool. Antiviral drug resistance in chronic hepatitis B can be caused by many factors, including the viral mutation frequency, the intrinsic mutability of the antiviral target site, the selective pressure exerted by the drug, the magnitude and rate of virus replication, the overall replication fitness of the mutant, the genetic barrier of the >compound and the availability of replication space. In the setting of HIV coinfection, the rate of replication is increased by one to two orders of magnitude, accelerating the emergence of drug resistance in this setting. The HBV genome is arranged into frame-shifted and overlapping reading frames in such a manner that antiviral drug-resistance-associated changes in the rt can result in changes in the viral envelope protein. These HBV isolates with altered surface antigens exhibit reduced binding of specific and neutralizing antibody and so have diagnostic and public health implications, especially in the setting of HIV coinfection where the risk of transmission is increased. Thus, prevention of resistance requires the adoption of strategies that effectively control virus replication, including the use of combination chemotherapy.

Mutational patterns of hepatitis B virus genome and clinical outcomes after emergence of drug-resistant variants during lamivudine therapy: Analyses of the polymerase gene and full-length sequences

It remains unclear whether mutational patterns of the hepatitis B virus (HBV) genome are associated with the development of severe hepatitis after the emergence of tyrosine-methionine-aspartate-aspartate (YMDD) variants during lamivudine treatment. Thirty patients with chronic hepatitis B who had YMDD variants during lamivudine therapy and were followed up subsequently while receiving lamivudine alone for at least 6 months were examined retrospectively. The lamivudine resistant mutations in the HBV polymerase gene were detected by a line probe assay, and the full-length sequences of HBV DNA were determined in some patients. Between months 5 and 33 of therapy, mutations from methionine to isoleucine at rt204 (rtM204I) were detected in 18 patients, and mutations from methionine to valine at rt204 (rtM204V) were detected in 12. The rtM204V mutations were always accompanied by mutations from leucine to methionine at rt180 (rtL180M), while rtM204I mutations were not. Baseline characteristics, alanine aminotransferase (ALT) levels, and HBV DNA levels within 6 months after the emergence of YMDD variants did not differ significantly between patients with rtM204I alone and those with rtL180M/rtM204V. No specific mutation was identified on full-length sequence analysis in three patients with a hepatitis flare. During long term follow-up, the addition of rtL180M to rtM204I was found in four patients 7-31 months after detecting the change at rt204 and was linked to increased ALT levels. In conclusion, mutational patterns of HBV DNA at the time of emergence of YMDD variants were apparently unrelated to the clinical outcomes in Japanese patients with chronic hepatitis B during lamivudine therapy.

[Hepatitis B and C virus antiviral resistance]

Infection by the hepatitis B (HBV) and C (HCV) viruses is a major cause of morbidity and mortality world-wide. The clinical outcomes of infection by these viruses (e.g., chronic hepatitis, liver cirrhosis, and hepatocellular carcinoma) depend on several factors related to the host and the viral agent. Among the latter, factors associated with the response to current antiviral therapies, such as the emergence of resistance mutants and the genotype responsible for the infection, are gaining increasing importance. As has been established for human immunodeficiency virus (HIV), the presence of resistance mutations in the viral polymerase constitutes the main problem for treating HBV infection with approved drugs and those recently applied. Methods have been developed to detect these mutations, as well as algorithms to predict the response to treatment. The outcome of treatment for HCV infection is highly influenced by the viral genotype, however, and our understanding of the molecular basis for the response to interferon in these patients has grown considerably in recent years.

Mutations affecting the replication capacity of the hepatitis B virus

The genetic variability of the hepatitis B virus (HBV) encounters two compounding forces: a high viral copy number produced during active replication and the lack of proofreading activity in the HBV polymerase, resulting in a high mutational rate. A large pool of quasispecies is generated in which the fittest virus, i.e. the virus that replicates best, becomes the dominant species. Immune and antiviral selection pressures result in vaccine/immunoglobulin escape mutants and antiviral resistant variants. Viruses encoding changes associated with antiviral resistance often have reduced replication in vitro, but the accumulation of additional mutations helps restore viral fitness. These compensatory mutations may occur not only in the polymerase gene but also in other genes such as the overlapping envelope gene, the precore gene, or in regulatory regions such as the basal core promoter. In this report we aim to review the new findings that have appeared in recent months.

Management of drug-resistant chronic hepatitis B

Lamivudine, adefovir, and entecavir are safe and effective orally administered inhibitors of hepatitis B virus (HBV) replication, but drug-resistant strains of HBV with point mutations in the HBV polymerase gene are being reported with prolonged treatment that can lead to viral rebound and serum alanine aminotransferase flares. Salvage treatment with antiviral agents including investigational nucleoside/nucleotide analogues is available but highlights the limitations and hazards of sequential antiviral monotherapy for chronic HBV. Studies of pegylated interferon combined with an antiviral agent or dual nucleoside/nucleotide combination therapy are awaited to minimize the incidence of drug-resistant HBV and improve long-term outcomes.

Comparison of Two Different Regimens of Combined Interferon-α2A and Lamivudine Therapy in Children with Chronic Hepatitis B Infection

Aim

To evaluate the efficacy of two regimens of combined interferon-α2a (IFN-α2a) and lamivudine (3TC) therapy in childhood chronic hepatitis B.

Methods

A total of 177 patients received IFN-α2a, 9 million units (MU)/m2 for 6 months. In group I (112 patients, 8.7±3.5 years), 3TC (4 mg/kg/day, max 100 mg) was started simultaneously with IFN-α2a, in group II (65 patients, 9.6±3.8 years) 3TC was started 2 months prior to IFN-α2a. 3TC was continued for 6 months after antiHBe seroconversion or stopped at 24 months in non-responders.

Results

Baseline alanine aminotransferase (ALT) was 134.2 ±34.1 and 147.0 ±45.3; histological activity index (HAI) was 7.4 ±2.7 and 7.1 ±2.3; and HBV DNA levels were above 2,000 pg/ml in 76% and 66% of patients in groups I and II, respectively ( P>0.005). Complete response was 55.3% and 27.6% in groups I and II, respectively ( P<0.01). AntiHBe seroconversion was higher and earlier, and HBV DNA clearance was earlier in group I ( P<0.05). HBsAg clearance was 12.5% and 4.6% and antiHBs seroconversion was 9.8% and 6.2% in groups I and II, respectively ( P>0.05). Breakthrough occurred in 17.9% and 24.6%; breakthrough times were 15.9 ±4.6 and 14.1 ±5.1 months; and relapse rates were 6.8% and none in groups I and II, respectively ( P>0.05, P>0.05, P>0.05). Responders had higher HAI (HAI>6) and higher pre-treatment ALT than non-responders.

Conclusion

Simultaneous 3TC+IFN-α2a yields a higher response and earlier antiHBe seroconversion and viral clearance than consecutive combined therapy. Relapse rate is low. Predictors of response are high basal ALT and high HAI scores. 3TC can be administered for 24 months without any side effect and breakthrough rate is comparable with previous studies.

Virologic response and resistance to adefovir in patients with chronic hepatitis B

BACKGROUND

The incidence and risk factors for adefovir-resistant HBV have not been clearly defined.

AIMS

To characterize the virologic response to adefovir, to determine the rate of adefovir resistance and to explore factors associated with initial virologic response (IVR) and adefovir resistance.

METHODS

All hepatitis B patients who received adefovir for > or =6 months at our center were prospectively monitored for virologic response and adefovir resistance.

RESULTS

Forty three patients were included; mean treatment duration was 18 months (range 6-45). Thirty four (79%) patients had prior lamivudine. IVR was observed in 44% patients and associated with higher pretreatment ALT (P = 0.05) and the absence of HBeAg (P = 0.02). Six (14%) patients were found to have adefovir-resistant mutations. The cumulative probability of genotypic resistance to adefovir at month 24 was 22%. Patients with adefovir resistance were more likely to have been switched from lamivudine to adefovir monotherapy (P = 0.01), to be older (P = 0.04), and to be infected with HBV genotype D (P = 0.02).

CONCLUSIONS

Roughly 50% of patients failed to achieve IVR on adefovir. The cumulative probability of adefovir resistance at 2 years was 22%. Our data suggest that combination of lamivudine and adefovir may prevent emergence of adefovir resistance in patients with lamivudine-resistant HBV.

Complete hepatitis B virus genome analysis in chronically infected children before and during lamivudine treatment

It is well established that during lamivudine treatment, mutations emerge within the polymerase gene but there is little information about the selection of other mutations in the whole hepatitis B virus (HBV) genome. The aim of this study was to investigate whether mutations outside this region are selected during lamivudine treatment. The complete HBV genomes of six HBsAg positive chronically infected children were isolated from the children's sera before, at the beginning and during lamivudine treatment, amplified and sequenced directly. A change in the mutation rate and type in periods with and without treatment for one and the same patient were examined longitudinally because blood samples were taken long before treatment started. During the testing period before treatment, 12 mutations occurred within 11.7 +/- 8.1 months in all genomes, resulting in a mutation rate of 1.1 x 10(-3) substitutions per site per year. During treatment with lamivudine, 20 mutations occurred in all patients within an average period of 7.6 +/- 1.2 months, giving an average mutation rate of 1.8 x 10(-3) substitutions per site per year. The 20 mutations observed during the treatment period occurred in only 4 of the patients and only 3 patients experienced nonsense mutations during lamivudine treatment. The mutations were spread across the entire genome with a non-significant cluster during treatment in the P-ORF (18 mutations vs. 7, P = 0.073) and S-ORF (11 vs. 2, P = 0.063). Mutations causing drug resistance did not emerge. This study describes the changes in the complete HBV genome in the spontaneous course of infection and during lamivudine treatment. An increased mutation rate and the occurrence of specific mutations could not be proven for the early phase of lamivudine treatment.

Management of Hepatitis B Patients with Antiviral Resistance

Drug resistance is an expected consequence of antiviral therapy for chronic hepatitis B because of the high rate of hepatitis B virus (HBV) replication, the lack of proofreading during reverse transcription of the pregenomic RNA and the low efficacy of available therapies in eliminating covalently closed circular HBV DNA. Mutations involving the YMDD motif of the catalytic domain of HBV reverse transcriptase have been reported in patients who have received lamivudine, emtricitabine and telbivudine. Drug-resistant mutations affecting other regions of HBV polymerase have also been reported, but at much lower rates in patients who have received adefovir dipivoxil or entecavir. Antiviral resistance is initially manifested as virological breakthrough infection. In most patients, this is followed by biochemical breakthrough and, in some patients, hepatitis flares and hepatic decompensation. Monitoring drug resistance may improve the management of patients with antiviral-resistant HBV and can guide the selection of salvage therapy. The optimal management of patients with antiviral-resistant HBV continues to evolve. The ideal approach is to prevent antiviral resistance through judicious use of antiviral therapy and the use of more potent antiviral agents, possibly in combination.

Evolution of hepatitis B virus sequence from a liver transplant recipient with rapid breakthrough despite hepatitis B immune globulin prophylaxis and lamivudine therapy

Recurrent hepatitis B virus (HBV) infection after liver transplantation can be prevented by prophylactic hepatitis B immune globulin (HBIG) and lamivudine therapy. However, reinfection may still occur due to the emergence of immune escape mutants and mutants of the YMDD motif. The full spectrum of mutations within the HBV genome during recurrent HBV infection remains to be documented. In this study, serial HBV isolates were characterized from a patient with lamivudine resistance prior to liver transplantation who developed recurrent HBV infection within 2 months of transplantation despite a high dose of HBIG and lamivudine therapy. Sequence analysis of full-length viral genome before transplantation revealed many point mutations as compared with a wild-type genotype C sequence, including the T1753G/A1762T/G1764A triple mutation in the basal core promoter and the G1896A nonsense mutation in the precore region. After transplantation and therapy, several point mutations in the HBV genome emerged or became dominant. These mutations caused L426I/L526M/M550I triple mutation (equivalent to L428I/L528M/M552I in previous reports) in the polymerase, and D144E mutation in the "a" determinant of HBsAg. Transfection experiments revealed that the D144E mutation reduced HBsAg affinity to anti-HBs, confirming its active role for immune escape. Our study suggests that mutations in the HBsAg (D144E) and the polymerase (L426I/L526M/M550I) of HBV genome may be responsible for viral breakthrough despite HBIG prophylaxis and lamivudine therapy.

YSDD: a novel mutation in HBV DNA polymerase confers clinical resistance to lamivudine

The emergence of drug-resistant virus in hepatitis B virus (HBV) patients treated with lamivudine is well documented. In this study, we determined the mutations occurring in the tyrosine-methionine-aspartate-aspartate (YMDD) amino acid motif of the HBV DNA polymerase gene, as well as upstream and downstream of this region, in patients with breakthrough virus during lamivudine therapy. Thirty-one Turkish patients (20 patients HBeAg positive, 11 patients HBeAg negative and anti-HBe positive) with chronic HBV infection who completed at least 104 weeks of lamivudine treatment were investigated. All patients received lamivudine, (150 mg/day), for 104 weeks, with or without 4 months of interferon (IFN) combination. HBV-specific sequences were amplified by polymerase chain reaction (PCR) from sera of patients with breakthrough virus, and the PCR products were directly analysed by sequencing. Breakthrough virus was detected in seven of the 31 patients (22.6%) between 9 and 18 months of therapy. Of the seven patients, six were HBeAg positive at baseline, and four had a double mutation consisting of rtM204V and rtL180M, while two had an rtM204I change. In one patient, two base substitutions at rt204 (ATG --> AGT; T to G and G to T) lead to a methionine to serine change (YMDD --> YSDD). This novel DNA pol mutation was detected at month 18 of lamivudine treatment. In addition, this new variant had the rtL180M mutation and a 12 base pair deletion in the pre-S1 region between nucleotides 43-54. The YSDD mutation was still present 6 months after lamivudine discontinuation. In vitro transfection studies also confirmed that the YSDD strain is resistant to lamivudine. In conclusion, the results indicate that, in addition to a Met --> Val and Met --> Ile change in YMDD, a Met --> Ser change at rt204 (YMDD --> YSDD) associated with the rtL180M change can also emerge during lamivudine treatment, which confers lamivudine resistance in vivo and in vitro, leading to virological breakthrough and ALT increases.

Treatment of chronic hepatitis B: current challenges and future directions

The clinical management of chronic hepatitis B infection has entered a new era with the introduction and widespread use of oral nucleoside analogues such as lamivudine and nucleotides such as adefovir dipivoxil. From this, new challenges have now emerged in terms of preventing antiviral drug resistance, promoting viral clearance and improving long-term survival. For example, the natural history of nucleoside or nucleotide analogue-associated hepatitis B virus resistant mutants has yet to be determined. Furthermore, the increasing prevalence of HBeAg negative disease with its reduced response to current therapies represents an ongoing challenge to attempts to improve standard of care. There is increasing recognition of the pivotal role that viral load and genotype, and their complex interactions with the host immune response, play in determining the outcome of these treatment interventions. The purpose of this paper is to highlight several key factors that should be considered in the context of future clinical research and management of chronic hepatitis B.

Comparisons of the Hbv and HIV Polymerase, and Antiviral Resistance Mutations

The antiviral treatment of chronic hepatitis B is limited by the selection of antiviral resistance mutations. Primary resistance to lamivudine occurs at rtM204I/V in the C Domain of the polymerase. Recently, resistance to adefovir has also been described in the D Domain at rtN236T. The treatment of patients with resistant virus without complete suppression can lead to the further selection of compensatory mutations. Thus, to gain an understanding of the hepatitis B virus (HBV) polymerase and also mutations associated with resistance, a three-dimensional model of the HBV reverse transcriptase core region based on homology with human immunodeficiency virus (HIV) was created. A comparative analysis of the HIV polymerase and the model of HBV polymerase was performed. In addition, the antiviral resistance mutations including potential compensatory mutations were mapped to determine their effect on the HBV polymerase model, especially in the nucleotide binding site.

Oral ganciclovir for treatment of lamivudine-resistant hepatitis B virus infection: a pilot study

Although liver disease seems to be stable in most patients who are infected with lamivudine-resistant mutant hepatitis B virus (HBV) in the short term, it may progress to more-advanced disease in some patients. In our pilot study, we investigated the efficacy of oral ganciclovir for the treatment of lamivudine-resistant HBV infection. Six patients infected with lamivudine-resistant HBV (3 patients had decompensated cirrhosis and 3 had chronic active hepatitis without cirrhosis) were included. Ganciclovir was administered at a dosage of 3 g daily for 6 months. Four of 6 patients completed the 6-month treatment period. Two patients with cirrhosis completed only 2 months of ganciclovir treatment because they died of cirrhosis complications. None of the patients had a > or =2-log(10) reduction of HBV DNA and complete alanine aminotransferase normalization at the end of their treatment regimens. In conclusion, 6 months of ganciclovir treatment is not effective for suppression of lamivudine-resistant HBV infection.

Hepatitis B virus resistance to lamivudine and its clinical implications

Lamivudine is the first orally available drug approved for treatment of chronic hepatitis B, but hepatitis B virus (HBV) resistance to lamivudine appears to be a sine qua non in the therapy of HBV. The mutations at the FLLA and YMDD motif in the domains B and C of HBV polymerase contribute to this resistance. These mutations are found at codon (or AA) rtL180M and rtM204V/I in the reverse transcriptase (RT) domain of the HBV polymerase for all genotypes according to a new standardized RT domain numbering system. The resistant HBV may be less replication-competent in vitro and in vivo, and it is rarely associated with markedly increased HBV replication or liver injury. Therefore, certain physicians favour continuing lamivudine therapy even after emergence of HBV resistance with the expectation of maintaining lower-than baseline HBV DNA, alanine amino-transferase, and histological improvement, and avoiding reversion to wild-type HBV until additional antiviral strategies are developed. Ultimately, once several antiviral agents are approved, combination strategy is likely to be incorporated in antiviral treatment for chronic HBV to suppress, prevent or minimize the emergence of resistant virus.

Kinetic analysis of wild-type and YMDD mutant hepatitis B virus polymerases and effects of deoxyribonucleotide concentrations on polymerase activity

Mutations in the YMDD motif of the hepatitis B virus (HBV) DNA polymerase result in reduced susceptibility of HBV to inhibition by lamivudine, at a cost in replication fitness. The mechanisms underlying the effects of YMDD mutations on replication fitness were investigated using both a cell-based viral replication system and an in vitro enzyme assay to examine wild-type (wt) and YMDD-mutant polymerases. We calculated the affinities of wt and YMDD-mutant polymerases for each natural deoxyribonucleoside triphosphate (dNTP) and determined the intracellular concentrations of each dNTP in HepG2 cells under conditions that support HBV replication. In addition, inhibition constants for lamivudine triphosphate were determined for wt and YMDD-mutant polymerases. Relative to wt HBV polymerase, each of the YMDD-mutant polymerases showed increased apparent K(m) values for the natural dNTP substrates, indicating decreased affinities for these substrates, as well as increased K(i) values for lamivudine triphosphate, indicating decreased affinity for the drug. The effect of the differences in apparent K(m) values between YMDD-mutant polymerase and wt HBV polymerase could be masked by high levels of dNTP substrates (>20 microM). However, assays using dNTP concentrations equivalent to those measured in HepG2 cells under physiological conditions showed decreased enzymatic activity of YMDD-mutant polymerases relative to wt polymerase. Therefore, the decrease in replication fitness of YMDD-mutant HBV strains results from the lower affinities (increased K(m) values) of the YMDD-mutant polymerases for the natural dNTP substrates and physiological intracellular concentrations of dNTPs that are limiting for the replication of YMDD-mutant HBV strains.

Susceptibility testing. Viral pathogens

The control of the global expansion and proliferation of the AIDS pandemic has been complicated by the emergence of resistant strains of HIV-1 to the many new antiviral drugs directed to the genes coding for reverse transcriptase and protease enzymes of the virus. Similarly, new drug regimens for the management of chronic hepatitis B and C infections have been complicated by the lack of sustained clinical responses recently associated with either nucleotide mutation (HBV) or specific genotype of the virus (HCV). Commercial systems for performing and interpreting genotypic analysis will facilitate the recognition of informative mutations, standardize results between laboratories, and produce informative and interpretative result formats for optimal treatment of patients. Drug-resistant strains of herpesviruses (HSV, VZV, CMV) are generally associated with prolonged treatment of these infections in immunocompromised patients. Ultimate relevance of genotypic assays for routine clinical practice will require correlation with phenotypic results and the outcomes of long-term studies associating clinical improvement with antiviral drugs with specific mutation patterns of these viruses.