Long-term monitoring shows hepatitis B virus resistance to entecavir in nucleoside-naïve patients is rare through 5 years of therapy

Patients with chronic hepatitis B virus (HBV) infection who develop antiviral resistance lose benefits of therapy and may be predisposed to further resistance. Entecavir (ETV) resistance (ETVr) results from HBV reverse transcriptase substitutions at positions T184, S202, or M250, which emerge in the presence of lamivudine (LVD) resistance substitutions M204I/V +/- L180M. Here, we summarize results from comprehensive resistance monitoring of patients with HBV who were continuously treated with ETV for up to 5 years. Monitoring included genotypic analysis of isolates from all patients at baseline and when HBV DNA was detectable by polymerase chain reaction (> or = 300 copies/mL) from Years 1 through 5. In addition, genotyping was performed on isolates from patients experiencing virologic breakthrough (> or = 1 log(10) rise in HBV DNA). In vitro phenotypic ETV susceptibility was determined for virologic breakthrough isolates, and for HBV containing novel substitutions emerging during treatment. The results over 5 years of therapy showed that in nucleoside-naïve patients, the cumulative probability of genotypic ETVr and genotypic ETVr associated with virologic breakthrough was 1.2% and 0.8%, respectively. In contrast, a reduced barrier to resistance was observed in LVD-refractory patients, as the LVD resistance substitutions, a partial requirement for ETVr, preexist, resulting in a 5-year cumulative probability of genotypic ETVr and genotypic ETVr associated with breakthrough of 51% and 43%, respectively. Importantly, only four patients who achieved < 300 copies/mL HBV DNA subsequently developed ETVr.

CONCLUSION

Long-term monitoring showed low rates of resistance in nucleoside-naïve patients during 5 years of ETV therapy, corresponding with potent viral suppression and a high genetic barrier to resistance. These findings support ETV as a primary therapy that enables prolonged treatment with potent viral suppression and minimal resistance.

Inhibition of hepatitis B virus polymerase by entecavir

Entecavir (ETV; Baraclude) is a novel deoxyguanosine analog with activity against hepatitis B virus (HBV). ETV differs from the other nucleoside/tide reverse transcriptase inhibitors approved for HBV therapy, lamivudine (LVD) and adefovir (ADV), in several ways: ETV is >100-fold more potent against HBV in culture and, at concentrations below 1 microM, displays no significant activity against human immunodeficiency virus (HIV). Additionally, while LVD and ADV are obligate DNA chain terminators, ETV halts HBV DNA elongation after incorporating a few additional bases. Three-dimensional homology models of the catalytic center of the HBV reverse transcriptase (RT)-DNA-deoxynucleoside triphosphate (dNTP) complex, based on the HIV RT-DNA structure, were used with in vitro enzyme kinetic studies to examine the mechanism of action of ETV against HBV RT. A novel hydrophobic pocket in the rear of the RT dNTP binding site that accommodates the exocyclic alkene moiety of ETV was predicted, establishing a basis for the superior potency observed experimentally. HBV DNA chain termination by ETV was accomplished through disfavored energy requirements as well as steric constraints during subsequent nucleotide addition. Validation of the model was accomplished through modeling of LVD resistance substitutions, which caused an eightfold decrease in ETV susceptibility and were predicted to reduce, but not eliminate, the ETV-binding pocket, in agreement with experimental observations. ADV resistance changes did not affect the ETV docking model, also agreeing with experimental results. Overall, these studies explain the potency, mechanism, and cross-resistance profile of ETV against HBV and account for the successful treatment of naive and LVD- or ADV-experienced chronic HBV patients.

Comprehensive evaluation of hepatitis B virus reverse transcriptase substitutions associated with entecavir resistance

Virologic resistance emerging during entecavir (ETV) therapy for hepatitis B virus (HBV) requires three substitutions in the viral reverse transcriptase (RT), signifying a high barrier to resistance. Two of these substitutions are associated with lamivudine resistance (LVDr) in the tyrosine-methionine-aspartate-aspartate (YMDD) motif (rtM204V and rtL180M), whereas the other occurs at one or more positions specifically associated with ETV resistance (ETVr): rtT184, rtS202, or rtM250. Although a variety of substitutions at these primary ETVr positions arise during ETV therapy, only a subset give rise to clinical virologic breakthrough. To determine the phenotypic impact of observed clinical and potential new ETVr substitutions, a comprehensive panel of clones containing every possible amino acid at the three primary ETVr positions in LVDr HBV was constructed and analyzed in vitro. A range of replication capacities was observed for the panel, but none of the mutations rescued replication of the LVDr mutant to the wild-type level. More clones with residue rtS202 substitutions were severely impaired than those at rtT184 or rtM250. A wide variety of ETV susceptibilities was observed, ranging from approximately eight-fold (no increase over the LVDr parent) to greater than 400-fold over the wild-type. A correlation was identified between clinically observed substitutions and those displaying higher in vitro replication and resistance, especially those from virologic breakthrough patients.

CONCLUSION

The high number of tolerated and resistant ETVr substitutions is consistent with models predicting that the mechanism for ETVr is through enhancement of LVDr changes in the RT deoxyribonucleotide triphosphate (dNTP)-binding pocket.

A novel small molecule inhibitor of hepatitis C virus entry

Small molecule inhibitors of hepatitis C virus (HCV) are being developed to complement or replace treatments with pegylated interferons and ribavirin, which have poor response rates and significant side effects. Resistance to these inhibitors emerges rapidly in the clinic, suggesting that successful therapy will involve combination therapy with multiple inhibitors of different targets. The entry process of HCV into hepatocytes represents another series of potential targets for therapeutic intervention, involving viral structural proteins that have not been extensively explored due to experimental limitations. To discover HCV entry inhibitors, we utilized HCV pseudoparticles (HCVpp) incorporating E1-E2 envelope proteins from a genotype 1b clinical isolate. Screening of a small molecule library identified a potent HCV-specific triazine inhibitor, EI-1. A series of HCVpp with E1-E2 sequences from various HCV isolates was used to show activity against all genotype 1a and 1b HCVpp tested, with median EC50 values of 0.134 and 0.027 µM, respectively. Time-of-addition experiments demonstrated a block in HCVpp entry, downstream of initial attachment to the cell surface, and prior to or concomitant with bafilomycin inhibition of endosomal acidification. EI-1 was equally active against cell-culture adapted HCV (HCVcc), blocking both cell-free entry and cell-to-cell transmission of virus. HCVcc with high-level resistance to EI-1 was selected by sequential passage in the presence of inhibitor, and resistance was shown to be conferred by changes to residue 719 in the carboxy-terminal transmembrane anchor region of E2, implicating this envelope protein in EI-1 susceptibility. Combinations of EI-1 with interferon, or inhibitors of NS3 or NS5A, resulted in additive to synergistic activity. These results suggest that inhibitors of HCV entry could be added to replication inhibitors and interferons already in development.

Hepatitis B virus quasispecies susceptibility to entecavir confirms the relationship between genotypic resistance and patient virologic response

BACKGROUND/AIMS

The efficacy of anti-viral therapy for chronic hepatitis B virus (HBV) is lost upon the emergence of resistant virus. Using >500 patient HBV isolates from several entecavir clinical trials, we show that phenotypic susceptibility correlates with genotypic resistance and patient virologic responses.

METHODS

The full-length HBV or reverse transcriptase gene was amplified from patient sera, sequenced, and cloned into an HBV expression vector. Entecavir susceptibilities of individual virus clones and patient quasispecies populations were analyzed in conjunction with the sequenced resistance genotype and the patient's virologic response.

RESULTS

Entecavir susceptibility decreased approximately 8-fold for isolates with various constellations of lamivudine resistance substitutions. The spectrum of additional substitutions that emerged during therapy at residues rtT184, rtS202, or rtM250 displayed varying levels of entecavir susceptibility according to the specific resistance substitutions and the proportion of resistant variants in the quasispecies. Phenotypic analyses of samples associated with virologic breakthrough confirmed the role of these residue changes in entecavir resistance. Additional longitudinal phenotypic analyses showed that decreased susceptibility correlated with both genotypic resistance and increased circulating HBV DNA.

CONCLUSIONS

HBV phenotypic analysis provides additional insight as part of a resistance monitoring program that includes genotypic analysis and quantification of circulating virus.

Inhibitors of HIV-1 attachment. Part 9: an assessment of oral prodrug approaches to improve the plasma exposure of a tetrazole-containing derivative

7-(2H-Tetrazol-5-yl)-1H-indole 3 was found to be a potent inhibitor of HIV-1 attachment but the compound lacked oral bioavailability in rats. The cause of the low exposure was believed to be poor absorption attributed to the acidic nature of the tetrazole moiety and, in an effort to address this liability, three more lipohilic tetrazole analogs, N-acetoxymethyl 4, N-pivaloyloxymethyl 5, and N-methyl 6, were evaluated as potential oral prodrugs in rats. Prodrug 5 was ineffective in improving the plasma concentration of 3 in vivo but compound 4 provided a 15-fold enhancement of the plasma concentration of 3. Most interestingly, oral dosing of analog 6 afforded a substantial increase in the plasma concentration of the parent in rats when compared to dosing of parent. This represents a novel example of a methyl tetrazole that acts as a prodrug for a free NH tetrazole-containing compound.

parC mutations in fluoroquinolone-resistant Borrelia burgdorferi

We have isolated in vitro fluoroquinolone-resistant mutants of the Lyme disease agent, Borrelia burgdorferi. Mutations in parC, which encodes a subunit of topoisomerase IV, were associated with loss of susceptibility to sparfloxacin, moxifloxacin, and Bay-Y3118, but not ciprofloxacin. This is the first description of fluoroquinolone resistance in the spirochete phylum.

High-throughput screening and rapid inhibitor triage using an infectious chimeric Hepatitis C virus

The recent development of a Hepatitis C virus (HCV) infectious virus cell culture model system has facilitated the development of whole-virus screening assays which can be used to interrogate the entire virus life cycle. Here, we describe the development of an HCV growth assay capable of identifying inhibitors against all stages of the virus life cycle with assay throughput suitable for rapid screening of large-scale chemical libraries. Novel features include, 1) the use of an efficiently-spreading, full-length, intergenotypic chimeric reporter virus with genotype 1 structural proteins, 2) a homogenous assay format compatible with miniaturization and automated liquid-handling, and 3) flexible assay end-points using either chemiluminescence (high-throughput screening) or Cellomics ArrayScan™ technology (high-content screening). The assay was validated using known HCV antivirals and through a large-scale, high-throughput screening campaign that identified novel and selective entry, replication and late-stage inhibitors. Selection and characterization of resistant viruses provided information regarding inhibitor target and mechanism. Leveraging results from this robust whole-virus assay represents a critical first step towards identifying inhibitors of novel targets to broaden the spectrum of antivirals for the treatment of HCV.