Metabolic studies on BMS-200475, a new antiviral compound active against hepatitis B virus

Synthesis of novel entecavir analogues having 4'-cyano-6''-fluoromethylenecyclopentene skeletons as an aglycone moiety as highly potent and long-acting anti-hepatitis B virus agent

Encouraged by our recent findings that 4'-cyano-deoxyguanosine (2), entecavir analogues 4 and 5 are highly potent anti-hepatitis B virus (HBV) agents, we designed and synthesized 6 having a hybridized structure of 4 and 5. The chiral quaternary carbon portion at the 4'-position, which is substituted by cyano- and 5'-hydroxymethyl groups, was stereospecifically constructed by radical-mediated 5-exo-dig mode cyclization of 10. The introduction of the fluorine atom into the 6''-position was achieved by radical-mediated stannylation of sulfide (E)-11 and subsequent electrophilic fluorination of (E)-12. The desired (E)-6 was obtained after the introduction of the guanine base into (E)-18 under Mitsunobu conditions and following global deprotection. The stereoisomer (Z)-6 was also prepared by the same procedure using (Z)-12. Compound (E)-6 showed highly potent anti-HBV activity (EC₅₀ = 1.2 nM) with favorable cytotoxicity (CC₅₀ = 93 μM).

Entecavir competitively inhibits deoxyguanosine and deoxyadenosine phosphorylation in isolated mitochondria and the perfused rat heart

Deoxyguanosine kinase (dGK) is reported responsible for the phosphorylation of deoxyadenosine (dA) and deoxyguanosine (dG) in the mitochondrial purine salvage pathway. Antiviral nucleoside analogs known as nucleoside reverse transcriptase inhibitors (NRTIs) must be phosphorylated by host enzymes for the analog to become active. We address the possibility that NRTI purine analogs may be competitive inhibitors of dGK. From a group of such analogs, we demonstrate that entecavir (ETV) competitively inhibited the phosphorylation of dG and dA in rat mitochondria. Mitochondria from the brain, heart, kidney, and liver showed a marked preference for phosphorylation of dG over dA (10-30-fold) and ETV over dA (2.5-4-fold). We found that ETV inhibited the phosphorylation of dG with an IC50 of 15.3 ± 2.2 μM and that ETV and dG were both potent inhibitors of dA phosphorylation with IC50s of 0.034 ± 0.007 and 0.028 ± 0.006 μM, respectively. In addition, the phosphorylation of dG and ETV followed Michaelis-Menten kinetics and each competitively inhibited the phosphorylation of the other. We observed that the kinetics of dA phosphorylation were strikingly different from those of dG phosphorylation, with an exponentially lower affinity for dGK and no effect of dA on dG or ETV phosphorylation. Finally, in an isolated heart perfusion model, we demonstrated that dG, dA, and ETV were phosphorylated and dG phosphorylation was inhibited by ETV. Taken together, these data demonstrate that dGK is inhibited by ETV and that the primary role of dGK is in the phosphorylation of dG rather than dA.

Voltammetric Methods Used in the Determination of Nucleoside Reverse Transcriptase Inhibitors

Background:

The Human Immunodeficiency Virus (HIV) has now been established as the causative agent of the Acquired Immunodeficiency Syndrome (AIDS) and exactly 25 antiretroviral drugs have been formally approved for clinical use in the treatment of AIDS. The life quality and duration of HIV-positive patients have increased with the usage of antiretroviral drugs in the treatment of AIDS. Nucleoside Reverse Transcriptase Inhibitors (NRTIs) are one of the subgroups of antiretroviral.

Objective:

The quantification of drugs is important, as they make positive contributions to dose adjustments in practice. Voltammetric methods are very powerful analytical methods used in the pharmaceutical industry because of the determination of therapeutic agents and/or their metabolites in clinical samples at extremely low concentrations (10-50 ng/ml).

Methods:

This review mainly includes the pharmacological properties and recent determination studies by voltammetric methods from pharmaceutical dosage forms and biological samples of eight NRTIs group antiretroviral drugs (zidovudine, abacavir, adefovir, entecavir, zalcitabine, didanosine, emtricitabine, lamivudine) that are used in the clinic and show electroactive properties, were performed.

Conclusion:

Due to the variety of working electrodes in voltammetric methods, it is possible to choose the electrode that best responds. In this way, the analysis of NRTIs was possible at lower concentrations in pharmaceuticals and biological samples with voltammetric methods in these studies without the necessity for the sample pre-treatment or time-consuming extraction steps. The voltammetric methods provide good stability, repeatability, reproducibility and high recovery for the analysis of the analyte. They could be used for the pharmacokinetic studies as well as for quality control laboratory studies.

Pharmacokinetic properties of a novel inosine analog, 4'-cyano-2'-deoxyinosine, after oral administration in rats

4′-cyano-2′-deoxyinosine (SK14-061a), a novel nucleoside analog based on inosine, has antiviral activity against the human immunodeficiency virus type 1 that has the ability to acquire resistance against many types of reverse transcriptase inhibitors based on nucleosides. The aim of this study was to investigate the pharmacokinetics studies after its oral administration to rats. For this purpose, we first developed and validated an analytical method for quantitatively determining SK14-061a levels in biological samples by a UPLC system interfaced with a TOF-MS system. A rapid, simple and selective method for the quantification of SK14-061a in biological samples was established using liquid chromatography mass spectrometry (LC-MS) with solid phase extraction. The pharmacokinetic properties of SK14-061a in rats after oral administration were then evaluated using this LC-MS method. SK14-061a was found to be relatively highly bioavailable, is rapidly absorbed from the intestinal tract, and is then mainly distributed to the liver and then ultimately excreted via the urine in an unchanged form. Furthermore, the simultaneous administration of SK14-061a with the nucleoside analog, entecavir, led to a significant alteration in the pharmacokinetics of SK14-061a. These results suggest that the SK14-061a has favorable pharmacokinetic properties with a high bioavailability with the potential for use in oral pharmaceutical formulations, but drug-drug interactions should also be considered.

Pharmacokinetics studies of 4'-cyano-2'-deoxyguanosine, a potent inhibitor of the hepatitis B virus, in rats

OBJECTIVES

4'-cyano-2'-deoxyguanosine (CdG), a novel nucleoside analogue, has a high degree of antiviral activity against the chronic hepatitis B virus (HBV). The objective of this study was to develop an analytical method for quantitatively determining CdG levels in biological samples by liquid chromatography-mass spectrometry (LC/MS) and to investigate the pharmacokinetic properties of CdG in rats after intravenous and oral administration.

METHODS

An analytical method using a UPLC system interfaced with a TOF-MS system was developed and validated. The pharmacokinetic properties after the intravenous and oral administration of CdG to rats were evaluated. In vivo pharmacokinetic interactions between CdG and entecavir were also investigated.

KEY FINDINGS

A rapid, simple and selective method for the quantification of CdG in biological samples was established using LC/MS with solid-phase extraction. In vivo pharmacokinetic studies of CdG in rats demonstrated that CdG is highly bioavailable, is rapidly absorbed from the intestinal tract, is then distributed to the liver rather than kidney and is ultimately excreted via the urine in an unchanged form. The co-administration of CdG and entecavir led to pharmacokinetic interactions with each other.

CONCLUSIONS

The data generated in this study provide support for the clinical development of CdG for use in the treatment of HBV.

A novel entecavir analogue constructing with a spiro[2.4]heptane core structure in the aglycon moiety: Its synthesis and evaluation for anti-hepatitis B virus activity

Synthesis of a novel 2'-deoxy-guanine carbocyclic nucleoside 4 constructed with spiro[2.4]heptane core structure in the aglycon moiety was carried out. Radical-mediated 5-exo-dig mode cyclization and following cyclopropanation proceeded efficiently to furnish the spiro alcohol 10. Subsequent Mitsunobu-type glycosylation between 13 and 14, deoxygenation of the 2'-hydroxyl group of 16 and deprotection of 17 gave the title compound 4. Compound 4 demonstrated moderate anti-HBV activity (EC₅₀ value of 0.12 ± 0.02 µM) and no cytotoxicity against HepG2 cells was observed up to 100 µM.

Design, Synthesis, and Evaluation of Anti-HBV Activity of Hybrid Molecules of Entecavir and Adefovir: Exomethylene Acycloguanine Nucleosides and Their Monophosphate Derivatives

Exomethylene acycloguanine nucleosides 4, 6 and its monophosphate derivatives 5, 7, and 8 have been synthesized. Mitsunobu-type coupling of 2-N-acetyl-6-O-diphenylcarbamoylguanine (11) with primary alcohols proceeded regioselectively to furnish the desired N(9)-substituted products in moderate yield. Evaluation of 4-8 for anti-HBV activity in HepG2 cells revealed that the phosphonate derivative 8 was found to exhibit moderated activity (EC50 value of 0.29 μM), but cytotoxicity (CC50 value of 39 μM) against the host cells was also observed.

Diastereoselective Synthesis of 6″-(Z)- and 6″-(E)-Fluoro Analogues of Anti-hepatitis B Virus Agent Entecavir and Its Evaluation of the Activity and Toxicity Profile of the Diastereomers

A method for the diastereoselective synthesis of 6″-(Z)- and 6″-(E)-fluorinated analogues of the anti-HBV agent entecavir has been developed. Construction of the methylenecyclopentane skeleton of the target molecules has been accomplished by radical-mediated 5-exo-dig cyclization of the selenides 6 and 15 having the phenylsulfanylethynyl structure as a radical accepting moiety. In the radical reaction of the TBS-protected precursor 6, (Z)-anti-12 was formed as a major product. On the other hand, TIPS-protected 15 gave (E)-anti-12. The sulfur-extrusive stannylation of anti-12 furnished a mixture of geometric isomers of the respective vinylstannane, whereas benzoyl-protected 17 underwent the stannylation in the manner of retention of configuration. Following XeF2-mediated fluorination, introduction of the purine base and deoxygenation of the resulting carbocyclic guanosine gave the target (E)- and (Z)-3 after deprotection. Evaluation of the anti-HBV activity of 3 revealed that fluorine-substitution at the 6″-position of entecavir gave rise to a reduction in the cytotoxicity in HepG2 cells with retention of the antiviral activity.

Changes in Mitochondrial Toxicity in Peripheral Blood Mononuclear Cells During Four-Year Administration of Entecavir Monotherapy in Chinese Patients with Chronic Hepatitis B

BACKGROUND

This study aimed to assess whether long-term entecavir monotherapy induces mitochondrial toxicity in patients with chronic hepatitis B (CHB).

MATERIAL AND METHODS

This was a prospective study in 34 antiviral treatment-naïve patients with CHB who received entecavir monotherapy and were followed up for 4 years. Blood samples were collected after 0, 2, 3, and 4 years of entecavir (ETC) monotherapy (ETC0, ETC2, ETC3, and ETC4, respectively). Mitochondrial DNA (mtDNA) contents were determined using real-time quantitative polymerase chain reaction (qRT-PCR) and mtDNA4977 depletions were detected using nested PCR. Levels of hepatitis B virus (HBV) DNA, alanine aminotransferase, alanine aminotransferase, hepatitis B e antigen (HBeAg), creatine kinase, urea nitrogen, and serum creatinine were recorded.

RESULTS

mtDNA contents at ETC0 (9.6±6.3) and ETC4 (10.3±6.2) were markedly higher than at ETC2 (0.8±0.5, P<0.01) and ETC3 (1.3±0.9, P<0.01), but there were no differences between ETC2 and ETC3 or between ETC0 and ETC4. MtDNA4977 depletion appeared in 79.4% cases at ETC2 and in 70.6% at ETC3, which were much higher than at ETC0 (32.4%, P<0.01) and ETC4 (8.8%, P<0.01), but there were no differences in mtDNA4977 depletion ratio between ETC2 and ETC3, or between ETC0 and ETC4. mtDNA content was negatively correlated to mtDNA4977 depletion (partial regression coefficient of -4.555, P<0.001, R2=0.315). mtDNA content was positively correlated with age (partial regression coefficient of 0.131, P=0.045).

CONCLUSIONS

Results suggest that during 4-year entecavir monotherapy for CHB, the mtDNA contents initially decreased and then increased, while the mtDNA4977 depletion rates first increased and then decreased.

Antiviral Agents☆

Antiviral agents are drugs approved in the USA by the Food and Drug Administration (FDA) for the treatment or control of viral infections. Available antiviral agents mainly target stages in the viral life cycle. The target stages in the viral life cycle are; viral attachment to host cell, uncoating, synthesis of viral mRNA, translation of mRNA, replication of viral RNA and DNA, maturation of new viral proteins, budding, release of newly synthesized virus, and free virus in body fluids.

Two important factors that can limit the utility of antiviral drugs are toxicity and the development of resistance to the antiviral agent by the virus. In addition, host phenotypic behaviors toward antiviral drugs because of either genomic or epigenetic factors could limit the efficacy of an antiviral agent in an individual. This article summarizes the most relevant pharmacologic and clinical properties of current antiviral agents, and targets for novel antiviral agents.

Study on the antiviral activity of San Huang Yi Gan Capsule against hepatitis B virus with seropharmacological method

Background

Seropharmacology arising recently is a novel method of in vitro pharmacological study on Chinese herb using drug-containing animal serum. As seropharmacology possesses the advantages of experiments in vitro and in vivo, it is increasingly applied in pharmacological research on Chinese medicine. However, some issues of seropharmacology remain controversial and need to be clearly defined. San Huang Yi Gan Capsule (SHYGC) is a Chinese herbal formula with antiviral property against hepatitis B virus (HBV), but little is known about the mechanism underlying its anti-HBV activity. The aim of the present study was to elucidate the action mechanism of SHYGC using seropharmacological method and systematically address the methodology of preparing drug-containing serum.

Methods

New Zealand rabbits were orally administrated SHYGC with various regimens, followed by preparation of SHYGC-containing rabbit sera with a variety of methods. After HBV-producing HepG2 2.2.15 cells were treated with SHYGC-containing sera or entecavir for 9 days, the levels of hepatitis B surface antigen (HBsAg) and HBV DNA and the activity of DNA Polymerase were determined in HepG2 2.2.15 cells-conditioned media.

Results

An optimally standardized method of preparing drug-containing serum was raised for seropharmacology, with which SHYGC was demonstrated to suppress HBsAg expression, HBV DNA replication and DNA Polymerase activity in a dose-dependent fashion.

Conclusions

This seropharmacological study shows SHYGC is a potentially powerful anti-HBV agent. Additionally, seropharmacology is a promising pharmacological method with a broad range of advantages, and it can be widely used in biomedical research, if combined with pharmacokinetics.

Entecavir for the treatment of patients with hepatitis B virus-related decompensated cirrhosis

INTRODUCTION

Chronic hepatitis B (CHB) infection is common and carries a significant risk for the development of cirrhosis, hepatic decompensation, and hepatocellular carcinoma. The goal of treatment in patients with CHB-related decompensated cirrhosis is to improve hepatic dysfunction and reduce mortality through the inhibition of viral replication. Several studies have now shown nucleot(s)ide analogs to be safe and effective in decompensated cirrhosis due to CHB.

AREAS COVERED

A review of the evidence for the use of entecavir in the treatment of decompensated hepatitis B cirrhosis is discussed.

EXPERT OPINION

Entecavir is an effective treatment option for most patients with CHB. In treatment naïve patients, it is a potent antiviral agent with a very low resistance rate, making it an excellent option for the treatment of decompensated hepatitis B cirrhosis. The use of entecavir monotherapy in patients with a known rtM204V lamivudine-resistant mutation should be avoided due to increased risk of developing entecavir resistance and failing treatment.

Synthesis of cyclopentanyl carbocyclic 5-fluorocytosine ((-)-5-fluorocarbodine) using a facially selective hydrogenation approach

An efficient synthetic route to biologically relevant (-)-5-fluorocarbodine 6 was developed. Direct coupling of N(6)-protected 5-fluorouracil 15 with cyclopentenyl intermediate 13, followed by formation of a macrocycle between the base and the carbocyclic sugar moiety, via ring-closing metathesis, allowed for a facial selective hydrogenation of the sugar double bond to give, exclusively, the desired 4'-β stereoisomer.

Selection of chronic hepatitis B therapy with high barrier to resistance

Antiviral drug resistance is a crucial factor that frequently determines the success of long-term therapy for chronic hepatitis B. The development of resistance to nucleos(t)ide analogues has been associated with exacerbations in liver disease and increased risk of emergence of multidrug resistance. The selection of a potent nucleos(t)ide analogue with a high barrier to resistance as a first-line therapy, such as entecavir or tenofovir, provides the best chance of achieving long-term treatment goals and should be used wherever possible. The barrier to resistance of a given nucleos(t)ide analogue is influenced by genetic barrier, drug potency, patient adherence, pharmacological barrier, viral fitness, mechanism of action, and cross-resistance. In countries with limited health-care resources, the selection of a therapy with a high barrier to resistance is not always possible and alternative strategies for preventing resistance might be needed, although limited data are available to support these strategies.

Update on entecavir in the management of severe forms of Hepatitis B

Despite the effective management of most cases of hepatitis B virus (HBV) infection, there is still much room for improvement in the treatment of more severe cases of hepatitis B, such as those occurring in patients with decompensated cirrhosis, in transplanted subjects, and in patients with exacerbations of HBV infection. Among the treatments currently approved for the therapy of hepatitis B, entecavir determines a rapid suppression of viral load. This drug is also associated with a high genetic barrier and an overall favorable safety profile. This review provides an overview of recent evidence related to the use of entecavir in the management of the most severe forms of hepatitis B. The results obtained for this drug in real-life clinical practice are also reviewed.

Iatrogenic mitochondriopathies: a recent lesson from nucleoside/nucleotide reverse transcriptase inhibitors

The use of nucleoside/nucleotide reverse transcriptase inhibitors (NRTIs) has revolutionized the treatment of infection by human immunodeficiency virus (HIV) and hepatitis-B virus. NRTIs can suppress viral replication in the long-term, but possess significant toxicity that can seriously compromise treatment effectiveness. The major toxicity of NRTIs is mitochondrial toxicity. This manifests as serious side effects such as myopathy, peripheral neuropathy and lactic acidosis. In general, it is believed that the mitochondrial pathogenesis is closely related to the effect of NRTIs on mitochondrial DNA polymerase-γ. Depletion and mutation of mitochondrial DNA during chronic NRTI therapy may lead to cellular respiratory dysfunction and release of reactive oxidative species, resulting in cellular damage. It is now apparent that the etiology is far more complex than originally thought. It appears to involve multiple mechanisms as well as host factors such as HIV per se, inborn mitochondrial mutation, and sex. Management of mitochondrial toxicity during NRTI therapy remains a challenge. Interruption of NRTI therapy and substitution of the causative agents with alternative better-tolerated NRTIs represents the mainstay of management for mitochondrial toxicity and its clinical manifestations. A range of pharmacological approaches has been proposed as treatments and prophylaxes.

Safety and efficacy of entecavir for the treatment of chronic hepatitis B

Entecavir is a cyclopentyl deoxyguanosine analog that was approved for the treatment of the hepatitis B virus (HBV) in 2005. In Phase III trials, it showed potent HBV suppression with drops of 6- to 7-log copies/mL in HBV DNA at 1 year. In addition, rates of genotypic resistance in nucleos(t)ide-naïve patients are low, reaching only 1.2% after 6 years. Safety and efficacy have been established in compensated cirrhosis and HIV-coinfected patients. Studies in decompensated cirrhosis also show efficacy. Because of potent viral suppression and a large genetic barrier to resistance, entecavir is now a first-line choice in most HBV treatment guidelines and has become an integral part of the HBV treatment armamentarium.

Binding of cordycepin monophosphate to AMP-activated protein kinase and its effect on AMP-activated protein kinase activation

It had been reported that cordycepin could activate AMP-activated protein kinase. One possible mechanism is that cordycepin mediated AMP-activated protein kinase activation by conversion into cordycepin monophosphate, which acts as an AMP analog to activate AMP-activated protein kinase. To confirm the aforementioned hypothesis, we investigate the binding of cordycepin monophosphate to AMP-activated protein kinase using molecular docking. The modeling results indicate that cordycepin monophosphate binds to AMP-activated protein kinase with high affinity. The hydrogen bonds provide attractive forces between molecules. Our results further identify the key residues contributing to the interaction. Also, the modeling results predict that cordycepin monophosphate and AMP would have similar binding modes with AMP-activated protein kinase. Further investigation of AMP-activated protein kinase activation in vitro provides the evidence that cordycepin monophosphate functioned as an AMP mimic to activate AMP-activated protein kinase.

Antiviral treatment of chronic hepatitis B virus (HBV) infections

While 25 compounds have been formally licensed for the treatment of HIV infection (AIDS), only seven licensed products are currently available for the treatment of chronic hepatitis B virus (HBV) infection: interferon-α, pegylated interferon-α, lamivudine, adefovir (dipivoxil), entecavir, telbivudine and tenofovir (disoproxil fumarate). In contrast to the treatment of HIV infections where the individual drugs are routinely used in combination, for the treatment of chronic HBV infection the individual drugs are generally used in monotherapy. In principle, combination drug therapy should allow reducing the likelihood of drug-resistant development.

Entecavir for the long-term treatment of chronic hepatitis B

Hepatitis B virus is a major cause of chronic liver disease worldwide and is associated with an increased risk of hepatocellular carcinoma, progressive hepatic fibrosis and end-stage liver disease. Suppression of HBV replication is recognized as the primary on-treatment goal of antiviral therapy, as reduction of serum HBV DNA to low or undetectable levels is highly likely to have a positive impact on long-term clinical outcomes in HBV-associated chronic liver disease. Entecavir is an oral nucleoside analogue that effectively inhibits HBV polymerase, resulting in rapid viral suppression. Long-term data on patients receiving entecavir for chronic hepatitis B have demonstrated high potency, a low incidence of antiviral drug resistance and good tolerability, making entecavir an ideal first-line agent for the treatment of chronic hepatitis B.

[Resistance in hepatitis B virus]

The development hepatitis B virus (HBV) polymerase inhibitors has revolutionised the treatment of chronic HBV infection. However, the emergence of resistance mutations can compromise their clinical efficacy and it is mandatory to know the mechanisms of these resistances, its clinical implications, strategies for prevention and how to deal with the rescue. Since HBV has a high degree of replication and a high error rate, during their life cycle it will produce a large number of punctual mutations in individuals with active replication. Due to the large size of the HBV genome, all the possible changes may occur daily and should be screened before starting any antiviral therapy. Therefore, in individuals infected with HBV there is a mixture of similar viruses that evolves over time (quasispecies), some of which are carriers of resistance mutations to antivirals, which explains why they can be selected quickly after exposure to drug. Of the five drugs approved in Europe for the treatment of hepatitis B, three of them (lamivudine, adefovir and entecavir) are likely to be affected directly by these mutations, as well as other active drugs, such as telbivudine, tenofovir and the emtricitabine. The characterization of the resistance mutations is helpful for the prevention and the optimization of antiviral therapies.

Entecavir: a potent antiviral with minimal long-term resistance in nucleoside-naive chronic hepatitis B patients

Entecavir has demonstrated safety and efficacy in the treatment of chronic hepatitis B infection. It is the prototype for the cyclopentane class of nucleoside/nucleotide chronic hepatitis B antiviral agents. It has a high potency and, due to its structural formula and mechanism of action, entecavir is associated with emergence of minimal resistance in the long-term treatment of nucleoside-naive patients. Research suggests that long-term treatment may be required for chronic hepatitis B patients, especially those who acquire HBV early in life, to achieve maximum viral suppression and improve outcomes. Several recent studies have evaluated the long-term safety, efficacy and development of resistance in nucleoside-naive patients treated with entecavir. Results indicate that the long-term use of entecavir is well tolerated and associated with continuous clinical improvement -- with an increasing number of patients achieving undetectable levels of HBV DNA, HBeAg seroconversion and minimal resistance. These data underscore the position of entecavir for first-line therapy and highlight its role in the long-term treatment of chronic hepatits B.

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.

The anti-HIV activity of entecavir: a multicentre evaluation of lamivudine-experienced and lamivudine-naive patients

BACKGROUND

Entecavir, an antiviral with potent anti-hepatitis B virus activity, was recently shown to have anti-HIV activity in three patients and the ability to select for the lamivudine-resistant HIV polymerase mutation M184V in a patient with prior antiretroviral therapy.

OBJECTIVES

To further characterize entecavir's anti-HIV activity and identify risk factors for selection of the M184V.

DESIGN

Retrospective cohort study.

METHODS

We evaluated the virological characteristics of HIV and hepatitis B virus in 17 HIV-hepatitis B virus coinfected patients (10 antiretroviral therapy-naive and seven antiretroviral therapy-experienced) prior to and during entecavir monotherapy. Descriptive statistics were used to assess changes in HIV RNA and hepatitis B virus DNA. Variables associated with development of the M184V were determined by univariate analysis.

RESULTS

Of the 17 patients, 13 (76%) demonstrated a reduction in HIV RNA by at least 0.5 log10 copies/ml. Of the remaining four patients, two had the M184V detected prior to entecavir therapy and the other two had wild-type HIV. The median reduction in HIV RNA for the cohort was 1.2 log10 copies/ml, which was similar in antiretroviral therapy-naive and antiretroviral therapy-experienced patients. The M184V mutation emerged in six patients receiving entecavir, including three antiretroviral therapy-naive patients. No other HIV mutations were consistently detected. Risk factors for the emergence of the M184V mutation were a decline in hepatitis B virus DNA (P = 0.04) and duration of entecavir use (P = 0.05).

CONCLUSION

Entecavir monotherapy in HIV-hepatitis B virus coinfected patients, including antiretroviral therapy-naive patients, has significant anti-HIV activity and can result in the development of the M184V variant. Entecavir should not be used in such co-infected patients without concomitant antiretroviral therapy.

Antiviral treatment of chronic hepatitis B virus infections: the past, the present and the future

A decade ago, standard therapy against chronic hepatitis B virus infections only consisted of lamivudine or IFN-alpha. Treatment with lamivudine and IFN has been compounded by, respectively, the emergence of drug-resistant virus strains and the appearance of serious side effects. In the last 10 years, hepatitis B treatment has made much progress. Several treatments are now licensed for the treatment of patients with chronic hepatitis B and others are under development. Here, we provide an overview of the potential and mode of action of anti-HBV agents that are currently available, and/or may become available in the near future. Foremost among these newer compounds are adefovir dipivoxil, entecavir and telbivudine.

The HBV drug entecavir - effects on HIV-1 replication and resistance

Entecavir, a drug approved by the Food and Drug Administration for the treatment of chronic hepatitis B virus (HBV) infection, is not believed to inhibit replication of human immunodeficiency virus type 1 (HIV-1) at clinically relevant doses. We observed that entecavir led to a consistent 1-log(10) decrease in HIV-1 RNA in three persons with HIV-1 and HBV coinfection, and we obtained supportive in vitro evidence that entecavir is a potent partial inhibitor of HIV-1 replication. Detailed analysis showed that in one of these patients, entecavir monotherapy led to an accumulation of HIV-1 variants with the lamivudine-resistant mutation, M184V. In vitro experiments showed that M184V confers resistance to entecavir. Until more is known about HIV-1-resistance patterns and their selection by entecavir, caution is needed with the use of entecavir in persons with HIV-1 and HBV coinfection who are not receiving fully suppressive antiretroviral regimens.

Entecavir pharmacokinetics, safety, and tolerability after multiple ascending doses in healthy subjects

A double-blind, placebo-controlled, multiple oral dose escalation study was conducted to investigate the pharmacokinetics, safety, and tolerability of entecavir in healthy subjects. Eight subjects were assigned to each of the 3 dose panels (0.1 mg, 0.5 mg, and 1 mg or matched placebo once daily for 14 days). Blood and urine samples were collected for pharmacokinetic analyses. Entecavir was rapidly absorbed, with peak plasma concentration occurring within 1 hour of dosing. Steady-state plasma concentrations of entecavir were achieved by 10 days following the initial dose. At steady state, the mean area under the plasma concentration-time curve over 1 dosing interval, increased approximately proportional to dose. Entecavir had a mean terminal half-life ranging from 128 to 149 hours and an effective half-life of approximately 24 hours. Elimination was predominantly through renal excretion, with mean urinary recovery ranging from 62% to 73%. Entecavir was safe and well tolerated when administered at doses ranging from 0.1 mg to 1 mg/d for 14 days.

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.

Entecavir: a new nucleoside analog for the treatment of chronic hepatitis B infection

BACKGROUND

Chronic hepatitis B infection carries considerable risk for the development of cirrhosis and hepatocellular carcinoma. Treatment options are increasing but are limited to interferon alfa-2b, pegylated interferon alfa-2a, lamivudine, adefovir dipivoxil, and entecavir. Entecavir, a nucleoside analog, is the newest oral antiviral approved in the United States for treatment of chronic hepatitis B.

OBJECTIVE

To review the available data for entecavir regarding its pharmacology, pharmacokinetics, safety, efficacy, and clinical use.

METHODS

A MEDLINE, EMBASE, and Cochrane search of the English-language literature from January 1997-May 2006 was performed. Preapproval studies provided by the manufacturer and abstracts from recent scientific meetings on infectious disease and hepatology were also reviewed.

RESULTS

Three phase III clinical trials representing more than 1600 subjects established the superior efficacy and equivalent safety of entecavir compared with lamivudine for treating patients who are hepatitis B early antigen (HBeAg) positive, HBeAg negative, or refractory to lamivudine. Entecavir resistance has not occurred in nucleoside-naïve patients but may develop in those who already possess lamivudine resistance mutations.

CONCLUSION

Trial results, along with previously published response rates for adefovir dipivoxil and interferon monotherapy, make entecavir the preferred first-line treatment option for patients with chronic hepatitis B who are nucleoside naïve, HBeAg positive or negative, and have compensated liver disease. Both entecavir and adefovir dipivoxil maintain activity against hepatitis B virus in patients with chronic hepatitis B who are refractory to lamivudine, and both agents are reasonable first-line treatment options. Longer trials involving nucleoside-naïve, lamivudine-refractory patients are needed to determine entecavir's optimal treatment duration, long-term safety, and durability of response, including rate of resistance.

Management of patients with HBeAg-negative chronic hepatitis B

Chronic hepatitis B (CHB) is one of the leading causes of morbidity and mortality worldwide. Although various drugs are available for the treatment of CHB, emergence of the hepatitis B e antigen (HBeAg)-negative mutant variant, specifically in Asia, the Middle East and southern Europe, is creating a new challenge as this variant is less responsive to available treatments. HBeAg-negative CHB rapidly progresses to cirrhosis and its related complications. This review discusses the available literature on the approved and under-trial treatment options and their respective efficacies for HBeAg-negative CHB.

Two-year assessment of entecavir resistance in Lamivudine-refractory hepatitis B virus patients reveals different clinical outcomes depending on the resistance substitutions present

Entecavir (ETV) is a deoxyguanosine analog approved for use for the treatment of chronic infection with wild-type and lamivudine-resistant (LVDr) hepatitis B virus (HBV). In LVD-refractory patients, 1.0 mg ETV suppressed HBV DNA levels to below the level of detection by PCR (<300 copies/ml) in 21% and 34% of patients by Weeks 48 and 96, respectively. Prior studies showed that virologic rebound due to ETV resistance (ETVr) required preexisting LVDr HBV reverse transcriptase substitutions M204V and L180M plus additional changes at T184, S202, or M250. To monitor for resistance, available isolates from 192 ETV-treated patients were sequenced, with phenotyping performed for all isolates with all emerging substitutions, in addition to isolates from all patients experiencing virologic rebounds. The T184, S202, or M250 substitution was found in LVDr HBV at baseline in 6% of patients and emerged in isolates from another 11/187 (6%) and 12/151 (8%) ETV-treated patients by Weeks 48 and 96, respectively. However, use of a more sensitive PCR assay detected many of the emerging changes at baseline, suggesting that they originated during LVD therapy. Only a subset of the changes in ETVr isolates altered their susceptibilities, and virtually all isolates were significantly replication impaired in vitro. Consequently, only 2/187 (1%) patients experienced ETVr rebounds in year 1, with an additional 14/151 (9%) patients experiencing ETVr rebounds in year 2. Isolates from all 16 patients with rebounds were LVDr and harbored the T184 and/or S202 change. Seventeen other novel substitutions emerged during ETV therapy, but none reduced the susceptibility to ETV or resulted in a rebound. In summary, ETV was effective in LVD-refractory patients, with resistant sequences arising from a subset of patients harboring preexisting LVDr/ETVr variants and with approximately half of the patients experiencing a virologic rebound.

Entecavir: a review of its use in chronic hepatitis B

Entecavir (Baraclude) is a novel nucleoside analogue of 2'-deoxyguanosine whose intracellular triphosphate form inhibits replication of the hepatitis B virus (HBV). In large, randomised, double-blind, phase III clinical trials in patients with chronic HBV infection, oral entecavir 0.5 or 1.0mg once daily for up to 96 weeks was superior to lamivudine 100 mg/day in improving hepatic histology, normalising aminotransferase levels and suppressing viraemia to levels undetectable by polymerase chain reaction (PCR) assay in nucleoside-naive hepatitis B e antigen (HBeAg)-negative (precore or core promoter mutants) and/or HBeAg-positive patients, and in lamivudine-refractory (persistent viraemia during lamivudine therapy) HBeAg-positive patients. In addition, the tolerability profile of entecavir was generally similar to that of lamivudine. Thus, entecavir is a primary option in the treatment of chronic HBV infection in both nucleoside-naive and lamivudine-refractory patients.

Ribavirin and mycophenolic acid markedly potentiate the anti-hepatitis B virus activity of entecavir

MPA [the active metabolite of the immuno-suppressive agent CellCept] and ribavirin markedly potentiate the anti-HBV activity of the guanine-based nucleoside analogue entecavir (ETV) against both wild-type HBV and a lamivudine-resistant variant. Ribavirin (in its 5'-monophosphate form) and MPA are inhibitors of IMP-dehydrogenase and cause depletion of intracellular dGTP pools. The active triphosphorylated form of ETV may inhibit more efficiently the priming reaction, reverse transcription and DNA-dependent DNA polymerase activity of the HBV polymerase in the presence of reduced levels of dGTP. The potential for enhanced ETV activity is supported by the observation that exogenously added deoxyguanosine reversed the potentiating effect of ribavirin and MPA. Our observations may have important implications for those (liver) transplant recipients that receive MMF as part of their immunosuppressive regimen and who, because of a de novo or a persistent infection with HBV need antiviral therapy such as ETV. Further studies will need to be conducted to determine if combining ribavirin (a compound used for the treatment of HCV infections) with ETV could have an advantage for the treatment of HBV infections, in particular in patients co-infected with HCV.

Entecavir for the treatment of chronic hepatitis B virus infection

OBJECTIVE

This article reviews the pharmacology/pharmacokinetics and therapeutic efficacy of entecavir, which was approved on March 29, 2005, for the management of adult patients with chronic hepatitis B virus (HBV) infection who have active viral replication and/or elevations in liver transaminases or signs of active liver disease on histologic examination. Potential drug interactions and adverse events associated with the use of entecavir are also reviewed.

METHODS

Relevant literature was identified through searches of MEDLINE (1996-July 2005) and BIOSIS (1993-July 2005). Search terms included, but were not limited to, entecavir, BMS-200475, hepatitis B, pharmacology, pharmacokinetics, adverse events, and therapeutic use. Further publications were identified from the reference lists of the identified articles and through correspondence with the manufacturer of entecavir.

RESULTS

Entecavir is highly selective for the HBV and inhibits all 3 steps of viral replication. Results of early studies indicated a 6% resistance potential after 48 weeks of therapy, although the potential may be higher in patients who harbor lamivudine-resistant mutants. The approved dosage in treatment-naive patients is 0.5 mg/d p.o., administered on an empty stomach; in patients who have failed lamivudine therapy or are known to harbor lamivudine-resistant mutants, the approved dosage is 1.0 mg/d p.o.. The oral tablet and solution can be used interchangeably. Entecavir is well absorbed orally, achieving a dose-related Cmax between 0.6 and 1.5 hours after administration. It is metabolized to a small extent and is not a substrate for the cytochrome P450 enzyme system. The mean elimination t(1/2) ranges from 77 to 149 hours in patients with normal kidney function. Entecavir is eliminated primarily in the urine via glomerular filtration and tubular secretion (62%-73%). No dose adjustment appears to be necessary in patients with moderate to severe liver disease alone. The potential for drug interactions with entecavir appears to be minimal, although medications that inhibit tubular secretion of drugs (eg, probenecid) may be expected to prolong serum concentrations of entecavir. One of the Phase III studies of entecavir found statistically significant benefits compared with lamivudine in terms of improvements in liver histology after 48 weeks of therapy (72% vs 62%, respectively; P<0.009), the proportions of patients with undetectable HBV DNA titers on branched DNA signal amplification assay after 48 weeks of therapy (91% vs 65%; P<0.001), and the proportion with undetectable HBV DNA on polymerase chain reaction (PCR) assay after 48 weeks of therapy (69% vs 38%; P<0.001). In another Phase III study, patients who had failed to respond to lamivudine therapy responded to entecavir: after 48 weeks of therapy, significant differences between entecavir and lamivudine were seen in histologic improvement (55% vs 28%; P<0.001) and the proportion of patients with undetectable HBV DNA on PCR assay (21% vs 1%; P<0.001). Adverse events associated with entecavir therapy were similar in character, severity, and incidence to those associated with placebo or lamivudine therapy. The most common adverse events in clinical trials of entecavir were headache (17%-23% of patients), upper respiratory tract infection (18%-20%), cough (12%-15%), nasopharyngitis (9%-14%), fatigue (10%-13%), dizziness (9%), upper abdominal pain (9%-10%), and nausea (6%-8%).

CONCLUSIONS

Entecavir is a new antiviral agent for the management of chronic HBV infection. Questions concerning the ideal length of therapy, long-term efficacy, and resistance rates over time await the results of ongoing clinical trials.

Entecavir: a new treatment option for chronic hepatitis B

Because of the slow kinetics of viral clearance and the spontaneous genetic variability of hepatitis B virus (HBV), antiviral therapy of chronic hepatitis B remains a clinical challenge. Despite the recent development of lamivudine, adefovir dipivoxil and pegylated interferon alpha for the treatment of chronic HBV infection, there is still a major need for new antiviral compounds. Entecavir, a guanosine analog, has been recently approved in US for the therapy of chronic hepatitis B and its registration is expected soon in Europe. Extensive studies have been performed to characterize its antiviral activity in enzymatic and tissue culture models, as well as in animal models of HBV infection. In clinical trails, entecavir administration was associated with a significantly more potent viral suppression compared to lamivudine, and a significant advantage in terms of biochemical and histological improvement compared to lamivudine. Entecavir was tolerated as well as lamivudine in these phase III trials. No case of resistance was detected after two years of therapy in nucleoside naive patients. Treatment of patients with lamivudine failure requires a higher dosage of entecavir and induces a significant decline in viraemia levels. However, 10% of these patients developed entecavir resistance after two years of therapy. The availability of entecavir as a new treatment option is providing clinicians more choice to keep both viral replication and liver disease under control. This provides new hope for improved treatment concepts for chronic HBV infection.

A review of entecavir in the treatment of chronic hepatitis B infection

BACKGROUND

Infection with the hepatitis B virus (HBV) affects two billion people worldwide, and an estimated 400 million people are chronically infected. Currently, FDA-approved regimens for the treatment of chronic HBV include interferon-alpha2b, peginterferon-alpha2a, lamivudine, adefovir dipivoxil, and recently, entecavir.

OBJECTIVE

The purpose of this review is to evaluate the pharmacokinetic and pharmacodynamic properties, and the clinical efficacy and safety of entecavir in the treatment of nucleoside-naĩve and nucleoside-resistant HBeAg-positive and HBeAg-negative chronic hepatitis B (CHB). SEARCH METHODOLOGY: Computerized searches of PubMed and International Pharmaceutical Abstracts from 1985 to July 10, 2005, were performed with the search headings: entecavir, BMS-200475, and chronic hepatitis B.

FINDINGS

Entecavir, a new deoxyguanosine analog, represents a third agent within the nucleoside/nucleotide HBV polymerase inhibitor class with distinct advantages over lamivudine and adefovir dipivoxil: it has a three-step mechanism of action, is the most potent inhibitor of HBV DNA polymerase, is not associated with any major adverse effects, and has a limited potential for resistance. In phase II and III clinical trials, entecavir was found to be superior to lamivudine for all primary endpoints evaluated in both nucleoside-naïve and lamivudine-resistant patients. Entecavir was effective in both HBeAg-positive and HBeAg-negative nucleoside-naïve patients. At this time, optimal duration of entecavir therapy is unknown.

CONCLUSION

Entecavir represents a new first- or second-line treatment option for patients chronically infected with HBV. Long-term efficacy and safety studies as well as studies of entecavir in combination with interferon products or other nucleoside/nucleotide analogs are eagerly awaited.

Drugs in development for hepatitis B

The management of chronic hepatitis B (CHB) has improved dramatically over the last decade with the development of new drugs such as lamivudine and adefovir dipivoxil, in addition to the now standard interferon (IFN)-alpha therapy. These new drugs can achieve a significant reduction or inhibit replication of hepatitis B virus (HBV) DNA during therapy. However, in the majority of patients, particularly in those who are hepatitis B e antigen (HBeAg)-negative, the sustained off-therapy suppression of HBV DNA is rare. For this reason, several new antiviral and immunomodulatory agents are currently being evaluated. Among the immunomodulatory agents, pegylated IFNalpha (peginterferon-alpha) has been shown to be more effective for HBeAg-positive CHB than either lamivudine or standard IFNalpha monotherapy, particularly in those patients infected by HBV genotypes A and B. The new antivirals entecavir, tenofovir disoproxil fumarate and telbivudine exhibit a more potent viral inhibitory effect than the currently approved drugs (IFNs, lamivudine and adefovir dipivoxil). However, the emergence of viral resistance has been witnessed and this could be one of the major limitations to the clinical use of these new drugs, particularly during prolonged therapy. In HBeAg-negative patients it is more and more common for oral antiviral therapy to be administered for prolonged periods, as the sustained off-therapy response rates of short-term therapy are very low. Different studies are currently evaluating combination therapy, using lamivudine with adefovir dipivoxil or peginterferon-alpha with lamivudine; the preliminary results show virological responses no better than those achieved by monotherapy. However, as combination therapy is associated with a low likelihood of developing HBV drug resistance, this could result in a higher virological response during prolonged therapy. In the near future the most realistic therapeutic option for the majority of patients with CHB will be long-term use of these new, more potent antiviral drugs, if they can achieve good safety profiles while maintaining low resistance rates at affordable costs.

Treatment of chronic hepatitis B

In the last years, marked progress has been made in the treatment of chronic hepatitis B. The efficacy of lamivudine, the first nucleoside analogue available, is limited by the high incidence of resistance. Adefovir, which was recently approved has a comparable efficacy with a very low frequency of resistance. However, adefovir needs to be indefinitely administered as withdrawal of therapy is generally associated with reactivation and sustained response is uncommon. Recent large randomized controlled trials showed that PEG IFNs induce relatively high sustained response rates both in HBeAg positive and HBeAg negative chronic hepatitis B. So far, the combination of PEG IFN with lamivudine, used simultaneously, is disappointing in terms of short-term efficacy. However, long-term efficacy needs to be assessed and different schedules of combination (for example sequential) need to be evaluated. A number of nucleoside analogues, with favourable toxicity profiles and a promise of increased effectiveness against HBV, are at various stages of clinical development. Results of phase III trials of entecavir and emtricitabine confirmed their efficacy. However, while entecavir is associated with a low incidences of resistance, emtricitabine is associated with a relatively high incidence of resistance which limits its use as a monotherapy. The efficacy and safety of new and more potent drugs like telbivudine and clevudine need to be confirmed. The future of chronic hepatitis B therapy seems to be in the combination of different drugs. Ideally, the optimal drugs to combine would meet the following criteria: they should have different sites of action on HBV DNA replication, a potent antiviral effect, an excellent safety profile and they should induce a sustained response with a limited duration of therapy. Indeed, the concept of combination therapy has been recently developed in order to increase efficacy and to decrease the occurrence of viral resistance. However, so far few combinations have been evaluated. No combination therapy demonstrated a benefit as compared with monotherapy. More potent drugs and new combinations together with the understanding of the mechanisms of resistance to therapy are important challenges to improve the efficacy of treatment and decrease in the future the global burden related to chronic hepatitis B.