Adefovir dipivoxil: A new antiviral agent for the treatment of hepatitis B virus infection

Reaction Kinetics of the Benzylation of Adenine in DMSO: Regio-Selectivity Guided by Entropy

The factors governing the regio-selectivity of the alkylation of adenine have been of interest for many years due to the biological importance of adenine derivatives, however, no reaction kinetic studies have been conducted. Herein, we report the rate constants and activation parameters of the benzylation of adenine under basic conditions in DMSO in the absence and presence of 15-crown-5 ether using real-time 1H NMR spectroscopy. The reaction is second-order for the formation of the N9- and N3-benzyladenine products, with a regio-selectivity factor 2.3 in favour of the N9-adduct. The Gibbs free energy of activation amounts to 87±2 kJ mol-1 for both reactions. The formation of the N9-adduct is more activated by 7 kJ mol-1, but its effect is offset by a less negative activation entropy, demonstrating that the long-contested reason for the regioselectivity in the benzylation of adenine is dominated by compensation of entropy and enthalpy in the transition state. The kinetic parameters obtained in the presence of the 15-crown-5 ether indicate that the crown ether forms a complex with an adenine-sodium ion-pair, increasing the activation barrier. However, the Gibbs free energy in the absence and presence of the crown ether remains constant.

Coordination Sites for Sodium and Potassium Ions in Nucleophilic Adeninate Contact ion-Pairs: A Molecular-Wide and Electron Density-Based (MOWED) Perspective

The adeninate anion (Ade-) is a useful nucleophile used in the synthesis of many prodrugs (including those for HIV AIDS treatment). It exists as a contact ion-pair (CIP) with Na+ and K+ (M+) but the site of coordination is not obvious from spectroscopic data. Herein, a molecular-wide and electron density-based (MOWED) computational approach implemented in the implicit solvation model showed a strong preference for bidentate ion coordination at the N3 and N9 atoms. The N3N9-CIP has (i) the strongest inter-ionic interaction, by -30 kcal mol-1, with a significant (10-15%) covalent contribution, (ii) the most stabilized bonding framework for Ade-, and (iii) displays the largest ion-induced polarization of Ade-, rendering the N3 and N9 the most negative and, hence, most nucleophilic atoms. Alkylation of the adeninate anion at these two positions can therefore be readily explained when the metal coordinated complex is considered as the nucleophile. The addition of explicit DMSO solvent molecules did not change the trend in most nucleophilic N-atoms of Ade- for the in-plane M-Ade complexes in M-Ade-(DMSO)4 molecular systems. MOWED-based studies of the strength and nature of interactions between DMSO solvent molecules and counter ions and Ade- revealed an interesting and unexpected chemistry of intermolecular chemical bonding.

Pharmacotherapy options for managing hepatitis B in children

INTRODUCTION

To eliminate viral hepatitis by 2030, the World Health Organization (WHO) launched the first global health sector strategy on viral hepatitis, with particular focus given to hepatitis B and C in 2016. To achieve the reduction of mortality in children, it is indispensable to know which children should be treated and how to treat them.

AREA COVERED

In this article, the authors review the antiviral treatment of children with chronic hepatitis B virus (HBV) infection including antivirals available for children with chronic HBV infection.

EXPERT OPINION

The approvals of nucleos(t)ide analogues (NAs) and pegylated interferon (PEG-IFN) for children have lowered a hurdle to the initiation of antiviral treatment in children. The international guidelines use nearly the same criteria of antiviral treatment for children with chronic HBV infection, but the WHO guidelines provide a cautious stance on the antiviral treatment of children. Not only PEG-IFN but also NAs with a high genetic barrier to drug resistance should be the first-line treatment for children. In settings with limited medical resources, NAs can be the first-line treatment for children. Although the concept of an 'immune-tolerant phase' is challenged, evidence is not sufficient to recommend the treatment of HBeAg-positive immune-tolerant children.

Investigation of the Structure and Dynamics of Antiviral Drug Adefovir Dipivoxil by Site-Specific Spin-Lattice Relaxation Time Measurements and Chemical Shift Anisotropy Tensor Measurements

Adefovir is regarded as a potential antiviral agent. However, it cannot be considered as a valuable drug candidate due to its high polarity that limits its permeability across the human intestinal mucosa. When the ribose phosphate group of adefovir is replaced by the isopolar phosphonomethyl ether functionality, it neutralizes the negative charge of the drug. This makes the drug lipid-soluble and potent to diffuse across the cell membrane. The prodrug adefovir dipivoxil is regarded as a potent antiviral drug against hepatitis B virus (HBV), human immunodeficiency virus (HIV), Rauscher murine leukemia virus (R-MuLV), murine cytomegalovirus (MCMV), herpes simplex virus (HSV), simian immunodeficiency virus (SIV), and feline immunodeficiency virus (FIV). The correlation between the structure and the dynamics of adefovir dipivoxil is determined by measuring the principal components of chemical shift anisotropy (CSA) tensor, site-specific spin-lattice relaxation time, and molecular correlation time at crystallographically different carbon nuclei sites. The CSA parameters, spin-lattice relaxation time, and molecular correlation time of phosphorous nucleus of the organophosphate group of adefovir dipivoxil molecule are also determined. The spin-lattice relaxation time of carbon nuclei varies from 1 to 107 s. The range of molecular correlation time also varies from 10-4 to 10-8 s. These remarkable diversities of motional dynamics of the molecules imply that there exist various motional degrees of freedom within this valuable drug and these motional degrees of freedom are independent of each other, which may be the reason for the biological activities exhibited by the drug. The correlation between structure and dynamics of such an important antiviral drug adefovir dipivoxil can be visualized by these types of extensive spectroscopic measurements, which will enlighten the path of inventing advanced medicine in the pharmaceutical industry, and it will also illuminate the understanding of the structure-activity relationships of antiviral drug.

Structural dynamics and susceptibility of anti-HIV drugs against HBV reverse transcriptase

Hepatitis B virus (HBV), a small enveloped DNA virus, attacks the human liver causing both acute and chronic diseases. Current therapeutic drugs use the nucleos(t)ide analogues (NAs) as a competitive inhibitor against HBV reverse transcriptase (HBV-RT), an essential enzyme pivotally involved in viral replication. Unfortunately, this treatment still causes the development of resistant variants of HBV against NAs. As HBV-RT is homologous to the human immunodeficiency virus reverse transcriptase (HIV-RT), it is reasonable to treat HBV-RT with anti-HIV drugs. In the present study, we aimed to investigate the structural dynamics and susceptibility of the known anti-HIV drugs (stavudine [d4T], didanosine [DDI], and zidovudine [ZDV]) against HBV-RT enzyme in comparison to the anti-HBV drug lamivudine (3TC) and deoxythymidine triphosphate (dTTP) substrate using several computational approaches. The ΔGbindresidue calculations revealed that seven polar residues (K32, R41, D83, S85, D205, N236, and K239) and three hydrophobic residues (A86, A87, and F88) of HBV-RT as well as the adjacent DNA strands play an important role in the ligand binding. In addition, the H-bond pattern of d4T is similar to that of 3TC, especially at the residues A86 and A87. Such interactions promote the favorable conformation of ligand in the HBV-RT binding pocket, while the several different conformations of ligand are found in the unbound state. The predicted binding free energy results based on QM/MM-GBSA and MM/GB(PB)SA methods suggested that the susceptibility towards HBV-RT of d4T and ZDV is higher than that of 3TC and dTTP. Altogether, this work sheds light on the potentiality of d4T and ZDV as a promising drug for HBV-infected patients harboring 3TC resistance.Communicated by Ramaswamy H. Sarma.

Adefovir dipivoxil induces DNA replication stress and augments ATR inhibitor-related cytotoxicity

Replication stress is a common feature of cancer cells. Ataxia telangiectasia-mutated (ATM) and Rad3-related (ATR) signalling, a DNA damage repair (DDR) pathway, is activated by regions of single-stranded DNA (ssDNA) that can arise during replication stress. ATR delays cell cycle progression and prevents DNA replication fork collapse, which prohibits cell death and promotes proliferation. Several ATR inhibitors have been developed in order to restrain this protective mechanism in tumours. It is known, however, that despite other effective anticancer chemotherapy treatments targeting DDR pathways, resistance occurs. This begets the need to identify combination treatments to overcome resistance and prevent tumour cell growth. We conducted a drug screen to identify potential synergistic combination treatments by screening an ATR inhibitor (VE822) together with compounds from a bioactive small molecule library. The screen identified adefovir dipivoxil, a reverse transcriptase inhibitor and nucleoside analogue, as a compound that has increased cytotoxicity in the presence of ATR, but not ATM or DNA-dependant protein kinase (DNA-PK) inhibition. Here we demonstrate that adefovir dipivoxil induces DNA replication stress, activates ATR signalling and stalls cells in S phase. This simultaneous induction of replication stress and inhibition of ATR signalling lead to a marked increase in pan-nuclear γH2AX-positive cells, ssDNA accumulation and cell death, indicative of replication catastrophe.

Safety, efficacy, and pharmacokinetics of pradefovir for the treatment of chronic hepatitis B infection

BACKGROUND & AIMS

Pradefovir is a liver targeted novel prodrug of adefovir (PMEA) developed to provide higher antiviral activity with reduced systemic toxicities. This study evaluated the tolerability, pharmacokinetics, and antiviral activity of pradefovir in patients with chronic hepatitis B (CHB) virus infection.

METHODS

Non-cirrhotic, treatment-naïve subjects with CHB were divided into five groups (10 patients each) and randomized within each group in a ratio of 6:2:2 to receive an ascending dose of 30, 60, 75, 90, or 120 mg pradefovir, 10 mg adefovir dipivoxil (ADV), or 300 mg tenofovir disoproxil fumarate (TDF) once a day for 28 days.

RESULTS

A total of 51 subjects were randomized and 49 subjects completed the study. The groups were well matched and included 39 males, of whom 71% were hepatitis B e-antigen-negative with a mean hepatitis B virus (HBV) DNA level of 6.4-7.16 log10 IU/mL. No subject experienced a serious adverse event or nephrotoxicity. The most frequently reported adverse event was asymptomatic reduction in blood cholinesterase levels in the pradefovir group which recovered without any treatment about 13 ± 7 days after drug discontinuation. This adverse event was not observed in the ADV and TDF groups. The mean changes in serum HBV DNA were -2.78, -2.77, -3.08, -3.18, -3.44, -2.34, and -3.07 log10 IU/mL at 30, 60, 75, 90, and 120 mg pradefovir, 10 mg ADV and 300 mg TDF, respectively, with plateau levels reached with 60 mg pradefovir. Pradefovir and its metabolite PMEA showed linear pharmacokinetics proportional to the dose. The half-life of PMEA in the pradefovir group was 11.47-17.63 h.

CONCLUSIONS

Short-term use of pradefovir was well tolerated. A decline in HBV DNA levels was superior to TDF at higher doses of pradefovir. 30-60 mg pradefovir is recommended for CHB treatment.

CLINICAL TRIAL NUMBER

CTR20150224.

Initial combination anti-viral therapy with lamivudine and adefovir dipivoxil decreases short-term fatality rate of hepatitis-B-virus-related acute-on-chronic liver failure

Background

Acute-on-chronic liver failure (ACLF) is a common serious hepatitis B virus (HBV)-related disease and has a poor prognosis. Until recently, initial combination antiviral treatment in ACLF patients was rarely reported. This study evaluated the effect of initial combination treatment with lamivudine and adefovir dipivoxil on the prognosis of HBV-related ACLF.

Methods

In this retrospective study, 131 eligible ACLF patients, including 61 treated with 100 mg lamivudine and 10 mg adefovir dipivoxil daily and 70 not treated with any nucleoside analogs (NAs), were selected and assigned into the NA and non-NA groups. All the patients received standard medicinal therapy. At weeks 0–4 and 12, serum markers for hepatic and renal functions were measured in all patients and accumulated fatality rates were calculated. Statistical analyses, including Student’s t test, χ² test and unconditional logistic regression analysis, were performed using SPSS version 17.0 software.

Results

Clinical data indicated that improvement of hepatic function was better in the NA than in the non-NA group. The accumulated fatality rate in the NA group was lower than in the non-NA group at weeks 2–4 and 12, and these differences were significant. Univariate analysis showed that age, prothrombin activity, model of end-stage liver disease (MELD) score, and treatment without NAs were risk factors for short-term survival of ACLF. Further research by unconditional logistic regression analysis identified that older age, high MELD score and treatment without NAs were independent risk factors for short-term survival of ACLF.

Conclusions

Initial combination antiviral treatment is effective in decreasing short-term fatality of HBV-related ACLF.

Liquid chromatography-tandem mass spectrometry method for the estimation of adefovir in human plasma: Application to a pharmacokinetic study

An analytical method based on solid phase extraction was developed and validated for analysis of adefovir in human plasma. Adefovir-d4 was used as an internal standard and Synergi MAX RP80A (150 mm×4.6 mm, 4 µm) column provided the desired chromatographic separation of compounds followed by detection with mass spectrometry. The method used simple isocratic chromatographic condition and mass spectrometric detection in the positive ionization mode. The calibration curves were linear over the range of 0.50-42.47 ng/mL with the lower limit of quantitation validated at 0.50 ng/mL. Matrix effect was assessed by post-column infusion experiment to monitor phospholipids and post-extraction addition experiment was performed. The degree of matrix effect for adefovir was determined as 7.5% and ion-enhancement in five different lots of human plasma was 7.1% and had no impact on study samples analysis with 4.5 min run time. The intra- and inter-day precision values were within 7.7% and 7.8%, respectively, for adefovir at the lower limit of quantification level. Validated bioanalytical method was successfully applied to clinical sample analysis.

Hepatitis B virus infection in children

For two decades, hepatitis B vaccine has been integrated into national routine childhood vaccination programs in almost all countries. The prevalence of HBsAg has decreased in children worldwide. However, there are children who miss the benefit of hepatitis B vaccine in some regions and countries. Long-term follow-up studies have revealed the clinical outcomes of chronic hepatitis B virus infection in children. A small percentage of chronically infected children develop liver cirrhosis and hepatocellular carcinoma. However, it is controversial who should be treated and when antiviral treatment should be initiated in children. Compared with adult studies, the data are insufficient to evaluate the pathogenesis of hepatitis B infection and the efficacy of antiviral treatment in childhood. New antiviral drugs have been approved for children and adults. Also, oral antiviral drugs are administered to pregnant women to reduce the hepatitis B virus mother-to-child transmission rate.

Targeting the Achilles heel of the hepatitis B virus: a review of current treatments against covalently closed circular DNA

Chronic infection with hepatitis B virus (HBV) often leads to the development of liver cancer and cirrhosis, creating immense sociological, clinical and economic burdens worldwide. Although current anti-HBV medications manage to control the disease progression and help restore normal liver functions, they often fail to eliminate the virus completely. A major reason for this failure is the presence of a stable viral genome in the hepatocyte nucleus: the covalently closed circular DNA (cccDNA). Targeting HBV cccDNA is a promising approach that could lead to a complete cure. Here, we review various research approaches that are directed toward eliminating HBV cccDNA. This is a brief, yet comprehensive, summary of current state-of-the-art developments in this emerging area of interest.

Systemic and Renal Pharmacokinetics of Adefovir and Tenofovir Upon Coadministration

Adefovir and tenofovir are nucleotide analogs that undergo renal secretion by the human renal organic anion transporter. The pharmacokinetics of tenofovir and adefovir following the administration of tenofovir disoproxil fumarate and adefovir dipivoxil alone and together were determined in 24 healthy subjects in an 8-day, open-label, fixed-sequence study. Subjects received oral doses of adefovir dipivoxil on days 1 and 8 and oral doses of tenofovir disoproxil fumarate on days 2 to 8. Pharmacokinetic sampling was performed on days 1, 7, and 8. The plasma pharmacokinetics of tenofovir and adefovir were unaltered upon coadministration. Furthermore, the renal clearances (CLrenal) of tenofovir and adefovir were unaffected by their coadministration. The plasma Cmax values of tenofovir and adefovir were 33-fold and 340-fold lower than their Km values for the human renal organic anion transporter. These results demonstrate that coadministration of tenofovir disoproxil fumarate and adefovir dipivoxil does not result in substantial changes to their individual pharmacokinetic profiles.

Quantification of lamivudine-resistant hepatitis B virus mutants by type-specific TaqMan minor groove binder probe assay in patients with chronic hepatitis B

BACKGROUND

Lamivudine (LAM)-resistant hepatitis B virus (HBV) with mutations in the polymerase region frequently appears after long-term use of LAM. Several methods allowing detection of mutant strains (YIDD, YVDD) have been reported, but they have no quantitative characteristics. In this study, we explored a unique approach for quantification of each mutant strain.

METHODS

A method for detection and quantification of wild and mutant strains was developed using realtime polymerase chain reaction and type-specific minor groove binder (MGB) probes, and tested in patients with chronic hepatitis B before and after additive treatment with adefovir dipivoxil (ADV).

RESULTS

A good correlation was confirmed in HBV DNA quantity obtained between the YMDD-specific MBG probe assay and Amplicor HBV Monitor assay results (P < 0.001), linear between 3 and 9 log copies/mL serum. Of 109 samples from patients with chronic hepatitis B tested by both these assays and conventional direct sequencing, 90 (88.2%) showed identical results. The assays successfully detected and quantified a single type of mutant in three of four patients with additive ADV treatment, and also two coexisting mutant types (YIDD and YVDD) in the remaining patient.

CONCLUSIONS

Our specific and sensitive method for detection and quantification of HBV DNA with the wild-type YMDD motif and its two mutant forms (YIDD and YVDD) appears to be clinically useful, especially in patients with multiple mutant HBV infections.

Substrate overlap between Mrp4 and Abcg2/Bcrp affects purine analogue drug cytotoxicity and tissue distribution

The use of probe substrates and combinations of ATP-binding cassette (ABC) transporter knockout (KO) animals may facilitate the identification of common substrates between apparently unrelated ABC transporters. An unexpectedly low concentration of the purine nucleotide analogue, 9-(2-(phosphonomethoxy)ethyl)-adenine (PMEA), and up-regulation of Abcg2 in some tissues of the Mrp4 KO mouse prompted us to evaluate the possibility that Abcg2 might transport purine-derived drugs. Abcg2 transported and conferred resistance to PMEA. Moreover, a specific Abcg2 inhibitor, fumitremorgin C, both increased PMEA accumulation and reversed Abcg2-mediated PMEA resistance. We developed Mrp4 and Abcg2 double KO mice and used both single KOs of Abcg2 and Mrp4 mice to assess the role of these transporters in vivo. Abcg2 contributed to PMEA accumulation in a variety of tissues, but in some tissues, this contribution was only revealed by the concurrent absence of Mrp4. Abcg2 also transported and conferred resistance to additional purine analogues, such as the antineoplastic, 2-chloro-2'-deoxyadenosine (cladribine) and puromycin, a protein synthesis inhibitor that is often used as a dominant selectable marker. Purine analogues interact with ABCG2 by a site distinct from the prazosin binding site as shown by their inability to displace the substrate analogue and photoaffinity tag [(125)I]iodoarylazidoprazosin. These studies show that Abcg2, like Mrp4, transports and confers resistance to purine nucleoside analogues and suggest that these two transporters work in parallel to affect drug cytotoxicity and tissue distribution. This new knowledge will facilitate an understanding of how Abcg2 and Mrp4, separately and in combination, protect against purine analogue host toxicity as well as resistance to chemotherapy.

HPLC method for determination of in vitro delivery through and into porcine skin of adefovir (PMEA)

A simple HPLC/UV method for the determination of the transdermal permeation and dermal penetration of a broad-spectrum antiviral drug adefovir (PMEA) was developed. The separation was achieved on a C18 column with the mobile phase composed of 10 mM KH2PO4 and 2 mM Bu4NHSO4 at pH 6.0 and 7% acetonitrile. The method was validated with respect to selectivity, linearity (0.1-50 microg/ml), precision, accuracy, and stability. Transdermal permeation of 2% PMEA was studied in vitro using the Franz diffusion cell and porcine skin. The flux values were 1.8, 3.0, and 0.6 microg/cm2/h from aqueous donor samples at pH 3.4 and 7.4, and isopropyl myristate, respectively. The respective skin concentrations at 48 h were 294, 263, and 971 microg/g from these vehicles. These results will serve as a lead for further studies on transdermal and topical delivery of antivirals from the group of acyclic nucleoside phosphonates.

Tubular transporters and clearance of adefovir

Adefovir is transported by the organic anion transporter (OAT1) and the multidrug resistant protein (MRP2, 4 and 5). We studied adefovir clearance in rat after inhibition of transporters by probenecid and in mutant transport-deficient (TR-) rats, in which MRP2 is lacking. After treatment by probenecid or placebo, pharmacokinetics of adefovir 10mg/kg was studied via population nonlinear mixed effect modeling. The fraction of drug excreted in the urine was low. Renal clearance of adefovir was significantly lower (P < 0.05) in probenecid TR- rats (0.03+/-0.02l/h) than in normal control (0.09+/-0.05l/h), in normal probenecid (0.10+/-0.07l/h) and in TR- control rats (0.13+/-0.07l/h). In vivo in rats MRP2 mutation alone did not affect adefovir clearance suggesting that MRP2 does not play a critical role in the secretion of adefovir. Additional pharmacological inhibition of transporters decreased renal clearance, which may reflect inhibition of compensating transport mechanisms activated when MRP2 is lacking.

Resistance of human hepatitis B virus to reverse transcriptase inhibitors: from genotypic to phenotypic testing

The treatment of HBV infected patients with analogues of nucleos(t)ides, including lamivudine and adefovir dipivoxil, has significantly increased the rate of anti-HBe seroconversion and therefore reduced the impact of chronic hepatitis B (CHB) on liver disease. Altogether, these antivirals have offered novel options for the treatment of patients who did not respond to previous therapy with interferon alpha, the only available treatment against CHB until 1998. However, therapies using analogues of nucleos(t)ides have been confronted with viral resistances which are often associated to with worsening of liver disease. Drug resistance is conferred by the appearance of one or several mutations within the HBV polymerase gene. These mutations confer to the mutant viral population a phenotypic advantage over the wild-type pretherapeutic viral quasispecies, as they induce a reduction of drug susceptibility of mutant strains in vivo. This reduction of drug susceptibility can be as well measured in vitro, i.e in cell culture, using phenotypic assays. The detection of these mutations has become of crucial importance to better adapt clinical option to the virological status of the patient. Genotypic and more recently phenotypic assays have been developed and both assays can be used for drug resistance testing. Genotypic assay gives information about already characterized mutations associated with viral resistance, while phenotypic testing measures the overall drug susceptibility of patient-derived viral strains in cell culture. These assays are described and their potential use in the clinical setting is discussed.

[Tubular transporters OAT1 and MRP2 and clearance of adefovir]

Adefovir is transported by the organic anion transporter (OAT1) and the multidrug resistant protein (MRP2, 4 and 5). We studied adefovir clearance in rat after inhibition of transporters by probenecid and in TR- rats, in which MRP2 is lacking. After treatment by probenecid or placebo, pharmacokinetics of adefovir 10 mg/kg was studied via population modeling (NONMEM). The fraction of drug excreted in the urine was low. Renal clearance of adefovir was significantly lower (P < 0.05) in probenecid TR- rats (0.03 +/- 0.02 l/hour) than in normal control (0.09 +/- 0.05 l/hour), in normal probenecid (0.10 +/- 0.07 l/hour) and in TR- control rats (0.13 +/- 0.07 l/hour). In vivo in rats MRP2 mutation alone did not affect adefovir clearance suggesting that MRP2 does not play a critical role in the secretion of adefovir. Additional pharmacological inhibition of transporters decreased renal clearance, which may reflect inhibition of compensating transport mechanisms activated when MRP2 is lacking.

Lamivudine prophylaxis in HBV carriers with haemato-oncological malignancies who receive chemotherapy

Reactivation of hepatitis B virus (HBV) is a well-recognized complication of chemo/immunosuppressive therapy in individuals who are HBV surface antigen-positive inactive carriers and in individuals with chronic HBV infection. Although it is well established that chemo/immunosuppressive therapy enhances HBV replication with a resultant increase in the viral load and disease activation, the role of prophylactic lamivudine therapy to prevent chemo/immunosuppressive therapy-induced HBV activation in HBV-positive individuals who are to receive chemo/immunosuppressive therapy remains controversial. The aims of the present article are: (i) to determine the effect of lamivudine prophylaxis in HBV carriers with haemato-oncological malignancies who require chemotherapy; (ii) to define the duration and safety of lamivudine in such individuals; and (iii) to identify the effect of lamivudine prophylaxis on the outcome of chemotherapy administered for the primary disease. The data currently available suggest that lamivudine prophylaxis prevents chemotherapy-induced HBV reactivation in HBV carriers with haemato-oncological malignancies who receive chemotherapy. Lamivudine is safe and tolerable in such individuals. The duration of lamivudine prophylaxis is not yet known; however, it would appear prudent to begin lamivudine at the time of the initiation of the chemotherapy and to continue it throughout the period of chemotherapy administration and for at least 1 and possibly 2 years following the discontinuation of the chemotherapy. Finally, the prophylactic use of lamivudine in inactive HBV carriers with haemato-oncological malignancy prevents interruptions in their treatment for primary disease as a result of HBV reactivation.

Entecavir for the treatment of chronic hepatitis B

Chronic infection with the hepatitis B virus remains a serious and life-threatening disease for approximately 5% of the world's population, despite the availability of effective vaccines. Although prognoses can be improved by chemotherapy, treatment options are limited and none has been consistently successful. Interferon-alpha, the longest established therapy, has limited efficacy, is slow-acting and frequently causes adverse effects. Newer drugs comprise of mainly nucleoside and nucleotide analogs. The two that are currently approved, lamivudine and adefovir dipivoxil, are well tolerated; both produce rapid and dramatic responses, but their effects may not be sustainable in the long-term due to the emergence of resistant virus. Development of resistance to lamivudine is approximately ten-times more frequent than development of resistance to adefovir dipivoxil (approximately 60 and 6%, respectively) during the first 3 years of therapy. Entecavir, a carbocyclic deoxyguanosine analog that is active against both lamivudine- and adefovir dipivoxil-resistant HBV, is in the vanguard of new antihepatitis B virus drugs that have progressed to Phase III clinical trials. It is the most potent antihepatitis B virus agent discovered to date.

Chronic hepatitis B and viral genotype: the clinical significance of determining HBV genotypes

The global health challenge posed by the hepatitis B virus (HBV) centres around the widespread distribution and the serious complications as a result of persistent infection with the virus. As with other chronic diseases mediated by pathogens such as human immunodeficiency virus (HIV) and hepatitis C virus (HCV), clinicians are searching for epidemiological, pathological and viral characteristics of HBV infection that may lead to more effective management of patients with chronic infection. Unlike HCV, the role of HBV genotype in disease progression, severity, response to therapy and drug resistance is still under investigation and just beginning to be clarified. This review examines the potential role of HBV genotype determination in the clinic with emphasis on how this genetic information may used to provide effective management for the treatment of patients with chronic hepatitis B.

Determination of adefovir in human plasma by liquid chromatography/tandem mass spectrometry: application to a pharmacokinetic study

A liquid chromatography/tandem mass spectrometry (LC/MS/MS) method was developed and validated to determine the concentrations of adefovir [9-(2-phosphonylmethoxyethyl)adenine, PMEA] in human plasma. After one-step protein precipitation of plasma samples by methanol, adefovir was analyzed by LC/MS/MS using positive electrospray ionization. Chromatography was performed on a C18 column. The extraction recoveries of adefovir were found to be 85.1-89.3%. Adefovir was stable under routine laboratory conditions. A minimal matrix effect resulting in a slight ionization enhancement of adefovir (<10.9%) was observed, which did not markedly affect the behavior of the calibrations curves and accuracy and precision data. The method had a chromatographic run time of 7.8 min and a linear calibration curve over the concentration range 1.5-90 ng/mL for adefovir. The lower limit of quantification of the method was 1.5 ng/mL. The intra- and inter-day precision was less than 8.4%. These results indicated that this LC/MS/MS method has high selectivity and efficiency, and acceptable accuracy, precision and sensitivity. The validated LC/MS/MS method has been successfully used in a pharmacokinetic study in healthy volunteers treated with oral adefovir dipivoxil at 10 and 20 mg.

Influence of hepatitis B virus genotypes on the response to antiviral therapies

Hepatitis B virus (HBV) has been classified into eight genotypes (A-H) based on genome sequence divergence. Genotypes of HBV have distinct geographical distributions, and two genotypes account for most HBV worldwide. Hepatitis B e antigen expression lasts longer and liver disease is more severe with graver outcomes in carriers of genotype C than B in Asia. Accumulating lines of evidence indicate a better response to interferon and lamivudine in patients with chronic hepatitis B who are infected with genotype B rather than C. The therapeutic response may differ, however, in patients infected with HBV of the same genotype. For example, the response to lamivudine is poorer in patients infected with subtype Ba, which contains a recombination with genotype C, than in those with subtype Bj without such a recombination. Influence of genotypes on therapeutic response needs to be examined in patients infected with the other genotypes, particularly in those with genotype A or D infection.