Adefovir dipivoxil: review of a novel acyclic nucleoside analogue

Phosphonylated Acyclic Guanosine Analogues with the 1,2,3-Triazole Linker

A novel series of {4-[(2-amino-6-chloro-9H-purin-9-yl)methyl]-1H-1,2,3-triazol-1-yl}alkylphosphonates and {4-[(2-amino-6-oxo-1,6-dihydro-9H-purin-9-yl)methyl]-1H-1,2,3-triazol-1-yl}alkylphosphonates as acyclic analogues of guanosine were synthesized and assessed for antiviral activity against a broad range of DNA and RNA viruses and for their cytostatic activity toward three cancerous cell lines (HeLa, L1210 and CEM). They were devoid of antiviral activity; however, several phosphonates were found slightly cytostatic against HeLa cells at an IC₅₀ in the 80–210 µM range. Compounds (1R,2S)-17k and (1S,2S)-17k showed the highest inhibitory effects (IC₅₀ = 15–30 µM) against the proliferation of murine leukemia (L1210) and human T-lymphocyte (CEM) cell lines.

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.

Inhibitors of Bacillus anthracis edema factor

Edema factor (EF) is a calmodulin (CaM)-activated adenylyl cyclase (AC) toxin from Bacillus anthracis that contributes to anthrax pathogenesis. Anthrax is an important medical problem, but treatment of B. anthracis infections is still unsatisfying. Thus, selective EF inhibitors could be valuable drugs in the treatment of anthrax infection, most importantly shock. The catalytic site of EF, the EF/CaM interaction site and allosteric sites constitute potential drug targets. To this end, most efforts have been directed towards targeting the catalytic site. A major challenge in the field is to obtain compounds with high selectivity for AC toxins relative to mammalian membranous ACs (mACs). 3'-(N-methyl)anthraniloyl-2'-deoxyadenosine-5'-triphosphate is the most potent EF inhibitor known so far (Ki, 10nM), but selectivity relative to mACs needs to be improved (currently ~5-50-fold, depending on the specific mAC isoform considered). AC toxin inhibitors can be identified in virtual screening studies based on available EF crystal structures and examined in cellular test systems or at the level of purified toxin using classic radioisotopic or non-radioactive fluorescence assays. Binding of certain MANT-nucleotides to AC toxins elicits large direct fluorescence- or fluorescence resonance energy transfer signals upon interaction with CaM, and these signals can be used to identify toxin inhibitors in competition binding studies. Collectively, potent EF inhibitors are available, but before they can be used clinically, selectivity against mACs must be improved. However, several methodological approaches, complementing each other, are now available to direct the development of potent, selective, orally applicable and clinically useful EF inhibitors.

Clonal evolution of hepatitis B virus polymerase gene mutations during lamivudine-adefovir combination treatment

AIM: To identify hepatitis B virus polymerase gene mutations during antiviral therapy using lamivudine-adefovir sequential monotherapy followed by lamivudine-adefovir combination therapy.

METHODS: The patient cohort included four adult chronic hepatitis B patients who had undergone sequential monotherapy, first with lamivudine (LMV) and then, after developing viral breakthrough, with adefovir (ADV) therapy. All of the patients had non-response or viral breakthrough after LMV-ADV sequential monotherapy, which resulted in the switching of their antiviral regimen to LMV-ADV combination therapy. Eleven serum samples from the four patients who showed non-response to rescue LMV-ADV combination therapy were collected sequentially at a time before the antiviral treatment and then during the LMV monotherapy, ADV monotherapy, and LMV-ADV combination therapy. For the genotypic analysis, the whole 1310-bp polymerase gene region was amplified, cloned and sequenced.

RESULTS: All patients had been previously treated with 100 mg of LMV once daily for a 15- to 26-mo period. The emergence of resistance mutations to LMV, such as rtM204V/I and/or rtL180M, were found in all patients. Their antiviral regimens were switched to ADV monotherapy as the second line treatment. All patients had viral breakthrough or non-response after the LMV-ADV sequential monotherapy. ADV-resistant mutations were detected after 13 to 19 mo of LMV-ADV sequential monotherapy. The rtA181V/T mutations were predominantly identified during the ADV treatment in the LMV-resistant patients. Twenty-seven of 38 clones were combined with an amino acid change at rt181; three clones had mutations in rt236 and one clone had a combined mutation. The rtA181V/T mutations were not suppressed by the LMV-ADV combination therapy. Thirty-nine of 64 clones showed an rtA181V/T mutation and six clones showed combined mutations in rt181 and rt236. Mutations in rt204 re-emerged during the combination treatment. The rt181 and rt204 mutations did not co-exist in one clone.

CONCLUSION: Add-on lamivudine therapy with adefovir for adefovir resistance may not suppress the pre-existing adefovir-resistant mutation that develops during lamivudine-adefovir sequential monotherapy.

Frequency and risk factors of renal impairment during long-term adefovir dipivoxil treatment in chronic hepatitis B patients

BACKGROUND AND AIM

There are insufficient data on renal safety during long-term adefovir dipivoxil (ADV) treatment. We aimed to elucidate the incidence and risk factors of renal impairment in chronic hepatitis B (CHB) patients treated with ADV.

METHODS

We retrospectively enrolled 687 CHB patients (51.4% with compensated cirrhosis) treated with ADV alone (18.2%) or in combination with lamivudine (81.8%) for more than 12 months. Renal function was measured using the estimated glomerular filtration rate (eGFR), and renal dysfunction was defined as mild (20-30% decrease), moderate (30-50%), or severe (more than 50%).

RESULTS

During the median treatment duration of 27 months, 72 patients (10.5%) developed renal impairment, which was mild in 77.8% of cases, moderate in 20.8% of cases, and severe in one patient. The cumulative incidence of renal impairment at 1, 3, and 5 years was 2.6%, 14.8%, and 34.7%, respectively. Modification of the dosing interval or discontinuation of ADV was required in seven and three patients, respectively, and none of them showed a further decline in the eGFR. Although a univariate analysis revealed age, the number of exposure to radio-contrast dye, liver cirrhosis, and hepatocellular carcinoma as risk factors of renal impairment, age was the only significant risk factor identified in the multivariate analysis (odds ratio = 1.048, 95% confidence interval = 1.019-1.076, P = 0.001).

CONCLUSIONS

Renal impairment in long-term ADV users was relatively frequent, but serious renal toxicity was rare, and all cases were safely managed. Careful monitoring of renal function is required, especially in older patients.

Low-dose tenofovir is more potent than adefovir and is effective in controlling HBV viremia in chronic HBeAg-negative hepatitis B

AIM: To study the efficacy of tenofovir disoproxil fumarate (TDF) at low dose in a small open trial of chronic hepatitis B patients with advanced stage disease.

METHODS: Eleven patients were treated with TDF 75 mg for a median period of 80 (range, 24-576) wk and then 7 cases were shifted to an adefovir 10 mg treatment group. All patients had been pre-treated with lamivudine: 5 had YMDD resistant mutants and 6 wild-type virus. When TDF was started, 4 patients had low-level viremia and 6 were PCR-negative.

RESULTS: During TDF treatment, PCR remained negative in 10 patients, transaminase levels were normal and no significant viral breakthrough was observed. The drug was well tolerated in all cases. When TDF 75 mg was substituted with adefovir 10 mg, 3 out of 7 patients had a persistent viral rebound (2700-130 000 copies/mL), in whom lamivudine had to be reintroduced.

CONCLUSION: Low-dose TDF monotherapy can control HBV viremia for an extended period of time without the emergence of resistance and is more potent than adefovir at the standard dosage. The use of a reduced dose of TDF could diminish the cost of therapy in low-income countries, but further studies in a larger population and in HBeAg-positive subjects are needed.

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.

Efficacy and safety of adefovir dipivoxil 20 mg daily in HBeAg-positive patients with lamivudine-resistant hepatitis B virus and a suboptimal virological response to adefovir dipivoxil 10 mg daily

BACKGROUND/AIMS

Some patients receiving adefovir at the approved dose of 10 mg daily for chronic hepatitis B have a "suboptimal" virological response characterized by a slow and moderate decrease in viral replication.

METHODS

We assessed the efficacy and safety of adefovir 20 mg daily in patients with hepatitis B e antigen-positive chronic hepatitis B resistant to lamivudine and a suboptimal virological response to adefovir 10 mg daily add-on.

RESULTS

No amino acid substitutions known to confer adefovir resistance were found in these patients. In the five treated patients, the switch from 10 mg to 20 mg of adefovir daily significantly improved antiviral efficacy (-1.78+/-0.28 log international units/mL versus -3.73+/-0.51 log international units/mL, respectively, p=0.0039), and alanine aminotransferase levels normalized in all but one of the patients. No signs of renal dysfunction occurred.

CONCLUSIONS

These results suggest: (i) that suboptimal responses to adefovir 10 mg daily are due to underdosing; and (ii) that increasing the adefovir dose to 20 mg daily is beneficial and safe in patients with lamivudine-resistant HBV and a suboptimal response to adefovir 10 mg daily, especially when alanine aminotransferase levels are elevated and/or the liver disease is severe or rapidly progressive. Careful monitoring of renal function is necessary.

Extended lamivudine treatment in patients affected by chronic active anti-Hbe positive hepatitis

The aim of the study was to evaluate the biochemical and virological response and the histological changes in 34 chronic hepatitis B anti-Hbe-positive patients treated with lamivudine at 100 mg/day orally for five years. Liver biopsies were performed in all patients at least 6 months before starting therapy and 3 months after the stop of treatment. After 12 months of therapy, 70.6% of patients showed evidence of HBV DNA clearance and normal ALT levels; 64.7% and 55.8% of patients maintained a complete response after two and three years of therapy, respectively, 47% after four years and 44.1% after five years. The histological activity index improved in 13%, remained unchanged in 61% and worsened in 26% of patients with tyrosine-methionine-aspartate-aspartate (YMDD) variants compared to 63, 27 and 0% without variants, respectively. The authors conclude that the clinical benefit of lamivudine is greatest for patients without YMDD variants after 5 years of extended treatment.

Strategies to improve oral drug bioavailability

Efforts to improve oral drug bioavailability have grown in parallel with the pharmaceutical industry. As the number and chemical diversity of drugs has increased, new strategies have been required to develop orally active therapeutics. The past two decades have been characterised by an increased understanding of the causes of low bioavailability and a great deal of innovation in oral drug delivery technologies, marked by an unprecedented growth of the drug delivery industry. The advent of biotechnology and consequent proliferation of biopharmaceuticals have brought new challenges to the drug delivery field. In spite of the difficulties associated with developing oral forms of this type of therapeutics, significant progress has been made in the past few years, with some oral proteins, peptides and other macromolecules currently advancing through clinical trials. This article reviews the approaches that have been successfully applied to improve oral drug bioavailability, primarily, prodrug strategies, lead optimisation through medicinal chemistry and formulation design. Specific strategies to improve the oral bioavailability of biopharmaceuticals are also discussed.