Inhibitory effect of 9-(2-phosphonylmethoxyethyl)-adenine (PMEA) on human and duck hepatitis B virus infection

Antiviral Chemotherapy in Avian Medicine-A Review

This review article describes the current knowledge about the use of antiviral chemotherapeutics in avian species, such as farm poultry and companion birds. Specific therapeutics are described in alphabetical order including classic antiviral drugs, such as acyclovir, abacavir, adefovir, amantadine, didanosine, entecavir, ganciclovir, interferon, lamivudine, penciclovir, famciclovir, oseltamivir, ribavirin, and zidovudine, repurposed drugs, such as ivermectin and nitazoxanide, which were originally used as antiparasitic drugs, and some others substances showing antiviral activity, such as ampligen, azo derivates, docosanol, fluoroarabinosylpyrimidine nucleosides, and novel peptides. Most of them have only been used for research purposes and are not widely used in clinical practice because of a lack of essential pharmacokinetic and safety data. Suggested future research directions are also highlighted.

The Elegance of the Acyclic Nucleoside Phosphonates (ANPs), Honorary Tribute to Antonín Holý, Who Passed Away on 16 July 2012, at the 10th Anniversary of His Death

My collaboration with Prof. Antonín Holý, that spans a period of 3–4 decades (1976–2012), led to the discovery of several acyclic nucleoside phosphonates (ANPs) which were clinically developed by Gilead Sciences: cidofovir, adefovir, and tenofovir. The latter was further converted to two orally bioavailable prodrug forms, TDF and TAF, and both TDF and TAF were further combined with other antiviral drugs, thus giving rise to a broad array of antiviral drug combinations for the treatment of HIV infections. TDF and TAF are both available for the treatment of hepatitis B virus (HBV) infections, and, in combination with emtricitabine, also applicable as Truvada^® and Descovy^®, respectively, for the prophylaxis of HIV infections.

Tribute to John C. Martin at the Twentieth Anniversary of the Breakthrough of Tenofovir in the Treatment of HIV Infections

At Bristol-Myers (BM) (1985–1990), John C. Martin started his HIV career with directing the clinical development of didanosine (ddI) and stavudine (d4T). During this period, he became aware of the acyclic nucleoside phosphonates (ANPs), such as (S)-HPMPA and PMEA, as potential antiviral drugs. Under his impulse, BM got involved in the evaluation of these ANPs, but the merger of BM with Squibb (to become BMS) incited John to leave BM and join Gilead Sciences, and the portfolio of the ANPs followed the transition. At Gilead, John succeeded in obtaining the approval from the US FDA for the use of cidofovir in the treatment of cytomegalovirus (CMV) retinitis in AIDS patients, which was reminiscent of John’s first experience with ganciclovir (at Syntex) as an anti-CMV agent. At Gilead, John would then engineer the development of tenofovir, first as TDF (tenofovir disoproxil fumarate) and then as TAF (tenofovir alafenamide) and various combinations thereof, for the treatment of HIV infections (i), TDF and TAF for the treatment of hepatitis B (HBV) infections (ii), and TDF and TAF in combination with emtricitabine for the prophylaxis of HIV infections (iii).

Screening and Evaluation of Novel Compounds against Hepatitis B Virus Polymerase Using Highly Purified Reverse Transcriptase Domain

Hepatitis B virus (HBV) polymerase seems to be very hard to express and purify sufficiently, which has long hampered the generation of anti-HBV drugs based on the nature of the polymerase. To date, there has been no useful system developed for drug screening against HBV polymerase. In this study, we successfully obtained a highly purified reverse transcriptase (RT) domain of the polymerase, which has a template/primer and substrate binding activity, and established a novel high-throughput screening (HTS) system using purified RT protein for finding novel polymerase inhibitors. To examine whether the assay system provides reliable results, we tested the small scale screening using pharmacologically active compounds. As a result, the pilot screening identified already-known anti-viral polymerase agents. Then, we screened 20,000 chemical compounds and newly identified four hits. Several of these compounds inhibited not only the HBV RT substrate and/ template/primer binding activity, but also Moloney murine leukemia virus RT activity, which has an elongation activity. Finally, these candidates did show to be effective even in the cell-based assay. Our screening system provides a useful tool for searching candidate inhibitors against HBV.

In Vitro and In Vivo Renoprotective Effects of Telbivudine in Chronic Hepatitis B Patients Receiving Nucleotide Analogue

AIM

Renal toxicity of adefovir disoproxil (ADV) and tenofovir disoproxil fumarate (TDF) is a significant concern in chronic hepatitis B (CHB) patients. Early observational clinical data suggested that telbivudine (LdT) might have renoprotective effects.

METHODS

In this prospective study, consecutive CHB patients on combined lamivudine (LAM) + ADV/TDF were switched to LdT + ADV/TDF at recruitment and were followed up for 24 months. Estimated glomerular filtration rate (eGFR) was calculated with the modification of diet in renal disease equation. The effects of LdT on cell viability and expression of kidney injury or apoptotic biomarkers were investigated in cultured renal tubular epithelial cell line HK-2.

RESULTS

Thirty-one patients (median age 55 years, 90.3% male) were recruited (54.8% TDF: 45.2% ADV). Serum HBV DNA was undetectable at all time points. Median eGFR was 70.2 (IQR 62.6-77.9) and 81.5 (IQR 63.6-99.1) mL/min/1.73 m² at baseline and 24 months, respectively (p < 0.001). Downstaging of chronic kidney disease was observed in eight (25.8%) patients and was more common in ADV-treated compared to TDF-treated patients (7/8 vs. 1/17, p = 0.011; OR 16, 95% CI 1.643-155.766, p = 0.017). In vitro data showed that adding LdT to ADV or TDF was associated with improved cell viability and lower expression of injury and apoptotic biomarkers compared with ADV or TDF alone. Treatment was prematurely discontinued in four(12.9%) patients due to myalgia.

CONCLUSIONS

Clinical and in vitro data suggest that LdT has renoprotective effects in patients on long-term ADV/TDF treatment. LdT may be considered as an adjuvant therapy in this special group of patients with renal impairment (NCT03778567).

Polypharmacology in HIV inhibition: can a drug with simultaneous action against two relevant targets be an alternative to combination therapy?

HIV infection still has a serious health and socio-economical impact and is one of the primary causes of morbidity and mortality all over the world. HIV infection and the AIDS pandemic are still matters of great concern, especially in less developed countries where the access to highly active antiretroviral therapy (HAART) is limited. Patient compliance is another serious drawback. Nowadays, HAART is the treatment of choice although it is not the panacea. Despite the fact that it suppresses viral replication at undetectable viral loads and prevents progression of HIV infection into AIDS HAART has several pitfalls, namely, long-term side-effects, drug resistance development, emergence of drug-resistant viruses, low compliance and the intolerance of some patients to these drugs. Moreover, another serious health concern is the event of co-infection with more than one pathogen at the same time (e.g. HIV and HCV, HBV, herpes viruses, etc). Currently, the multi-target drug approach has become an exciting strategy to address complex diseases and overcome drug resistance development. Such multifunctional molecules combine in their structure pharmacophores that may simultaneously interfere with multiple targets and their use may eventually be more safe and efficacious than that involving a mixture of separate molecules because of avoidance or delay of drug resistance, lower incidence of unwanted drug-drug interactions and improved compliance. In this review we focus on multifunctional molecules with dual activity against different targets of the HIV life cycle or able to block replication, not only of HIV but also of other viruses that are often co-pathogens of HIV. The different approaches are documented by selected examples.

Entecavir Combined With Adefovir Ameliorates Patients With Chronic Hepatitis B Who Fail to Respond to Nucleotide (Acid) Analog Monotherapy

The aim of this study was to evaluate the efficacy and safety of entecavir (ETV) combined treatment with adefovir (ADV) on chronic hepatitic B (CHB) patients who failed to respond to nucleotide (acid) analog (NA) treatment. On this basis, the possible factors in the combined treatment of these patients will be analyzed. The safety, biochemical index, and the possible factors that might affect the ETV and ADV combined treatment at different points in time were retrospectively analyzed. The biochemical index included the following: virological response, hepatitis B virus (HBV) DNA decline, primary nonresponse, biochemical response, and the hepatitis B virus E antigen/hepatitis B virus E antibody seroconversion rate. There were 94 CHB patients and compensated liver cirrhosis patients who received ETV plus ADV treatment for over 12 weeks after failure of treatment with NAs. The authors have also investigated 76 CHB patients (80.9%) and 18 hepatitis B cirrhosis patients (19.1%) in this study. The HBV DNA baseline was 4.4 ± 1.4 log10 IU/mL, and the positive rate of HBeAg before salvage treatment was 78.7% (74/94). The sample sizes were 94, 78, 42, 10, 6, and 1 for follow-up of 24, 48, 96, 144, 192, and 240 weeks, respectively. The virological responses (HBV DNA < 2 log10 IU/mL) and biochemical responses were 52.1%, 74.3%, and 90.4% and 63.1%, 61.6%, and 81.1%, respectively, at 24, 48, and 96 weeks, which showed significant differences (P < 0.001 and P < 0.005, respectively). The HBV DNA decline was presented as mean ± SEM, which were 1.53 ± 1.23, 1.75 ± 1.37, 2.07 ± 1.54, and 2.39 ± 1.77 log10 IU/mL at 12, 24, 48, and 96 weeks, respectively. They showed significant differences compared with the baseline (χ = 8.084, P < 0.05). The rate of primary nonresponse was 30.9% (29/94), and the primary treatment failure rates were 26.6% (25/94), 24.4% (19/78), and 4.8% (2/42) at 24, 48, and 96 weeks, respectively. They all have statistical difference (P = 0.011 < 0.05). There were 23 patients who experienced virological breakthrough after the HBV DNA levels were undetectable, whereas after follow-up for 12-24 weeks, the HBV DNA levels were back to undetectable again. ETV plus ADV treatment is an efficient and safe treatment for CHB and compensated liver cirrhosis patients who experienced NA treatment failure. The high quantity of baseline HBV DNA level is a risk factor for poor efficacy of salvage treatment.

HPMPC (cidofovir), PMEA (adefovir) and Related Acyclic Nucleoside Phosphonate Analogues: A Review of their Pharmacology and Clinical Potential in the Treatment of Viral Infections

The acyclic nucleoside phosphonate (ANP) analogues are broad-spectrum antiviral agents, with potent and selective antiviral activity in vitro and in vivo. The prototype compounds are: ( S)-1-(3-hydroxy-2-phosphonylmethoxypropyl)cytosine (HPMPC, cidofovir), which is active against a wide variety of DNA viruses; 9-(2-phosphonylmethoxyethyl)adenine (PMEA, adefovir), which is active against retro-, herpes- and hepadnaviruses, and ( R)-9-(2-phosphonylmethoxypropyl) adenine (PMPA), which is active against retro- and hepadnaviruses. The antiviral action of the ANP analogues is based on a specific interaction of the active diphosphorylated metabolite with the viral DNA polymerase. The long intracellular half-life of the active metabolite accounts for the optimal efficacy in infrequent dosing schedules. The potential of HPMPC as a broad-spectrum anti-DNA virus agent, as originally observed in vitro and in vivo, has been confirmed in clinical trials. HPMPC has recently been commercially released in the USA for the treatment of cytomegalovirus retinitis in AIDS patients. In addition, topical systemic HPMPC is being (or will be) explored for use against other herpesviruses (i.e. herpes simplex virus, Epstein-Barr virus, or varicella-zoster virus), by adenoviruses, or by human papilloma- or polyomaviruses. Intravenous HPMPC is associated with dose-dependent nephrotoxicity, that should be counteracted by prehydration and concomitant administration of probenecid, and by the application of an infrequent dosing schedule. The oral prodrug of PMEA, bis(pivaloyloxymethyl)-PMEA, is currently being evaluated in patients infected with human immunodeficiency virus (HIV) or hepatitis B virus. Finally, preclinical data on the efficacy of PMPA in animal retrovirus models point to its potential usefulness against HIV infections, when given either prophylactically or therapeutically in the treatment of established HIV infections.

Comparative Anti-Retrovirus and Anti-Hepadnavirus Activity of Three Different Classes of Nucleoside Phosphonate Derivatives

The prototype compounds of three different classes of nucleoside phosphonates [i.e. 9-(2-phosphonomethoxyethyl)adenine (PMEA), 9-(2-phosphonomethoxyethoxy)adenine (PMEoA) and 9-[(2R,5R)-2,5-dihydro-5-(phosphonomethoxy)-2-furanyl]adenine (D4API)] were investigated and compared for their antiviral activities. The three test compounds showed a marked inhibition of human immunodeficiency virus type 1 (HIV-1) and type 2 (HIV-2) in CEW and MT-4 cell cultures [50% effective concentration (EC50): 0.8-14μm]. D4API was 2- and 15-fold more inhibitory than PMEA and PMEoA, respectively. In contrast, the activity of PMEA against human hepatitis B virus (HHBV) in human hepatoma Hep G2 2.2.15 cells was 5- and 10-fold more pronounced than the activities of PMEoA and D4API, respectively (EC50 1.2μm versus 10 and 6 μm, respectively). The inhibitory activity of D4API against Moloney murine sarcoma virus (MSV)-induced C3H/3T3 cell transformation was superior to the activities of PMEA and PMEoA by at least one order of magnitude (EC50 for D4API 1.3μM, versus 2.8 and 14 μM for PMEA and PMEoA, respectively). The markedly greater inhibitory effect of D4API on MSV in vitro was in agreement with our In vivo findings that D4API inhibited MSV-induced tumour formation in newborn mice and delayed the MSV-associated animal death at a lower dose than PMEA or PMEoA. Both PMEA and D4API emerged as promising compounds that warrant further investigation for their anti-retrovirus and anti-hepadnavirus activities in vivo.

Remofovir Mesylate: A Prodrug of PMEA with Improved Liver-Targeting and Safety in Rats and Monkeys

Adefovir dipivoxil (Hepsera™), a first-line therapy for chronic hepatitis B, is an esterase-activated prodrug of PMEA. Dose-limiting nephrotoxicity necessitates suboptimal dosing at 10 mg/day. Remofovir mesylate (MB06866Q) (Hepavir B) is a CYP3A4-activated prodrug of PMEA based on the HepDirect™ technology that targets PMEA to the liver. In a whole body autoradiography study in rats after oral dosing (30 mg/kg) of [14C]adefovir dipivoxil or [14C]remofovir mesylate, remofovir yielded 15 times higher concentrations of radioactivity in the liver than adefovir dipivoxil, but only one-third of the concentrations in the kidney. After oral dosing (4 mg/kg) of the same radiolabelled agents in cynomolgus monkeys, remofovir mesylate yielded 60 times higher levels of total radioactivity in the liver, but only two-thirds of total radioactivity levels in the kidney. Thus, remofovir mesylate may provide better efficacy and reduced nephrotoxicity. In portal vein-cannulated rats (30 mg/kg) after a single oral dose of [14C]adefovir dipivoxil or [14C]remofovir mesylate, no PMEA was detectable in rat portal plasma early after dosing, indicating that intestinal CYP3A4 does not play a role in conversion of remofovir mesylate to PMEA. The portal/systemic extraction ratio was quite high in both models, suggesting good liver-targeting properties. Portal and systemic remofovir/PMEA ratio indicates that the liver is the site of conversion of remofovir to PMEA. 28-Day toxicity studies demonstrated renal toxicity in rats at doses of 100 mg/kg or higher with no safety concerns at 30 mg/kg and acceptable safety in monkeys at doses up to 60 mg/kg. Thus, in rats and non-human primates, remofovir mesylate has liver-targeting properties and is safer than adefovir dipivoxil.

Adefovir dipivoxil in the treatment of chronic hepatitis B virus infection

Adefovir dipivoxil (Hepsera, Gilead Sciences) is a prodrug of adefovir, with potent antiviral activity against hepatitis B virus. Adefovir dipivoxil therapy, 10 mg daily for 48 weeks, is effective in hepatitis B e antigen-positive and -negative chronic hepatitis B. In hepatitis B e antigen-negative chronic hepatitis B, adefovir dipivoxil was recently found to maintain its efficacy even after 3 years of therapy. Adefovir dipivoxil is effective in patients with compensated or decompensated chronic viral B liver disease, and in pre- and post-transplant hepatitis B virus patients who develop resistance to lamivudine (Epivir, GlaxoSmithKline). It is well-tolerated and safe even after the third year of long-term therapy, and is associated with low rates of viral resistance. All these characteristics make adefovir dipivoxil an important drug for the treatment of hepatitis B virus infection and an excellent candidate for long-term maintenance therapy in chronic viral B liver disease.

A multi-targeted drug candidate with dual anti-HIV and anti-HSV activity

Human immunodeficiency virus (HIV) infection is often accompanied by infection with other pathogens, in particular herpes simplex virus type 2 (HSV-2). The resulting coinfection is involved in a vicious circle of mutual facilitations. Therefore, an important task is to develop a compound that is highly potent against both viruses to suppress their transmission and replication. Here, we report on the discovery of such a compound, designated PMEO-DAPym. We compared its properties with those of the structurally related and clinically used acyclic nucleoside phosphonates (ANPs) tenofovir and adefovir. We demonstrated the potent anti-HIV and -HSV activity of this drug in a diverse set of clinically relevant in vitro, ex vivo, and in vivo systems including (i) CD4⁺ T-lymphocyte (CEM) cell cultures, (ii) embryonic lung (HEL) cell cultures, (iii) organotypic epithelial raft cultures of primary human keratinocytes (PHKs), (iv) primary human monocyte/macrophage (M/M) cell cultures, (v) human ex vivo lymphoid tissue, and (vi) athymic nude mice. Upon conversion to its diphosphate metabolite, PMEO-DAPym markedly inhibits both HIV-1 reverse transcriptase (RT) and HSV DNA polymerase. However, in striking contrast to tenofovir and adefovir, it also acts as an efficient immunomodulator, inducing β-chemokines in PBMC cultures, in particular the CCR5 agonists MIP-1β, MIP-1α and RANTES but not the CXCR4 agonist SDF-1, without the need to be intracellularly metabolized. Such specific β-chemokine upregulation required new mRNA synthesis. The upregulation of β-chemokines was shown to be associated with a pronounced downmodulation of the HIV-1 coreceptor CCR5 which may result in prevention of HIV entry. PMEO-DAPym belongs conceptually to a new class of efficient multitargeted antivirals for concomitant dual-viral (HSV/HIV) infection therapy through inhibition of virus-specific pathways (i.e. the viral polymerases) and HIV transmission prevention through interference with host pathways (i.e. CCR5 receptor down regulation).

To contain the HIV-1 epidemic, it is necessary to develop antivirals that prevent HIV-1 transmission. It is well known that HIV infection might be accompanied by other pathogens, which often are engaged with HIV-1 in a vicious circle of mutual facilitation. One of the most common of these pathogens is herpes simplex virus (HSV) type 2. Since there is an urgent need for a next generation antivirals that are multi-targeted, we can now report on the development of the first antiviral of this new generation that efficiently suppresses both HIV-1 and HSV-2. We found that the dual-targeted antiviral drug affects several targets for viral replication. It uniquely combines in one molecule three important abilities: (i) to efficiently suppress HSV-encoded DNA polymerase, (ii) to efficiently suppress HIV-1-encoded reverse transcriptase, and (iii) to stimulate secretion of CC-chemokines that downregulate the HIV-1 coreceptor CCR5. The compound suppresses both viruses in a wide-range of in vitro, ex vivo, and in vivo experimental models. The ability of one molecule to suppress HIV-1 and HSV-2 by combining direct activity against their two key enzymes and indirect immunomodulatory effects is unique in the antiviral field.

Treatment of HBeAg positive chronic hepatitis B: interferon or nucleoside analogues

Interferon alpha has restricted efficacy in as much as only a proportion of patients show a response. However, in appropriately selected HBeAg-positive and HBeAg-negative patients, sustained suppression of viral replication can be achieved, and HBeAg or even HBsAg seroconversion can be attained. Thus, finite course of interferon alpha can be successful, and offer an advantage to patient. Interferon (IFN) remains a benchmark therapy for chronic hepatitis B. The main advantages of IFN-α over nucleoside analogues are the absence of resistance and the possibility of immune-mediated clearance of hepatitis B. Unfortunately, side effects preclude the use of interferon alpha in substantial proportions of patients, and prolonged maintenance therapy to suppress hepatitis B virus (HBV) is not feasible. Nucleoside analogues are given by mouth, once per day, and the safety, potency and efficacy have improved and facilitated treatment. However, maintenance of long-term suppression is required for the majority of patients. In general, treatment of chronic hepatitis B should target patients with active disease and viral replication, preferably before the signs and symptoms of cirrhosis or significant injury has occurred. Current EASL guidelines suggest that treatment be based on the evaluation of three criteria: Serum aminotransferase levels, serum HBV DNA levels and histological grade and stage. Many questions remain unanswered on the optimal treatment of patients with chronic hepatitis B with a nucleoside vs interferon alpha. Both forms of treatment have benefits and the choice should be selected and tailored. Stopping or futility rules can be implemented in patients who fail interferon. Recent data suggest the safety and efficacy of nucleoside analogues in the third trimester of pregnancy to reduce the risk of transmission from mothers to their children.

Direct acting antivirals for the treatment of chronic viral hepatitis

The development and evaluation of antiviral agents through carefully designed clinical trials over the last 25 years have heralded a new dawn in the treatment of patients chronically infected with the hepatitis B and C viruses, but not so for the D virus (HBV, HCV, and HDV). The introduction of direct acting antivirals (DDAs) for the treatment of HBV carriers has permitted the long-term use of these compounds for the continuous suppression of viral replication, whilst in the case of HCV in combination with the standard of care [SOC, pegylated interferon (PegIFN), and ribavirin] sustained virological responses (SVRs) have been achieved with increasing frequency. Progress in the case of HDV has been slow and lacking in significant breakthroughs.This paper aims to summarise the current state of play in treatment approaches for chonic viral hepatitis patients and future perspectives.

Another ten stories in antiviral drug discovery (part C): "Old" and "new" antivirals, strategies, and perspectives

The ten stories told here deal with (i) ribavirin as an inhibitor of IMP dehydrogenase and (ii) ribavirin, in combination with pegylated interferon, as the present "standard of care" for hepatitis C; (iii) S-adenosylhomocysteine hydrolase inhibitors as antiviral agents; (iv) new adamantadine derivatives for the treatment of influenza A virus infections; (v) 5-substituted 2'-deoxyuridines (i.e. IDU, TFT) for the treatment of herpes simplex virus (HSV) infections; (vi) acyclic guanosine analogues (e.g. acyclovir) for the treatment of HSV infections; (vii) OMP decarboxylase inhibitors (i.e. pyrazofurin) and CTP synthetase inhibitors (i.e. cyclopentenylcytosine) as possible antiviral agents; (viii) the future of cidofovir (and alkoxyalkyl esters thereof) and ST-246 as potential antipoxvirus agents; (ix) the two decade journey from tivirapine to rilpivirine in the ultimate therapy of HIV infections; and (x) the extension of the therapeutic application of tenofovir disoproxil fumarate (Viread) to the treatment of hepatitis B virus infection, in addition to HIV infection.

Current antiviral therapies for chronic hepatitis B

Among current treatment options for chronic hepatitis B, nucleoside/nucleotide analog therapy has better tolerability and most patients respond to the therapy, while interferon (IFN) therapy has rather severe side-effects and a lower response rate. However, nucleoside/nucleotide analog therapies have problems of the emergence of drug resistance and poor sustainability of response after discontinuation. After the first nucleoside/nucleotide analog lamivudine, adefovir and entecavir are now utilized in many countries. Adefovir has efficacy for lamivudine resistant patients and current data suggests that adding adefovir to ongoing lamivudine is better than switching to adefovir in terms of viral suppression and the occurrence of resistance. Entecavir can be the first choice for naïve patients, although cross-resistance has been known for lamivudine resistant patients and mutational screening should take place before using entecavir with such patients. Many other new nucleoside/nucleotide analogs are being developed such as telbivudine, clevudine and tenofovir; the details of each drug will be disclosed in near future.

Antiviral Chemistry & Chemotherapy's Current Antiviral Agents FactFile (2nd Edition): Retroviruses and Hepadnaviruses

There are at present exactly 25 compounds that have been formally approved for the treatment of retrovirus (that is HIV) infections: seven nucleoside reverse transcriptase inhibitors (NRTIs), one nucleotide reverse transcriptase inhibitor (NtRTI), four non-nucleoside reverse transcriptase inhibitors (NNRTIs), 10 protease inhibitors (PIs), one core-ceptor inhibitor (CRI), one fusion inhibitor (FI) and one integrase inhibitor (INI). Other compounds expected to be approved for the treatment of HIV infections in the near future are the NNRTI rilpivirine, the CRI vicriviroc and the INI elvitegravir. To obtain synergistic activity, enable lower dosage levels, thus minimizing toxic side effects, and particularly to reduce the risk of drug resistance development, common wisdom dictates that the HIV inhibitors should be used in drug combination regimens. Although, given the number of compounds available, the drug combinations that could be concocted are uncountable, only one triple-drug combination has so far been formulated as single pill to be taken orally once daily, namely Atripla® containing the NtRTI tenofovir disoproxil fumarate, the NRTI emtricitabine and the NNRTI efavirenz. Here, we document these approved compounds along with other HIV-active compounds and, for the first time, compounds whose principal activity is against hepatitis B virus. The logic of this new division being the enzymatic similarity between the reverse transcriptase of HIV and hepatitis B virus; the strategies for the development of antiviral agents to combat them have much in common.

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.

Metabolic activation of pradefovir by CYP3A4 and its potential as an inhibitor or inducer

Metabolic activation of pradefovir to 9-(2-phosphonylmethoxyethyl)adenine (PMEA) was evaluated by using cDNA-expressed CYP isozymes in portal vein-cannulated rats following oral administration and in human liver microsomes. The enzyme induction potential of pradefovir was evaluated in rats following multiple oral dosing and in primary cultures of human hepatocytes. The results indicated that CYP3A4 is the only cDNA-expressed CYP isozyme catalyzing the conversion of pradefovir to PMEA. Pradefovir was converted to PMEA in human liver microsomes with a K(m) of 60 microM, a maximum rate of metabolism of 228 pmol/min/mg protein, and an intrinsic clearance of about 359 ml/min. Addition of ketoconazole and monoclonal antibody 3A4 significantly inhibits the conversion of pradefovir to PMEA in human liver microsomes, suggesting the predominant role of CYP3A4 in the metabolic activation of pradefovir. Pradefovir at 0.2, 2, and 20 microM was neither a direct inhibitor nor a mechanism-based inhibitor of CYP3A4, CYP2D6, CYP2C9, CYP2C19, CYP2E1, and CYP1A2 in human liver microsomes. In rats, the liver was the site of metabolic activation of pradefovir, whereas the small intestine did not play a significant role in the metabolic conversion of pradefovir to PMEA. Daily oral dosing (300 mg/kg of body weight) to rats for 8 days showed that pradefovir was not an inducer of P450 enzymes in rats. Furthermore, pradefovir at 10 microg/ml was not an inducer of either CYP1A2 or CYP3A4/5 in primary cultures of human hepatocytes.

Pharmacokinetics of pradefovir and PMEA in healthy volunteers after oral dosing of pradefovir

The pharmacokinetics of pradefovir and adefovir, 9-(2-phosphonylmethoxyethyl) adenine (PMEA), was evaluated in healthy male volunteers after oral dosing of pradefovir (10, 30, or 60 mg). Pradefovir was absorbed rapidly. The maximum serum concentration, the area under the concentration-time curve between 0 and 96 hours after dosing (AUC(0-96)), and the area under the plasma concentration versus time curve from time 0 to infinity (AUC(0-infinity)) of pradefovir and PMEA increased with the dose of pradefovir. The ratio of PMEA to pradefovir for AUC(0-96) and AUC(0-infinity) ranged from 1.4 to 1.8. Renal clearance of pradefovir (18-31 L/h) increased with the dose of pradefovir and was greater than glomerular filtration. The fraction of total body clearance due to renal clearance was low (0.045 to 0.083), suggesting that metabolic clearance played a significant role in the clearance of pradefovir in man. In addition, an evaluation of the food effect was conducted at the 30-mg dose. The results indicate that food intake has no effect on the extent of exposure of pradefovir and PMEA but may decrease the rate of systemic availability of PMEA.

Inhibition of hepatitis B virus gene expression and replication by helioxanthin and its derivative

Chronic hepatitis B virus (HBV) infection continues to be an important worldwide cause of morbidity and mortality. All the currently approved therapeutic drugs have their limitations: interferon-alpha (IFN-alpha) has limited efficacy and a high incidence of adverse effects; nucleoside analogues are very efficient HBV DNA inhibitors, but resistance occurs eventually. Therefore, it is important to develop new non-nucleoside/nucleotide agents with different modes of action that can be used for antiviral combination therapy. Here, we report on a novel class of compounds, helioxanthin and its derivative 5-4-2, which had potent anti-HBV activities in HepG2.2.15 cells, with the EC50s of 1 and 0.08 microM, respectively. The lamivudine-resistant HBV, L526M/M550V double mutant strain, was also sensitive to helioxanthin and 5-4-2. This class of compounds not only inhibited HBV DNA, but also decreased HBV mRNA and HBV protein expression. The EC50 of HBV DNA inhibition was consistent with the EC50 of HBV 3.5 Kb transcript inhibition, which was 1 and 0.09 microM for helioxanthin and 5-4-2 respectively. Western blot analysis of cell lysate from HepG2.2.15 cells showed that the core protein expression decreased in a dose-dependent manner after drug treatment. In conclusion, helioxanthin and 5-4-2 are potentially unique new anti-HBV agents, which possess a different mechanism of action from existing therapeutic drugs. Both compounds inhibited HBV RNA and protein expression in addition to inhibiting HBV DNA.

Single-dose pharmacokinetics and metabolism of [14C]remofovir in rats and cynomolgus monkeys

Single-dose pharmacokinetics and metabolism of [(14)C]remofovir was studied in rats and monkeys following intravenous (i.v.) and oral administration (30 mg/kg of body weight). Oral absorption and bioavailability were 29.7 and 5.42% in rats and 65.6 and 19.4% in monkeys, respectively. Following i.v. administration, the elimination half-life for remofovir was 0.7 h in both rats and monkeys. Total body clearance was 5.85 liters/h/kg in rats and 2.60 liters/h/kg in monkeys; apparent volume of distribution was 5.99 liters/kg in rats and 2.70 liters/kg in monkeys. Following oral administration, remofovir was extensively converted to 9-(2-phosphonylmethoxyethyl)adenine (PMEA) and other metabolites in both species. In rats, excretion of total radioactivity in urine accounted for 61.8% of the i.v. dose and 12.9% of the oral dose, while in monkeys it accounted for 43.3% of the i.v. dose and 34.9% of the oral dose. Following i.v. dosing of [(14)C]remofovir, fecal excretion of radioactivity accounted for 37.5% of the dose in rats and 17.4% of the dose in monkeys, indicating significant biliary excretion of the drug in animals. PMEA and metabolite A were the major urinary metabolites in both species after i.v. and oral administration of remofovir.

Adefovir dipivoxil in chronic hepatitis B infection

Adefovir dipivoxil, an acyclic nucleotide analogue, is effective for the treatment of chronic hepatitis B in both hepatitis B e antigen (HBeAg)-positive and -negative patients, with improvement in liver histology, hepatitis B virus (HBV) DNA levels, alanine aminotransferase levels, and HBeAg seroconversion (for HBeAg-positive patients). It is also effective against lamivudine-resistant strains of hepatitis B mutations. It has been studied in pre- and post-liver transplant patients. Compared to lamivudine, adefovir dipivoxil is associated with a much lower risk of emergence of drug-resistant HBV. Adefovir-associated resistant virus is susceptible to lamivudine therapy. The recommended dose of adefovir dipivoxil 10 mg is associated with low risk of nephrotoxicity. Adefovir dipivoxil can be recommended as a first-line treatment but can also be used in patients with chronic hepatitis B infection who are failing lamivudine therapy.

Adefovir dipivoxil: review of a novel acyclic nucleoside analogue

Adefovir dipivoxil (ADF) is a novel acyclic nucleoside analogue that has recently been approved for the treatment of chronic hepatitis B virus (HBV). Adefovir was initially assessed at higher doses for the treatment of human immunodeficiency virus (HIV) infection. However, in these studies, nephrotoxicity proved a dose-limiting side effect. Large randomised controlled studies have recently shown that ADF results in histological, virological and biochemical improvement in both hepatitis B e antigen (HBeAg)-positive and HBeAg-negative chronic HBV. While the rate of HBeAg seroconversion at 1 year (12%) was lower than both lamivudine and interferon, this increases with prolonged treatment. The clinical improvements occurred without serious side effects or the development of resistance at the dose of 10 mg daily, in treatment trials of up to 2 years, although resistance has now been observed. In addition, the drug is efficacious in HBV/HIV co-infection and hepatitis B-infected liver transplant recipients, particularly in those who have developed lamivudine resistance. ADF can be added as a treatment option to existing treatment options (interferon-alpha and lamivudine) and assumes a role in the ongoing management of chronic HBV. The optimal use of ADF as either a monotherapy or as part of combination therapy requires further assessment.

Selective inhibition of anthrax edema factor by adefovir, a drug for chronic hepatitis B virus infection

Edema factor (EF), a key virulence factor in anthrax pathogenesis, has calmodulin (CaM)-activated adenylyl cyclase activity. We have found that adefovir dipivoxil, a drug approved to treat chronic infection of hepatitis B virus, effectively inhibits EF-induced cAMP accumulation and changes in cytokine production in mouse primary macrophages. Adefovir diphosphate (PMEApp), the active cellular metabolite of adefovir dipivoxil, inhibits the adenylyl cyclase activity of EF in vitro with high affinity (K(i) = 27 nM). A crystal structure of EF-CaM-PMEApp reveals that the catalytic site of EF forms better van der Waals contacts and more hydrogen bonds with PMEApp than with its endogenous substrate, ATP, providing an explanation for the approximately 10,000-fold higher affinity EF-CaM has for PMEApp versus ATP. Adefovir dipivoxil is a clinically approved drug that can block the action of an anthrax toxin. It can be used to address the role of EF in anthrax pathogenesis.

Evolution of hepatitis B viral load and viral genome sequence during adefovir dipivoxil therapy

Phase II and III clinical trials of adefovir dipivoxil (ADV) for the treatment of chronic hepatitis B have shown that this hepadnavirus polymerase inhibitor is well tolerated and effectively suppresses hepatitis B virus (HBV) replication. We therefore analysed the evolution of viral load and the emergence of HBV polymerase mutants in a 22-patient subgroup from a phase III clinical trial of ADV for the treatment of HBeAg-positive chronic hepatitis B. HBV DNA serum titres were quantified using a real-time polymerase chain reaction (PCR) assay with molecular hybridization probes. Emergence of polymerase mutants was assessed by direct sequencing of the viral reverse transcriptase domain after PCR amplification of HBV DNA isolated from serum. Our results indicated that ADV therapy effectively suppressed HBV replication in these patients (median serum HBV decrease at week 48 of treatment = 4.3 log10 copies/mL). The initial drop of HBV DNA titres in serum at week 12 of ADV therapy seemed to be predictive of subsequent HBe seroconversion (P = 0.059). Neither viral breakthrough nor the selection of drug resistant mutants were observed during the study period. Our results showed that ADV administration for 48-72 weeks effectively suppresses HBV replication without the emergence of resistant viral mutants.

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

BACKGROUND

Hepatitis B virus (HBV) infection is a major problem worldwide and an important cause of chronic liver disease and cirrhosis. Currently available treatments include interferon alfa-2b, lamivudine, and adefovir dipivoxil. Adefovir dipivoxil is an acyclic nucleotide analogue that was developed in part to improve on the limitations of earlier therapies.

OBJECTIVE

This article is a review of available data on the clinical pharmacology, virology, efficacy, tolerability, and clinical use of adefovir dipivoxil.

METHODS

A search of the English-language literature indexed on MEDLINE from 1966 to July 2003 was performed using the terms adefovir, PMEA, and Bis-POM PMEA. Pertinent abstracts from scientific meetings on infectious diseases and hepatology were also included. The manufacturer of adefovir dipivoxil provided additional information. These materials were supplemented by US Food and Drug Administration briefing documents and other unpublished materials. In vitro and preclinical studies were included in the review, as were Phase II and III clinical trials.

RESULTS

In vitro, adefovir dipivoxil concentrations exceed those necessary to inhibit both wild-type and lamivudine-resistant isolates of HBV. In clinical trials, adefovir dipivoxil was clinically and virologically effective in patients in whom lamivudine therapy had failed due to the presence of lamivudine-resistant HBV. The drug was generally well tolerated. The risk of nephrotoxicity, the most notable adverse effect of adefovir dipivoxil at previously used higher doses, has been substantially reduced at the currently recommended dosage of 10 mg/d.

CONCLUSION

Based on the data reviewed adefovir dipivoxil is an effective and well-tolerated alternative for the treatment of HBV infection, including disease that is lamivudine resistant.

Antiviral effect of adefovir in combination with a DNA vaccine in the duck hepatitis B virus infection model

BACKGROUND/AIMS

Combination of antiviral drugs with immunotherapeutic approaches may be a promising approach for the treatment of chronic hepatitis B. We used the duck HBV (DHBV) infection model to evaluate the efficacy of the combination of adefovir with DNA-immunization by comparison with the respective monotherapies.

METHODS

Pekin ducks chronically infected with DHBV received adefovir treatment alone or in association with intramuscular immunization with a plasmid (pCI-preS/S) expressing the DHBV large envelope protein. Ducks immunized with pCI-preS/S plasmid alone and two control groups receiving empty plasmid injections or no treatment were followed in parallel.

RESULTS

All animals treated with adefovir showed a marked drop in viremia titers during drug administration, followed by a rebound of viral replication after drug withdrawal. Eight weeks after the third DNA boost, the median of viremia within the duck group receiving the combination therapy tended to be lower compared to that of the other groups. In addition, our results suggest a trend to an additive effect of adefovir and DNA vaccine since a 51% decrease in DHBV DNA was observed in autopsy liver samples from combination therapy group, whereas pCI-preS/S or adefovir monotherapies decreased intrahepatic viral DNA by 38 and 14%, respectively. This effect was sustained since it was observed 12 weeks after the end of therapy.

CONCLUSIONS

Our results suggest that combination of adefovir with DNA-vaccine may be able to induce a sustained antiviral effect in vivo.

Adefovir Dipivoxil

Adefovir dipivoxil (Hepsera) is an oral prodrug of the nucleotide analogue adefovir. It is indicated for the treatment of chronic hepatitis B in adults. Adefovir dipivoxil 10 mg/day significantly improved histological, biochemical and virological outcomes in hepatitis B e antigen (HBeAg)-positive and -negative patients, and serological outcomes in HBeAg-positive patients. In two trials, the proportion of adefovir dipivoxil recipients showing histological improvement in the liver was approximately twice that for placebo recipients. In two trials in patients chronically infected with lamivudine-resistant hepatitis B virus (HBV), switching to or adding adefovir dipivoxil was significantly more effective at reducing serum HBV DNA levels than continuing lamivudine monotherapy. In treatment-naive patients, 1 year's treatment with adefovir dipivoxil plus lamivudine had similar efficacy to lamivudine plus placebo; however, lamivudine-resistant HBV emerged in significantly more patients receiving lamivudine plus placebo. Adefovir dipivoxil has also shown efficacy in noncomparative trials in patients with decompensated liver disease, patients co-infected with HIV and patients pre- or post-liver transplantation. Within 96 weeks of treatment with adefovir dipivoxil, a resistance-conferring mutation emerged in viral isolates from 1.6% of patients. In vitro, these isolates remained sensitive to lamivudine, while lamivudine-resistant HBV isolates remained sensitive to adefovir dipivoxil. Adefovir dipivoxil 10 mg/day is generally well tolerated. In a pooled analysis of 48-week data from two trials, there was no marked difference in adverse events or laboratory abnormalities between adefovir dipivoxil and placebo recipients. Within 96 weeks of treatment with adefovir dipivoxil, >1% of patients with adequate renal function developed an increase in serum creatinine levels of >/=0.5 mg/dL above baseline. Within 48 weeks of treatment, increases in serum creatinine levels of >/=0.5 mg/dL above baseline were observed in 13% of pre- and post-liver transplantation patients who generally had renal insufficiency or risk factors for renal dysfunction at baseline. Most patients continued treatment with dosage adjustments.

CONCLUSION

Oral adefovir dipivoxil is effective and generally well tolerated in HBeAg-positive and -negative patients chronically infected with wild-type or lamivudine-resistant HBV. Few resistant HBV mutants have emerged to date. Data from ongoing long-term studies are awaited with interest. Existing treatment options for patients with chronic hepatitis B are limited in both number and effectiveness; the proven efficacy, good tolerability profile and apparently low potential for resistance of adefovir dipivoxil make it a promising new option in the management of this disease.

Resistance surveillance in chronic hepatitis B patients treated with adefovir dipivoxil for up to 60 weeks

Current therapies for chronic hepatitis B virus (HBV) infection do not provide adequate long-term control of viral replication in the majority of patients. Monotherapy with nucleoside analogs, such as lamivudine and famciclovir, is effective for short periods but results in the emergence of drug-resistant HBV in a substantial number of patients within 1 year of therapy. Adefovir dipivoxil (ADV) has demonstrated clinical activity against wild-type and lamivudine-resistant HBV, but it is unclear whether resistance mutations will emerge after long-term therapy with this drug. To determine whether extended treatment with ADV led to the emergence of drug-resistant populations of HBV, we analyzed virus isolated from patients currently enrolled in a long-term open-label study. The reverse transcriptase domain of HBV polymerase was amplified and sequenced from patients that had received a cumulative exposure of up to 60 weeks of ADV. During our analyses, several previously unreported amino acid substitutions were observed in the reverse transcriptase domain of HBV. Importantly, none of the observed mutations occurred in more than 1 patient, nor were they associated with an adefovir-resistant phenotype in vitro. Furthermore, none of the patients from whom these mutant viruses were isolated had evidence of virologic rebound. In conclusion, these results, although based on a limited number of patients, suggest that treatment with ADV does not lead to the emergence of resistant virus after up to 60 weeks of therapy.

Antiretrovirus activity of a novel class of acyclic pyrimidine nucleoside phosphonates

A novel class of acyclic nucleoside phosphonates has been discovered in which the base consists of a pyrimidine preferably containing an amino group at C-2 and C-4 and a 2-(phosphonomethoxy)ethoxy (PMEO) or a 2-(phosphonomethoxy)propoxy (PMPO) group at C-6. The 6-PMEO 2,4-diaminopyrimidine (compound 1) and 6-PMPO 2,4-diaminopyrimidine (compound 11) derivatives showed potent activity against human immunodeficiency virus (HIV) in the laboratory (i.e., CEM and MT-4 cells) and in primary (i.e., peripheral blood lymphocyte and monocyte/macrophage) cell cultures and pronounced activity against Moloney murine sarcoma virus in newborn NMRI mice. Their in vitro and in vivo antiretroviral activity was comparable to that of reference compounds 9-[(2-phosphonomethoxy)ethyl]adenine (adefovir) and (R)-9-[(2-phosphonomethoxy)-propyl]adenine (tenofovir), and the enantiospecificity of (R)- and (S)-PMPO pyrimidine derivatives as regards their antiretroviral activity was identical to that of the classical (R)- and (S)-9-(2-phosphonomethoxy)propyl purine derivatives. The prototype PMEO and PMPO pyrimidine analogues were relatively nontoxic in cell culture and did not markedly interfere with host cell macromolecular (i.e., DNA, RNA, or protein) synthesis. Compounds 1 and 11 should be considered attractive novel pyrimidine nucleotide phosphonate analogues to be further pursued for their potential as antiretroviral agents in the clinical setting.

Inhibitory effect of adefovir on viral DNA synthesis and covalently closed circular DNA formation in duck hepatitis B virus-infected hepatocytes in vivo and in vitro

The elimination of viral covalently closed circular DNA (CCC DNA) from the nucleus of infected hepatocytes is an obstacle to achieving sustained viral clearance during antiviral therapy of chronic hepatitis B virus (HBV) infection. The aim of our study was to determine whether treatment with adefovir, a new acyclic nucleoside phosphonate, the prodrug of which, adefovir dipivoxil, is in clinical evaluation, is able to suppress viral CCC DNA both in vitro and in vivo using the duck HBV (DHBV) model. First, the effect of adefovir on viral CCC DNA synthesis was examined with primary cultures of DHBV-infected fetal hepatocytes. Adefovir was administered for six consecutive days starting one day before or four days after DHBV inoculation. Dose-dependent inhibition of both virion release in culture supernatants and synthesis of intracellular viral DNA was observed. Although CCC DNA amplification was inhibited by adefovir, CCC DNA was not eliminated by antiviral treatment and the de novo formation of CCC DNA was not prevented by pretreatment of the cells. Next, preventive treatment of experimentally infected ducklings with lamivudine or adefovir revealed that both efficiently suppressed viremia and intrahepatic DNA. However, persistence of viral DNA even when detectable only by PCR was associated with a recurrence of viral replication following drug withdrawal. Taken together, our results demonstrate that adefovir is a potent inhibitor of DHBV replication that inhibits CCC DNA amplification but does not effectively prevent the formation of CCC DNA from incoming viral genomes.

Evolving therapies for the treatment of chronic hepatitis B virus infection

Despite the availability of prophylactic vaccines lamivudine and IFN-alpha, chronic hepatitis B remains an enormous global health problem. Several promising nucleosides/nucleotides are undergoing clinical trials, including adefovir dipivoxil, the latter of which is active against lamivudine-resistant hepatitis B virus (HBV). In addition to nucleosides/nucleotides, it will be important to develop new agents with different modes of action. Novel small molecule inhibitors, as well as gene therapy approaches, have produced encouraging results in vitro and in animal models. Additional immunomodulatory therapies, including thymosin-alpha 1, IL-12 and several therapeutic vaccines, are also being explored. Combination therapy with multiple nucleosides/nucleotides and other agents will play an important role in the treatment of hepatitis and may help achieve complete viral suppression, host-mediated elimination of infected cells and lasting immunity.

Drug discovery and development of antiviral agents for the treatment of chronic hepatitis B virus infection

A safe and effective vaccine for hepatitis B virus (HBV) has been available for nearly twenty years and currently campaigns to provide universal vaccination in developing countries are underway. Nevertheless, chronic HBV infection remains a leading cause of chronic hepatitis worldwide and there is a strong need for safe and effective antiviral therapies. Attempts to identify and develop antiviral agents to treat chronic HBV infection remains focused on nucleoside analogs such as 3TC (lamivudine), adefovir dipivoxil, (bis-POMPMEA), and others. However, advances in our understanding of the molecular biology of HBV and the development of new assays for HBV polymerase activity, such as the reconstitution of active HBV polymerase in vitro, should facilitate large screening efforts for non-nucleoside reverse transcriptase inhibitors. Recent advances have furthered our understanding of clinical resistance to lamivudine, have provided new approaches to treatment, and have offered new perspectives on the major challenges to the identification and development of antiviral agents for chronic HBV infection. Here, in an update to our previous review article that appeared in this series [59a], we focus on recent advances that have occurred in the areas of virus structure and replication, in vitro viral polymerase assays, cell culture systems, and animal models.

Cross-resistance testing of antihepadnaviral compounds using novel recombinant baculoviruses which encode drug-resistant strains of hepatitis B virus

Long-term nucleoside analog therapy for hepatitis B virus (HBV)-related disease frequently results in the selection of mutant HBV strains that are resistant to therapy. Molecular studies of such drug-resistant variants are clearly warranted but have been difficult to do because of the lack of convenient and reliable in vitro culture systems for HBV. We previously developed a novel in vitro system for studying HBV replication that relies on the use of recombinant baculoviruses to deliver greater than unit length copies of the HBV genome to HepG2 cells. High levels of HBV replication can be achieved in this system, which has recently been used to assess the effects of lamivudine on HBV replication and covalently closed circular DNA accumulation. The further development of this novel system and its application to determine the cross-resistance profiles of drug-resistant HBV strains are described here. For these studies, novel recombinant HBV baculoviruses which encoded the L526M, M550I, and L526M M550V drug resistance mutations were generated and used to examine the effects of these substitutions on viral sensitivity to lamivudine, penciclovir (the active form of famciclovir), and adefovir, three compounds of clinical importance. The following observations were made: (i) the L526M mutation confers resistance to penciclovir and partial resistance to lamivudine, (ii) the YMDD mutations M550I and L526M M550V confer high levels of resistance to lamivudine and penciclovir, and (iii) adefovir is active against each of these mutants. These findings are supported by the limited amount of clinical data currently available and confirm the utility of the HBV-baculovirus system as an in vitro tool for the molecular characterization of clinically significant HBV strains.

Lamivudine resistance in hepatitis B: mechanisms and clinical implications

Lamivudine (beta-L-(-)-2',3'-dideoxy-3'-thiacytidine) has been a major breakthrough in the care of patients with hepatitis B. With prolonged monotherapy the development of resistance is an increasingly recognized problem that limits the long term efficacy of this nucleoside analogue. The most common mutations associated with lamivudine resistance occur within the highly conserved YMDD motif in the C domain of the viral polymerase and are often associated with a compensatory mutation in the proximal B domain. The structural and functional relationship of resistance mutations is reflected in different in vitro sensitivities to lamivudine and changes in replication capacities. During prolonged lamivudine treatment there can be successive changes of different resistant mutants (genotypic succession) or a single mutant can remain the dominant viral species. In patients treated for chronic hepatitis B infection the cumulative incidence of viral resistance reaches over 50% after 3 years. Most patients will have lower serum HBV DNA levels after the emergence of resistance which is ascribed to the decreased replication capacity of these mutants. Although severe flares and ongoing HBe antigen seroconversion can occur in these patients with lamivudine-resistant HBV, the impact of continued therapy on the long-term outcome is still insufficiently studied. In the setting of liver transplantation for HBV-associated disease the clinical course after the emergence of viral resistance is variable but still may lead to disease progression and graft failure. Analogous to the success of combination therapies to delay the emergence of antiviral-resistant HIV, it will be important to combine anti-HBV agents with additive or synergistic antiviral properties and different resistance profiles for future de novo combination therapies for hepatitis B infection.

The polymerase L528M mutation cooperates with nucleotide binding-site mutations, increasing hepatitis B virus replication and drug resistance

After receiving lamivudine for 3 years to treat chronic hepatitis B, 67-75% of patients develop B-domain L528M, C-domain M552I, or M552V mutations in the HBV polymerase that render hepatitis B virus (HBV) drug-resistant. The aim of this study was to evaluate the influence of these mutations on viral replication and resistance to antiviral agents. We investigated the replication fitness and susceptibility of the wild-type and five mutant HBVs (L528M, M552I, M552V, L528M/M552I, and L528M/M552V) to 11 compounds [lamivudine, adefovir, entecavir (BMS-200475) (+)-BCH-189 (+/-)-FTC (racivir) (-)-FTC (emtricitabine) (+)-FTC, L-D4FC, L-FMAU (clevudine), D-DAPD, and (-)-carbovir] by transfecting HBV DNA into hepatoma cells and monitoring viral products by Southern blotting. The replication competency of the single C-domain mutants M552I and M552V was markedly decreased compared with that of wild-type HBV. However, addition of the B-domain mutation L528M restored replication competence. Only adefovir and entecavir were effective against all five HBV mutants, and higher doses of these compounds were necessary to inhibit the double mutants compared with the single mutants. The B-domain mutation (L528M) of HBV polymerase not only restores the replication competence of C-domain mutants, but also increases resistance to nucleoside analogues.

Synthesis and Cytostatic Activity of N-[2-(Phosphonomethoxy)alkyl] Derivatives of N6-Substituted Adenines, 2,6-Diaminopurines and Related Compounds

N6-Substituted adenine and 2,6-diaminopurine derivatives of 9-[2-(phosphonomethoxy)- ethyl] (PME), 9-[(R)-2-(phosphonomethoxy)propyl] [(R)-PMP] and enantiomeric (S)-PMP series were synthesized by reactions of primary or secondary amines with 6-chloro-9-{[2-(diisopropoxyphosphoryl)methoxy]alkyl}purines (26-28) or 2-amino-6-chloro-9-{[2-(diisopropoxy- phosphoryl)methoxy]alkyl}purines (29-31) followed by treatment of the diester intermediates32with bromo(trimethyl)silane and hydrolysis. Diesters32were also obtained by reaction ofN6-substituted purines with synthons23-25bearing diisopropoxyphosphoryl group. Alkylation of 2-amino-6-chloropurine (9) with diethyl [2-(2-chloroethoxy)ethyl]phosphonate (148) gave the diester149which was analogously converted toN6-substituted 2,6-diamino- 9-[2-(2-phosphonoethoxy)ethyl]purines151-153. Alkylation ofN6-substituted 2,6-diaminopurines with (R)-[(trityloxy)methyl]oxirane (155) followed by reaction of thus-obtained intermediates156with dimethylformamide dimethylacetal and condensation with diisopropyl [(tosyloxy)methyl]phosphonate (158) followed by deprotection of the intermediates159gaveN6-substituted 2,6-diamino-9-[(S)-3-hydroxy-2-(phosphonomethoxy)propyl]purines160-163. The highest cytostatic activityin vitrowas exhibited by the followingN6-derivatives of 2,6-diamino-9-[2-(phosphonomethoxy)ethyl]purine (PMEDAP): 2,2,2-trifluoroethyl (53), allyl (54), [(2-dimethylamino)ethyl] (68), cyclopropyl (75) and dimethyl (91). In CCRF-CEM cells, the cyclopropyl derivative75is deaminated to the guanine derivative PMEG (3) which is then converted to its diphosphate.

Characterization of novel human hepatoma cell lines with stable hepatitis B virus secretion for evaluating new compounds against lamivudine- and penciclovir-resistant virus

L-Nucleoside analogs are new therapeutic agents for treatment of chronic hepatitis B. However, their clinical application was limited by the emergence of viral resistance. It is important to develop a new system to evaluate drug cross-resistance and to test new agents that may overcome resistant virus. In this report, three cell lines HepG2-WT10, HepG2-SM1, and HepG2-DM2 are presented; these cell lines were established by transfection of HepG2 cells with unique fully functional 1.1x hepatitis B virus (HBV) genomes: wild-type HBV-adr and its L526M and L526MM550V variants, respectively. We have demonstrated that these genomes have different susceptibilities to lamivudine [L(-)SddC] and penciclovir (PCV). By examining HBV RNA transcription, antigen expression, progeny DNA replication, and viral susceptibilities to L(-)SddC, PCV, and other nucleoside analogs, it is concluded that the cell lines are able to stably produce L(-)SddC- and PCV-sensitive and -resistant HBV virions. In addition, the relative susceptibilities of the wild-type and mutant HBV produced from the stably transfected cell lines to several anti-HBV nucleoside analogs were also examined and found to be about the same as those found by using a transient infection system. PMEA [9-(2-phosphonylmethoxytehyl)-adenine] and QYL685 are able to suppress L(-)SddC- and PCV-resistant HBV. In conclusion, this cell culture system is a novel and useful tool for evaluating anti-HBV compounds and biologics.