Comparative effects of adefovir and selected nucleoside inhibitors of hepatitis B virus DNA polymerase on mitochondrial DNA in liver and skeletal muscle cells

The utility of a differentiated preclinical liver model, HepaRG cells, in investigating delayed toxicity via inhibition of mitochondrial-replication induced by fialuridine

During its clinical development fialuridine caused liver toxicity and the death of five patients. This case remains relevant due to the continued development of mechanistically-related compounds against a back-drop of simple in vitro models which remain limited for the preclinical detection of such delayed toxicity. Here, proteomic investigation of a differentiated, HepaRG, and proliferating, HepG2 cell model was utilised to confirm the presence of the hENT1 transporter, thymidine kinase-1 and -2 (TK1, TK2) and thymidylate kinase, all essential in order to reproduce the cellular activation and disposition of fialuridine in the clinic. Acute metabolic modification assays could only identify mitochondrial toxicity in HepaRG cells following extended dosing, 2 weeks. Toxic effects were observed around 10 μM, which is within a range of 10-15 X approximate Cmax. HepaRG cell death was accompanied by a significant decrease in mitochondrial DNA content, indicative of inhibition of mitochondrial replication, and a subsequent reduction in mitochondrial respiration and the activity of mitochondrial respiratory complexes, not replicated in HepG2 cells. The structural epimer of fialuridine, included as a pharmacological negative control, was shown to have no cytotoxic effects in HepaRG cells up to 4 weeks. Overall, these comparative studies demonstrate the HepaRG model has translational relevance for fialuridine toxicity and therefore may have potential in investigating the inhibition of mitochondrial replication over prolonged exposure for other toxicants.

Mitochondrial toxicity and body shape changes during nucleos(t)ide analogues administration in patients with chronic hepatitis B

Our study purpose was to evaluate mitochondrial (mt)DNA and RNA in peripheral blood mononuclear cells (PBMCs) and body shape changes (BSC) in HBV-infected patients. mtDNA and mtRNA were measured in PBMCs. The presence of BSC was evaluated through a questionnaire and clinical evaluation. A total of 157 subjects were enrolled, of these 107 were HBV-infected patients, 54 receiving nucleoside analogues (NAs, Group A), 53 naive to antivirals (Group B) and 50 age-sex matched controls (Group C). All HBV-treated patients had negative HBV–DNA. Twenty (37,0%) received lamivudine + adefovir, 20 (37.0%) tenofovir, 2 (3.7%) lamivudine and 12 (22.2%) entecavir. Therapy median duration was 38 months (IQR 20–60) in NA-treated patients. Group A showed significantly higher mtDNA/nuclear (n) DNA ratio (p = 0.000008) compared to Group C and Group B (p = 0.002). Group B showed significantly higher mtDNA/nDNA ratio compared to Group C (p = 0.017). Group A and B had significantly lower mtRNA/nRNA ratio compared to Group C (p = 0.00003 and p = 0.00006, respectively). Tenofovir and entecavir showed less impact compared to lamivudine + adefovir. mtDNA/nDNA ratio positively (Rho = 0.34, p < 0.05) and mtRNA/nRNA ratio negatively (Rho = −0.34, p < 0.05) correlated with therapy duration. BSC were significantly more frequent in Group A [10/54 (18.5%)] compared to Group B [3/53 (5.6%, p = 0.04)] and Group C [0/50, (p = 0.0009)]. In conclusion, long-term NA therapy was associated both to mitochondrial toxicity and BSC, showing significant differences in mtDNA and mtRNA levels. Tenofovir and entecavir showed lower impact on alterations, compared to 1^st generation NA.

Editor's Highlight: Comparative Renal Safety Assessment of the Hepatitis B Drugs, Adefovir, Tenofovir, Telbivudine and Entecavir in Rats

The aim of this study was to determine the relative safety of 4 antiviral drugs (telbivudine, tenofovir, adefovir, and entecavir) against hepatitis B virus with respect to kidney function and toxicity in male Sprague Dawley rats. The antiviral drugs were administered once daily for 4 weeks by oral gavage at ∼10 and 25-40 times the human equivalent dose. Main assessments included markers of renal toxicity in urine, magnetic resonance imaging (MRI) of kidney function, histopathology, and electron microscopic examination. Administration of adefovir at 11 and 28 mg/kg for 4 weeks caused functional and morphological kidney alterations in a time- and dose-dependent manner, affecting mainly the proximal tubules and suggesting a mechanism of toxicity related to mitochondrial degeneration/depletion. Of note, the observed adefovir-induced reduction of kidney function was not detected by the standard method of glomerular filtration rate (GFR) measurements (clearance rate of the endogenous marker, creatinine), thereby emphasizing the superiority of MRI in terms of sensitive detection of GFR in rats. For the low dose of 300 mg/kg of tenofovir, minor kidney effects such as nuclear enlargement in the tubular epithelium, and hyaline droplets accumulation were detected, which was also observed for the low dose (11 mg/kg) of adefovir. No assessments could be done at the higher dose of 600/1000 mg/kg tenofovir due to gastrointestinal tract toxicity which prevented treatment of the animals for longer than 1 week. Entecavir at 1 and 3 mg/kg and telbivudine at 600 and 1600 mg/kg caused no toxicologically relevant effects on the kidney.

Liquid oral suspension adefovir dipivoxil (GS-02-526): an update on treatments for hepatitis B infection

Though the global epidemiology of hepatitis B virus infection has declined due to effective immunization, chronic hepatitis B (CHB) remains a serious public health problem and there is still a need for more treatment options that are efficient, safe and simple for different kinds of CHB patients. Adefovir dipivoxil (ADV) liquid suspension (GS-02-526), as a new form of oral ADV, not only has competent antiviral efficacy, but is also more convenient for patients with swallowing difficulties or patients with impaired renal function requiring dosage adjustment. The clinical data evaluating the safety, tolerability and antiviral activity of liquid suspension of ADV as well as its tablet are summarized in this article. The availability of liquid oral suspension of ADV would allow more patients to receive timely and reasonable antiviral treatments.

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.

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

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

Evaluation of mitochondrial toxicity in Marmota himalayana treated with metacavir, a novel 2',3'-dideoxyguanosine prodrug for treatment of hepatitis B Virus

Metacavir (PNA) is a novel synthetic nucleoside analogue for the treatment of hepatitis B virus (HBV). Our recent studies showed that PNA, a prodrug of 2',3'-dideoxyguanosine (ddG), exhibited lower mitochondrial toxicity in long-term cultures of HepG2 cells. In the current study, we examined the long-term effects of PNA on mitochondrial toxicity in Marmota himalayana (Himalayan marmot). Himalayan marmots were treated daily with oral PNA (50 or 100 mg/kg), ziduvidine (AZT) (100 mg/kg), or water (control) for 90 days. PNA treatment did not alter the body weight or plasma lactate acid level. In livers from the animals treated with PNA at 100 mg/kg/day, histopathology showed mild steatosis or small focal liver cell necrosis. Electron microscopy also showed minor proliferation and partial mitochondrial swelling with crista reduction. Measurement of respiratory chain complex enzyme activity and mitochondrial DNA (mtDNA) content revealed no significant differences in skeletal muscle, liver, and kidney tissues between animals treated with PNA and controls. In contrast, in Himalayan marmots treated with AZT we observed delayed toxicity, including lactic acidosis, severe hepatic steatosis, obvious mitochondrial damage, and significant decreases in respiratory chain complex enzyme activity and mtDNA content. This is similar to the delayed toxicity syndrome observed previously in animals and humans. In summary, PNA treatment did not alter mitochondrial enzyme activity or mtDNA content. This suggests that PNA could pose a very low risk for adverse mitochondrion-related effects. However, long-term hepatotoxic effects of PNA were observed, and this indicates a need for continued monitoring of PNA-associated hepatotoxicity in clinical trials.

In vitro mitochondrial toxicity of metacavir, a new nucleoside reverse transcriptase inhibitor for treatment of hepatitis B virus

Therapy with nucleoside reverse transcriptase inhibitors (NRTIs) can be associated with mitochondrial toxicity. In vitro studies have been used to predict the predisposition for and characterize the mechanisms causing mitochondrial toxicity. Metacavir (PNA) is a novel synthetic nucleoside analog for oral administration with potent and specific antiviral activity against hepatitis B virus (HBV). We assessed the potential for mitochondrial toxicity of PNA in long-term cultures of HepG2 hepatoma cells by measuring mitochondrial function (through lactate secretion), levels of mitochondrial DNA (mtDNA), and the activities of respiratory-chain complexes I to IV. Exposure of HepG2 cells to PNA at concentrations up to 50 μM for 15 days resulted in no deleterious effect on cell proliferation, levels of lactate or mtDNA, or enzyme activities of respiratory-chain complexes I to IV. In contrast, dideoxycytosine at 10 μM and zidovudine at 50 μM have significant effects on cell proliferation, levels of lactate and mtDNA, and enzyme activities of respiratory-chain complexes I to IV. However, PNA at a supratherapeutic concentration of 250 μM could result in significant alterations in the levels of mtDNA and the activities of respiratory-chain complex enzymes, revealing evidence of the potential mitochondrial toxicity of PNA. In summary, these in vitro results indicate that the potential for PNA to interfere with mitochondrial functions is low.

Nonclinical safety profile of telbivudine, a novel potent antiviral agent for treatment of hepatitis B

Telbivudine is a novel nucleoside drug recently approved for the treatment of patients with chronic hepatitis B. Its nonclinical safety was evaluated in a comprehensive program of studies, including safety pharmacology, acute and chronic toxicity, reproductive and developmental toxicity, genotoxicity, and carcinogenicity. There were no test article-related effects observed in an in vitro hERG assay or in a core battery of safety pharmacology studies (central nervous system, respiratory, and cardiovascular safety pharmacology studies). Telbivudine was well tolerated in rats and in monkeys following single oral doses up to 2,000 mg/kg/day. Except for equivocal axonopathic findings in monkeys and occasional incidences of emesis, soft feces, and minor changes in body weight and food consumption, there was no target organ toxicity observed in mice, rats, or monkeys following oral administration for up to 3, 6, or 9 months, respectively, at doses up to 3,000 mg/kg/day. Axonopathy in the sciatic nerves and in the spinal cords of monkeys dosed at 1,000 mg/kg/day observed in a 9-month study was considered equivocal, as the role of telbivudine in the injury could not be determined. Slightly higher incidences of abortion and premature delivery observed in rabbits dosed at 1,000 mg/kg/day were considered secondary to maternal toxicity. There was no evidence of genotoxicity or carcinogenicity. These results suggest that telbivudine has a favorable safety profile and support its use in patients with chronic compensated hepatitis B viral infection.

Entecavir for treatment of hepatitis B virus displays no in vitro mitochondrial toxicity or DNA polymerase gamma inhibition

Therapy with nucleoside reverse transcriptase inhibitors (NRTIs) can be associated with mitochondrial toxicity. In vitro studies have been used to predict the predisposition for and characterize the mechanisms causing mitochondrial toxicity. Entecavir (ETV) is an approved deoxyguanosine nucleoside for the treatment of chronic hepatitis B virus (HBV) infection that exhibits potent activity against viral reverse transcriptase. We assessed the potential for mitochondrial toxicity of ETV in long-term cultures of HepG2 hepatoma cells by measuring mitochondrial function (through lactate secretion), levels of mitochondrial DNA (mtDNA), and levels of mitochondrial proteins COX II and COX IV. Furthermore, we tested the activity of ETV-triphosphate (ETV-TP) against mitochondrial DNA polymerase gamma (Pol gamma) in vitro. ETV concentrations as high as 100 times the maximal clinical exposure (C(max)) did not affect cell proliferation, levels of lactate, mitochondrial DNA, or mitochondrial proteins throughout the 15-day culture. The lack of mitochondrial toxicity was consistent with the finding that ETV-TP was not recognized by mitochondrial DNA Pol gamma and failed to be incorporated into DNA or inhibit the polymerase assay at the highest levels tested, 300 microM. Combinations of ETV with each of the other HBV NRTI antivirals, adefovir, tenofovir, and lamivudine at 10 times their respective C(max) levels also failed to result in cellular or mitochondrial toxicity. In summary, cell culture and enzymatic studies yielded no evidence that would predict mitochondrial toxicity of ETV at exposure levels in excess of those expected to be achieved clinically.

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.

Antiviral therapy: nucleotide and nucleoside analogs

For the management of HBV infection, an increasing number of nucleotide and nucleoside analogs are active against wild-type HBV and some against HBV with YMDD and other compensatory mutations. Table 2 depicts the IC50 and susceptibilities of HBV to various antiviral agents. The dichotomy between in vitro and in vivo susceptibilities to YMDD mutants is due to a change in IC50 between wild-type and mutant virus. Thus a drug may have less activity in vitro but at doses used in vivo show activity against YMDD and other compensatory mutations. Some HBV drugs share activity against HIV, which may be useful in the co-infected patient. Other nucleoside analogs are in various stages of development, including MCC-478 and DAPD. In the future, clinicians will have a plethora of reagents to chose from, and combination therapies may be invoked.

Effects of 9-beta-D-arabinofuranosylguanine on mitochondria in CEM T-lymphoblast leukemia cells

The nucleoside analog 9-beta-D-arabinofuranosylguanine (araG) is presently evaluated in clinical trials for therapy of T-cell lymphoid malignancies. AraG is a substrate for the mitochondrial deoxyguanosine kinase and we have recently shown that araG is predominantly incorporated into mitochondrial DNA (mtDNA). In this study we have investigated the effects of araG on mtDNA content and function. Although araG was incorporated into mtDNA, no decrease in mtDNA levels or effect on the expression of the mtDNA encoded cytochrome c oxidase was detected. Cells depleted of mtDNA were resistant to araG, but the mechanism of resistance was not specific for nucleoside analogs incorporated into mtDNA. Furthermore, the results suggest that the cells need to pass the S-phase in order for araG to induce caspase-dependent apoptosis. In summary, our findings suggest that the incorporation of araG into mtDNA does not cause the acute cytotoxicity of araG.

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.