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

The molecular feature of abnormal fetal neuromuscular development after maternal use of telbivudine or tenofovir during pregnancy in rodent model

AIMS

Maternal treatment with nucleoside analogues such as telbivudine (LdT) and tenofovir disoproxil fumarate (TDF) has been applied worldwide. However, administration of LdT or TDF during pregnancy may affect the fetal neuromuscular development. We conducted the current study to investigate the histological pathology and transcriptomic changes pertaining to the neuromuscular system of the newborn exposed to LdT or TDF during pregnancy in rodent model.

MAIN METHODS

Pregnant C57/BL6 mice were randomly divided into three arms and administered either with LdT solution (0.1 ml, 78 mg/kg/d), TDF solution (0.1 ml, 39 mg/kg/d) or normal saline solution (0.1 ml). Pups in each arm were weighed and sacrificed after birth. Both sides of quadriceps femoris muscle of the newborn were obtained. The histological observation was conducted under light microscope. The transcriptional profiling was analyzed with RNA sequencing (RNA seq).

KEY FINDINGS

Four types of morphological abnormalities of the newborn neuromuscular system, being clusters of rhabdomyoblasts, skeletal muscle fibrosis, rhabdomyolysis and necrosis and immature muscle fiber bundles, were noted in both LdT group and TDF group. Moreover, both groups showed significantly decreased gross cross-sectional area of muscle fiber and significantly increased percentage of muscle lesion area. RNA seq identified a total of 164 differentially expressed genes (DEGs) essential to fetal neuromuscular development. These DEGs were involved in calcium regulation, phospholipid activity, muscle cell development, the functioning of mitochondria/endoplasmic reticulum/lysosome/cytoskeleton, the regulation of arachidonic acid and the development of nervous system.

SIGNIFICANCE

Our findings suggest maternal administration of LdT or TDF lead to abnormal neuromuscular development in offspring mice. Further study should be encouraged to investigate the down-stream signaling pathways.

Insights into the mechanisms of telbivudine-induced myopathy associated with mitochondrial dysfunction

As a nucleotide analogue (NA), telbivudine was widely used in the treatment for chronic hepatitis B (CHB) by interfering with reverse transcriptase of hepatitis B virus. However, the use of NAs for hepatitis B treatment has been accompanied by numerous reports highlighting the occurrence of neuromyopathy, particularly in the case of telbivudine. This study aimed to investigate the underlying mechanisms responsible for telbivudine-induced myopathy. We established animal and cell models of telbivudine-induced myopathy using C57BL/6 mice and C2C12 cells, respectively. Our findings revealed that telbivudine significantly reduced mitochondrial DNA (mtDNA) copy number and caused increase of oxidative stress. Telbivudine treatment significantly inhibited mitochondrial complex I and IV expression, impairing the oxidative phosphorylation function of the respiratory chain. Modified Gomori trichrome (MGT) staining of the muscle sections displayed an increase in ragged red fibers (RRFs), indicating abnormal mitochondrial accumulation. In conclusion, our study provides compelling evidence suggesting that telbivudine-induced myopathy is associated with mitochondrial toxicity and impaired energy metabolism. The observed muscle pathology, depletion of mtDNA, elevation of oxidative stress and altered mitochondrial function support the hypothesis that telbivudine disrupts mitochondrial homeostasis, ultimately leading to muscle damage. This may be also a common mechanism for NAs to cause neuromyopathy.

Infant immune response to hepatitis B vaccine after fetal exposure to telbivudine

Whether telbivudine (LdT) treatment to pregnant women with hepatitis B surface antigen (HBsAg) affects infant immune response to hepatitis B vaccine (HepB) has not been investigated. A total of 127 HBsAg positive mothers and their neonates were enrolled and followed up at 11–13 months. Mothers took LdT (LdT group) or did not receive antiviral therapy (control group). Infant anti-HBs, immune cells and cytokines were measured after HepB was administered according to 0-1-6 procedure. We performed a 1:3 propensity score matching (PSM). Immune indexes in the two groups were compared. Baseline characteristics of mother-baby pairs were comparable in LdT group and control group. Infant anti-HBs geometric mean concentration (GMC) did not differ significantly between the two groups [767.70 (745.35) vs. 711.90 (819.60), P = .599]. There was no difference between the two groups in infant positive rate of anti-HBs [97.8% (91/93) vs. 97.1% (33/34), P = .999] and strong positive rate of anti-HBs [40.9% (38/93) vs. 44.1% (15/34), P = .742]. Infants with negative, low, medium, and high anti-HBs levels were similarly distributed between the two groups (P = .511). No differences in proportion of helper T cells, cytotoxic T cells, B cells, myeloid dendritic cells, and plasmacytoid dendritic cells of infants (P > .05) were detected between the two groups. Children in the LdT and control group had comparable levels of interleukin-2, interleukin-4, interleukin-6, interleukin-10, interleukin-12, interferon-α, interferon-γ and tumor necrosis factor-α (P > .05). Intrauterine exposure to LdT was safe to infant immune response to HepB after birth.

Predicting target-ligand interactions with graph convolutional networks for interpretable pharmaceutical discovery

Drug Discovery is an active research area that demands great investments and generates low returns due to its inherent complexity and great costs. To identify potential therapeutic candidates more effectively, we propose protein–ligand with adversarial augmentations network (PLA-Net), a deep learning-based approach to predict target–ligand interactions. PLA-Net consists of a two-module deep graph convolutional network that considers ligands’ and targets’ most relevant chemical information, successfully combining them to find their binding capability. Moreover, we generate adversarial data augmentations that preserve relevant biological backgrounds and improve the interpretability of our model, highlighting the relevant substructures of the ligands reported to interact with the protein targets. Our experiments demonstrate that the joint ligand–target information and the adversarial augmentations significantly increase the interaction prediction performance. PLA-Net achieves 86.52% in mean average precision for 102 target proteins with perfect performance for 30 of them, in a curated version of actives as decoys dataset. Lastly, we accurately predict pharmacologically-relevant molecules when screening the ligands of ChEMBL and drug repurposing Hub datasets with the perfect-scoring targets.

Real-Life State of Anti-Hepatitis B Virus Drug Choice in Child-Bearing Age Male Patients and Effect on Fertility and Fetal Safety

Research on effects of anti-hepatitis B virus (HBV) nucleoside analogs on male fertility and birth defects is limited and safety of nucleoside analogs in pregnancy is still a concern. Chronic hepatitis B (CHB) patients in Guangdong province were surveyed using a structured questionnaire. We collected data including medication type, fertility, and birth defects. Moreover, a survey of the knowledge of antiviral nucleoside analogs safety in fertility of male patients was conducted among physicians nationwide. Semen samples of 30 patients were collected. We screened 1050 HBV-positive male patients. Reasons for not receiving antivirals in 150 patients were "did not meet criteria for antiviral therapy," fertility, and financial. Furthermore, 900 participants received antivirals (85.71%, 900/1050), including 792 patients with children and 15.15% (120/792) took anti-HBV treatment when preparing for pregnancy. Based on whether they received antiviral therapy during conception or not, we divided patients into two groups. In the child-bearing age group, 88.33% (106/120) of patients received telbivudine (LDT), whereas the other group mainly received entecavir (ETV) (87.20%, 586/672). No significant difference occurred in birth defect incidence rates between both groups. Furthermore, 558 physicians completed questionnaires. Reasons that influenced drug selection were "patient's condition," "fertility demand," "financial condition," and "compliance." Telbivudine was the first-choice drug (32.80%, 183/558) while tenofovir (TDF) was the second (2.69%, 15/558). Additionally, 61.47% of physicians considered telbivudine or tenofovir as the first choice for male patients who met antiviral criteria, whereas 19% suggested delayed therapy and follow-up until childbirth. No significant changes occurred in semen volume, concentration, mobility, and percentage before and after administration of anti-HBV nucleoside analogs, which did not affect male fertility and birth defect incidence while the desire for pregnancy influenced drug selection and timing of administration. Further research on the effects of analogs on male fertility and fetal safety is required.

Tryptophan 32 mediates SOD1 toxicity in a in vivo motor neuron model of ALS and is a promising target for small molecule therapeutics

SOD1 misfolding, toxic gain of function, and spread are proposed as a pathological basis of amyotrophic lateral sclerosis (ALS), but the nature of SOD1 toxicity has been difficult to elucidate. Uniquely in SOD1 proteins from humans and other primates, and rarely in other species, a tryptophan residue at position 32 (W32) is predicted to be solvent exposed and to participate in SOD1 misfolding. We hypothesized that W32 is influential in SOD1 acquiring toxicity, as it is known to be important in template-directed misfolding. We tested if W32 contributes to SOD1 cytotoxicity and if it is an appropriate drug target to ameliorate ALS-like neuromuscular deficits in a zebrafish model of motor neuron axon morphology and function (swimming). Embryos injected with human SOD1 variant with W32 substituted for a serine (SOD1^W32S) had reduced motor neuron axonopathy and motor deficits compared to those injected with wildtype or disease-associated SOD1. A library of FDA-approved small molecules was ranked with virtual screening based on predicted binding to W32, and subsequently filtered for analogues using a pharmacophore model based on molecular features of the uracil moiety of a small molecule previously predicted to interact with W32 (5'-fluorouridine or 5'-FUrd). Along with testing 5'-FUrd and uridine, a lead candidate from this list was selected based on its lower toxicity and improved blood brain barrier penetrance; telbivudine significantly rescued SOD1 toxicity in a dose-dependent manner. The mechanisms whereby the small molecules ameliorated motor neuron phenotypes were specifically mediated through human SOD1 and its residue W32, because these therapeutics had no measurable impact on the effects of UBQLN4^D90A, EtOH, or tryptophan-deficient human SOD1^W32S. By substituting W32 for a more evolutionarily conserved residue (serine), we confirmed the significant influence of W32 on human SOD1 toxicity to motor neuron morphology and function; further, we performed pharmaceutical targeting of the W32 residue for rescuing SOD1 toxicity. This unique residue offers future novel insights into SOD1 stability and toxic gain of function, and therefore poses an potential target for drug therapy.

Interferon alpha antagonizes the anti-hepatoma activity of the oncolytic virus M1 by stimulating anti-viral immunity

Alpha virus M1 is an oncolytic virus that targets zinc-finger antiviral protein (ZAP)-defective cancer cells, and may be useful for treatment of hepatocellular carcinoma (HCC). Most of HCC patients have hepatitis and need long-term antiviral medication. Thus, it is necessary to clarify whether anti-virus medicines influence oncolytic effect of M1. We examined the effect of drugs used to treat hepatitis B/C on M1-mediated oncolysis in vitro and in vivo. Interferon (IFN)-α induces expression of antiviral IFN-stimulated genes (ISGs) in HCC cells with moderate sensitivity to M1 virus. This leads to reduced replication of M1, and blocking of M1-mediated apoptosis. The antagonistic effect of IFN-α is positively related with the expressive level of ISGs. We also examined a population of 147 HCC patients. A total of 107 patients (73%) had low ZAP expression in liver tissues relative to adjacent tissues. Among these 107 patients, 77% were positive for hepatitis B and 2% were positive for hepatitis C. A combination of M1 virus and IFN should be avoided in those patients with HBV or HCV infection, of who ZAP expression is low but ISGs expression is moderate. In conclusion, this study provides a basis for anti-viral regimens for HCC patients with hepatitis B or C who are given oncolytic virus M1.

Motor development delay in offspring is associated with prenatal telbivudine exposure

Telbivudine is an orally nucleoside analog with potent and specific antihepatitis B virus (HBV) activity, and it has been reported to block mother-to-infant transmission. However, few studies have focused on the safety of prenatal exposure for offspring development.This is a prospective noninterventional study. Participants were enrolled during delivery through the Women's Hospital of Zhejiang University School of Medicine between January 2012 and September 2013. Neonate's umbilical cord arterial blood (UCAB) was collected after delivery. Hepatitis B virus DNA copy, HBV serology, alanine aminotransferase (ALT), creatine kinase (CK), creatinine (CRE), and blood urea nitrogen (BUN) were measured. The development of the offspring was evaluated by the Chinese Revision of Bayley Scales of Child Development (BSCD-CR) at 12 to 24 months old.Around 30 and 31 chronic hepatitis B mothers were recruited in untreated group (non-LdT group) and telbivudine-treatment group (LdT group), respectively, and 2 children (one in non-LdT group and 1 in LdT group) were lost in follow-up. Sixty-one normal women and their children were recruited as a normal control (control group). Compared with non-LdT group, telbivudine treatment effectively blocks HBV transmission from mother to infant. However, CK in UCAB was significantly increased in the LdT group. Moreover, children with prenatal telbivudine exposure showed lower level of serum creatinine than non-LdT group, reduction of psychomotor developmental index and increased risk of motor development delay.Prenatal telbivudine exposure is correlated with motor development delay in offspring.

Telbivudine attenuates UUO-induced renal fibrosis via TGF-β/Smad and NF-κB signaling

Renal fibrosis yields decreased renal function and is a potent contributor to chronic kidney disease (CKD). Telbivudine (LdT) is an anti-hepatitis B virus (HBV) drug that has been found to steadily improve renal function, but the mechanism of drug action is unclear. One explanation is that LdT impacts inflammatory or fibrotic pathways. In this study, we investigated renal protection by LdT in a rat model of unilateral ureteral obstruction (UUO). UUO rats received oral gavage of LdT (1, 1.5, or 2g/kg/day) for 5weeks. Kidney tissues were examined histopathologically with hematoxylin and eosin and Masson's trichrome stain. To assess proliferation of myofibroblasts and matrix accumulation, α-smooth muscle actin (α-sma) and collagen type I and III were detected. Interleukin-1 (IL-1) and tumor necrosis factor (TNF)-α were evaluated as a measure of proinflammatory cytokines. Transforming growth factor (TGF)-β and nuclear factor-κB (NF-κB) were considered the canonical signaling components in our investigation of the underlying mechanism of LdT action. Histopathology results indicated that LdT ameliorates renal injury and matrix accumulation. Expression of α-sma and collagen I/III as well as key fibrotic signaling factors in the TGF-β/Smad pathway were downregulated. In addition, LdT suppressed the release of IL-1 and TNF-α and decreased the expression of NF-κB by inhibiting toll-like receptor 4. Taken together, these findings indicate that LdT can attenuate renal fibrosis and inflammation via TGF-β/Smad and NF-κB pathways in UUO.

Chronic hepatitis B: a critical discussion

Telbivudine (LdT) is an antiviral agent currently used in the treatment of chronic hepatitis B virus, which was first approved by the US FDA in 2006. The safety of LdT is of great importance for patients that receive long-term treatment for this condition. It has been confirmed that patients treated with LdT have significantly elevated creatine kinase levels. However, the mechanism responsible for this adverse reaction is unclear. This review summarizes the current literature of the adverse reactions of LdT and the possible mechanisms that are involved in chronic hepatitis B infection. Thus, we aim to provide guidance on best practices in using LdT and to provide evidence of the possible mechanisms of LdT-associated adverse reactions.

Telbivudine prevents vertical transmission of hepatitis B virus from women with high viral loads: a prospective long-term study

BACKGROUND & AIMS

Hepatitis B virus (HBV) infection is a leading cause of liver diseases. We investigated the efficacy and safety of telbivudine in preventing transmission of HBV from hepatitis B e antigen-positive pregnant women with high viral loads to their infants in an open-label study.

METHODS

We performed a prospective study of 450 hepatitis B e antigen-positive pregnant women with HBV DNA levels greater than 10(6) IU/mL; 279 women received telbivudine (600 mg/d) during weeks 24 to 32 of gestation, and 171 women who were unwilling to take antiviral drugs participated as controls. All newborns were vaccinated with a recombinant HBV vaccine and hepatitis B immune globulin, according to a standard immunoprophylaxis procedure. Mother-to-child transmission of HBV was determined by detection of hepatitis B surface antigen and HBV DNA in the infant 6 months after birth.

RESULTS

None of the infants whose mothers were given telbivudine tested positive for of hepatitis B surface antigen at 6 months, compared with 14.7% of infants in the control group (P = 5.317 × 10(-8)). Levels of HBV DNA also decreased among women given telbivudine; 40 of 172 (23.2%) women given telbivudine had undetectable HBV DNA levels before delivery, compared with none of the controls. A significantly higher proportion of women given telbivudine had undetectable levels of HBV DNA in cord blood (99.1%) than controls (61.5%; P = 1.195 × 10(-22)). No severe adverse events or complications were observed in women or infants.

CONCLUSIONS

Telbivudine significantly reduces vertical transmission of HBV from pregnant women to their infants; it is safe and well tolerated by women and infants. Antiretroviral Pregnancy Registry Health Care Providers ID: 26592; Government number: Natural Science Foundation of China (NSFC) 30830090, 30972598; and Third Military Medical University Key Project for Clinical Research: 2012XLC05).

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.

Chronic hepatitis B virus infection and pregnancy

Planning of pregnancy and management of chronic hepatitis B virus during pregnancy includes recognition of maternal virological status, assessment of liver disease severity and minimization of risk for mother to infant transmission of infection. Decisions regarding the use of antivirals during pregnancy need to be individualized. Monitoring for infection and immunization in newborns is also important. For mothers on antiviral therapy, breastfeeding is not recommended.

Antiviral therapy for hepatitis B infection during pregnancy and breastfeeding

It is estimated there are 350-400 million people worldwide chronically infected with HBV. Many of these are women and of reproductive age. As such, they may face therapeutic decisions regarding antiviral therapy and the implication this may have on future or current pregnancies. This article reviews the data of all antivirals licensed for use against hepatitis B infection regarding teratogenicity, carcinogenicity, clinical experience during pregnancy, placental transfer and excretion in breast milk.

Cell fate control gene therapy based on engineered variants of human deoxycytidine kinase

The safety of cell therapy applications can be enhanced by the introduction of Cell Fate Control (CFC) elements, which encode pharmacologically controlled cellular suicide switches. CFC Gene Therapy (CFCGT) offers the possibility of establishing control over gene-modified cells (GMCs) with regards to their proliferation, differentiation, or function. However, enzymes commonly employed in these approaches often possess poor kinetics and high immunogenicity. We describe a novel CFCGT system based on engineered variants of human deoxyCytidine Kinase (dCK) that overcomes limitations of current modalities. Mutants of dCK with rationally designed active sites that make them thymidine-activating were stably introduced into cells by recombinant lentiviral vectors (LVs). Transduced cells maintained growth kinetics and function. These dCK mutants efficiently activate bromovinyl-deoxyuridine (BVdU), L-deoxythymidine (LdT), and L-deoxyuridine (LdU), which are otherwise not toxic to wild-type cells. We show that mutant dCK-expressing Jurkat, Molt-4, and U87mg cells could be efficiently eliminated in vitro and in xenogeneic leukemia and tumor models in vivo. We also describe a fusion construct of the thymidine-activating dCK to the cytoplasmic tail-truncated LNGFR molecule and applications to in vivo eradication of primary human T cells. This novel CFCGT system offers unique plasticity with respect to the wide range of prodrugs it can potentiate, and can be used as a reliable safety switch in cell and gene therapy.

Telbivudine: a review of its use in compensated chronic hepatitis B

Telbivudine (Sebivo®; Tyzeka®) is a synthetic nucleoside analogue that inhibits replication of hepatitis B virus (HBV). It is used in the treatment of adults with chronic hepatitis B (CHB) with evidence of viral replication and persistently elevated serum ALT and/or AST levels, and/or histological evidence of active disease. Telbivudine is a potent antiviral that provides effective and sustained viral suppression in patients with compensated CHB. In clinical trials, treatment outcomes were improved significantly more with telbivudine 600 mg once daily than with lamivudine 100 mg or adefovir 10 mg once daily, and telbivudine-treated patients had significantly less viral resistance than lamivudine-treated patients. Increasing rates of hepatitis B e antigen (HBeAg) seroconversion were achieved in HBeAg-positive patients during periods of up to 4 years continuous telbivudine treatment, and seroconversion was durable in most patients throughout a 2-year, off-treatment follow-up, indicating the potential for a finite treatment period in this group of patients. Telbivudine is associated with a medium genetic barrier to resistance and, as patients with undetectable HBV DNA levels have significantly improved outcomes, it is recommended that HBV DNA levels are monitored at week 24 (and 6 monthly thereafter), with the addition of a nucleoside/nucleotide analogue without cross resistance (such as adefovir dipivoxil) if viraemia is present to reduce the risk of resistance (Roadmap concept). Telbivudine was generally well tolerated in clinical trials for periods of up to 4 years, and has a similar tolerability profile to that of lamivudine. A minority of telbivudine-treated patients experience creatinine kinase elevation, usually transient, and myopathy occurs rarely. In modelled cost effectiveness studies in several Asian countries, treatment with telbivudine Roadmap was cost effective in HBeAg-positive patients. Thus, telbivudine provides a valuable treatment option in CHB, particularly when administered using the Roadmap concept in HbeAg-positive patients.

Structural and kinetic characterization of human deoxycytidine kinase variants able to phosphorylate 5-substituted deoxycytidine and thymidine analogues

The physiological role of human deoxycytidine kinase (dCK) is to phosphorylate deoxynucleosides required for DNA synthesis, with the exception of thymidine. Previous structural analysis of dCK implicated steric factors, specifically the thymine methyl group at the 5-position, that prevent thymidine phosphorylation by dCK. This hypothesis is supported by the observation that mutations that enlarge the active site cavity in proximity to the nucleoside 5-position endow dCK with the ability to phosphorylate thymidine. However, in conflict with this hypothesis was our discovery that the cytidine analogue 5-methyldeoxycytidine (5-Me-dC), an isostere of thymidine, can indeed be phosphorylated by wild-type (WT) dCK. To reconcile this seemingly contradicting observation, and to better understand the determinants preventing thymidine phosphorylation by WT dCK, we solved the crystal structure of dCK in complex with 5-Me-dC. The structure reveals the active site adjustments required to accommodate the methyl group at the 5-position. Combination of kinetic, mutagenesis, and structural data suggested that it is in fact residue Asp133 of dCK that is most responsible for discriminating against the thymine base. dCK variants in which Asp133 is replaced by an alanine and Arg104 by select hydrophobic residues attain significantly improved activity with 5-substituted deoxycytidine and thymidine analogues. Importantly, the ability of the designer enzymes to activate 5-substitued pyrimidines makes it possible to utilize such nucleoside analogues in suicide gene therapy or protein therapy applications that target cancer cells.

Management of chronic hepatitis B in pregnancy

Providing appropriate treatment and follow-up to hepatitis B virus (HBV)-infected mothers and their newborns is critical in preventing HBV mother-to-child transmission (MTCT) and eradicating HBV infection. Although highly effective in preventing MTCT, standard passive-active immunoprophylaxis with hepatitis B immunoglobulin and the hepatitis B vaccine may have a failure rate as high as 10% to 15%. Antiviral treatment has been used during pregnancy and may decrease MTCT. Several issues must be addressed in future clinical studies before universal recommendations for antiviral therapy for pregnant women can be made.

Absence of effect of telbivudine on cardiac repolarization: results of a thorough QT/QTc study in healthy participants

The effect of telbivudine on cardiac repolarization was evaluated in healthy participants at clinical and supratherapeutic doses. Sixty-two participants were enrolled, stratified by sex, and randomized according to a crossover design among 4 treatment sequences: placebo, a single moxifloxacin 400-mg dose as positive calibrator, and telbivudine 600 and 1800 mg/d for 7 days. Intensive time-matched electrocardiogram measurements and pharmacokinetic sampling were performed at baseline and on day 7 over 24 hours. For telbivudine and moxifloxacin, time-matched, placebo-adjusted change from baseline in QT was calculated and corrected using Fridericia's formula (QTcF). While moxifloxacin produced the expected significant changes in QTcF, none of the changes in QTcF for either doses of telbivudine exceeded 5 ms, and none of the associated upper 1-sided 95% confidence intervals (CI) exceeded the limit of 10 ms. There was no increase in QTcF with increasing plasma telbivudine. The supratherapeutic dose of telbivudine was well tolerated with a safety profile similar to the clinical dose despite a 3-fold increase in plasma exposure. This study therefore met the criteria for a negative thorough QT study. Telbivudine had no adverse effect on cardiac repolarization in healthy participants, even at supratherapeutic exposure, suggesting a broad safety margin.

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

Telbivudine was recently approved for the treatment of chronic hepatitis B. Phase III studies indicated its antiviral potency with 6- to 6.5-log copies/mL reductions in hepatitis B DNA levels at year 1, comparable to other potent agents such as entecavir or tenofovir. Genotypic resistance rates, however, reached 25% at year 2 in hepatitis B e-antigen positive subjects and 11% in hepatitis B e-antigen negative subjects, preventing it from becoming a preferred first-line drug for hepatitis B. Furthermore, its signature resistance mutation (a change from methionine to isoleucine at position 204 in the reverse transcriptase domain of the hepatitis B polymerase) also confers cross-resistance to entecavir, lamivudine, and emtricitabine. Telbivudine is well tolerated, with elevations in creatine phosphokinase being the most common abnormality observed in clinical trials. Most often, elevations were asymptomatic. Future research in hepatitis B will focus on the best ways to use existing therapies, including telbivudine, sequentially or in combination in order to maximize viral suppression and minimize the development of antiviral resistance.

Telbivudine in the treatment of chronic hepatitis B

INTRODUCTION

The treatment of chronic hepatitis B virus (HBV) infection has been revolutionized in the past decade by the increased availability of effective antiviral agents. Telbivudine is an L-nucleoside that is structurally related to lamivudine and has recently been approved for use in patients with chronic HBV infection. Telbivudine is highly selective for HBV DNA and inhibits viral DNA synthesis with no effect on human DNA or other viruses. This article reviews the pharmacology, pharmacokinetics, therapeutic efficacy and safety of telbivudine, and discusses its place in the current armamentarium against HBV.

METHODS

Relevant publications were identified from searches of Medline and PubMed between 2000 and 2008, using the search terms "hepatitis B/HBV," "telbivudine/LdT," "beta-L-thymidine," "pharmacokinetics," "safety," "adverse events," and "resistance." The reference lists of retrieved articles were searched for relevant studies.

RESULTS

Phase 3 clinical studies demonstrate that telbivudine is superior to lamivudine over a 2-year period in hepatitis B e-antigen (HBeAg)-positive and HBeAg-negative patients. Telbivudine was associated with a statistically significantly greater reduction in HBV DNA, greater proportion of alanine aminotransferase normalization, and greater histological response than lamivudine. Furthermore, telbivudine use resulted in fewer cases of treatment failure and less virological resistance than lamivudine. However, after 2 years of therapy, telbivudine resistance was appreciable (25%) and considerably higher than that seen with other new antivirals such as tenofovir and entecavir. Overall, telbivudine was found to be safe, although grade 3 or 4 adverse events, including elevations in creatine kinase, were more commonly found in patients receiving telbivudine than lamivudine. Telbivudine is not active against lamivudine-resistant HBV.

CONCLUSIONS

Telbivudine is a new antiviral agent joining the armamentarium against HBV. It is superior to lamivudine in terms of therapeutic response and resistance profile. However, concerns about resistance with long-term use, along with inferior cost-effective analyses, have relegated telbivudine to a second-line agent in the management of chronic HBV infection.