REVIEW

Pharmacotherapy of Chronic Hepatitis B with Entecavir Emilio Palumbo Clinic of Paediatric, Hospital of Sondrio, Italy. Abstract: Actually three nucleotide/nucleoside analogues are used for chronic hepatitis B: lamivudine, adefovir dipivoxil, entecavir and telbivudine. Lamivudine and adefovir are advantageous for oral administration and safety, but they induce a sustained response after withdrawal of therapy in only a minority of patients. Telbivudine is a new drug and further studies are need to evaluate its real efficacy. Entecavir, a cyclopentyl guanosine analog, is a potent inhibitor of HBV-DNA polymerase and it inhibits both priming and elongation steps of viral DNA replication. In phase II and III clinical trials, entecavir was found to be superior to lamivudine for all primary endpoints evaluated in both nucleoside-naive and lamivudine-resistant patients and it was effective in both HBeAg-positive and HBeAg-negative nucleoside-naive patients. Only one trial has evidenced cases of viral resistance to entecavir. The approved dosage in treatment-naive patients is 0.5 mg/day orally, while in patients who have failed lamivudine therapy or are known to harbour lamivudine-resistant mutants, the approved dosage is 1.0 mg/day. Keywords: entecavir, hepatitis B, pharmacology, pharmacokinetics

Despite the use of HBV vaccine, chronic hepatitis B virus (HBV) infection occurs in approximately 5% of the global population. This infection may lead to chronic hepatitis, cirrhosis and hepatocellular carcinoma in 25% to 40% of infected patients, it is among the principle 10 causes of death throughout the world.1,2 Until recently, the only generally approved treatment for chronic hepatitis B was alpha-interferon, but it has demonstrated moderate efficacy in terms of sustained response (biochemical, virological and histological). In fact, only 20% to 40% of treated patients responded to therapy, with lower percentages (∼10%) among patients infected with precore-mutant strains of HBV (HBeAb HBV-DNA positive).3,4 This form is prevalent in mediterranean area (∼90% of the all patients with HBV infection) and it is due to a mutation at nucleotide 1896 in the precore region of the HBV-DNA genome. The result of this mutation is a stop codon that blocks HBeAg synthesis but still permits HBV replication and hepatitis B core antigen production, leading to persistence of viremia and persistent or intermittent elevated serum alanine aminotransferase (ALT) levels with frequent evolution into cirrhosis and hepatocellular carcinoma. The suboptimal response of this form to alpha-interferon with a high rate of non-responders or relapses has led to the research and development of new antiviral drugs to be used as alternative therapies. The use of nucleot(s)ide analogues is a milestone in the treatment of chronic hepatitis B (CHB) and they are considered as an alternative to interferon-treatment. The FDA of the USA approved the use of lamivudine in adult patients in 1998, adefovir dipivoxil in 2002. These agents are advantageous for oral administration and safety, but they induce a sustained response (after withdrawal of therapy) in only a minority of patients. In addition, the long-term efficacy of lamivudine is limited by the frequent emergence of drug-resistant HBV mutants.5–7 Adefovir is associated with a low frequency of resistance but its antiviral effect is not optimal.8 For these reasons new drugs for treatment of chronic hepatitis B are needed. A recent study has evidence entecavir is a selective inhibitor of HBV-DNA and it is less effective against lamivudine-resistant mutants than against wild-type HBV effective in combating lamivudine-resistant mutant. This review focuses on the pharmacodynamic and pharmacokinetic characteristics and on the efficacy and tolerability of entecavir in the treatment of chronic hepatitis B. Relevant literature was identified through searches of MEDLINE (2002-May 2007).

Pharmacodynamic and Pharmacokinetic

Entecavir, a guanosine nucleoside analogue with activity against HBV polymerase, is efficiently phosphorylated to the active triphosphate (TP) form, which has an intracellular half-life of 15 hours. By competing Correspondence: Emilio Palumbo, Via dell’ Arc. Michele, 4 71100 Foggia Italy. Tel: +39 0881 685023; Fax: + 39 0881 685023; Email: [email protected] Copyright in this article, its metadata, and any supplementary data is held by its author or authors. It is published under the Creative Commons Attribution By licence. For further information go to: http://creativecommons.org/licenses/by/3.0/. The authors grant exclusive rights to all commercial reproduction and distribution to Libertas Academica. Commercial reproduction and distribution rights are reserved by Libertas Academica. No unauthorised commercial use permitted without express consent of Libertas Academica. Contact [email protected] for further information.

Clinical Medicine: Therapeutics 2009:1 11–15

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Palumbo

with the natural substrate deoxyguanosine TP, entecavir-TP functionally inhibits the 3 activities of the viral polymerase: (1) priming of the HBV polymerase, (2) reverse transcription of the negative strand DNA from the pregenomic messenger RNA, and (3) synthesis of the positive strand HBV DNA. The entecavir-TP Ki for HBV DNA polymerase is 0.0012 μM. Entecavir-TP is a weak inhibitor of cellular DNA polymerases α, β, and δ with Ki values of 18 to 40 μM. In addition, high exposures of entecavir had no relevant adverse effects on γ polymerase or mitochondrial DNA synthesis in HepG2 cells (Ki ⬎ 160 μM).9 Entecavir inhibited HBV DNA synthesis (50% reduction, EC50) at a concentration of 0.004 μM in human HepG2 cells transfected with wild-type HBV. The median EC50 value for entecavir against LVDr HBV (rtL180M and rtM204V) was 0.026 μM (range 0.010–0.059 μM). In contrast, no clinically relevant activity was noted against HIV type 1 (EC50 value ⬎ 10 μM) grown in cell culture. Recombinant viruses encoding adefovir-resistant mutations at either rtN236T or rtA181V remained fully susceptible to entecavir. In HBV combination assays in vitro, abacavir, didanosine, lamivudine, stavudine, tenofovir or zidovudine were not antagonistic to the anti-HBV activity of entecavir over a wide range of concentrations. In HIV antiviral assays, entecavir was not antagonistic to the in vitro anti-HIV activity of these six NRTIs at ⬎4 times the Cmax of entecavir. Relative to wild-type HBV, LVDr viruses containing rtM204 V and rtL180 M substitutions within the reverse transcriptase exhibit 8-fold decreased susceptibility to entecavir. Incorporation of additional entecavir resistant amino acid changes rtT184, rtS202 and/or rtM250 decreases entecavir susceptibility in cell culture. Substitutions observed in clinical isolates (rtT184 A, C, F, G, I, L, M or S; rtS202 C, G or I; and/or rtM250 I, L or V) further decreased entecavir susceptibility 16- to 741-fold relative to wild-type virus. The entecavir resistant substitutions at residues rtT184, rtS202 and rtM250 alone have only a modest effect on entecavir susceptibility, and have not been observed in the absence of lamivudine resistant substitutions in more than 1000 patient samples sequenced. Resistance is mediated by reduced inhibitor binding to the altered HBV reverse transcriptase, and resistant HBV exhibits reduced replication capacity in cell culture.5 Entecavir is rapidly absorbed with peak plasma concentrations occurring between 0.5–1.5 hours. 12

The absolute bioavailability has not been determined. Based on urinary excretion of unchanged drug, the bioavailability has been estimated to be at least 70%. There is a dose-proportionate increase in Cmax and AUC values following multiple doses ranging from 0.1–1 mg. Steady-state is achieved between 6–10 days after once daily dosing with ≈2 times accumulation. Cmax and Cmin at steady-state are 4.2 and 0.3 ng/ml, respectively, for a dose of 0.5 mg, and 8.2 and 0.5 ng/ml, respectively, for 1 mg. The tablet and oral solution were bioequivalent in healthy subjects; therefore, both forms may be used interchangeably.5 Administration of 0.5 mg entecavir with a standard high-fat meal (945 kcal, 54.6 g fat) or a light meal (379 kcal, 8.2 g fat) resulted in a minimal delay in absorption (1–1.5 hour fed vs. 0.75 hour fasted), a decrease in Cmax of 44%–46%, and a decrease in AUC of 18%–20%. The lower Cmax and AUC when taken with food is not considered to be of clinical relevance in nucleoside-naive patients but could affect efficacy in lamivudine-refractory patients. The estimated volume of distribution for entecavir is in excess of total body water. Protein binding to human serum protein in vitro is ≈13%. Entecavir is not a substrate, inhibitor or inducer of the CYP450 enzyme system. Following administration of 14C-entecavir, no oxidative or acetylated metabolites and minor amounts of the phase II metabolites, glucuronide and sulfate conjugates, were observed. Entecavir is predominantly eliminated by the kidney with urinary recovery of unchanged drug at steady-state of about 75% of the dose. Renal clearance is independent of dose and ranges between 360–471 ml/min suggesting that entecavir undergoes both glomerular filtration and net tubular secretion. After reaching peak levels, entecavir plasma concentrations decreased in a bi-exponential manner with a terminal elimination half-life of ≈128–149 hours. The observed drug accumulation index is ≈2 times with once daily dosing, suggesting an effective accumulation half-life of about 24 hours. Pharmacokinetic parameters in patients with moderate or severe hepatic impairment were similar to those in patients with normal hepatic function. Entecavir clearance decreases with decreasing creatinine clearance.6,7

Efficacy and Tolerability

In the first study (Lai CL et al) a 24-week, doubleblind, randomized, multicenter, phase II clinical trial, the safety and efficacy of entecavir (0.01 mg/day, Clinical Medicine: Therapeutics 2009:1

Entecavir for HBV

0.1 mg/day, or 0.5 mg/day orally) were compared with lamivudine (100 mg/day orally). A total of 169 treatment-naive patients chronically infected with HBV (HBeAg and anti-HBe positive) were evaluated for efficacy. Compared with lamivudine, entecavir reduced HBV DNA by an additional 0.97 log(10) at the 0.1-mg/day dose and an additional 1.28 log(10) at the 0.5-mg/day dose. A clear dose-response relationship was observed for entecavir with the higher doses showing significantly greater viral suppression. In patients treated with entecavir 0.5 mg/day, 83.7% had an HBV-DNA level below the lower limit of detection of the Quantiplex branched DNA assay (⬍2.5 pg/ml), compared with 57.5% treated with 100 mg/day lamivudine. In both treatment arms, very few patients achieved HBeAg loss and/or seroconversion by week 22. More patients treated with the 0.1-mg/day and 0.5-mg/day doses of entecavir had normalization of alanine transaminase levels at week 22 compared with lamivudine, without a significant statistical difference. Entecavir was well tolerated and side effects were similar to lamividine. This study showed that entecavir has potent antiviral activity against HBV at 0.1-mg/day and 0.5-mg/day doses, both of which were superior to lamivudine in patients affected by chronic B hepatitis, both HBeAg and anti-HBe-positive.9 In a phase III clinical trial Sherman M et al. randomized a total of 286 patients affected by chronic HBeAg-positive hepatitis with lamivudine-resistance to switch to entecavir 1 mg daily (141 patients) or continue lamivudine 100 mg daily (145) for a minimum of 52 weeks. Histological improvement occurred in 55% (68/124) of entecavir-treated treated for 52 weeks vs. 28% (32/116) of lamivudine-treated patients. More patients on entecavir than lamivudine achieved the composite end point: 55% (77/141) vs. 4% (6/145), respectively. Mean change from baseline in HBV DNA was −5.11 log(10) copies/mL for entecavir-treated patients and −0.48 log(10) copies/mL for lamivudine-treated patients. HBV DNA was determined by PCR-Real-Time (⬍1.000 copies/ml). Virologic rebound because of entecavir resistance substitutions occurred in 2 of 141 of entecavir-treated patients, and genotypic evidence of resistance was detected in 10 patients. The safety profile of entecavir was comparable to lamivudine with fewer ALT flares on treatment.10 In another phase 3, double-blind trial Lai CL et al. randomly assigned 648 patients with HBeAg-negative chronic hepatitis B who had not previously been treated with a nucleoside analogue to receive 0.5 mg Clinical Medicine: Therapeutics 2009:1

of entecavir or 100 mg of lamivudine once daily for a minimum of 52 weeks. The primary efficacy end point was histological improvement with a decrease by at least two points in the Knodell necroinflammatory score, without worsening of fibrosis. Histological improvement after 48 weeks of treatment occurred in 208 of 296 patients in the entecavir group who had adequate baseline liver-biopsy specimens that could be evaluated (70%), as compared with 174 of 287 such patients in the lamivudine group (61%). More patients in the entecavir group than in the lamivudine group had undetectable serum hepatitis HBV-DNA levels according to a polymerase-chainreaction assay (⬍1000 copies/ml, 90% vs. 72%) and normalization of alanine aminotransferase levels (78% vs. 71%, P = 0.045). The mean reduction in serum HBV DNA levels from baseline to week 48 was greater with entecavir than with lamivudine. There was no evidence of resistance to entecavir. Safety and adverse-event profiles were similar in the two groups. Therefore, this study evidences as among patients with HBeAg-negative chronic hepatitis B who had not previously been treated with a nucleoside analogue, the rates of histological improvement, virologic response, and normalization of alanine aminotransferase levels were significantly higher at 48 weeks with entecavir than with lamivudine.11 Chang TT et al. randomly assigned 715 patients affected by HBeAg-positive chronic hepatitis B who had not previously received a treatment to receive either 0.5 mg of entecavir or 100 mg of lamivudine once daily for a minimum of 52 weeks. Also in this study the primary efficacy end point was histological improvement with a decrease by at least two points in the Knodell necroinflammatory score, without worsening of fibrosis at week 48, while secondary end points included a reduction in the serum HBV DNA level, HBeAg loss and seroconversion, and normalization of the alanine aminotransferase level. Histological improvement after 48 weeks occurred in 226 of 314 patients in the entecavir group (72%) and 195 of 314 patients in the lamivudine group (62%). More patients in the entecavir group than in the lamivudine group had undetectable serum HBV DNA levels according to a polymerase-chain-reaction assay (⬍1.000 copies/ml, 67% vs. 36%) and normalization of alanine aminotransferase levels (68% vs. 60%). The mean reduction in serum HBV DNA from baseline to week 48 was greater with entecavir than with lamivudine (6.9 vs. 5.4 log copies per milliliter). HBeAg seroconversion occurred in 21% of entecavir-treated patients and 18% of those treated 13

Palumbo

with lamivudine. No viral resistance to entecavir was detected. Safety was similar in the two groups. In this study among patients with HBeAg-positive chronic hepatitis B, the rates of histological, virologic, and biochemical improvement are significantly higher with entecavir than with lamivudine in patients affected by chronic HBeAg-positive hepatitis. The safety profile of the two agents is similar, and there is no evidence of viral resistance to entecavir.12 The aim of a recent study was to assess the efficacy and safety of entecavir in kidney- and livertransplant recipients with chronic hepatitis B virus (HBV) infection. Ten male transplant patients with chronic HBV infection (eight kidney- and two livertransplant patients), who have become adefovir (n = 9) or lamivudine-resistant (n = 1) were given entecavir at 0.5 to 1 mg/d. All patients were HBs Ag positive: six were HBe Ag(−)/HBe Ab(+), and four were HBe Ag(+)/HBe Ab(−). After a median followup of 16.5 months, entecavir therapy was associated with a significant decrease in HBV DNA viral load and rate of HBV DNA clearance was 50% in both HBeAg(+) and HBeAg(−) patients. The study showed a decline of eGFR from 62 to 48 ml/min in patients with renal transplant. This is doubtful since the patients had received a renal transplant and many reasons for declining renal function, however, a cautionary remark might be justified.13 The efficacy and safety of entecavir in patients with chronic hepatitis B and advanced liver fibrosis/ cirrhosis was assessed from three large, randomized, multicenter, phase III studies. These studies enrolled patients (⬎/ = 16 yr) with chronic hepatitis B, elevated alanine aminotransferase (ALT) levels, and compensated liver disease. Two trials enrolled nucleos(t)ide-naive patients randomized to at least 48 weeks of treatment with entecavir 0.5 mg/day or lamivudine 100 mg/day. The third trial randomized lamivudine-refractory patients to 48 weeks of entecavir 1 mg/day or lamivudine 100 mg/day. The majority of patients were previously treated with interferon. The efficacy and safety in patients with advanced liver fibrosis/cirrhosis were examined for consistency with those seen in the overall study populations. Of the 1,633 treated patients, 245 had advanced liver fibrosis/cirrhosis (120 entecavir and 125 lamivudine). Among entecavir-treated patients with advanced liver fibrosis, improvement in Ishak fibrosis was observed in 57% of nucleos(t)ide-naive HBeAg-positive patients, 59% of nucleos(t)ide-naive HBeAg-negative patients, and 43% of lamivudinerefractory HBeAg-positive patients versus 49%, 14

53%, and 33% of lamivudine-treated patients with advanced liver fibrosis. The overall trends in other histologic, virologic, biochemical, and serologic outcomes in entecavir- versus lamivudine-treated patients with advanced liver fibrosis/cirrhosis were consistent with those observed in the overall study populations in each trial. The treatment was well tolerated. This data confirms that the performance of entecavir relative to that of lamivudine in patients with advanced liver fibrosis/cirrhosis was consistent with the relationship observed in the overall treated population.14 A recent randomized, double-blind, multicenter study in Japan evaluated the efficacy and safety of two doses of entecavir in adult patients with lamivudine-refractory chronic hepatitis B infection. Eighty-four patients with chronic hepatitis B who were refractory to lamivudine therapy were switched from lamivudine to daily oral doses of 0.5 mg entecavir (41 patients) or 1 mg entecavir (43 patients) for 52 weeks. The proportions of patients achieving the primary end-point (⬎ or = 2 log(10) reduction in HBV-DNA from baseline by polymerase chain reaction assay or undetectable HBV-DNA levels [⬍400 copies/mL] at week 48) were 90% and 93% for entecavir 0.5 mg and 1 mg, respectively, with 33% of patients in each dosing group achieving ⬍400 copies/mL. The mean reduction in HBV-DNA from baseline was 3.58 and 3.75 log(10) copies/mL for entecavir 0.5 mg and 1 mg, respectively. High proportions of patients achieved alanine aminotransferase normalization at week 48 (0.5 mg 86%, 1 mg 78%). Histological improvement was observed in most patients (0.5 mg 52%, 1 mg 60%). Virological breakthrough (increase in HBV-DNA of ⬎ or = 1 log(10) copies/mL from nadir) was observed in one patient but was not associated with selection of entecavir-associated resistance substitutions. Entecavir was well tolerated, with no patients discontinuing the study drug due to adverse events.15 In another experiment a total of 286 patients were randomized and treated with entecavir 1 mg (n = 141) or continued lamivudine 100 mg (n = 145). At week 52, 77 entecavir-treated patients who had a protocoldefined virologic response (HBV branched DNA [bDNA] ⬍0.7 MEq/mL but HBeAg-positive) continued blinded therapy for up to 96 weeks. Patients were assessed for efficacy, safety, and emerging resistance. Cumulative proportions of all treated patients who achieved confirmed efficacy endpoints were also analyzed. Between week 48 and the end of dosing, the proportions of patients with Clinical Medicine: Therapeutics 2009:1

Entecavir for HBV

HBV DNA ⬍ 300 copies/mL by polymerase chain reaction increased from 21% to 40%, and alanine aminotransferase normalization (⬍ or = 1x upper limit of normal) increased from 65% to 81%. In the second year, HBeAg seroconversion was achieved by 10% of patients. Of the 77 patients in the second year treatment cohort, entecavir resistance emerged in six patients, and seven experienced virologic breakthrough (five with genotypic resistance acquired before year 2). The safety profile of entecavir in the second year of therapy was consistent with that reported during year 1. Through 96 weeks of treatment, 1 mg entecavir resulted in continued clinical benefit in lamivudine-refractory HBeAgpositive chronic hepatitis B patients with a safety profile comparable to lamivudine.16 Using ⬎500 patient HBV isolates from several entecavir clinical trials, Baldick CJ et al. show that phenotypic susceptibility correlates with genotypic resistance and patient virologic responses. The full-length HBV or reverse transcriptase gene was amplified from patient sera, sequenced, and cloned into an HBV expression vector. Entecavir susceptibilities of individual virus clones and patient quasispecies populations were analyzed in conjunction with the sequenced resistance genotype and the patient’s virologic response. Entecavir susceptibility decreased approximately 8-fold for isolates with various constellations of lamivudine resistance substitutions. The spectrum of additional substitutions that emerged during therapy at residues rtT184, rtS202, or rtM250 displayed varying levels of entecavir susceptibility according to the specific resistance substitutions and the proportion of resistant variants in the quasispecies. Phenotypic analyses of samples associated with virologic breakthrough confirmed the role of these residue changes in entecavir resistance. Additional longitudinal phenotypic analyses showed that decreased susceptibility correlated with both genotypic resistance and increased circulating HBV DNA.17

Conclusion

The goal of therapy for patients with HBV infection is to prevent the progression of liver disease to cirrhosis and hepatic cell cancer. Among analogue nucleos(t)ise, lamivudine i is limited by the high frequency of resistance. Adefovir has the advantages of a very low frequency of resistance and effectiveness both in wild-type and lamivudine-resistant HBV infection. Entecavir has been proved effective Clinical Medicine: Therapeutics 2009:1

against wild-type and lamivudine-resistant mutants. The treatment most likely needed for chronic hepatitis B is to use multiple agents and entecavir must be used in combination with lamivudine or adefovir. We do not have information on treating interferon resistant patients with entecavir. Futher studies are needed to confirm the rate of resistance to this drug.

Disclosure

The author reports no conflicts of interest.

References

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