Entecavir for the treatment of chronic hepatitis B virus infection.

OBJECTIVE This article reviews the pharmacology/pharmacokinetics and therapeutic efficacy of entecavir, which was approved on March 29, 2005, for the management of adult patients with chronic hepatitis B virus (HBV) infection who have active viral replication and/or elevations in liver transaminases or signs of active liver disease on histologic examination. Potential drug interactions and adverse events associated with the use of entecavir are also reviewed. METHODS Relevant literature was identified through searches of MEDLINE (1996-July 2005) and BIOSIS (1993-July 2005). Search terms included, but were not limited to, entecavir, BMS-200475, hepatitis B, pharmacology, pharmacokinetics, adverse events, and therapeutic use. Further publications were identified from the reference lists of the identified articles and through correspondence with the manufacturer of entecavir. RESULTS Entecavir is highly selective for the HBV and inhibits all 3 steps of viral replication. Results of early studies indicated a 6% resistance potential after 48 weeks of therapy, although the potential may be higher in patients who harbor lamivudine-resistant mutants. The approved dosage in treatment-naive patients is 0.5 mg/d p.o., administered on an empty stomach; in patients who have failed lamivudine therapy or are known to harbor lamivudine-resistant mutants, the approved dosage is 1.0 mg/d p.o.. The oral tablet and solution can be used interchangeably. Entecavir is well absorbed orally, achieving a dose-related Cmax between 0.6 and 1.5 hours after administration. It is metabolized to a small extent and is not a substrate for the cytochrome P450 enzyme system. The mean elimination t(1/2) ranges from 77 to 149 hours in patients with normal kidney function. Entecavir is eliminated primarily in the urine via glomerular filtration and tubular secretion (62%-73%). No dose adjustment appears to be necessary in patients with moderate to severe liver disease alone. The potential for drug interactions with entecavir appears to be minimal, although medications that inhibit tubular secretion of drugs (eg, probenecid) may be expected to prolong serum concentrations of entecavir. One of the Phase III studies of entecavir found statistically significant benefits compared with lamivudine in terms of improvements in liver histology after 48 weeks of therapy (72% vs 62%, respectively; P<0.009), the proportions of patients with undetectable HBV DNA titers on branched DNA signal amplification assay after 48 weeks of therapy (91% vs 65%; P<0.001), and the proportion with undetectable HBV DNA on polymerase chain reaction (PCR) assay after 48 weeks of therapy (69% vs 38%; P<0.001). In another Phase III study, patients who had failed to respond to lamivudine therapy responded to entecavir: after 48 weeks of therapy, significant differences between entecavir and lamivudine were seen in histologic improvement (55% vs 28%; P<0.001) and the proportion of patients with undetectable HBV DNA on PCR assay (21% vs 1%; P<0.001). Adverse events associated with entecavir therapy were similar in character, severity, and incidence to those associated with placebo or lamivudine therapy. The most common adverse events in clinical trials of entecavir were headache (17%-23% of patients), upper respiratory tract infection (18%-20%), cough (12%-15%), nasopharyngitis (9%-14%), fatigue (10%-13%), dizziness (9%), upper abdominal pain (9%-10%), and nausea (6%-8%). CONCLUSIONS Entecavir is a new antiviral agent for the management of chronic HBV infection. Questions concerning the ideal length of therapy, long-term efficacy, and resistance rates over time await the results of ongoing clinical trials.