Entecavir: a new nucleoside analogue for the treatment of chronic hepatitis B.

Infection with hepatitis B virus (HBV) is extremely widespread - it infects two billion people out of the six billion world population. It is estimated that between 350 and 400 million people are chronically infected with HBV. Chronic HBV infection leads to development of complications, such as cirrhosis and hepatocellular carcinoma (HCC), which arise in 15-40% of patients. HBV-related liver disease and its complications result in approximately one million deaths each year. The ultimate goals of chronic hepatitis B (CHB) therapy are decreases in the incidence of cirrhosis, end-stage liver disease and HCC. The following six medications are currently approved by the U.S. Food and Drug Administration for the treatment of CHB: interferon (INF)-alpha2b, pegylated INF-alpha2a, lamivudine, adefovir dipivoxil, entecavir and, recently, telbivudine. Interferon therapy has many contraindications and commonly causes multiple intolerable adverse effects. Lamivudine therapy leads to increased development of resistant mutations with each year of use. Entecavir, a new guanosine nucleoside analogue with specific activity against HBV DNA polymerase, represents a third agent within the nucleoside/nucleotide HBV polymerase inhibitor class. It has distinct advantages over lamivudine and adefovir dipivoxil: it has a three-step mechanism of action, is the most potent inhibitor of HBV DNA polymerase, is not associated with any major adverse effects and has a limited potential for resistance. In clinical trials, entecavir was superior to lamivudine in all primary endpoints in both nucleoside-naive and lamivudine-refractory hepatitis B e antigen (HBeAg)-positive and HBeAg-negative patients. Preliminary data support entecavir efficacy in patients with cirrhosis and HIV/HBV coinfected patients. No resistance occurred after two years of entecavir therapy in nucleoside-naive patients. Up to 9% resistance developed in patients with documented prior lamivudine resistance during 96 weeks of entecavir therapy. Currently, entecavir should be considered a first- or second-line treatment option for the management of HBeAg-positive or -negative nucleoside-naive or lamivudine-refractory CHB patients.