Hepatitis B and C viruses: molecular identification and targeted antiviral therapies.

Four agents are in clinical development for the treatment of chronic hepatitis B infection. These nucleoside analogs are incorporated into the growing DNA chain and terminate replication. Lamivudine, a cytadine analog that inhibits the synthesis of negative strand DNA from pre-genomic RNA, predictably inhibits replication and improves liver enzymes and histology in infected individuals. Following cessation of treatment, relapse is common, and genetic causes of viral resistance have been described. Other drugs for HBV infection include famciclovir, a guanosine analog that has also shown to suppress replication in immunocompetent as well as in immunocompromised patients; lobucavir, a guanosine analog; and adevfovir, an adenine nucleotide analog. The future of drug therapy against HBV likely includes combination agents with one or more nucleoside/nucleotide analogs and immune stimulants, such as interferon, or therapeutic vaccines. Recent advances in the treatment of HCV have been less impressive. An effective vaccine is greatly needed yet development in the near future is unlikely. Recommendations for therapy of chronic HCV have been proposed following the National Institutes of Health Consensus Conference. Interferon alpha is advised in patients with elevated serum alanine aminotransferases and liver histology demonstrating active hepatitis, regardless of level of pretreatment viremia or infecting genotype. Therapy should be continued for three months, at which time response should be assessed. If a biochemical and/or virological response has been achieved, treatment should be continued for a year. Trials are underway to evaluate interferon in combination with ribavirin. Recent identification of the crystalline structure of the HCV NS3 protease promises development of effective inhibitors of this critical viral enzyme.