New antiherpesvirus agents. Their targets and therapeutic potential

Of the large number of agents under development for the treatment of herpes virus infections [herpes simplex virus types 1 and 2 (HSV-1 and HSV-2), varicella zoster virus (VZV), cytomegalovirus (CMV), Epstein-Barr virus (EBV)], only ten have apparently reached clinical development. Aciclovir was approved for the treatment of HSV infections over 10 years ago, and it remains an important and reliable antiviral agent. Recent approvals in some countries of valaciclovir for VZV infection and famciclovir for both HSV and VZV infections demonstrate the rapidity of change in this field. Intravenous ganciclovir and foscarnet are approved for the treatment of CMV infection in the immunocompromised patient. Five of the antiherpetic drugs under current clinical development are nucleoside analogues or their prodrugs; another is a phosphorylated nucleoside (nucleotide). Four of the nucleoside agents-penciclovir, famciclovir, valaciclovir and lobucavir-are being developed for the management of HSV and VZV infections. Valaciclovir is also being developed for the prevention of CMV infections and famciclovir and lobucavir for the treatment of hepatitis B virus infection. Oral ganciclovir, lobucavir, ISIS 2922 and cidofovir are being developed for the suppression of CMV infections in immunocompromised patients. Sorivudine has been studied in VZV infections. n-Docosanol is under development for HSV infections, and cidofovir is being developed for both HSV and CMV infections, as well as for treatment of other viral diseases. Traditionally, the adverse effects associated with anti-CMV compounds have been more difficult to manage and are acceptable clinically only because of the severity of the underlying infection and lack of safer therapeutic alternatives. In general, toxicity issues continue to be problematic in the anti-CMV arena, although newer agents have improved the situation to some extent. In contrast, the safety of anti-HSV compounds has traditionally been excellent, establishing a safety standard that must be met by newer agents entering the field.