Acyclovir--and beyond

Over the past 15 years, acyclovir has become established as standard therapy for the management of herpes simplex virus infections, but there are areas where improvements might be made. Acyclovir has a relatively low oral bioavailability. As a result, valaciclovir, the L-valine ester of acyclovir, is being developed. This new drug produces enhanced plasma levels of acyclovir following oral dosing, which will not only allow more convenient dosing for the treatment of herpes simplex virus and varicella zoster virus (VZV) infections, but also mean that valaciclovir has the potential for superior clinical efficacy over acyclovir. This may broaden the potential utility of the drug to include human cytomegalovirus prophylaxis. Other new drugs in the antiherpes area include penciclovir and its pro-drug famciclovir, which have antiviral characteristics similar to acyclovir but no clinical benefit over and above that seen with acyclovir has been demonstrated. The synthesis of new specific antiherpes compounds has led to the discovery of a novel nucleoside analogue, 882C87, which has significantly greater activity against VZV than acyclovir. The compound also has a longer plasma half-life than acyclovir which may permit less frequent dosing.