The most successful antiviral agent to date, aciclovir, was a serendipitous discovery. It was manufactured as an anticancer drug but was found to have good in vitro activity against herpes simplex virus, and after clinical trials it was licensed for use in the 1980s. Subsequently, the antivirals have been designed and manufactured with specific viral targets in mind that will inhibit viral replication.

One disadvantage of the antivirals that act by DNA polymerase inhibition is that they affect the cellular DNA replication at the same time, and therefore can cause cytotoxic side effects such as nausea, vomiting, bone marrow suppression etc. For this reason they are not licensed to be used in pregnancy (except in life threatening situations or in HIV cases where risk to the fetus is outweighed by the benefit of treatment) because of risk of fetal teratogenesis. These side effects are limited in certain drugs, such as aciclovir, because the drug is preferentially activated in virus-infected cells and has little or no side effects on normal uninfected cells (see below).Table 50.1 gives examples of antiviral drugs targeted to different steps in the viral life cycle.

Human immunodeficiency virus (HIV)

See below under antiretrovirals.

Herpes viruses

Drugs: aciclovir, valaciclovir, penciclovir/famciclovir, ganciclovir, foscarnet, cidofavir, rituximab.