An antiviral drug, or antiviral agent, are "agents used in the prophylaxis or therapy of virus diseases. Some of the ways they may act include preventing viral replication by inhibiting viral DNA polymerase; binding to specific cell-surface receptors and inhibiting viral penetration or uncoating; inhibiting viral protein synthesis; or blocking late stages of virus assembly." While antiviral drugs are intended for systemic use by ingestion or injection of the drug, virucides are used exclusively on objects and surfaces and, when possible, topically. Antiviral drugs are used extensively to treat HIV, hepatitis C, herpes and other devastating viruses. The protease inhibitor Atazanavir, popularly known as AZT, was the first widely used treatment for HIV.
They can be divided into three non-exclusive categories: protease inhibitors, anti-metabolites and reverse transcriptase inhibitors, based on the ways in which they disrupt viral replication cycles. In addition some antivirals bind to specific cell-surface receptors and inhibit viral penetration or uncoating.
Protease inhibitors, as the name suggests, inhibit the function of protease enzymes created by viruses. The DNA or RNA in viruses are coded to produce large polyproteins, which need to be cleaved into smaller functional proteins by viral proteases before infectious, mature virus particles can be formed. For example, West Nile virus and the Dengue fever virus produce a single polyprotein that must be cleaved into ten separate proteins. The antiviral drugs amprenavir, atazanavir, indinavir, nelfinavir, ritonavir, saquinavir and tipranavir are protease inhibitors.
Anti-metabolites are chemicals that mimic natural biochemical building blocks, and most often are analogs of the nucleotides used to make DNA and RNA. Anti-metabolites act as DNA or RNA chain terminators during the replication or translation of viral DNA or RNA. Often, the drugs are missing one of the necessary linkage groups, such as a 3'- or 5'-hydroxyl group in the sugar, so that the drug gets incorporated into new viral RNA or DNA, but the next nucleotide base cannot be added because a linkage group is missing. Anti-metabolites also bind competitively with genuine metabolites in the active sites of the polymerases that create viral nucleic acids, and thus also slow down production of viral nucleic acids by clogging up the polymerases.
Reverse transcriptase inhibitors
Many of the anti-metabolites are also reverse transcriptase inhibitors (RTIs). They inhibit the reverse transcriptase enzymes by binding to them either irreversibly by forming covalent linkages, or in competition with natural biochemicals. This class of drugs stops the conversion of viral RNA into DNA.