Human immunodeficiency virus (HIV), the virus that causes AIDS, is a retrovirus. Like other retroviruses, HIV contains reverse transcriptase, an enzyme that converts viral RNA into DNA. This DNA is integrated into the DNA of the host cell, where it replicates. Reverse transcriptase (RT) inhibitors work by blocking the action of reverse transcriptase. There are two groups of RT inhibitors. Nucleoside RT inhibitors (e.g., zidovudine, didanosine, zalcitabine, lamivudine, and stavudine) must be phosphorylated to become active. These drugs mimic the normal nucleosides and block reverse transcriptase. Because the different nucleoside RT inhibitors mimic different purines and pyrimidines, use of two of the drugs in this group is more effective than one alone. The second group of RT inhibitors are the non-nucleoside inhibitors (e.g., delaviridine, efanvirenz, and nevirapine), which do not require activation and, because they act through a different mechanism, exhibit a synergistic inhibition of HIV replication when used with the nucleoside RT inhibitors.

A significant challenge with the use of RT inhibitors is the development of resistance; because HIV replicates continuously at a very high rate, there are many chances for mutation and hence the emergence of a virus resistant to many drugs. To combat the emergence of resistant virus, a class of HIV drugs called nucleotide RT inhibitors (e.g., tenofovir) has been developed. These drugs are “preactivated”; that is, they are already phosphorylated and require less cellular processing. Otherwise, they are similar to nucleoside RT inhibitors and non-nucleoside RT inhibitors.

Protease inhibitors (e.g., ritonavir, saquinavir, and indinavir) block the spread of HIV to uninfected cells by inhibiting the viral enzymes involved in the synthesis of new viral particles. Because they act at a different point in the life cycle of HIV, use of a protease inhibitor with an RT inhibitor suppresses replication better than either drug alone. Protease inhibitors also slow the emergence of resistant virus. The principal adverse effects of protease inhibitors are nausea and diarrhea. Long-term use can bring on a syndrome known as lipodystrophy (wasting of peripheral fat, accumulation of central fat, hyperlipidemia, and insulin resistance).

Another class of HIV drugs is the fusion inhibitors (e.g., enfuvirtide). Fusion inhibitors work by blocking the HIV virus from entering human cells. Serious side effects include allergic reactions and infections at sites where the medicine is given intravenously.

Other types of drugs designed to combat HIV include CCR5 antagonists and post-attachment inhibitors, which block different types of molecules on the immune cell surface to prevent HIV from entering the cells, and integrase inhibitors, which block the ability of HIV to replicate. Pharmacokinetic enhancers, although not antiviral drugs themselves, may be used to augment the effectiveness of anti-HIV drugs.

Anti-RSV drugs

Respiratory syncytial virus (RSV) causes a potentially fatal lower respiratory disease in children. The only pharmacological therapy available for treatment of the infection is the nucleoside analogue ribavirin, which can be administered orally, parenterally, or by inhalation. Ribavirin must also be activated by phosphorylation in order to be effective. An injectable humanized monoclonal antibody is available for prevention of RSV infection in high-risk infants and children. It provides passive immunity and must by given by intramuscular injection once a month during RSV season.

Interferons

Interferons represent a group of nonspecific antiviral proteins produced by host cells in response to viral infections as well as in response to the injection of double-stranded RNA, some protozoal and bacterial components, and other chemical substances. Interferon results in the production of a protein that prevents the synthesis of viral components from the viral nucleic acid template. The interferons are of interest because they have broad-spectrum antiviral activity and because they inhibit the growth of cancer tissue. However, the use of interferon is limited by adverse effects, a relative lack of efficacy, and the requirement for local or intravenous administration.