Treatments That Suppress HIV

Drugs that interfere with the activity of a retrovirus such as HIV are generally known as antiretrovirals. Nearly all antiretroviral medications currently approved to treat HIV infection target two viral enzymes used by the virus to replicate itself. These enzymes, reverse transcriptase and protease, are involved in different stages of viral replication. A new treatment approved in the past year works in a completely new way by preventing the virus from entering the human immune cells.

Four classes of antiretroviral drugs have been developed to interfere with the activity of these viral enzymes and slow down the multiplication of the virus. These are:

• Nucleoside analog reverse transcriptase inhibitors (NRTIs). NRTIs interrupt an early stage of HIV replication by interfering with the activity of reverse transcriptase. AZT (zidovudine), the first drug approved for treating HIV infection, is an NRTI, as are zalcitabine (ddC), didanosine (ddI), stavudine (d4T), lamivudine (3TC), and abacavir.
• Non-nucleoside reverse transcriptase inhibitors (NNRTIs). NNRTIs also work by hindering the action of reverse transcriptase. This class of drugs includes delavirdine, nevirapine, and efavirenz.
• Protease inhibitors. Protease inhibitors interrupt a later stage of viral replication. This class of drugs includes saquinavir, indinavir, ritonavir, nelfinavir, and amprenavir.
• Fusion inhibitors. Fusion inhibitors prevent HIV from entering human immune cells. The only fusion inhibitor approved to date is enfuvirtide.

Studies have found that various combinations of antiretroviral drugs are more effective in suppressing HIV than antiretroviral drugs used alone. Experts refer to one common treatment approach, usually involving a protease inhibitor combined with two other antiretroviral drugs, as "highly active antiretroviral therapy" or HAART.

Drug combinations, or drug "cocktails," also can help reduce the risk that drug-resistant HIV will develop. When drug resistance occurs, medications that initially succeeded in suppressing the replication of HIV in the patient's body loose their effectiveness. Enfuvirtide works in a unique way that reduces the likelihood of cross-resistance with other HIV drugs.

Antiretroviral drugs have side effects that can limit their use in some people.

• AZT, for example, may result in a loss of blood cells.
• Protease inhibitors can cause nausea, diarrhea, and other symptoms.