Protease inhibitors (PIs) are a class of drugs used to treat or prevent infection by viruses, including HIV and Hepatitis C. PIs prevent viral replication by inhibiting the activity of HIV-1 protease, an enzyme used by the viruses to cleave nascent proteins for final assembly of new virons.

Protease inhibitors have been developed or are presently undergoing testing for treating various viruses:

• HIV/AIDS: antiretroviral protease inhibitors (saquinavir, ritonavir, indinavir, nelfinavir, amprenavir etc.)
• Hepatitis C: experimental agents: BILN 2061 (All clinical trials of BILN 2061 have been suspended due to cardiac issues), VX 950 (trade name Telaprevir), or SCH 503034

Given the specificity of the target of these drugs there is the risk, as in antibiotics, of the development of drug-resistant mutated viruses. To reduce this risk it is common to use several different drugs together that are each aimed at different targets.

Antiretrovirals
Protease inhibitors were the second class of antiretroviral drugs developed. In all cases, patents remain in force until 2010 or beyond.

What are Protease Inhibitors?
When HIV infects a CD4 cell in a person's body, it copies its own genetic code into the cell's DNA. The CD4 cell is then "programmed" to make new HIV genetic material and HIV proteins. The proteins must be cut up by the HIV protease—a protein-cutting enzyme—to make functional new HIV particles. PIs block the protease enzyme and prevent the cell from producing new viruses. It is recommended that they be used in combination with at least two other HIV drugs to treat HIV infection.