A protease inhibitor used against human immunodeficiency virus (HIV). Lopinavir competitively inhibits the HIV-1 protease, an enzyme that mediates the cleavage of Gag, Gag-Pol and Nef precursor polypeptides into their mature proteins, including protease, reverse transcriptase, and integrase. Inhibition of HIV-1 protease prevents cleavage of the viral polyprotein precursor and results in the release of immature, noninfectious virions. or A drug used with another drug, called ritonavir, to treat infection with HIV (the virus that causes AIDS). It is also being studied in the treatment of some types of cancer. Lopinavir blocks the ability of HIV to make copies of itself and may help some anticancer drugs work better. It is a type of anti-HIV agent and a type of protease inhibitor.