The mesylate salt form of saquinavir, a synthetic peptidomimetic substrate with antiviral property. Saquinavir selectively binds to and inhibits human immunodeficiency virus type 1 (HIV-1) protease, an aspartic protease that cleaves viral gag and gag-pol polyprotein (precursors for viral proteases, reverse transcriptase, and integrase). Inhibition of HIV-1 protease prevents functional viral proteins to be cleaved from the viral polyprotein precursor and results in the release of immature, noninfectious virions.