protease inhibitor, any of a class of drugs that interfere with replication of the AIDS virus (HIV), by blocking an enzyme (protease) necessary in the late stages of its reproduction. Clinical trials of the protease inhibitor indinavir have shown it to be especially beneficial in combination with the anti-HIV drugs AZT and 3TC, which act by blocking a different enzyme, reverse transcriptase. Saquinavir, the first member of the class to be marketed, was approved for use in 1995 by the Food and Drug Administration.