Due tothedevelopmentofdrugresistanceandotherlimitationsinthe treat- ment of AIDS patients with reverse transcriptase (RT) inhibitors like zidovu- dineandothers, itbecamenecessarytoexploreantiviralagentsactingontar- getsotherthan RT. Inthepastfewyears, hundredsofHIVproteaseinhibitoLs have been synthesized and tested. Among these protease inhibitors, saquinavir, ritonavir, indinavir and nelfinavir have been marketed during 1995-1997. In this review, emphasis is placed on the development of HIV protease inhibitors as antiviral agents against HIV, structure-activity rela- tionship (SAR) analysis ofsaquinavirand relatedcompounds, comparisonof four marketed HIV protease inhibitors, and future prospect in developing new anti-HIV drugs. 2 Introduction HIV protease inhibitors 3 HIV protease as a target for chemotherapy HIV protease was first suggested as a potential target for AIDS therapy by Kramer et a1. in 1986 [5]. HIV protease is a proteolytic enzyme responsible for cleaving large numbers of amino acid sequences. This enzyme regulates conversionoftheselargeaminoacid sequencesintobiologicallyactive struc- tural and functional protein products. Specifically, HIV protease is responsi- the enzymatic processing of the gagand gag-pol genes of HIV, which ble for encode for functional core proteins and viral enzymes (reverse transcriptase, ribonuclease H, integrase, and HIV protease). The polyproteins encoded by the gagand gag-pol genes undergo post-translational processing by HIV pro- tease to form functional protein products as the viral particles budding out from infected cells. Therefore, inhibition of HIV protease by a protease inhibitor results in the release ofimmature, noninfectious viral particles [4].