Inhibitor of calpain I (Ki=120nM), calpain II (Ki=230nM), cathepsin B (Ki=100nM), and cathepsin L (Ki=0.6nM). Inhibits activation-induced programmed cell death and restores defective immune responses in HIV+ donors. Also prevents nitric oxide production by activated macrophages by interfering with transcription of the inducible nitric oxide synthase gene.