PS-341 (Pyz-Phe-boroLeu) is the active ingredient in Bortezomib. PS-341 is a reversible, cell permeable, potent, and selective proteasome inhibitor (Ki=0.6 nM). It binds the catalytic site of the 26S proteasome with high affinity and specificity. Inhibits proliferation of a number of tumor cell lines (IC50=7 nM). Inhibits TNF synthesis and FGF-induced angiogenesis.