ADCs Toxins, Lactacystin is a cell-permeable, potent and selective proteasome inhibitor. A Streptomyces metabolite that is thought to bind irreversibly to the active site N-terminal threonine residue of the catalytic β-subunit of the 20S proteasome, thereby inhibiting its chymotrypsin and trypsin-like activities. Lactacystin induces neurite outgrowth in Neuro 2a neuroblastoma cells and has been reported to induce apoptosis in human monoblast U937 cells.