NPI-0052 inhibits the chymotrypsin-like (CT-L), caspase-like (C-L), and trypsin-like (T-L) activity of human erythrocyte-derived 20S proteasomes. Also known as salinosporamide A, NPI-0052 is a novel chemical entity discovered during the fermentation of Salinispora tropica NPS021184, a marine actinomycete, and is manufactured by saline fermentation. NPI-0052 was shown in nonclinical studies to have increased potency and duration of biological effects compared with bortezomib and may provide a significant therapeutic advantage, particularly if toxicity is less at therapeutic doses.