NPI-0052 is the second proteasome inhibitor with potential for clinical use since the FDA approval of bortezomib. NPI-0052 is a novel, orally active, non-peptide small molecule inhibitor discovered by Nereus Pharmaceuticals during the fermentation of a new marine Gram-positive actinomycete, Salinospora sp. In human erythrocyte derived 20S proteasomes, with EC50 values in the picomolar and nanomolar range, NPI-0052 inhibits all three proteolytic activities: the chymotrypsin-like, the trypsin-like and caspase-like activities. In the present study, exposure of 1uM NPI-0052 for 1 h inhibited the chymotryptic and caspase-like activities by greater than 90% in Jurkat and ML-1 cells. The trypsin-like activity was also inhibited to a lesser extent. NPI-0052 demonstrated varying degrees of apoptosis in cell lines representative of AML (ML-1), ALL (Jurkat), and Burkitt lymphoma (BL-41) and in mononuclear cells isolated from CLL and ALL patients, as measured by propidium iodide staining and subsequent FACS analysis. In addition, treatment of Jurkat cells with NPI-0052 resulted in activation of caspase-3 and cleavage of poly ADP-ribose polymerase (PARP). Further experiments revealed that caspase activity might be initiated differently in myeloid versus lymphoid leukemia cell lines. NPI-0052 caused cleavage of caspase-8 as demonstrated by SDS-PAGE analysis and when combined with an inhibitor specific for caspase-8 (IETD-fmk), Jurkat cells (of lymphoid origin) were protected against NPI-0052 induced apoptosis whereas ML-1 (of myeloid origin) were not. The cleaved product of Bid was detected by immunoblotting in NPI-0052 treated Jurkat cells, suggesting amplification of caspase-8 activity through mitochondria. NPI-0052 induced loss of mitochondrial membrane potential and release of cytochrome c in Jurkat cells. Cell lines of lymphocytic origin exposed to 4h of NPI-0052 resulted in increased levels of peroxide and superoxide prior to cell death. Furthermore, the antioxidant, N-acetyl cysteine (NAC), conferred protection in Jurkat cells against NPI-0052 induced apoptosis. CLL and Ph+ ALL patient material confirmed that lymphocytes from these patients are protected from NPI-0052 induced apoptosis by antioxidants. In summary, NPI-0052 inhibits all three major proteolytic activities of 20S proteasome in leukemia cells and induces apoptosis in leukemic cells and patient samples. The cytotoxic effects of NPI-0052 in leukemia and lymphoma cells warrant further testing to determine if this compound is clinically effective.