The pharmacodynamic profile of NPI-0052 is cell-type specific

AACR Annual Meeting-- Apr 12-16, 2008; San Diego, CA 3257 The purpose of this study was to compare the pharmacodynamic profile of the proteasome inhibitor NPI-0052 in different cell types. NPI-0052 is a potent, small molecule proteasome inhibitor that inhibits the chymotrypsin-like (CT-L), trypsin-like (T-L) and caspase-like (C-L) activities of the 20S proteasome in vitro and in vivo with different kinetics when compared to Velcade® (Chauhan et al, Cancer Cell, 2005, 8:407) or PR-171 (Demo et al, Cancer Res, 2007, 67:6383). Due to the different profiles, it was considered important to evaluate the inhibition and recovery of proteasome activities after repeated administration of NPI-0052, which may contribute to understanding efficacy, dose and schedule in clinical studies. Preclinical studies demonstrated that after intravenous or oral administration to mice, NPI-0052 inhibited the CT-L, T-L and C-L activities of whole blood 20S proteasomes in a dose-dependent manner. Furthermore, a single administration of NPI-0052 resulted in sustained whole blood proteasome inhibition with significant recovery of the proteasome activity by day 7. Repeated intravenous administration of NPI-0052 in rats and primates demonstrated a dose-dependent inhibition of the CT-L activity, which was most potent at 24 hr post dosing and partially recovered over a period of 7 days. A similar trend was observed after second and third weekly treatments. Since whole blood cell lysates mainly represent red blood cells, which are anucleated cells with a half-life of about 80 days and lack the capability to generate new proteasomes, we assessed the effects of NPI-0052 on nucleated cells, focusing on peripheral blood mononuclear cells (PBMCs) that have a half life of 2-5 days. Rats were treated with NPI-0052, blood was collected at various time points after dosing, and PBMCs were isolated. Both whole blood and PBMC lysates were prepared and the CT-L activity of the 20S proteasome was determined. Results demonstrated a different recovery profile of the 20S proteasome CT-L activity in PBMCs as compared to whole blood, with activity in PBMCs markedly recovering within 48-72 hours after dosing.These results clearly demonstrate the difference between cell types in the duration of proteasome inhibition following NPI-0052 administration, which is probably secondary to the ability of PBMCs to synthesize new proteasomes and their relatively short half-life in blood. It will be of interest to correlate these findings with those in patients. NPI-0052 is currently being evaluated in Phase I clinical trials for treatment of patients with solid tumors, lymphomas, or multiple myeloma (MM). The 20S proteasome activity is being evaluated in PWB, PBMC and plasma for all tumor types, and also in CD138+ MM cells and CD138- bone marrow derived cells in the case of multiple myeloma patients.