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Evaluating the inhibition and recovery of proteasome activities after repeated administration of the proteasome inhibitor NPI-0052 is important to guide the dosing and scheduling for IND-enabling safety studies and clinical trials. NPI-0052 is a potent, small molecule proteasome inhibitor that contains an unusual bicyclic γ-lactam-β-lactone ring structure. Our recent study demonstrated that NPI-0052 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 (Chauhan et al, Cancer Cell, Nov, 2005) with different kinetics compared to Velcade®. NPI-0052 induces apoptosis of CD138+ multiple myeloma (MM) cells freshly isolated from patients relapsing after multiple therapies including, Dexamethasone, Thalidomide and in 7 of 7 patients refractory to Velcade®. NPI-0052 does not significantly decrease the viability of normal human lymphocytes even at increased concentrations (20nM). In contrast, Bortezomib significantly decreases the survival of human lymphocytes even at low concentrations (6-10 nM). A series of studies were conducted in rodents and primates to define the activities of NPI-0052. When administrated intravenously (i.v.) or orally (p.o.) to mice, NPI-0052 inhibited the CT-L, T-L and C-L activities of whole blood 20S proteasomes in a dose-dependent manner. Of note, Velcade® inhibited the CT-L activity, did not inhibit the T-L activity and markedly inhibited C-L activity, which recovered over 7 days. Further studies in rats treated with NPI-0052 (i.v.) at four dose levels weekly for three weeks, demonstrated a dose-dependent inhibition of CT-L activity of whole blood 20S proteasomes 24 hr post-dosing which markedly recovered over 7 days. A similar trend was observed after second and third weekly treatments. In addition, administration of NPI-0052 (i.v.) in primates resulted in a dose-dependent, sustained whole blood 20S proteasome inhibition, which recovered by day 7. A sensitive bioanalytical plasma assay for NPI-0052 was developed and pharmacokinetic profiling in primates demonstrated a terminal half-life of 25-30 minutes. Treatment of MM.1S MM-bearing BNX-mice with NPI-0052 at 0.25 mg/kg (p.o.) or 0.15 mg/kg (i.v.) was well tolerated, resulted in significant inhibition of tumor growth and prolonged survival. Histological analysis of the tumor inoculation site at day 300 showed no recurrence of tumor in 57% of NPI-0052-treated mice along with disappearance of plasma cells in the NPI-0052 versus vehicle control-treated mice. These results demonstrate different pharmacodynamic profiles for NPI-0052 and Velcade® and support the clinical use of NPI-0052 on a weekly regimen.

[Proc Amer Assoc Cancer Res, Volume 47, 2006]