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NPI-0052 is a potent, structurally novel, small molecule proteasome inhibitor isolated from the fermentation broth of a Salinosporasp. NPI-0052 shares a common bicyclic γ-lactam-β-lactone ring structure with Omuralide and PS-519, two known 20S proteasome inhibitors. NPI-0052 specifically inhibits all three major proteolytic activities of purified human 20S proteasome with EC50 values ranging from 3 nM to 400 nM. Furthermore, NPI-0052 demonstrates potent anti-tumor activity against a panel of human tumor cell lines. Among the tested cell lines, NPI-0052 is the most active against the multiple myeloma (MM) cell lines. The effects of NPI-0052 were also investigated on MM cells freshly isolated from patients relapsing after multiple prior therapies including Dexamethasone, Velcade® (Bortezomib) and Thalidomide. Results from DNA fragmentation assays demonstrated that NPI-0052 induces apoptosis in patient MM cells refractory to conventional and Bortezomib therapies. In contrast, NPI-0052 did not affect the viability of normal human peripheral blood mononuclear cells. When analyzed in vivo, NPI-0052 inhibits the chymotrypsin-like activity of murine whole blood 20S proteasomes in a dose-dependent manner after oral or i.v. administration. The effect of NPI-0052 on the 20S proteasome activity in whole blood cells was also evaluated after repeated oral dosing. The weights of the mice were monitored to access tolerability of the dosing schedules. These studies showed that twice weekly oral dosing of NPI-0052 at 0.25 mg/kg and 0.5 mg/kg for up to seven consecutive treatment resulted in minimal body weight loss, while a sustained inhibition of the 20S proteasome was established. Treatment of MM tumor bearing mice with NPI-0052 at 0.25 mg/kg or 0.5mg/kg was very well tolerated and resulted in significant inhibition of MM tumor growth, prolonged survival and a high percentage of tumor cures in these mice. In summary, NPI-0052 inhibits all three major protease-like activities of the 20S proteasome and induces apoptosis of patient MM cells resistant to conventional and Bortezomib therapies in vitro. NPI-0052 is orally active, inhibits MM tumor growth, prolongs survival and resulted in tumor cured in vivo.

[Proc Amer Assoc Cancer Res, Volume 46, 2005]