The Proteasome Inhibitor NPI-0052 in Combination with Bortezomib Induces Antitumor Activity in Waldenstrom Macroglobulinemia.

Background: Waldenstrom’s Macroglobulinemia (WM) is an incurable low-grade lymphoplasmacytic lymphoma. Recently, Bortezomib has demonstrated clinical activity in patients with relapsed WM. The purpose of this study was to investigate the in vitro effect of the new proteasome inhibitor NPI-0052 (Nereus Pharmaceuticals, CA), alone and in combination with Bortezomib, in WM.

Methods: WM cell lines (BCWM1 and WSU-WM) and IgM secreting low-grade lymphoma cell lines (MEK1, RL, Namalwa) were used. Bone marrow primary CD19+ malignant cells were obtained from WM patients after informed consent. Inhibition of growth was measured using MTT assays. DNA synthesis was measured using the thymidine incorporation assay. Apoptosis was determined using Apo2.7 staining, followed by flow cytometric analysis. Determination of the additive or synergistic effect of the combination was calculated using the CalcuSyn software (Biosoft, MO) based on the Chou-Talalay method with the combination index (CI)<1.0 indicating synergism.

Results: NPI-0052 induced cytotoxicity and inhibition of DNA synthesis, with an IC50 of 20–30nM in all cell lines tested at 24 hrs. NPI-0052 induces 50% apoptosis at 48 hrs. Similar effects were demonstrated in primary CD19+ WM cells. Bortezomib induced cytotoxicity and inhibition of DNA synthesis with an IC50 of 20–30nM in all cell lines tested. Importantly, the combination of NPI-0052 and Bortezomib induced significant inhibition of proliferation compared to each agent alone, specifically at the combination of 2.5nM and 5nM of Bortezomib and 10nM of NPI-0052. For example, Bortezomib single agent at 2.5nM and 5 nM induced 8% and 13% cytotoxicity in BCWM.1 in vitro, respectively, while the combination Bortezomib and NPI-0052 10nM induced a synergistic cytotoxic effect, 44% (CI=0.7) and 58% (CI=0.47), respectively. The combination effect of NPI-0052 20 nM with Bortezomib 2.5nM and 5nM was also synergistic, with 61% (CI=0.7) and 66% (CI=0.7) cytotoxicity, respectively. Similar effects were observed in 3 primary CD19+ WM cells obtained from patients’ bone marrow.

Conclusion: NPI-0052 has significant anti-tumor activity against WM in vitro, especially in combination with Bortezomib. These results provide the framework to further study the potential therapeutic role of NPI-0052 in WM. *XJ and XL are co-first authors.