Preferential Inhibition of Malignant Cell Growth by CDDO in Waldenström Macroglobulinemia.

Waldenström macroglobulinemia (WM) is a B cell disorder with a highly variable clinical outcome, where some patients remain asymptomatic, while others have significant symptoms and require therapeutic intervention. Clinical symptoms include infiltration of lymphoplasmacytic cells into the bone marrow, production of a monoclonal IgM protein, anemia, lymphadenopathy, and serum hyperviscosity. Despite the introduction of multiple chemotherapeutic regimens over the past several decades, WM remains an incurable disease.

2-cyano-3,12-dioxoolean-1,9-dien-28-oic acid (CDDO) and its methyl ester derivative (CDDO-Me) and imidazolide derivative (CDDO-Im) are synthetic triterpenoids derived from oleanolic acid. These compounds have been shown to induce apoptosis of several tumor cell types including breast cancer, lung cancer, ovarian cancer, melanoma, osteosarcoma, leukemia, and multiple myeloma cells. The goal of this study was to evaluate the potential role of synthetic triterpenoids in WM.

Preliminary studies on malignant B cells indicated that CDDO-Im induced the greatest amount of cell death and we therefore used this derivative of CDDO for our studies. CD19+ CD138+ cells from bone marrow biopsy specimens obtained from WM patients were isolated by positive selection and were treated with varying concentrations of CDDO-Im (62.5 nM to 750 nM ) and cell viability was determined after 24 hours (n=3). Compared to the nil control 47% of the malignant cells remained viable at a CDDO-Im concentration of 62.5 nM and only 11% remained viable at 125 nM CDDO-Im. To determine if CDDO-Im had specific toxic effects on non-malignant cells, we cultured CD19- CD138- cells from WM patient bone marrows with CDDO-Im and found that non-malignant cells were less sensitive to the drug, 80% being viable at 62.5 nM and 65% being viable at 125 nM. Similarly, we found that normal peripheral blood B cells and CD19+ CD138+ bone marrow B cells from healthy donors were less sensitive to CDDO-Im. Compared to the nil control 93% of the CD19+ CD138+ bone marrow B cells and 70% of the peripheral blood B cells remained viable at a CDDO-Im concentration of 62.5 nM and 95% and 68% remained viable at 125 nM CDDO-Im respectively. We next examined the effect of CDDO-Im on WM cell proliferation and found that CDDO-Im inhibited cell proliferation in a dose-dependent manner. Similar to the viability assays, there was a differential effect of CDDO-Im on malignant and non-malignant cells. Compared to the nil control, at 125 nM, there was a complete inhibition of malignant cell growth, while approximately 40% of the non-malignant cells remained proliferative. To determine the mechanism of cell death, CD19+ CD138+ cells were cultured in the presence or absence of various doses of CDDO-Im for 6 hours and cell lysates were examined for cleavage of PARP. There was evidence of PARP cleavage in a dose-dependent manner, suggesting that CDDO-Im induced malignant cell death occurs through a caspase-dependent mechanism.

In summary, the synthetic triterpenoid CDDO-Im decreased the viability of WM B cells in a dose-dependent manner, and CDDO-Im had a greater effect on the viability of the malignant cells compared to non-malignant cells from the same WM patients. CDDO-Im also inhibited malignant cell growth in a dose-dependent manner and the mechanism of CDDO-Im mediated cell death appears to be a caspase-mediated event. Overall, our data indicate that CDDO-Im may have potential efficacy in WM patients.