Effects of combretastatin A-4 prodrug against a panel of malignant human B-lymphoid cell lines

Combretastatin A-4 (CA-4) is one of a family of compounds isolated from the South African willow tree Combretum caffrum. CA-4 was found to be active against murine melanoma and a variety of other human solid tumors. For the first time, we report the effect of CA-4 against a panel of malignant human B-lymphoid cell lines [early pre-B acute lymphoblastic leukemia (Reh), diffuse large cell lymphoma (WSU-DLCL2), chronic lymphocytic leukemia (WSU-CLL) and Waldenstrom's macroglobulinemia (WSU-WM)]. Our results indicate, using the prodrug form of CA-4, a concentration-dependent growth inhibition in all tested cell lines, although WSU-DLCL2 was more sensitive. Exposure to 4 nM CA-4 for 96 h induced 77% growth inhibition in Reh, 86% in WSU-CLL and 92% in WSU-WM. When used against the WSU-DLCL2 cell line, this same concentration of CA-4 was completely toxic. Morphological examination showed CA-4 induced the formation of giant, multinucleated cells, a phenomenon commonly found in mitotic catastrophe. Only minimal numbers of cells showing characteristics of apoptosis were detected. In WSU-DLCL2 cells, CA-4 (3 nM) induced the highest apoptosis (5%) after 48 h, while the percentage of dead cells was approximately 47%. Exposure of Reh, WSU-CLL, WSU-WM and WSU-DLCL2 cells for 24 h to 5 nM CA-4 induced 19, 28, 57 and 75% G2/M arrest, as determined by flow cytometry, respectively. Based on these preliminary studies, we believe that mitotic catastrophe is the predominant mechanism by which CA-4 induces cell death rather than apoptosis. Further studies to elucidate the mechanisms of CA-4 activity in vitro and in vivo are currently under investigation in our laboratory.