Abstract 2993: Vinblastine induces very rapid apoptosis in chronic lymphocytic leukemia and mantle cell lymphoma

Proceedings: AACR 101st Annual Meeting 2010‐‐ Apr 17‐21, 2010; Washington, DC Chronic lymphocytic leukemia (CLL) and mantle cell lymphomas are initially responsive to traditional therapies, but invariably relapse and are generally considered incurable. CLL is caused by an accumulation of mature resting B cells, due to a defect in cellular apoptosis rather than increased cell division. Most CLL cells maintain high levels of the anti-apoptotic protein BCL2 which protects them from apoptotic death, suggesting that they may be sensitive to BCL2-targeting drugs. Freshly isolated lymphocytes from CLL patients were exposed to the BH3 mimetic ABT-737, targeting BCL2 and BCL-XL anti-apoptotic proteins. Comparable to previous reports, ABT-737 displayed acute concentration-dependent toxicity, with low levels of ABT-737 (10 nM) inducing apoptosis as early as 3 hours. ABT-737 sensitivity has been attributed to its ability to displace Bim from BCL2/BCL-XL leading to intrinsic apoptosis. Consistent with this mechanism, we report three mantle cell lymphoma cell lines that are deficient in Bim are resistant to ABT-737. However, many other cell lines containing Bim are also resistant to ABT-737. This resistance has been attributed to the presence of MCL-1; however the Bim-deficient Mino cell line has very low MCL-1 levels yet is still resistant to ABT-737. Surprisingly, the Bim-deficient mantle cell lymphoma cell lines and fresh CLL lymphocytes were also acutely sensitive to vinblastine, a microtubule disrupting drug. Apoptosis induced by vinblastine was dose-dependent with the pharmacologically relevant concentrations of 100-200 nM vinblastine killing CLL cells within 6 hours. The normally recognized mechanism of action of vinblastine suggests that cells are only sensitive in mitosis, yet we find that non-dividing CLL cells are acutely sensitive to this drug. Vinblastine-induced death was both caspase- and c-jun N-terminal kinase (JNK)- dependent, with pan-caspase inhibitor zVAD-fmk and JNK Inhibitor VIII both protecting from vinblastine. In contrast, acute apoptosis induced by ABT-737 is caspase-dependent but JNK-independent. Furthermore, the presence of stromal cells, utilized to mimic the tumor environment of the lymph node, failed to protect CLL cells from ABT-737 or vinblastine. CLL sensitivity to either ABT-737 or vinblastine was independent of CD38 status and previous clinical treatment (untreated or fludarabine, cyclophosphamide and rituximab treated, relapsed patients). These results suggest that vinblastine acts via a JNK-dependent mechanism to kill CLL acutely, while ABT-737 kills acutely via a Bim-dependent mechanism. A number of other leukemia and lymphoma cell lines studied were not acutely sensitive to either ABT-737 or vinblastine. These findings suggest that ABT-737 and vinblastine, alone or in combination, may be beneficial to patients with refractory or minimal residual CLL and mantle cell lymphoma. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 2993.