Comparison of in Vitro Apoptotic Response of Chronic Lymphocytic Leukemia (CLL) Cells to Bcl-2 Antagonist ABT-737 and IAP Antagonist BV6.

Abstract 4386

Altered expression of Bcl-2-family and IAP-family proteins has been considered to play central roles in apoptosis dysregulation in cancer and leukemia, promoting malignant cell expansion and contributing to chemoresistance. Anti apoptotic Bcl-2-family proteins Bcl-2 and Bcl-X_L as well as IAP proteins, including XIAP, have been thoroughly validated as drug discovery targets for cancer and strategies for inhibiting these proteins have been devised based on mimicking their endogenous antagonists, as represented by ABT-737, a fully synthetic Bcl-2/Bcl-X_L antagonist developed at Abbott Laboratories, and BV6, bivalent SMAC mimetics developed at Genentech Inc. Chronic lymphocytic leukemia (CLL) is a quintessential example of a human malignancy caused by defective programmed cell death. Over-expression of the Bcl-2 protein is one of the most consistent and prominent etiological factors associated with this disease, whereas over-expression of XIAP protein has been demonstrated in a majority of CLL patients. In this study, we evaluated the pro-apoptotic effects of ABT-737 and BV6 on CLL cells in side-by-side comparisons. We found that the CLL cells of 31 of 42 patients were highly sensitive to ABT-737, inducing potent dose-dependent killing of cells with an IC50 = 52.5 ± 22.5 (mean ± std error). In contrast, the CLL cells of 11 of 42 patients were relatively resistant to ABT-737, requiring higher concentrations to induce apoptosis (IC50 >1 μM). Preliminary statistical analysis was performed to correlate ABT737-sensitive and -resistant CLL with various other characteristics, including patient treatment status, ZAP-70 expression, and VH mutation status, but no significant correlation was observed. In contrast, the CLL cells from all 42 patients examined were relatively resistant to the IAP antagonist BV6, which required high concentrations to induce apoptosis (IC50 > 1 μM). Similar results were obtained using other SMAC mimetic compounds. Analysis of CLL cells for endogenous SMAC protein expression and for the subcellular location of SMAC protein demonstrated that the leukemia cells of 18 of 23 patients expressed high-levels of SMAC, as assessed by immunoblotting. Furthermore, SMAC was located in the cytosolic fraction of these leukemia cells in 7 of 18 cases examined. However, CLL cells with high-levels of cytosolic SMAC had little evidence of concomitant release of cytochrome c from mitochondria and demonstrated little spontaneous apoptosis in vitro. These results suggest that the leukemia cells of some CLL patients may already express cytosolic SMAC, which might explain the relative insensitivity of CLL cells to SMAC mimetics such as BV6. Taken together, these studies suggest that Bcl-2 antagonists such as ABT-737, but not SMAC mimetics such as BV6, may have promising therapeutic activity for most CLL patients. (NIH CA-081534)