ABT-737 Is a Useful Component of Combinatory Chemotherapies for Chronic Myelogenous Leukemias with Diverse Drug Resistance Mechanisms.

Chronic myelogenous leukemia (CML) is characterized by its refractoriness to various apoptotic insults by Bcr-Abl tyrosine kinase (TK)-mediated signalling. Although imatinib mesylate (IM), a Bcr-Abl TK inhibitor, has markedly improved the therapeutic outcomes of CML, additional or alternative molecular targeting strategies are still needed. Since the interplay of anti-apoptotic Bcl-2 proteins and BH3-only proteins, such as Bim and Bad, is crucial for regulating the cellular fate of Bcr-Abl⁺ leukemic cells (Kuroda J et al, PNAS , 2006; Cell Death Differ, 2007), the direct targeting of anti-apoptotic Bcl-2 proteins by the use of a BH3-only protein mimetic is an attractive approach for treating CML. We here investigated the activity of ABT-737, a mimic of BH3-only proteins that inhibit anti-apoptotic Bcl-2, Bcl-X_L and Bcl-w, but not Mcl-1 or A1, against CML. The Annexin-V-staining study, the assay for mitochondrial outer membrane potential and the internucleosomal fragmentation assay revealed that ABT-737 potently induced apoptosis in CML cell lines (BV173, K562, KCL22, KT-1, MEG-01 and MYL) with the IC₅₀ for induction of cell death by 48 h of treatment ranging from 0.04 to 4.06 μM. ABT-737 was also effective in killing primary CML samples in vitro. ABT-737 prolonged the survival of mice xenografted with a CML cell line, BV173, demonstrating its in vivo bioactivity. Higher expression of Bcl-2, Bcl-X_L, or Mcl-1 reduced cell killing by ABT-737 in each cell line, but we found no correlation between the sensitivities to ABT-737 and the specific expression patterns of Bcl-2 family proteins among different cell lines. The levels of Bcr-Abl and Lyn, a member of Src kinase family associated with apoptosis resistance in CML, also varied among the cell lines, and we found no consistent relationship between the sensitivity to ABT-737 and the expression levels of these proteins Thus, the cell killing effect of ABT-737 in CML must be determined in part by other drug resistance mechanisms, such as high expression of Bcr-Abl, overexpression of P-glycoprotein (P-gp), a drug-efflux pump, and/or their combination. Importantly, ABT-737 augmented the cell killing effect of IM in CML cell lines with high levels of anti-apoptotic Bcl-2 family proteins (Bcl-2, Bcl-X_L, or even Mcl-1), Bcr-Abl, P-gp, or Lyn, unless leukemic cells harboured IM-insensitive Abl kinase domain (KD) mutations. Moreover, the combination of ABT-737 with homoharringtonine, an herbal-derived anti-CML therapeutic that potently reduces Mcl-1 within 6 hours in vitro, dramatically enhanced the killing by ABT-737 in CML cells with diverse drug resistance mechanisms, including IM-insensitive Abl KD mutations, such as T315I mutation. These results suggest that ABT-737 could be a useful component of combinatory chemotherapies for CML.