Abstract
The treatment of adult acute lymphoblastic leukemia (ALL) remains unsatisfactory. A potential hope is now given to Philadelphia-positive cases by targeted treatment modalities. Among other pathways involved in cell proliferation, we have recently demonstrated (Blood2007; 109:5473) the unfavorable role of ERK1/2 phosphorylation as an independent predictor of complete remission (CR) in adult ALL, suggesting the potential therapeutic value of other targeted therapies. The B-cell leukemia/lymphoma 2 (Bcl-2) family of proteins are important regulators of apoptosis and are frequently found aberrantly expressed, particularly in lymphoid malignancies. The role of Bcl-2 overexpression in tumorigenesis and chemoresistance prompted us to investigate whether the inhibition of the antiapoptotic function may result also in ALL in an attractive therapeutic strategy. In this study, we thus investigated the cell cycle and apoptotic effects of ABT-737 (kindly provided by Abbott Laboratories), a Bcl-2 (BH3) inhibitor, on both lymphoid leukemia cell lines and primary adult and childhood ALL cells. The lymphoid leukemia cell lines CEM and MOLT-4 were exposed to increasing concentrations of ABT-737 (from 0.1 to 1 μM) up to 72 hours. A dose- and time-dependent cell growth arrest and induction of apoptosis was found. In fact, measuring the subG0/1 peak at 48 hours, the levels of apoptosis increased in the CEM cell line from 14.1% (DMSO) to 34.4%, 64.5%, 86.5% and 98.6% at ABT-737 concentrations of 0.1, 0.25, 0.5 and 1 μM, respectively. Similarly, 48 hours of exposure to ABT-737 increased in MOLT-4 the Annexin V-positive cells from 7.2% to 64.2%. The effects of ABT-737 were then examined on primary blasts from 9 ALL patients (6 adults and 3 children). Bone marrow aspirates with a blast infiltration >70% were obtained at diagnosis from patients broadly characterized for clinical and biological parameters, as well as therapeutic response. ALL cells were cultured in vitro with ABT-737 (at increasing concentrations from 0.01 to 1 μM) for 24 hours. A significant decrease in viability was observed at 0.01 μM (p=0.008) with a remarkable dose-dependent increase of apoptosis. In fact, Annexin V-positive cells increased from a mean baseline value of 16.8% ± 8.8 to 43.6% ± 22.8 (p=0.04), 66% ± 21.3 (p=0.0001), 70.3% ± 26.9 (p=0.04), 74.6% ± 18.9 (p=0.03) and 76.2% ± 11.8 (p<0.0001) in the presence of ABT-737 at 0.01, 0.1, 0.25, 0.5 and 1 μM, respectively. A significant cell killing was demonstrated in all samples (9/9), including Ph-positive ALL. No significant cell cycle changes were instead detected even at higher concentration of ABT-737. In summary, our study shows for the first time a potent growth-inhibitory and pro-apoptotic activity of the Bcl-2 antagonist ABT-737, at nanomolar concentrations, on primary cells from adult and childhood ALL samples. These results prompt to further extend pre-clinical studies in the different biologically-defined subset of ALL and suggest a potential clinical development of a Bcl-2 family inhibitor in this disease.
