Abstract Chemotherapeutic regimens for pediatric acute lymphoblastic leukemia (ALL) treatment entail the use of glucocorticoids such as prednisolone (PRED). Around 5% of patients fail to attain remission due to drug resistance. We have previously shown that the pro-apoptotic BH3-only BCL-2 family member BIM was up-regulated in only in PRED-sensitive cells. After the specific silencing of BIM by siRNA, inhibition of apoptosis was observed and validated by levels of cleaved PARP and caspase activity, providing a clear indication of BIM's significance in PRED induced apoptosis. In this study, protein expression levels of BIM were further evaluated using patient bone marrow. We also examined the ability of two BH3 mimetics, GX15-070 (obatoclax) and ABT-737, to induce apoptosis in ALL cell lines and the effect of drug combinations. Patient bone marrow samples were obtained at diagnosis (Day 0) and after 7 days (Day 8) of PRED monotherapy. Response to PRED was defined as Good (PGR, peripheral blast count < 1000 /µl), and Poor (PPR, peripheral blast count ≥1000 /µl). Twenty-five PGR and five PPR patients were included in this study. Using western immunoblotting, the protein expression of BIM was reproducibly observed in patient samples. The value of ODBim/ODActin at Day 8 was significantly higher than that on Day 0 (p-Value=0.0006) for PGR patients. In contrast, this value did not differ greatly in PPR patients. At Day 0, BIM expression levels of the entire patient cohort was comparable, but levels were significantly different at Day 8 (p-Value=0.01). These results indicate that the upregulation of BIM was induced by PRED in ALL patients and further confirms the essential role of BIM in this apoptotic pathway. Treatment with GX15-070 and ABT-737 as single agents yielded an increase in cell death in a dose dependent manner both in PRED-sensitive and -resistant cell lines. Apoptosis induced by both GX15-070 and ABT-737 could be inhibited by caspase-9 inhibitor (Z-LEHD-FMK) and caspase-3 inhibitor (Z-DEVD-FMK). Simultaneous exposure of ALL cells to PRED and GX15-070 or ABT-737 resulted in synergistic levels of cell death in SUP-B15 and RS4;11 cell lines with combination index (CI) values that are significantly less than one (as calculated by CalcuSyn software version 2.1). This highlights the possibility of enhancing cell death in ALL cells even with a reduction in drug dosage. In conclusion, BH3-only BCL-2 family member BIM, is vital for PRED-induced apoptosis in ALL. BH3 mimetics GX15-070 and ABT-737 can induce apoptosis in ALL cells regardless of PRED response. The synergism between PRED and BH3-mimetics is likely to minimize severe side effects, and more importantly, provide alternative therapies especially for patients that are resistant to conventional chemotherapeutic drugs in the clinic. 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 1033.