Targeting BCL-2 and BCR-Abl Activity in Ph+ALL

BACKGROUND:  Treatment of adult Philadelphia chromosome positive acute lymphoblastic leukemia (Ph⁺ALL) remains a challenge. Tyrosine kinase inhibitors (TKI’s) have greatly improved treatment options for Ph⁺ALL, leading to an increased number of patients eligible for curative hematopoietic stem cell transplant. A current induction regimen for Ph⁺ALL is the combination of the targeted dual Abl/Src TKI dasatinib and a corticosteroid.  It is thought that corticosteroids work synergistically with TKI’s by modulating the BCL-2 family of proteins, leading to apoptosis.  However corticosteroids have to be tapered after the first 21 days of induction due to significant toxicities, particularly in older adults. Meanwhile, patients remain on TKI monotherapy until the time of transplant, rendering them susceptible to the development of resistance.  Recently BCL-2 inhibitors have been shown to have single agent efficacy in B cell malignancies with relatively low toxicity and our lab has shown in vitro effects in primary patient Ph⁺ALL samples (unpublished data). Therefore we investigated the potential combination of dasatinib with the BCL-2 inhibitor ABT737 as a targeted combination in Ph+ALL.

METHODS: Drug efficacy in vitro was determined using the Ph⁺ALL cell line SupB15 and the CML cell line K562.  Cells were incubated with varying concentrations of dasatinib, ABT737 or in combination for 72 hours.  Cell viability was assessed with the colorimetric MTS assay, and synergy was calculated using Calculsyn software.   Xenografted cells from a patient with Ph⁺ALL were assessed under identical conditions. Apoptosis was assessed with annexin V staining. Expression of the BCL family proteins BCL-2 and MCL-1 were assessed via immunoblot.  

RESULTS: The IC₅₀ of dasatinib and ABT737 in SupB15 were 8.8nM and 5.9nM, respectively. The IC50 of equimolar combination was 0.42nM, and synergistic with combination index (CI) values between 0.15 and 0.49 (<1=synergy).   Primary Ph⁺ALL xenograft cells showed a similar pattern of synergy to the dasatinib + ABT737 combination, with CI values between 0.01 and 0.38. Combination treatment increased apoptosis as measured by Annexin V staining.  In contrast, the CML cell line K562 was not sensitive to ABT737 as a single agent and there was no enhanced efficacy by adding this agent to dasatinib, suggesting this combination is specific for Ph+ALL.  This was supported by increased BCL-2 and low MCL-1 protein expression in SupB15 and xenografted Ph+ ALL cells, whereas K562 had low expression of BCL-2 and high levels of MCL-1. 

CONCLUSIONS: These data verify that the combination of BCL-2 and BCR-Abl targeting in Ph+ ALL is synergistic in vitro laying the foundation for further evaluation in vivo for adult Ph⁺ALL.  Combination targeted therapies may offer the potential for greater and longer responses without the morbidity associated with cytotoxic chemotherapy, particularly in older adults.