Relapse remains a major problem in pediatric acute lymphoblastic leukemia (ALL). Adding a drug that can modulate drug resistance to combination chemotherapy used for therapy of ALL could enhance the durability and frequency of complete remissions. We evaluated ABT-737, a small-molecule Bcl-2 family protein inhibitor, in combination with vincristine, dexamethasone, and L-asparaginase (VXL) in ALL cell lines (1 established at multiple-relapse) using the DIMSCAN cytotoxicity assay. Multi-log synergistic cytotoxicity (combination index <1) was seen in 7 of 7 ALL cell lines with ABT-737 + L-asparaginase or vincristine, in 5 of 7 with ABT-737 + dexamethasone, and in 5 of 7 with ABT-737 + VXL. ABT-737 combined with L-asparaginase induced greater mitochondrial depolarization (JC-1 staining), mitochondrial cytochrome c release, activation of Bax + Bid, caspase activation (immunoblotting), and eventually apoptosis (annexin V staining), than did either drug alone. ABT-737 + VXL was tested against systemic human xenografts in NOD/SCID mice of pediatric ALL from patients at diagnosis (ALL-7) or at relapse (ALL-19). Mice were monitored weekly by flow cytometry for % human CD45+ (%huCD45+) cells in peripheral blood. When the %huCD45+ cells reached 1%, mice were randomized into 4 groups: (1) vehicle control; (2) ABT-737 (25 mg/kg/day x 5 days); (3) vincristine (0.15 mg/kg q7 days) + dexamethasone (5 mg/kg x 5 days) + L-asparaginase (1,000 IU/kg x 5 days) (VXL), or (4) VXL plus ABT-737. Therapy was given by intraperitoneal injection for 4 weeks. Events were defined as %huCD45+ cells > 25% or toxicity-related events. Leukemia Growth Delay (LGD) was calculated as the median event-free survival of treated minus control groups. ABT-737 alone achieved LGDs of 26.6 days (ALL-7) and 2.6 days (ALL-19). VXL achieved LGDs of 71.6 days (ALL-7) and 36.5 days (ALL-19). VXL + ABT-737 achieved LGDs of 115.6 days (ALL-7) and 67.7 days (ALL-19). The LGDs for ABT-737 + VXL were 17.4 days (ALL-7) or 28.6 days (ALL-19) greater than the sum of individual LGDs for VXL and ABT-737. In conclusion, ABT-737 synergistically enhanced cytotoxicity of VXL in ALL cell lines and enhanced activity of VXL against ALL xenografts. Combining VXL with an inhibitor of the Bcl-2 family of proteins warrants clinical investigation in relapsed ALL.

98th AACR Annual Meeting-- Apr 14-18, 2007; Los Angeles, CA