Background: DNA damaging agents are the most common chemotherapy for human cancers, and combining them with agents that modulate resistance mechanisms can enhance activity. Etoposide, cyclophosphamide, and doxorubcin are DNA-damaging agents that are employed against a variety of cancers including recurrent pediatric acute lymphoblastic leukemia (ALL). We determined if the BH-3 mimetic pan-Bcl-2 inhibitor ABT-737 could enhance the cytotoxicity of DNA damaging agents in ALL cell lines. Methods: Cytotoxicity was determined using a fluorescence-based digital imaging assay (DIMSCAN). Changes in various proteins were detected by immunoblotting. Apoptotic cells were detected by flow cytometry using Annexin-V and propidium iodide. Mutational status of TP53 was evaluated by sequencing PCR amplified DNA. Results: Combining ABT-737 with etoposide, cyclophosphamide (as 4-HC), or doxorubicin caused synergistic cytotoxicity (combination index values: <0.1) in 3 of 7 tested ALL cell lines, COG-LL-317 and CCRF-CEM (both TP53 mutated) and COG-LL-319 (TP53 wild-type). Synergistic cytotoxicity was not observed for ABT-737 + the DNA damaging drugs in Molt-3, Molt-4, RS4-11, or NALM-6 and was independent of p53 function. Sensitivity to ABT-737 as a single agent or Mcl-1 expression (J Natl Can Inst 100:580-595, 2008) did not correlate with ability of ABT-737 to synergize the DNA damaging agents. The mechanism of synergy was investigated for ABT-737 + etoposide. Etoposide + ABT-737 caused greater apoptosis than either agent alone (64% ± 2.9 for ABT-737, 85% ± 7.3 for etoposide, 94% ± 5.9 for combination; p<0.001 for ABT-737 vs combo and p=0.039 for etoposide vs combination). ABT-737 + etoposide increased cytochrome c release and Smac/Diablo, decreased XIAP in cytosol, and caused greater activation of caspases than either drug alone. Conclusion: ABT-737 enhanced etoposide, cyclophosphamide, or doxorubicin cytotoxicity via enhancing apoptosis in in vitro models of ALL.

Citation Information: In: Proc Am Assoc Cancer Res; 2009 Apr 18-22; Denver, CO. Philadelphia (PA): AACR; 2009. Abstract nr 3990.

100th AACR Annual Meeting-- Apr 18-22, 2009; Denver, CO

American Association for Cancer Research
2009