In spite of improvements in the last decade, relapses still occurs in the majority of ALL patients, with a long-term survival rate of only 30–40%. Recently, small-molecule inhibitors have been developed for targeting deregulated signal transduction pathways involved in proliferation and apoptosis. Current evidence identifies the Raf/MEK/ERK, the PI3K/AKT/mTOR, and the Bcl-2 pathways as potentially relevant targets for therapeutic intervention. We have previously demonstrated that constitutive ERK phosphorylation is an independent predictor of failure to achieve complete remission in adult ALL (

Blood
2007
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109
:
5473
) and thus we evaluated the in vitro activity of MEK-inhibitors. However, neither PD98059 nor PD0325901 affected cell growth, cell cycle distribution, and/or apoptosis in ALL cell lines (IC50 >1 μM for PD0325901). These results were confirmed in primary ALL samples, in which PD98059 significantly (P=0.012) inhibited ERK phosphorylation in 8/12 samples, without inducing cell cycle changes or apoptosis. We next investigated the activity of the mTOR inhibitor Temsirolimus. Temsirolimus displayed a biphasic dose-response in a panel of different ALL cell lines, with a flat curve (35–55% of inhibition) at concentrations ranging between 1 and 5,000 nM and more pronounced growth inhibition at concentrations ≥ 10 μM. The CEM cell line was the most sensitive (IC50: 7 nM), Jurkat showed intermediate sensitivity (IC50: 200 nM), while MOLT-4 were resistant (IC50 > 20,000 nM). Cell growth inhibition was associated with inhibition of cell cycle progression, while apoptosis induction was observed in less than 15% of the cells even at the highest concentration of Temsirolimus (20 μM). We then investigated the cell cycle and apoptotic effects of ABT-737 (kindly provided by Abbott Laboratories to A.T.), a Bcl-2/Bcl-xL (BH3 mimetic) inhibitor, in ALL cell lines and in 10 primary samples. ABT-737 showed a potent dose- and time-dependent growth-inhibitory activity in MOLT-4 (IC50: 198 nM), associated with loss of mitochondrial membrane potential, caspase activation, Bcl-2 cleavage, and ultimately apoptosis induction. Conversely, CEM cells proved resistant (IC50: 12,000 nM). We also found that ABT-737 was highly effective in primary adult and childhood ALL, independent of their chromosomal abnormalities, with a significant decrease in viability (p=0.008) and a remarkable induction of apoptosis (from a mean baseline value of 16.8±8.8% to 43.6±22.8%, p=0.04) at 10 nM ABT-737. A dose-dependent down-regulation of Bcl-2 and Bcl-xL expression was observed in sensitive samples, but not in the only resistant one. In summary, our study shows that ALL cells, do not undergo apoptosis in response to single-pathway inhibition, suggesting the presence of multiple, redundant pathways that preserve leukemic cell survival. The only notable exception was the Bcl-2/Bcl-xL inhibitor ABT-737. Studies are ongoing to identify mechanism-driven combinations of agents that would disrupt multiple signal transduction pathways, resulting in synergistic killing and, ultimately, in novel therapeutic strategies for ALL.

Topics:
acute lymphocytic leukemia, bcl2 gene, mtor serine-threonine kinases, signal transduction, mechlorethamine, abt-737, bcl-xl protein, temsirolimus, 1-phosphatidylinositol 3-kinase, caspases

Disclosures: No relevant conflicts of interest to declare.

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2008, The American Society of Hematology
2008
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