Prelinical Evaluation of Lyn Kinase Inhibition for the Treatment of B Cell Chronic Lymphocytic Leukemia (B-CLL).

Abstract 3447

Poster Board III-335

B-CLL is characterized by the accumulation of mature B lymphocytes incapable of undergoing apoptosis. Although several mechanisms have been implicated in the apoptotic defects in B-CLL cells, the signal transduction pathways underlying these defects remain unresolved.

Lyn kinase is known to be a negative regulator of apoptosis, and is linked to chemo-resistance. Our preliminary study indicated that Lyn activity was 4- to 7-fold higher in primary B cells from six randomly-selected B-CLL patients than in normal B cells from healthy donors. Treatment of the B-CLL cells for 4 h with 10 μM concentration of a Lyn-specific inhibitor peptide targeting a unique interaction site within Lyn (Cancer Res. 2004;64:1058) resulted in 40 to 50% inhibition of Lyn kinase activity compared to negligible inhibition with control peptide. Further, treatment of the B-CLL cells for 16 to 20 h with the Lyn inhibitor peptide at 5 μM and 10 μM concentrations, decreased the cell viability by ∼25% and ∼50%, respectively, compared with no peptide or the control peptide. Fludarabine is one of the key chemotherapeutic agents for B-CLL and is known to induce cell death by apoptosis. The combined treatment with Lyn inhibitor peptide (5 μM) and fludarabine (2 μg/ml), decreased cell viability by ∼50% compared to ∼25% decrease with fludarabine or Lyn inhibitor peptide alone. In addition, the combined treatment showed ∼75% increase in caspase-3/7 activity compared to ∼25% increase with the Lyn inhibitor peptide or fludarabine alone. Because, overexpression of antiapoptotic proteins is correlated with apoptotic resistance of B-CLL cells, we examined the effect of Lyn inhibitor peptide on the changes in the expression of antiapoptotic genes, myeloid cell leukemia-1 (Mcl-1), x-linked inhibitor of apoptosis (XIAP), and B-cell leukemia/lymphoma-2 (Bcl-2). Treatment of B-CLL cells with 10 μM Lyn inhibitor peptide for 16 to 20 h resulted in more than 50% decreases in the expression of all the three antiapoptotic genes. Further, the Lyn inhibitor peptide markedly inhibited NF-κB activation (∼60%) and VEGF production (∼80%), both strongly implicated in the apoptotic resistance of B-CLL cells. Collectively, our results suggest that targeting Lyn kinase pathway with a clinically relevant Lyn kinase inhibitor may have a therapeutic potential for B-CLL.