BACKGROUND: Lymphoid enhancer factor-1 (lef-1) is overexpressed in B-cell chronic lymphocytic leukemia (CLL) when compared with normal B cells and transcribes several genes implicated in the pathogenesis of CLL. We therefore hypothesize that antagonism of lef-1 might lead to killing of CLL cells. We used two small molecule inhibitors of Wnt/β-catenin/lef-1 signaling (CGP049090 and PKF115-584) to test our hypothesis. DESIGN AND METHODS: Enriched CLL cells and healthy B cells were used in this study. Small interfering RNA (siRNA)-mediated knockdown of lef-1 in primary CLL cells was done using nucleofection, and 50% lethal concentration (LC₅₀) of two small molecules was assessed using ATP-based cell viability assay. Apoptotic response was investigated in time course experiments with different apoptotic markers. Specificity of the small molecules was demonstrated by coimmunoprecipitation experiments for the lef-1/β-catenin interaction. In vivo studies were done in JVM-3 subcutaneous xenograft model. RESULTS: Inhibition of lef-1 by siRNA leads to increased apoptosis of CLL cells and inhibited proliferation of JVM-3 cell lines. The two small molecule inhibitors (CGP049090 and PKF115-584) efficiently kill CLL cells (LC₅₀<1 µM), whereas normal B cells were not significantly affected. Coimmunoprecipitation showed a selective disruption of β-catenin/lef-1 interaction. In vivo studies exhibited tumor inhibition of 69% with CGP049090 and 57% with PKF115-584 when compared with vehicle-treated controls, and the intervention was well tolerated. CONCLUSIONS: We have demonstrated that targeting lef-1 is a new and selective therapeutic approach in CLL. CGP049090 or PKF115-584 may be attractive compounds for CLL and other malignancies that deserve further (pre)clinical evaluation.