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Purpose: There is evidence that vascular endothelial growth factor (VEGF) is a critical microenvironmental factor that exerts angiogenesis-independent effects on the survival of chronic lymphocytic leukemia (CLL) cells. Vatalanib and pazopanib are potent orally available VEGF receptor tyrosine kinase inhibitors. We investigated the efficacy and selectivity of both compounds in CLL cells, simulated potential combination with conventional cytostatics, and tested the effect of both substances on CLL-like tumor xenografts.Experimental Design: Primary CLL and normal peripheral blood cells were tested for viability after incubation with varying concentrations of both inhibitors. Further, phosphorylation status of VEGF receptor on treatment, caspase activation, and poly(ADP-ribose) polymerase cleavage were assessed. Combinations of each inhibitor with fludarabine, vincristine, and doxorubicin were analyzed for possible synergistic effects in vitro. For in vivo testing, mice grafted with the CLL-like cell line JVM-3 were treated orally with each inhibitor.Results: Vatalanib and pazopanib decreased phosphorylation of the VEGF receptor, along with induction of apoptosis in CLL cells in clinically achievable concentrations. Healthy B cells were only mildly affected. Immunoblots showed downregulation of the antiapoptotic proteins XIAP and MCL1, whereas poly(ADP-ribose) polymerase cleavage was increased. Combinations with conventional cytostatic agents resulted in synergistic effects. Treatment of xenografted mice with 100 mg/kg of body weight for 21 days resulted in tumor inhibition rates of 76% (vatalanib) and 77% (pazopanib). In two mice, a total tumor eradication could be observed. No gross systemic toxicity occurred.Conclusion: We conclude that VEGF inhibition is a promising new therapeutic approach in CLL. Vatalanib and pazopanib seem to be effective and safe candidates to be further evaluated for this purpose.

Chronic lymphocytic leukemia (CLL) cells feature a pronounced apoptotic resistance. The vascular endothelial growth factor (VEGF) possesses a role in this apoptotic block, although underlying functional mechanisms and the involvement of the microenvironment are unclear. In this study, the VEGF status in CLL was assessed by enzyme-linked immunosorbent assay and immunofluorescence. VEGF receptor 2 (VEGFR2) phosphorylation was determined flow cytometrically and by immunofluorescence. For coculture, CLL cells were cultivated on a monolayer of the bone marrow-derived stromal cell (BMSC) line HS5. Secreted VEGF was neutralized using the monoclonal antibody mAb293 (R&D Systems, Minneapolis, MN, USA). To block protein secretion, we used Brefeldin A. VEGF was downregulated in BMSCs by small interfering RNA (siRNA), and we assessed survival by annexin V-fluorescein isothiocyanate (FITC)/propidium iodide (PI) staining. CLL cells express and secrete VEGF and possess phosphorylated VEGFR2. This positive VEGF status is not sufficient to prevent spontaneous apoptosis in vitro. Coculture with BMSCs, which secrete vast amounts of VEGF, maintains in vitro CLL cell survival. Blockage of secreted VEGF using the monoclonal antibody mAb293 significantly reduced the survival support for cocultured CLL cells. Both general blockage of protein secretion by Brefeldin A in BMSCs, but not in CLL cells, and siRNA-mediated downregulation of VEGF in BMSCs, significantly reduced the coculture-mediated survival support for CLL cells. It can be concluded that BMSC-derived proteins and VEGF, in particular, but not CLL cell-derived VEGF, is essentially involved in the coculture-mediated survival support for CLL cells. Hence, therapeutic targeting of VEGF signaling might be a promising approach to overcome the apoptotic resistance CLL cells feature within their natural microenvironment.

Wnt signaling is crucial for cell proliferation and differentiation. It represents a complex network with mechanisms of self‐regulation through positive and negative feedback. Recent increasing interest in this signaling pathway has led to the discovery of many new proteins that down‐regulate Wnt activity. Here, we provide a short description of the most important and best‐studied inhibitors, group them according to the target molecule within the Wnt cascade, and discuss their clinical potential. Although most of the inhibitors discussed here may also interact with proteins from other signaling pathways, we focus only on their ability to modulate Wnt signaling.

There is a growing body of evidence that Wnt signaling, which is already known to play a critical role in various types of cancer, also has a vital function in B cell neoplasias, particularly in chronic lymphocytic leukemia (CLL). It is known that Wnt proteins are overexpressed in primary CLL cells and several physiological inhibitors are partly inactivated in this disease. Furthermore, β-catenin is upregulated upon Wnt stimulation and cooperates with the transcription factor lymphoid enhancer binding factor-1 (LEF-1). LEF-1 is excessively overexpressed in CLL cells by more than 3,000-fold compared to normal B cells. Moreover, LEF-1 could be identified as an important regulator of pathophysiologically relevant genes in CLL, and several Wnt/β-catenin signaling components substantially influence CLL cell survival. In this review we summarize the current state of knowledge about Wnt/β-catenin/LEF-1 signaling in CLL. Following a short overview of current treatment concepts in CLL, we briefly describe Wnt signaling in human cancers. We then discuss recent progress in understanding regulation of the Wnt/β-catenin/LEF-1 signaling pathway in this disease. Based on the present scientific evidence we highlight which components of this important signaling pathway could serve as therapeutic targets in CLL. We then present previous results gained from experimental approaches to target different parts of the Wnt/β-catenin/LEF-1 cascade. Together with potentially promising approaches we also critically reflect on the kind of difficulties and problems that may arise using such strategies.