Abstract 867: Therapeutic potential of small molecule SYK inhibitors for treatment of primary B cell acute lymphoblastic leukemia

Abstract

Abstract Background: B-cell acute lymphoblastic leukemia (B-ALL) is the most common childhood cancer. Intensified and central nervous system (CNS)-directed chemotherapy has significantly improved outcomes for pediatric patients but are associated with late-effect morbidities. Moreover, ∼20% pediatric and a higher frequency of adult patients suffer relapses that are often fatal. Thus there is a need to develop therapies that target signaling abnormalities in B-ALL, which may reduce complications of CNS leukemia and decrease long-term morbidities. Rationale: Using a p53-/- SCID mouse model of B-ALL we observed pre-B cell receptor (pre-BCR)-independent activation of the spleen tyrosine kinase (SYK) and found that it was crucial for the proliferation and survival of these leukemias. We then asked whether abnormal SYK activation occurs in human B-ALL and whether these cells are sensitive to small molecule SYK inhibitors. Methods: Viably frozen diagnostic B-ALL samples from children (n=54) and adults (n=42) tested for sensitivity to SYK inhibitors R406 (Astra-Zeneca) and BAY61-3606 in a short-term in vitro proliferation assay. Phospho-flow cytometry was also performed to quantify phosphorylation of SYK and other signaling proteins in B-ALL samples. The R406 pro-drug (Fostamatinib: Fosta) was used in a xenotransplant assay to determine therapeutic potential of SYK inhibition in vivo. Results: Phospho-flow cytometry profiling of primary B-ALL samples revealed prominent phosphorylation of SYK (Y348) and downstream signaling proteins that was decreased by SYK inhibitors. Furthermore, SYK inhibitors significantly attenuated proliferation of pre-BCR-negative and pre-BCR-positive B-ALL samples indicating that SYK was required for their survival and proliferation. In contrast, FLT3 or SRC inhibitors did not inhibit proliferation of pediatric and adult B-ALL samples. Importantly, siRNA-mediated SYK knockdown also reduced proliferation of B-ALL cell lines. Therefore, we tested the therapeutic potential of SYK inhibition using xenotransplantion. NOD.SCID.gamma C-/- (NSG) mice were injected intrafemorally with primary B-ALL samples (n=9) and fed chow containing either vehicle (AIN-76A diet) or Fosta (AIN-76A diet with 2g Fosta/kg). Leukemia burden was assessed 4-8 weeks post-transplantation. Mice given the Fosta diet had significantly reduced numbers of leukemic blasts in their injected femurs, other bones, spleens and CNS as compared to vehicle-treated mice. In addition, Fosta treatment reduced spleen, liver and kidney weight in ALL-transplanted mice. Conclusion: SYK signaling is vital to B cell acute lymphoblastic leukemia survival; small molecule SYK inhibitors have therapeutic potential in poor-prognosis and relapsed B-ALL. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 867. doi:1538-7445.AM2012-867