Antitumor Activity of Small-Molecule SYK Inhibitor R788 and Fludarabinin Mono- and Combined Therapy in a Human B-CLL Xenograft Model.

Abstract 2378

Poster Board II-355

B cell chronic lymphocytic leukemia (B-CLL), one of the most common leukemia in adults, is characterized by the accumulation of mature B cells expressing CD19 and CD5. Improved understanding of CLL has lead to new prognostic tools and therapeutic options. The protein kinase Syk as a key mediator of proximal B-cell receptor (BCR) signalling is constitutively phosphorylated in CLL B cells. For these novel therapeutics, functional in vivo models are highly valuable. We have established a cell line-based, disseminated B-CLL model in NOD/SCID-IL2-receptor-gamma-chain^-/- (NSG) mice. In the current study, SYK inhibitor R788 (60mg/kg/day (d); applied daily) was evaluated in mono- and in combined therapy with Fludarabin (8mg/kg/day; applied 5 consecutive days in two cycles) as a well established compound in B-CLL treatment regimens in comparison with a control group. Equal parts of MEC1 cells were injected intravenously and into the peritoneal cavity of NSG mice and respective therapies were started 7 days after implantation. Tumor growth was monitored with flow-cytometry (FACS), daily monitoring of B-CLL symptoms and fluorescence-based in vivo imaging (FI). Tumor inhibition was calculated as the proportional reduction of mean B-CLL cell infiltration at the respective compartment of the test- compared to the control-group (in %). MEC1 cells engrafted predominantly in bone marrow (BM; take rate = 100%), but were as well detectable in spleen (33%) and peripheral blood (PB; 8%). At the respective doses and schedules the examined compounds were well tolerated in tumor-bearing mice. No acute toxicity could be observed and maximal body weight loss was below 15% in mono- and combined therapy. Tumor development was clearly reduced by R788 (optimal T/C of 48% on day 18), albeit to a lesser extend then standard anticancer agent Fludarabin (optimal T/C of 19% on day 18). Combined therapy induced no synergistic effects showing an optimal T/C value of 42% on day 19. Thus, MEC1 engraftment into NSG mice is a valuable in vivo model for B-CLL which exhibits high reproducibility and take-rates in relevant compartments closely mimicking the clinical situation. Collection of whole-body FI data proved to be a time- and animal-saving analysis that allows to closely monitor B-CLL growth. Further investigations will optimize the very promising antitumor activity of R788 and evaluate the potentially synergistic effect of R788 with additional well-established B-CLL therapeutics.