The Syk Inhibitor R788 (FosD) Inhibits Tumor Growth in the TCL1 Transgenic Mouse Model of CLL by Blocking Antigen-Dependent BCR Signaling.
Authors: Suljagic Mirza , Longo Pablo G , Laurenti Luca , Efremov Dimitar G 
Published: October 2019

Abstract
Abstract 887
CLL B-cells depend on various signals from the microenvironment for survival and proliferation. Among these, antigenic stimuli that are propagated through the B-cell receptor (BCR) are considered particularly important for the development and progression of CLL, suggesting that the BCR signaling pathway could be an important target for therapeutic intervention. We have previously characterized some of the critical components of the signaling pathway downstream of the BCR in CLL B cells and identified the protein tyrosine kinase Syk as a promising therapeutic target. In a recent study we showed that CLL B-cells frequently have increased basal/constitutive Syk activity and are moderately sensitive to the cytotoxic effect of the selective Syk inhibitor R406 [Gobessi et al, Leukemia 2009]. More importantly, the survival signal induced by sustained BCR engagement was completely abolished by R406, suggesting that this compound may exert an even greater effect in vivo by inhibiting antigen-dependent Syk activation. We have now tested this possibility in the Eμ-TCL1 transgenic mouse model of CLL. Aged Eμ-TCL1 mice develop CD5+ B-cell leukemias that, similar to aggressive human CLL, show features of an antigen-driven process, including expression of stereotyped BCRs and reactivity with common autoantigens and microbial agents [Yan et al, Proc Natl Acad Sci USA 2006]. For our experiments we used a TCL1 leukemia (TCL1-002) that does not grow in vitro, but can be propagated in syngeneic recipients in vivo. TCL1-002 cells express an unmutated stereotyped BCR encoded by the VH12/VK4 combination, which reacts with phosphatidylcholine, an autoantigen exposed on the surface of senescent erythrocytes.
In vitro experiments showed that R406 is not cytotoxic for TCL1-002 cells, although it completely inhibited both the basal and BCR-induced activation of signaling pathways downstream of Syk. The absence of a direct cytotoxic effect provided a unique opportunity to investigate whether inhibition of BCR signaling will affect leukemia growth in vivo. For this purpose, 1×107 TCL1-002 cells were injected intraperitoneally in 18 syngeneic mouse recipients. Three days later treatment was started in 8 mice with R788, which is the water-soluble prodrug of R406, at a daily dose of 80mg/kg during 18 consecutive days. Because of the rapid clearance of the drug (serum half-life <2 hours) R788 was administered in 3 divided doses at 4 hour intervals. Two weeks after the end of treatment leukemia developed in all mice from the control group (median WBC counts 131×106/ml, range 12-300×106/ml), whereas all R788-treated mice showed normal WBC numbers (median 6×106/ml, range 3-8×106/ml, P<0.001). Three weeks later all mice in the control group had died (median survival 46 days), whereas all mice in the R788 group were still alive and only two of them had detectable leukemic cells. R788 also showed some efficacy in the treatment of mice with overt TCL1-002 leukemias (WBC >50×106/ml). Whereas all mice from the control group (n=9) died between 6 and 18 days from the beginning of therapy, 4 out of 9 mice from the R788 group survived for more than 33 days. The mechanism of R788 activity was primarily related to inhibition of leukemic cell proliferation, as evidenced by a substantial decrease in the percentage of Ki67-positive cells after 7 days of treatment (30% before, 5% after therapy, P<0.001).
To investigate whether R788 will also be effective against other TCL1 tumors we treated five TCL1 mice with preleukemic mono- or oligoclonal B-cell expansions during a four week period. R788 reduced the percentage of CD5+/B220+ cells in 2 cases, whereas in 2 other cases the percentage increased. Interestingly, the pattern of clonal Ig gene rearrangements changed during therapy, suggesting that only certain TCL1 clones are sensitive to R788 treatment. In summary, this study shows that R788 can effectively inhibit the growth of certain TCL1 tumors and provides the first in vivo experimental evidence suggesting that inhibition of antigen-dependent BCR signaling could be an effective therapeutic approach in CLL.

Disclosures:
No relevant conflicts of interest to declare.