SYK inhibition with fostamatinib in Waldenström's macroglobulinemia: In vitro cell based assays.

Abstract

e19047 Background: Waldenström's macroglobulinemia (WM) is characterized by accumulation of IgM secreting neoplastic B cells in the bone marrow (BM). B-cell antigen receptor (BCR) signaling plays a pivotal role in the regulation of integrin-mediated retention of lymphoma cells in the BM. Signaling through the BCR leads to activation of spleen tyrosine kinase (SYK) and downstream kinases which play a role in lymphoma pathobiology. The Bruton’s tyrosine kinase (BTK) inhibitor ibrutinib targets BCR-controlled signaling and integrin-mediated adhesion in WM cells, but patients with WM may become refractory to ibrutinib treatment. As SYK signaling is upstream of BTK, we explored the effect of the SYK inhibitor R406 (the active metabolite of fostamatinib) on BCR mediated signaling and cell adhesion in WM. Methods: MWCL-1 cells were treated with R406, CAL101 (PI3 kinase inhibitor) or ibrutinib at concentrations up to 10µM and stimulated with anti-IgM antibodies or PMA. The cells were transferred to plates coated with fibronectin to assess cell adhesion. For the proliferation assays, MWCL-1 cells or primary B cells were pretreated with kinase inhibitors, followed by stimulation of primary B cells with anti-IgM. Results: Anti-IgM or PMA alone stimulated the adhesion of MWCL-1 on fibronectin. The addition of R406 had a comparable effect to ibrutinib on inhibition of IgM mediated cell adhesion (IC50 = 0.32µM), consistent with the effect of both compounds on BCR mediated signaling. Neither compound affected PMA induced cell adhesion, indicating that the effect on IgM mediated cell adhesion was not due to toxicity. Lack of off-target toxicity was further confirmed in the MWCL-1 proliferation assays where the compounds showed less potency than in BCR-mediated assays. Comparable inhibition of adhesion and proliferation was seen with CAL101. Conclusions: Preliminary preclinical studies with R406 show that SYK is involved in BCR mediated signaling and cell adhesion in WM. CAL101 mediated inhibition of adhesion further confirmed the role of SYK, which is upstream of PI3K. R406 was as effective as ibrutinib in blocking WM cell adhesion. Further studies of SYK inhibition with fostamatinib as a potential treatment option for WM are warranted.