Selective inhibition of spleen tyrosine kinase (SYK) with a novel orally bioavailable small molecule inhibitor, RO9021, impinges on various innate and adaptive immune responses: implications for SYK inhibitors in autoimmune disease therapy

Abstract

IntroductionSpleen tyrosine kinase (SYK) is a key integrator of intracellular signals triggered by activated immunoreceptors, including Bcell receptors (BCR) and Fc receptors, which are important for the development and function of lymphoid cells. Given the clinical efficacy of Bcell depletion in the treatment of rheumatoid arthritis and multiple sclerosis, pharmacological modulation of Bcells using orally active small molecules that selectively target SYK presents an attractive alternative therapeutic strategy.MethodsA SYK inhibitor was developed and assayed in various in vitro systems and in the mouse model of collagen-induced arthritis (mCIA).ResultsA novel ATP-competitive inhibitor of SYK, 6-[(1R,2S)-2-Amino-cyclohexylamino]-4-(5,6-dimethyl-pyridin-2-ylamino)-pyridazine-3-carboxylic acid amide, designated RO9021, with an adequate kinase selectivity profile and oral bioavailability, was developed. In addition to suppression of BCR signaling in human peripheral blood mononuclear cells (PBMC) and whole blood, FcγR signaling in human monocytes, and FcϵR signaling in human mast cells, RO9021 blocked osteoclastogenesis from mouse bone marrow macrophages in vitro. Interestingly, Toll-like Receptor (TLR) 9 signaling in human Bcells was inhibited by RO9021, resulting in decreased levels of plasmablasts, immunoglobulin (Ig) M and IgG upon B-cell differentiation. RO9021 also potently inhibited type I interferon production by human plasmacytoid dendritic cells (pDC) upon TLR9 activation. This effect is specific to TLR9 as RO9021 did not inhibit TLR4- or JAK-STAT-mediated signaling. Finally, oral administration of RO9021 inhibited arthritis progression in the mCIA model, with observable pharmacokinetics (PK)-pharmacodynamic (PD) correlation.ConclusionsInhibition of SYK kinase activity impinges on various innate and adaptive immune responses. RO9021 could serve as a starting point for the development of selective SYK inhibitors for the treatment of inflammation-related and autoimmune-related disorders.

Suboptimal B‐cell antigen receptor signaling activity in vivo elicits germinal center counterselection mechanisms

“…The deficiency in DC antigen presentation is in line with a recent report showing that conditional ablation of Syk in DCs abrogates FcγR-mediated cross priming of diabetogenic T cells [26]. Given the importance of Syk for B-cell maintenance and the reported regulatory activity of B cells in autoimmune conditions [27], interference with Syk function in human B cells is currently employed as a new avenue in the pharmacological treatment of rheumatoid arthritis and multiple sclerosis [28].…”

“…The deficiency in DC antigen presentation is in line with a recent report showing that conditional ablation of Syk in DCs abrogates FcγR-mediated cross priming of diabetogenic T cells [26]. Given the importance of Syk for B-cell maintenance and the reported regulatory activity of B cells in autoimmune conditions [27], interference with Syk function in human B cells is currently employed as a new avenue in the pharmacological treatment of rheumatoid arthritis and multiple sclerosis [28].We therefore hypothesize that those B fl/Zap-70 cells that start to lose Syk expression before they recognize antigen complexes on, for example, follicular DCs are increasingly penalized for cyclic dark zone reentry as BCR binding avidity/affinity is not efficiently translated into prosurvival pathway activity and efficient antigen presentation (see a hypothetical model in Supporting Information Fig. 4).We provide evidence for such a model by showing that a significantly attenuated pErk1/2 level, a central downstream target of the Ras pathway, is only to be found among excluded non-GC B cells, whereas Syk-sufficient cells remain in the competitive cycle.…”

Cutting Edge: The UNC93B1 Tyrosine-Based Motif Regulates Trafficking and TLR Responses via Separate Mechanisms

“…For example, YxxL of UNC93B1 could serve as a so-called hemITAM motif. Upon tyrosine phosphorylation ITAM or hemITAM motifs recruit Src Homology 2 domain containing proteins, such as spleen tyrosine kinase, which was previously suggested to regulate endosomal TLR signaling (21, 22). …”

The Plasmacytoid Dendritic Cell as the Swiss Army Knife of the Immune System: Molecular Regulation of Its Multifaceted Functions

“…However, these drugs do not convey maintenance of disease control in the majority of patients due to inefficient NFκB inhibition in pDC(107), thereby preventing GC-induced pDC death and consequently ongoing IFNα production. Improved therapeutic advantage may be gained by treating SLE patients with inhibitors of Syk (108), BTK (109) or TLR(110). Future intervention may aim at altering expression of miR-29b/c, involved in TLR-inhibited GC-induced pDC apoptosis by directly targeting Mcl-1 and Bcl-2(111).…”