Genetic mutation and pharmacological inhibition of Bruton's tyrosine kinase (Btk) both have been shown to prevent the development of collagen-induced arthritis (CIA) in mice, providing a rationale for the development of Btk inhibitors for treating rheumatoid arthritis (RA). In the present study, we characterized a novel Btk inhibitor, 6-cyclopro-, in vitro and in rodent models of immune hypersensitivity and arthritis. We demonstrated that RN486 not only potently and selectively inhibited the Btk enzyme, but also displayed functional activities in human cell-based assays in multiple cell types, blocking Fc receptor cross-linking-induced degranulation in mast cells (IC 50 ϭ 2.9 nM), Fc␥ receptor engagement-mediated tumor necrosis factor ␣ production in monocytes (IC 50 ϭ 7.0 nM), and B cell antigen receptor-induced expression of an activation marker, CD69, in B cells in whole blood (IC 50 ϭ 21.0 nM). RN486 displayed similar functional activities in rodent models, effectively preventing type I and type III hypersensitivity responses. More importantly, RN486 produced robust anti-inflammatory and boneprotective effects in mouse CIA and rat adjuvant-induced arthritis (AIA) models. In the AIA model, RN486 inhibited both joint and systemic inflammation either alone or in combination with methotrexate, reducing both paw swelling and inflammatory markers in the blood. Together, our findings not only demonstrate that Btk plays an essential and conserved role in regulating immunoreceptor-mediated immune responses in both humans and rodents, but also provide evidence and mechanistic insights to support the development of selective Btk inhibitors as small-molecule disease-modifying drugs for RA and potentially other autoimmune diseases.

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A novel approach to design selective spleen tyrosine kinase (Syk) inhibitors is described. Inhibition of spleen tyrosine kinase has attracted much attention as a mechanism for the treatment of autoimmune diseases such as asthma, rheumatoid arthritis, and SLE. Fostamatinib, a Syk inhibitor that successfully completed phase II clinical trials, also exhibits some undesirable side effects. More selective Syk inhibitors could offer safer, alternative treatments. Through a systematic evaluation of the kinome, we identified Pro455 and Asn457 in the Syk ATP binding site as a rare combination among sequence aligned kinases and hypothesized that optimizing the interaction between them and a Syk inhibitor molecule would impart high selectivity for Syk over other kinases. We report the structure-guided identification of three series of selective spleen tyrosine kinase inhibitors that support our hypothesis and offer useful guidance to other researchers in the field.

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.