Specific Btk inhibition suppresses B cell– and myeloid cell–mediated arthritis

Abstract

Bruton's tyrosine kinase (Btk) is a therapeutic target for rheumatoid arthritis, but the cellular and molecular mechanisms by which Btk mediates inflammation are poorly understood. Here we describe the discovery of CGI1746, a small-molecule Btk inhibitor chemotype with a new binding mode that stabilizes an inactive nonphosphorylated enzyme conformation. CGI1746 has exquisite selectivity for Btk and inhibits both auto- and transphosphorylation steps necessary for enzyme activation. Using CGI1746, we demonstrate that Btk regulates inflammatory arthritis by two distinct mechanisms. CGI1746 blocks B cell receptor-dependent B cell proliferation and in prophylactic regimens reduces autoantibody levels in collagen-induced arthritis. In macrophages, Btk inhibition abolishes FcγRIII-induced TNFα, IL-1β and IL-6 production. Accordingly, in myeloid- and FcγR-dependent autoantibody-induced arthritis, CGI1746 decreases cytokine levels within joints and ameliorates disease. These results provide new understanding of the function of Btk in both B cell- or myeloid cell-driven disease processes and provide a compelling rationale for targeting Btk in rheumatoid arthritis.

Analysis of the Effects of the Bruton's tyrosine kinase (Btk) Inhibitor Ibrutinib on Monocyte Fcγ Receptor (FcγR) Function

“…The results demonstrated that ibrutinib did not adversely affect the phagocytic ability of monocytes despite its ability to block Fc␥R-mediated TNF␣ production by monocytes. We also tested the effects of the more selective Btk inhibitor CGI-1746 (28) and found that it was equal in its ability to block Fc␥R-mediated cytokine production in monocytes (data not shown). This is in contrast to the findings with NK cells, where ibrutinib, but not CGI-1746, blocked NK-cell antibody-dependent cellular cytotoxicity because activated NK cells express Itk, which is not inhibited by CGI-1746 (18).…”

“…Btk inhibition results in reduced Fc␥R-mediated cytokine production in monocytes and macrophages (19,27,28). However, the effect of reduced Btk function on phagocytosis is less clear, with studies showing significant (29 -31) or only marginal (32) effects.…”

“…Intracellular Calcium Flux Is Required for Fc␥R-mediated Cytokine Production but Not for Phagocytosis-One major signaling event downstream of Btk that is blocked by ibrutinib and other Btk inhibitors is calcium flux (19,28). Here we tested the effects of ibrutinib within the context of calcium signaling and actin polymerization.…”

Pathogenetic insights from the treatment of rheumatoid arthritis

“…Genetic absence of Btk in humans is associated with impaired B cell development and agammaglobulinemia. Small molecule inhibitors of Btk are available and currently used for the treatment of mantle cell lymphoma, while others are in early development for RA [58]. The variable responses thus far detected to kinase inhibitors however reflects our relatively limited understanding of the true cellular hierarchy of these pathways at the synovial pathogenetic level.…”

Targeting DAMP Activation of Toll-Like Receptors: Novel Pathways to Treat Rheumatoid Arthritis?

“…These include Tocilizumab (anti-IL-6R) (Fleischmann et al, 2006;Jones et al, 2010) and Anakinra, a recombinant IL-1 receptor antagonist, as well as Bevacizumab, an antibody that targets vascular endothelial growth factor (VEGF) and hence may reduce neovascularisation that pannus formation depends upon. A variety of small molecule inhibitors designed to target critical elements of the B cell receptor, T cell receptor or cytokine signalling pathways such as inhibitors of IKK2, PDE4 and Btk (Bruton's tyrosine kinase), have shown interesting results in some animal models of arthritis, as have clinical trials with the syk inhibitor R788 (Podolin et al, 2005;Lindstrom et al, 2010;Di Paolo et al, 2011). The p38 inhibitors however, which showed such promise in animal models have not lived up to expectations in clinical trials and have not progressed beyond phase II (Genovese, 2009).…”