Article

R406, an Orally Available Spleen Tyrosine Kinase Inhibitor Blocks Fc Receptor Signaling and Reduces Immune Complex-Mediated Inflammation

Rigel Pharmaceuticals, South San Francisco, CA 94080, USA.
Journal of Pharmacology and Experimental Therapeutics (Impact Factor: 3.86). 11/2006; 319(3):998-1008. DOI: 10.1124/jpet.106.109058
Source: PubMed

ABSTRACT Recent compelling evidence has lead to renewed interest in the role of antibodies and immune complexes in the pathogenesis of several autoimmune disorders, such as rheumatoid arthritis. These immune complexes, consisting of autoantibodies to self-antigens, can mediate inflammatory responses largely through binding and activating the immunoglobulin Fc receptors (FcRs). Using cell-based structure activity relationships with cultured human mast cells, we have identified the small molecule R406 [N4-(2,2-dimethyl-3-oxo-4H-pyrid[1,4]oxazin-6-yl)-5-fluoro-N2-(3,4,5-trimethoxyphenyl)-2,4-pyrimidinediamine] as a potent inhibitor of immunoglobulin E (IgE)- and IgG-mediated activation of Fc receptor signaling (EC(50) for degranulation = 56-64 nM). Here we show that the primary target for R406 is the spleen tyrosine kinase (Syk), which plays a key role in the signaling of activating Fc receptors and the B-cell receptor (BCR). R406 inhibited phosphorylation of Syk substrate linker for activation of T cells in mast cells and B-cell linker protein/SLP65 in B cells. R406 bound to the ATP binding pocket of Syk and inhibited its kinase activity as an ATP-competitive inhibitor (K(i) = 30 nM). Furthermore, R406 blocked Syk-dependent FcR-mediated activation of monocytes/macrophages and neutrophils and BCR-mediated activation of B lymphocytes. R406 was selective as assessed using a large panel of Syk-independent cell-based assays representing both specific and general signaling pathways. Consistent with Syk inhibition, oral administration of R406 to mice reduced immune complex-mediated inflammation in a reverse-passive Arthus reaction and two antibody-induced arthritis models. Finally, we report a first-inhuman study showing that R406 is orally bioavailable, achieving exposures capable of inhibiting Syk-dependent IgE-mediated basophil activation. Collectively, the results show R406 potential for modulating Syk activity in human disease.

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    • "In fact, several SYK inhibitors have been developed and some of them have entered clinical trials. These include fostamatinib, also called R-788 (Braselmann et al., 2006; Weinblatt et al., 2014) and PRT062607 (Simmons, 2013). Fostamanitib , an oral SYK inhibitor, has been successfully used in a phase II clinical trial for treatment of patients with rheumatoid arthritis (Weinblatt et al., 2010; Weinblatt et al., 2014). "
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    • "The relatively low rate of responses observed after dasatinib treatment when compared to kinase inhibitors targeting BTK, SYK, or PI3Kδ may be due to the fact that LYN functions in vivo as a negative regulator of BCR signaling (Hibbs et al., 1995; Nishizumi et al., 1995) (Table 1). Fostamatinib (FosD, R788) is an orally available inhibitor of SYK (Braselmann et al., 2006). In vitro, its bioactive form called R406 reduces CLL cell migration, chemokine secretion and BCR signaling (Quiroga et al., 2009). "
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    • "We thus wondered whether the detected BCR opening is associated with the Syk/ITAM signal amplification process we have described previously (Rolli et al., 2002) and that was recently confirmed by another study (Mukherjee et al., 2013). To test this, we pretreated splenic B cells with the Src-family kinase inhibitor PP2, or the Syk inhibitor R406, prior to their activation with Lat-A (Braselmann et al., 2006). While the PP2-treatment only delays the opening of the IgM-BCR oligomer, this process no longer occurs in the presence of the Syk inhibitor R406 (Figure 5A,B). "
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