Background Bruton's tyrosine kinase (BTK) is a member of the Tec family kinases, and is expressed in B cells, monocytes/macrophages, mast cells, basophils and osteoclast1,2. BTK is a critical effector molecule that activates B cell receptor signaling pathway in B cells and Fc-γ receptor signaling pathway in macrophages3. In osteoclast, RANKL (receptor activator of NF-kB ligand) binds to its receptor RANK, and induces differentiation and bone resorption of osteoclast through BTK-PLCγ signaling pathway. These various effector cells are reported to be associated with disease progression of rheumatoid arthritis4,5. Therefore, BTK would be an attractive target for a therapeutic agent in autoimmune disease such as rheumatoid arthritis (RA). We discovered a novel synthesized compound which inhibits BTK with a highly selectivity. Objectives We report characteristics of a highly selective and potent BTK inhibitor, TAS5315, in vitro and in vivo models for RA. Methods The kinase selectivity of TAS5315 was evaluated in a series of biochemical assays. The expression of CD69, a lymphocyte activation marker, stimulated by anti-IgM in the mouse splenic B cells was measured using flow cytometry. The bone resorption activity of mouse osteoclast was evaluated using osteoclast culture kit. Male DBA/1 mice were injected on day-27 and-6 with an emulsion of complete Freund's adjuvant and bovine type II collagen. TAS5315 (0.05, 0.1, 0.2 and 0.4 mg/kg) was administrated orally once daily for 15 consecutive days. Results TAS5315 showed a potent inhibitory activity against anti-IgM-induced phosphorylation of BTK with an IC50 in sub nmol/L range. TAS5315 also inhibited only 4 kinases (TEC, BMX, TXK and ITK) out of 276 off-target kinases with high inhibitory activity (over 80% at 100 nmol/L). The up-regulation of CD69 on stimulated mouse splenic B cells was suppressed by TAS5315 in a dose-dependent manner (IC50 =0.2 nmol/L). In pit formation assay, TAS5315 also inhibited bone resorption activity of osteoclast in a dose-dependent manner (IC50 =2.2 nmol/L). In a mouse collagen-induced arthritis model, TAS5315 dose-dependently and significantly decreased the clinical score in arthritic mice compared with that in vehicle-treated mice, with an ED50 value of 0.12 mg/kg. The treatment group with TAS5315 (0.4 mg/kg) completely ameliorated arthritic symptoms on day 14. All four paws of CIA mice were then examined by histopathological analysis. TAS5315-treated mice had a marked reduction in the severity of inflammation, pannus, cartilage destruction and bone destruction in a dose-dependent manner. In the micro CT analysis of hind paws of CIA mice, TAS5315 (1mg/kg)-treated mice showed a remarkable recovery of bone mineral density compared with vehicle-treated mice. Conclusions Our study demonstrates that TAS5315 is a highly selective BTK inhibitor and has significant efficacy in the mouse CIA model. These data suggests that TAS5315 could be a promising new therapeutic agent for RA. References Curr Opin Immunol. 2000;12, 282-288, Int Arch Allergy Immunol. 2004; 134, 65-78, Rheumatology (Oxford). 2013; 52, 1155-1162, Arthritis Res Ther. 2011; 134, 3380-3391, Nat Rev Immunol. 2007; 7, 191-201 Disclosure of Interest F. Hosoi Employee of: Taiho pharmaceutical co. ltd., S. Iguchi Employee of: Taiho pharmaceutical co. ltd., Y. Yoshiga Employee of: Taiho pharmaceutical co. ltd., R. Kaneko Employee of: Taiho pharmaceutical co. ltd., Y. Nakachi Employee of: Taiho pharmaceutical co. ltd., D. Akasaka Employee of: Taiho pharmaceutical co. ltd., K. Yonekura Employee of: Taiho pharmaceutical co. ltd., Y. Iwasawa Employee of: Taiho pharmaceutical co. ltd., E. Sasaki Employee of: Taiho pharmaceutical co. ltd., T. Utsugi Employee of: Taiho pharmaceutical co. ltd.