Background/Purpose: 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 osteoclast^1,2. BTK regulates cell proliferation and survival of B cells, and the expression of inflammatory factors 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 pathway⁴. These various effector cells are reported to be associated with disease progression of rheumatoid arthritis^4,5. Therefore, BTK would be a potential target for a therapeutic agent in rheumatoid arthritis.

Methods:  The phosphorylation of BTK and the expression of CD69 on B cells, which were stimulated by anti-IgM, were examined using western blotting or flow cytometry. In macrophages, the expression of inflammatory factors induced by IgG was measured with ELISA assay. The bone resorption activity of mouse osteoclast was evaluated using osteoclast culture kit. To establish collagen-induced arthritis, male DBA/1 mice were injected on day-27 and-6 with an emulsion of complete Freund’s adjuvant and bovine type II collagen. On day 0, mice were randomized into treatment groups. TAS5315 was administrated orally once daily for 15 consecutive days. Disease severity was evaluated by clinical score of paw swelling, and the bone destruction of ankle was also measured by micro-CT imaging analysis at the end of treatment period. The scoring of inflammation, pannus, cartilage and bone damage in four joints of the CIA mice was performed by a single blinded pathologist using a modified Mankin score system.

Results:  TAS5315 showed a potent inhibitory activity against anti-IgM-induced phosphorylation of BTK with an IC50 in sub nmol/L range. 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). TAS5315 inhibited the expression of TNFα and CCL3 stimulated by IgG in macrophages, and also suppressed bone resorption activity of osteoclast induced by RANKL. 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. In the 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. TAS5315-treated mice also showed a remarkable recovery of bone mineral density compared with vehicle-treated mice by the micro CT analysis of hind paws of CIA mice.

Conclusion:  Our study demonstrates that a novel BTK inhibitior, TAS5315, shows significant efficacy in an animal model of rheumatoid arthritis. These data suggests that TAS5315 could be a promising novel therapeutic agent for RA by inhibiting inflammation and bone destruction.

References:1. Curr Opin Immunol. 2000;12, 282-288, 2. Int Arch Allergy Immunol. 2004; 134, 65-78, 3. Rheumatology (Oxford). 2013; 52, 1155-1162, 4. Arthritis Res Ther. 2011; 13, 3380-3391, 5. Nat Rev Immunol. 2007; 7, 191-201

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/tas5315-a-novel-brutons-tyrosine-kinase-btk-inhibitor-demonstrates-potent-efficacy-in-an-animal-model-of-rheumatoid-arthritis/