SAT0056\u2005TAS5315, A NOVEL BRUTON’S TYROSINE KINASE INHIBITOR, DEMONSTRATES POTENT EFFICACY AGAINST BONE DESTRUCTION IN AN ANIMAL MODEL FOR RHEUMATOID ARTHRITIS

Abstract

Background Bone erosions and cartilage damages are a pathological hallmark of rheumatoid arthritis (RA) and are associated with poor functional outcome1. Aberrant activations of osteoclasts induced by receptor activator of nuclear factor κB ligand (RANKL)2 are involved in the bone erosions of RA. It has also been recently shown that under chronic inflammatory conditions such as RA, inflammatory cytokines in joints induce pathological osteoclast differentiation and cause excessive bone resorption independent of RANKL-RANK signaling3. The Bruton’s tyrosine kinase (BTK) signaling pathway plays a pivotal role in inflammatory response and bone resorption4. Thus, targeting BTK may be efficacious against not only inflammation but also bone erosion by regulating activation of effector cells such as B cells, macrophages and osteoclasts in RA. We have already shown the inhibitory effects of TAS5315 on RANKL-dependent osteoclast activation. However, it remained uncertain whether TAS5315 inhibits osteoclast activation induced by inflammatory cytokines. Objectives In this study, we evaluated the effects of TAS5315 on osteoclast activation induced by inflammatory cytokines (in vitro) and the infiltration of tartrate-resistant acid phosphatase (TRAP)-positive osteoclasts in the joint of a mouse CIA model. Methods In vitro biochemical assay was performed using available kinase assay panels. The BioMAP® Diversity PLUS panels were used to determine the profile of TAS5315 in primary human cell systems. The effects of TAS5315 on osteoclasts were assessed by examining osteoclast-mediated bone resorption. TAS5315 was orally administrated once a day in an established mouse CIA model. TRAP-positive osteoclasts were counted manually. Bone mineral density (BMD) and bone erosion were assessed using micro-CT analysis. The mechanical properties of the tibia were evaluated by a compression test of proximal metaphysis using a material-testing machine. Results TAS5315 selectively inhibited the enzyme activity of BTK and had less off target inhibition against other kinases. In BioMAP® systems, TAS5315 decreased the production of IgG and the expression of cytokines (TNF-α, IL-6, IL-17A). TAS5315 also inhibited osteoclast-mediated bone resorption induced by inflammatory cytokines. On the other hand, anti-RANKL antibody did not inhibit bone resorption induced by inflammatory cytokines. Futhermore, in the mouse CIA model, TAS5315 significantly ameliorated paw swelling and pathological changes. TAS5315 significantly decreased the infiltration of TRAP-positive osteoclasts in the joint and also showed improvement of BMD and bone erosion by time-dependent micro-CT analysis. In vehicle-treated mice, the mechanical strength of tibia was decreased compared with normal mice. TAS5315-treated mice recovered the decreased parameters of the mechanical strength compared with vehicle-treated mice. These data suggests that TAS5315 improves bone erosion in murine model for RA through direct inhibition of osteoclast activation induced by inflammatory cytokines as well as RANKL. Conclusion Our study demonstrates that TAS5315 would be an attractive RA therapeutic drug that could improve bone destruction as well as inflammation. Reference [1] Nat Rev Rheumatol. 2012;8,656-64. 2: Nat Rev Rheumatol. 2015;11,189-94. 3: Arthritis Rheumatol. 2016;68,2889-2900. 4: Drug Discov Today. 2014;19,1200-4. Disclosure of Interests None declared