The anti‐allergic compound tranilast attenuates inflammation and inhibits bone destruction in collagen‐induced arthritis in mice

Background and purpose:  Recent findings suggest the importance of mast cells in the pathogenesis of rheumatoid arthritis and their potential as a therapeutic target. Tranilast is an anti‐allergic compound with a potent membrane‐stabilizing effect on mast cells and a wide range of anti‐inflammatory effects, thus may be advantageous in the treatment of arthritis. Here, we have evaluated the effects of tranilast on the progression of collagen‐induced arthritis in mice. Experimental approach:  Tranilast (400 mg·kg −1 ·day −1 ) was orally administered for 8 weeks to mice with established collagen‐induced arthritis. Arthritis was assessed by clinical signs and X‐ray scores. In paw tissue, the numbers of mast cells and osteoclasts were measured by histological analysis, and several inflammatory factors were assessed by RT‐PCR and Western blot analysis. * Key results:  TNF‐α‐positive mast cells were present extensively throughout the inflamed synovium of vehicle‐treated arthritic mice, with some mast cells in close proximity to osteoclasts in areas of marked bone and cartilage destruction. Tranilast significantly reduced clinical and X‐ray scores of arthritis and decreased numbers of TNF‐α‐positive mast cells and mRNA levels of TNF‐α, chymase (mouse mast cell protease 4), tryptase (mouse mast cell protease 6), stem cell factor, interleukin‐6, cathepsin‐K, receptor activator of nuclear factor‐κB, and of receptor activator of nuclear factor‐κB‐ligand, but increased interleukin‐10 mRNA level in paws of arthritic mice. Osteoclast numbers were decreased by treatment with tranilast. Conclusions and implications:  Tranilast possesses significant anti‐rheumatic efficacy and, probably, this therapeutic effect is partly mediated by inhibition of mast cell activation and osteoclastogenesis.