Therapeutic promise of proteinase-activated receptor-2 antagonism in joint inflammation

Biological therapies such as tumor necrosis factor-α inhibitors have advanced the treatment of rheumatoid arthritis, but one-third of patients do not respond to such therapy. Furthermore, these inhibitors are now usually administered in combination with conventional disease-modifying antirheumatic drugs, suggesting they have not achieved their early promise. This study investigates a novel therapeutic target, proteinase-activated receptor (PAR)-2, in joint inflammation. Intra-articular carrageenan/ kaolin (C/K) injection in mice resulted in joint swelling that was associated with synovial PAR ₂ up-regulation. Inhibiting receptor up-regulation using small interfering RNA technology, as confirmed by immunoblotting, substantially reduced the inflammatory response in the joint. Serine proteinase-induced joint swelling was mediated primarily via PAR₂ activation, since the response to exogenous application of trypsin and tryptase was absent in PAR ₂ knockout mice. Furthermore, serine proteinase inhibitors were effective anti-inflammatory agents in this model. Disrupting proteolytic activation of PAR₂ using antiserum (B5) directed to the receptor cleavage/activation site also attenuated C/K-induced inflammation, as did the similarly targeted PAR₂ monoclonal antibody SAM-11. Finally, we report the activity of a novel small molecule PAR₂ antagonist, N ¹-3-methylbutyryl-N⁴-6-aminohexanoyl-piperazine (ENMD-1068), that dose dependently attenuated joint inflammation. Our findings represent a major advance in collectively identifying PAR₂ as a novel target for the future treatment of arthritis.