In earlier proof of concept studies, we reported a key role for a G-protein-coupled receptor, PAR₂, in chronic joint inflammation¹ and, demonstrated that a novel small molecule PAR₂ antagonist, ENMD-1068, dose dependently attenuated acute carageenan/kaolin-induced inflammation of the murine knee joint². The aim of the present study was two-fold: to test the hypothesis that antagonism of PAR₂ offered therapeutic potential in the collagen-induced model of rheumatoid arthritis (CIA), and to assess the potency and selectivity of ENMD-1068. All procedures involving animals were performed in strict accordance with the Animals (Scientific Procedures) Act 1986. Initial synovial perfusion studies using a Laser Doppler Imager (LDI; Moor Instruments Ltd) were conducted in adult male C57/6J/Bl mice to assess the potency and selectivity of ENMD-1068. Collagen and Freunds Complete Adjuvant (MD Biosciences) were used to induce CIA in DBA1 mice, with booster (collagen in PBS) given at day 21. From the first day of response, mice were treated once daily for 7 days with vehicle, 4 mg or 16 mg of ENMD-1068. Arthritic index (arbitrary) and paw pad thicknesses were recorded daily and confirmed by blinded observation. Late responders (> day 33) were excluded. ENMD-1068 reduced PAR₂ agonist, but not PAR₁ agonist-induced synovial hyperaemia. Dose response studies also indicated low potency of the compound for PAR₂. Furthermore, ENMD-1068 had no effect on basal synovial perfusion in normal mice. Daily subcutaneous administration of ENMD-1068 significantly reduced the arthritic index compared to vehicle treatment (P<0.0001, 2-way ANOVA). At day 7 post-response, arthritic indices were: vehicle group 7.1 ± 0.9; 4 mg group 4.2 ± 1.0; 16 mg group 2.8 ± 0.5. This study supports ENMD-1068 as a selective but low-potency antagonist for PAR₂, with therapeutic efficacy in ameliorating collagen-induced murine arthritis. PAR₂ may therefore represent a novel target in the treatment of rheumatoid arthritis.