FR167653 is a potent inhibitor of p38 mitogen‐activated protein kinase (MAPK) and inhibits tumor necrosis factor α (TNFα) and interleukin‐1β (IL‐1β) production in inflammatory cells. In this study we investigated the effect of FR167653 on collagen‐induced arthritis (CIA).

Rats with CIA were subcutaneously injected with FR167653 (32 mg/kg/day) starting on the day of the booster injection (day 7) in the prophylactic treatment group and after the onset of arthritis (day 21) in the therapeutic treatment group. Hind‐paw swelling, body weight, radiographic and histologic scores, and osteoclast number were evaluated. Cytokine levels in the serum and tissue were assessed by enzyme‐linked immunosorbent assays. Flow cytometric analysis of T lymphocytes from bone marrow was performed. The effect of FR167653 on in vitro osteoclast formation induced by soluble receptor activator of nuclear factor κB ligand (sRANKL) and TNFα was examined.

Swelling of hind paws and loss of weight occurred in the CIA rats, but this was not evident in the prophylactic treatment group. Therapeutic treatment also significantly reduced paw swelling. The mean radiographic and histologic scores as well as the osteoclast numbers were significantly lower in the treatment group than in the CIA rats without treatment. FR167653 treatment reduced the serum levels of TNFα and IL‐1β, lowered the IL‐1β concentration in the ankle joints, and decreased the CD4−,CD8a+ T cell population in bone marrow. Furthermore, FR167653 inhibited the osteoclast‐like cell differentiation induced by both sRANKL and TNFα in vitro.

FR167653 prevents the onset of arthritis in a prophylactic treatment model and suppresses the progression of joint destruction in a therapeutic treatment model, suggesting that p38 MAPK is a potential therapeutic target for rheumatoid arthritis.