To evaluate the effects of SB 242235, a potent and selective inhibitor of p38 mitogen‐activated protein (MAP) kinase, on joint integrity in rats with adjuvant‐induced arthritis (AIA).

Male Lewis rats with AIA were orally treated either prophylactically (days 0–20) or therapeutically (days 10–20) with SB 242235. Efficacy was determined by measurements of paw inflammation, dual‐energy x‐ray absorptiometry for bone mineral density (BMD), magnetic resonance imaging (MRI), micro–computed tomography (CT), and histologic evaluation. Serum tumor necrosis factor α (TNFα) in normal (non‐AIA) rats and serum interleukin‐6 (IL‐6) levels in rats with AIA were measured as markers of the antiinflammatory effects of the compound.

SB 242235 inhibited lipopolysaccharide‐ stimulated serum levels of TNFα in normal rats, with a median effective dose of 3.99 mg/kg. When SB 242235 was administered to AIA rats prophylactically on days 0–20, it inhibited paw edema at 30 mg/kg and 10 mg/kg per day by 56% and 33%, respectively. Therapeutic administration on days 10–20 was also effective, and inhibition of paw edema was observed at 60, 30, and 10 mg/kg (73%, 51%, and 19%, respectively). Significant improvement in joint integrity was demonstrated by showing normalization of BMD and also by MRI and micro‐CT analysis. Protection of bone, cartilage, and soft tissues was also shown histologically. Serum IL‐6 levels were decreased in AIA rats treated with the 60 mg/kg dose of compound.

Symptoms of AIA in rats were significantly reduced by both prophylactic and therapeutic treatment with the p38 MAP kinase inhibitor, SB 242235. Results from measurements of paw inflammation, assessment of BMD, MRI, and micro‐CT indicate that this compound exerts a protective effect on joint integrity, and thus appears to have disease‐modifying properties.