Recovery process of arthritis induced by 6-sulfanilamidoindazole (6SAI) in rats.

6-Sulfanilamidoindazole (6SAI) is known to induce not only an acute arthritis but also serositis and arteritis which resemble those induced by some vasodilators in rats. In this study, the recovery process of ankle lesions was examined histopathologically for up to 12 weeks of recovery period in rats bearing arthritis induced by administration of 6SAI (500 mg/kg) for 2 weeks. At 2 weeks of 6SAI-treatment, exudative synovitis and exudative/edematous periarthritis with marked formation of granulation tissues and periosteal reactive bone formation were noted in the ankles, but no remarkable neutrophil infiltration was detected in those lesions. The ankle swelling induced by 6SAI diminished by 4 weeks of recovery period, and the elevated plasma fibrinogen levels were normalized by 2 weeks of recovery period. Although fibrosis and newly-formed periosteal bone were still observed after 2 weeks of recovery period, no inflammatory lesion was detected at that point. At 4 or 12 weeks of recovery periods, the ankles showed an almost normal appearance. These results indicate that 6SAI-induced arthritis is reversible in nature and does not develop into chronic phase.