Plasma and synovial fluid concentrations of sulphasalazine and two of its metabolites in rheumatoid arthritis.

A study was made of plasma and synovial fluid levels of sulphasalazine, one of its dissociation products--sulphapyridine and a metabolite of the latter--acetyl sulphapyridine in patients with rheumatoid arthritis (RA) who were in a steady state on sulphasalazine therapy. Combined sulphapyridine levels were significantly higher than those of sulphasalazine both in plasma and synovial fluid. Synovial fluid levels of both drugs correlated with their plasma levels and were generally slightly lower. Some patients accumulated sulphasalazine and sulphapyridine in the synovial fluid and the mean concentration of sulphasalazine was higher in the fluid than in the plasma. The explanation for this is uncertain. The concentration of combined sulphapyridine in synovial fluid was related to local joint inflammation and more active systemic disease. No consistent association was found between sulphasalazine levels and local or systemic activity. The higher sulphapyridine levels in synovial fluid found in this study suggest the possibility that this moiety could play a more active role in RA than it does in inflammatory bowel disease.