Comparison of the single dose pharmacokinetics of sulphasalazine in rheumatoid arthritis and inflammatory bowel disease.

The pharmacokinetics of sulphasalazine and its principal metabolites in rheumatoid arthritis (RA) and inflammatory bowel disease (IBD) were compared. Patients with RA had a significantly greater concentration of plasma sulphapyridine than patients with IBD (medians 14.0 micrograms/ml and 7.4 micrograms/ml respectively). Patients with RA also tended to maintain a higher plasma sulphapyridine concentration with time, as determined by the area under the curve (AUC), but a lower plasma sulphasalazine AUC than patients with IBD. It is suggested that more sulphasalazine may be presented to the lower bowel for cleavage to sulphapyridine and 5-aminosalicylic acid in patients with RA than in IBD. Patients with RA may also have impaired metabolism of sulphapyridine as a consequence of their disease. Together these factors may contribute to higher peak circulating sulphapyridine concentrations and may be responsible for the higher incidence of side effects of sulphasalazine treatment in patients with RA than in patients with IBD.