Sulfasalazine and 5-aminosalicylic acid inhibit contractile leukotriene formation

Sulfasalazine, used therapeutically in the treatment of ulcerative colitis, is cleaved in vivo to form 5-aminosalicylic acid (5-ASA) and sulfapyridine. In an isolated preparation of rat peritoneal cells both sulfasalazine (IC50 = 0.15 mM) and 5-ASA (IC50 = 2.3 mM), but not sulfapyridine, inhibited calcium ionophore-stimulated formation of contractile leukotriene activity. This activity, although not identified directly, is attributable to a mixture of leukotriene C4 and leukotriene D4 (commonly referred to as SRS-A, or slow-reacting substance of anaphylaxis). Reference compounds evinced expected activities (IC50 = 0.024 mM for phenidone, IC50 = 0.3 microM for nordihydroguaiaretic acid, IC50 = 0.033 mM for BW 755C), whereas para-aminosalicylic acid and thiosalicylic acid were inactive. These properties of sulfasalazine may contribute to its therapeutic efficacy in vivo.