Review article: mode of action and delivery of 5-aminosalicylic acid - new evidence.

The effectiveness of sulfasalazine depends on the splitting of the diazo bond in the molecule by the action of bacteria in the large bowel, releasing the pharmacologically active moiety, 5-aminosalicylic acid. The development of pH-dependent, delayed-release formulations of 5-aminosalicylic acid abolished the toxicity associated with the sulfapyridine part of sulfasalazine. 5-aminosalicylic acid is now believed to act by activating a class of nuclear receptors involved in the control of inflammation, cell proliferation, apoptosis and metabolic function, the gamma form of peroxisome proliferator-activated receptors. These receptors are expressed at particularly high levels in colon epithelial cells, where their expression appears to be at least in part stimulated by gut bacteria. Other drugs known to act via peroxisome proliferator-activated receptor-gamma, such as rosiglitazone and the selective peroxisome proliferator-activated receptor-gamma ligand GW1929, can be displaced from their binding sites on the peroxisome proliferator-activated receptor-gamma molecule by 5-aminosalicylic acid at concentrations of 5-aminosalicylic acid that correspond with the concentrations found in the lumen of ulcerative colitis patients taking oral mesalazine. Genetically engineered heterozygous knockout mice (peroxisome proliferator-activated receptor-gamma+/-) are particularly susceptible to colonic inflammation, and inflammation is more severe in these mice, in response to chemicals that induce experimental colonic ulcers. In these experimental models, 5-aminosalicylic acid is ineffective in peroxisome proliferator-activated receptor-gamma+/- mice. This new insight provides a mechanistic foundation for the possibility that long-term treatment with 5-aminosalicylic acid can reduce the risk of colorectal cancer in patients with ulcerative colitis.