Colorimetric determination of 5-aminosalicylic acid and its N-acetylated metabolite on urine and feces.

A simple and convenient colorimetric method is described for the quantitative determination of 5-aminosalicylic acid (5-ASA) and N-acetyl-5-ASA in urine and feces after oral administration of salicylazosulfa-pyridine (SASP), the drug of choice in the treatment of ulcerative colitis. N-acetyl-5-ASA is extracted directly from the acidified biological specimen, deacetylated, and the liberated 5-ASA subjected to a modified Bratton-Marshall reaction. The 5-ASA present in the specimen must be acetylated with acetic anhydride prior to extraction. The violet colored product of the Bratton-Marshall reaction has a lambdamax of 560 nm and conforms to Beer's law over the concentration range of 0-70 umg/ml. Average recoveries (+/- S.D., N = 6) OF 5-ASA added to rat and human urine and rat fecal homogenates were 91.6 +/- 4.9%, 102 +/- 6.0%, and 71.0 +/- 4.8%, respectively. Interference by SASP and its sulfapyridine metabolities is negligible. As demonstrated, the colorimetric method is of sufficient sensitivity for application in most metabolic and pharmacokinetic studies conducted with SASP in laboratory animals and man.