chapter  12
Treatment of Active Crohn's Disease with Salazopyrine and Derivatives of Aminosalicylic Acid (5-ASA)
Pages 20

A. Metabolism When taken orally, 20% to 30% of the drug is absorbed at the level of the upper intestinal tract, and approximately 70% to 80% reaches the colon where the azo linkage is cleaved by the intestinal bacteria, thus liberating SP and 5-ASA (1). SP is rapidly absorbed by the colonic mucosa, metabolized by the processes of acetylation, hydroxylation, and glucuronidation, and subsequently excreted in the urine as such or in the form of metabolites (2). Traces of SP are found in the blood between three and five hours after oral intake (3). High blood levels of this molecule appear to be responsible for the majority of side effects caused by SASP, depending mainly on slow or rapid acctylator phenotype (4). Only a minimal part of 5-ASA is absorbed at the colonic level, rapidly acctylated, and excreted in the urine (2). The greater part, therefore, remains in contact with the colonic mucosa where it exerts its topical anti-inflammatory action and is then eliminated in the feces (5).