Olsalazine-13C6 is intended for use as an internal standard for the quantification of olsalazine by GC- or LC-MS. Olsalazine (T20615) is an orally bioavailable prodrug form of the anti-inflammatory agent 5-aminosalicylic acid that is cleaved by bacterial azo reductases in the gut to generate active 5-ASA.1