Pharmacokinetic studies of benzalazine

The pharmacokinetic properties of benzalazine ((2-hydroxy-5-[(4-carboxyphenyl)azo]benzoic acid, CAS 64896-26-0), a new agent for the treatment of ulcerative colitis and Crohn's disease of the large intestine, were investigated. From jejunal loops of rats in situ no noteworthy absorption of benzalazine was observed. All attempts to demonstrate metabolic conversion of benzalazine in mucosal homogenate of the small intestine of rats were without any success. In faecal suspensions, the half-life of the metabolic conversion of benzalazine was determined as 15 min and the formation of the metabolite 5-aminosalicylic acid (5-ASA) was demonstrated qualitatively. 72 h after single oral administration of 300 mg benzalazine/kg b.w. to rats, an average of 71.83% of the administered dose was recovered in urine and faeces. Only a small amount of unmetabolized benzalazine was excreted with urine and faeces (0.75% and 1.47% of the administered dose, respectively). The benzalazine metabolite 5-ASA and the 5-ASA metabolite acetyl-5-aminosalicylic acid (Ac-5-ASA) were excreted mainly with the faeces (29.22% and 20.66% of the administered dose, respectively) and only in small amounts with the urine (2.54% and 11.06% of the administered dose, respectively).