Investigation of the degradation mechanism of 5-aminosalicylic acid in aqueous solution.

The solution degradation of the antiinflammatory agent 5-aminosalicylic acid (5-ASA) was investigated in order to elucidate a mechanism for degradation. Two degradation pathways were considered: decarboxylation by analogy to 4-aminosalicylic acid (4-ASA) decomposition and oxidation from consideration of 5-ASA's aromatic ring substitution pattern (i.e., relation to p-aminophenol). The oxidation of 5-ASA was investigated using cyclic voltammetry and flow electrolysis. These studies showed that 5-ASA is more easily oxidized than is 4-ASA and that 5-ASA undergoes a two-electron, two-proton oxidation consistent with formation of 5-ASA-quinoneimine (5-ASA-QI). This oxidation is followed by subsequent complex chemistry. The decomposition of 5-ASA in solution was examined under a variety of conditions. 5-ASA decomposes most rapidly under conditions promoting oxidation and is most stable under conditions tending to inhibit oxidation. Decarboxylation was not found to be a significant degradation pathway.