Identification of reaction products between drug substances and excipients by HPLC–SPE–NMR: Ester and amide formation between citric acid and 5-aminosalicylic acid

The reaction between the high-dose drug substance 5-aminosalicylic acid (5-ASA) and the excipient citric acid during storage of an experimental enema preparation has been studied and three isobaric reaction products, i.e., an ester and an amide with non-symmetrically substituted citric acid moieties and a symmetrical amide, were identified by combined use of HPLC–SPE–NMR and HPLC–MS. After storage for 1 week at 70 °C, approximately 5% of the 5-ASA present in the formulation was transformed into these impurities. Storage of the enema for 32 months at room temperature led to loss of approximately 10% of the original amount of 5-ASA, with the ester as the main reaction product.