preparing the following four aminoacidprodrugs of 5-ASA: 5-(N-L-aspartylamino)-2-salicylic acid, disodium salt (18), 5-(N-L-glutamylamino)-2-salicylic acid, disodium salt (19), [(5-aminosalicyl)-L-prolyl]-L-leucine, sodium salt (25), and [[5-(N-L-glutamylamino)salicyl]-L-prolyl]-L-leucine, disodium salt (28). In these compounds, the peptide bond is selectively split by the intestinal brush border aminopeptidase