Dextran-5-aminosalicylic acid conjugate (dextran-5-ASA) was in vitro-evaluated as a polymeric colon-spe-cific prodrug of 5-aminosalicylic acid (5-ASA). Chemical stability of dextran-5-ASA in the pH 1.2 or 6.8 buffer solutions was investigated at 37 for 6 hrs. The dextran backbone was not degraded and no 5-ASA release was detected. Moreover, dextran-5-ASA neither liberated 5-ASA in the homogenates of the small intestine of rats nor was transported across Caco-2 cell monolayers, suggesting no significant loss of dextran-5-ASA during transit through the upper intestine. Furthermore, incubation of dextran-5-ASA in 10% cecal contents of rats released about 37% and 55% of 5-ASA bound to dextran in 8 hr and 24 hr, respectively. While that with either esterase or dextranase failed to liberate 5-ASA from the polymeric prodrug, incubation of dextran-5-ASA with both esterases and dextranse released 5-ASA up to about 24% of 5-ASA bound to dextran. These results suggest that, after oral administration of dextran-5-ASA, the polymeric prodrug is delivered specifically to and releases 5-ASA in the large intestine, and reveal that the 5-ASA release by cleavage of the ester bond requires precedent depolymerization of the dextran backbone.