Methods of modifying therapeutic compounds such as drugs to be substrates for active transporters expressed in epithelial cells lining the lumen of the human colon are disclosed. The transporters expressed in the human colon include the sodium dependent multi-vitamin transporter (SMVT), and monocarboxylate transporters 1 and 4 (MCT 1 and MCT 4). The modified compounds can themselves be pharmacologically active, or upon cleavage of a chemical moiety after uptake from the colon, can be metabolized to form a compound that is pharmacologically active (e.g., a prodrug). The modified compounds disclosed herein are suitable for use in extended release oral dosage forms, particularly those that release drug over periods of greater than about 2-4 hours following administration.