Participants have a colonoscopy (an examination of the colon using a lighted tube) before starting the study medication. Colon tissue samples are biopsied during the procedure. Following the colonoscopy, patients are randomly assigned to receive either STA-5326 mesylate or placebo (sugar or dummy pill). They take four tablets a day and are seen in the clinic once a week (days 1, 8, 15, 22 and 29) for the following tests and procedures: