CORE: A Study of OPC-6535 With Asacol® in Maintaining Ulcerative Colitis (UC) Remission

Inclusion Criteria: * Male or female subjects, 18 to 80 years of age, inclusive. * Subjects with a prior diagnosis of ulcerative colitis, established by colonoscopy. * Subjects currently in remission. * Subjects who have undergone treatment for a flare of ulcerative colitis, with symptomatic onset of remission occurring no more than 52 weeks from the Screening Period. * If subjects are taking sulfasalazine or 5-ASA products prior to entry, the dose must be stable for at least 6 weeks prior to the Screening Period. NOTE: During the study, use of these drugs will be discontinued. * Subjects having undergone colonoscopy with pan-colonic surveillance biopsies negative for dysplasia within 1 year of the Screening Visit if at increased risk of colorectal cancer (≥ 8 year history of ulcerative colitis). * Female or male subjects who are surgically sterilized or who are prepared to and agree to practice a double-barrier form of birth control from the Screening Period through 30 days (females) and 90 days (males), respectively, from the last dose of study medication. Females who are more than 12 months post-menopausal are also eligible to participate in the study. Exclusion Criteria: * Subjects who have active disease. * Subjects who have any other clinically significant disease(s) or condition/procedure(s). * Subjects who have had major gastrointestinal surgery including, but not limited to, a colostomy, an ileostomy or previous colonic surgery other than appendectomy. * Subjects who have used corticosteroids or topical agents (corticosteroid or 5-ASA enemas, suppositories, foams) within 6 weeks of the Screening Period. * Female subjects who are pregnant or lactating. * Other exclusion criteria may apply.