The purpose of The PROSPECT Study is to evaluate an investigational medication for the treatment of moderate to severe ulcerative colitis. This study is being conducted at up to 38 clinical research centers in the US, Canada, and Belgium, and is open to male and female patients 12 years and older. Participants in the study will have a number of visits to a research center over a five-month period. All study related care and medication is provided to qualified participants at no cost: this includes all visits, examinations, and laboratory work.