Additional Study Information: The purpose of this study is to see how safe and effective study drug GS-5745 is in treating people with moderate to severe Ulcerative Colitis. This is a randomized, double-blind, placebo-controlled study. Placebo-controlled means that you may receive an injection with no medicine in it, but looks like GS-5745. Randomized means you will be selected by chance (like flipping a coin) to receive either one of three treatments which will be administered in weekly injections. If you are eligible to take part in Group 1 of this study, you will be randomized and will have a one in three chance of receiving one of the following treatments: 150 mg GS-5745, 150 mg GS-5745 and placebo, switching each week or Placebo.