A study with an 8 week open label phase study followed by a year long placebo controlled maintenance phase in subjects with active mild to moderate ulcerative colitis (UC), with a modified Mayo Score 4-10 and an endoscopy subscore of 2-3, taking mesalamine (or equivalent) as a concomitant medication. Subjects are required to be in clinical remission or clinical response to enter the year long maintenance phase. This study will help evaluate if HMPL-004 is effective in subjects maintaining clinical remission following successful induction therapy achieving clinical remission or clinical response.