A two-phase multi-center study with an open label 8-week induction phase and a randomized, double-blind, placebo controlled maintenance phase to evaluate the efficacy and safety of HMPL-004 for a 52-week maintenance therapy in adults with mild to moderate UC and who have demonstrated a sub-optimal response to mesalamine treatment. The study participants will consist of subjects who have completed either the HMPL-004-03 or HMPL-004-05 induction study, complemented by the enrollment of additional subjects in the open label induction phase who meet the entry criteria of this study. . Subjects entering this maintenance study from one of the HMPL-004 induction studies will have their modified Mayo baseline score from that study as their baseline score for this study. All subjects enrolled in the open label induction phase of the study will be re-evaluated for clinical response and remission after the 8-week open label 2400 mg HMPL-004 treatment period. Clinical responders and remitters will be randomized 2:1 (HMPL-004: placebo) to HMPL-004 1800 mg/day or placebo group for the 52-week maintenance phase of the study.