 The study is designed as a randomized, double-blind, double-dummy, placebo-controlled, sequential dose escalating trial, with an active control ASACOL group as internal control. Up to three cohorts Cohorts 1-3, each consisting of approximately 20 patients with mild-moderately active ulcerative colitis not limited to the rectum, will be included, one for each dose level of GSK1399686 to be tested. Within a cohort, patients will be randomized in a 3:1:1 ratio to receive GSK1399686 once daily over 4 weeks, followed by 2 weeks dosing with placebo, placebo, or ASACOL t.i.d. for 6 weeks, respectively. 