This is an open label exploratory study to investigate the safety of 400 milligram (mg ) twice a day (b.i.d.) GSK2586184 in patients with moderate to severe, active ulcerative colitis (UC). Study medication will be administered orally (as tablets), twice daily, for up to 8 weeks (56 days). Study medication will be taken with food. Each subject will have 6 out-patient visits: Screening (Day -30 to -1); Baseline and Start of treatment (Day 1); Week 2 (Day 14); Week 4 (Day 28); Week 8 (Day 56); and Follow-up (Week 12; Day 84). Visit windows for weeks 2, 4 and 8 will be + 2 days. The primary objective of this study is to assess the safety and tolerability of GSK2586184. The primary endpoints to measure safety are laboratory tests (including haematology, clinical chemistry and serum creatinine), vital signs, 12-lead electrocardiogram (ECG), physical examination, and adverse event reporting. These are standard measurements to evaluate safety.