To demonstrate non-inferiority in efficacy of FE 999315 to mesalazine in patients with mild to moderate active ulcerative colitis after 8 weeks of treatment.

Inclusion Criteria:

• Male and female patients, 16 to 75 years old, diagnosed with ulcerative colitis.
• Diagnosis of ulcerative colitis in active phase of mild to moderate entity.
• Female patients must fulfill at least one of the following criteria: Post-menopausal (women ≥45 years with no menstrual period for at least 12 months without an alternative medical cause), surgically sterile, using a medically approved contraception throughout the trial period or her male partner using medically approved contraception throughout the trial period.
• Male patients must agree to use medically approved contraception throughout the trial period.

Exclusion Criteria:

• Patients with limited distal proctitis.
• Patients with infectious colitis.
• Patients with history of colectomy.
• Patients with severe diseases in other organs and systems.
• Evidence or history of toxic megacolon.
• Women who wish to become pregnant during the trial period