An Efficacy and Safety Study of Mongersen (GED-0301) in Subjects With Active Ulcerative Colitis

Eligible subjects will have the Baseline Visit (Week 0/ Visit 2) and receive the following treatments: * Induction Phase - GED-0301 160 mg once daily (QD) for 8 weeks; * Extension Phase - GED-0301 160 mg on alternating dosing schedule (GED-0301 160 mg QD for 4 weeks, followed by 4 weeks without GED-0301 treatment) for an additional 44 weeks. Subjects who do not achieve at least a 20% decrease in partial mayo score (PMS) from baseline at Week 12 will be discontinued from the study. Clinical, safety, and pharmacokinetic (PK) data will be evaluated on an ongoing basis, however, if the response to treatment is lower than expected (eg, ≤2 subjects achieving clinical remission based on modified Mayo score (MMS)) when 50% of the subjects complete Week 8, or discontinue before Week 8, the study team will review available data (clinical, safety, and PK) to evaluate if the study conduct should be modified. This evaluation will be based on clinical judgment and the following guidance * Consider starting a new cohort of subjects using a QD dose up to 320 mg if there is endoscopic or histologic evidence of proximal colon benefit but limited or no distal colon drug exposure/efficacy. Also, there is evidence of potential overall efficacy (total Mayo score (TMS), MMS,PMS) outcomes and acceptable safety (adverse events (AEs)/vitals/clinical laboratory test results) and exposure (PK). * Consider to terminate the study if there is no evidence of drug exposure/efficacy in the colon observed by endoscopy or biopsy nor evidence of potential overall efficacy (TMS, MMS, PMS) outcomes or unacceptable safety(AEs/labs/vitals) or exposure (PK). * Continue the study with the GED-0301 160 mg QD dose. If the GED-0301 160 mg QD dose group is discontinued and a new dose group is added, an additional 40 subjects will be enrolled in the new dose group. Subjects enrolled subsequent to the decision to adjust the dose of GED-0301, will receive the following treatments: * Induction Phase - GED-0301, up to 320 mg QD, for 8 weeks; * Extension Phase- GED-0301, up to 320 mg on alternating dosing schedule (GED-0301, up to 320 mg QD for 4 weeks, followed by 4 weeks without GED-0301 treatment) for an additional 44 weeks. Subjects who do not achieve at least a 20% decrease in the PMS from baseline at Week 12 will be discontinued from the study. Actively enrolled subjects will not be affected by the dose adjustment. Subjects receiving corticosteroids at baseline will start tapering their corticosteroids at Week 8 (the end of the Induction Phase) if they achieve clinical response, defined as a decrease from baseline of at least 2 points and at least 25%, along with a reduction in the RBS of at least 1 point or an absolute RBS ≤ 1 in the MMS. The endoscopy subscore assessed by the investigator will be used for the calculation of the Week 8 MMS. The study will consist of 4 phases: * Screening Phase - up to 4 weeks * Induction Phase - 8 weeks * Extension Phase - 44 weeks * Observational Follow-up Phase - 4 weeks Subjects who complete the Extension Phase, and those subjects who prematurely discontinue from the study for any reason, will enter the post-treatment Observational Follow-up Phase, the 4-week period after the last dose of Investigational Product(IP). The study will be conducted in compliance with International Conference on Harmonisation (ICH) Good Clinical Practices (GCPs).