Study of the Safety and Efficacy of Zoenasa® Versus Mesalamine Enema in Subjects With Left-Sided Ulcerative Colitis

Inclusion Criteria: * Male and female subjects are eligible if they are ≥ 18 years of age and ≤ 64 years. * They have a documented history of idiopathic ulcerative colitis based on endoscopic and/or histologic findings involving the left side of the colon, with mild to moderate active disease. * Eligible subjects will have a documented history of ulcerative colitis, and a modified UCDAI score of 4-10, inclusive, with a Physician's rating of disease score of 2 points or less (mild or moderate active ulcerative colitis), rectal bleeding score of 1 or more (based on subject diary), and mucosal appearance score (based on endoscopy) of 1 point or more at baseline. * Laboratory data: * White blood cell count between 4.0 - 12.0 K/mm3 * Platelet count: 150 - 500 K/mm3 * Hemoglobin \> 10.0 g/dL * Total bilirubin \< 1.5 mg/dL * Aspartate aminotransferase \< 100 u/dL * Alanine aminotransferase \< 100 u/dL * Alkaline phosphatase \< 250 u/dL * Blood urine nitrogen \< 40 mg/dL * Creatinine \< 1.5 mg/dL * Satisfies one of the following: * Female subjects of childbearing potential must have a negative urine pregnancy test at screening; surgically sterile, post-menopausal, abstinent, or patient or partner agree to use a medically appropriate form of birth control from screening to until 1 month after the last dose of study medication. * Male subjects must be surgically sterile, abstinent, or patient or partner compliant with a contraceptive regimen from screening to until 1 month after the last dose of study medication. * They are able to understand and sign a written informed consent form, which must be obtained prior to initiation of study procedures. Exclusion Criteria: * They have documented history of proximal or universal ulcerative colitis, proctitis or active proctitis confined to 15cm or less from the anal verge. * They demonstrate signs and symptoms of fulminant colitis, bowel stricture, toxic megacolon, an anticipated need for blood transfusion for gastrointestinal bleeding, or demonstrate evidence of peritonitis. * They receive a Physician's rating of disease severity as part of the modified UCDAI of 3 (severe disease) or an aggregate score of 11 or greater. * They have shown prior documented history of evidence of high grade dysplasia on biopsy from endoscopic examinations. * Their stool contains enteric pathogens or Clostridium difficile toxins. * They have a history of recurrent Clostridium difficile infection. * They have prior history of biologic therapy within the previous 4 years. * They have received systemic steroids or immunosuppressants within the previous 4 weeks. * Treatment in the last 14 days that included antibiotic, antifungal, antiparasitic medications, or rectally administered steroids (e.g. Cortenema®) or mesalamine enema (Rowasa®). * Treatment in the last 7 days that included mesalamine (5-ASA) via oral administration (e.g. Asacol®, Lialda®, balsalazide, etc). * They have a history of cancer (defined as malignancy within 5 years except for squamous cell or basal cell cancers of the skin), asthma, or bronchospasm. * Positive pregnancy test or lactating subjects. * There is evidence of chemical substance abuse. * They have had repeated anti-inflammatory drug treatment (longer than 3 days at doses that exceed those available without a prescription) within the previous 7 days (with exception of aspirin at doses of 325mg/day or less for prophylaxis of cardiac disease), or initiated new non-steroidal anti-inflammatory (NSAID) treatment within the last 30 days. * They have a known allergy to N-acetylcysteine or mesalamine, or have a history of serious AEs related to their use (including, but not limited to pancreatitis or hepatitis). * They have a history of failure to retain enemas. * Other clinically significant diseases that could interfere with the protocol compliance appear. These would include clinically important hematological, renal, hepatic, metabolic, psychiatric, central nervous system (CNS), pulmonary or cardiovascular disease. * Use of any investigational medication within the previous 90 days. * Any condition which the study physician judges to preclude safe participation in the study or to confound the evaluation of the study outcome.