Efficacy and Safety Study of ABX464 as Maintenance Therapy in Patients With Moderate to Severe Ulcerative Colitis

Inclusion Criteria: * Patients must have completed the 16-week induction treatment period (ABX464-103); * Patients are able and willing to comply with study visits and procedures as per protocol; * Patients should understand, sign and date the written voluntary informed consent form prior to any protocol-specific procedures are performed; * Patients should be affiliated to a social security regimen (for French sites only); * Females and males receiving the study treatment (potentially in combination with immunosuppressant) and their partners must agree to use a highly effective contraceptive method during the study and for 6 months (180 days) after end of study or early termination. Contraception should be in place at least 2 weeks prior to screening. Women must be surgically sterile (bilateral oophorectomy, bilateral salpingectomy, or hysterectomy) or in the postmenopausal state (no menses for 12 months without an alternative medical cause) or if of childbearing potential must use a highly effective contraceptive method. Women of childbearing potential (WOCBP) will enter the study after confirmed menstrual period and a negative pregnancy test. Highly effective methods of contraception include true abstinence, intrauterine device (IUD) or hormonal contraception aiming at inhibition of ovulation, intrauterine hormone releasing system, bilateral tubal ligation, vasectomized partner. True abstinence is defined when this is in line with the preferred and usual lifestyle of the patient. In each case of delayed menstrual period (over one month between menstruations) confirmation of absence of pregnancy is required. This recommendation also applies to WOCBP with infrequent or irregular menstrual cycle. Female and male patients must not be planning pregnancy during the trial and for 6 months post completion of their participation in the trial. In addition, male patients should use condom during the trial and for 6 months (180 days) post completion of their participation in the study. Male patients must not donate sperm as long as contraception is required. Criteria that should be met by patients at week 48 to be eligible for 48 additional weeks of study treatment. * Patients should be in clinical response. Clinical response is defined as: a reduction in Modified Mayo Score ≥ 2 points and ≥ 30 % from baseline (induction) with an accompanying decrease in rectal bleeding sub-score ≥ 1 point or absolute rectal bleeding sub-score ≤ 1 point. * Patients able and willing to continue the study treatment and who are compliant with study visits and procedures and who signed the update of the written voluntary informed consent. Exclusion Criteria: * Patients who had major protocol deviation(s) in the induction study; * Patients who permanently discontinued study the treatment in induction study (ABX464-103) because of an adverse event (AE) regardless of relatedness to investigational product; * Patients who have developed any major illness/condition or evidence of an unstable clinical condition (except UC) that, in the investigator's judgment, will substantially increase the risk to the participant if he or she participates in the study; * Patients with any other severe acute or chronic medical or psychiatric condition or laboratory or electrocardiogram (ECG) abnormality that may increase the risk associated with study participation or investigational product administration or may interfere with the interpretation of study results and, in the judgment of the investigator, would make the patient inappropriate for entry into this study; * Patients who are participating or plan to participate in other investigational studies (other than induction study) during the study.