Mercaptopurine Therapy in Ulcerative Colitis

Inclusion Criteria: 1. Confirmed diagnosis of UC by endoscopy and histopathology 2. Patients between 18 and 80 years of age 3. Active disease, despite oral treatment with at least 2g/day 5-ASA 4. Treatment with oral corticosteroids is required Exclusion Criteria: 1. Prior treatment with thiopurines 2. Prior treatment with biologics (e.g. anti-TNF agents and vedolizumab) 3. Current pregnancy (a pregnancy test will be performed if necessary according to the treating physician) 4. Chronic Obstructive Pulmonary Disease (COPD) 5. Acute coronary heart disease 6. (Bacterial) gastroenteritis has to be treated first 7. Coagulation disorders 8. Active malignancy 9. History of colonic dysplasia/cancer 10. Extensive colonic resection, i.e. subtotal colectomy with \<15 cm colon in situ 11. Concomitant therapy with drugs interfering with MP metabolism, like allopurinol, ribavirin or anti-epileptics. 12. Known systemic fungal infections or parasitic infections have to be treated first 13. Known duodenal or ventricular ulcus 14. Substance abuse, such as alcohol (\> 80 gram/day - one standard glass contains 10 gram of alcohol), I.V. drugs and inhaled drugs. If the subject has a history of substance abuse, to be considered for inclusion into the protocol, the subject must have abstained from using the abused substance for at least 2 years. Subjects receiving methadone within the past 2 years are also excluded 15. Positive tuberculosis screen (when a screening is performed at the discretion of the treating physician) 16. Active hepatitis B virus or hepatitis C virus infection defined as a positive anti-HCV, HBsAg and/or anti-HBcore screening. 17. Leucopenia (Neutrophil count \< 1,8x10\^9/L) 18. Thrombopenia (Platelets \< 90x10\^9/L) 19. Elevated liver enzymes (\>2x ULN) 20. Abnormal renal function (eGFR\< 30 mL/min) 21. Other conditions which in the opinion of the investigator may interfere with the subject's ability to comply with the study procedure