Treatment of Imminent Haematological Relapse in Patients With AML and MDS Following Allogeneic Stem Cell Transplantation With 5-azacitidine (Vidaza®)

Inclusion Criteria: Screening phase: * Age \> 18 years * Patients with CD34+ AML or MDS post-allogeneic HSCT * Written patient consent after consultation Treatment phase * AML/MDS: donor chimerism \< 80% in the CD34+ subpopulation following allogeneic HSCT in patients with CD34+ AML or MDS, but with no haematological relapse (blasts \< 5% in bone marrow) * Leukocytes \> 3 Gpt/l and platelets \> 75 Gpt/l (transfusion-independent) Exclusion Criteria: * Known intolerance to 5-azacitidine or mannitol * Uncontrollable infectious disease * Patients with active hepatitis B or C or HIV infection * Severe hepatic function impairment (ASAT and ALAT may not be above three times the normal value) or hepatic cirrhosis, or malignant hepatic tumour * Renal function impairment (creatinine \> twice the normal value, creatinine clearance \< 50 ml/min) * Pregnancy or lactation * Women of childbearing age, except for those who meet the following criteria: * postmenopausal (12 months natural amenorrhoea) * postoperative (6 weeks after bilateral ovarectomy with or without hysterectomy) * regular and correct use of a contraceptive method with an error rate \< 1% per year (e.g. implants, depot injections, combined oral contraceptives, intrauterine device - IUD, whereby hormonal coils with a Pearl Index of \< 1% are safer than copper coils) * sexual abstinence * Partner vasectomy * Men who do not use one of the following for contraception: * sexual abstinence * post vasectomy * condoms * Participation of the patient in a drug trial outside the indication of allogeneic transplantation up to four weeks before study initiation * Addictive or other illnesses that prevent the person concerned from comprehending the nature and impact, as well as potentical consequences of the clinical trial * Evidence that the patient may intentionally not comply with the protocol, e.g. lack of cooperation With the exception of a known allergic reaction or intolerance to 5-azacitidine, these criteria do not apply to the screening phase.