Allogeneic Stem Cell Transplantation vs. Conventional Therapy as Salvage Therapy for Relapsed / Progressive Patients With Multiple Myeloma After First-line Therapy (AlloRelapseMM)

Inclusion Criteria: Patients eligible for study inclusion must meet criteria 1- 7 at registration and all of the following criteria before randomization: Multiple Myeloma Age 18 - 65 years A signed informed consent form must be obtained before participation in the study Age 66 - 70 years, if comorbidity index according to Sorror score = 0 and ECOG ≤ 1 1st relapse/ progression according to IMWG criteria after first-line therapy (consisting of induction therapy followed by autologous transplantation once or twice and maintenance therapy), Additionally: meeting the need for treatment based on the SLiM criteria Negative pregnancy test in female patients Maximum of 1 cycle salvage therapy prior to study inclusion Availability of a fully compatible stem cell donor (HLA-ident. Sibling or 10/10 MUD or 9/10 MMUD if mismatch affects DQB) after 3 cycles salvage therapy CR/PR or SD according to IMWG-criteria after 3 cycles salvage therapy within the study Exclusion Criteria: Patients are excluded from the study if any one of criteria 1-6 are met at registration and if criterion 7 is met before randomization: Non-sufficient organ function defined as: Bilirubin (in the absence of Meulengracht's disease), SGPT or SGOT ≥3 higher than normal values Cardiac ejection fraction ≤ 50% GFR < 30 ml/min DLCO < 50% and/or continuous oxygen dependency Active hepatitis B or C infection or uncontrolled HIV infection Other, active malignant disease Prior treatment with allogeneic stem cells Participation in a clinical trial or taking an IMP within 30 days or five times the half-life of the IMP, whichever is longer, prior to registration Positive serum pregnancy test at screening and before first treatment or breastfeeding PD under salvage therapy