Tafasitamab (MOR00208) in Pediatric Patients With Relapsed or Refractory Acute B Lineage Leukemia

Inclusion Criteria: * Age ≥ 3 years and \< 18 years at enrollment * B-lineage (CD19 positive) ALL (B, pro-B, pre-B or c-ALL) * Patients must have either * underwent a first allogeneic stem cell transplantation after relapse with one of the following very high-risk somatic molecular alterations: * KMT2A::AFF1 \[t(4;11) rearrangement * TP53 alteration (mutation/deletion) * low hypodiploidy (\<40 chromosomes, evident or masked) * TCF3-PBX1 \[t(1;19)\] * TCF3::HLF \[t(17;19)\] and irrespective of MRD after SCT or * underwent a first allogeneic stem cell transplantation or a CAR T-cell therapy with newly emerging or persistent MRD load posttransplant / post CAR T- cell-treatment or * have received stem cell transplantation without having reached a sufficient molecular remission prior to transplant (defined as MRD ≥10E-4) irrespective of MRD after SCT or * underwent a second or subsequent allogeneic stem cell transplantation irrespective of MRD after SCT * Females of childbearing potential (FCBP1) must agree * to utilize two reliable forms of contraception simultaneously or practice complete abstinence from heterosexual contact for at least 3 months before starting study drug, while participating in the study (including dose interruptions), and for at least 3 months after study treatment discontinuation and must agree to regular pregnancy testing during this timeframe * to abstain from breastfeeding during study participation and 3 months after study drug discontinuation. * Males must agree * to use a latex condom during any sexual contact with FCBP while participating in the study and for 3 months following discontinuation from this study, even if he has undergone a successful vasectomy * to refrain from donating semen or sperm during study participation and for 3 months after discontinuation from this study treatment. Exclusion Criteria: * Frank relapse (\>5% leukemic blasts) * Philadelphia chromosome-positive (Ph+) ALL * Ejection fraction \<25% on echocardiography * Cystatin C-clearance \<40ml/min * Liver function abnormalities with bilirubin \>4 mg/dL and elevation of transaminases higher than 400 U/L * Severe infection (HIV, Chronic active viral hepatitis), tests have to be conducted at screening * Acute GvHD III-IV or extensive chronic GvHD * The following immunosuppressive drugs (≥ 1 week of administration): steroids ≥ 1mg/kg body weight, cytostatics (except intrathecal/ intracerebroventricular application for CNS treatment) * Application of other experimental therapy modalities in the last 4 weeks * Significant psychiatric disabilities, uncontrolled seizure disorders or severe peripheral neuropathy/ leukoencephalopathy * Signs of autoimmune disease (i.e. idiopathic thrombocytopenic purpura, autoimmune hemolytic anemia) * Subjects that do not agree to refrain from donating blood while on study drug * Concurrent severe or uncontrolled medical disease which by assessment of the treating physician could compromise participation in the study * Women during pregnancy and lactation * History of hypersensitivity to the investigational medicinal product or to any drug with similar chemical structure or to any excipient present in the pharmaceutical form of the investigational medicinal product.