• Patients with CD38 positive T-ALL fitting either to the definitions for cohort 1 or cohort 2:

⁃ Cohort 1: In relapse or with primary refractory disease defined as ≥5% blasts in bone marrow after at least three chemotherapy cycles (induction I-II, consolidation I) with the following additional specifications:

• early relapse within 12 months from first achievement of CR or

• late relapse later than 12 months from first achievement of CR or

• primary refractory disease without any CR or

• any relapse after stem cell transplantation or

• any refractory relapse, defined as no response to at least one salvage therapy or

• any second or later relapse and

• Availability of patient material with blast cells (bone marrow or peripheral blood) for central MRD assessment or availability of respective predefined marker.

⁃ Cohort 2: In complete hematological remission (defined as less than 5% blasts in bone marrow and no evidence of extramedullary disease) after at least three chemotherapy cycles (induction I-II, consolidation I)

• Detection of quantifiable MRD at a level of ≥10-4, either as molecular failure without prior achievement of molecular remission or molecular relapse after prior achievement of molecular remission

• MRD assay at the central reference lab with at least one marker a minimum sensitivity of 10-4

• MRD detection for study inclusion after an interval of at least 2 weeks from last systemic chemotherapy including antibody therapy

• (in patients without clonal molecular MRD marker, MRD testing can be based on flow-cytometry established in reference laboratory)

⁃ ECOG status:

• Cohort 1: 0-2

⁃ Age ≥ 18 years Evidence of a personally signed and dated informed consent indicating that the patient has been informed of all pertinent aspects of the study Patient must be willing and able to comply with scheduled visits, treatment plan, laboratory tests and other study procedures

⁃ Regeneration from last chemotherapy defined as follows:

⁃ Cohort 1:

• Platelets ≥10.000/uL (platelet transfusion allowed)

• Hemoglobin ≥ 7.5 g/dl (red blood cell transfusion allowed)

⁃ Cohort 2:

• Neutrophils ≥1.000/uL

• Platelets ≥50.000/uL

• Hemoglobin ≥9 g/dl

⁃ Adequate liver function defined as follows:

• Bilirubin ≤ 1.5 ULN (unless Gilbert Meulengracht disease or classified as result of liver infiltration by investigator)

• AST and ALT ≤ 2.5 x ULN (unless classified as result of liver infiltration by investigator)

⁃ Adequate renal function defined as follows:

• Serum creatinine ≤ 2 x ULN

• Any serum creatinine level associated with a calculated creatinine clearance ≤ 40 mL/min

• Negative pregnancy test in women of childbearing potential (WOCBP)

• WOCBP must commit to either abstain continuously from heterosexual sexual intercourse or to use 2 methods of reliable birth control simultaneously.

• Men who are sexually active with a WOCBP must agree to use a barrier method of contraception

• Participation in the registry of the German Multicenter Study Group for Adult ALL (GMALL)