Lineage switch under blinatumomab treatment of relapsed common acute lymphoblastic leukemia without MLL rearrangement

Abstract Blinatumomab is a bispecific T‐cell engaging αCD19 antibody used in refractory or relapsed B‐cell precursor acute lymphoblastic leukemia (ALL). Recently, lineage switch to a myeloid phenotype has been described following CD19 targeting treatment in three pediatric patients with mixed lineage leukemia (MLL) rearranged ALL. We report the case of a female who received blinatumomab for a first relapse of ALL without MLL alterations. She suffered from a second relapse early after hematopoietic stem cell transplantation and was treated with blinatumomab again. During this treatment, the leukemia lost CD19 expression as well as nearly all other B‐cell markers, while still harboring the initial minimal residual disease marker, and switched to a myeloid phenotype.