INTRODUCTION
Acute undifferentiated leukemia (AUL) is a rare subtype of acute leukemia that expresses neither lymphoid nor myeloid lineage-specific markers. AUL is reported to be 0.2–1% of all acute leukemias.1 Due to the rarity of AUL, an optimal treatment regimen has not been established. According to the treatment protocol for mixed phenotype leukemia, first-line chemotherapy is usually acute lymphoblastic leukemia (ALL)-type therapy, with patients who cannot achieve complete remission (CR) considered for acute myeloid leukemia (AML)- or combined-type therapy;2,3however, when first- and second-line treatment are ineffective, the choice of alternative therapy is very limited.
In the 2016 revised World Health Organization (WHO) classification, multiple antigens are required for B-lineage identification, including a combination of CD19 with CD79a, cytoplasmic (cy) CD22, and/or CD10.4 Expression of limited B-lineage-associated antigens, such as CD22, has been reported in some cases of AUL.5
CD22 is a glycoprotein widely expressed on normal B cells, as well as B-ALL cells.6 Inotuzumab ozogamicin (InO) is a CD22-targeted antibody-drug conjugate of calicheamicin, which is approved for treatment of relapsed/refractory CD22-positive B-cell ALL. Bhojwani et al. reported that use of InO for 51 children with relapsed/refractory ALL, who were heavily pretreated, resulted in 67% of patients achieving CR, and InO was well-tolerated;7however, no study has reported the efficacy of InO for CD22-positive AUL. Here, we report a case of CD22-positive AUL resistant to ALL- and AML-type therapy that achieved minimal residual disease (MRD)-negative CR using InO and who was able to undergo stem cell transplantation.