FMS‐like tyrosine kinase 3 (FLT3) is a type III receptor tyrosine kinase that plays an important role in hematopoietic cell survival, proliferation and differentiation. The most clinically important point is that mutation of the FLT3 gene is the most frequent genetic alteration and a poor prognostic factor in acute myeloid leukemia (AML) patients. There are two major types of FLT3 mutations: internal tandem duplication mutations in the juxtamembrane domain (FLT3‐ITD) and point mutations or deletion in the tyrosine kinase domain (FLT3‐TKD). Both mutant FLT3 molecules are activated through ligand‐independent dimerization and trans‐phosphorylation. Mutant FLT3 induces the activation of multiple intracellular signaling pathways, mainly STAT5, MAPK and AKT signals, leading to cell proliferation and anti–apoptosis. Because high‐dose chemotherapy and allogeneic hematopoietic stem cell transplantation cannot sufficiently improve the prognosis, clinical development of FLT3 kinase inhibitors expected. Although several FLT3 inhibitors have been developed, it takes more than 20 years from the first identification of FLT3 mutations until FLT3 inhibitors become clinically available for AML patients with FLT3 mutations. To date, three FLT3 inhibitors have been clinically approved as monotherapy or combination therapy with conventional chemotherapeutic agents in Japan and/or Europe and United states. However, several mechanisms of resistance to FLT3 inhibitors have already become apparent during their clinical trials. The resistance mechanisms are complex and emerging resistant clones are heterogenous. Further basic and clinical studies are required to establish the best therapeutic strategy for AML patients with FLT3 mutations.

Allogeneic hematopoietic stem cell transplantation (allo-SCT) using post-transplant cyclophosphamide (PTCy) is increasingly performed. We conducted a multicenter phase II study to evaluate the safety and efficacy of PTCy-based HLA-haploidentical peripheral blood stem cell transplantation (PTCy-haploPBSCT) after busulfan-containing reduced-intensity conditioning. Thirty-one patients were enrolled; 61% patients were not in remission and 42% patients had a history of prior allo-SCT. Neutrophil engraftment was achieved in 87% patients with a median of 19 days. The cumulative incidence of grades II to IV and III to IV acute graft-versus-host disease (GVHD) and chronic GVHD at 1 year were 23%, 3%, and 15%, respectively. No patients developed severe chronic GVHD. Day 100 nonrelapse mortality (NRM) rate was 19.4%. Overall survival, relapse, and disease-free survival rates were 45%, 45%, and 34%, respectively, at 1 year. Subgroup analysis showed that patients who had a history of prior allo-SCT had lower engraftment, higher NRM, and lower overall survival than those not receiving a prior allo-SCT. Our results suggest that PTCy-haploPBSCT after busulfan-containing reduced-intensity conditioning achieved low incidences of acute and chronic GVHD and NRM and stable donor engraftment and low NRM, particularly in patients without a history of prior allo-SCT.

Key Points KIT exon 17 mutation is a poor prognostic factor in AML patients with RUNX1-RUNX1T1, but not in those with CBFB-MYH11. NRAS mutation is a poor prognostic factor in AML patients with CBFB-MYH11.

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We analyzed 3 hematopoietic stem cell transplant (HSCT) recipients with inherited chromosomally integrated human herpesvirus-6 (inherited CIHHV-6). Cases 1 (inherited CIHHV-6A) and 2 (inherited CIHHV-6B) were inherited CIHHV-6 recipients. Case 3 received bone marrow from a donor with inherited CIHHV-6B. Following HSCT, HHV-6B was isolated from Case 1. HHV-6A and -6B messenger RNAs were detected in Cases 1 and 3.

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Late graft failure (LGF) without evidence of residual recipient cells is a serious complication after allogeneic hematopoietic stem cell transplantation (allo-SCT) and often requires stem cell infusion from the same donor when the patient fails to respond to conventional therapies. We screened the peripheral blood (PB) of 14 patients who developed donor-type LGF at 2 to 132 months after allo-SCT for the presence of the markers for immune-mediated bone marrow (BM) failure. Increased glycosylphosphatidyl inositol-anchored protein-deficient (GPI-AP) leukocytes, which accounted for .009% to 0.147% of the total granulocytes, were detected in 5 patients (severe aplastic anemia, n = 2; follicular lymphoma, n = 1; acute lymphoblastic leukemia, n = 1; myelodysplastic syndromes; n = 1) and 4.7% to 81.2% HLA-allele-lacking leukocytes (HLA-LLs) were detected in 2 patients (acute myelogenous leukemia, n = 1; and myelodysplastic syndromes, n = 1). Three of the 5 patients with increased GPI-AP leukocytes were treated with antithymocyte globulin (ATG), and 2 patients achieved transfusion independence. These results suggest that immune mechanisms that are similar to acquired aplastic anemia underlie condition of approximately one-half of the patients with donor-type LGF, and that in patients with increased GPI-AP cells, donor-derived hematopoiesis may be restored by ATG therapy alone without donor stem cell infusion.