Ex Vivo T Cell-Depleted Haploidentical HCT for Children with Acute Leukemia after a Reduced-Intensity Preparative Regimen Containing Low-Dose TBI

Background: Recent advances in haploidentical hematopoietic cell transplantation (HHCT) enabled this transplant using haploidentical family donor to be a viable option for pediatric patients lacking matched related or unrelated donor. In our center, HHCT using ex vivo T cell-depleted (TCD) grafts after reduced-intensity conditioning (RIC) was conducted since 2008. The safety and efficacy of this transplantation modality for pediatric with acute leukemia were investigated.

Methods: Thirty-one pediatric patients with acute leukemia received ex vivo T cell-depleted HHCT at Asan Medical Center Children's Hospital between July 2008 and June 2016. Four patients received CD3-depleted grafts and 27 received TCRαβ-depleted stem cells. Among 31 patients, 9 had ALL (3 CR1, 6 CR2-3), 22 had AML (18 CR1-3, 4 NR). Seven patients had relapsed after previous allogeneic HCT. All 31 patients underwent a uniform RIC regimen consisting of low-dose total body irradiation (LD-TBI; 600 cGy), fludarabine (FLU; 180 mg/m²), cyclophosphamide (CY; 100 mg/kg), and rabbit anti-thymocyte globulin (r-ATG; 3 mg/kg).

Results: The median age at HHCT was 14 years (range, 1-19). All 31 patients achieved sustained neutrophil engraftment at a median of 10 days (range, 9-17) post-transplant. The cumulative incidence of acute GVHD grade II-III and III were 30% and 21%, respectively. None developed grade IV. Two of 26 evaluable patients developed extensive chronic GVHD. As of July 2016, 18 of the 31 patients survive free of disease with a median follow-up of 26 months (range, 2-98 months). Ten patients have died. Causes of death were relapse (n=9) and disseminated tuberculosis (n=1). Only one patient died of non-relapse cause, leading to TRM of 5.3% at 1 year. EFS and OS at 2 years for all patients were 51% and 60%, respectively. Sixteen patients with AML who received a first HHCT in any CR showed a favorable outcome (EFS of 85%), whereas, 6 patients with ALL who received a first HHCT in CR showed a poor EFS of 28%. In addition, all patients (6 with AML and 3 with ALL) who received a subsequent HCT in CR or were not in remission developed relapse.

Conclusions: This study demonstrated that our ex vivo T cell-depleted HHCT using RIC is a feasible therapy with low TRM for pediatric patients with acute leukemia. The outcome of patients with AML who received their first transplant in CR was excellent in this treatment modality. However, the outcome of ALL was poor suggesting that more intensified conditioning regimen may be required for those diseases. Furthermore, an innovative treatment strategy is warranted to improve the outcome for patients with relapsed or refractory acute leukemia.