Allogeneic Hematopoietic Stem Cell Transplantation (allo-HSCT) with Different Intensity Conditioning Regimens in Children with Very High Risk Acute Lymphoblastic Leukemia (VHR ALL): A Single-Center Experience

Background: Allo-HSCT with myeloablative conditioning (MAC) has traditionally been performed in patients with ALL and has been associated with significant transplant-related morbidity and mortality. Reduced-intensity conditioning (RIC) is used in patients not eligible for conventional conditioning therapy due to poor performance status and severe toxic complications after previous chemotherapy. RIC is a well-established treatment strategy in adult patients with comorbidities or in elderly patients. In contrast to the adults, the RIC pediatric experience is still sparse.

The aim: to compare efficacy of reduced intensity conditioning (RIC) and myeloablative conditioning (MAC) for allo-HSCT in children suffering from very high risk (VHR) ALL.

Patients and methods: This was a retrospective analysis performed in 233 patients (pts) with ALL (age - from 4 month till 18 y.o. (mediana 11 y.o.)), who received allo-HSCT at R.M. Gorbacheva Memorial Institute of Children Oncology, Hematology and Transplantation between 12/2000 and 12/2017. MAC (busulfan or treosulphan containing) was used in 159 pts: 1st CR - 26 pts, 2nd CR -57 pts, 3rd or 4th CR 23 - pts, relapse -53 pts. Allo-HSCT with RIC was performed in 74 pts: 1st CR - 14 pts, 2nd CR -29 pts, 3^rd or 4th CR - 12 pts, relapse -20 pts. RIC consisted of fludarabine (30 mg/m²/d x 5 days) + melfalan (70 mg/m²/d x 2 days) or fludarabine (30 mg/m²/d x 5 days) + busulfan (4 mg/kg/d x 2 days). The disease status was not different between groups (p=0.674) Indication for RIC allo-HSCT was poor performance status (Lanskoy/Karnovsky<70%), organ dysfunction due to previous therapy or infectioous complications at the moment of allo-HSCT. Allo-HSCT from matched related donor was performed in 41 pts, from matched unrelated donor - in 131 pts, haploidentical - in 61 pts. Post-transplant Cy was included in GVHD prophylaxis regimens in 102 pts (44%).

Results: Median follow-up was 5 years for МАС, 4.5 years for RIC. In both groups the median time to neutrophil engraftment >=0,5x10⁹/l was D+18 (range D+8-48). Transplant engraftment was observed in 64 pts (86%) after RIC and 150 pts (93%) after MAC (p=0,224). OS pts after RIC allo-HSCT performed in 1 CR 70% and after MAC - 85% (p=0,043), in 2 CR 56% after RIC and 49% after MAC (p=0,436), in 3 or 4 CR 34% after RIC and 25% after MAC (p=0,394). OS in patients with active disease was 16% in RIC-group and 18% in MAC-group (p=0,432). The transplant-related mortality rate was 18% after RIC and 20% after MAC (p=0,40). Risk of relapse was 37% after RIC and 42% after MAC (p=0,39). Acute GVHD II-IVgr developed in 32% pts after RIC and 33% after MAC (p=0,883), early infections were diagnosed in 63% in RIC-group, and 59% in MAC-group (p=0,356). Early toxic complications were observed (CTC): mucositis III-IV gr in 13% pts after RIC and in 54% pts after MAC (p=0,001), nephrotoxicity in 7% pts and in 21% pts (p=0,042), hepatitis gr II-III in 24% pts and in 57% pts, respectively (p=0,034), neurological complications in 22% pts and 42% pts, resp. (p=0,046).

Conclusion: RIC allo-HSCT of VHR ALL in 2,3 or 4 CR pts ≤ 18 y.o. is effective and comparable with MAC allo-HSCT. Early toxic complications after RIC were less frequent.

Possibly, the use of RIC can maintain efficacy while minimizing regimen-related toxicity in children who have undergone previous intensive treatment.