A Novel Chemotherapy-Based Allogeneic Hematopoietic Stem Cell Transplant Regimen for Very Young Children with Leukemia.
Authors: Prockop Susan E. , Kernan Nancy A. , Klein Elizabeth G , Kobos Rachel , Scaradavou Andromachi , Small Trudy N. , O'Reilly Richard J. , Boulad Farid 
Published: October 2019

Abstract
Abstract 3149
Young children in need of allogeneic hematopoietic stem cell transplant (HSCT) are at increased risk of unacceptable side effects from total body irradiation (TBI) and have historically been considered candidates for non-TBI containing regimens. However, disease free survival (DFS) has been poorer in cohorts of very young patients transplanted without TBI and novel chemotherapy based regimens are needed. We report results in a cohort of 14 children all under three years of age at the time of transplant (6 – 32 months; median 19.8 months) using a clofarabine-based ablative regimen. Fourteen patients in this age group have undergone transplant with a regimen consisting of clofarabine 20 mg/m2/day × 5, thiotepa 10 mg/Kg/day × 1 and melphalan 70 mg/m2/day × 2. All patients had high risk disease. Seven (7) pts were transplanted for ALL, 6 for AML and 1 for JMML. Patients with ALL or AML in first remission (CR1) or CR2, were categorized as patients with good risk disease while all other pts were considered as poor risk irrespective of all other factors. Transplant risk was good for 6/7 with ALL, and 3/6 with AML. The patient with JMML had stable disease. Stem cell grafts consisted of unmodified bone marrow (BMT) (N=6), double cord blood (dCBT) (N=7) and T cell depleted PBSCT (N=1). Donors were matched unrelated (N=5) or mismatched unrelated (N=9) including 7 double umbilical cord blood grafts, and one T cell depleted graft. Graft versus host disease (GvHD) prophylaxis was with tacrolimus and methotrexate for unmodified BMT, tacrolimus and mycophenolate for dCBT or T cell depleted HSCT. Two patients died early post transplant of infection (1) and acute GvHD (1). Neutrophil engraftment for the 13 evaluable patients was at a median of 13 days (10 –29 days) for PBSC and BM grafts and 17.5 days (12 –23 days) for recipients of CB grafts. Platelet engraftment for the 12 evaluable patients was at a median of 23 days (16 – 36 days) for recipients of PBSC and BM grafts and 43.5 days (36 –66 days) for recipients of CB grafts. In all five patients developed grade II-IV GvHD, and two patients chronic GvHD. Seven patients developed transaminitis which resolved in all cases. No patients developed Grade IV mucositis. One patient (AML) died after relapsing 5.5 months post transplant. Two patients are alive after relapsing at 1.3 months (AML) and 10.8 months (JMML) post-transplant. Nine of the 14 patients are alive in continuous complete remission seven of whom are greater than 36 months from transplant (40.2 – 71 months). The seven patients without chronic GvHD have had robust immune reconstitution, have responded to vaccination, and continue to meet growth and developmental milestones. Only one patient (transplanted at 14 months of age) has mild neurocognitive deficits. This novel chemotherapy based regimen is associated with durable engraftment of unmodified and cord blood HSCT grafts and promising disease free survival in very young children with leukemia. Based on the low toxicity profile in this cohort of patients higher dosing of clofarabine will be explored as a possible way to improve leukemia remission in the highest risk patients.

Disclosures:
Off Label Use: Clofarabine.