Efficacy of Reduced Intensity Conditioning (RIC) with FLU-BU-ATG and Allogeneic Hematopoietic Stem Cell Transplantation (HSCT) for Pediatric ALL.

Reduced Intensity Conditioning (RIC) regimens prior to allogeneic HSCT have gained acceptance in the treatment of adults with myelodysplasia, leukemia and multiple myeloma. RIC reduces the risk for regimen related morbidity and mortality enabling patients with pre-existing medical conditions that would have been precluded from allogeneic HSCT to attempt a curative approach. The resilience of pediatric patients (pts) following high-dose conditioning regimens and the concern that ALL cells are inherently more resistant to a graft-vs-leukemia effect have limited accrual of pediatric ALL pts to RIC protocols despite the potential benefit for reduced long-term morbidity. We report the experience of 10 pediatric ALL pts (6 M, 4 F, median age 9.5 years) treated for recurrent ALL with RIC and allogeneic HSCT. A uniform RIC regimen comprised of fludarabine, 30 mg/m² for 6 consecutive days (days −10 through −5), followed by intravenous busulfan, 0.8 – 1 mg/kg for 8 doses or targeted AUC 4000 μMol*min for 2 doses (days −5 and −4) and equine ATG, 40 mg/kg or rabbit ATG, 2 mg/kg for 4 days (days −4 through −1) was administered. Pts with prior CNS involvement received whole brain (2400 cGy) and spinal (1800 cGy) irradiation immediately prior to the RIC. Stem cell sources included 7 unrelated donors and 3 matched sibs. 9 of 10 stem cell donations were peripheral blood stem cells (PBSC). The median cell doses infused were 6.5 x 10⁸ MNC/kg and 4.2 x 10⁶ CD34+ cells/kg. Graft-versus-host disease (GVHD) prophylaxis was cyclosporin A (CsA) alone in 5 patients, CsA and mycophenolate mofetil in 5 pts. Growth factor support was not used. Each of the pts had at least two very high-risk features--prior HSCT (n = 6), CR > 3/refractory disease (8), prior CNS disease (6), Ph+ (2), pre-exisiting neurologic (1) or cardiac (1) dysfunction or aspergillous infection (1). Full donor chimerism was achieved in 9 of 10 with a median time to reach an ANC >500/μl of 16 days (range 11–62) and an unsupported platelet count > 20,000/μl was achieved in 8 of 10 at a median of 25 days (15–67). 2 pts developed Gr IV acute GVHD, 2 of 5 pts surviving more than 100 days developed chronic GVHD. Only 3 patients have relapsed: 1 refractory T-ALL pt recurred day +27 and 2 Ph+ pts had a molecular relapse day +61 and +196. The latter pt is in subsequent continuous molecular remission for over 1 year on imatinib therapy. 6 pts have died, 5 in the first 100 days of HSCT from complications of GVHD (2), relapse (1), pulmonary failure (in 1 pt S/p 3 prior allogeneic HSCT) and PTLD (1). 1 pt succumbed from complications of chronic GVHD day +756. The RIC regimen and supportive care are primarily an outpatient experience. During the first 30 days post HSCT, pts spent an average of only 9 days in hospital (23 of the first 100 days). Despite very high-risk features, 4 of 10 pts survive (3 CCR) at a median of 500 days post HSCT. Thus, RIC and allogeneic HSCT also offers promise for efficacy in pediatric ALL.