Outcome of Second Transplants in Pediatric Patients with Hematological Malignancies After a Hematopoietic Stem Cell Transplant (HSCT) From a Different Donor. Assessment of Chimerism by Real -Time PCR to Determine the Risk of Relapse

 A retrospective analysis of 13 patients (9 males, 4 females) aged 1-16 (median, 9) years  were transplanted at Lurie Children's between 2002 to 2011 (AML n= 2, ALL n=11).  Donor sources for the 1^st HSCT  were 8 match related sibling (MRS) and 5 unrelated (2/5 cord blood) and for the 2^nd HSCT were 5 MRS and 8 unrelated. Two conditioning regimens;  A: fractionated TBI 1200cGy in 150cGy fractions from day -7 to -4, Etoposide 1000 mg/m²  from day -3 and Cyclophosphamide 60 mg/Kg day -3 to day -1, 1^st HSCT (n=7) and 2^nd HSCT (n=6), B: Fludarabine 30 mg/m²/day from day -10 to -6, Busulfan (dose based on PK of the test dose to achieve an AUC 4000microM-min/day ) day -5, -4 and ATG (rabbit ATG) 2 mg/kg from day -4 to -9,  1^stHSCT (n=3) and 2^nd HSCT (n=10).  GVHD prophylaxis was regimen specific for regimen A (CSA/Tacro, short course MTX ± ATG) and regimen B (CSA/Tacro, MMF ±ECP). Chimerism was evaluated by real-time PCR (Applied Biosystems 7500) using AlleleSEQR  (Cellera), weekly after the second week, full donor chimerism (FDC) was defined as >98% ± 1%. Median time from diagnosis to 1^st HSCT was 202 days. The median time from the 1^st to the 2^nd HSCT was 531 days.  ANC > 500 /µL was achieved at a median of 15 (10-39) days in the 1^st, compared to 18.5 (10-25) days after the 2^nd (p= 0.7). Un-sustained platelet count >20.0/ µL was attained at a median of 16.5 (1-54) days and 19 (0-54) days at 1^st and 2^nd (p= 0.8), respectively. FDC was achieved at a median of 34.5 (12-63) days in 13/13 after the 1^st  and a median of 44 (22-108) days in 10/13 after the 2^nd. The median follow-up after the 2^nd HSCT was 1259 (35-3508) days. The Kaplan- Meier estimates of survival at 3 and 5 years were 69.2% and 43.3%, respectively (median follow up of 1735 days). Seven patients (1 with partial chimerism, 6 with FDC) are alive after the 2^nd HSCT, free of disease with a median follow up of 1562 (range 915-3508) days. Six patients have expired, three of cGVHD complications in remission (3 with FDC) and three patients expired from progressive leukemia (2 with partial chimerism). Patients who relapse after the 1^st myeloablative HSCT can be successfully treated with a 2^nd HSCT from a different donor. Patients who did not achieve a FDC after the 2^nd transplant were at a higher risk of relapse. One patient with partial chimerism has remained in CR after developing graft failure after the 2^nd HSCT, but achieving FDC from the first donor.  Patients who developed cGVHD after the 2^nd transplant with a FDC did not relapse but died of cGVHD complications in remission. All but one of the surviving patients has cGVHD. Chimerism analysis is the index for which the risk of relapse is determined. Larger number of patients will be needed to confirm these preliminary observations.