Allogeneic Stem Cell Transplantation from Two Loci Mismatched (<= 6/8) Unrelated Donors in Acute Leukemia: On Behalf of the ALWP of the EBMT

Background Data on outcome of allogeneic transplantation (HSCT) from mismatched unrelated donors (MMUD) (≤6/8) in leukemic patients (pts) are limited. Methods We assess HSCT results in pts with AML/ALL in CR transplanted between 2000 and 2017 from ≤6/8 MMUD (2-4 HLA mismatches at the allelic level at loci A, B, C and DR) reported to the EBMT. Results A total of 465 pts were allocated, AML-320 and ALL-145. Median age was 50.2 years (range, 18-71.8) and 34.8 years (range, 18.2-62.9); 54% and 64% were males, respectively. Median F/U was 63 and 75 months, respectively. 69% and 66% were in CR1, while 31% and 34% were in CR2. 40% of AML pts had intermediate risk cytogenetics, 10% poor risk and 5% good risk (missing 45%),respectively. 37% of ALL pts were Ph positive, 22% Ph negative and 18% T ALL (17% B ALL with no Ph data,6% missing data). The number of HLA mismatches (HLAMM) was 6/8 - in 82% and 85%,5/8- in 11.5% and 12% and 3-4 in 6% and 3% of AML and ALL pts, respectively. 85% of AML pts and 77% of ALL pts received PBSC graft. Conditioning regimen was myeloablative in the majority of pts (89% of AML and 61% of ALL pts). 82% and 77% underwent in vivo TCD, respectively. GVHD prophylaxis was CSA/MTX in 51% and 67% and CSA/MMF in 21% and 10% of AML and ALL pts, respectively. Engraftment was achieved in 97% and 95% of pts, respectively. The incidence of acute (a) GVHD grade (Gr) II-IV and Gr III-IV was 34% and 41% and 14% and 21% for AML and ALL pts, respectively. Total and extensive chronic(c) GVHD rates at 2y were 38% and 38% and 23% and 14%, respectively. The main causes of death were disease recurrence (37% in both), GVHD (29% and 22%) and infections (22% and 26%).The 2 and 5y outcomes in the entire population were: RI-28% and 33%, NRM-28% and 30%; LFS-44% and 37%, OS- 50% and 41% and GRFS- 33% and 27%, respectively. More than 2 HLA mismatches were associated with a higher incidence of aGVHD Gr II-IV (29% vs 15%, respectively).In MVA, RI was lower for ALL vs AML HR (95% CI) 0.60, p=0.02. Disease status (CR2 vs CR1) was poor prognostic factor for RI, LFS and OS. Age was prognostic factor for NRM, LFS and OS. HLA mismatch at locus DR was poor prognostic factor, giving increased NRM: HR (95% CI) 1.68, p=0.02, and lower LFS: HR (95% CI), 1.42, p=0.03and OS HR (95% CI) 1.46, p=0.03and higher GVHD Gr II-IV HR (95% CI) 1.48, p=0.048. Results of MVA in AML pts in CR1 were similar. GVHD prophylaxis CSA/MMF in comparison to CSA/MTX resulted in higher NRM HR (95% CI) 2.2,p= 0.005 and lower GRFS HR (95% CI) 1.58, p=0.02.Notably, in 154 pts with AML in CR1 transplanted from ≤6/8 UD with T cell depletion c mismatches was associated with higher incidence of cGVHD HR (95% CI) 2.52, p=0.002. Summary and Conclusions HSCT from ≤6/8 MMUD transplantation can provide 40% 5y OS, 37% LFS and GRFS of 27% in leukemic pts. DR mismatch is a poor prognostic factor. cGVHD incidence at 5y is 41% with 21% being severe and is associated in AML pts with HLA C mismatches. CSA and MTX is the preferred GVHD prophylaxis.