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PV and AL contributed equally Multiple studies from the EBMT registry and others have shown excellent survival rates after allogeneic haematopoietic stem cell transplantation (HSCT)for Wiskott-Aldrich syndrome (WAS) patients (Ozsahin et al, Blood 2008). The importance of myeloid engraftment for full disease correction has also been demonstrated (Moratto et al, Blood 2011). However, the vast majority of HSCTs in these studies were performed with (oral) busulfan/cyclophosphamide-based conditioning and in the early 2000 years or before. In 2005, the inborn errors working party (IEWP) of EBMT and ESID first recommended busulfan/fludarabine (BuFlu) or treosulfan/fludarabine (TreoFlu) based conditioning for primary immunodeficiencies such as WAS, with some centers deciding to add thiotepa (TT) to the conditioning. We performed a retrospective analysis via the EBMT and SCETIDE registries of WAS patients transplanted between 01/01/20006 and 12/31/2016 with these two regimens. The primary objective was to compare the overall (OS) and event-free survival (EFS) after HSCT with either BuFlu±TT or TreoFlu±TT conditioning. Secondary objectives included the influence of either conditioning regimen on acute and chronic GVHD, the degree of donor chimerism, incidence of secondary procedures after HSCT (2nd HSCT, stem cell boost, DLI, gene therapy or splenectomy) and rates of disease-specific complications after HSCT. At the time of this interim analysis, 174 patients were included, 92 (53%) with BuFlu±TT and 82 (47%) with TreoFlu±TT conditioning with a median age of 1.57 years (range 0.21-29.96) at HSCT and a median follow-up of 32.9 months (range 1.5-128.9). The donor was an HLA-matched sibling (MSD) in 30, a matched related donor (MRD) in 5, a matched unrelated donor (MUD, 9/10 or 10/10) in 105, a mismatched unrelated donor (MMUD, 5 years had a worse OS as compared to those 5 years or younger at HSCT (74.9% vs. 90.8%; log-rank test p=0.005). The type of donor had no influence on OS: 96.4% for MSD/MFD, 86.8% for MUD/MMUD and 87.7% for MMFD (log-rank test p=0.4). Whole blood chimerism was complete (>90% donor) in 60/75 evaluable patients (80%) at last follow-up or before secondary procedure (if a patient had one), 39/40 (98%) in the BuFlu±TT group and 21/35 (60%) in the TreoFlu±TT group. Twenty-six patients required a secondary procedure: stem cell boost in 4 patients, donor lymphocyte infusion in 9, 2nd HSCT in 15 and splenectomy in 1. Twenty-two of these 26 (84.6%) are alive and 14 of 16 with available chimerism data have a complete donor chimerism (>90%donor) at last follow-up. The 2-year cumulative incidence (CI) of secondary procedures was higher at 33.9% in the TreoFlu±TT versus 12.8% in the BuFlu±TT group (Gray's test p=0.017), and 2-year EFS (secondary procedure or death as event) was 61.4% in the TreoFlu±TT and 75.0% in the BuFlu±TT group (log-rank test p=0.2). Grade II-IV acute GVHD had the same incidence in both groups (100 day CI 24.4% vs. 26.3%; Gray's test p=0.849) and chronic GVHD of any grade was borderline more frequent in the TreoFlu±TT group (2 year CI 17.2% vs 6.7%; Gray's test p=0.054). The cumulative incidence of disease-specific complications occurring more than 6 months post HSCT (severe infections, bleeding, autoimmunity) was not different between the two groups (6.5% vs. 6.4%; Gray's test p=0.92). There was no malignancy reported after HSCT except for one EBV-post-transplant lymphoproliferative disorder (PTLD) 2.7 months after HSCT. In summary, HSCT with either BuFlu±TT or TreoFlu±TT conditioning reliably cures almost 90% of patients with WAS regardless of donor type. WAS-related complications are very rare events more than 6 months post HSCT. More patients required secondary procedures after treosulfan-based than busulfan-based conditioning. These data confirm the feasibility and efficacy of the regimens currently recommended by the IEWP. Disclosures Slatter: Medac: Other: Travel assistance. Chiesa:Gilead: Consultancy; Bluebird Bio: Consultancy. Kalwak:Sanofi: Other: travel grants; medac: Other: travel grants. Locatelli:Novartis: Consultancy, Membership on an entity's Board of Directors or advisory committees; bluebird bio: Consultancy; Bellicum: Consultancy, Membership on an entity's Board of Directors or advisory committees; Amgen: Honoraria, Membership on an entity's Board of Directors or advisory committees; Miltenyi: Honoraria. Sykora:Aventis-Behring: Research Funding; medac: Research Funding. Zecca:Chimerix: Honoraria. Veys:Pfizer: Honoraria; Servier: Research Funding; Novartis: Honoraria.

INTRODUCTION: Allogeneic hematopoietic cell transplantation (HCT) is a diffuse curative option for transfusion dependent thalassemia (TDT) and sickle cell disease (SCD). To verify transplant activity, distribution, demography, policies and outcomes the Hemoglobinopathy Registry was established inside the European Group for Blood and Marrow Transplantation (EBMT). After a previous analysis limited to TDT for the 2000-2010 period data (BMT 2016; 51:536-41), we performed an updated report considering TDT and SCD patients transplanted in the last eighteen years (years 2000-2017). METHODS: Data on pediatric patients transplanted between Jan 1st, 2000 through Dec 31st, 2017 were extracted by the EBMT promise Hemoglobinopathy registry database. Only first transplants were considered. Data are expressed as median with range unless specifically indicated. Survival probabilities were calculated with the method of Kaplan and Meier and expressed as means and 95% confidence intervals (95%CI). Differences between survival probabilities were tested by means of the log-rank test. RESULTS: In the above-specified period 3856 consecutive pediatric patients affected by TDT (2936, 76%) or SCD (920, 24%) were transplanted in 166 HCT centers distributed in 36 countries in Europe, Asia and Africa. Median age at transplant was 7.2 years (range 0.48-17.9). 3342 (87%) transplants were performed on patients Figure 1 reports pediatric transplant activity inside the EBMT showing an increased numbers of patients transplanted after year 2010. After a median follow up of 24 months, the 2 years overall survival (OS) and event-free survival (EFS) were 91% (95%CI 90-92) and 86% (95%CI 85-87) for the entire population. In TDT, OS and EFS were 90% (95%CI 89-92) and 84% (95%CI 82-85), respectively. In SCD, OS and EFS were 94% (95%CI 92-96) and 92% (95%CI 90-94), respectively. In both diseases no outcome difference was recorded on the basis of year of transplant (data not shown). Source of hematopoietic cells was bone marrow in 70% of TDT transplants and 81% of SCD transplants. In both diseases better results were recorded with the use of bone marrow versus peripheral blood [OS 91% versus 85% (P< 0.001); 95% versus 84% (P= 0.004) for TDT and SCD, respectively]. Similar results were recorded when EFS was analyzed. 176 patients received hematopoietic cells from single cord blood donors. Of them only 21 were from an unrelated donor. 119 patients received cord blood + bone marrow. Results of related HLA identical cord blood were similar to that of HLA identical related bone marrow (data not shown). Transplantation from an HLA identical sibling offered the best results in OS and EFS compared to other donor options (P< 0.001), both for TDT and SCD. Transplant results by donor type are reported in table 1. The threshold age for optimal transplant outcomes is confirmed 14 years, with OS 92% (95%CI 91-93), EFS 87% (95%CI 85-88) versus OS 85% (95%CI 82-89), EFS 81% (95%CI 77-85) ( P The two years incidence of cumulative extensive chronic graft versus host disease was 4.1% in TDT and 4.4 % in SCD (P=ns). CONCLUSIONS: Allogeneic HCT for TDT and SCD is a widely available curative approach; the procedure has been increasing and internationally performed during the years with excellent results. The emerging gene therapy approach will have to be compared to these well-established results. Disclosures Zecca: Chimerix: Honoraria. Forni:Novartis: Other: travel expenses, Research Funding; Celgene: Research Funding; Roche: Consultancy; Shire: Research Funding; Apopharma: Other: DSM Board. Bader:Medac: Patents & Royalties, Research Funding; Neovii: Research Funding; Novartis: Consultancy, Speakers Bureau; Riemser: Research Funding; Cellgene: Consultancy. Angelucci:Vertex Pharmaceuticals Incorporated (MA) and CRISPR CAS9 Therapeutics AG (CH): Other: Chair DMC; Novartis: Honoraria, Other: Chair Steering Comiittee TELESTO Protocol; Roche Italy: Other: Local (national) advisory board; Jazz Pharmaceuticals Italy: Other: Local ( national) advisory board; Celgene: Honoraria, Other: Chair DMC.