Outcome of HLA-Mismatched Unrelated Donor Allogeneic Stem Cell Transplantation Depends On the Intensity of the Conditioning Regimen

Abstract 3393 Poster Board III-281 Background: HLA-mismatched unrelated donors are increasingly becoming a graft source for both standard myeloablative (MAC) and reduced intensity conditioning (RIC) hematopoetic stem cell transplantation (HSCT). Patients and methods: We retrospectively analysed the effects of HLA-mismatching in 553 patients undergoing MAC (n = 342) or RIC (n = 211) unrelated donor HLA-matched (n = 289) or -mismatched (n = 264) HSCT with antithymocyte globulin as part of conditioning. Patient characteristics well matched between HLA-matched and –mismatched groups of MAC and RIC patients. Results: Median follow-up was 1946 days for MAC patients and 765 days for RIC patients. In MAC patients, there was no difference in the incidence of aGvHD, treatment related mortality (TRM) and overall survival (OS) between recipients of HLA-matched vs. –mismatched allografts. In RIC patients on the contrary, the incidence of aGvHD II-IV (46%, vs. 32.0% p = 0.05), III-IV (18%, % vs. 9.0 p = 0.07) and TRM (35% vs. 20%, p = 0.04) were higher and OS lower (31% vs. 50%, p = 0.001) for recipients of HLA-mismatched vs. –matched transplants. In the RIC patients, female donor gender (RR: 1.8; p = 0.02) and HLA-mismatch (RR: 1.8; p = 0.02) negatively influenced TRM while female donor gender (RR: 1.58, p = 0.03), HLA-mismatch (RR: 1.82, p = 0.003) and bad risk disease (RR: 2.14, p < 0.001) negatively impacted OS. Conclusion: Our analyses surprisingly reveal that the positive effect of ATG in HLA-mismatched transplants is only limited to patients undergoing standard myeloablative conditioning. HLA-mismatching and female donor gender negatively impact outcome of RIC allogeneic unrelated donor HSCT despite the use of ATG, highlighting the need for improved strategies. Acknowledgments We thank the staff of the BMT unit for providing outstanding care to our patients and the medical technicians for their excellent work in the BMT laboratory. Disclosures: Off Label Use: Antithymocyte globulin for prevention of severe GVHD.