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Systemic mastocytosis (SM) is a rare hematologic neoplasm characterized by abnormal growth and accumulation of tissue mast cells (MC) in various organ systems, including bone marrow (BM). Indolent and advanced forms of SM have been described. Whereas patients with ISM have a normal or near normal life-expectancy, patients with advanced SM, including those suffering from mast cell leukemia (MCL) have a poor prognosis. In these patients, neoplastic MC are usually resistant against conventional drugs and various targeted drugs. In rapidly progressive aggressive SM (ASM) and MCL, polychemotherapy followed by allogeneic hematopoietic stem cell transplantation (alloHCT) has been proposed. However, outcome of alloHCT in advanced SM is unknown, and it also remains uncertain whether clinically relevant graft-versus-SM (GVSM) effects may occur in these patients, as only sporadic case reports have been published. We performed a retrospective multi-center analysis to evaluate the outcome of alloHCT in patients with advanced SM. Fifty-four advanced SM patients receiving SCT in 32 transplantation centers in Europe and America were identified between 1990 and 2013. The median patient age was 45 years. Donors were: HLA identical siblings (31), unrelated donors (URD) (15), umbilical cord blood donors (UCB) (2), and haploidentical donors (1). In 5 patients, stem cell source was not defined (5). Thirty-four patients received myeloablative conditioning (MAC) and 18 received reduced intensity conditioning regimens (RIC). In 2 patients, conditioning regimen was not specified. Indications for alloHCT were SM with an associated clonal hematologic non-mast cell lineage disease (SM-AHNMD) (n=32), MCL (n=13, including one with MCL-AHNMD), 8 with ASM and 1 with myelomastocytic leukemia (MML). The most prevalent AHNMD was acute myeloid leukemia (AML, n=16). With follow-up of 35-6180 (median 365) days, SM responses (defined as ≥50% decrease in BM mast cells ± decrease in serum tryptase ± regression of other organ manifestations) were observed in 39 patients (72%), including complete responses (CR) documented in 12 patients (22%). Eleven patients had stable disease, whereas 4 patients (7%) progressed immediately after alloHCT (primary resistance). In addition, 10 patients progressed (5 of them within 100 days) after an initial response. Progression was most frequently seen in MCL patients (n=6, 50%). In the AHNMD group, only 8 patients relapsed/progressed (25%). The overall survival (OS) and SM progression-free survival (PFS) at 1 year were 63% and 50% for all patients, 77% and 68% for SM-AHNMD, 63% and 50% for ASM, and 25% and 17% for MCL, respectively. The strongest predictive variable associated with inferior survival was a diagnosis of MCL. Other factors associated with poor outcome were: Karnofsky performance status ≤70%, ≥2 SM regimens given before alloHCT (e.g., steroids, cladribine, chemotherapy, tyrosine kinase inhibitor), donor source (alternative donors-UCB and haploidentical compared to sibling or URD), SM progression within the first 100 days, normal cytogenetics (compared to t(8;21) (q22;q22), and RIC (compared to MAC). The following variables were not associated with poor outcome: patient and donor age, recipient-donor sex match status, graft source (BM vs. peripheral stem cells), BM mast cell percentage at time of alloHCT, and CR status of AML or SM response at time of alloHCT. This largest multi-center analysis of results in advanced SM provides evidence for clinical efficacy of alloHCT, presumably because of a GVSM effect of alloHCT (achieving CR, and response to donor-lymphocyte infusions and RIC alloHCT). However, responses varied among different SM categories: while patients with SM-AHNMD enjoyed excellent outcomes, the OS for MCL patients in general, was poor. Nevertheless it is remarkable that 3 of 13 patients with MCL – an otherwise fatal disease with a median survival of Disclosures: Vercellotti: Sangart Inc.: Research Funding; Seattle Genetics: Research Funding. Akin:Novartis: Consultancy. Valent:Novartis: Consultancy, Honoraria, Research Funding.