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Introduction Acute and chronic graft vs. host disease (a/cGVHD) are major causes of treatment failure and non-relapse mortality (NRM) in multiple myeloma (MM) patients (pts) undergoing allogeneic hematopoietic cell transplantation (allo-HCT). Use of post-transplant cyclophosphamide (PTCy) is now an established method for GVHD prophylaxis after HLA haplo-identical (haplo)-HCT. We previously reported data in MM pts undergoing haplo-HCT (EBMT/CIBMTR) and showed that PTCy was associated with promising overall survival (OS) (Sahebi et al., BBMT 2019). However, data using PTCy for GVHD prophylaxis in other donor types are very limited in MM. Methods We evaluated PTCy as GVHD prophylaxis in MM pts who underwent a first allo-HCT using matched related (MRD), matched unrelated (MUD), mismatched related or unrelated (MMRD/MMUD, one antigen), and haplo donors within EBMT centers. OS and progression free survival (PFS) were determined by means of the Kaplan-Meier estimator. Neutrophil and platelet engraftment, relapse and NRM, and a/cGVHD were analyzed individually in a competing risks framework with relapse and death as competing events. Cox proportional hazards regression was used in the multivariable analyses. All estimates include 95% confidence intervals. All included covariates are listed in the Table. Patient Characteristics Between 2012-2018, a total of 295 MM pts received PTCy as GVHD prophylaxis. Median age at transplant was 55 yrs. Allo-HCT was given at a median interval of 34.9 mo from MM diagnosis. The conditioning regimen included reduced intensity (RIC, 193, 65.4%) and myeloablative (MAC, 102, 34.6%). All pts except 10 (3.4%) had prior autologous HCT; all of these 10 pts received a haplo-HCT. GVHD prophylaxis included PTCy + cyclosporine A or tacrolimus +/- mycophenolate mofetil in the majority of patients (239, 81%), and this combination was used in nearly every patient with haplo-HCT. Overall, peripheral blood was used as the stem cell source in about 80% of pts, but bone marrow was used in nearly 40% of haplo-HCTs. Results (Median and 95% confidence interval are provided)1) Median time to neutrophil engraftment was 19 d (18-19 d) with no apparent difference among donor types.2) Median time to platelet engraftment was 23 d (21-26 d), whereas haplo-HCT engraftments were longer (27 d (25-33 d)).3) NRM at 1 yr was 18% (12-22%) and at 2 yrs was 19% (14-24%), with no significant difference among different donor types. Age < 50 yrs significantly decreased NRM to 9% (2-16%, p=0.027) at 2 yrs.4) Cumulative incidence of grade II-IV aGVHD at +100 d was 30% (25-36%), and 1 yr cGVHD was 27% (21-32%), with no apparent difference among donor types.5) OS was 63% (57-69%) at 1 yr and 51% (45-58%) at 2 yrs for the whole group, with no statistical difference among different donor types after a median follow-up of 26.1 mo. Disease status at transplant < PR significantly decreased OS to 35% (22-47%, p=0.005) at 2 yrs.6) PFS was 42% (36-49%) at 1 yr and 26% (20-32%) at 2 yrs for the whole group without apparent significant difference among donor types. Disease status at transplant < PR significantly decreased PFS to 16% (6-26%, p=0.028) at 2 yrs.7) However, in multivariable analyses, donor type using haplo, HR 1.65 (0.56-1.67, p=0.03), was associated with increased mortality in addition to disease status at allo-HCT < PR, HR 1.93 (1.25-2.97, p=0.003). Finally, MUD was associated with an improved PFS, HR=0.63 (0.4-0.99, p=0.04). Disease status < PR, HR=1.85 (1.24-2.74, p=0.002) was again associated with inferior PFS. Summary PTCy in MM patients undergoing allo-HCT throughout donor types resulted in a low incidence of aGVHD grade II-IV of 30% and cGVHD of 27%, with OS of 51% and PFS of 26% at 2 yr. Our first donor comparison using PTCy revealed improved survival in matched (MRD and MUD) versus haplo allo-HCT after adjusting for other risk factors. Disclosures Blaise: Jazz Pharmaceuticals: Honoraria. Tarella:TG-therapeutics: Research Funding; ImmunoGen: Research Funding; ADC Therapeutics: Membership on an entity's Board of Directors or advisory committees, Research Funding. McDonald:venetoclax advisory board in South Africa (in CLL context): Consultancy; Alberts Cellular Therapy: Current Employment. Milpied:Roche: Honoraria, Other: Travel support; Astellas: Honoraria; Gilead Sciences: Other: consultancy or advisory role; Celgene: Other: Travel support; Sandoz: Honoraria, Other: consultancy or advisory role; Janssen: Honoraria. Yakoub-Agha:Celgene: Honoraria; Novartis: Honoraria; Gilead/Kite: Honoraria, Other: travel support; Janssen: Honoraria; Jazz Pharmaceuticals: Honoraria. Schonland:Janssen: Honoraria, Other: travel support to meetings, Research Funding; Prothena: Honoraria, Other: travel support to meetings, Research Funding; Takeda: Honoraria, Other: travel support to meetings, Research Funding.

Mantle cell lymphoma (MCL) is an aggressive form of mature B-cell non-Hodgkin's lymphoma (NHL), accounting for nearly 6% of NHL cases. Currently, MCL patients are treated with aggressive chemo-immunotherapy regimens followed mostly by consolidation with autologous stem cell transplantation and maintenance rituximab. Despite these intensive therapies, MCL prognosis remains poor, with a median overall survival of 6-7 years, with most of the patients developing the refractory or recurrent disease. Thus, there is a need for novel and more effective, less toxic therapies for MCL. Epidermal growth factor-like 7 (EGFL7) is a protein secreted by endothelial cells and plays a critical role in angiogenesis. Our lab was the first to demonstrate a role for EGFL7 in hematologic malignancies, demonstrating that EGFL7 is increased in leukemic blasts of AML patients and that anti-EGFL7 treatment alone results in prolonged survival of leukemic mice (Papaioannou et al., 2017). While EGFL7 has been shown to play a role in some hematological malignancies, its role in MCL has not been investigated. Therefore, we assessed EGFL7 expression levels in MCL patients compared to healthy controls using the publicly available dataset GSE46846. We found significant increases in EGFL7 in malignant B cells from MCL patients compared to healthy individuals (p To examine the therapeutic potential of targeting EGFL7 in MCL cells, we treated patient-derived xenograft (PDX) cells (n=3) with an anti-EGFL7 blocking antibody (Parsatuzumab) in vitro. We found an increase in apoptosis of MCL PDX cells compared to IgG control (15-50% vs. 0.5-2.4%, respectively), p Our lab has previously shown that EGFL7 binds to the Epidermal growth factor receptor (EGFR) in AML (Bill et al., 2020). Knowing the importance of EGFR in lymphoma, we validated the binding of EGFL7 to EGFR in MCL cells by performing an immunoprecipitation (IP) assay on protein lysates from PDX cells (n=2) and Jeko1 cells. We found that EGFL7 protein was significantly enriched in protein fractions pulled down using anti-EGFR antibody compared to IgG. Conversely, we transfected Jeko1 cells with Flag-tagged EGFL7 plasmid and performed IP using anti-Flag antibody. EGFR protein was significantly enriched in the protein fraction pulled down using an anti-Flag antibody compared to IgG. Next, we examined the association between EGFL7 and EGFR expression in primary MCL patients and found that EGFR positively correlates with EGFL7 expression (n=122, r=0.1533). Further, Anti-EGFL7 treatment decreased phospho-AKT protein levels in PDX cells and MCL cell lines compared to IgG control, suggesting blocking EGFL7 abrogates EGFR mediated downstream signals. In conclusion, this is the first report describing a role for EGFL7 in MCL growth and/or survival by modulating the EGFR-AKT signaling pathway and targeting EGFL7 using an anti-EGFL7 blocking antibody as a novel treatment to improve the outcome for MCL patients. Figure 1 Figure 1. Disclosures No relevant conflicts of interest to declare.

Background: Post-transplant cyclophosphamide (PTCy) has been shown to significantly reduce transplant related mortality (TRM) post hematopoietic stem cell transplantation (HSCT) and is being increasingly used for acute myelogenous leukemia (AML) patients (pts) undergoing HSCT including from unrelated donors (UD). These publications mainly include pts transplanted in first complete remission (CR1). We recently reported outcome in 1879 AML pts transplanted in second CR (CR2) with conventional graft-versus-host disease (GVHD) prophylaxis (Leukemia 2020). Results may differ in transplantation with PTCy as GVHD prophylaxis, eliminating proliferating alloreactive T cells, and upregulating T regulatory cells while suppressing host natural killer (NK) cells immediately after transplantation, changing the biology and characteristics of the transplantation. Methods: The study aim was to assess outcome of adult AML pts, aged ≥18 years in CR2 undergoing HSCT from a 9-10/10 UD with PTCy, in 2010-2019.Statistics included multivariate analysis (MVA) adjusting for potential confounding factors was performed using a Cox's proportional-hazards regression model for main outcomes. Results: In total, 127 pts were included. Median follow-up was 19.2 (95% CI, 14.7-28) months (mos). Median age was 45.5 (range, 18.2-71.3) years. 54.3% were male. Cytogenetic risk (MRC classification) was favorable, intermediate, and adverse in 15.7%, 55.9%, and 5.5% of pts, respectively (missing data-22.8percentage) Median year of transplantation was 2017. Time from diagnosis to transplantation was 20.4 (range, 4.1-182.4) months. All pts were at CR2 at time of transplantation. Donors were 10/10 and 9/10 UD in 60.6% and 39.4% of pts, respectively. 77.8% and 47.2% of the pts and donors, respectively, were cytomegalovirus (CMV) seropositive. Conditioning was myeloablative in 50.4% and reduced intensity in 49.6%. The most frequent (61.4%) conditioning consisted of busulfan and fludarabine. All pts received PTCy as anti GVHD prophylaxis in combination with immunosuppression, which was cyclosporine A /mycophenolate mofetil (MMF) in 21.3% and MMF/tacrolimus in 23.6%. 33.9% of the pts received In vivo T-cell depletion. Grafts were peripheral blood in 93.7% and bone marrow in 6.3% of transplants. Karnofsky performance score (KPS) was > 90 in 71.2% and the hematopoietic cell transplantation-specific comorbidity index (HCT-CI) was zero in 61.5% of the pts. Engraftment was achieved by 97.6% with day (d) 60 absolute neutrophil count (ANC) > 0.5 x 10 9/L in 96.8%, and d 180 incidence of acute (a) GVHD II-IV and III-IV was 26.2% and 9.2%, respectively. The 2-year total and extensive chronic (c) GVHD was 34.3% and 13.8 %, respectively. The 2-year non-relapse mortality (NRM) was 17.2%. The 2-year relapse incidence (RI) was 21.1%. RI was the main cause of death in 41% of pts who died, followed by infections (23.1%) and GVHD (20.5%). The 2-year leukemia-free survival (LFS), overall survival (OS) and GVHD-free, relapse-free survival (GRFS) was 61.7%, 65.2% and 49.3%, respectively (Figure). In MVA time from diagnosis to transplant was significant prognostic factor for RI, LFS, OS and GRFS hazard ratio (HR) =0.19 (95% CI 0.07-0.48, p Conclusions: Outcome of AML pts undergoing HSCT from a 9-10/10 MUD in CR with PTCy as GVHD prophylaxis, are similar to previous reports using conventional GVHD prophylaxis with 26% of pts developing aGVHD ,34% cGVHD and NRM of 17%. These results are also similar to those we previously observed in pts undergoing HSCT from UD with PTCy while in CR1. Figure 1 Figure 1. Disclosures Labopin: Jazz Pharmaceuticals: Honoraria. Kulagin: X4 Pharmaceuticals, Alexion, Apellis, Biocad: Research Funding; Novartis, Generium, Sanofi, Roche, Johnson & Johnson, Pfizer: Speakers Bureau. Blaise: Jazz Pharmaceuticals: Honoraria. Yakoub-Agha: Jazz Pharmaceuticals: Honoraria. Ciceri: IRCCS Ospedale San Raffaele: Current Employment. Mohty: Sanofi: Honoraria, Research Funding; Pfizer: Honoraria; Novartis: Honoraria; Takeda: Honoraria; Jazz: Honoraria, Research Funding; Janssen: Honoraria, Research Funding; Gilead: Honoraria; Celgene: Honoraria, Research Funding; Bristol Myers Squibb: Honoraria; Astellas: Honoraria; Amgen: Honoraria; Adaptive Biotechnologies: Honoraria.

INTRODUCTION: Myelodysplastic Syndrome (MDS) is a heterogenous disease which is almost incurable without an allogeneic hematopoietic cell transplantation (allo-HCT). Within the revised international scoring system (R-IPSS), MDS with poor and very poor cytogenetics have a much worse outcome after allo-HCT. The very poor cytogenetic subgroup refers to patients harboring more than 3 abnormalities and is therefore a highly heterogenous group. We have shown in acute myeloid leukemia (AML) that beyond complex karyotype, specific adverse cytogenetic features such as 7q abnormalities (abn7q), 5q abnormalities (abn5q), 17p abnormalities (abn17p) and monosomal karyotype (MK) worsen the outcomes after allo-HCT. We have therefore retrospectively reviewed MDS with very poor cytogenetics and studied the impact of adverse cytogenetic features on outcomes after transplant. METHODS: We selected MDS patients who underwent allo-HCT between 2001 and 2018 from a matched related or unrelated donor, for whom a full cytogenetic report was available in the EBMT registry. We then stratified them according to the presence of abn7q, abn5q, abn17p, MK and the number of abnormalities (≤5, 6-9 and ≥10). Graft-versus-host disease (GvHD) and relapse-free survival (GRFS) was defined as survival without grade II-IV acute GvHD, extensive chronic GvHD or relapse. RESULTS: A total of 154 patients were identified in the registry. One hundred twenty-three patients (81%) had MDS with excess of blasts and 4 (3%) had secondary AML. Median age was 59 years (interquartile range (IQR), 51-64) and the median follow-up was 38 months (95% confidence interval (CI), 34-60). The time from diagnosis to allo-HCT was a median of 6 months (IQR, 4-8). Two thirds of patients received a reduced-intensity conditioning regimen (N = 103, 67%) and 87 patients had a matched unrelated donor (57%). Almost all patients were in first complete remission at time of transplant (N= 149, 97%). Regarding specific cytogenetic features, 87 patients had abn7q (57%), 99 abn5q (64%), 59 abn17p (38%) and 120 MK (78%) with considerable overlap between groups. The 2-year overall survival (OS) and progression-free survival (PFS) was 34% (95% CI 26-42%) and 24% (95% CI 17-31%), respectively. The 2-year cumulative incidence of relapse and non-relapse mortality (NRM) was 59% (95% CI 51-67%) and 18% (95% CI 12-24%), respectively. The cumulative incidence of grade II-IV acute GvHD and chronic GvHD was 33% (95% CI 25-40%) and 44% (95% CI 36-53%) by day 100 and 2 years respectively. The 2-year GRFS was 12% (95% CI 6-17%). The presence of abn5q was associated with a significantly decreased PFS of 17% (95% CI 9-25%) versus 36% (95% CI 23-49%); p=0.05) and GRFS (6% (95% CI 1-11%) versus 23% (95% CI 11-34%); p=0.04). The presence of abn7q was associated with significantly increased NRM (25% (15-34%) versus 9% (2-16%); p=0.02) which did not translate into OS. There were no specific cytogenetic features that had an independent impact on the cumulative incidence of relapse, but age over 55 years did increase the relapse risk (65: 66% (95% CI 50-83%); p=0.03). A continuous effect was also observed (per decade increase: HR=1.24, 95%CI 1.02-1.52; p=0.03). Patients with an interval of more than 6 months from diagnosis to allo-HCT had almost double the OS (45% (95% CI 32-58%)) compared to patients with an interval less than 6 months (27% (95% CI 17-37%); p=0.04), however a continuous effect was not observed. CONCLUSION: MDS with very poor cytogenetics according to R-IPSS is a very bad group with dismal outcomes after allo-HCT. Within this high-risk group, specific adverse cytogenetic features such as the number of abnormalities, abn7q, abn5q, abn17p or MK did not stratify outcomes further, except for abn5q which was associated with a decreased PFS. Our results might be explained in part by the low number patients and by the over-representation of adverse features within this cohort. Despite that, advancing age was associated with increased relapse. Whilst allo-HCT remains the best therapeutic option for this very high-risk patient group, efforts should focus on post-transplant preemptive intervention strategies to prevent relapse. Disclosures Byrne: Incyte: Honoraria. Schroeder: Celgene: Honoraria, Other: Travel support, Research Funding. Blaise: Jazz Pharmaceuticals: Honoraria. Hayden: Jansen, Takeda: Other: Travel, Accomodation, Expenses; Amgen: Honoraria. Scheid: Janssen: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees; BMS: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Amgen: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees; Takeda: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Abbvie: Honoraria; Roche: Consultancy; Novartis: Honoraria, Membership on an entity's Board of Directors or advisory committees. Yakoub-Agha: Jazz Pharmaceuticals: Honoraria.

Introduction: Asparaginase is an essential part of the treatment of acute lymphoblastic leukemia (ALL). Hypersensitivity occurred in 16.8% (clinical allergy (13.8%) or silent inactivation (3.0%)) of patients treated with pegylated asparaginase (PEG-asp) in the NOPHO ALL2008 cohort (Hoejfeldt et al 2018). Hypersensitivity (clinical allergy or silent inactivation) is the most common cause of truncated asparaginase therapy, and truncated treatment has been associated with decreased event-free survival (Silverman et al 2001). Asparaginase encapsulated in erythrocytes (eryaspase) is an alternative formulation of asparaginase aiming to prolong the half-life of asparaginase and to reduce toxicity e.g. hypersensitivity, since the erythrocyte membrane prevents activation of the immune system and protects asparaginase against elimination. A phase 2 study has demonstrated prolonged asparaginase enzyme activity (AEA) and significantly reduced incidence of hypersensitivity in patients with ALL, who had a prior exposure to other asparaginase preparations (NCT01518517). The aim of the NOR-GRASPALL 2016 study is to evaluate the safety and efficacy of eryaspase in combination with multiagent chemotherapy according to the NOPHO ALL2008 protocol and the ALLTogether Pilot study. Antileukemic treatment in these protocols include 4-8 doses of PEG-asp as first line treatment. Methods: NOR-GRASPALL 2016 is a phase 2 multinational multicentre trial conducted in the Nordic/Baltic countries. It is a single-arm study for non-high-risk patients with ALL and hypersensitivity to PEG-asp. Eryaspase (150 U/kg) is scheduled to complete the intended course of asparaginase (1-7 doses). AEA-measurements are used as biomarkers for treatment efficacy. Results: Since August 2017, 36 children and two adults were included in the study. Median age was 6.0 years (IQR: 3.3;8.7). 37 patients (97.4%) had clinical allergy to PEG-asp of whom 59.5% (n=22) had a severe allergic reaction. One patient (3.3%) was included due to silent inactivation. AEA-measurements were available in all patients but one (n=37), and in none of these patients AEA was detectable following PEG-asp treatment. In total, 171 doses of eryaspase were administered. A total of 34 of the 36 patients (94.7%) had AEA >100 U/L and 27 patients (71.1%) had AEA-levels >400 U/L 14 days after first eryaspase administration (expected nadir). The median AEA-level was 798 U/L [IQR: 387;864]. In total 90.7% of all samples collected 14 days after eryaspase administration had AEA >100 U/L and 69.3% had AEA >400 U/L. The median AEA-level was 621U/L [IQR: 331;962]. Adverse events with relation to eryaspase were reported in 8 of 36 patients (22%). Six patients experienced a possible allergic reaction to eryaspase; one patient had a severe allergic reaction, three patients developed a rash and two patients had "drug fever", deemed related to eryaspase. Three of these six patients (50%) had low AEA-levels after developing allergic symptoms, the remaining patients (50%) had AEA-levels comparable with all other patients in the study. Four patients experienced adverse events related to eryaspase; two patients with mild hyperlipidaemia and two with hepatoxicity. No other severe adverse events with relation to eryaspase have been reported. Final study results will be provided at the meeting. Conclusion: Eryaspase consistently demonstrated prolonged AEA in patients who developed hypersensitivity reactions to PEG-asp. Treatment with eryaspase was well tolerated. We conclude that eryaspase is a promising alternative to PEG-asp in case of hypersensitivity. Disclosures Schmiegelow: Jazz Pharmaceuticals: Other: Speaker and/or Advisory Board Honoraria ; Amgen: Other: Speaker fee; Medscape: Other: Speaker fee; Servier: Other: Educational grant. Speaker and/or Advisory Board Honoraria . Albertsen:Erytech Pharma: Other: Sponsor of the investigator initiated study: NOR-GRASPALL 2016. No financial benefits..

Background: PCR of rearranged antigen receptor genes is the method of choice for MRD quantification in ALL. Although FCM-MRD is faster and biologically more informative than PCR, the analysis requires a high level of training. The only larger published studies using FCM-MRD based stratification (Borowitz, Blood, 2008 and 2015) showed a clear association with clinical outcome in BCP-ALL. However, MRD analyses were centralized and these studies included only one MRD-based stratification (MRD levels at the end of induction). Patients and methods: We examined FCM-MRD as stratification tool in BCP-ALL at various timepoints in a large-scale multicenter (18 MRD centers) study. A total of 1487 patients with BCP-ALL (1298 children (younger than 18 years) and 189 adults (18-45 years) are included in the study and were treated according to the NOPHO ALL2008 protocol between July 2008 and February 2016. The median follow-up time for patients in first remission was 51 months (IQR 32-75). MRD was measured by FCM and/or real time quantitative PCR on days 15, 29 (end of induction) and 79 (for standard (SR) and intermediate risk (IR) patients) and prior to and after high risk blocks. A 6-colour FCM analysis including 3 standardized antibody combinations was used and performed in 18 laboratories. Patients were stratified by FCM-MRD, or by PCR-MRD if no FCM-MRD marker was available. End-of-induction MRD (cut-off 10-3) was used to stratify patients to standard risk (SR) vs intermediate risk (IR) or IR vs high risk consolidation therapy (in case of WBC > 100 x 109/L at diagnosis). Patients with MRD >=2.5x10-1 on day 15 were stratified to high risk block therapy. Patients with MRD >=5x10-2 on day 29 or day 79/post high risk-2 block MRD >=10-3 were stratified to HSCT. Primary outcomes were 5year event-free survival (5y EFS) and 5year cumulative incidence of relapse (5y CIR). Results: Only two patients (0.14% of total) had neither an informative FCM nor a PCR marker, and an informative FCM marker combination for MRD monitoring was identified in 96.2% of patients. There was a significant correlation between FCM- and PCR-MRD levels on day 15 (r=0.77, p Based on FCM-MRD only, the median MRD level on day 15, 29 and 79/post high risk-2 block was 5x10-3, 1.1x10-4, and below detection limit, respectively. Adults had significantly higher MRD levels at all time-points (p The day 29 FCM-MRD level was closely associated with clinical outcome and a higher hazard of relapse was seen independently for a FCM-MRD >=10-3 (hazard ratio (HR) 2.4, CI 1.6-3.7, p18 year (HR 3.0, CI 1.7-5.3, p=100 (HR 2.7, CI 1.6-4.6, p=0.0001), and B-other (HR 2.1, CI 1.2-3.5, p=0.0052) or high risk B-ALL cytogenetic aberration (rearranged KMT2A/iAMPchr21/hypodiploid) (HR 3.2, CI 1.6-6.1, p=0.0006) (multivariate cause-specific Cox regression, n=1328). Patients with a day 79 FCM-MRD >=10-4 and =10-4 and =10-4 and Patients with day 15 FCM-MRD =10-3 and Conclusion: FCM-MRD performed in a multi-center setting is a clinically useful method for disease monitoring and MRD-based treatment stratification in BCP-ALL. Moreover, FCM-MRD is a reliable indicator of outcome in BCP-ALL independently of other key risk factors. Residual disease >=10-4 and Disclosures No relevant conflicts of interest to declare.

Thanks to the recent developments in transplant procedures, an increasing number of patients with acute myeloid leukemia (AML) with a poor Karnofsky Performance Status (KPS) score are currently offered an allogeneic hematopoietic stem cell transplantation (allo-HCT). Nevertheless, little data is available about outcome of this fragile population. We report here the results of a retrospective study designed to evaluate outcome of patients with AML undergoing allo-HCT with KPS score ≤80%. The analysis included patients with AML aged ≥18 years, undergoing allo-HCT in first remission between 2000 and 2018, with a KPS score at the time of transplant between 50% and 80%. Patients who received manipulated grafts or had incomplete data about cytogenetics were excluded. Conditioning intensity was defined according to EBMT definitions. A total of 2,963 patients were identified. Median age at transplant was 55 years (18-77 years). Median year of transplant was 2014. The KPS score was =80% in 85% of the patients and Cumulative incidence of grade II-IV and III-IV acute GVHD (aGvHD) was 26% and 8%, respectively. The 2-year cumulative incidence of chronic GVHD (cGvHD) and severe cGVHD was 38% and 18%. Non-relapse mortality (NRM) and relapse incidence (RI) at 2 years were 19% and 27%, respectively. Notably, in the subgroup of patients with KPS On multivariate analysis, transplant from a MSD was associated with a reduced risk of aGvHD (10/10 UD HR 1.8, 9/10 UD HR 2.4, haplo HR 1.9, CB HR 3.4, p In order to compare outcome following MAC and RIC conditioning the analysis was restricted to patients receiving transplant from MSD or UD. Patients with a KPS score =80% or In conclusion, allo-HCT is feasible in patients with acute myeloid leukemia in first remission and KPS score Disclosures Labopin: Jazz Pharmaceuticals: Honoraria. Kröger:Sanofi-Aventis: Research Funding; Celgene: Honoraria, Research Funding; DKMS: Research Funding; JAZZ: Honoraria; Medac: Honoraria; Neovii: Honoraria, Research Funding; Novartis: Honoraria, Research Funding; Riemser: Research Funding. Socie:Alexion: Consultancy. Blaise:Pierre Fabre medicaments: Honoraria; Jazz Pharmaceuticals: Honoraria; Sanofi: Honoraria; Molmed: Consultancy, Honoraria. Byrne:Ariad/Incyte: Honoraria, Speakers Bureau. Esteve:Astellas: Consultancy, Speakers Bureau; Novartis: Consultancy, Research Funding, Speakers Bureau; Celgene: Consultancy, Speakers Bureau; Amgen: Consultancy; Daiichi Sankyo: Consultancy; Jazz Pharmaceuticals: Consultancy; Roche: Consultancy; Pfizer: Consultancy. Mohty:Jazz Pharmaceuticals: Honoraria, Research Funding.

Background: Central nervous system (CNS) involvement is common in childhood acute lymphoblastic leukemia (ALL). Today all patients receive intensive prophylactic CNS-directed therapy, which is associated with short- and long-term neurotoxicity. The conventional method for diagnosing CNS leukemia is by microscopic examination of cytospin preparations of cerebrospinal fluid (CSF). Both high (CNS3: ≥5 x 109 cells/L) and low level CNS disease (CNS2: Methods: Cytospin results were collected from local oncology clinics for patients treated according to the Nordic Society of Pediatric Hematology (NOPHO) ALL2008 protocol from July 2008 to December 2017 (N=1841). CSF samples for FCM were obtained from patients enrolled from September 2012 to December 2017 at 17 participating centers (N=669). CSF samples were collected into Transfix® tubes (Intermedico Ltd., Hellerup, Denmark) and shipped to Rigshospitalet, Copenhagen, Denmark for centralized analysis as previously described (Levinsen at al., Pediatr Blood Cancer 2016). FCM positivity required detection of ≥10 cells with a leukemia-associated phenotype. CSF FCM results were blinded to the treating physician. Time to relapse was analyzed using Cox proportional hazards models with delayed entry at remission and with death and second malignant neoplasm as competing events. Results: At diagnosis, 241 of 1841 (13.1%; CNS2: 8.9% and CNS3: 4.2%) patients were positive by cytospin, whereas 167 of 669 (25.0%) patients were positive by FCM. FCM positivity was not associated with shipment duration (median: 2 days). In total, 191 of the 669 patients (28.6%) were CNS positive by cytospin and/or FCM. CNS positivity was associated with higher white blood cell (WBC) count at diagnosis, T-cell ALL, lack of t(12;21), and traumatic lumbar puncture (TLP) (all comparisons: p < 0.001). However, patients with CNS positivity did not differ in their end of induction bone-marrow minimal residual disease (MRD) levels compared to the remaining patients (cytospin: median MRD 1.2 x 10-4 vs. 0.9 x 10-4, p = 0.058; FCM: median MRD 2.0 x 10-4 vs. 1.7 x 10-4, p = 0.62). During follow-up, 30 patients relapsed with nine of the relapses involving the CNS and 16 relapses isolated to the bone-marrow. The 4-year cumulative incidence of any relapse was higher for patients who were CNS positive by cytospin (16.1% vs. 8.9%), FCM (18.6% vs. 6.0%), and combined cytospin/FCM (19.0% vs. 5.5%) (Figure 1a). Simple Cox regressions yielded relapse hazard ratio (HR) estimates of CNS positivity at diagnosis of 2.4 for cytospin (95% CI 1.0-5.7, p = 0.04), 3.5 for FCM (95% CI 1.7-7.2, p < 0.001), and 4.1 for combined cytospin/FCM (95% CI 2.0-8.7, p < 0.001). Cytospin and/or FCM was associated with increased risk of both isolated bone-marrow relapse (16 events, HR 3.1, 95% CI 1.2-8.4, p = 0.024) and of any CNS relapse (9 events, HR 4.9, 95% CI 1.2-19.5, p = 0.025). In a multiple Cox model stratified by immunophenotype and risk group, predictors of relapse were age (per year: HR 1.1, 95% CI 1.0-1.2, p = 0.003), WBC at diagnosis (per doubling: HR 1.3, 95% CI 1.1-1.6, p = 0.005), and cytospin and/or FCM (HR 3.2, 95% CI 1.4-7.2, p = 0.006) (Figure 1b). Cytospin and/or FCM was associated with a significantly higher risk of relapse for BCP-ALL (N=581; 23 relapses; HR 4.5, 95% CI 1.9-10.4, p = < 0.001). For T-cell ALL no significant association was observed (N=85; 7 relapses; HR 1.6, 95% CI 0.3-8.2, p = 0.58), however the number of T-cell ALL was too small to allow reliable conclusions. In a simple Cox model, TLP was associated with a significantly higher risk of relapse than no TLP (HR 2.7, 95% CI 1.3-5.9, p = 0.011), but this was only the case for patients who were positive by FCM (FCM pos: HR 2.7, 95% CI 1.0-7.0, p = 0.043; FCM neg: 0.7, 95% CI 0.1-5.0, p = 0.68). Conclusion: FCM markedly increases the detection rate for CNS involvement at diagnosis, and distinguish between patients at high and low risk of relapse. Diagnosis of CNS leukemia by combined cytospin and FCM analysis should be the standard for accurate classification of CNS status, which will enable better stratification of CNS-directed and systemic therapy. Disclosures No relevant conflicts of interest to declare.

Mixed hematopoietic chimerism permits durable tolerance of T, B and NK cells to xenoantigens in a rat to mouse bone marrow transplant model. However, it is unclear whether tolerance of human NK cells to pig xenoantigens can be induced by mixed hematopoietic chimerism. We assessed the tolerance of human NK cells towards pig cells in a humanized mouse model with established pig and human mixed xenogeneic chimerism. Pig and human mixed chimeras (MCs) were generated by injection of pig bone marrow cells to irradiated pig cytokine (IL3, GMCSF and SCF) transgenic NOD-scid common gamma chain knockout (NSG) mice followed by injection of human fetal liver CD34+ cells 3 day later. In the control group, only human CD34+ cells were transplanted. 12 weeks post-transplant, hydrodynamic injection of plasmid encoding human Flt3L followed by injection of three rounds of recombinant IL15/IL-15 receptor alpha Fc complex was given to promote human NK cell reconstitution. The control non–mixed chimeric group (Non-MC) received the same treatment without pig cells. 12 days following induction of human NK cell reconstitution, human NK cells from both MC and Non-MC mice were isolated from the spleen and their cytotoxic responses in vitro to pig cells were determined in a chromium release assay. In addition, the presence of pig cells in various tissues of the chimeric mice was studied. While human NK cells were usually undetectable in peripheral blood prior to the injection of human Flt3L plasmid and IL-15/IL-15 receptor alpha-Fc complex, they were detected by 5 days post-injection of IL-15 protein. 12 days post-induction of human NK cell reconstitution, pig cells remained detectable in peripheral blood, spleen, bone marrow and liver in the chimeric mice together with human NK cells. The co-existence of human NK cells and pig cells suggested that human NK cells in MCs might be tolerant to pig cells. Consistent with this notion, cytotoxicity assays showed that human NK cells from MCs demonstrated decreased killing of pig PBMC blasts compared to NK cells from Non-MC mice. Importantly, killing of K562 cells by NK cells from MCs mice was also decreased compared to that of Non-MC mice, suggesting that human NK cell tolerance to pig cells induced by mixed chimerism was associated with global hyporesponsiveness, as we have previously observed in a rat-to-mouse bone marrow transplantation model. Moreover, higher percentages of CD56highCD16low and lower percentages of CD56lowCD16high human NK cell subsets were observed in bone marrow of chimeric mice than in non-chimeric mice, indicating that the development of human NK cells in bone marrow might be altered by the presence of pig cells. In summary, our data suggest that mixed xenogeneic chimerism may induce tolerance of human NK cells towards porcine cells, but the tolerance may be associated with global hyporesponsiveness. Disclosures No relevant conflicts of interest to declare.

Background: Central nervous system (CNS)-directed treatment has reduced risk of CNS relapse for childhood acute lymphoblastic leukemia (ALL), which accounts for 30-40% of initial relapses. Compared with CNS-negative patients, patients with CNS leukemia (>5 leukocytes/µL cerebrospinal fluid (CSF) and lymphoblasts) suffer from more CNS+ relapses. Conventional cytology (CC) is specific (>95%), but nonsensitive ( Method: We prospectively assessed centralised multi-parameter flow cytometry (FCM) of fixated CSF versus local CC in Nordic/Baltic childhood ALL. Diagnostic samples from 172 children aged 0-18 years with de novo and eight children with relapsed ALL were investigated in 297 CSF samples from 180 patients. Kinetics of disappearance of leukemic cells in the CSF was evaluated until day 15. Antibody-combinations reflected the immunophenotype of leukemic blasts in bone marrow. Result: Of 172 newly diagnosed patients, 51 (30%) had CSF involvement by FCM, while CC was positive in 16 patients (9%) (p Conclusion: Leukemic blasts are present in spinal fluid of one third of newly diagnosed ALL. CSF involvement is associated with other higher risk characteristics. The prognostic value of these findings awaits prospective evaluation. Disclosures No relevant conflicts of interest to declare.