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Background Sickle cell disease (SCD) is one of the most common and devastating inherited blood disorders characterized by a single nucleotide mutation in the beta-globin chain leading to the production of mutant hemoglobin S (HbS). HbS polymerizes upon deoxygenation causing red blood cells (RBC) to sickle which results in extremely painful episodes of vaso-occlusive crisis (VOC), severe hemolytic anemia, chronic multiorgan failure and a reduced life span. An important metabolic feature directly associated with RBC sickling is increased intracellular levels of the glycolytic intermediate 2,3-diphosphyglycerate (2,3-DPG) which promotes deoxygenation by lowering the oxygen affinity of hemoglobin (Hb). Pyruvate kinase R (PKR) is a key enzyme in RBC metabolism, generating adenosine triphosphate (ATP) to maintain energy homeostasis, membrane integrity and deformability, and modulates 2,3-DPG levels. Mitapivat (AG-348) is an oral, small molecule allosteric activator of PKR and shows promise as an anti-sickling agent in addition to its effect in PK deficiency and thalassemia. The safety and efficacy of mitapivat in subjects with SCD was evaluated in the dose finding period of this ongoing phase 2 study. Methods The ESTIMATE study is a phase 2, open-label study in which subjects ≥16 years with SCD (HbSS, HbS/β0, HbS/β+) with a baseline hemoglobin >6.1 g/dL and ≤11.1 g/dL, no chronic transfusion and adequate organ function were eligible. In the 8-week Dose Finding Period, initial dosing of mitapivat was 20 mg twice daily (BID). Subjects received a maximum of two sequential dose escalations of mitapivat (i.e. from 20 mg BID to 50 mg BID and 100 mg BID) depending on safety. The primary endpoints were safety, evaluated by frequency and severity of adverse events (AEs), and efficacy of mitapivat on RBC sickling. RBC sickling was evaluated by change in Point of Sickling (PoS), the pO2 at which sickling occurs as measured by oxygen gradient ektacytrometry on the Lorrca (RR Mechatronics). Secondary endpoints included changes in hematological parameters, levels of 2,3-DPG and ATP, Hb-oxygen affinity (p50) and surrogate markers of organ damage and mortality. Subjects who safely tolerated mitapivat and showed evidence of clinical improvement, were eligible to continue a 52-week follow-up period (Fixed Dose Extension Period). Results Six subjects have been enrolled as of September 2020 and completed the Dose Finding Period. All had homozygous HbSS except one patient who had HbS/β0-thalassemia. Baseline characteristics were: median age of 36 years (range 20-59 years), 4 (66.7%) were female and 5 (83.3%) were on stable-dose hydroxyurea. All subjects received dose escalation to a maximum dose of 100 mg BID. No serious adverse events (SAEs) occurred. Adverse events (AEs) were mild and often transient, with the most common treatment emergent AEs: transaminase increase (n=3 [50.0%], Grade 1), gastrointestinal disorders including dyspepsia, diarrhea and abdominal discomfort (n=3 [50.0%], Grade 1) and headache (n=2 [33.3%], Grade 1). One VOC occurred without hospital admission and did not require dose reduction or discontinuation. Table 1 summarizes the anti-sickling effect as well as the hematological and biochemical response to mitapivat treatment. Sickling occurred at lower pO2 levels in all 6 patients during the Dose Finding Period reflected by a significant decrease in treatment week 8 mean PoS compared to baseline. 5/6 subjects (83.3%) achieved a Hb increase of ≥1 g/dL during this period, which was accompanied by a decrease in hemolytic markers. Consistent with activation of PKR, 2,3-DPG levels decreased and ATP levels increased. Additional results including biomarker data will be presented. Conclusion Mitapivat demonstrated an adequate safety profile during the 8-week Dose Finding Period in patients with SCD. The data show promising efficacy in terms of a decrease in the pO2 at which RBCs start to sickle, as well as increase in Hb from baseline and a concomitant decrease in markers reflecting hemolysis. The observed changes in 2,3-DPG and ATP levels are consistent with the proposed mechanism of the drug. The study is ongoing and further data including follow-up data, patient-reported outcomes, PKR activity and thermostability will be reported at a later stage. Figure 1 Figure 1. Disclosures van Dijk: Agios Pharmaceuticals: Research Funding; Axcella Health: Research Funding. Rab: Axcella Health: Research Funding; Agios Pharmaceuticals: Research Funding. Rijneveld: Servier: Research Funding; Amgen: Research Funding. Nur: Celgene: Speakers Bureau; Novartis: Research Funding, Speakers Bureau; Roche: Speakers Bureau. Schutgens: CSL Behring: Research Funding; Novo Nordisk: Research Funding; OctaPharma: Research Funding; Pfizer: Research Funding; Shire/Takeda: Research Funding; Bayer: Research Funding. Wijk: Agios Pharmaceuticals: Membership on an entity's Board of Directors or advisory committees, Research Funding; Axcella health: Research Funding; Global Blood Therapeutics: Membership on an entity's Board of Directors or advisory committees, Research Funding. Van Beers: Pfizer: Research Funding; Novartis: Research Funding; Agios Pharmaceuticals: Membership on an entity's Board of Directors or advisory committees, Research Funding; RR Mechatronics: Research Funding.

Red blood cell (RBC) transfusions are of vital importance in patients with sickle cell disease (SCD). However, a major complication of transfusion therapy is alloimmunization. The low-affinity Fcg receptors, expressed on immune cells, are important regulators of antibody responses. Genetic variation in FCGR genes has been associated with various auto- and alloimmune diseases. The aim of this study was to evaluate the association between genetic variation of FCGR and RBC alloimmunization in SCD. In this case-control study, DNA samples from 2 cohorts of transfused SCD patients were combined (France and The Netherlands). Cases had a positive history of alloimmunization, having received ‡1 RBC unit. Controls had a negative history of alloimmunization, having received ‡20 RBC units. Single nucleotide polymorphisms and copy number variation of the FCGR2/3 gene cluster were studied in a FCGR-specific multiplex ligation-dependent probe amplification assay. Frequencies were compared using logistic regression. Two hundred seventy-two patients were included (130 controls, 142 cases). The nonclassical open reading frame in the FCGR2C gene (FCGR2C.nc-ORF) was strongly associated with a decreased alloimmunization risk (odds ratio [OR] 0.26, 95% confidence [CI] 0.11-0.64). This association persisted when only including controls with exposure to ‡100 units (OR 0.30, CI 0.11-0.85) and appeared even stronger when excluding cases with Rh or K antibodies only (OR 0.19, CI 0.06-0.59). In conclusion, SCD patients with the FCGR2C.nc-ORF polymorphism have over a 3-fold lower risk for RBC alloimmunization in comparison with patients without this mutation. This protective effect was strongest for exposure to antigens other than the immunogenic Rh or K antigens.

Despite recent advances in treatment of B-acute lymphoblastic leukemia (B-ALL) there is still a need for novel targeted therapies. The tetraspanin CD9 is expressed in 60-80% of B-ALL and correlates with adverse prognosis. Recently, the mouse CD9 antibody ALB6 was shown to induce leukemia rejection in NOD/SCID mice. However, clinical development of ALB6 and other CD9-targeting antibodies was hampered by their CD9 mediated induction of platelet aggregation. It is known that CD9 is still expressed on tumor cells after treatment with chemotherapy or blinatumomab (Leung, 2019; Linder, 2016). AT1412 is a fully human antibody isolated from B cells of a patient that was cured from stage IV metastatic melanoma (Verdegaal, 2011). AT1412 targets CD9, without inducing platelet aggregation in vitro or thrombosis in cynomolgus monkeys after intravenous administration at therapeutic dose levels. By crystallography AT1412 was shown to bind a unique epitope preventing homodimerization of CD9, distinctly different from other CD9 antibodies. AT1412 binds a majority of patient B-ALL samples, but not T-ALL and induces ADCC and ADCP of CD9 positive B-ALL primary cells and the level of cytotoxicity significantly correlated with that of AT1412 binding. In both NSG and immunodeficient mice harboring a human immune system (HIS mice) AT1412 demonstrated a strong, dose-dependent tumor rejection of B-ALL, most pronounced in the extramedullary sites. In HIS mice AT1412 treatment led to an accumulation of T cells and CD14+ myeloid cells at the tumor sites. To support clinical development, pre-clinical safety of AT1412 was evaluated in cynomolgus monkeys. AT1412 demonstrated a half-life of 8.5 days, supporting 2-3 weekly administration in humans. Besides transient thrombocytopenia no other pathological deviations were observed. No effect on coagulation parameters, bruising or bleeding were observed macro- or microscopically. The thrombocytopenia is reversible, and its recovery accelerated in those animals developing anti-drug antibodies. Taken together, we demonstrate that CD9 on B-ALL cells can be successfully targeted by AT1412. AT1412 targets a unique epitope and does not induce thrombosis. Pre-clinical safety assessment is supporting that AT1412 can be safely administered. A First in Human clinical study is scheduled to start early 2021 in human solid tumors to determine safety and efficacy. AT1412 efficacy will be evaluated in B-ALL in expansion cohorts. Leung, K.T., Zhang, C., Chan, K.Y.Y., Li, K., Cheung, J.T.K., Ng, M.H.L., Zhang, X.-B., Sit, T., Lee, W.Y.W., Kang, W., et al. (2019). CD9 blockade suppresses disease progression of high-risk pediatric B-cell precursor acute lymphoblastic leukemia and enhances chemosensitivity. Leukemia. Linder, K., Gandhiraj, D., Hanmantgad, M., Seiter, K., and Liu, D. (2016). Complete remission after single agent blinatumomab in a patient with pre-B acute lymphoid leukemia relapsed and refractory to three prior regimens: HyperCVAD, high dose cytarabine mitoxantrone and CLAG. Exp. Hematol. Oncol. 5, 5-8. Verdegaal, E.M.E., Visser, M., Ramwadhdoebé, T.H., van der Minne, C.E., van Steijn, J. a Q.M.J., Kapiteijn, E., Haanen, J.B. a G., van der Burg, S.H., Nortier, J.W.R., and Osanto, S. (2011). Successful treatment of metastatic melanoma by adoptive transfer of blood-derived polyclonal tumor-specific CD4+ and CD8+ T cells in combination with low-dose interferon-alpha. Cancer Immunol. Immunother. 60, 953-963. Disclosures Schotte: AIMM Therapeutics: Current Employment, Current equity holder in private company. Villaudy:AIMM Therapeutics: Current Employment, Current equity holder in private company. Levie:AIMM Therapeutics: Current Employment, Current equity holder in private company. Go:AIMM Therapeutics: Current Employment, Current equity holder in private company. Yasuda:AIMM Therapeutics: Current Employment, Current equity holder in private company. Frankin:AIMM Therapeutics: Current Employment, Current equity holder in private company. Cercel:AIMM Therapeutics: Current Employment, Current equity holder in private company. van Hal-van Veen:AIMM Therapeutics: Current Employment, Current equity holder in private company. van de Berg:AIMM Therapeutics: Current Employment, Current equity holder in private company. Fatmawati:AIMM Therapeutics: Current Employment, Current equity holder in private company. Kedde:AIMM Therapeutics: Current Employment, Current equity holder in private company. Claassen:AIMM Therapeutics: Current Employment, Current equity holder in private company. Rijneveld:Amgen: Research Funding; Servier: Research Funding. Spits:AIMM Therapeutics: Current equity holder in private company, Ended employment in the past 24 months, Membership on an entity's Board of Directors or advisory committees. van Eenennaam:AIMM Therapeutics: Current Employment, Current equity holder in private company.

Background: Blinatumomab is approved for the treatment of adult patients (pts) with relapsed/refractory Philadelphia chromosome-negative B-cell precursor acute lymphoblastic leukaemia (R/R Ph- BCP-ALL). We present interim 3-year results of a multi-country observational study in R/R Ph- BCP-ALL pts, including the subset with late first relapse (LFR, defined as first remission duration of ≥12 months [mos]). LFR pts were excluded from the pivotal studies of R/R Ph- BCP-ALL and data is limited on blinatumomab treatment outcomes in this population. Methods: This study included adult pts who initiated blinatumomab between 22 March 2017 and 12 February 2020 in routine clinical practice. Pts who received blinatumomab in a clinical trial or an expanded access program were excluded. Informed consent was sought according to local guidelines. Data was extracted from medical records: pts were categorized as LFR by the treating clinicians. Pts were followed-up until data cut-off date, death, loss-to-follow-up, or withdrawal: whichever came first. Denominators of percentages excluded pts with missing data unless indicated. Kaplan-Meier (KM) estimates were calculated to investigate survival outcomes at 24 mos: relapse-free survival (RFS) defined as the interval between complete remission with full/partial/incomplete recovery of peripheral blood counts (CR/CRh/CRi) and relapse (bone marrow blasts >5%) or death; and overall survival (OS) defined as the interval from blinatumomab initiation to death. Cumulative Incidence Function (CIF) estimates were calculated for: mortality (independent of relapse) following allogeneic haematologic stem cell transplant (alloHSCT), and relapse (independent of unrelated death) after alloHSCT. Results: A total of 118 R/R Ph- pts were included: median age was 45.5 years (interquartile range [IQR]: 29.0, 58.0) with 47.5% (n=56) being female. For the subset of R/R Ph- pts with LFR (n=38), median age was 34.5 years (IQR: 23.0, 50.0) with 50.0% being female. Among all pts: 22% (n=26) had previous HSCT (28.9% [n=11] in the LFR subset), 100% (n=118) were treated with ≥1 prior anti-cancer therapy, 42.4% (n=50) underwent first salvage, 8.5% (n=10) second salvage, and 0.8% (n=1) third salvage. Within 2 blinatumomab cycles, 73.7% (n=87) R/R Ph- pts achieved CR/CRh/CRi: among CR/CRh/CRi pts with evaluable minimal residual disease (MRD, n=44), 45.5% had MRD response (Table). The majority (78.5%, n=62) of pts proceeded to alloHSCT, of whom 64.5% (n=40) had achieved CR/CRh/CRi and had no additional myelosuppressive therapy (Table). The KM estimates for RFS and OS at 24 mos were 50.0% (95% confidence interval [CI]: 37.0, 62.0) and 58.0% (95% CI: 46.0, 68.0), respectively. In pts who achieved CR/CRh/CRi and had no additional therapy before proceeding to alloHSCT, the CIF estimates for relapse and mortality at 1 year following alloHSCT were 8.0% (95% CI: 2.0, 20.0) and 13.0% (95% CI: 5.0, 26.0) respectively. In the LFR subset, 78.9% (n=30) achieved CR/CRh/CRi within 2 cycles: among evaluable pts (n=15) 73.3% (n=11) had MRD response (Table). Most pts (83.3%, n=25) proceeded to alloHSCT, of whom 77.3% (n=17) had achieved CR/CRh/CRi and had no additional myelosuppressive therapy (Table). At 24 mos, the KM estimates for RFS and OS were 70.0% (95% CI: 47.0, 85.0) and 74.0% (95% CI: 51.0, 87.0), respectively. In pts who achieved CR and proceeded to alloHSCT without other therapy, the CIF estimates for relapse and mortality at 1 year were both 7.0% (95% CI: 0.0, 27.0). Among R/R Ph- pts, 89.8% (n=106) had adverse events (AEs) occurring between blinatumomab initiation and ≤30 days following the final infusion, 36.4% (n=43) reported serious AEs (SAEs) and 5.9% (n=7) fatal events: 1 fatal AE was blinatumomab-related. AEs were reported in 86.8% of LFR pts (n=33), and 28.9% (n=11) of pts experienced an SAE (no fatal events occurred). In both subgroups, the most commonly reported AEs (in >34% of pts) were infusion reactions (including cytokine release syndrome), serious infections, and neurological events (Table). Conclusion: In real-world clinical practice, blinatumomab was an effective therapy in pts with R/R Ph- ALL including those with LFR. Almost three quarters of pts achieved CR/CRh/CRi within two cycles. Most pts proceeded to alloHSCT, the majority in CR/CRh/CRi and without additional myelosuppressive therapy. The safety data are consistent with the established safety profile of blinatumomab. Disclosures Rijneveld: Servier: Research Funding; Amgen: Research Funding. Fracchiolla:Amgen: Honoraria, Membership on an entity's Board of Directors or advisory committees, Other: Travel, accommodations, expenses, Speakers Bureau; Pfizer: Honoraria, Membership on an entity's Board of Directors or advisory committees, Other: Travel, accommodations, expenses, Speakers Bureau; Gilead: Honoraria, Membership on an entity's Board of Directors or advisory committees, Other: Travel, accommodations, expenses, Speakers Bureau; ABBVIE: Honoraria, Membership on an entity's Board of Directors or advisory committees, Other: Travel, accommodations, expenses. Šálek:Amgen: Consultancy, Honoraria, Research Funding. Chiaretti:Amgen: Membership on an entity's Board of Directors or advisory committees; Incyte: Membership on an entity's Board of Directors or advisory committees; Pfizer: Membership on an entity's Board of Directors or advisory committees; Shire: Membership on an entity's Board of Directors or advisory committees. Alam:Amgen: Current Employment, Current equity holder in publicly-traded company. Pezzani Grueter:Amgen: Current Employment, Current equity holder in publicly-traded company, Other: Travel, accommodations, expenses. Mohammad:Amgen: Current Employment, Current equity holder in publicly-traded company. Kormany:Amgen: Current Employment, Current equity holder in publicly-traded company. Kreuzbauer:Amgen: Current Employment, Current equity holder in publicly-traded company. Rambaldi:Omeros: Honoraria, Other: TRAVEL, ACCOMMODATIONS, EXPENSES (paid by any for-profit health care company); Amgen: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Other: TRAVEL, ACCOMMODATIONS, EXPENSES (paid by any for-profit health care company). Research grant from Amgen Inc.; Pfizer: Honoraria, Membership on an entity's Board of Directors or advisory committees, Other: TRAVEL, ACCOMMODATIONS, EXPENSES (paid by any for-profit health care company). Advisory board and speaker fees from Pfizer.; Novartis: Honoraria, Membership on an entity's Board of Directors or advisory committees, Other: TRAVEL, ACCOMMODATIONS, EXPENSES (paid by any for-profit health care company); Astellas: Honoraria, Other: TRAVEL, ACCOMMODATIONS, EXPENSES (paid by any for-profit health care company); Roche: Honoraria, Membership on an entity's Board of Directors or advisory committees, Other: Support of parent study and funding of editorial support. Received travel support., Research Funding; BMS/Celgene: Honoraria, Other: TRAVEL, ACCOMMODATIONS, EXPENSES (paid by any for-profit health care company); University of Milan: Current Employment; Gilead: Honoraria, Membership on an entity's Board of Directors or advisory committees, Other: Travel support from Gilead.; Sanofi: Honoraria, Other: TRAVEL, ACCOMMODATIONS, EXPENSES (paid by any for-profit health care company).

Red blood cell (RBC) transfusions are of vital importance in patients with sickle cell disease (SCD). However, a major complication of transfusion therapy is alloimmunization. The low-affinity Fcγ receptors, expressed on immune cells, are important regulators of antibody responses. Genetic variation in

Background: Iron overload is an emerging and underestimated problem in management of patients with hereditary anemia characterized by chronic hemolysis or ineffective erythropoiesis in the absence of regular red cell transfusions. Liver iron overload was present in 65% of the patients with hereditary hemolytic anemia who were never transfused. (Van Straaten et al., Br J Haematol. 2018) Iron-loading results from inadequately high intestinal iron absorption in response to low hepcidin and ultimately leads to iron overload in liver and other organs. Iron chelation treatment is recommended, however drug-related toxicity is considerable, and treatment with iron chelators is expensive. Alternative treatment options for anemic patients not tolerating iron chelating agents are not available. For adequate iron absorption, a low luminal pH is required to solubilize dietary iron salts, and to promote the reduction of dietary Fe(III) to Fe(II) by ferriredutases as only Fe(II) can be absorbed. Proton pump inhibitors (PPI) block gastric acid secretion and thereby impede iron absorption. No studies are available that quantify effects of PPIs on dietary iron absorption in human. However, PPIs have shown to minimize phlebotomy requirements in patients with hereditary hemochromatosis. (Vanclooster et al., Gastroenterology. 2017) Here we report the design of the currently ongoing PPI SHINE AGAIN study that evaluates the efficacy and safety of PPI treatment in patients with non-transfusion dependent hereditary anemias. Methods: The PPI SHINE AGAIN is a phase 3, multicentre, randomized, placebo-controlled, cross-over clinical trial (Netherlands Trial Register, identifier [NL6659]). Transfusion independent adults with a form of hemolytic or dyserythropoietic anemia and mild to moderate iron overload are randomized in a 1:1 ratio to start with either esomeprazole 40 mg (administered orally BID) or placebo. Mild to moderate iron overload is defined as baseline liver iron content (LIC) of 3-15 mg Fe/g dry weight as measured by T2* MRI (MRQuantif Software Université de Rennes, https://imagemed.univ-rennes1.fr/en/mrquantif/quantif.php) without or on stable iron chelation therapy with no expected dose adjustments over the next 2 years. Additional criteria included baseline Hb ≤ 11.3 g/dL, expected to receive less than 4 red cell transfusions in the following 12 months, and no phlebotomies. The trial consists of two treatment periods of 12 months each (Figure), directly starting after baseline MRI. The primary study end-point is effectiveness of PPI treatment defined as difference in delta LIC after one-year treatment with esomeprazole compared to one-year treatment with placebo. Key secondary endpoints include tolerability, quality of life (assessed by EQ5D-5L questionnaire) and cost-effectiveness (assessed by iMTA Productivity Cost Questionnaire and iMTA Medical Consumption Questionnaire). Inclusion in the PPI SHINE AGAIN was completed in April 2019. Thirty patients are enrolled (non-transfusion dependent β-thalassemia n=10; pyruvate kinase deficiency n=8; congenital dyserythropoietic anemia n=3; sideroblastic anemia n=3; SCD n=2; HbH disease n=2; hereditary elliptocytosis n=1; G6PD deficiency n=1) with a median age of 44 years (IQR 28, 54). Results are expected Q3 2020. The trial is funded by ZonMW, The Netherlands Organization for Health and Research Development, and by the Innovatiefonds Zorgverzekeraars. Disclosures Nur: Novartis Pharmaceuticals: Consultancy. van Wijk:Agios Pharmaceuticals: Consultancy, Research Funding; RR Mechatronics: Research Funding. van Beers:Agios Pharmaceuticals, Inc.: Membership on an entity's Board of Directors or advisory committees, Research Funding; Novartis: Consultancy, Research Funding; Pfizer: Research Funding; RR Mechatronics: Research Funding.

Despite rapid advances in immunotherapeutic options for precursor B-acute lymphoblastic leukemia (ALL), outcomes remain poor especially for adult ALL and relapsed pediatric ALL. With conventional chemotherapy, remission percentages in adult ALL range from 75 to 90%, but relapse rates are high and long-term leukemia-free survival ranges between 35-70% depending on age and risk group. The introduction of CD19 targeting immunotherapy has significantly improved patient outcomes in (relapsed) B-ALL. However, tumor escape via downregulation of CD19 occurs in a significant number of patients. Therefore an ongoing urgency remains for the identification of additional or alternative immunotherapeutic targets for the treatment of ALL. AT1412 is an antibody that was identified from the peripheral blood memory B cell pool of a patient cured of metastatic melanoma after adoptive T-cell therapy, using a B cell immortalization technology (AIMSelect) with ectopic Bcl-6 and Bcl-xL expression as described previously [Kwakkenbos et al. Nat. Med. 2010]. The antibody was selected based on differential binding to melanoma cells as compared to healthy melanocytes and was shown to be successful in killing melanoma cells in vitro and in vivo [manuscript submitted]. In addition to melanoma, AT1412 binds other tumor types including B-ALL, gastric, colon- and pancreatic cancer. The target of AT1412 is the tetraspanin CD9, which is expressed by more than half of all B-ALL. Expression of CD9 has been correlated with adverse prognosis [Liang et al. Cancer Biomark. 2018]. We assessed binding of this human CD9 antibody to a panel of ALL cell lines using flow cytometry. Binding of AT1412 to the B-ALL cell lines SUP-B15, MHH-CALL-2 and CCRF-SB varied as expected based on the CD9 levels that we detected using a commercial CD9 antibody. AT1412 induced antibody dependent cellular cytotoxicity (ADCC) on these cells, in line with the level of AT1412 binding. No binding was seen to the T-ALL cell line Jurkat. Importantly, these findings were confirmed in primary ALL samples, obtained prospectively at diagnosis from a cohort of patients with T- or B-ALL (n=30). AT1412 showed binding to 61% of B-ALL samples but not to T-ALL samples. The potential of AT1412 to induce ADCC was tested on patient samples from the same panel. Remarkably, AT1412 induced ADCC of all B-ALL samples it bound to (8 out of 14) and of none of the T-ALL samples. Cytotoxicity significantly correlated with the level of AT1412 binding. These findings were supported by the observation that AT1412 induced B-ALL cell death when a freshly drawn whole bone marrow sample from a patient with newly diagnosed B-ALL was cocultured with AT1412. AT1412-induced cell death of B-ALL blasts occurred without affecting the monocytic, granulocytic and lymphocytic populations. This cell death was not observed when this patient's ALL blasts were incubated with AML-targeting antibodies. Remarkably, AT1412 induced cell death in the absence of added effector cells or other (chemo)therapeutic agents, while the bone marrow sample contained over 80% blasts and as little as 3% lymphocytes. We are currently investigating the in vivo efficacy of the antibody in a humanized immune system mouse model with human B-ALL. Taken together, the majority of precursor B-ALL blasts express CD9 and expression of CD9 is associated with a dismal outcome. Our data demonstrate that CD9 can be successfully targeted by the human CD9 antibody AT1412, suggesting that AT1412 has the potential to be developed as a therapeutic antibody for B-ALL. AT1412 is currently being advanced through preclinical development. Disclosures De Jong: AIMM Therapeutics: Employment. Levie:AIMM Therapeutics: Employment. Schotte:AIMM Therapeutics: Employment, Equity Ownership, Patents & Royalties: Patent WO2017119811A1. Pos:AIMM Therapeutics: Patents & Royalties: Patent WO2017119811A1. Go:AIMM Therapeutics: Employment, Patents & Royalties: Patent WO2017119811A1. Yasuda:AIMM Therapeutics: Employment, Equity Ownership. Cercel:AIMM Therapeutics: Employment. van Hal-van Veen:AIMM Therapeutics: Employment. Frankin:AIMM Therapeutics: Employment. Villaudy:AIMM Therapeutics: Employment, Equity Ownership, Patents & Royalties: Patent WO2017119811A1. van Helden:AIMM Therapeutics: Employment, Equity Ownership, Patents & Royalties: Patent WO2017119811A1. van Eenennaam:AIMM Therapeutics: Employment. Spits:AIMM Therapeutics: Employment, Equity Ownership, Patents & Royalties: Patent WO2017119811A1. Hazenberg:AIMM Therapeutics: Other: Employment/equity of partner/spouse.

Introduction: Amongst patients with sickle cell disease (SCD) the leading cause of death is the acute chest syndrome (ACS). This pneumonia-like complication frequently occurs during or shortly after a vaso-occlusive crisis (VOC). In pediatric patients hospitalized for VOC, incentive spirometry has demonstrated to prevent the development of ACS. This study was designed to determine if a comparable effect of incentive spirometry can be demonstrated in adult patients with SCD. Furthermore, we aimed to validate the ability of the Bartolucci score to identify patients at risk of ACS and assessed the value of procalcitonin as a potential biomarker for ACS . In addition, clinical characteristics and laboratory results were determined to identify potential risk factors. Methods: In this multicenter prospective randomized trial, we included consecutive adult patients (≥18 yr) admitted for VOC presenting with chest or back pain above the diaphragm. Patients were randomly assigned to spirometry or control group. Patients presenting with ACS were excluded. A chest radiograph was performed 5 days after admission, or sooner when clinically indicated, in order to diagnose pulmonary abnormalities. ACS was defined as a new infiltrate/atelectasis combined with pulmonary symptoms. At presentation, procalcitonin plasma levels were assessed and the Bartolucci risk score was calculated to determine to the risk of developing ACS for each patient. In addition, clinical and laboratory parameters were compared between patients with and without ACS during admission. Results: In total 66 episodes of hospitalization for VOC in 48 patients were included. Median age was 26 years and 46 of the hospitalizations concerned patients with a severe genotype (HbSS/HbSβ0 thalassemia) versus 20 hospitalization with a mild genotype (HbSC/HbSβ+thalassemia). The overall incidence of ACS in this study cohort was 19.7%. In the spirometry group, ACS was diagnosed in 5/34 (14.7%) hospitalizations compared to 8/32 (25%) hospitalizations in the control group (OR 0.5 [0.15-1.8]; P=.293). Twelve of the 13 ACS episodes occurred in patients with a severe genotype. The Bartolucci risk score could be calculated for 50 hospitalizations. The scores area under the curve (AUC) was 0.747 (P=.013), with a negative predictive value (NPV) of 94% and a positive predictive value (PPV) of 31%. No difference in procalcitonin plasma levels were found between patients with and patients without ACS (0.52 ± 1.56 μg/ml versus 0.56 ± 1.44 μg/ml, respectively). At baseline, hemoglobin levels were significantly lower while LDH plasma levels, leukocyte and platelet counts were significantly higher in ACS hospitalizations as compared to non-ACS hospitalizations. Patients who developed ACS showed significantly more documented fever during admission (61.5% vs 17.0%) and a longer length of hospital stay (median 10.0 days vs 4.5 days). Conclusion: Incentive spirometry did not significantly reduce the development of ACS in this prospective study in adult patients with SCD admitted with VOC and pain above the diaphragm. Procalcitonin plasma levels and the Bartolucci score could not accurately identify patients that at risk to develop ACS, but a low score appeared to be a reliable tool to identify patients with a low risk of ACS. Disclosures No relevant conflicts of interest to declare.

Somatic mutations in isocitrate dehydrogenase 1 and 2 (IDH1 and IDH2) were recently demonstrated in acute myeloid leukemia (AML), but their prevalence and prognostic impact remain to be explored in large extensively characterized AML series, and also in various other hematologic malignancies. Here, we demonstrate in 893 newly diagnosed cases of AML mutations in the IDH1 (6%) and IDH2 (11%) genes. Moreover, we identified IDH mutations in 2 JAK2 V617F myeloproliferative neoplasias (n = 96), a single case of acute lymphoblastic leukemia (n = 96), and none in chronic myeloid leukemias (n = 81). In AML, IDH1 and IDH2 mutations are more common among AML with normal karyotype and NPM1mutant genotypes. IDH1 mutation status is an unfavorable prognostic factor as regards survival in a composite genotypic subset lacking FLT3ITD and NPM1mutant. Thus, IDH1 and IDH2 mutations are common genetic aberrations in AML, and IDH1 mutations may carry prognostic value in distinct subtypes of AML.

Urokinase-type plasminogen activator (uPA) and its receptor uPAR are compo- nents of the fibrinolytic system and are important for an adequate immune re- sponse to respiratory tract infection, in part through their role in the migration of inflammatory cells. PA inhibitor-1 (PAI-1) is the predominant inhibitor of soluble and receptor-bound uPA. To determine the role of PAI-1 in host defense against pneumococcal pneumonia, the following studies were performed: (1) Patients with unilateral community-acquired pneumo- nia demonstrated elevated PAI-1 concen- trations together with decreased PAactiv- ity in bronchoalveolar lavage fluid (BALF) obtained from the infected, but not from the contralateral, site. (2) Mice with Strep- tococcus pneumoniae pneumonia dis- played elevated PAI-1 protein and mRNA levels in their lungs. (3) PAI-1 gene- deficient mice, however, had an unaltered immune response to pneumococcal pneu- monia, as measured by cell recruitment into lungs, bacterial outgrowth, and sur- vival. Furthermore, plasminogen-gene- deficient mice also had an unremarkable defense against pneumococcal pneumo- nia. These data indicate that pneumonia is associated with inhibition of thefibrino- lytic system at the site of the infection secondary to increased production of PAI-1; an intact fibrinolytic response is not required for an adequate host re- sponse to respiratory tract infection, how- ever, suggesting that the previously de- scribed role of uPA and uPAR are restricted to their function in cell migra- tion. (Blood. 2003;102:934-939)

BCR-ABL1 and BCR-ABL1-like acute lymphoblastic leukemia (ALL) are two major pre-B cell acute leukemia subtypes characterized by genetic alterations affecting lymphoid-specific transcription factors. Studies examining the chain of genetic events necessary to develop leukemia established that the BCR-ABL1 fusion gene and kinase-activating BCR-ABL1-like lesions are initiating events, however, insufficient for leukemia development. Secondary genetic events targeting B cell development genes are therefore an essential requirement for overt ALL. A recent study (Papaemmanuil et al, Nat. Genet., 2014) revealed that illegitimate RAG-mediated recombination is the predominant mutational mechanism establishing these secondary genetic events in ETV6-RUNX1 ALL. Of note, ETV6-RUNX1ALL is mainly restricted to pediatric cases and it remains unanswered whether this mutational process also plays a prominent role in adult ALL pathogenesis. We carried out a detailed genomic characterization to determine whether aberrant RAG activity is also a prominent mutational driver in certain adult B cell ALL (B-ALL) subtypes. Diagnostic material of 53 unselected B-ALL cases and matched remission specimens were characterized using DNA mapping arrays to discern copy number alterations (CNAs). We observed multiple BCR-ABL1/BCR-ABL1-like patients with abundant genetic lesions and selected 5 cases for targeted sequencing of CNA boundaries to determine whether these lesions were driven by RAG-mediated recombination. Whole genome sequencing (WGS) for a single BCR-ABL1-like patient was used to asses this mutational mechanism genome-wide. In total 64 structural variants (SVs) could be analyzed at base-pair level. De novo motif detection on breakpoint sequences revealed the prominence of the heptamer CACAGTG (E-value=5.68x10-91), a constituent of the recombination signal sequence (RSS), present in 121 out of 128 breakpoints (94.5%). RSS detection revealed that 58 out of 64 SVs (90.6%) had a cryptic RSS (cRSS) on one or both sides of the lesion. Incorporation of non-templated sequences was observed for 54 out of the 64 (84.4%) SVs. Superimposition of breakpoints on chromatin marks revealed a strong enrichment for active promoters and enhancers (p < 2.2x10-16). WGS data revealed cRSS motifs and incorporation of non-templated sequences for 23 out of 26 SVs (88.5%). Integrative analysis of all 6 cases confirmed 125 unique SV breakpoints strongly enriched for the active chromatin marks H3K4me3 and H3K27ac. STAT5 binding, a postulated regulator of V(D)J recombination, is similarly enriched at the breakpoints. Promiscuous binding of RAG1 and RAG2 was previously noted in human thymocytes and murine pre-B cells (Teng et al, Cell, 2015). Strikingly, the breakpoints are frequently bound by RAG2 in human thymocytes. In total 66 out of 125 breakpoints could be translated to the murine genome and revealed a strong enrichment of RAG1 and RAG2 binding at homologous positions in murine pre-B cells. Exhaustive mutation detection revealed complex somatic mutations within cRSS motifs, which are rare V(D)J recombination products introduced by erroneous cleavage and error-prone repair (open-and-shut joints). Strikingly, 4 out of 6 BCR-ABL1/BCR-ABL1-like cases had mutations in the BTLA promoter-situated cRSS, frequently in combination with a RAG-mediated deletion of the other allele (Figure 1). Genomic screening in 142 B-ALL patients confirmed 8 additional cases with BTLA promoter mutations, predominantly (6 out of 8) belonging to the BCR-ABL1/BCR-ABL1-like subgroups. We provide strong evidence that aberrant RAG activity plays a pivotal role in the development of BCR-ABL1/BCR-ABL1-like adult ALL. We demonstrate that breakpoints are strongly enriched for RAG binding implying a predisposition for illegitimate V(D)J recombination. Importantly, we report on a novel mutational mechanism introducing mutations in cRSS motifs through open-and-shut joints, frequently resulting in the biallelic inactivation of BTLA. Proliferation and V(D)J recombination during pre-B cell development is orchestrated by the interplay of IL7R and pre-BCR signalling. Strikingly, most kinase-activating lesions constitutively activate these signalling cascades and could enact, in concert with BTLA inactivation, constant proliferation, pro-survival and V(D)J recombination-initiating signals with disastrous consequences. Disclosures No relevant conflicts of interest to declare.

Patients with sickle cell disease (SCD) suffer from frequent and severe episodes of pain that are associated with hospitalizations, impaired quality of life and increased mortality. Current treatment options are scarce. Oxidative stress appears to play a pivotal role in the pathophysiology of SCD. Pilot studies have demonstrated that administration of the antioxidant N-acetylcysteine (NAC) effectively reduces markers of oxidative stress in SCD and may decrease the hospitalization rate for painful crises. NAC is a safe, inexpensive and well-tolerated drug that has been used for years for various indications. The primary aim of this study was to evaluate the effect of NAC on the frequency of daily pain in patients with SCD. We conducted a randomized (1:1), double-blind, placebo-controlled, parallel-group trial at 11 sites across The Netherlands, Belgium and the United Kingdom. Patients were eligible for participation if they were ≥12 years of age, had either HbSS, HbSC, HbSβ⁰- or HbSβ⁺-thalassemia with a history of at least 1 painful crisis per year over the 3 years prior to enrolment. At randomization patients were assigned to receive either oral NAC 600 mg twice daily or placebo for a total duration of 6 months. Patients had monthly checkups during the study. The primary endpoint was the rate of SCD related pain days per patient year, assessed daily with the use of pain diaries. Secondary endpoints included the rate of days with painful crises, admission days, hospitalizations and days with home analgesic use, the severity of pain, the time to first painful crisis and hospitalization, the number of adverse events and the effect on quality of life and various blood markers. The primary intention-to-treat analysis of this study was limited to patients with a minimal completed diary observation time of 110 days. Sensitivity analyses were done in both all randomized patients as well as a subset with ≥80 days of observation time. Lastly, an additional per protocol analysis was performed on patients with adequate adherence (≥80% of tablets used) and ≥110 days of completed diary observation time. A total of 96 patients were randomized of which 67 patients met the minimum observation time of 110 days (27 in the NAC and 40 in the placebo arm). Inclusions were stopped before reaching the estimated sample size of 116 patients due to imposed time restrictions by the main funder of this study. Groups were well balanced for baseline characteristics. The proportion of patients adherent to the assigned study medication regimen was low in both treatment groups (53% in the NAC and 50% in the placebo arm). The rate of SCD related pain days per patient year was 61.4 in the placebo group and 61.6 in the NAC group (rate ratio 0.98; 95% confidence interval [CI] 0.54-1.78; P=.98). Moreover, there were no significant differences in the secondary endpoints between groups. These findings were consistent in the sensitivity analyses. In the total study population, more patients reported gastro-intestinal events in the NAC group (42%) as compared to the placebo group (15%, P In conclusion, treatment with oral NAC did not provide any clinical benefit over placebo in this phase 3 study, possibly due to low compliance rates. Therefore, a potential effect of NAC in SCD cannot fully be excluded based on these results. In an adherent subset of patients we did observe a reduction of days with painful crises and a trend to reduction in other pain related endpoints in the NAC treatment arm. To corroborate these findings additional blood sample analysis of pathophysiological markers is currently being performed. This may further elucidate on the dose-effect relation of NAC in SCD. Disclosures Fijnvandraat: Bayer: Research Funding; CSL Behring: Research Funding.

Background: Value-based health care (VBHC) according to Porter et al. is a novel approach that aims to improve health care by identifying and systematically measuring both medical and patient-centered health care outcome. Thus, including the well-being of the patient as related to physical and mental status, social functioning, and occupational consequences. By applying generic and disease specific outcome measures, health care providers are able to benchmark their center with others in order to compare care strategies and outcome, and to make informed choices with regard to optimization of care, necessary investments and possible cost reductions. Erasmus University Medical Center aims to apply this system in 80% of the disease expert centers in the upcoming three years. SickleCell Disease (SCD) is a chronic hereditary disease characterized by severe anemia, unpredictable episodes of extreme pain, cumulative organ damage and high health care utilization rates. In addition, multiple psychosocial problems influence complexity of health care delivery. Therefore, SCD is an excellent model to explore the effects of VBHC. Our hospital has a large Comprehensive Sickle Cell Care Centre. Aim: To identify ten outcome measures in SCD patients and the validated tools to quantify these measures. This is not possible without definition of baseline patient characteristics of influence on patient outcome. Methods: In six well-prepared sessions, our multidisciplinary team, including both adult and pediatric care takers defined relevant patient outcome measures and most important patient characteristics, in live sessions and surveys in collaboration with patients and parents. Results: Ten outcome measures were identified (Table 1) and discussed with patients and parents. Eight patient characteristics were defined with effect on outcome (Table 2). Eight validated tools to quantify outcome measures were described (Table 3). Specific tools and evaluation modes will be integrated into electronic patient files (Table 3). Conclusions: We believe VBHC is a valuable strategy to optimize patient care and to facilitate informed decision making with regard to health care investments. We have made a first step by identification of patient outcome measures and important baseline patient characteristics according to VBHC methodology. The coming year, outcome will be measured, thus enabling comparisons in the near future with other (inter)national centers. Table 1. Overview of most relevant patient outcome measures Mortality Anxiety Veno-occlusive crises per year Admissions per year Pain related to veno-occlusive crises Caregiver burden Complications Quality of life Social functioning Self-efficacy Table 2. Overview of baseline patient characteristics Genotype Age Born < or >January 2007* Socio-economic status Hemoglobin level Communication Comorbidity Mode of treatment Treatment exclusively in University Medical Center *Neonatal screening for SCD was implemented >January 1st, 2007 Table 3. Overview of tools to quantify patient outcome measures Target group Tool Patient Outcome Measures All ages Electronic patient file · Mortality · Complications · Admissions per year · Veno-occlusive crises per year · 'Door-to-needle-time Emergency Department >18 yrs8-18 yrs0-4 yrs EQ-5D- 5 level version (EQ-5D-5L) Pediatric Quality of Life Inventory (PEDsQL)TNO-AZL Preschool children Quality of Life (TAPQOL) · Quality of life · Social functioning · Anxiety and insecurity 18 yrs Health Care Related Quality of Life (CAREQOL-7D) Long-term Orientation (LTO) · Caregiver burden All ages Visual Analogue Scale (VAS) · Pain related to veno-occlusive crisis >18 yr Hospital Anxiety and Depression Scale (HADS) · Anxiety and insecurity All ages Sickle Cell Self-Efficacy Scale (SCSES) · Self-efficacy Disclosures Cnossen: Pfizer: Other: travel funding, Research Funding; Baxter: Other: Travel Funding, Research Funding; Bayer: Other: travel funding, Research Funding; CSL Behring: Other: travel funding, Research Funding; Novo Nordisk: Research Funding; Novartis: Research Funding.

Background In 1995 the European Group for the Immunological Classification of Leukemias (EGIL) presented guidelines to classify bilineage and biphenotypic acute leukemias (BAL). Subsequently, the WHO in 2008 defined mixed phenotype acute leukemias (MPAL), as leukemias coexpressing antigens of both lymphoid and myeloid lineage, and leukemias with distinct blast populations of more than one lineage. However, a classifying diagnosis towards myeloid or lymphoid leukemia based on an extensive molecular-biological characterization could provide much better distinction, which could be useful for selecting AML or ALL like treatment approaches. We investigated whether extensive molecular analysis might improve a diagnosis of BAL/MPAL into predominant AML or ALL. Patients and methods For this study 25 adults, diagnosed between 2000 and 2009 with BAL/MPAL were identified. Flowcytometry and cytogenetics were performed at diagnosis. In addition, molecular analysis was performed to identify BCR-ABL, MLL, AML1-ETO, CBF-MYH11, SIL-TAL1, SET-NUP, NUP-ABL fusion transcripts, FLT3-ITD, CEBPA, NPM1, NOTCH1 mutations and IKZF1, CDKN2A, CDKN2B deletions. Furthermore, multiplex PCR assays to detect immunoglobulin (Ig) and T-cell receptor (TCR) gene rearrangements were performed. Gene expression profiles (GEP) were generated, and results were compared to an AML/ALL classifier profile. A classifying diagnosis of either ALL or AML was made if at least two of the 5 assays applied, were highly suggestive for a specific lineage without explicit contradicting results of the remaining assays. Results Twenty-four of the 25 BAL/MPAL cases could be diagnosed into predominant AML or ALL by extensive molecular analysis (Table 1). In 11 cases the WHO2008 criteria classified BAL cases as AML or ALL, and extensive molecular analysis also indicated the same lineage. Adding standard cytogenetic and molecular analysis improved the WHO classification in another 4 patients. Remarkably, extensive molecular analysis clearly showed preference for either AML or ALL in all of the MPAL cases. As a result, we were able to improve a diagnosis in all MPAL cases. Only 1 of the 25 patients remained unclassified. Statistical analysis did not show significant correlation between the different assays. Twelve patients received AML-based chemotherapy, including 3 patients ultimately classified as ALL. Moreover, among 12 patients who received ALL type therapy, 1 was retrospectively classified as AML. In general, outcome appeared very poor with a 3 year overall survival of 32% (Kaplan-Meier estimate, 95%CI: 14%-51%). Conclusions This study suggests that a differential diagnosis of AML versus ALL is partly improved by applying the WHO2008 criteria instead of the EGIL classification. However, extensive molecular analysis, including GEP and Ig/TCR, more strongly enabled the identification of the lineage of origin, ultimately resulting in a classifying diagnosis in 24 out of 25 patients. So far no assay appeared as golden standard but different complementary techniques showed additive value. Collectively these results strongly argue for advanced, centralized diagnostic procedures in patients with acute leukemia of ambiguous lineage, which may subsequently enable the development of specific therapeutic approaches in these poor-risk patients. Table 1 Patient characteristics Age (year) EGIL WHO 2008 Molecular aberrations GEP probability Ig/TCR Classification AML ALL 51 TM MPAL * 0,000 1,000 POS ALL 52 TM MPAL FLT3-ITD 0,970 0,030 NEG AML 20 BM MPAL MLL-AF4 CEBPA 0,000 1,000 POS ALL 18 BM MPAL IKZF1 CDKN2A-2B 0,000 1,000 POS ALL 49 BM MPAL BCR-ABL IKZF1 ND ND POS ALL 57 BM MPAL CDKN2A 0,000 1,000 POS ALL 40 BM MPAL CDKN2A-2B ND ND POS ALL 23 T MPAL CDKN2A ND ND POS ALL 24 B MPAL * 0,000 1,000 POS ALL 65 B MPAL BCR-ABL IKZF1 CDKN2A ND ND POS ALL 58 BM AML * 1,000 0,000 NEG AML 52 BM AML NPM1 ND ND NEG AML 59 BM AML FLT3-ITD NPM1 ND ND NEG AML 41 BM AML FLT3-ITD CEBPA NPM1 ND ND POS AML 69 BM AML MLL ND ND NEG AML 18 BM AML CBF-MYH11 ND ND POS AML 71 BM # FLT3-ITD CDKN2A-2B 0,709 0,291 NEG AML 67 BM # * 0,003 0,997 POS ALL 15 BM ALL IKZF1 CDKN2A-2B ND ND POS ALL 23 BM # * 0,998 0,002 POS ? 32 BTM # * 0,938 0,062 POS AML 27 BTM ALL IKZF1 0,001 0,999 POS ALL 69 BTM ALL CDKN2A-2B 0,098 0,902 NEG ALL 16 BTM ALL CDKN2A-2B ND ND POS ALL 54 TM ALL NOTCH1 0,148 0,852 NEG ALL # Unclear classification according to WHO2008 * No abnormalities Abbreviations: ND, not determined. Disclosures No relevant conflicts of interest to declare.

The thrombomodulin–protein C–protein S (TM-PC-PS) pathway exerts anticoagulant and anti-inflammatory effects. We investigated the role of TM in the pulmonary immune response in vivo by the use of mice with a mutation in the TM gene (TMpro/pro) that was earlier found to result in a minimal capacity for activated PC (APC) generation in the circulation. We here demonstrate that TMpro/pro mice also display a strongly reduced capacity to produce APC in the alveolar compartment upon intrapulmonary delivery of PC and thrombin. We monitored procoagulant and inflammatory changes in the lung during Gram-positive (Streptococcus pneumoniae) and Gram-negative (Klebsiella pneumoniae) pneumonia and after local administration of lipopolysaccharide (LPS). Bacterial pneumonia was associated with fibrin(ogen) depositions in the lung that colocalized with inflammatory infiltrates. LPS also induced a rise in thrombin-antithrombin complexes in bronchoalveolar lavage fluid. These pulmonary procoagulant responses were unaltered in TMpro/pro mice, except for enhanced fibrin(ogen) deposition during pneumococcal pneumonia. In addition, TMpro/pro mice displayed unchanged antibacterial defense, neutrophil recruitment, and cytokine/chemokine levels. These data suggest that the capacity of TM to generate APC does not play a role of importance in the pulmonary response to respiratory pathogens or LPS.

Background Transfusion of red blood cells (RBC) is a common intervention to treat and prevent complications in sickle cell disease (SCD). However, frequent transfusions may lead to erythrocyte alloimmunization, thereby complicating donor matching procedures and posing patients at risk for hemolytic transfusion reactions. Little information is available about the risk of alloimmunization of sickle cell patients living in European countries. In the Netherlands extensive matching procedures to prevent alloimmunization were introduced a decade ago, but the effect on alloimmunization has not been evaluated yet. Aims The primary aim of this study is to evaluate the cumulative incidence of first alloantibody formation in a Dutch cohort of transfused SCD patients, and to compare this with a general Dutch RBC-transfused population. In addition, the effect of extended RBC matching protocols on the incidence of alloantibody formation in SCD and potential clinical determinants of alloimmunization will be assessed. Methods We conducted a retrospective cohort study and collected data on SCD patients (genotypes HbSS, HbSC, HbSβ0 and HbSβ+ thalassemia), diagnosed in three Dutch Sickle Cell Treatment Centers that received non-extended matched (ABO, RhD) RBC transfusions between 1984-2004 and extended matched (at least ABO, Rhesus phenotype, Kell) RBC transfusions between 2004-2011. In addition, we compared this population with a general population of 3 042 patients that received non-extended matched (ABO, RhD) RBC transfusions between 2005-2009 in the Leiden University Medical Center (Zalpuri et al. 2012). Cohorts were not matched for ethnicity. Alloimmunization risk was calculated as Kaplan-Meier incidence with cumulative number of transfusions as time variable. The association with the clinical determinants gender, SCD-phenotype and ethnicity was analyzed with Cox-regression analysis. Results A total of 291 SCD patients received 7 957 RBC units. Alloantibody formation occurred in 52 (17.9%) patients. The cumulative incidence of alloimmunization was 9% after 5 RBC units, 15% after 10, 24% after 20 and 34% after 40 RBC units. Multivariate analysis, correcting for the cumulative number of transfusions, demonstrated a significantly increased risk of alloantibody formation in our SCD cohort when compared to a general population of transfused patients (HR 7.5 (95% CI: 5.06-11.14), where the cumulative incidence of alloimmunization was 1.1% after 5, 2.4% after 10, 3.4% after 20 and 6.5% after 40 RBC units. No association could be demonstrated between alloantibody formation and clinical determinants such as gender, SCD-phenotype or ethnicity. However, a significant reduction in alloimmunization was observed in SCD patients that received their first transfusion from the year 2004 onwards, after preventive matching for Rhesus phenotype and Kell was introduced for SCD patients (HR 0.48 (95% CI: 0.24-0.97)). Conclusion The overall rate of first RBC alloantibody formation in our cohort was 17.9% and the risk of alloimmunization increased substantially with an increasing number of RBC transfusions. A unique comparison with a general cohort of Dutch transfused patients demonstrates a significantly higher risk of alloantibody formation in SCD, acknowledging earlier findings. This may partially be explained by differences in RBC antigens between patients of African descent and the predominantly Caucasian donors. Besides the number of RBC units, no other clinical risk factors for allo-immunization in SCD could be identified. The effectiveness of extended RBC matching protocols in the prevention of alloimmunization for chronically transfused patients in the participating centers was confirmed. Disclosures: No relevant conflicts of interest to declare.