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Overcoming drug resistance and targeting cancer stem cells remain challenges for curative cancer treatment. To investigate the role of microRNAs (miRNAs) in regulating drug resistance and leukemic stem cell (LSC) fate, we performed global transcriptome profiling in treatment-naive chronic myeloid leukemia (CML) stem/progenitor cells and identified that miR-185 levels anticipate their response to ABL tyrosine kinase inhibitors (TKIs). miR-185 functions as a tumor suppressor: its restored expression impaired survival of drug-resistant cells, sensitized them to TKIs in vitro, and markedly eliminated long-term repopulating LSCs and infiltrating blast cells, conferring a survival advantage in preclinical xenotransplantation models. Integrative analysis with mRNA profiles uncovered PAK6 as a crucial target of miR-185, and pharmacological inhibition of PAK6 perturbed the RAS/MAPK pathway and mitochondrial activity, sensitizing therapy-resistant cells to TKIs. Thus, miR-185 presents as a potential predictive biomarker, and dual targeting of miR-185-mediated PAK6 activity and BCR-ABL1 may provide a valuable strategy for overcoming drug resistance in patients.

Aging is associated with significant changes in the hematopoietic system, including increased inflammation, impaired hematopoietic stem cell (HSC) function, and increased incidence of myeloid malignancy. Inflammation of aging (“inflammaging”) has been proposed as a driver of age-related changes in HSC function and myeloid malignancy, but mechanisms linking these phenomena remain poorly defined. We identified loss of miR-146a as driving aging-associated inflammation in AML patients. miR-146a expression declined in old wild-type mice, and loss of miR-146a promoted premature HSC aging and inflammation in young miR-146a–null mice, preceding development of aging-associated myeloid malignancy. Using single-cell assays of HSC quiescence, stemness, differentiation potential, and epigenetic state to probe HSC function and population structure, we found that loss of miR-146a depleted a subpopulation of primitive, quiescent HSCs. DNA methylation and transcriptome profiling implicated NF-κB, IL6, and TNF as potential drivers of HSC dysfunction, activating an inflammatory signaling relay promoting IL6 and TNF secretion from mature miR-146a−/− myeloid and lymphoid cells. Reducing inflammation by targeting Il6 or Tnf was sufficient to restore single-cell measures of miR-146a−/− HSC function and subpopulation structure and reduced the incidence of hematological malignancy in miR-146a−/− mice. miR-146a−/− HSCs exhibited enhanced sensitivity to IL6 stimulation, indicating that loss of miR-146a affects HSC function via both cell-extrinsic inflammatory signals and increased cell-intrinsic sensitivity to inflammation. Thus, loss of miR-146a regulates cell-extrinsic and -intrinsic mechanisms linking HSC inflammaging to the development of myeloid malignancy.

Introduction Pulmonary embolism (PE) is a leading cause of cardiovascular morbidity worldwide. The risk of early death in the setting of untreated PE may be as high as 30%. However, diagnostic and therapeutic advances in recent years have led to a progressive decline in global PE-related mortality and recent data describing rates of in-hospital death following PE suggest a mortality rate of approximately 5-15%. Moreover, strategies directed at stratification of PE severity have been shown to safely identify a sub-group of low-risk patients (up to 30-50% of all patients) for whom outpatient management is feasible without the need for hospital admission. Avoiding hospitalisation for low-risk PE patients is associated with improved patient satisfaction and avoids exposing patients to the risks associated with hospital admission. Ambulatory PE management would also be predicted to lead to significant healthcare cost-savings. However ambulatory care models for low-risk PE appear to be under-utilised despite these potential benefits. Barriers to implementation include access to outpatient follow-up services and the perceived risks associated with this model of care. The Ireland East Hospital Group (IEHG) is the largest hospital network in the Republic of Ireland, consisting of 11 hospitals (including large academic centres, community general hospitals and the national maternity hospital). The IEHG serves a population of over 1.1 million individuals. We sought to determine the frequency of admissions to hospital with PE and to assess key outcomes, including length-of-stay (LOS) and in-hospital mortality within this population. Methods Data pertaining to PE diagnosis from January 2018 to December 2020 were obtained from NQAIS Clinical (National Quality Assessment and Improvement System; an electronic reporting tool which is populated with anonymised data extracted from the hospital in-patient enquiry system). This system compiles diagnostic data on all patients by ICD-10 code at the time of discharge. For the purposes of this analysis the ICD-10 codes I26.0 and I26.9 were used to identify patients with PE and only admission episodes where PE was the primary diagnosis were included; cases of 'secondary PE' (historical PE or hospital-acquired) were excluded. Projected population figures, extrapolated from Census 2016 data, were obtained from Health Atlas Ireland (an open-source application providing access to datasets developed by the Health Intelligence Unit of the Health Service Executive of Ireland). Results During the 3-year study period, 958 in-patient episodes occurred where PE was recorded as the primary diagnosis, corresponding to an incidence of 0.37 per 1000 adults per annum (95% CI 0.35 to 0.40). The incidence was highest in the over 85 years age-group (1.07 per 1000 per annum; 95% CI 0.80 to 1.33). PE was more common in women in all age-groups apart from the 46-65 years age group [males: 0.51 (95% CI 0.44-0.51) vs females: 0.36 (95% CI 0.3-0.42) per 1000]. In 82.7% of episodes, the ultimate discharge destination was to home. In 5.3% the discharge destination was a nursing home and 4.6% were transferred to another hospital. The all-cause in-hospital mortality rate was 3.1% (30 fatalities; 18 females, 12 males). Most deaths occurred in the 66-85 years age-group (n=14), with 9 fatalities in the age >85 years group and 7 fatal PE events in the 46-65 years age-group. Average hospital LOS was 7.8 days. 8.9% of inpatient episodes resulted in same-day discharge. In 55.9% of episodes, discharge occurred after day 4. Those discharged to home had an average length of stay of 6.31 days, while patients awaiting nursing home facilities averaged 26.5 days. Conclusion The incidence of acute presentation with PE within this population is consistent with international reports. The rate of in-hospital mortality compares favourably with these international standards. The mortality rate may reflect improvements in PE care but may also reflect the inclusion of a significant number of 'low-risk' individuals in the analysis (many of whom may have been suitable for outpatient management). The mortality rate might also reflect increased detection of small, low-risk distal PE (as a result of advances in diagnostics). In any event, these data suggest that more widespread implementation of outpatient PE management is likely to be feasible and would represent an opportunity for improved resource utilisation. Disclosures Ni Ainle: Leo Pharma: Research Funding; Actelion: Research Funding; Daiichi-Sankyo: Research Funding; Bayer Pharma: Research Funding. Kevane: Leo Pharma: Research Funding.

Introduction: Acute lymphoblastic leukemia (ALL) is the most common type of pediatric cancer. Unfortunately, approximately 15% of children with high-risk B-cell ALL (B-ALL) relapse after frontline chemotherapy. Treatment of relapsed/refractory B-ALL is still challenging, and more effective novel therapies are urgently warranted. Blinatumomab, a first in class bispecific antibody therapeutic, has demonstrated superiority compared to standard chemotherapy in patients with B-ALL and has a manageable toxicity profile. Blinatumomab functions by binding to CD19 expressed on B-cells and CD3 expressed on T-cells, resulting in T-cell-mediated killing of CD19-positive cells common in B-cell malignancies. Despite remarkable efficacy and a manageable toxicity profile compared to standard-of-care chemotherapy, blinatumomab poses unique healthcare system challenges related to preparation, administration, toxicity monitoring, and medication error prevention. The drug's success in helping patients achieve complete remission relies on its continuous and uninterrupted administration. In order to ensure that it is delivered in the safest and most effective manner, education on its unique logistical and administration challenges is imperative. Objectives: The primary objective of this study is to describe and share the 6 years of institutional experience on the outpatient delivery of blinatumomab for the management of pediatric patients with B-ALL as per Children's Oncology Group protocols, as well as to retrospectively analyze the safety of this novel 28-day home-based therapy. Methods: A multidisciplinary team composed of physicians, nurses, and pharmacists was created to address administration challenges associated with blinatumomab infusions. Although blinatumomab requires a 28-day continuous infusion, it is not necessary for patients to remain hospitalized for the entire cycle. To ensure tolerability prior to discharge, patients are monitored closely during the first 3 days of Cycle 1 and 2 days of Cycle 2 for signs of cytokine release syndrome and neurological toxicities. Once discharged, they are seen every 96/72 hours for bag changes in either an outpatient hematology/oncology unit or by home health for those off study. Results: A total of 16 patients were treated with blinatumomab between May 2015 and June 2021; 10 were newly diagnosed and 6 were in first relapse. Of the 26 total infusions, 24 were successfully completed without significant adverse reactions. Two patients treated for relapsed disease had to discontinue therapy; one experienced neurotoxicity within 72 hours of blinatumomab infusion initiation and the other developed refractory disease and was switched to another protocol. No adverse events were observed in the home setting. Discussion: The team was successfully able to transform the original inpatient-only blinatumomab protocol to the outpatient setting. Retrospective analysis over 6 years demonstrates a clinically significant reduced rate of complications of blinatumomab administration in comparison to previous reports (Amicucci et al. 2021), which can be attributed to careful multidisciplinary team planning and delivery. This study confirms the feasibility of a home-based continuous blinatumomab infusion without adverse effects on safety. Additionally, this outpatient protocol leads to cost savings associated with reduced length of stay and an overall improved quality of life for pediatric patients able to receive therapy at home with their caregivers. Disclosures No relevant conflicts of interest to declare.

INTRODUCTION: The recent introduction of new therapies for people with hemophilia A and B (HA and HB) mandates careful monitoring of the safety of these treatments. The primary aim of ATHN 7: A Natural History Study of the Safety, Effectiveness, and Practice of Treatment for People with Hemophilia (NCT03619863) is to monitor the safety of current hemophilia therapies. METHODS: ATHN 7 is a longitudinal, prospective cohort study being conducted at 26 American Thrombosis and Hemostasis Network (ATHN)-affiliated sites. The study is approved by central and local institutional review boards. Any person with a diagnosis of congenital hemophilia A or B (factor VIII or IX activity < 50%) who receives care at a participating site is eligible for inclusion. Participants and/or a parent/guardian sign informed consent and assent prior to participation. Adverse events, including those events designated by the European Haemophilia Safety Surveillance group as well as other adverse events of special interest, are recorded and monitored. Demographic and clinical information are collected at baseline and at least quarterly through participant interview and medical record review. Descriptive statistics of medical history and demographic data as well as longitudinal data are used to characterize the study population. RESULTS: As of June 2021, a total of 391 participants had enrolled in ATHN 7. The median age of participants was 22.5 ± 17.8 years. Males represented 97.7% (382/391) of participants. Eighty-five percent (333/391) were not Hispanic. White participants comprised 82.6% (323/391) of the study population, while 7.9% (31/391) were Black/African American, 3.8% (15/391) were Asian, 1.8% (7/391) were of mixed race, and 3.8% (15/391) were of other or unknown race. The primary diagnosis of participants included 70.1% (274/391) with severe HA, 15.3% (60/391) with moderate HA, 7.2% (28/391) with mild HA, 4.1% (16/391) with severe HB, 2.0% (8/391) with moderate HB, and 1.0% (4/391) with mild HB. Nineteen percent (71/391) of participants had a documented history of an inhibitor at the time of enrollment with a mean peak inhibitor titer of 96.7 ± 214.3 Bethesda Units. Substitution therapy with a non-factor molecule was the primary medication for 43.4% (162/391) participants, while 4.3% (16/391) utilized a plasma-derived clotting factor concentrate, 28.4% (196/391) utilized a standard half-life, recombinant factor concentrate, 18.3% (126/391) utilized an extended half-life recombinant factor concentrate, 4.1% (28/391) utilized a bypassing agent, and 19.3% (133/391) utilized some other hemostatic agent. Continuous prophylaxis was the primary regimen for 79.1% (295/391) participants. The remaining 19.6% (73/391) of participants were on episodic therapy. Within the cohort of 391 participants, a total of 14 adverse events have been reported in 8 participants. No adverse events of special interest were reported. Redness or rash at injection site accounted for 64.2% (9/14) of the reported adverse events, all in participants receiving emicizumab. Two participants experienced bruising or bleeding while on emicizumab. Two participants on emicizumab were diagnosed with malignancy, neither found to be attributed to their therapy. One participant developed an allergic reaction to their standard half-life recombinant factor concentrate. DISCUSSION: Open to enrollment in January 2019, ATHN 7 has now collected 18 months of longitudinal safety data for therapies used to treat HA and HB in 391 participants, an addition of 12 months and 24 participants since our last report. As the study was designed to enhance representation of those utilizing substitution therapy, almost half of the participants were on emicizumab for prophylaxis of bleeding. Adverse events attributable to hemophilia therapy was limited to minor skin reactions. As previously reported, a single participant experienced delayed bleeding after closed head trauma (despite factor replacement and normal intracranial imaging at the time of the event), providing potentially important information on the risk of delayed bleeding from significant trauma in those receiving emicizumab. Over the coming year, participants in ATHN 7 will be transitioned to ATHN's new longitudinal natural history study, ATHN TRANSCENDS (NCT04398628). Disclosures Buckner: Pfizer: Honoraria; Takeda: Honoraria; Bayer: Honoraria; Sanofi: Honoraria; Spark: Honoraria; Genetech: Honoraria; CSL Behring: Honoraria; Novo Nordisk: Honoraria; Tremeau Pharmaceuticals: Consultancy, Honoraria; BioMarin: Consultancy, Honoraria; uniQure: Consultancy, Honoraria; American Thrombosis: Membership on an entity's Board of Directors or advisory committees; Hemostasis Network: Membership on an entity's Board of Directors or advisory committees. Carpenter: Hemophilia and Thrombosis Research Society: Membership on an entity's Board of Directors or advisory committees; Kedrion Pharmaceuticals: Honoraria; Genentech: Honoraria; Novo Nordisk: Honoraria. Raffini: Genentech: Consultancy; CSL Behring: Consultancy; HEMA Biologics: Consultancy; XaTek: Consultancy; Bayer: Consultancy. Zia: Takeda: Consultancy; Hemophilia and Thrombosis Research Society: Membership on an entity's Board of Directors or advisory committees. Recht: Hema Biologics: Consultancy; Kedrion: Consultancy; Novo Nordisk: Consultancy; Octapharma: Consultancy; Pfizer: Consultancy; Sanofi: Consultancy; Takeda: Consultancy; uniQure: Consultancy; Foundation for Women and Girls with Blood Disorders, Partners in Bleeding Disorders: Speakers Bureau; American Thrombosis and Hemostasis Network: Current Employment; Oregon Health & Science University: Current Employment; Genentech: Consultancy; CSL Behring: Consultancy; Catalyst Biosciences: Consultancy.

Background: Clinical researchers affiliated with the American Thrombosis and Hemostasis Network (ATHN) conduct multi-institutional, observational cohort studies assessing the safety and effectiveness of various interventions for people affected by bleeding and clotting disorders. In parallel with the growth of ATHN's clinical studies, the number of new therapies for all congenital and acquired hematologic conditions, not just those for bleeding and clotting disorders, is increasing. Some of the recently FDA-approved therapies for congenital and acquired hematologic conditions have yet to demonstrate long-term safety and effectiveness. In addition, results from well-controlled, pivotal studies often cannot be replicated once a therapy has been approved for general use. With this explosion of potential new therapies on the horizon, it is imperative that clinicians and clinical researchers in the field of non-neoplastic hematology have a uniform, secure, unbiased, and enduring method to collect long-term safety and efficacy data. The overarching objective of ATHN Transcends (NCT04398628) is to characterize the safety, effectiveness, and practice of treatments for all people with congenital and acquired hematologic disorders in the United States. Study Design and Methods: ATHN Transcends is a longitudinal, natural history, observational cohort study being conducted at approximately 150 ATHN-affiliated sites (ATHN Affiliate Network). Participants will be followed for a minimum of 15 years. Specific data will be collected for participants enrolled in cohort-specific arms. Each participant will be assigned in a single cohort: Hemophilia, Von Willebrand Disease, Congenital Platelet Disorder, Rare Bleeding Disorders, Bleeding Not Otherwise Specified, Thrombosis/Thrombophilia, or Non-Neoplastic Hematologic Conditions (see Figure 1). Inclusion criteria include: any age, any congenital or acquired non-neoplastic hematologic disorder, having a bleeding phenotype with an unknown diagnosis, or having a connective tissue disorder with a bleeding tendency. Exclusion criteria include not qualifying for a cohort and the unwillingness or inability to give informed consent or assent. Data will be collected at baseline, every three months, annually and at study exit (Figure 2). Participant biologic samples will be collected at enrollment and yearly thereafter. Safety endpoints mirror those collected by the European Haemophilia Safety Surveillance and include allergic and other acute events, treatment-emergent side effects of therapy, transfusion-transmitted infections, inhibitor development, thrombosis and cardiovascular events, malignancies, neurologic events, and deaths. Adverse events of special interest including thrombotic microangiopathies, anti-drug antibodies, unanticipated bleeding, and hospitalizations will be collected. A panel of patient-reported outcomes (Table 1) will be collected at baseline and annually. Descriptive statistics will be calculated to analyze the primary and secondary outcomes. If there are questions involving multiple cohorts, given there is adequate power to make comparisons between cohorts, for a discrete outcome, we will report estimated difference in percentage between cohorts as well as the 95% confidence interval (CI) of such a difference. If the outcome measurement is a continuous variable, we will report the mean difference and its associated 95% CI. Discussion: The key advantages of an independent study conducted by the ATHN Affiliate Network include the ability to observe participants on a variety of treatment regimens regardless of regimen dosing, frequency, or time of initiation; the ability to observe participants on recently FDA-approved therapies as well as continued monitoring of well-established therapies; the ability to enrich the ATHNdataset; and the ability to be the initial cohort study involving potentially all ATHN-affiliated sites to provide the infrastructure for all congenital and acquired hematologic disorders-related sub-studies in cooperation with other funders, including federal, foundation, academic and industry sources. ATHN Transcends received central institutional review board approval in April 2020. The protocol is currently being rolled out throughout the ATHN Affiliate Network. Figure 1 Figure 1. Disclosures Malec: HEMA Biologics: Consultancy; Genentech: Consultancy; Sanofi: Consultancy, Research Funding; Pfizer: Consultancy; CSL Behring: Consultancy; Takeda: Consultancy. Recht: uniQure: Consultancy; Takeda: Consultancy; Sanofi: Consultancy; Pfizer: Consultancy; Octapharma: Consultancy; Novo Nordisk: Consultancy; Kedrion: Consultancy; Hema Biologics: Consultancy; Genentech: Consultancy; CSL Behring: Consultancy; Catalyst Biosciences: Consultancy; Foundation for Women and Girls with Blood Disorders, Partners in Bleeding Disorders: Speakers Bureau; American Thrombosis and Hemostasis Network: Current Employment; Oregon Health & Science University: Current Employment.

Extensive research has led to recent approval of novel therapies such as mylotarg, venetoclax, glasdegib and CC486, and small molecule inhibitors against actionable mutations such as ivosidenib (IDH1), enasidenib (IDH2), gliteritinib and midostaurin (FLT3) in AML. However, the mainstay of treatment in AML remains unchanged since the 1970s. There is a significant unmet need for AML patients that fail to respond to or relapse after standard-of-care (SOC) treatments including allogeneic stem cell transplantation and targeting actionable mutations. In addition, a large fraction of SOC patients invariably relapse due to persistence of chemotherapy-resistant leukemia stem cells (LSCs) or immune evasion. Therefore, identification of unique therapies that preferentially target elusive LSCs and promote immune responses to AML to prevent relapse are highly sought after. Unlike, targeting acute lymphoblastic leukemia (ALL) with CD19 or CD22 with various modalities, when developing AML therapies, it is of paramount importance to differentiate LSCs from hematopoietic stem cells (HSCs) to lessen or abolish unavoidable cytopenias. Leukocyte-associated immunoglobulin-like receptor 1 (LAIR-1) is an immune checkpoint receptor on T cells and myeloid cells that delimits immune cell activation through binding to endogenous collagen ligands. In addition, LAIR-1 is universally expressed on AML blasts and may sustain AML survival signals. We demonstrated using multi-color flow cytometry that LAIR-1 is highly expressed in AML blasts (n=9 of 9) and that LAIR-1 expression in LSCs (markers: CD34 +CD38 -CD90 -CD45RA +/- or CD34 -CD117 +CD244 +/-) is high compared with negligible expression of LAIR-1 in HSCs (markers: CD34 +CD38 -CD90 +CD99 -) (n=3) (Figure 1). Based on these findings, we hypothesized that a LAIR-1 monoclonal antibody (mAb) would disrupt LAIR-1 mediated survival signaling and preferentially target LAIR-1 expressing AML LSCs and blast cells but not HSCs. To test this, we developed a novel LAIR-1 targeting mAb with a functional human IgG 1 isotype that blocks LAIR-1 binding to its ligands (including collagens, complement component C1q, MBL and SP-D) To characterize the anti-leukemic effect of the LAIR-1 mAb we performed an in vitro antibody dependent cell cytotoxicity (ADCC) assay with LAIR-1 expressing AML cells (MOLT4 and MV-4-11). Compared with isotype control, the LAIR-1 mAb significantly increased leukemia cell death (MV411 = 17% above isotype, and MOLT4 = 29.24% above isotype at 15 µg/ml), suggesting that the LAIR-1 mAb confers ADCC activity against LAIR1 + AML cells (Figure 2). To elucidate if the LAIR-1 mAb has a direct signaling effect on LAIR-1 + AML cells, a colony forming unit assay using primary AML cells was carried out. Interestingly, the LAIR-1 mAb inhibited colony formation by AML CD34 + cells (40-60% decreased compared with isotype control, N=4), but not normal CD34 + cells. These data suggests that our LAIR-1 mAb stimulated LAIR-1 signaling that inhibits LSC self-renewal. We then tested the in vivo anti-leukemia effect of the mAb in cell line derived xenograft (CDX) models (immune deficient mice transplanted with MV-4-11 expressing luciferase). In vivo bioluminescence imaging indicated that the LAIR-1 mAb significantly inhibited in vivo AML growth (91% reduction of total flux)(Figure 3). A significant increase in cell death was observed in the presence of the mAb in the blood (47%), spleen (89.4%) and bone marrow (27.6%). Similar to the anti-leukemic effect in CDX AML models, the LAIR-1 mAb significantly suppressed in vivo growth of AML patient derived xenografts (5 different primary AML donors) (10-90% human CD33 + AML cells in isotype control treatment vs 0.5-5% CD33 + AML cells in anti-LAIR-1 treatment, N=3) (Figure 4), while minimally impacting normal immune cells. Taken together, our studies suggest that the LAIR-1 mAb we generated is a novel AML immunomedicine that preferentially eradicates AML LSCs and blasts while preserving healthy HSCs through disruption of AML survival signals and clearance of AML through ADCP and ADCC. Additional studies are currently evaluating if this novel LAIR-1 mAb has other mechanisms of action that contribute to overall in vivo activity, including reduction of AML niche implantation, regulation of bone marrow homing and regulation of anti-tumor immunity. Figure 1 Figure 1. Disclosures Tian: NextCure: Ended employment in the past 24 months. Paucarmayta: NextCure: Current Employment. Lovewell: NextCure: Current Employment. Maloveste: NextCure: Current Employment. Copeland: NextCure: Current Employment. O'Neill: NextCure: Current Employment. Patel: NextCure: Current Employment. Liu: NextCure: Current Employment, Current holder of stock options in a privately-held company. Myint: NextCure: Current Employment, Current holder of stock options in a privately-held company. Langermann: NextCure: Current Employment, Current holder of stock options in a privately-held company. Flies: NextCure: Current Employment, Current holder of stock options in a privately-held company. Kim: Nextcure: Research Funding.

BACKGROUND Waldenström Macroglobulinaemia (WM) is an indolent, IgM-producing lymphoplasmacytic lymphoma (LPL) that infiltrates bone marrow (BM) and other tissues. Only symptomatic patients (with IgM-related complications, cytopenias, constitutional symptoms, bulky extramedullary disease) require treatment. Symptomatic BM lesions are not a recognised manifestation of WM and, if present, may raise the possibility of high-grade transformation. AIMS To characterise the clinical, radiological and histological features of focal bone marrow lesions (FBML) identified in WM patients with unrelenting pain in bones and/or joints, and outline implications for treatment. METHODS This study retrospectively reviewed investigations performed at the time of FBMLpresentation, including MRI of the site of pain, total body FDG-PET/CT, review of histology from BM trephine biopsy of the posterior superior iliac spine (PSIS) and CT-guided core biopsy of the identified FBML, and routine blood panels. RESULTS The 6 patients identified (Table 1) presented with localised skeletal pain at different stages of their WM disease course: 1 at initial WM diagnosis, 1 after 10 years of observation, 3 in remission following R-chemotherapy, and 1 during active treatment with R-chemotherapy. Median age at diagnosis of WM was 57 years (41-64 years). Median time from diagnosis to presentation with FBML was 28 months (0-120 months). MRI demonstrated well-defined areas of abnormal signal within the medullae of symptomatic bones and no evidence of cortical or trabecular bone involvement (Fig. 1A), with T1 hypointensity, diffusion restriction and mild STIR hyperintensity in excess of background BM changes. The FBML showed a predilection for the lower limbs (knees, ankles and/or feet involvement in 3 patients, 50%) and periarticular spaces (5 patients, 83%). They were CT-occult and only mildly FDG-avid. In all cases, core biopsies of the FBML showed heavy BM infiltration with diffuse interstitial infiltrate of small, mature lymphocytes (Fig. 1B, 1C) expressing CD20, CD79a and IgM; CD138 staining highlighted scattered interstitial plasma cells ( On blood panels, LDH, ALP and platelet count were normal and Hb was >100g/L in all but the 1 patient with active WM (who had Hb 79g/L and ALP 149IU/L). There was no corresponding rise in paraprotein at the time of FBML presentation. In all cases, the detection of FBML prompted initiation/escalation of systemic treatment despite no evidence of progression using conventional criteria; this resulted in resolution of pain and disappearance of the T1 hypointense lesions on MRI in all cases. CONCLUSION Painful, LPL-dense, infiltrative lesions confined to the BM of affected appendicular bones are an unreported manifestation in WM. They are clinicopathologically different from the osteolytic lesions seen in myeloma and the painless, diffuse BM infiltration seen in low-grade lymphomas and can induce unrelenting focal skeletal pain. Based on our findings, such cases should be evaluated for focal bone marrow lesions; MRI is the preferred modality as lesions are CT-occult. They do not necessarily represent high-grade transformation. This small yet comprehensive series suggests they constitute a hitherto undescribed novel indication for systemic therapy irrespective of the general disease status. Disclosures D'Sa: Janssen:Honoraria, Research Funding;BeiGene:Honoraria, Research Funding;Sanofi:Honoraria.

Background: Myelodysplastic syndrome (MDS) is a clonal hematopoietic neoplasm that results in bone marrow failure and frequently leads to acute myeloid leukemia (AML). Hypomethylating agents (HMA) are the only FDA-approved treatment for MDS and among few options for chemotherapy-ineligible patients with AML. There are limited options for patients in whom HMA therapy fails and who are not candidates for allogeneic stem cell transplant, so therapies that complement or restore sensitivity to HMAs are needed. Studies have shown increases in bone marrow microvessel density (MVD) and angiogenic markers in patients with MDS and AML. A decrease in MVD has been shown to correlate with response to hypomethylating agents in MDS. A receptor-ligand interaction, comprised of receptor EphB4 and membrane localized ligand EphrinB2, mediates angiogenesis in normal tissue and appears to be a target unique to many cancer types. We have previously shown EphB4 to be highly expressed and a driver of leukemic cell survival in a subset of AML patients. Our group has developed a human fusion protein, sEphB4-HSA, that blocks bidirectional signaling induced by EphB4-EphrinB2 interaction to inhibit tumor cell proliferation and angiogenesis. In phase I clinical trials of sEphB4-HSA in various tumor types, there were no myelosuppressive effects and minimal toxicity. Given its safety in phase I and potential to inhibit leukemic cell proliferation and angiogenesis, we proposed a pilot trial to evaluate the safety of sEphB4-HSA in combination with HMAs in MDS and AML patients who have failed treatment with HMAs. Methods: This pilot study was designed to enroll 6 patients with relapsed/refractory intermediate or high-risk MDS and 6 patients with AML refractory to or relapsed to HMA treatment and who are deemed unfit for chemotherapy. Treatment consisted of sEphB4-HSA 15 mg/kg IV every 2 weeks in combination with the FDA-approved HMA most recently or currently being used for treatment (decitabine 20mg/m2 IV/1hr on days 1 to 5 every 28 days or azacitidine 75mg/m2 SC or IV on days 1 to 7 every 28 days). Patients were treated for as long as they were receiving clinical benefit up to 12 months. The primary endpoint was toxicity and tolerability of sEphB4-HSA in combination with HMA. Toxicity was assessed and graded after each cycle according to the CTCAE version 4. Tolerability was defined as the ability to complete two cycles of treatment without the occurrence of dose-limiting toxicity. A secondary efficacy endpoint was to assess best overall response, based on the IWG Working Group Criteria for MDS and AML, during the first two cycles of treatment. Enrollment was stopped after 7 patients due to expiration of funding. Results: Three patients with intermediate-risk MDS were treated for a median duration of 6 cycles (2-12) and 4 patients with AML were treated for 2 cycles. Median age was 75.5 years (67.9-84.8) and 57.1% were male. HMA included azacitadine in 6 patients and decitabine in one patient. There were no dose-limiting toxicities. There were 8 grade 3/4 events attributed to HMA, which included: neutropenia (2), thrombocytopenia (3), and leukopenia (3). There were 3 grade 3/4 events attributed to sEphB4, which included: febrile neutropenia (1), leukopenia (1), and hypertension (1). Of the MDS patients, 2 had stable disease, 1 patient after 2 cycles and 1 patient after 4 cycles. One patient achieved a hematologic improvement-erythroid after 6 cycles. AML patients had no disease response. Reasons for treatment discontinuation were death (1), disease progression (2), patient's decision (1), physician's decision (2), and hospice (1). Notably, a comparison of bone marrow biopsies at baseline and after 8 weeks of treatment demonstrated a decrease in MVD (Figure 1). Discussion: This pilot study found sEphB4 in combination with HMAs to be tolerable with no significant toxicity beyond that expected with HMA therapy and associated with potential clinical benefit in MDS patients. Improvement in abnormal bone marrow MVD may indicate a potential for sEphB4-HSA plus HMA therapy to alter the malignant microenvironment in MDS/AML. Disclosures No relevant conflicts of interest to declare.

Background: Novel coronavirus infection (SARS CoV-2 or COVID-19) is associated with a high risk of thrombotic complications, including macro- and micro-thrombi in major organs, leading to increased morbidity and mortality. Anticoagulant use, mainly heparin, which has both anticoagulant and anti-inflammatory properties, has been suggested as potentially beneficial. However, the optimal dose of anticoagulant for patients with COVID-19 is unknown. Establishing the optimal thromboprophylaxis strategy and determining the role of biomarkers for patient risk stratification may help to improve outcomes in COVID-19. Methods: This single-center retrospective cohort study is part of an ongoing Quality Improvement project on the use of an anti-factor Xa-driven heparin protocol, which includes a low-dose intravenous (IV) unfractionated heparin (UFH) option, being conducted at our medical center. Data on the type, dose, and indication for anticoagulation as well as outcomes including thrombosis, bleeding and survival was collected for inpatients diagnosed with COVID-19 between mid-March and June 15, 2020. To address COVID-coagulopathy we developed a d-dimer-based anticoagulation protocol for patients with COVID-19 (Figure 1). We recorded anticoagulant use as either standard prophylactic, escalated prophylactic (low-dose intravenous unfractionated heparin titrated to achieve an anti-factor Xa level of 0.1-0.3 anti-Xa units or enoxaparin 0.5mg/kg subcutaneously every 12 hours) or standard therapeutic dose used during the hospitalization. The primary endpoints assessed were ISTH-defined major and clinically relevant non-major bleeding (CRNMB) events and survival. Secondary endpoints included incidence of breakthrough thrombosis and duration of hospitalization. Results: A total of 263 patients with COVID-19 were reviewed. Of these, 68.44% of patients received prophylactic, 12.55% escalated prophylactic and 19.01% therapeutic dosage. Of total, 129 (49%) were receiving ICU level of care. No major bleeding events were observed. The incidence of CRNMB was 4.56% in the whole cohort, which did not differ significantly between the escalated prophylactic and therapeutic groups (12% and 12.12%, respectively). Patients treated with standard prophylaxis had less CRNMB (1.11%), but this was not statistically significant in a multivariate analysis that included other confounding factors such as age, sex, ethnicity, BMI, comorbidity, HASBLED bleeding risk, and sepsis induced coagulopathy score (SICS). The mortality rate was 12.6% in the whole cohort (7.22%, 21.21% and 26% in prophylactic, escalated prophylactic, and therapeutic dosage, respectively). Factors significantly associated with increased mortality included age and ICU level of care (HR 1.10, 95%CI [1.05, 1.15] and HR 20.42, 95%CI [2.84, 146.72], respectively). The use of therapeutic dose heparin and high-flow nasal cannula demonstrate a survival benefit in multivariate analysis (HR 0.13, 95%CI [0.04,0.44] and HR 0.23, 95%CI [0.07, 0.72], respectively; Figure 2). Breakthrough thrombosis occurred in 7 (2.66%) patients; 1 (0.56%), 1 (3.03%) and 5 (10%)) in prophylactic, escalated prophylactic and therapeutic dosage, respectively but very few diagnostic tests were performed during this time period. Duration of hospitalization was significantly longer in the therapeutic dose group when compared to escalated prophylaxis and standard prophylactic groups. Conclusion: In this cohort of inpatients with COVID-19, there were no major bleeding events related to any dose of heparin or LMWH prophylaxis. By multivariate analysis, implementation of a d-dimer-titrated anticoagulation strategy was not associated with increased CRNMB. Therapeutic dose heparin based on a d-dimer-driven anticoagulation protocol was associated with a survival benefit in COVID19-infected patients. Limitations of this study include the retrospective observational nature and a lack of a uniform diagnostic protocol for patients with suspected VTE. Although no significant difference in bleeding events were observed in our study subgroups, randomized clinical trials are necessary to determine optimal thromboprophylaxis strategy in the COVID-19 population. Disclosures No relevant conflicts of interest to declare.

Introduction Hypersensitivity reactions (HSRs) can be induced by many medications, including chemotherapy agents and monoclonal antibodies (mAbs). Virtually all chemotherapeutics have the potential to initiate infusion reactions. Allergies to chemotherapeutics and biologic agents usually manifest with skin symptoms (rash, itching, and swelling), angioedema, or even potentially fatal anaphylactic reactions. Medication allergies may lead to discontinuation of life-prolonging therapies and therefore worsen prognosis. For clinical situations where hypersensitivity reactions prevent administration of a critical medication, desensitization can be an extremely useful addition to the clinical toolkit. In the early 2000s, multiple clinical trials showed that Rapid Drug Desensitization (RDD) for chemotherapy and mAbs was safe and effective in improving clinical outcomes and allowing patients to remain on preferred first-line therapy. Desensitization is challenging however, and this is reflected in variable success rates reported in the literature. One recent review noted successful desensitization in anywhere from 20 to 75% of cases (Ruggiero et al. 2017). This wide range is likely due to different methods of desensitization used in various studies. There are multiple variables influencing the success of desensitization, including the starting dose, the infusion rate, and the number of increments. Desensitization protocols are often complex requiring multiple changes in the rate and volume of medication administered between steps--making it difficult to complete them correctly. We have developed a new, simpler desensitization method that does not require multiple changes in the infusion rate and volume, thus reducing clinical confusion and leading to improved patient outcomes. Methods We developed a standardized protocol for rapid desensitization in patients who had hypersensitivity reactions to carboplatin or rituximab. This new desensitization method utilizes the same rate, administration time, and volume for each dose, only changing the drug concentration gradually between steps. This method helps minimize the chance of errors that occur due to repeatedly reprogramming the rate and volume of the infusion. A total of 10 patients were treated under this desensitization protocol between November 2017 and June 2020 for a total of 76 desensitization episodes. Initial desensitization occurred in the medical intensive care unit and subsequent infusions of carboplatin took place in an outpatient hematology-oncology setting. We retrospectively analyzed safety and efficacy of this novel desensitization protocol through review of treatment records. Results Of the 76 desensitizations performed, no adverse reactions occurred during desensitization and patients received their full target dose in all cases. No breakthrough reactions were noted during subsequent infusions of carboplatin or rituximab in desensitized patients. To prevent breakthrough reactions, antihistamines, corticosteroids and a leukotriene modifier were administered as pre-medications. Patients undergoing carboplatin desensitization were able to be switched from a 12-step protocol to a 10-step protocol with the same 100 % success rate in completing the entire planned cycle of chemotherapy. Discussion This retrospective analysis demonstrated a statistically and clinically significant reduced rate of breakthrough reactions in comparison to previous reports (Brennan et al. 2009). The other benefits of this novel method include improvement in safety of administration and cost savings. Disclosures No relevant conflicts of interest to declare.

BACKGROUND Bendamustine and Rituximab (BR) therapy is recommended in international consensus guidelines for treatment of Waldenström Macroglobulinaemia (WM) in both frontline and relapsed settings. The optimal dose and schedule of Bendamustine is not well established. Dose options include 90mg/m2 or 70mg/m2 on days 1&2 of each of 6 cycles (total Bendamustine dose of 1080mg/m2 and 840mg/m2, respectively) +/- reduction from 6 to 4 cycles. AIMS Determine response rates (IWWM criteria), PFS and toxicity following BR in frontline and relapsed settings Determine the impact of depth of response and Bendamustine dose on PFS METHODS A multicentre, retrospective cohort analysis was undertaken of consecutive WM patients treated with BR in the frontline or relapsed settings. Data were collected from 17 sites across 4 countries, including UK centres participating in the Rory Morrison Registry. RESULTS Data from Sep 2010 to May 2020 were collected for 250 patients. Frontline (n=139, 55.6%) and relapsed (n=111, 44.4%) cohorts were matched in terms of sex, age at commencement of BR, ECOG score, and baseline parameters including: haemoglobin, platelet count, bone marrow infiltration, and presence of adenopathy, splenomegaly and extranodal disease. At a median follow up of 37 months, disease progression had occurred in 25 frontline patients (18.0%) and 48 relapsed patients (43.2%; p Major responses (≥PR) differed significantly between frontline and relapsed cohorts (91.4% vs 73.9%, respectively; p Best response significantly impacted on PFS and OS. Two-year predicted PFS rates for those achieving CR/VGPR vs PR were 96% vs 82%, respectively (p=0.002, Fig. 1A). Median OS was 83 months after achieving CR/VGPR, 65 months after achieving PR/MR and 28 months after achieving SD/PD (p Relapsed patients had received 1 (n=53, 47.7%), 2 (n=28, 25.2%) or ≥3 (n=30, 27.0%) prior therapies. PFS was significantly longer in the frontline cohort compared with recipients of ≥2 prior lines of treatment, but not between cohorts treated frontline vs after 1 prior therapy line (Fig. 1B). Rates of toxicity-related treatment truncation were significantly lower in the frontline setting (17.3% vs 35.1%; p Frontline patients received higher total Bendamustine doses than relapsed patients (median total dose 1080mg/m2 vs 720mg/m2; p Age did not significantly affect tolerated dose in the frontline cohort, with similar median total Bendamustine doses in the CONCLUSION Outcomes for WM patients following BR are excellent. Frontline patients tolerate higher doses of Bendamustine, and achieve deeper responses and longer PFS than relapsed patients. In both settings, attaining CR/VGPR results in superior PFS and OS. Our data clearly delineate the Bendamustine doses required to achieve optimum PFS. At frontline, 6 cycles of 90mg/m2 on days 1&2 is superior to lower doses with respect to response and PFS. In the relapsed cohort, maximum response and PFS benefit are seen with 4 cycles of 90mg/m2 on days 1&2; a starting dose of 70mg/m2 on days 1&2 is also sufficient provided 5-6 cycles are administered. Disclosures Menne: Kyowa Kirin: Other: Travel expenses; AstraZeneca: Research Funding; Pfizer: Honoraria, Other; Atara: Honoraria; Novartis: Honoraria, Research Funding; Kite/Gilead: Honoraria, Other: Travel expenses; Celgene: Honoraria, Other: Travel expenses; Daiichi Sankyo: Honoraria; Amgen: Honoraria, Other: Travel expenses; Janssen: Honoraria, Research Funding; Takeda: Honoraria; Bayer: Other: Travel expenses; Roche: Honoraria. Talaulikar:Janssen: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding, Speakers Bureau; Amgen: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Roche: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Takeda: Research Funding. Pratt:Binding Site Ltd: Other: Personal fees; Amgen: Other: Personal fees; Janssen: Other: Personal fees; Celgene: Other: Personal fees; Takeda: Other: Personal fees; Gilead: Other: Personal fees; Sanofi-Genzyme: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Beigene: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Karyopharm: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding. Manos:Bristol-Myers Squibb: Other: Conference sponsorship. Gavriatopoulou:Genesis Pharma: Consultancy, Honoraria; Amgen: Consultancy, Honoraria; Janssen: Consultancy, Honoraria; Takeda: Consultancy, Honoraria; Karyopharm: Consultancy, Honoraria. Minnema:Kite, a Gilead Company: Speakers Bureau; Celgene: Other: travel support, Research Funding; Servier: Consultancy; Amgen: Consultancy. El-Sharkawi:Roche: Other: Conference fees; Janssen: Honoraria, Membership on an entity's Board of Directors or advisory committees. Kersten:Kite, a Gilead Company: Consultancy, Honoraria, Other: travel support; Novartis: Consultancy, Honoraria, Other: travel support; Miltenyi: Consultancy, Honoraria, Other: travel support; Roche: Research Funding; Takeda: Research Funding; Celgene: Research Funding. McCarthy:Abbvie: Membership on an entity's Board of Directors or advisory committees; Janssen: Honoraria. Bishton:Janssen: Consultancy; Takeda: Other: Travel/accommodations/expenses, Research Funding; Roche: Other: Travel/accommodations/expenses, Research Funding; Gilead: Other: Travel/accomodations/expenses, Research Funding; AbbVie: Research Funding. Follows:Roche: Consultancy, Other: Paid lecturing; Karyopharm: Consultancy, Other: Paid lecturing; Janssen: Consultancy, Other: Paid lecturing; Abbvie: Consultancy, Other: Paid lecturing; Bristol Myers Squibb: Consultancy, Other: Paid lecturing; Astrazeneca: Consultancy, Other: Paid lecturing. Wechalekar:Janssen, Takeda, Caelum, Celgene: Honoraria, Membership on an entity's Board of Directors or advisory committees. D'Sa:BeiGene: Honoraria, Research Funding; Janssen: Honoraria, Research Funding; Sanofi: Honoraria.

Despite genetic heterogeneity, myelodysplastic syndromes (MDSs) share features of cytological dysplasia and ineffective hematopoiesis. We report that a hallmark of MDSs is activation of the NLRP3 inflammasome, which drives clonal expansion and pyroptotic cell death. Independent of genotype, MDS hematopoietic stem and progenitor cells (HSPCs) overexpress inflammasome proteins and manifest activated NLRP3 complexes that direct activation of caspase-1, generation of interleukin-1β (IL-1β) and IL-18, and pyroptotic cell death. Mechanistically, pyroptosis is triggered by the alarmin S100A9 that is found in excess in MDS HSPCs and bone marrow plasma. Further, like somatic gene mutations, S100A9-induced signaling activates NADPH oxidase (NOX), increasing levels of reactive oxygen species (ROS) that initiate cation influx, cell swelling, and β-catenin activation. Notably, knockdown of NLRP3 or caspase-1, neutralization of S100A9, and pharmacologic inhibition of NLRP3 or NOX suppress pyroptosis, ROS generation, and nuclear β-catenin in MDSs and are sufficient to restore effective hematopoiesis. Thus, alarmins and founder gene mutations in MDSs license a common redox-sensitive inflammasome circuit, which suggests new avenues for therapeutic intervention.

Background: Idiopathic erythrocytosis (IE) is an entity characterized by a persistently elevated hemoglobin, variable erythropoietin (EPO) level, absence of janus kinase 2 (JAK2) mutations suggestive of polycythemia vera (PV) and no identifiable secondary cause. Previous studies have compared IE to PV, showing a lower incidence of venous thrombosis and leukemic transformation in IE but similar incidence of arterial events. PV is known to be associated with constitutional symptoms and splenomegaly, while hereditary erythrocytosis can be associated with recurrent headaches and fatigue. A comprehensive assessment of clinical features including symptoms in IE has not been performed. Methods: Patients signed informed consent to participate in an observational study approved by the Institutional Review Board. Enrollment criteria included: age 18 years or older; hemoglobin level greater than 16 g/dL on two occasions at least 3 months apart or greater than 15 g/dL if undergoing phlebotomy; negative testing for JAK2 mutations; and negative work up for secondary causes of erythrocytosis. Baseline assessment included history and physical exam, vital signs, pulse oximetry, and body mass index. Baseline laboratory exams included a complete blood cell count, complete metabolic panel, C-reactive protein, iron panel, ferritin, hemoglobin A1C, erythropoietin level. Abdominal ultrasound was performed to evaluate for splenomegaly. The Myeloproliferative Symptom Assessment Form (MPN-SAF) was used to assess for the presence and severity of a broad range of symptoms that may be expected to occur in patients with IE. The MPN-SAF was administered at baseline and every 6 months thereafter. Results: 35 patients had data available for analysis. Patient characteristics are shown in Table 1. The most prevalent co-morbid conditions were those known to be associated with cardiovascular disease risk and metabolic syndrome, including hepatic steatosis identified on abdominal ultrasound in 63% of patients. Three (8.6%) patients had a history of venous or arterial thrombosis. Two (5.8%) patients had a history of lymphoma (NK/T-cell and Hodgkin). Three patients (8.6%) had a first-degree relative with a myeloproliferative neoplasm (chronic myelomonocytic leukemia, essential thrombocytosis and polycythemia vera) and one patient had a son with IE and history of stroke. 16 (46%) patients were taking aspirin and 11 (31%) had undergone phlebotomy within 3 months of study enrollment. Patients reported the following symptoms on the MPN-SAF at baseline: fatigue (77%), early satiety (57%), difficulty sleeping (57%), numbness/tingling (51%), headaches (49%), concentration problems (40%), itching (40%), bone pain (37%), night sweats (37%), depression (37%), abdominal pain (37%), abdominal discomfort (37%), inactivity (37%), problems with sexual desire/function (34%), dizziness/lightheadedness (31%), cough (26%), fever (17%), and unintentional weight loss (17%). Fatigue carried the highest average symptom intensity (3.77, SD 3.17). Discussion: In this study, we describe the clinical features associated with IE in a multiracial cohort. Patients with IE are frequently symptomatic and have a high incidence of hepatic steatosis by ultrasound. Disclosures No relevant conflicts of interest to declare.

The combination of pomalidomide (POM) and dexamethasone (DEX) was evaluated for relapsed/refractory primary central nervous system lymphoma (PCNSL) and primary vitreoretinal lymphoma (PVRL) to determine the maximal tolerated dose (MTD) of POM as the primary objective, and overall response rate (ORR), progression-free survival (PFS), and safety profile as secondary objectives. A cohorts-of-3 study design was used with a dose-escalation schedule consisting of POM (3, 5, 7, or 10 mg) orally daily for 21 days every 28 days and DEX 40 mg orally every week. After 2 cycles, POM was continued alone until disease progression, intolerance, or subject withdrawal. Following MTD determination, the MTD cohort was expanded. Twenty-five of 29 patients with the median of 3 prior treatments were eligible for assessment as per international PCNSL collaborative group criteria. The MTD of POM was 5 mg daily for 21 days every 28 days. Whole-study ORR was 48% (12 of 25; 95% confidence interval [CI], 27.8%, 68.7%) with 6 complete response (CR), 2 complete response, unconfirmed (CRu), and 4 partial response (PR). MTD cohort ORR was 50% (8 of 16; 95% CI, 24.7%, 75.4%) with 5 CR, 1 CRu, and 2 PR. Median PFS was 5.3 months (whole study) and 9 months (for responders). One patient had pseudoprogression. Grade 3/4 hematologic toxicities included neutropenia (21%), anemia (8%), and thrombocytopenia (8%). Grade 3/4 nonhematologic toxicities included lung infection (12%), sepsis (4%), fatigue (8%), syncope (4%), dyspnea (4%), hypoxia (4%), respiratory failure (8%), and rash (4%). POM/DEX treatment is feasible with significant therapeutic activity against relapsed/refractory PCNSL and PVRL. This trial was registered at www.clinicaltrials.gov as #NCT01722305.