ALBERT

Description: Key Points Tcl1 cooperates with the NF-kB pathway in the pathogenesis of the aggressive form of CLL.

Description: Dematin is a relatively low abundance actin binding and bundling protein associated with the spectrin–actin junctions of mature erythrocytes. Primary structure of dematin includes a loosely folded core domain and a compact headpiece domain that was originally identified in villin. Dematin’s actin binding properties are regulated by phosphorylation of its headpiece domain by cyclic adenosine monophosphate–dependent protein kinase. Here, we used a novel gene disruption strategy to generate the whole body dematin gene knockout mouse model (FLKO). FLKO mice, while born at a normal Mendelian ratio, developed severe anemia and exhibited profound aberrations of erythrocyte morphology and membrane stability. Having no apparent effect on primitive erythropoiesis, FLKO mice show significant enhancement of erythroblast enucleation during definitive erythropoiesis. Using membrane protein analysis, domain mapping, electron microscopy, and dynamic deformability measurements, we investigated the mechanism of membrane instability in FLKO erythrocytes. Although many membrane and cytoskeletal proteins remained at their normal levels, the major peripheral membrane proteins spectrin, adducin, and actin were greatly reduced in FLKO erythrocytes. Our results demonstrate that dematin plays a critical role in maintaining the fundamental properties of the membrane cytoskeleton complex.

Description: Background: The availability of ibrutinib, an oral Bruton's tyrosine kinase inhibitor, has transformed the management of CLL. For older patients, first-line ibrutinib (± obinutuzumab) has shown advantage over chlorambucil (± obinutuzumab; Burger, NEJM 2015; Moreno, Lancet Oncol 2018) or bendamustine/rituximab (Woyach, NEJM 2019). However, its high cost raises concerns about potential disparities in access. Other concerns in older patients include the risk of atrial fibrillation (AFib), ventricular arrhythmias (VArr), or central nervous system hemorrhage (CNSh) associated with ibrutinib. We examined treatment patterns and outcomes among Medicare beneficiaries with CLL after the approval of ibrutinib in 2014. Methods: Using the linked SEER-Medicare registry, which covers ~34% of the US population, we identified Medicare beneficiaries with CLL diagnosed in 2007-2015, who had complete claims (including outpatient prescriptions), and who received systemic treatment in 2007-2016. We identified ibrutinib use as first or subsequent lines of therapy. We classified common first-line regimens as: 1) rituximab alone, 2) bendamustine ± CD20 antibody, 3) obinutuzumab ± chlorambucil, 4) other cytotoxic agents, and 5) ibrutinib (± CD20 antibody). We calculated total spending (observed in SEER-Medicare data) on CLL drugs per year, described duration of ibrutinib therapy, survival after starting ibrutinib, and the cumulative incidence function (CIF) for AFib, VArr, and CNSh during ibrutinib therapy. Using time-split survival analysis, we calculated the incidence rate ratio (IRR) for these events among patients starting ibrutinib compared with those managed without ibrutinib. All estimates report 95% confidence intervals (CI). Results: Ibrutinib was administered to 594 (18.2%) of 3,271 beneficiaries with CLL (at median age of 77 years [y]), including 196 in first line. The proportion of patients receiving ibrutinib as first-line therapy increased from 7% in 2014 to 36% in 2016 (Fig. A). As a result, Medicare spending on ibrutinib skyrocketed, surpassing all other CLL chemotherapeutics in 2016 (total $29.2M for ibrutinib, $18.5M for other drugs combined; Fig. B). Median gross cost of the first ibrutinib prescription was $9,301 (interquartile range [IQR], $8,635-10,233). Median patient's out-of-pocket (OOP) responsibility was $2,296 (IQR, $30-2,739), markedly lower for beneficiaries with the low-income subsidy ($4) than those without it ($2,538). Despite this variation, first-line ibrutinib use was not associated with any socio-demographic factor, including the low-income subsidy (P=.67 in χ2 test), age (P=.95), sex (P=.52), race/ethnicity (P=.31), or prevalent poverty (P=.35). Median duration of ibrutinib therapy was 13.6 months [mo.] (95%CI, 11.7-16.1), similar in first and subsequent line (P=.93, Fig. C). Median overall survival after starting ibrutinib was 30.3 mo. (95%CI, 22.3-not reached) in first line, and 24.6 mo. (95%CI, 21.6-28.7) in subsequent lines (Fig. D; P=.08). Median survival after stopping ibrutinib was 12.4 mo. (95%CI, 7.5-20.6) in first line, and 7.5 mo. (95%CI, 4.6-11.1) in subsequent lines (P=.06). The CIF of AFib on ibrutinib was 15.7% at 1 y (95%CI, 12.3-19.4), and 23.7% at 2 y (95%CI, 18.9-28.8; Fig. E). The incidence of AFib was significantly higher among patients receiving ibrutinib compared with other treatments (IRR, 1.65; 95%CI, 1.37-2.00). The CIF of VArr at 1 y was 4.4% (95%CI, 2.8-6.5%), also significantly increased (IRR, 1.62; 95%CI, 1.10-2.39). The CIF of CNSh at 1 y was 1.7% (95%CI, 0.8-3.0) (Fig. F), and significantly increased on ibrutinib (IRR, 4.48; 95%CI, 2.31-8.68). Among patients starting ibrutinib, 11.4% were on anticoagulants, 19.2% had prior atrial fibrillation, and 10.8% were on CYP3A4 inhibitors interacting with ibrutinib. Conclusions: Ibrutinib has been rapidly adopted for treatment of Medicare beneficiaries with CLL, resulting in major shifts in spending on CLL drugs. Despite high OOP costs, the use of first-line ibrutinib is not associated with sociodemographic characteristics, suggesting that therapeutic decisions are determined by clinical factors. The real-world survival of older patients on ibrutinib is poorer than seen in clinical trials, and rates of AFib, VArr and CNSh are higher. The use of ibrutinib in patients with pre-existing Afib or at risk for bleeding because of anticoagulation warrants particular attention. Disclosures Olszewski: Spectrum Pharmaceuticals: Research Funding; Adaptive Biotechnologies: Research Funding; TG Therapeutics: Research Funding; Genentech: Research Funding. Davids:Acerta Pharma, Ascentage Pharma, Genentech, MEI pharma, Pharmacyclics, Surface Oncology, TG Therapeutics, Verastem: Research Funding; AbbVie, Astra-Zeneca, Genentech, Janssen, MEI, Pharmacyclics, Syros Pharmaceuticals, Verastem: Consultancy; Research to Practice: Honoraria; AbbVie, Acerta Pharma, Adaptive, Biotechnologies, Astra-Zeneca, Genentech, Gilead Sciences, Janssen, Pharmacyclics, TG therapeutics: Membership on an entity's Board of Directors or advisory committees.

Description: Background: NLPHL is a rare, biologically distinct subtype of Hodgkin lymphoma (HL) characterized by late recurrences and risk of transformation to non-Hodgkin histology. The incidence of NLPHL is markedly increased among black patients (pts), but they were not distinguished in prior studies, and data are lacking about specific features in this group. Relative benefits of various treatment modalities in early-stage NLPHL are also controversial. Radiation therapy (RT) is endorsed by guidelines, but one large study suggested a survival advantage of combined modality therapy (CMT) over historical controls treated with RT alone (Savage et al., Blood 2011:118:4585). The objective of this study was to compare clinical features of NLPHL in black and other pts, and to describe treatment patterns and survival using the National Cancer Data Basea registry capturing over 70% of incident cancers in the United States (US). Methods: We identified NLPHL and classical HL cases diagnosed between 1998 and 2011, with survival data available for the 1998-2006 cohort. Receipt of chemotherapy (ChT), RT or CMT as initial course of treatment was recorded. Stage was categorized as limited (Ann Arbor I, IIA) or advanced (IIB, III, IV). Overall survival (OS) was the primary outcome and was compared using log-rank tests stratified by stage, sex and age group. Trends were assessed using log-linear regression. All confidence intervals (CI) were set at 95%. Results: Among the NLPHL pts (N=2,656), 23% were of black race (N=602), compared with 10% of classical HL cases (N=66,369). Most NLPHL pts presented with limited stage disease (66%), without any difference between races (P=0.79). Stage migration was evident between 1998 and 2003 with the proportion of stage III/IV cases increasing by 13.2% per year (CI, 3.3-24.0, P=0.008). The male-to-female ratio was 1.0 (CI, 0.9-1.2) in black pts, whereas pronounced male predominance was present in others (ratio 2.5, CI, 2.2-2.7). Black pts were also on average younger (median age 42 years, versus 45 for others, P=0.0001), and their NLPHL more often originated from the axillary nodes (25%, versus 17% for other races, P=0.0003). Median follow-up was 7.1 years. OS at 7 years was 90.1% (CI, 87.7-92.1) for limited- and 80.1% (CI, 75.6-83.8) for advanced-stage NLPHL, with no difference between the races (P=0.15, Fig. A). In limited disease, RT alone was used in 55%, 31% and 27% of pts with stage IA, IB and IIA, respectively, while CMT was used in 21%, 33% and 35%, and ChT alone in 12%, 24% and 31%, respectively. Black pts were more likely to have no treatment recorded (P

Description: Background: Ibritumomab tiuxetan (IbT) is a radioimmunoconjugate linking rituximab with 90Y isotope. IbT is approved for treatment of relapsed/refractory indolent B-cell lymphomas, or for consolidation after chemotherapy in follicular lymphoma (FL), yet its use has been limited by concern about treatment-related myeloid neoplasms (tMDS/AML: acute myeloid leukemia and myelodysplastic syndrome). The risk of tMDS/AML may differ among older patients who experience significant competing mortality. Our objective was to use population-based data to examine real-life patterns of use of IbT, and associated incidence of tMDS/AML among older patients with B-cell lymphomas. Methods: From the SEER-Medicare registry, which covers about 28% of the United States population, we selected fee-for-service Medicare beneficiaries 〉60 years [y] old, with B-cell lymphomas diagnosed between 2001-2015, who received systemic therapy. We identified the use of IbT, purine analogues, alkylating agents, and rituximab. We defined tMDS/AML as myeloid neoplasms diagnosed 〉2 months from the start of first-line therapy for lymphoma, or use of tMDS/AML-directed chemotherapy. We estimated incidence rate and incidence rate ratio (IRR) for tMDS/AML after IbT exposure using a time-split survival model. By matching IbT recipients with non-recipients according to age, sex, lymphoma histology, and observation time from the start of lymphoma therapy, we compared the cumulative incidence function of tMDS/AML using competing risk models stratified by matched groups. We fitted the univariate model, and model adjusting for prior exposure to purine analogues or alkylators. Results: Among 43,945 Medicare beneficiaries (median age of 76 y) treated for diffuse large B-cell (DLBCL, 48%), FL (22%), mantle cell (7%), marginal zone (10%), or unspecified B-cell lymphoma (14%), 428 (1%) received IbT, at median 18 months from first-line chemotherapy (interquartile range [IQR], 8-34). IbT was used most frequently in FL (50%) and DLBCL (23%), and was applied as first-line treatment in 30 patients (7%). With median follow-up of 8.5 y, 618 cases of tMDS/AML were recorded (15 among IbT recipients, and 603 among non-recipients). Median latency from the start of first-line lymphoma therapy to tMDS/AML was 38 months (IQR, 18-65). The cumulative incidence rate of tMDS/AML after IbT exposure was 2.8% at 5y (95%CI, 1.2-4.4) and 3.9% at 10y (95%CI, 1.8-6.0). No tMDS/AML was observed among patients who received IbT as first-line treatment, before chemotherapy exposure. In the time-split survival analysis, the incidence of tMDS/AML was 0.39 per 100 person-years (95%CI, 0.18-0.60) without IbT exposure, and 1.02 (95%CI, 0.56-1.71) after IbT exposure, with an IRR of 3.22 (95%CI, 1.80-5.26). There was no significant heterogeneity in the IRR by lymphoma histology (Mantel-Haenszel test of homogeneity, P=.47). In the matched analysis (Fig A), the cumulative incidence of tMDS/AML was significantly higher among patients receiving IbT (subhazard ratio [SHR], 2.16; 95%CI, 1.32-3.55). However, the association became non-significant after adjusting for prior exposure to myelotoxic chemotherapy (SHR, 1.58; 95%CI, 0.93-2.68). Prior exposures to a purine analogue (SHR 3.83; 95%CI, 2.96-4.95) or an alkylator (SHR, 1.39; 95%CI, 1.11-1.75) remained significantly associated with increased risk of tMDS/AML. Median overall survival after tMDS/AML diagnosis was 0.6y (95%CI, 0.5-0.7), and it did not differ according to IbT exposure status (P=.62; Fig B). Conclusions: The cumulative incidence of secondary tMDS/AML in this large population-based cohort is similar to reports from clinical trials (Czuczman et al. J Clin Oncol. 2007), and lower than in some prior observational series (Epperla N, et al., Br J Haematol 2017). Our results support the use of IbT for older patients with indolent lymphomas. Exposure to IbT was associated with an increased incidence of tMDS/AML, but it was no longer significant after adjustment for prior exposure to myelotoxic chemotherapy. This result suggests that the increased incidence of tMDS/AML is a cumulative effect of IbT with other myelotoxic agents. Caution should thus be exercised when using IbT after exposure to chemotherapy. The fact that no tMDS/AML were observed in patients receiving IbT as first line therapy suggests that earlier use of IbT may be preferable from the point of view of the tMDS/AML risk. Disclosures Olszewski: Genentech: Research Funding; TG Therapeutics: Research Funding; Spectrum Pharmaceuticals: Research Funding; Adaptive Biotechnologies: Research Funding. OffLabel Disclosure: Ibritumumab tiuxetan - for treatment of any B-cell lymphoma in any setting

Description: Abstract 3326 Immune thrombocytopenia (ITP) is a relatively prevalent disease in dogs with significant morbidity and mortality. Canine ITP is clinically analogous to human ITP, with heterogeneity in bleeding manifestations in individuals with similar platelet counts. With a view to ultimately investigate this bleeding heterogeneity, we set out to develop a canine model of ITP. There are currently no existing large animal models of ITP. An induced canine ITP model would be representative of ITP without the confounding co-morbidities seen in clinical cases. Since spontaneous ITP occurs in both dogs and humans, the dog is an ideal translational model. We hypothesized that 2F9, a murine IgG2a monoclonal antibody to the canine platelet glycoprotein GPIIb (a common target of autoantibodies in ITP), would induce predictable dose-dependent thrombocytopenia (TCP) in healthy dogs. 2F9 had not been previously administered in vivo. We produced highly purified 2F9 and αYFA antibodies from the 2F9 hybridoma (gift of David Wilcox, Blood Research Institute, Wisconsin) and an isotype control murine anti-yellow fever antibody (αYFA) hybridoma. A dose titration (2 dogs) and a dose repeatability study (3 dogs) were performed in healthy adult research dogs by repeated intravenous infusion (≤ 6 doses) of 2F9 antibody until a target nadir of 5–30 × 103 platelets/μl was reached. Platelet counts were performed hourly until the platelet count reached the desired nadir range (t=0 hrs), after which complete blood counts were performed at 2, 4, 6, 8, 12, 24 hours, then q 24 hours for 10 days. The following were evaluated throughout the study: physical examination, buccal mucosal bleeding time (BMBT, baseline and t=0 only), serum cytokines and chemokines (INFγ, Interleukin (IL) 2, 6, 7, 8, 10, 15, 18, KC, IP-10, MCP-1, GM-CSF, TNFα; Milliplex CCYTOMAG-90K), fibrinogen, and D-dimers. Specificity of the 2F9 effect was confirmed by IV infusion of the isotype control (αYFA) to 3 dogs at the highest cumulative effective dose of 2F9 (167 μg/kg); all parameters were measured as above (t=0 hrs was one hour after αYFA dosing). Within 2 hours of a median cumulative 2F9 administration of 63 μg/kg (range 50.0–166.6 μg/kg), all dogs developed profound TCP (range 11–28 × 103/μl). Compared to the control group, platelet nadir was significantly lower (median (range): 6 (4–11) × 103/μl vs. 200 (179–209) × 103/μl; p= 0.036) and change in platelet count from baseline to nadir was significantly greater in the 2F9-treated group (median (range): 238 (179–325) × 103/μl vs. 4 (0–10) × 103/μl; p=0.036) (Fig 1); p-values were calculated using the exact Wilcoxon rank-sum test. Platelet nadir was in our target range and platelet count remained 〈 40 × 103/μl in all 2F9-treated dogs for 24 hours. Dosing was predictable: in each dog, after an initial dose of 50 μg/kg 2F9, the second dose needed to reach the target nadir could be accurately calculated from the initial platelet decrease. 2F9-treated dogs developed a range of clinical bleeding from none to petechiae, ecchymoses, melena, and hematuria. At t=0 hrs, BMBT increased 3–8 fold in treated dogs, compared to 〈 2 fold in control dogs. Dogs had no changes in vital signs or demeanor and did not require any transfusion support. The model does not appear pro-thrombotic as fibrinogen and D-dimers were similar over time in 2F9-treated vs. control dogs. 2F9 infusion also generated negligible systemic inflammation, as assessed by white blood cell count and serum cytokine measurement. Unexpectedly, however, serum IL8 tracked faithfully with platelet count, demonstrating that platelets are a major source of serum IL8 in dogs (Fig 2). Although α granules are known to contain IL8, platelets have not been previously described as a significant serum IL8 source. Since IL8 is an important neutrophil chemokine, our finding may illuminate a novel mechanism of platelet-neutrophil cross-talk. In summary, we have developed a novel large animal ITP model that is highly representative of the spontaneous disease. Like naturally-occurring ITP, dogs demonstrate bleeding heterogeneity despite similar platelet counts (data not shown). We expect our model to lead to further insights into bleeding mechanisms in ITP. Ultimately, understanding what factors predispose certain patients to bleed will allow us to exploit these factors therapeutically as novel ITP treatments. Disclosures: No relevant conflicts of interest to declare.

Description: Background: Benefits of hospice care for patients with advanced solid cancers have been extensively studied, but they have not been comprehensively documented in hematologic malignancies (HM). Surveys indicate that hematologists harbor concerns about adequacy of hospice services for patients with HM (Odejide et al., Cancer 2017; Hui et al., Ann Oncol 2015). A common perception is that these patients often receive chemotherapy until the very end of life (EOL) and are destined to die in the acute care setting (Sekeres & Gerds, Cancer 2015). They also experience a barrier to hospice use when transfusion dependence develops (LeBlanc et al., Blood 2018). In this context, some authors have questioned whether standard measures of EOL care quality proposed by the National Quality Forum (NQF) should apply to patients with HM, and whether the use of hospice services can improve such measures. Our objective was to describe EOL care quality measures derivable from Medicare administrative claims among patients with HM who did or did not use hospice services. Methods: From the population-based linked SEER-Medicare registry, we selected fee-for-service beneficiaries with leukemias (acute or chronic), myeloma, myeloproliferative neoplasms (MPN), myelodysplastic syndrome (MDS), or lymphoma (any subtype), who died in 2007-2011. We identified beneficiaries who used hospice services, and ascertained claims-based indicators of aggressive EOL care corresponding to select NQF care quality measures: 1) death in acute care hospital, 2) number of days spent in acute care hospital within 30 days of death, 3) intensive care unit (ICU) admission within 30 days of death, 4) use of chemotherapy within 14 days of death, and 5) Medicare spending for care within the last 30 days of life. Binary outcomes were compared in multivariable robust Poisson models (reporting relative risk, RR), count outcomes in negative binomial models, and costs in a log-gamma model. All estimates were reported with 95% confidence intervals (CI). Models were adjusted for age, sex, race, marital status, Medicaid co-insurance (indicator of low socio-economic status), prevalent poverty in the county of residence, comorbidity index, poor performance status indicator, calendar year, and survival from diagnosis. Results: We identified 13,556 decedents with leukemia, 7,910 with myeloma, 9,543 with MPN/MDS, and 22,990 with lymphoma, who had median age at death of 78 years (interquartile range [IQR] 72-84). Overall, 47% of patients used hospice services, enrolling at median 22 months from diagnosis (IQR, 5-59; varying from 13 in leukemia to 28 in lymphoma). Median length of stay on hospice was 9 days (IQR, 3-27), varying from 8 to 10 days between different HM. Overall, 39% of patients died in hospital settings (from 36% in myeloma to 41% in leukemia). Median number of days spent in hospital at EOL was 4 in all types of HM. ICU admissions within the last month of life occurred in 39% of patients, and chemotherapy was administered to 10% in the last 14 days (varying from 7% in MPN/MDS to 13% in leukemia). These measures significantly differed between hospice enrollees and non-enrollees (Table). In multivariable models, use of hospice services was associated with a significant decrease in the aggressiveness of EOL care, as follows: 95% decrease in inpatient deaths (adjusted RR, 0.05; 95%CI, 0.05-0.06), 48% decrease in days spent in hospital (adjusted relative count, 0.62; 95%CI 0.60-0.63), 44% decrease in the risk of ICU stay (adjusted RR, 0.56; 95%CI, 0.54-0.57), 47% decrease in the use of chemotherapy (adjusted RR, 0.53; 95%CI, 0.50-0.57), and 38% decrease in mean Medicare spending at EOL (adjusted relative cost, 0.62; 95%CI, 0.60-0.63). Conclusions: The proportion of Medicare beneficiaries with HM who die in inpatient setting (39%) or are admitted to ICU (39%) in the last month of life is higher than in a contemporary population of Medicare beneficiaries with cancer (22% and 27%, respectively, in 2009 per Teno et al., JAMA 2013). Across the spectrum of HM, which vary in prognosis and clinical course, use of hospice services is associated with markedly improved measures of EOL care quality, as well as lower Medicare spending in the last month of life. However, only 47% of beneficiaries with HM use hospice services (compared with 59% in Teno et al.). Chemotherapy use at EOL was uncommon, challenging the notion that pervasive chemotherapy is a barrier to hospice use in HM. Disclosures Olszewski: Genentech: Research Funding; Spectrum Pharmaceuticals: Consultancy, Research Funding; TG Therapeutics: Research Funding.

Description: Background: Among many causes of thrombocytopenia acquired during hospitalization, heparin-induced thrombocytopenia (HIT), an immune complex-mediated thrombogenic state with high morbidity and mortality, is one of the most feared etiologies. The clinical '4T' score is an established method to assess risk for HIT and guide immediate management(Lo, Juhl et al. 2006). However, the 4T score is not consistently used by clinicians, leading to potential over-testing and suboptimal management of suspected HIT. Our objective in this Quality Improvement (QI) project was to implement a mandatory 4T calculator (4TC) into the electronic medical record (EMR) to improve the positive predictive value of HIT antibody (HITA) testing, decrease the incidence of testing of low-risk patients, and to improve clinical management of suspected HIT. Methods: We developed a 4TC that assisted clinicians by integrating EMR-derived data on heparin exposure and the five most recent platelet counts with a link to an online calculator as well as clinical suggestions (Figure). As a part of this QI project, clinicians were required to enter the calculated 4T score before ordering a polyspecific HITA assay, which serves as the screening test for HIT in our institution. We reviewed laboratory records for all HITA tests performed over a six-month period, including 3 months before and after the implementation of a "hard stop" for HITA order in the EMR. We extracted data on the ordering hospital service team, 4T score documented in notes (if any), as well as quality of care indicators including rates of clinically recommended heparin cessation and initiation of alternative anticoagulation in the subgroup of patients with intermediate or high 4T score at the time of HITA order. In this QI project, we refrained from any statistical testing and summarized pre-specified outcome measures: incidence of HITA testing, positive predictive value of the HITA assay, and quality of care indicators. Results: We identified 99 (53 prior to the 4TC and 46 post) cases of HITA orders in the study period, among which there were 4 cases of confirmed HIT (either with positive IgG or positive serotonin-release assay [SRA]). Three HIT cases occurred after the 4TC was implemented. After the implementation of the 4TC, the positive predictive value for the HITA assay increased from 9% to 21%. The rate of clinically appropriate heparin order discontinuation increased from 72% to 88%, as did the initiation of alternative anticoagulation from 14% to 27%. HITA assay was most commonly ordered by the internal medicine service (35% of tests), non-cardiac intensive care service (30%), coronary care and cardiothoracic surgery services (21%), neurology (5%), and general surgery/other services (9%). Implementation of the 4TC did not significantly change this distribution, but the overall incidence of HITA orders decreased from 4.0 to 3.4 per 1000 admissions. We observed a high frequency of confirmatory SRA testing requested (18%), which is the reflex confirmatory test performed at our institution, even in cases of negative polyspecific HITA or HIT IgG. Conclusions: Addition of a 4T score calculator into the EMR which provides immediate feedback on patient's prior heparin exposure and recent platelet counts allows clinicians to consistently use the 4T score when considering HIT. Importantly, it also helps to educate non-hematologists on the proper management of HIT. Improved predictive value of the HITA assay suggests that our 4TC led to ordering HITA in a more appropriate patient population. We also demonstrated improved quality of care for suspected HIT, with increased rates of heparin cessation and initiation of alternative anticoagulation in patients with intermediate or high pre-test probability. Identifying clinical teams that order a disproportionate number of HITA assays will help us to direct education to improve clinical management. This QI project has led to other areas of improvement, particularly with regard to rational ordering of the SRA. At only three months of review since implementation of the 4TC, we have noted an improvement in the management of suspected HIT and a slight reduction in the incidence of testing. With continued review, we hope to demonstrate further improvement, as practitioners continue to use the 4TC and rely on clinical rationale rather than reflexive laboratory testing when evaluating thrombocytopenia in the hospital. Disclosures Olszewski: TG Therapeutics: Research Funding; Spectrum Pharmaceuticals: Consultancy, Research Funding; Genentech: Research Funding. Reagan:Alexion: Honoraria; Takeda Oncology: Research Funding; Pfizer: Research Funding.

Description: Background: Immunotherapy with CD19 specific chimeric antigen receptor (CAR) T cells for relapsed/refractory acute lymphoblastic leukemia (ALL) demonstrated a minimal residual disease (MRD) negative remission rate of 93% on the phase 1 portion of Pediatric Leukemia Adoptive Therapy (PLAT)-02. However, the 1-year event free survival (EFS) was 50.8%, largely due to recurrence of disease which was frequently associated with early loss of CAR T cell persistence. Subjects on PLAT-02 with

Description: Different MPNs have distinct rates of malignant transformation (PMF〉PV〉ET). Although PV and ET can arise from hematopoietic stem/progenitor cells (HSPCs) with similarly activated JAK-STAT oncogenic pathway, the transformation rate into secondary myelofibrosis and leukemia is higher for PV than for ET. However, the underlying reasons are not fully clear. Whereas in some cases secondary mutations might cause transformation, it remains unclear whether distinct bone marrow (BM) microenvironments can influence the progression of MPNs or any preleukemic disorder. Previous studies have suggested that normal BM niches close to bone (endosteal) promote HSPC quiescence, whereas non-endosteal vessels permit transmigration of activated HSPCs. We hypothesized that PV and ET HSPCs might expand in different BM niches, which could thereby influence disease progression. To address this question, we performed combined two-photon and confocal real-time intravital microscopy in the skull BM of WT mice transplanted with HSPCs from WT mice or MPN mice carrying the same driver mutation (JAK2V617F) but showing phenotypically distinct diseases (ET or PV). ET HSPCs resembled WT HSPCs in their homing and engraftment near bone (22±15 and 26±13 μm, respectively) 3 days after i.v. injection into lethally-irradiated WT mice. In contrast, PV HSPCs located significantly further (31±21 μm) from bone, which was independently confirmed in a distinct PV model. The different homing of ET and PV HSPCs was similarly observed in non-irradiated WT recipients and was therefore independent of myeloablative conditioning. Following engraftment, ET and PV HSPCs preferentially expanded in endosteal and non-endosteal locations, respectively, over 7 months' follow up. Importantly, the asymmetric expansion of HSPCs in ET and PV was confirmed in human BM trephines. Human CD34+ HSPCs were significantly closer to bone in ET than in PV patients (86±2 vs. 109±6 μm). Together, these results suggest that mutated HSPCs preferentially occupy distinct BM niches in ET and PV. Murine ET and PV HSPCs also differed in their dynamic interactions with the microenvironment. ET HSPCs migrated significantly faster (1 μm/min) than PV HSPCs (0.8 μm/min) and covered longer tracks after 1 h (57±2 and 35±1 μm, respectively). Moreover, ET HSPCs (but not PV HSPCs) migrated faster when closer to bone, suggesting an exploratory strategy of ET HSPCs to find endosteal niches. Separation of endosteal and non-endosteal BM fractions revealed increased abundance of integrin β3+ HSPCs in the endosteal BM of ET mice carrying JAK2V617F or CALRdel52/del52 mutations, but not in different PV models (despite generally sharing oncogenic JAK-STAT activation). Furthermore, competitive gravity adhesion assay and interference reflection microscopy showed that ET HSPCs are more adhesive to ECM substrates of integrin β3, suggesting that integrin β3 might trigger endosteal lodging of ET HSPCs. Asymmetric HSPC expansion caused differential microenvironmental remodeling possibly explaining differences in the pathophysiology and secondary outcomes of ET and PV. Non-endosteal sinusoids were dilated in different PV (but not ET) models, whereas CD31hiSca1hi arterioles and aberrant bone-forming integrin β1+ blood vessels increased only in ET mice. Similarly, sinusoidal vessels showed increased coverage by the integrin β1 ligand laminin α4 only in ET mice. Consequently, increased bone (μCT), osteoblasts and osteoclasts were found in ET but not PV mice. Increased expression of vascular-derived bone-forming factors (such as Bmp1 and Dll4) downstream of endothelial laminin α4/integrin β1 signaling might trigger osteosclerosis in ET mice. Finally, we tested whether HSPC location might directly impact ET progression. In a separate study we found that β3-adrenergic receptor (AR) signaling regulates Cxcl12-dependent BM HSPC localization (ASH abstract ID 116015). Therefore, we transplanted WT or ET donor BM cells into WT and β3-AR KO mice. Measurement of peripheral blood counts over 28 weeks showed that the microenvironment lacking β3-AR significantly worsened thrombocytosis and leukocytosis in ET, which correlated with redistribution of HSPCs and their progeny away from bone towards central BM. Altogether, these results suggest that differential interactions with the microenvironment might impact disease progression in MPNs and possibly in other preleukemic disorders. Disclosures Mead: Cell Therapeutics: Consultancy; Bristol-Myers Squibb: Consultancy; ARIAD: Consultancy; Novartis: Consultancy, Honoraria, Research Funding, Speakers Bureau; Celgene: Research Funding; Elstar: Research Funding; Evotek: Research Funding.