ALBERT

Description: Background: Chronic ITP in children is an autoimmune disorder characterized by increased platelet destruction and suboptimal platelet production. Pediatric patients with chronic ITP that completed a romiplostim phase 1/2 or phase 3 study could enroll in this open-label long-term extension; 5.2 years of data are reported. Methods: All patients received SC romiplostim once weekly. The initial dose was the final dose from the parent study or 1 µg/kg (for patients previously receiving placebo or who had not received romiplostim for 〉24 weeks), adjusted from 1-10 µg/kg to target platelet counts of 50-200×109/L. The incidence of adverse events (AEs) was the 1° endpoint. If approved by investigators, patients who maintained platelet counts ≥50×109/L for ≥4 consecutive weeks at a stable dose could subsequently administer doses by self or caregiver. Patients who turned 18 years of age could remain on study. Results: A total of 66 patients (phase 1/2, n=12; phase 3, n=54) entered the extension; 65 received romiplostim for ≤269 weeks (5.2 years); 1 patient withdrew consent before treatment. At baseline, median (range) age was 11 (3-18) years; 56% were female; 61% were white, 14% African American, and 14% Hispanic/Latino; 9.1% had prior splenectomy. Median (range) treatment duration was 57.9 (1-269) weeks; median number of doses was 53.0 (1-268). Median (range) average weekly romiplostim dose was 5.5 (0.1-10.0) µg/kg, including escalation to stable dose, median maximum dose was 8.0 (1.0-10.0) µg/kg, and median most frequent dose was 6.0 (0-10) µg/kg. The median dose fell from 6 to 1 µg/kg; after ~week 140 (n≤9), the median dose fluctuated (Fig 1). Thirteen patients discontinued treatment: consent withdrawn (n=6), noncompliance (2), administrative decision (2), nonresponse (2), and per protocol (1). Fifty-two (79%) patients continued in the study; none withdrew due to AEs. After the first study week median platelet counts remained 〉50×109/L (Fig 2); for 15 patients (23%), the first study week was the first week receiving romiplostim (ie, previously received placebo). Fifty-six (86%) patients (or caregivers) self-administered romiplostim. Twenty-one (32%) patients received rescue medications on 63 occasions (for low platelet counts [n=35], bleeding/bruising [17], pre- or post-procedure [9], and other [2]), including IVIG (n=10), prednisone (9), aminocaproic acid (3), tranexamic acid (2), methylprednisolone (2), and platelet transfusion (1). Patients required rescue treatment during the first 3 months (27/63 instances), 〉3-6 months (9), 〉6-9 months (6), 〉9-12 months (7), and after 1 year (14) in the extension. Five previous placebo recipients received rescue medication, mostly during the first 3 months (10/14 instances). Three patients achieved remission (platelet counts ≥50×109/L for 24 weeks with no ITP treatments): 1) 9-year-old boy with ITP for 9 years, after 4.3 years of romiplostim, entered remission for the last 2.1 years as of this datacut; 2) 13-year-old boy with ITP for 8.5 years, after 3.3 years of romiplostim, entered remission for the last 1.1 years; and 3) 17-year-old girl with ITP for 8.2 years, after 5.9 years of romiplostim, entered remission for the last 44 weeks. Thirty-four serious AEs occurred in 14 patients, including pyrexia (n=3), epistaxis (2), and thrombocytopenia (2); 3 were deemed treatment-related (anemia, epistaxis, and thrombocytopenia), and none led to discontinuation of romiplostim. Five patients had life-threatening AEs, including thrombocytopenia (n=2) and infection, decreased platelet counts, and subcutaneous abscess (1 each); none were fatal or deemed treatment-related. Bleeding AEs occurred in 47 patients; 3 were deemed treatment-related by the investigator (gingival bleeding, petechiae, and epistaxis). Bleeding AEs occurring in ≥4 patients included contusion (n=25), epistaxis (21), petechiae (16), gingival bleeding (9), mouth hemorrhage (6), ecchymosis (5), and hemorrhage, injection-site bruising, and purpura (4 each). No thrombotic events were reported. There were no peripheral blood abnormalities suggestive of malignancy to warrant a bone marrow examination in any patient. There was no development of anti-TPO or anti-romiplostim antibodies. Conclusion: In this ongoing open-label extension of children with chronic ITP, romiplostim for ≤5.2 years maintained platelet counts with a safety profile similar to that seen in past studies. Disclosures Tarantino: BPL: Membership on an entity's Board of Directors or advisory committees; Grifols: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding, Speakers Bureau; Novo Nordisk: Membership on an entity's Board of Directors or advisory committees, Research Funding, Speakers Bureau; Octapharma: Membership on an entity's Board of Directors or advisory committees; Baxter: Membership on an entity's Board of Directors or advisory committees, Research Funding; Amgen, Inc: Membership on an entity's Board of Directors or advisory committees; Pfizer: Membership on an entity's Board of Directors or advisory committees; Cangene: Research Funding; UptoDate, Inc.: Patents & Royalties: royalties. Off Label Use: Romiplostim is indicated for use in adults with chronic immune thrombocytopenia who have had an insufficient response to corticosteroids, immunoglobulins, or splenectomy. The use of romiplostim in children with ITP is investigational.. Bussel:Eisai: Membership on an entity's Board of Directors or advisory committees, Research Funding; Sysmex: Research Funding; Immunomedics: Membership on an entity's Board of Directors or advisory committees, Research Funding; IgG of America: Research Funding; GlaxoSmithKline: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Cangene: Membership on an entity's Board of Directors or advisory committees, Research Funding; Amgen, Inc: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding. Blanchette:Baxter Corporation: Honoraria, Membership on an entity's Board of Directors or advisory committees, Other: Data Safety Monitoring Board, Research Funding; Novo Nordisk: Honoraria, Membership on an entity's Board of Directors or advisory committees; Pfizer: Honoraria, Membership on an entity's Board of Directors or advisory committees; Octapharma: Other: Data Safety Monitoring Board; Bayer Healthcare: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding. Morales:CSL Behring: Membership on an entity's Board of Directors or advisory committees; Baxter: Membership on an entity's Board of Directors or advisory committees. Nie:Amgen Inc.: Employment, Other: Stockholder. Eisen:Amgen Inc: Employment, Other: stock ownership.

Description: Background: The indications for treating children with immune thrombocytopenia (ITP) remain controversial. A valid and reliable bleeding assessment tool could assist in objectively quantifying bleeding and influence treatment decisions. Both the ITP Bleeding Score (IBLS) and the ITP-Bleeding Assessment Tool (BAT) have been developed to assess bleeding severity in ITP. The IBLS scores bleeding severity using an 11-item tool with grading from 0 to 2 and the 18-item BAT grades from 0 to 3 or 4. The BAT includes some types of bleeding not represented on the IBLS, such has intramuscular hematomas. To date, no data describe how these two measures compare when measuring bleeding associated with ITP. Objective: To describe and compare bleeding as assessed by both the IBLS and BAT and to correlate bleeding severity with platelet counts in a cohort of children with ITP. Methods: A longitudinal observational cohort of children ages 〉1 and 〈 18 years with ITP, were enrolled from 2013-2015 in the Pediatric ITP Consortium of North America ICON1 trial. All children were enrolled prior to starting a new second line monotherapy (not IVIG, steroids or anti-D). At enrollment, bleeding was assessed using the IBLS in all children. A subset of children also underwent a BAT assessment. Grades of bleeding were described and compared between tools and agreement in grading was assessed. Severity was correlated with platelet count using Spearman's correlation calculation. Results: 118 children were enrolled from 21 ICON centers. 54% had chronic ITP and the median age was 11.4y (range 1.2-17.8). The mean platelet count was 28 x 109/l (SD 57) and 88% had a baseline platelet count of

Description: Background: Romiplostim, a thrombopoietin (TPO) receptor agonist approved for children and adults with chronic ITP, was evaluated in children with ITP in a ≤3-year open-label trial. Interim results were previously reported (Grainger et al., Blood 2017 130:2334). Here we present updated results as of 27 Mar 2019. Methods: Eligible children from 17 countries with ITP for ≥6 months and screening platelet count ≤30×109/L (or uncontrolled bleeding) received SC romiplostim (1 μg/kg titrated to 10 μg/kg to maintain platelet counts of 50-200×109/L). In Europe, bone marrow was evaluated at baseline and after 1 (day 365 ± 4 weeks) or 2 (day 730 ± 4 weeks) years. The primary endpoint was % time with a platelet response (platelet count ≥50×109/L, no rescue therapy in preceding 4 weeks) in months 0-6. Results: A total of 203 patients (pts) received ≥1 dose; the median (interquartile range [IQR]) age was 10 (6-13) and median (IQR) platelet count 14 (7-23.5×109/L). The median (IQR) duration of treatment was 145 (39-156) weeks, median (IQR) % of time with a platelet response in months 0-6 was 50% (17-83%), with 88% (179/203) of pts having a platelet response at least once (Fig. 1A). In all pts, the median (IQR) % of time with an increase in platelet counts ≥20×109/L above baseline from week 2 until the end of treatment was 79% (39-92%). Median and lower quartile platelet counts were both consistently 〉50×109/L from week 12 and 48, respectively, and did not vary by age. Eleven pts maintained platelet counts ≥50×109/L without ITP medications (including romiplostim) for ≥24 weeks; median (IQR) time to onset was 50 (24-80) weeks after starting romiplostim. During the study, 60 (30%) pts received rescue therapy, typically within weeks 1-36, and 3 underwent splenectomy. With a total exposure of 428.7 patient-years, median (IQR) average weekly romiplostim dose over the entire study was 6.9 (4.6-8.9) µg/kg; 8.5 (5.0-10.0) μg/kg at 1 year (n=144) and 6.0 (3.0-10.0) μg/kg at 2 years (n=129; Fig. 1B). Self-administration was initiated in 68% of pts. Ninety-five pts (46.8%) discontinued treatment: reasons included lack of efficacy (n=43 [21.2%]), patient request (n=15 [7.4%]), adverse event (AE; n=9 [4.4%]), and neutralizing antibodies (NAb; n=7 [3.4%]). AEs occurred in 192 pts (94.6%); the most frequent were epistaxis (38.4%), headache (37.9%), and nasopharyngitis (36.9%). Serious AEs (SAEs) occurred in 59 (29.1%) pts, including epistaxis (5.9%), decreased platelet count (4.4%), and thrombocytopenia and NAb (2% each); 8 pts had treatment-related SAEs (NAbs [n=4], headache and abdominal pain [each n=2], and presyncope [n=1]). Bleeding occurred in 69% of pts over the study, decreasing over time, with bleeding in 18% of pts from week 144 to the end of treatment. Bleeding-related AEs occurring in 〉10% of pts were epistaxis (38.4%), petechiae (23.6%), hematoma (20.7%), contusion (19.2%) and gingival bleeding (10.3%). CTCAE grade ≥3 bleeding events occurred in 20 pts (9.9%) and included epistaxis (n=9 [4.4%]), and persistent ITP (n=3 [1.5%]). In pts with no evidence of NAbs at baseline, there were 7 cases of NAbs to romiplostim (2 were transient) and 1 transient NAbs to TPO; only 1 pt, with NAbs to romiplostim, had a reduced therapeutic effect. Of 75 European pts with evaluable baseline bone marrow biopsies [modified Bauermeister scores: grade 0 (no reticulin, n=16), 1 (fine fibers, n=54), or 2 (fine fiber network, n=5)], 27 had evaluable on-study biopsies after 1 year and 36 after 2 years (Table). Of these, 5 pts developed increased reticulin at year 1 and 17 at year 2. One pt had an increase in modified Bauermeister score from baseline of ≥2 grades (increase from grade 0 to 2), 4 pts had an increase in 1 grade, 1 a decrease in 2 grades, and 3 a decrease in 1 grade in year 1. In year 2, 15 pts had an increase in 1 grade and 3 pts a decrease in 1 grade. No pts developed collagen and no bone marrow abnormalities were detected. Conclusion: Over the course of the study with 〉30 months of treatment, on a median dose of 6.9 μg/kg, 88% of children had a platelet response, median platelet counts were ≥20×109/L above baseline 79% of the time and 〉50×109/L from week 12. An important new safety signal over 429 patient-years was the 3.4% of NAbs to romiplostim; children develop NAbs with romiplostim more frequently than adults. Bone marrow findings showed that children, like adults, did not develop clinically important fibrosis. Disclosures Grainger: Amgen: Consultancy, Speakers Bureau; Ono: Consultancy; Alexion: Consultancy; ITP Support Association: Other: medical advisor; Octapharma: Consultancy; Biotest: Consultancy; Novartis: Consultancy, Speakers Bureau. Bussel:Tranquil: Honoraria, Membership on an entity's Board of Directors or advisory committees; Novartis: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Momenta Pharmaceuticals: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees; Dova Pharmaceuticals: Consultancy, Membership on an entity's Board of Directors or advisory committees; UCB: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees; argenx: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees; Regeneron: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees; RallyBio: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees; Kezar Life Sciences: Consultancy, Membership on an entity's Board of Directors or advisory committees; Physician Education Resource: Speakers Bureau; 3S Bio: Speakers Bureau; Rigel: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees; GSK: Honoraria, Membership on an entity's Board of Directors or advisory committees; Amgen: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees. Tarantino:Novo Nordisk: Consultancy, Membership on an entity's Board of Directors or advisory committees, Other: Investigator initiated study, Speakers Bureau; Bleeding and Clotting Disorders Institute: Employment; Pfizer: Other: PI for program grant; Amgen: Consultancy, Membership on an entity's Board of Directors or advisory committees, Other: Clinical Trial PI; Grifols: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding, Speakers Bureau; Takeda: Consultancy, Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Michael Tarantino, MD SC: Other: President, Owner- Private Practice ; Genentech: Consultancy, Membership on an entity's Board of Directors or advisory committees. Cooper:Novartis: Honoraria, Other: clinical trial; Principia: Honoraria, Other: clinical trial; UCB: Other: clinical trial; Rigel: Honoraria, Other: clinical trial; Amgen: Honoraria, Other: clinical trial. Despotovic:Novartis: Research Funding; Dova: Honoraria. Wang:Amgen: Employment. Eisen:Amgen: Employment, Other: stock ownership. Bowers:Amgen: Employment, Other: stock ownership.

Description: Background: Autoimmune neutropenia (AIN) is the most common chronic neutropenia of childhood. Despite the expectation of a benign clinical course, many such patients undergo extensive evaluation to rule out congenital neutropenia or malignant disease, are followed within a subspecialty hematology clinic until resolution, and are given strict infectious precautions including emergency center (EC) assessment during febrile episodes. Data on healthcare utilization and rate of serious infections in young patients with AIN is limited. Objective: To evaluate the utilization of routine subspecialty and emergency healthcare and incidence of serious bacterial infections in young children with AIN at a large tertiary care children's hospital. Methods: All patients with a diagnosis code of leukopenia, neutropenia, or AIN followed within the outpatient hematology clinic of a large tertiary hospital from 2014 to 2016 were identified. Manual review of the electronic medical record was performed to assess patient eligibility and perform data collection. Patients age ≤5 years, with absolute neutrophil count (ANC) ≤500/µL persisting for ≥3 months, and a clinical diagnosis of AIN as determined by a hematology provider were included. Patients were excluded if they were lost to follow up prior to neutropenia resolution, had AIN secondary to underlying systemic autoimmune or immunologic disorder, neutropenia persisting beyond age 5 years, or if the etiology of neutropenia was unclear. Data on clinical assessment, laboratory parameters, immune and infectious evaluations, and total number of outpatient and EC assessments for fever were collected. Results of anti-neutrophil antibody testing were reviewed but no patients were included or excluded based on those results. EC visits for evaluation of fever in 3 representative patients were reviewed for charge capture data. All charges during the reviewed time period remained constant. Results: From 2014 to 2016, 46 patients (20 male [44%], median age at diagnosis 12 months) diagnosed with AIN met eligibility criteria and were followed in the outpatient hematology clinic for a total of 450 outpatient visits (Table 1). Median duration from initial identification of neutropenia to discharge from hematology clinic was 19 months (range 6 to 85 months). Thirteen patients (28%) had bone marrow evaluation to rule out infiltrative disease or marrow failure etiologies. Nine patients (20%) had genetic testing, the majority of which (n=6) were targeted sequencing for congenital neutropenia. Thirty-six patients (78%) had one or more EC encounters for evaluation of fever, and the cohort had a combined 125 such encounters. Of 155 blood cultures drawn, 3 returned positive. Positive cultures resulted in repeat blood cultures being drawn and hospital admission for antibiotics and clinical monitoring. However, all were determined to be contaminants that did not require treatment. Total charges associated with EC visits for the cohort are estimated at $471,375 based on a cost of $3771 per visit (Table 2). Those patients with ANC

Description: Background: Evans Syndrome (ES) describes the simultaneous or sequential occurrence of 2 or more autoimmune cytopenias - most often autoimmune hemolytic anemia (AIHA) and immune thrombocytopenia (ITP), but also autoimmune neutropenia (AIN) - which are often refractory to therapy and chronic in nature. Historically, ES has been attributed to idiopathic autoantibody production, but recent advances in our understanding of this disease have revealed associations with more well-described underlying disorders of immune regulation when properly investigated. These include autoimmune lymphoproliferative syndrome (ALPS) and more novel immune dysregulation disorders such as Cytotoxic T Lymphocyte Antigen-4 (CTLA-4) defects, among the more historically well-known systemic autoimmune diseases (such as systemic lupus erythematosus [SLE]) and common variable immunodeficiency (CVID). Importantly, recent data shows improved outcomes among ES patients in whom underlying immune dysregulation is identified and for whom appropriately targeted immunomodulating treatments are then utilized. Despite this, the majority of underlying and potentially targetable pathologies driving ES in children remain undiagnosed; and treatment strategies are therefore empiric, resulting in sub-optimal outcomes among this population of patients. Aims: We aim to better describe the diagnostic rates, treatment strategies, and clinical outcomes among a large cohort of pediatric patients with this rare and poorly studied disease, in order to identify barriers to appropriate and timely diagnostic investigation, to highlight areas for future research, and to better inform clinical practices. Methods: We completed a retrospective chart review including pediatric ES patients followed at 3 large tertiary centers over a recent 6-year period (2012 - 2018). All patients meeting eligibility criteria (those aged 6 months - 21 years, with 2 documented autoimmune cytopenias as defined by current international expert committee standards, and not having undergone solid organ or stem cell transplantation) were reviewed in accordance with IRB-approved protocols. Chi-squared test or Fisher's exact test were utilized to compare nonparametric categorical data. Mann-Whitney U-test was used to compare nonparametric continuous data. A Kruskal-Wallis one way ANOVA test was used to compare ranks between more than two categories. Statistical analyses were performed using SPSS Statistics 24 (IBM, Armonk, NY). A Bonferroni correction was applied to correct for multiple comparisons. A p

Description: Hydroxyurea therapy has proven laboratory and clinical efficacies for children with sickle cell anemia (SCA). When administered at maximum tolerated dose (MTD), hydroxyurea increases fetal hemoglobin (HbF) to levels ranging from 10% to 40%. However, interpatient variability of percentage of HbF (%HbF) response is high, MTD itself is variable, and accurate predictors of hydroxyurea responses do not currently exist. HUSTLE (NCT00305175) was designed to provide first-dose pharmacokinetics (PK) data for children with SCA initiating hydroxyurea therapy, to investigate pharmacodynamics (PD) parameters, including HbF response and MTD after standardized dose escalation, and to evaluate pharmacogenetics influences on PK and PD parameters. For 87 children with first-dose PK studies, substantial interpatient variability was observed, plus a novel oral absorption phenotype (rapid or slow) that influenced serum hydroxyurea levels and total hydroxyurea exposure. PD responses in 174 subjects were robust and similar to previous cohorts; %HbF at MTD was best predicted by 5 variables, including baseline %HbF, whereas MTD was best predicted by 5 variables, including serum creatinine. Pharmacogenetics analysis showed single nucleotide polymorphisms influencing baseline %HbF, including 5 within BCL11A, but none influencing MTD %HbF or dose. Accurate prediction of hydroxyurea treatment responses for SCA remains a worthy but elusive goal.

Description: Key Points PK deficiency manifests a broad spectrum in anemia severity that moderately improves after splenectomy. Close attention to monitoring for iron overload, gallstones, and other complications is recommended in all patients with PK deficiency.

Description: Abstract 2131 Background: Over the past 15 years, numerous prospective and cross-sectional clinical trials have demonstrated that hydroxyurea has both laboratory and clinical efficacy for pediatric patients with sickle cell anemia (SCA). In infants, toddlers, children, and adolescents, hydroxyurea administered at maximum tolerated dose (MTD) leads to significant increases in hemoglobin (Hb) concentration, MCV, and %HbF along with simultaneous decreases in neutrophils, reticulocytes, total bilirubin, and serum lactate dehydrogenase. Clinical benefits include reduction in acute vaso-occlusive events (pain and acute chest syndrome), and emerging data suggest protection against chronic organ damage with a low risk of genotoxicity. For individual patients, however, the %HbF response to hydroxyurea and the MTD dose itself are highly variable. Currently there are no accurate predictors of the final %HbF or the MTD dose. Methods: To address the phenotypic variability, the Hydroxyurea Study of Long-Term Effects (HUSTLE, NCT00305175) was designed to capture prospectively first-dose (20 mg/kg) pharmacokinetics (PK) of hydroxyurea including maximum serum concentration (Cmax), area under the concentration curve (AUC), apparent oral clearance (CL/F), half-life (t1/2), and apparent volume of distribution (V/F). A consistent dose escalation schedule was then used to achieve a stable MTD, at which time PK studies were repeated and hydroxyurea responses (%HbF and MTD dose) were recorded along with other pharmacodynamics parameters. Results: After written informed consent, a total of 98 pediatric patients commenced hydroxyurea, 65 of whom reached MTD with complete paired PK information. In the majority of patients (39 of 65), a ‘fast/slow’ absorption phenotype identified at first-dose PK analysis was retained at MTD, supporting the concept of a genetic basis for this variation. Inter-individual PK variability was substantially greater than intra-individual variability; for example, the coefficient of variation (%CV) for CL/F was 44.4% on day 1 and 42.3% at MTD among all subjects, but averaged only 12.8% within the same subject. The CL/F was partly dependent on the rate of absorption but was most strongly influenced by subject weight and serum creatinine. At MTD, the average AUC was 114 ± 22.3 mg·h/mL, and the %CV of AUC was reduced from 24.8% on first-dose PK to 19.5% at MTD, likely reflecting dose titration toward a common degree of myelosuppression. In an attempt to predict the MTD dose toward this target AUC using data available at baseline, ‘best-fit’ equations were developed such as the following with or without PK data: Predicted MTD (mg/kg/day) = 31.9 - [17.1*Baseline Creatinine] - [0.14*Baseline BMI] + [0.0036*Baseline ARC] Predicted MTD (mg/kg/day) = 32.6 - [15.1*Baseline Creatinine] - [0.16*Baseline BMI] - [0.56*First-dose half-life] + [0.0034*Baseline ARC] - [0.48*PK phenotype (fast=0, slow=1)] A simpler equation for predicting MTD dose using only baseline creatinine and weight was then developed: Predicted MTD (mg) = 400 - [1000*Baseline Creatinine] + [21*weight] Conclusions: The relatively small intra-individual PK variability at hydroxyurea MTD compared to baseline PK studies, coupled with a similar degree of drug exposure when patients achieve a stable MTD, supports future investigation to predict the optimal hydroxyurea dose prior to the first dosing. Prediction equations of the MTD should be tested prospectively against standard dose-escalation schedules. Disclosures: Off Label Use: Hydroxyurea for children with sickle cell disease.

Description: Background Wiskott-Aldrich syndrome (WAS) is a rare X-linked disorder characterized by combined immunodeficiency, eczema, microthrombocytopenia, infections, autoimmunity and lymphoma. Gene therapy (GT) using autologous CD34+ cells is an emerging alternative treatment with advantages over standard allogeneic hematopoietic stem cell transplant for patients who lack well matched donors, avoiding graft-versus-host-disease. An initial experience with gene therapy using a γ-retroviral vector showed correction of hematological defects in 9/10 patients, but was aggravated by development of leukemia in 7 of them. We report the outcomes of a phase I/II clinical trial in which 5 WAS patients underwent GT using a self-inactivating lentiviral (SIN-LV) vector expressing the human WAS cDNA under the control of a 1.6kB fragment of the human WAS promoter. Subjects and Methods Five patients with severe WAS (clinical score 3-5) were enrolled at a median age of 1.8 years (1.4 - 8 years) at a single pediatric tertiary care center. WAS protein (WASP) was absent or markedly decreased in 2 and 3 subjects, respectively. Purified CD34+ cells from mobilized peripheral blood (n = 4) or both mobilized peripheral blood and bone marrow (n = 1) were transduced ex-vivo with the SIN-LV vector and re-infused after conditioning with busulfan (target AUC of 70-80 mg*h/L) and fludarabine (120mg/m2). The median dose of CD34+ cells infused was 9.8 x 106 cells/kg (6.3 - 24.9 x 106 cells/kg) with a mean vector copy number (VCN) of 1.7 copies/cell in CD34+ cells (0.54 - 3.37). In addition to eczema, thrombocytopenia and WAS-related infections in all patients, two subjects also had autoimmunity pre-GT, manifested as skin vasculitis and autoimmune cytopenias. Results All 5 subjects were alive and well at median follow-up of 4.8 years (2.5 - 5.9 years). Multi-lineage vector gene marking was sustained over time. All subjects had improvement or resolution of eczema and none have had any intercurrent severe infectious events. WASP expression measured by flow cytometry in T cells was increased over baseline in all patients, but remained below normal levels and correlated with VCN and cell dose received. Proliferation of T cells in response to anti-CD3, which was initially defective in 4/5 patients, improved post-GT. Humoral immune deficiency was also ameliorated, as evidenced by independence from Ig replacement and vaccine responses in those tested. All subjects remained platelet transfusion-free and none have had severe bleeding events. Platelet levels increased to 〉50 x 103 cells/uL in two patients with a VCN ≥2 in transduced stem cells and myeloid VCN ~1 copy/cell in neutrophils; the other 3 subjects sustained platelet counts 20% of the transgene-marked cell population. To date, there have been no malignancies reported, either related to GT or WAS itself. Conclusion In summary, our data confirm and extend the safety and efficacy of GT in correcting disease manifestations associated with WAS, as seen in other studies using SIN-LV. Higher VCN in the drug product and in transduced stem cells correlated with better reconstitution of platelets and myeloid function. In contrast to other groups, we found in our study that patients with poor lymphocyte reconstitution post-GT may be at risk of ongoing autoimmunity despite high-level gene marking. Disclosures London: ArQule, Inc: Consultancy; United Therapeutics: Consultancy. Despotovic:Novartis: Research Funding; Amgen: Research Funding; Dova: Honoraria. Forbes:Takeda: Consultancy. Galy:Genethon: Employment. Williams:Novartis: Membership on an entity's Board of Directors or advisory committees; bluebird bio: Other: License of certain IP relevant to hemoglobinopathies. Potential for future royalty/milestone income. Received payment in past through BCH institutional licensing agreement., Research Funding; Orchard Therapeutics: Membership on an entity's Board of Directors or advisory committees, Other: Co-founder, potential for future royalty/milestone income, Research Funding; Alerion Biosciences: Membership on an entity's Board of Directors or advisory committees, Other: Co-founder. OffLabel Disclosure: CliniMACS technology for CD34+ cell selection