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Background: Children with sickle cell disease (SCD) are at increased risk of cerebrovascular events that can impact neurocognitive development and quality of life (Colombatti 2016). Transcranial Doppler ultrasound (TCD) is a validated screening tool to identify pediatric SCD patients with the highest risk of stroke to start on a preventive chronic blood transfusion regimen (Estcourt 2020; Inusa 2019). High TCD velocities are an indication to start disease-modifying treatments or consider disease-curative options in children with SCD (Khemani 2019). However, real-world pediatric data on the correlation between hematological variables and TCD results are scarce (Salama 2020). We aimed to evaluate the distribution of TCD velocities in a pediatric natural history cohort and investigate their correlation with hematological variables and treatments. Methods: We performed a retrospective analysis on data from a prospective pediatric cohort followed from January 1,2009, to December 31, 2020 (censoring date). Standard care includes annual TCD from 2 years of age. We used transcranial Doppler imaging (TCDi) and classified results according to STOP criteria, considering terminal internal carotid artery (TICA) and middle cerebral artery (MCA) time-averaged maximum mean velocities (TAMMVs). Only complete exams with right and left measures available for both vessels were included. Hematological, clinical, and treatment variables were available from the natural history cohort database. Patients were divided according to genotype: HbSS/HbSβ 0 or HbSC/HbSβ +. Two-sample and Welch t-tests for unequal variances were used to compare mean hemoglobin (Hb) values and hemolysis markers in patients with and without abnormal/conditional TCDi results. Fisher and chi-square tests were used to compare categorical variables. Linear regression models were used to assess the effects of MCA and TICA TAMMVs as continuous variables on Hb. Odds ratios (ORs) for neurological events at different Hb levels were estimated using generalized estimated equations (GEE) with a binomial distribution, logistic function, and exchangeable correlation structure, allowing for correlation among repeated observations for the same patient. Multivariable GEE including characteristics and treatment variables were used to evaluate the association between neurological events and Hb. Results: Of the 182 SCD patients in the cohort, 169 had assessments of cerebral vasculopathy, and 155 had evaluable TCDi (583 exams). The median follow-up of the entire cohort was 79.8 months (range: 2.1-298.6 months) (interquartile range [IQR]: 36.9-126.3 months). The median age at the censoring date was 13.4 years (IQR: 9.1-17.5 years); 130 were HbSS/HbSβ 0, and 25 were HbSC/HbSβ +. Basic demographic characteristics of the cohort are in Table 1. The distribution of TCDi results was significantly different between genotypes (P We detected a linear correlation between TICA/MCA TAMMVs and Hb (Figure 1A and 1B). Univariate analysis showed significant inverse correlation between abnormal/conditional TCDi results and Hb considered as a continuous variable (OR: 0.484, P Conclusions: This analysis from our natural history cohort shows a significant inverse correlation between Hb and MCA and TICA velocities, supporting the beneficial effect of higher Hb levels in reducing TAMMV. Disease-modifying therapies increasing Hb and reducing hemolysis could be helpful in reducing TAMMV in children with SCD. Funding: This study was supported by Global Blood Therapeutics. Figure 1 Figure 1. Disclosures Agodoa: Global Blood Therapeutics: Current Employment, Current equity holder in publicly-traded company. Beaubrun: Global Blood Therapeutics: Current Employment, Current equity holder in publicly-traded company. Biffi: BlueBirdBio: Consultancy, Other: Advisory Board. Colombatti: Global Blood Therapeutics: Research Funding; Addmedica: Consultancy; Forma Therapeutics: Consultancy; Novartis: Consultancy; NovoNordisk: Consultancy; BlueBirdBio: Consultancy; Global Blood Therapeutics: Consultancy; BlueBirdBio: Research Funding.

Background: Hydroxyurea (HU) and chronic transfusion therapy (CTT) are the only disease modifying therapies (DMTs) currently available as standard of care in the UK and Italy for patients with sickle cell disease (SCD), with hematopoietic stem cell transplant (HSCT) available to a small fraction of patients. Few registries capture long-term follow up of real-world outcomes among patients with SCD and there is a need for natural history studies demonstrating morbidity and mortality under existing standard of care. The East London Newborn Sickle Cohort Study (ELNSCS) at the Royal London Hospital and the Sickle Cell Disease Cohort (SCDC) at the University of Padova (UNIPD) collect biomarker and clinical data on patients followed comprehensively at two expert referral centers and provide an opportunity to understand SCD real-world outcomes. Here, we report severe acute complications under standard of care during years when HU and CTT were widely available and accepted. Methods: Inclusion of patients in the ELNSCS required being born in a designated region in London, diagnosed by newborn screening between 1983-2018 and, for this analysis, followed during 2015-2018. Inclusion in the SCDC required being followed, for this analysis, at the UNIPD during 2016-2019. Analyses were restricted to patients with β Sβ S, β Sβ 0, and, for ELNSCS, β Sβ +. Data were entered into clinical databases at each site and subsequently validated against institutional records. Severe vaso-occlusive events (VOEs) were defined as events requiring admission (inpatient, ED, or hematology day unit) for acute chest syndrome (ACS), acute painful crisis, acute hepatic/splenic sequestration, acute ischaemic stroke, dactylitis and priapism. Statistical analysis was aligned between both sites. Patients were categorized into three mutually exclusive treatment groups (HU, CTT, no treatment) according to treatment during study period. Results: One hundred seventy-two patients in the ELNSCS and 62 patients at UNIPD met study inclusion criteria in the four-year analysis period. HbSS genotype accounted for 161 (93.6%) of ELNSCS cohort and 58 (93.5%) of UNIPD cohort. Median age at study entry was 11.6 (range 0.2 - 31.4) years in the ELNSCS and 7.66 (range 0.12 - 19.96) years at UNIPD. Median age differed across treatment groups; HU group tended to be younger, while CTT group tended to be older. According to institutional standards of care, 47 (27.3%) patients from the ELNSCS and 50 (80.6%) from UNIPD received HU, 53 (30.8%) from the ELNSCS and 8 (12.9%) from UNIPD received CTT, and 72 (41.9%) from the ELNSCS and 4 (6.5%) from UNIPD received no treatment during the four year study period. Differences in treatment allocation reflect slightly different patient populations and approaches to care at the two centers. Severe VOEs persist among patients in all three treatment groups at both sites. Of those receiving HU, 83.0% from the ELNSCS and 68.0% at UNIPD had ≥1 severe VOE, including 21.3% from the ELNSCS and 44.0% at UNIPD experiencing ≥1 ACS event. The rate of severe VOEs in the HU group was 0.75 (range 0 - 39.5) per patient year from the ELNSCS and 0.49 (range 0 - 3.00) per patient year from UNIPD. HU dosing and adherence will be explored using data collected on hematologic parameters. In the ELNSCS, of those receiving CTT, 60.4% experienced ≥1 severe VOE (20.8% had ≥1 ACS event); of those receiving no therapy, 56.9% experienced ≥1 severe VOE (11.1% had ≥1 ACS event). At UNIPD, severe VOEs were observed in 62.5% of the CTT group and 50% of the no treatment group, though sample sizes were very small. C onclusions: Despite receiving expert care in accordance with local and international guidelines at two large academic centers, a significant sub-group of patients continue to experience severe VOEs. Results show that real-world usage of HU and CTT may not be optimized and, even if optimized, some patients may continue to experience severe acute complications including ACS. Both cohorts confirm that implementation of existing standard of care is insufficient to prevent significant morbidity in patients with SCD. Findings suggest the need to introduce DMT early in life to reduce and prevent acute complications and minimize disease progression. There is a persistent need for maximizing effective DMTs, as well as further developing curative therapies such as HSCT and gene therapy for both pediatric and adult patients. Figure 1 Figure 1. Disclosures Colombatti: Novartis: Membership on an entity's Board of Directors or advisory committees; Novo Nordisk: Membership on an entity's Board of Directors or advisory committees; Global Blood Therapeutics: Membership on an entity's Board of Directors or advisory committees, Research Funding; BlueBirdBio: Membership on an entity's Board of Directors or advisory committees, Research Funding. Chawla: BlueBirdBio: Current Employment. Puri-Sharma: BlueBirdBio: Current Employment. Walls: BlueBirdBio: Current Employment. Kommera: BlueBirdBio: Current Employment. Telfer: Novo Nordisk: Membership on an entity's Board of Directors or advisory committees; Emmaus: Membership on an entity's Board of Directors or advisory committees; Novartis: Membership on an entity's Board of Directors or advisory committees; Pfizer: Membership on an entity's Board of Directors or advisory committees; Addmedica: Membership on an entity's Board of Directors or advisory committees; ApoPharma: Membership on an entity's Board of Directors or advisory committees; Global Blood Therapeutics: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; BlueBirdBio: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Terumo: Honoraria; Roche: Membership on an entity's Board of Directors or advisory committees.