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Introduction Strokes are a preventable cause of neurological morbidity and premature death, particularly in children with sickle cell anemia (SCA) living in low-resource countries. If untreated, 50% of children with SCA their first overt ischemic stroke will have a recurrent stroke within two years of the event. In high-income countries, ASH 2020 guidelines recommend indefinite regular blood transfusion therapy for secondary stroke prevention (Blood Adv. 2020). Unfortunately, regular blood transfusion therapy is not a feasible option for children with SCA in sub-Saharan Africa due to the high cost of monthly blood transfusion, limited blood supply, and unsafe transfusion practices. Also, children who receive regular blood transfusions will ultimately require daily iron chelation at a cost that is prohibitive to most families in low-income settings. One randomized controlled trial provided evidence that HU therapy may be an effective therapy for secondary prevention of strokes when compared to no therapy (Blood. 2012;119(17):3925-3932). In the SWiTCH trial, the incidence rate of stroke recurrence in the group randomly allocated to receive maximum tolerated dose HU therapy was significantly higher than the group randomly assigned to receive blood transfusion therapy (5.6 and 0 events per 100 person-years, respectively, but considerably lower when compared to children not treated with any treatment, approximately 28 events per 100 person-years (Niger Postgrad Med J. 2013;20(3):181-187). Given the practical limitations for regular blood transfusion therapy, we tested the hypothesis that for secondary stroke prevention among children with SCA and acute overt ischemic stroke, fixed moderated dose HU therapy (~20 mg/kg/day) results in 80% relative risk reduction when compared to fixed low-dose HU therapy (10 mg/kg/day) in a randomized controlled trial (SPRINT Trial; NCT02675790). Methodology In phase III controlled trial, partially blind d controlled trial, we randomly assigned children 1 - 16 years of age with SCA and a new-onset of ischemic stroke (within 1 month) to receive fixed moderate-dose HU therapy at 20 mg/kg/day or fixed-low dose HU therapy at 10 mg/kg/day) with a monthly follow-up for at least 36 months. The primary endpoint was a recurrence of overt stroke or transient ischemic attack. Myelosuppression was assessed with monthly CBCs. Adherence to hydroxyurea was based on an increase in MCV from baseline and monthly pill count return, as a percent of dispensed pills. Results A total of 101 children with SCA were randomly assigned to fixed low- (~10 mg/kg/day) or moderate- (~20 mg/kg/day) dose hydroxyurea. The mean age was 6.6 years; 55.4% were males, and the median follow up was 1.6 years (IQR 1.0 - 2.3). The DSMB stopped the trial early due to the futility of the primary endpoint. In the fixed low- and moderate-dose groups, the incidence rates of recurrent strokes per 100 person-years were 7.1 and 6.0, respectively, incidence rate ratio of 0.85 (95% CI: 0.20 - 3.34), p=0.999. The incidence rates of mortality per 100 person-years in the fixed low dose and moderate- fixed-dose groups were 2.38 and 3.63, respectively, with an incidence rate ratio of 1.53 (95% CI: 0.18 - 18.30), p=0.98. No participant had hydroxyurea therapy stopped because of myelosuppression. As a measure of adherence, MCV from baseline to endpoint increased 6.2 fl and 13.3 fl in the fixed low- and moderate-dose groups, respectively, p=0.025; returned pills during the trial were 3.3% and 3.5% in the fixed low- and moderate-dose groups, respectively, p= 0.76. Conclusion For secondary stroke prevention, in a randomized controlled trial in children with SCA and new onset of ischemic strokes, fixed low-dose, when compared to fixed moderate-dose hydroxyurea therapy, demonstrated no difference in the incidence rate of stroke recurrence. Fixed low-dose hydroxyurea for secondary prevention of strokes in Nigeria provided similar stroke recurrence rate, when compared to the SWiTCH Trial (Blood. 2012;119(17):3925-3932) with the maximum tolerated dose of hydroxyurea of 7.0 and 5.7 events per 100 person-years, respectively. For secondary stroke prevention, in low-income settings without access to indefinite regular blood transfusion therapy, fixed low-dose hydroxyurea of at least 10 mg/kg/day with biannual CBC is a new evidence-based strategy to prevent strokes and minimize unnecessary laboratory testing. Disclosures DeBaun: Global Blood Therapeutics (GBT): Membership on an entity's Board of Directors or advisory committees; Novartis: Membership on an entity's Board of Directors or advisory committees. OffLabel Disclosure: fixed low and moderate dose hydroxyurea for primary stroke prevention in sickle cell

Introduction: In children with sickle cell anemia (SCA) without transcranial Doppler (TCD) screening, the incidence rates of ischemic strokes is approximately the same among children living in low- and high- low-resource settings (Pediatr Neurol. 2019;95:73-78.) with a prevalence of ~ 11%. However, in high-income settings, the standard use of TCD ultrasonography, coupled initially with monthly blood transfusion therapy has dropped the stroke prevalence to < 1%. In a low-income setting, such as Nigeria, where 50% of children in the world with SCA are born (150,000 per year), initial monthly blood transfusion therapy is not practical for most children. In the Stroke Prevention in Nigeria (SPIN) Feasibility Trial (NCT01801423), fixed moderate-dose hydroxyurea was associated with a decreased rate of strokes in children with SCA and abnormal time-averaged mean of the maximum velocity (TAMMV) TCD measurements (≥200cm/sec) when compared to no treatment in the STOP Trial, 0.76 and 10.7 strokes per 100 person-years, repsectively (Am J Hematol. 2020). Based on the success of the SPIN trial, plus the challenges of real-world implementation of a government-supported primary stroke prevention programs for estimated 40,0000 children with SCA in three states in Nigeria, we tested the hypothesis that fixed-moderate dose (~20 mg/kg/day) hydroxyurea therapy for primary stroke prevention results in a 66% relative risk reduction (9 to 3 events per 100 person-years) when compared to fixed low-dose hydroxyurea (~10 mg/kg/day) therapy in a randomized controlled trial (The SPRING Trial; NCT02560935). Methods: In this partial-blind controlled phase III trial, we randomly assigned children between 5 and 12 years of age with SCA and a TCD time-averaged mean of the maximum velocity (TAMMV) ≥ 200 cm/sec measured independently twice or TAMMV ≥220 cm/sec once at study screening to receive fixed low-dose or fixed moderate-dose hydroxyurea. The primary endpoint was a clinical stroke or a transient ischemic attack (TIA). Myelosuppression was assessed with monthly complete blood counts (CBCs). Adherence to hydroxyurea was primarily based on an increase in MCV from baseline and monthly pill count return as a percent of dispensed pills. Hemoglobin F levels were measured at baseline, annually and upon trial exit. To evaluate the safety of hydroxyurea in the trial, children attending the same SCA clinics with TCD (TAMMV) <200 cm/sec at study screening were prospectively followed with biweekly phone calls and annual research visits. Results: A total of 220 children (mean age: 7.5 years, 51.8% female) were randomly assigned to fixed low- (10 mg/kg/day) or moderate- (20 mg/kg/day) dose hydroxyurea, and were followed for a median of 2.4 years (IQR 2.0-2.8). NINDS Clinical Trials leaders stopped the trial early because of futility for the primary outcome. In the fixed low- and moderate-dose hydroxyurea groups, the incidence rates of strokes per 100 person-years were 1.19 and 1.92 respectively, with an incidence rate ratio of 1.60 (95% CI: 0.31-10.34), p = 0.768. The incidence rate ratio of mortality when comparing the children treated with low- and moderate- fixed-dose hydroxyurea to the non-elevated TCD group (no hydroxyurea therapy, n= 211) was 1.97 (95% CI: 0.64-6.02) and 0.47 (95% CI: 0.05-2.38), p = 0.265 and 0.545, respectively. Returned pills during the trial was 5.4% and 4.8% in the fixed low- and moderate-dose groups, respectively, p= 0.144. MCV from baseline to endpoint increased 1.5fl and 7.2 fl in the fixed low- and moderate-dose groups, respectively, p<0.001. Upon exit from the trial 29.4% and 66.7% of the fixed- low and moderate -dose groups, respectively, had either hemoglobin level ≥ 9.0 g/dl, or a fetal hemoglobin level ≥ 20%. Conclusions: For primary stroke prevention in children with SCA, fixed low-dose, when compared to fixed moderate-dose hydroxyurea therapy, demonstrated no difference in the incidence rate of strokes. Both fixed low- and moderate -dose hydroxyurea doses are superior to no treatment for primary stroke prevention with abnormal TCD values. In partnership with Katsina, Kano, and Kaduna health department's leaders in Nigeria, 9 distinct SCA and primary stroke prevention clinics have been established, with the provision of free fixed low-dose hydroxyurea therapy (Bond Chemical, Nigeria; $0.15 per 500 mg) for abnormal TCD values, and biannual CBCs as standard care ,for over 40,000 children with SCA. Disclosures DeBaun: Global Blood Therapeutics (GBT): Membership on an entity's Board of Directors or advisory committees; Novartis: Membership on an entity's Board of Directors or advisory committees. OffLabel Disclosure: fixed low and moderate dose hydroxyurea for primary stroke prevention in sickle cell