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Background: BCL11A is a key transcription factor that suppresses the production of fetal hemoglobin (HbF) in red blood cells (RBCs), leading to the production of adult Hb (HbA). In diseases with hemoglobin production defects such as b-thalassemia, or in sickle cell disease (SCD), HbF upregulation could ameliorate anemia and reduce transfusion requirements, such as in β-thalassemia, or reduce clinical complications, including vaso-occlusive crises (VOCs), in SCD. To induce potentially curative levels of HbF in erythrocytes, we used the ex vivo CRISPR-Cas9-based gene-editing platform to edit the erythroid enhancer region of BCL11A in hematopoietic stem and progenitor cells (HSPCs), producing CTX001. Aims: CLIMB THAL-111 (NCT03655678) and CLIMB SCD-121 (NCT03745287) are multi-center, first-in-human studies of CTX001 for transfusion-dependent b-thalassemia (TDT) and SCD, respectively. Here, we present available safety and efficacy results from all patients with at least 3 months of follow-up from both studies as of July 2020. Methods: Patients (aged 18 to 35 years) with TDT receiving packed red blood cell (pRBC) transfusions of ≥100 mL/kg/year or ≥10 units/year in the previous 2 years, and those with severe SCD, defined as ≥2 VOCs/year requiring medical care in the previous 2 years, were eligible. Peripheral CD34+ HSPCs were collected by apheresis after mobilization with G-CSF (filgrastim) and plerixafor (for TDT) or plerixafor alone (SCD). The erythroid enhancer region of BCL11A was edited in CD34+ cells using a specific CRISPR guide RNA and Cas9 nuclease. Prior to CTX001 infusion on Day +1, patients received myeloablation with 4 days of busulfan. Patients were monitored for stem cell engraftment and hematopoietic recovery, adverse events, total Hb and HbF production, hemolysis, F-cells, pRBC transfusion requirements (TDT), and VOCs (SCD) during follow-up. Results: Data are presented for patients with TDT (N=5; RBC transfusion history range: 23.5 to 61 units/year; CTX001 post-infusion follow-up through Months 15, 6, 4, 4, and 3, respectively) and with SCD (N=2; 7 VOCs/year and 7.5 VOCs/year, respectively, annualized over 2 years prior to consent; CTX001 post-infusion follow-up through Months 12 and 3, respectively). In the patients with TDT, median neutrophil engraftment occurred on Day +32 (range: +27 to +36); median platelet engraftment occurred on Day +37 (range: +34 to +52). In the patients with SCD, neutrophil engraftment occurred on Day +30 and Day +22 and platelet engraftment occurred on Day +30 and Day +33, respectively. All patients demonstrated increases in total Hb and HbF over time (Figure). Patients with TDT ceased receiving pRBC transfusions soon after CTX001 infusion, with the last pRBC transfusion occurring between 0.9 and 1.9 months after CTX001 infusion. The first patient with TDT who received CTX001 has remained transfusion-free for over 15 months. Patients with SCD have had no VOCs since CTX001 infusion. The first SCD patient who received CTX001 has remained free of VOCs for over 1 year. In all 7 patients, the safety profile after CTX001 infusion was generally consistent with busulfan myeloablation. Four serious adverse events (SAEs) related or possibly related to CTX001 were reported in 1 patient with TDT: headache, haemophagocytic lymphohistiocytosis (HLH), acute respiratory distress syndrome, and idiopathic pneumonia syndrome. All 4 of these SAEs occurred in the context of HLH and were either resolved or clinically improving at the time of this analysis. No other CTX001-related SAEs were reported in the other patients with TDT or in any patients with SCD. Conclusions: These data demonstrate that CTX001, a first-in-human, CRISPR-Cas9-modified autologous HSPC product, has resulted in increases in HbF and total Hb in the first 7 patients infused. All patients infused with CTX001 demonstrated hematopoietic engraftment with a post-infusion safety profile generally consistent with myeloablation. All 5 patients with TDT have been transfusion-free since ~2 months after CTX001 infusion and the 2 patients with severe SCD have had no VOCs during follow-up after CTX001 infusion. These early data demonstrate that CTX001 is a potential functional cure for the treatment of TDT and SCD. Data will be updated for the presentation. Data from these ongoing studies were submitted on behalf of the CLIMB THAL-111 and CLIMB SCD-121 Investigators. Figure Disclosures Frangoul: Vertex Pharmaceuticals: Membership on an entity's Board of Directors or advisory committees. Bobruff:CRISPR Therapeutics: Current Employment, Current equity holder in publicly-traded company. Cappellini:BMS: Honoraria; CRISPR Therapeutics, Novartis, Vifor Pharma: Membership on an entity's Board of Directors or advisory committees; Genzyme/Sanofi: Honoraria, Membership on an entity's Board of Directors or advisory committees. Fernandez:CRISPR Therapeutics: Current Employment, Current equity holder in publicly-traded company. Grupp:Juno/BMS: Other; Cellectis: Other; TCR2: Other: SAB; Servier: Research Funding; Janssen/JnJ: Consultancy; CBMG: Consultancy; Humanigen: Consultancy; GlaxoSmithKline: Consultancy; Roche: Consultancy; CRISPR Therapeutics/Vertex Pharmaceuticals: Other; Allogene: Other; Kite/Gilead: Research Funding; Novartis: Consultancy, Other: SSC, Research Funding; Adaptimmune: Other: SAB; Jazz: Other: SSC. Handgretinger:Amgen: Honoraria. Ho:CRISPR Therapeutics: Current Employment, Current equity holder in publicly-traded company. Imren:Vertex Pharmaceuticals Incorporated: Current Employment, Current equity holder in publicly-traded company. Kattamis:Agios: Consultancy; Vertex: Membership on an entity's Board of Directors or advisory committees; Ionis: Membership on an entity's Board of Directors or advisory committees; Genesis Pharma SA: Membership on an entity's Board of Directors or advisory committees; Vifor: 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, Research Funding, Speakers Bureau; Celgene/BMS: Honoraria, Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Apopharma/Chiesi: Honoraria, Speakers Bureau. Lekstrom-Himes:Vertex Pharmaceuticals Incorporated: Current Employment, Current equity holder in publicly-traded company. Locatelli:Medac: Speakers Bureau; Miltenyi: Speakers Bureau; Bellicum Pharmaceutical: Membership on an entity's Board of Directors or advisory committees; Novartis: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Amgen: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Jazz Pharmaceeutical: Speakers Bureau. Lu:Vertex Pharmaceuticals Incorporated: Current Employment, Current equity holder in publicly-traded company. de Montalembert:Bluebird bio: Honoraria, Membership on an entity's Board of Directors or advisory committees; Vertex: Honoraria, Membership on an entity's Board of Directors or advisory committees; Novartis: Honoraria, Membership on an entity's Board of Directors or advisory committees; Addmedica: Honoraria, Membership on an entity's Board of Directors or advisory committees. Mulcahey:Vertex Pharmaceuticals Incorporated: Current Employment, Current equity holder in publicly-traded company. Shanbhag:Vertex Pharmaceuticals Incorporated: Current Employment, Current equity holder in publicly-traded company. Sheth:Agios: Consultancy, Research Funding; Celgene/BMS: Consultancy, Research Funding; La Jolla: Research Funding; Acceleron: Consultancy; Bluebird Bio: Consultancy; Novartis: Consultancy, Research Funding; DisperSol Technologies: Research Funding; Terumo: Research Funding; Vertex Pharmaceuticals/CRISPR Therapeutics: Membership on an entity's Board of Directors or advisory committees. Soni:CRISPR Therapeutics: Current Employment, Current equity holder in private company. Steinberg:Vertex Pharmaceuticals/CRISPR Therapeutics: Membership on an entity's Board of Directors or advisory committees; Fulcrum Therapeutics: Membership on an entity's Board of Directors or advisory committees; DSMB: Membership on an entity's Board of Directors or advisory committees; Imara: Membership on an entity's Board of Directors or advisory committees. Weinstein:CRISPR Therapeutics: Current Employment, Current equity holder in publicly-traded company. Wu:Bayer: Research Funding; Novo Nordisk: Membership on an entity's Board of Directors or advisory committees; Octapharma: Membership on an entity's Board of Directors or advisory committees; CSL Behring: Membership on an entity's Board of Directors or advisory committees; Bioverativ: Membership on an entity's Board of Directors or advisory committees; Sanofi: 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; Roche: Membership on an entity's Board of Directors or advisory committees.