Exagamglogene Autotemcel for Transfusion-Dependent β-Thalassemia

Background: Exagamglogene autotemcel (exa-cel) is a non-viral cell therapy designed to reactivate fetal hemoglobin (HbF) via ex vivo CRISPR-Cas9 gene-editing of autologous CD34+ hematopoietic stem and progenitor cells (HSPCs) at the erythroid-specific enhancer region of BCL11A in patients (pts) with transfusion-dependent β-thalassemia (TDT). Here we report that in a pre-specified interim analysis, the pivotal CLIMB THAL-111 trial of exa-cel met primary and key secondary endpoints.

Methods: CLIMB THAL-111 is an ongoing, 24-month (mo), phase 3 trial of exa-cel in pts age 12-35y with TDT and a history of ≥100 mL/kg/y or ≥10 U/y packed red blood cell (RBC) transfusions in the 2y before screening. Primary endpoint is transfusion independence defined as proportion of pts maintaining a weighted average hemoglobin (Hb) ≥9 g/dL without RBC transfusion for ≥12 consecutive mos (TI12). Key secondary endpoint is proportion of pts maintaining a weighted average Hb ≥9 g/dL without RBC transfusion for ≥6 consecutive mos (TI6). Evaluable pts had ≥16 mos of follow-up after exa‑cel infusion. Evaluation of TI12 and TI6 started 60 days after last RBC transfusion for post-transplant support or TDT management. Pts completing trial enrolled in long-term follow-up Study 131. Mean (SD) shown except where noted.

Results: As of 16 Jan 2023, 52 pts (mean age 21.5[range 12-35]y; 18[34.6%] age ≥12 to <18y; 31[59.6%] with severe genotypes [β⁰/β⁰ or β⁰/β⁰-like], median annualized transfusion volume 201.0 mL/kg) received exa-cel; median follow-up 20.4 (range 2.1-48.1) mos. Following infusion, all pts engrafted neutrophils and platelets (median 29 and 44 days, respectively). Of the 35 pts evaluable for primary and key secondary endpoints, 32 (91.4%) achieved TI12 and TI6 (95% CI: 76.9%, 98.2%; P<0.0001). Pts achieving TI12 stopped transfusions 35.2 (SD, 18.5) days after exa-cel infusion and remained transfusion independent for 22.5 (range, 13.3, 45.1) mos (Fig). For 3 pts not achieving TI12, one had reductions in annualized RBC transfusion volume of 83.9%, while the others have been transfusion-free for 7.3 mos and 4.0 mos starting 60 days after the last transfusion. For all pts, total Hb was 11.4 g/dL at Month 3 (≥12g/dL Month 6 onward) and HbF was 7.7 g/dL at Month 3 (≥ 10 g/dL Month 6 onward) with pancellular distribution (≥95% RBCs expressing HbF Month 6 onward). Proportion of edited BCL11A alleles was stable over time in bone marrow CD34⁺ and peripheral blood nucleated cells. Pts not yet evaluable and with sufficient follow-up were also transfusion-free. Quality-of-life (QOL) measures showed clinically significant improvements.

All pts had ≥1 adverse event (AE), most were Grade 1 or 2; 46 (88.5%) pts had AEs of Grade 3 or 4 severity. Most common AEs were febrile neutropenia (61.5%), headache (53.8%), and stomatitis (50.0%). Most AEs and serious AEs (SAEs) occurred within first 6 mos after infusion. Two pts had SAEs considered related to exa-cel: headache, hemophagocytic lymphohistiocytosis (HLH), acute respiratory distress syndrome and idiopathic pneumonia syndrome (latter also considered related to busulfan) all in the context of HLH (n=1) and delayed engraftment and thrombocytopenia (both also considered related to busulfan) (n=1), which all resolved. There were no deaths, discontinuations, or malignancies.

Conclusions: The CLIMB THAL-111 trial met primary and key secondary endpoints, with exa-cel treatment resulting in early and sustained increases in Hb and HbF leading to transfusion independence in >90% of pts with TDT and improved QOL. Safety profile of exa‑cel was generally consistent with myeloablative busulfan conditioning and autologous transplantation. These results show exa-cel has the potential to deliver a one-time functional cure to pts with TDT.