Long Term Outcomes of 63 Patients with Transfusion-Dependent β-Thalassemia (TDT) Followed up to 7 Years Post-Treatment with betibeglogene autotemcel (beti-cel) Gene Therapy and Exploratory Analysis of Predictors of Successful Treatment Outcomes in Phase 3 Trials

Background: One-time treatment with beti-cel ex vivo gene therapy (GT) adds functional copies of a modified HBB gene, β^A-T87Q, to autologous CD34+ hematopoietic stem and progenitor cells (HSPCs) through transduction with BB305 lentiviral vector (LVV), thereby addressing the underlying genetic cause of Transfusion-Dependent β-Thalassemia (TDT). Here, we evaluate the long-term efficacy and safety profile of beti-cel across phase 1/2, phase 3, and long-term follow-up (LTF) studies. We also present an exploratory analysis of predictors of successful treatment outcomes in patients from phase 3 studies.

Methods: Patients with TDT underwent HSPC collection followed by pharmacokinetic-adjusted myeloablative busulfan conditioning and beti-cel infusion. Patients were followed for 2 yrs in the parent phase 1/2 (HGB-204, NCT01745120; HGB-205, NCT02151526) and phase 3 (HGB-207, NCT02906202; HGB-212, NCT03207009) studies; enrollment and all beti-cel infusions have been completed. Patients then entered a 13-yr LTF study, LTF-303 (NCT02633943). Transfusion independence (TI) was defined as weighted average hemoglobin (Hb) ≥ 9 g/dL without packed red blood cell transfusions for ≥ 12 months. An exploratory multivariate analysis was also carried out in patients enrolled in the phase 3 trials to identify predictors of clinical outcomes.

Results: As of August 2021, 63 patients had received beti-cel and were followed for a median of 41.4 (9.0–87.5) months across the 4 parent studies and LTF. The median time until neutrophil and platelet engraftment was 23 (13–39) and 45 (19–191) days, respectively, and lymphocyte subsets were generally within normal range after beti-cel infusion.

Peripheral blood vector copy number (VCN), levels of GT–derived adult Hb (HbA^T87Q), and total Hb were stable and durable across studies and higher in phase 3 vs phase 1/2 studies following optimized beti-cel drug product (DP) manufacturing. HbA^T87Q levels were stabilized by Month 6 and contributed to stable levels of total Hb. Total unsupported Hb levels ≥ 9g/dL at Month 6 were significantly associated with achieving TI (p<0.0001) (March 2021 data cut).

One-time beti-cel GT was followed by durable TI across ages and genotypes with up to 7 yrs follow-up. A trend towards more patients achieving TI was observed in phase 3 (34/38; 89.5%) (Fig 1A) vs phase 1/2 (15/22; 68.2%). All patients who achieved TI (N=49) continue to remain TI as of last follow-up. Patients who achieved TI had reductions in markers of ineffective erythropoiesis and iron overload. Overall, 21/37 patients who restarted chelation stopped chelation at 24 months; 25% (12/49) of patients who achieved TI utilized phlebotomy for iron removal, including 6 patients who also received iron chelation.

Overall, 18% (11/63) of patients experienced ≥1 adverse event (AE) considered related or possibly related to beti-cel; the most common DP-related AEs were abdominal pain (5/63 [8%]) and thrombocytopenia (3/63 [5%]), but none were observed beyond 2 yrs post-infusion, supporting a favorable long-term safety profile. Veno-occlusive liver disease was reported in 11% (7/63) of patients and resolved after appropriate treatment. No malignancies, insertional oncogenesis, vector-derived replication competent lentivirus, or clonal predominance was observed. The safety of the beti-cel treatment regimen largely reflected the known side effects of hematopoietic stem cell collection and the busulfan conditioning regimen.

An exploratory multivariate analysis in patients from phase 3 studies (n=37; March 2021 data cut) identified that the percentage of DP cells transduced with the BB305 LVV (%LVV+ cells) was the best predictor of clinical outcomes (Fig 1B). DP VCN and %LVV+ cells are current release assays for beti-cel.

Summary: beti-cel is a potentially curative GT for patients with TDT across ages and genotypes through the achievement of TI and normal or near normal Hb levels. An exploratory multivariate analysis demonstrated that DP transduction efficiency was the most predictive attribute of TI, a key endpoint for clinical outcomes.