Long-term stability of posttranscriptional genetic silencing of BCL11A using a shmiR vector in Sickle Cell Disease

Sickle cell disease (SCD) is characterized by chronic hemolysis, painful vaso-occlusive episodes (VOE) and end organ damage. High levels of fetal hemoglobin (HbF) attenuate the disease phenotype. We used a lentivirus vector (LVV) expressing an shRNA embedded in a microRNA (shmiR) targeting BCL11A in erythrocytes to induce HbF in a first-in-human pilot study in SCD. The purpose of the study was to assess hematopoietic stem/progenitor cells (HSPCs) collection, transduction parameters, safety, HbF induction and durability. Eleven eligible patients with SCD had HSC collection. Plerixafor-mobilized peripheral blood HSCs required for manufacturing were obtained in one mobilization cycle for 10/11 subjects and 11/11 patient products were successfully manufactured with a median time to release of product of 39 days. Ten patients were infused with autologous HSCs transduced with the shmiR vector. Engraftment occurred in all 10 patients. With a median follow-up of 58 months (range: 35-82) after infusion, no adverse events attributed to the gene vector have occurred. Transduction efficiency was 93.1%. One patient demonstrated low engraftment of transduced cells and had suboptimal HbF induction. In the remaining 9 patients, at 2 years post-treatment peripheral blood demonstrated 71% F cells with 11.9 pg HbF/F cells, both stable in 9 patients with ≥48 months follow-up. All patients who had VOEs prior to gene therapy demonstrated sustained mitigation of pain events. These data demonstrate excellent manufacturing efficiency and safety, with efficacy of targeting BCL11A using a shmiR LVV, and long-term durability of the shmiR vector, leading to a pivotal multi-site phase 2 trial currently underway (NCT05353647).