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We report feasibility and safety for the first subject in a first-in-human trial of gene therapy for severe hemophilia A with factor VIII inhibitor history (NCT03818763). Eligible were males, ≥18 years, had severe hemophilia A with historical or active exogenous factor VIII inhibitor (≥0.6 BU/ml). Excluded were those at risk for thrombosis, bone marrow disorder or immune tolerance induction ≤30 days of enrollment. Autologous CD34+ cells were selected using a cell-isolation platform (CliniMacs Plus, Miltenyi Biotech) and transduced with a lentiviral vector encoding factor VIII driven by an ITGA2B gene promoter for platelet specific expression and storage of the encoded protein in platelet α-granules1,2 (Supplementary Appendix, available with the full text of this letter at NEJM.org).Transduced cells were infused after reduced-intensity cytoreduction (fludarabine [120 mg/m2] and melphalan [120 mg/m2])3 and a washout period ≥24 hours. The primary endpoints were feasibility (≥4 × 106/kg transduced clinical grade CD34+cells, viability ≥70% and undetectable microbial contamination) and safety, hematopoietic recovery ≤28 days after infusion and absence of ≥ grade 3 toxicity (CTCAE version 5.0). The participant was a 29-year old with factor VIII inhibitor history (2.6 BU/ml). His prophylaxis included standard half-life, then extended half-life factor VIII concentrates before switching to emicizumab 1.5 mg/kg (150 mg) weekly 13 months prior to enrollment. He had a history of one target joint (left ankle) which remained active at enrollment.

old with factor VIII inhibitor history (2.6 BU/ml). His prophylaxis included standard half-life, then extended half-life factor VIII concentrates before switching to emicizumab 1.5 mg/kg (150 mg) weekly 13 months prior to enrollment. He had a history of one target joint (left ankle) which remained active at enrollment. The cell product, yielded 6.73 × 106/kg total viable CD34+ cells post-transduction and vector copy number by qPCR of 1.16 copies/cell. Megakaryocyte factor VIII:C levels were 0.00 mU/106 cells before transduction and 101.32 mU/106 cells after transduction. Release criteria were satisfied and hematopoietic recovery was achieved 15 days after infusion. Emicizumab was discontinued 3.6 months after infusion. Secondary and exploratory assessments are summarized in Table 1. Longitudinal integration site analysis showed that none of the samples contained expanded transduced clones that exceeded 20% of the sample’s total inferred cells. Cell populations were highly polyclonal without enrichment of integration sites near cancer-associated genes.4 Annualized bleeding rate was 10 before gene therapy and zero after (table 1).

alysis showed that none of the samples contained expanded transduced clones that exceeded 20% of the sample’s total inferred cells. Cell populations were highly polyclonal without enrichment of integration sites near cancer-associated genes.4 Annualized bleeding rate was 10 before gene therapy and zero after (table 1). Our approach takes advantage of the main function of platelets, which is to mediate the primary response to vascular injury. Factor VIII synthesis and storage within platelet α-granules allow direct delivery of factor VIII to the bleeding site for hemostasis. In conclusion, the first participant in this first-in-human trial with 24-months of follow up confirms feasibility and safety with preliminary evidence in humans that a ITGA2B gene promoter is an effective driver of platelet-derived factor VIII expression. Additional follow-up will clarify effects beyond 24 months.5