Phase 1 pilot study for hemophilia-A: AAV8 vector with prophylactic tacrolimus-glucocorticoid achieves therapeutic FVIII activity

# Phase 1 pilot study for hemophilia-A: AAV8 vector with prophylactic tacrolimus-glucocorticoid achieves therapeutic FVIII activity *Published: 2026 Mar 11* This prospective phase 1 pilot study evaluated the safety and efficacy of GS001, an adeno-associated virus serotype 8 (AAV8)-based vector encoding B-domain-deleted factor VIII with exploratory assessment of prophylactic immunosuppression. Twelve adult severe hemophilia A participants were enrolled. Participants received GS001 at either 2×10¹² (low dose cohort) or 4×10¹² (high dose cohort) vg/kg following prophylactic treatment with prednisone alone or prednisone plus tacrolimus starting from one week before GS001 infusion. Median (range) follow-up was 156.0 (144.0-208.0) / 110.5 (104.0-130.0) weeks (2×10¹² / 4×10¹² vg/kg). Transgene expression peaked at 5-7 weeks post-infusion. In low dose cohort, at week 144.0, median (range) factor VIII activity was 5.6 (0.5-52.0) IU/dL (n = 6). In high dose cohort, at week 104.0, median (range) factor VIII activity was 42.7 (29.4-92.1) IU/dL (n = 6) and all six participants sustained factor VIII levels above 29 IU/dL. Both cohorts showed significant reductions in annualized bleeding rate and factor VIII consumption with grade 1-2 adverse events. Alanine aminotransferase elevations occurred in 3/6 and 5/6 low/high-dose participants and most were grade 1. The Single-cell RNA analysis demonstrated that the prednisone plus tacrolimus pretreatment effectively inhibited CD8+ T cell immune responses in both cohorts. Overall, GS001 infusion was safe and well-tolerated, achieving a significant increase in factor VIII level. Pre-treatment with glucocorticoids and tacrolimus may benefit by suppressing induced immune responses, thereby enhancing transgene expression without increasing the risk of infection. This study was registered at www.clinicaltrials.gov as # NCT04728841. DOI: 10.1038/s41392-026-02599-3