FVIII expression and inhibitor development after intravenous temporal vein infusion of Lenti-CMV-cFVIII and Lenti-HCR/hAAT-cFVIII into BALB/c neonatal mice. (A-B) An initial cohort of 5 BALB/c hemophilia A neonates was treated via temporal vein infusion with 100 μL of 2 × 10⁷ IU of Lenti-CMV-cFVIII. (D-E) BALB/c hemophilia A neonates (n = 6) treated via the temporal vein with 2 × 10⁷ IU of Lenti-HCR/hAAT-cFVIII. (A,D) Plasma levels of cFVIII activity were determined by a chromogenic assay. (B,E) Anti-cFVIII inhibitory antibodies were measured by the Bethesda assay. Throughout the duration of the study, hemophilia A mice that had been infused with 2 × 10⁷ IU of Lenti-CMV-cFVIII via the temporal vein as neonates were used as controls for positive inhibitor development. The inhibitor titers for these mice are shown in panel C.