FVIII activity and inhibitor titers after intraperitoneal or subcutaneous administration of lentiviral vectors to neonatal hemophilia A mice. Neonatal BALB/c hemophilia A mice were treated with 2 × 10⁷ IU of Lenti-HCR/hAAT-cFVIII intraperitoneally (A-B) or subcutaneously (C-D). (A,C) Plasma levels of FVIII in mice treated via these delivery routes and (B,D) levels of neutralizing anti-cFVIII antibodies as measured by a Bethesda assay (note the different inhibitor axes). Five of the mice that were injected subcutaneously with Lenti-HCR/hAAT-cFVIII and which developed neutralizing anti-cFVIII antibodies that ranged between 23 and 95 BU were used as positive controls for inhibitor development.