Factor VIII with a 237 amino acid B-domain has an extended half-life in F8-knockout mice

Background Factor VIII consists of the A1-domain, A2-domain, B-domain, A3-domain, C1-domain, and C2-domain. FVIII with an intermediate-length B-domain of 226 amino acids (aa) has previously been evaluated in murine gene therapy studies. Objective To characterize FVIII with intermediate-length B-domains in vitro and in vivo in F8-knockout (KO) mice. Methods and results FVIII molecules with B-domains of 186-240aa had longer half-lives in F8-KO mice than FVIII molecules with shorter or longer B-domains. FVIII with a B-domain containing the 225 N-terminal aa fused to the 12 C-terminal aa of the wild-type B-domain (FVIII-237) had a 1.6-fold extended half-life in F8-KO mice as compared with FVIII with a 21aa B-domain (FVIII-21). The in vitro and in vivo activity of FVIII-237 were comparable to those of FVIII-21, as was binding to von Willebrand factor. Cell binding to LDL receptor-related protein 1 (LRP-1)-expressing cells was markedly reduced for FVIII-237 as compared with FVIII-21, whereas the affinity for LRP-1 was not reduced in surface plasmon resonance (SPR) studies. FVIII-21 cell binding and internalization could be inhibited by a fragment consisting of the 226 N-terminal aa of the FVIII B-domain, and SPR analysis suggested that this B-domain fragment might bind with weak affinity to FVIII-21. Conclusion Reduced cell binding of FVIII-237 might explain the observed extended half-life in F8-KO mice. This may contribute to the increased FVIII levels measured in murine gene therapy studies using FVIII constructs with similar B-domain lengths.