Correction of bleeding in experimental severe hemophilia A by systemic delivery of factor VIII-encoding mRNA

The treatment or prevention of bleeding in patients with hemophilia A relies on replacement therapy with different factor VIII (FVIII)-containing products or on the use of by-passing agents, i.e., activated prothrombin complex concentrates or recombinant activated factor VII. Emerging approaches include the use of bispecific anti-factor IXa/factor X antibodies, anti-tissue factor pathway inhibitor antibodies, interfering RNA to antithrombin, and activated protein C-specific serpins or gene therapy. The latter strategies are, however, hampered by the short clinical experience and potential adverse effects including the absence of tight temporal and spatial control of coagulation and the risk of uncontrolled insertional mutagenesis. Systemic delivery of mRNA allows endogenous production of the corresponding encoded protein. Thus, injection of erythropoietin-encoding mRNA in a lipid nanoparticle formulation resulted in increased erythropoiesis in mice and macaques. Here, we demonstrate that a single injection of in vitro transcribed B domain-deleted FVIII-encoding mRNA to FVIII-deficient mice enables endogenous production of pro-coagulant FVIII. Circulating FVIII:C levels above 5% of normal levels were maintained for up to 72 h, with an estimated half-life of FVIII production of 17.9 h, and corrected the bleeding phenotype in a tail clipping assay. The endogenously produced FVIII did however exhibit low specific activity and induced a potent neutralizing IgG response upon repeated administration of the mRNA. Our results suggest that the administration of mRNA is a plausible strategy for the endogenous production of proteins characterized by poor translational efficacy. The use of alternative mRNA delivery systems and improved FVIII-encoding mRNA should foster the production of functional molecules and reduce their immunogenicity.