AbstractA novel mechanism for extending the circulatory half-life of coagulation factor VIII (FVIII) has been established and evaluated preclinically. The FVIII binding domain of von Willebrand factor (D′D3) fused to human albumin (rD′D3-FP) dose dependently improved pharmacokinetics parameters of coadministered FVIII in all animal species tested, from mouse to cynomolgus monkey, after IV injection. At higher doses, the half-life of recombinant FVIII (rVIII-SingleChain) was calculated to be increased 2.6-fold to fivefold compared with rVIII-SingleChain administered alone in rats, rabbits, and cynomolgus monkeys, and it was increased 3.1-fold to 9.1-fold in mice. Sustained pharmacodynamics effects were observed (ie, activated partial thromboplastin time and thrombin generation measured ex vivo). No increased risk of thrombosis was observed with coadministration of rVIII-SingleChain and rD′D3-FP compared with rVIII-SingleChain alone. At concentrations beyond the anticipated therapeutic range, rD′D3-FP reduced the hemostatic efficacy of coadministered rVIII-SingleChain. This finding might be due to scavenging of activated FVIII by the excessive amount of rD′D3-FP which, in turn, might result in a reduced probability of the formation of the tenase complex. This observation underlines the importance of a fine-tuned balance between FVIII and its binding partner, von Willebrand factor, for hemostasis in general.