Hypothesis for reduced FVIII efficacy at high concentrations of rD′D3-FP in human plasma. (A) FVIII in human plasma (≈0.3 nM) exists primarily bound to its endogenous carrier protein VWF (≈25 nM dimer) as a result of its low KD (≈0.1 nM). The fast on- and off-rate of this interaction allows switching of FVIII to rD′D3-FP. At micromolar concentrations, most FVIII is expected to be bound to rD′D3-FP. (B) Upon activation of FVIII, its affinity for VWF is drastically reduced, resulting in its dissociation from rD′D3-FP. Some portion of this free FVIIIa is suggested to bind membranes to form the tenase complex. However, at high micromolar concentrations of rD′D3-FP, rebinding of FVIIIa might result in reduced availability of FVIIIa and, in turn, the observed loss of efficacy. HSA, human serum albumin.