Creating a DDAVP Releasable Pool of Factor VIII: Its Impact on the Gene Therapy of Hemophilia A † 786

The administration of DDAVP has been know for many years to result in a marked increase in plasma von Willebrand factor (vWF) and Factor VIII (FVIII). This occurs within minutes of intravenous administration and suggests a pre-existing pool of both proteins. The two cells in the body that can synthesize vWF are the endothelial cell and the megakaryocyte which respectively store vWF in the endothelial cell Weibel-Palade Body and the platelet α-granule. Most investigations have suggested that the primary source of the increased plasma vWF is the endothelial cell, but there is no recognized storage source of FVIII. In fact, the cell that produces FVIII is not definitively determined. Transplantation studies suggest that liver, spleen, and lymph nodes may be organ sources of FVIII but the tissue source is not well defined. To further define the nature of FVIII synthesis, storage and release, we studied two patients with severe hemophilia A undergoing prophylactic administration of recombinant FVIII and a type 3 von Willebrand disease (vWD) patient receiving regular infusions of vWF without significant FVIII. These individuals were administered 0.3 μg/kg DDAVP intravenously before and following the administration of the respective therapeutic product to determine if the DDAVP-releasable pool of FVIII was established. Plasma assays of vWF and FVIII demonstrated that there was no measurable release of FVIII into plasma in either disorder. In the vWD patient, the endogenous production of FVIII was at a normal level when the vWF was exogenously administered. Yet, there was no increase of FVIII following DDAVP. These studies suggested that the storage pool of FVIII was vWF dependent and that the storage reservoir of both proteins may need to be in the same cell - presumably the same cell in which they were synthesized. Since we have demonstrated that FVIII co-localizes with vWF when the gene for FVIII is inserted into an endothelial cell, we postulate that the DDAVP-releasable pool of FVIII is in a vWF containing cell. This most likely would occur in a subpopulation of endothelial cells that are not yet defined. Furthermore, gene therapy of hemophilia A may need to be targeted to the vascular endothelium to produce a physiologic storage pool that is protected from the proteolytic activity of plasma proteases.