Bio-Distribution of Recombinant Factor VIIa: A Possible Explanation for the Hemostatic Effect of rFVIIa in Prophylaxis.

Abstract 3480

Poster Board III-417

Frequent spontaneous joint bleedings in severe hemophilia leads to chronic arthopathy, which significantly reduces the quality of life of these patients. Recent clinical studies suggest that the prophylactic use of rFVIIa markedly reduced the number of bleeding incidents, hospitalization time and days of immobility in hemophilic patients with inhibitors. Interestingly the prophylactic effect of rFVIIa was maintained during the 3-month post-prophylactic follow up (Konkle et al., J Thromb Haemost 2007; 5:1904-13). Given the short biological half-life rFVIIa (∼2 to 3 h), it is unclear how rFVIIa prophylaxis continues to reduce the number of hemorrhagic events in the post-treatment period. It had been hypothesized that rFVIIa administered pharmacologically may reach the extravascular spaces where it could be retained for extended time periods and therefore continue to be available at the site of injury to contribute to the hemostatic plug (Hedner, J Thromb Haemost 2006; 4:2498-500). The present study was aimed to examine the extravascular distribution of pharmacologically administered rFVIIa. Recombinant mouse FVIIa was tagged with AF488 fluorophore (AF488-FVIIa) and the AF488-FVIIa was administered into anesthetized male C57BL/6 mice through the tail vein (120 mg/kg b.w.). At different time intervals following AF488-FVIIa administration (from 10 min to 7 days), mice were exsanguinated and various tissues were collected. The tissues were fixed, processed, sectioned and examined for rFVIIa by immunohistochemistry by using specific antibodies directed against AF488. We also examined the distribution of AF488-rFIX under similar experimental settings. The immunohistochemistry data revealed that rFVIIa given at a pharmacological dose enters the extravascular compartment and accumulates differentially in different tissues. The tissue and cell localization appear to be specific for rFVIIa as we found significant differences between rFVIIa and rFIX distribution following their administration to mice. rFVIIa, immediately following administration, was found associated with the endothelium lining of large blood vessels. This may reflect rFVIIa binding to EPCR as FVIIa has been shown to bind to EPCR and endothelial cells lining large blood vessels have been shown to express EPCR. In contrast to rFVIIa, rFIX failed to associate with the endothelial cells. Within one hour, rFVIIa bound to endothelial cells was transferred to the perivascular tissue surrounding the blood vessels and thereafter diffused throughout the tissue. Although we have noted rFVIIa association with the endothelium in many vascular beds (e.g., bone joints, liver, skin, heart and kidney), it is conspicuously absent in the lung and brain. In liver, rFVIIa was localized initially to the portal vein and the sinusoidal capillaries. rFVIIa was also accumulated in hepatocytes. Although rFVIIa levels in the liver were decreased 6 h after administration, traces of rFVIIa were still seen at both 3 and 7 days following rFVIIa administration. In the bone, large amounts of rFVIIa accumulated in the zone of calcified cartilage for 6 h following administration and were decreased only slightly thereafter. A significant amount of rFVIIa was retained in the zone of calcified cartilage even a week following the administration. The common finding of the present study is that rFVIIa in extravascular spaces was mostly localized to the regions that contain TF expressing cells. For example, in the skin, the sebaceous gland and hair follicles that are rich with TF were shown to sequester FVIIa for a longer time period than other regions where TF is absent. In the kidney, FVIIa was retained in glomeruli that express high levels of TF. Overall, the present study suggests that pharmacologically administered rFVIIa readily associates with the endothelium, probably by binding to EPCR, and then enters into extravascular spaces probably by EPCR-mediated transcytosis where it likely binds to TF and is consequently retained for extended time periods. This may explain the prolonged pharmacological effect of rFVIIa.