Extravascular FIX and coagulation

Abstract

This review summarizes the evidence that collagen IV binding is physiologically important, and that the extravascular compartment of FIX is composed of type IV collagen. As we have previously demonstrated, 7 days post-infusion, FIX_WT (BeneFIX) is able to control bleeding as well as the same dosage of Alprolix in hemophilia B mice, tested using the saphenous vein bleeding model (Alprolix is a chimeric FIX molecule joined at its C terminus to a Fc domain). Furthermore, we have shown that in hemophilia B mice, doses of BeneFIX or Alprolix (up to a dose of 150 IU/kg) have increased bleeding-control effectiveness in proportion to the dose up to a certain limit: higher doses are no more effective than the 150 IU/kg dose. These studies suggest that in hemophilia B mice, tested using the saphenous vein bleeding model, three things are true: first, extravascular FIX is at least as important for coagulation as is circulating FIX; second, measuring circulating levels of FIX may not be the best criterion for designing new "longer lasting" FIX molecules; and third, trough levels are less diagnostic for FIX therapy than they are for FVIII therapy.

Keywords: Coagulation; Collagen IV; FIX; Hemophilia B.

Fig. 1

Comparison of the ability of different concentrations of bovine factor IX to displace baboon factor IX from its extravascular pool. Redrawn from Stern et al. Br J. Haemat. 66:227. (1987). Adult baboons were infused with the indicated amounts of bovine FIX, and the amount of baboon FIX in the plasma was measured over the first 15 min post-infusion

Fig. 2

Comparison of different doses of factor IX to effect hemostasis seven days post-infusion in hemophilia B mice. From: Cooley et al. Blood. 128:286. (2016). Doses of 50, 100, 150, 250, and 500 IU/kg of either Alprolix or Benefix were infused into hemophilia B mice. After 7 days, the ability of the mice to maintain hemostasis was evaluated by the saphenous vein bleeding model. At least 20 mice were used for each dose

Fig. 3

Comparison of the hemostatic ability of wild-type mice and hemophilia B mice whose endogenous factor IX was exchanged for FIX_K5A. From: Cooley et al. Blood. 128:286. (2016). A mouse expressing FIX_K5A from its endogenous promoter was compared to a wild-type mouse using the saphenous vein bleeding model. The activity level of the FIX_K5A in the plasma of this mouse was slightly higher than that of the wild-type mouse. The mouse expressing FIX_K5A nevertheless has a bleeding diathesis

Fig. 4

An examination of the ability of FIX (BeneFIX) to effect hemostasis 7 days post-infusion, after the ability of a first injury to effect clotting has been exhausted. From: Cooley et al. Blood. 128:286. (2016). Seven days post-infusion, the right saphenous vein was injured and the number of times that re-clotting ensued was determined. After the first injury was unable to continue clotting, the contralateral vein was injured and hemostatic efficacy was again measured. Presumably, any residual plasma FIX would have been used up during the first injury had plasma FIX been required for clotting

Fig. 5

A fit of Stern’s data [Stern et al. Br J. Haemat. 66:227. (1987)] to a simple set of differential equations describing equilibrium. Stern’s data was digitized and fit to a simple differential equation with the assumption that, during the first 10 min after infusion, only equilibrium between FIX and type IV collagen occurs. The numbers are at least consistent with in vitro measurements of the binding constants between FIX and collagen IV