High Affinity Binding of FVIII to VWF Is Not Required for the Haemostatic Effect of FVIII In Vivo

Abstract 1182

VWF protects FVIII from clearance in the circulation and is believed to ensure location of platelets to the site of injury. However, it is unknown if binding of FVIII to VWF has a role in localizing and thereby also facilitating the effect of FVIII in vivo. In the present study, a FVIII variant, FVIII-Y1680F, lacking the high affinity binding to VWF (Leyet et al. JCB 1991; 15; 740) was used to evaluate the binding of FVIII to VWF in clot formation hemophilia A mice in vivo.

Binding of the FVIII variant to immobilized VWF was evaluated by surface plasmon resonance showing a 50–25 fold reduction in the K_d for FVIII-Y1680F compared to wt FVIII. Furthermore, pharmacokinetic studies in hemophilia A mice indicated that FVIII-Y1680F is basically devoid of VWF binding in vivo. The circulating half-life decreased from 7–8 hours for wt FVIII to 0.5 hours for FVIII-Y1680F (see figure) which is comparable to the half-life of wt FVIII in VWF knockout mice (0.5 hours). Using a chromogenic assay the specific activity of FVIII-Y1680F was 9200 IU/mg similar to that of wt FVIII confirming normal activity FVIII-Y1680F after cleavage with thrombin and removal of VWF.

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As the short half-life may influence the haemostatic effect of FVIII-Y1680F in vivo, a 40 kDa PEG moiety was attached to the O-glycan in the B-domain of the FVIII variant. This re-establishes the circulating half-life (6.1 hours) to that of wt FVIII without affecting the specific activity in vitro. The haemostatic effect of 40K-O-PEG FVIII-Y1680F was subsequently used to investigate if high affinity VWF binding of FVIII influences its haemostatic effect in vivo.

The acute haemostatic effect of 40K-O-PEG-FVIII-Y1680F was compared to wt FVIII (Advate^®) in the tail bleeding model in hemophilia A mice at doses equivalent to the 50% of the maximal effect and at maximal efficacy (20 and 280 IU/kg; Elm et al., Hemophilia 2011 epub). The blood loss was significantly reduced at both doses with comparable effect of 40K-O-PEG-FVIII-Y1680F and wt FVIII (see see figure, * indicates significant differences compared to vehicle treated hemophilia A mice). This indicates that the lack of VWF binding does not interfere with the haemostatic properties of FVIII in this particular model. To further support these data, the haemostatic effect of 40K-O-PEG-FVIII-Y1680F was tested in the FeCl₃ injury model (2.5, 5 and 10 IU/kg) in hemophilia A mice. No clot formation was observed in vehicle mice and 40K-O-PEG-FVIII-Y1680F normalized dose dependently the clot formation time comparable to wt FVIII.

In conclusion, the current data suggest that the haemostatic effect of FVIII in vivo is not dependent on high affinity binding of FVIII to VWF.