Summary.  Haemostatic effect of compounds for treating haemophilia can be evaluated in various bleeding models in haemophilic mice. However, the doses of factor VIII (FVIII) for normalizing bleeding used in some of these models are reported to be relatively high. The aim of this study was to establish a sensitive venous bleeding model in FVIII knock out (F8‐KO) mice, with the ability to detect effect on bleeding at low plasma FVIII concentrations. We studied the effect of two recombinant FVIII products, N8 and Advate®, after injury to the saphenous vein. We found that F8‐KO mice treated with increasing doses of either N8 or Advate® showed a dose‐dependent increase in the number of clot formations and a reduction in both average and maximum bleeding time, as well as in average blood loss. For both compounds, significant effect was found at doses as low as 5 IU kg−1 when compared with vehicle‐treated F8‐KO mice. Normalization of maximum bleeding time was found at doses equal to or above 10 IU kg−1 N8 or Advate®, corresponding to plasma concentrations of approximately 10% of the level in wild type mice. The present study adds a new model to the armamentarium of bleeding models used for evaluation of pro‐coagulant compounds for treatment of haemophilia. Interestingly, the vena saphena model proved to be sensitive towards FVIII in plasma levels that approach the levels preventing bleeding in haemophilia patients, and may, thus, in particular be valuable for testing of new long‐acting variants of e.g. FVIII that are intended for prophylaxis.

Prophylaxis in severe haemophilia significantly increases health-related quality of life for patients, but the dosing frequency still constitutes a challenge. Thus, there is a need for new treatment options, utilizing compounds with longer duration of action, while still maintaining potency. The objective of this study was to evaluate the acute and prolonged effects of a new glycoPEGylated recombinant factor VIII (rFVIII) (N8-GP) in a venous bleeding model in haemophilia A mice and to compare the efficacy and potency to turoctocog alfa (rFVIII). Following intravenous administration of turoctocog alfa or N8-GP to normal and FVIII-deficient mice, bleeding time and blood loss from a saphenous vein incision were evaluated in an acute dose-response study and a duration of action study. In the acute setting, N8-GP dose dependently reduced the number and duration of bleeding episodes as well as blood loss compared to FVIII-deficient mice, reaching statistical significance at doses as low as 5-10 U kg(-1) . In the duration of action study, a significantly prolonged and maintained effect of N8-GP was found for up to 48 h after dosing, whereas the effect of rFVIII was no longer present for any end-points 24 h after dosing. Seventy-two hours after dosing, no significant effect of either compound was found. This study shows a prolonged haemostatic effect of N8-GP compared to rFVIII supporting other recent studies that N8-GP may hold a potential to increase the quality of life for patients with haemophilia A by reducing dosing frequency.