Mechanisms of action of recombinant activated factor VII in the context of tissue factor concentration and distribution.

Supraphysiological concentrations of recombinant activated factor VII (rVIIa, NovoSeven) are used to control bleeding in hemophilia. Current experimental evidence suggests that rVIIa may increase thrombin generation via two pathways: one being tissue factor (TF)-dependent and another being activated platelet-dependent. Contribution of TF to the rVIIa action may justify different administration profiles of rVIIa. In the present study, thrombin and fibrin generation and spatial clot formation assays in platelet-free hemophilia A and normal plasma were used to investigate this contribution. By varying the concentration of TF and the way it becomes available to plasma, we obtained the following results. Activation of clotting with less than 5 pmol/l of TF facilitates thrombin and fibrin generation at low, but not at supraphysiological rVIIa concentrations. Activation with more than 13 pmol/l of TF saturates thrombin and fibrin generation kinetics, making it insensitive to rVIIa. rVIIa minimally modulates clot growth on the surface of TF-expressing fibroblasts. On the contrary, rVIIa produces spontaneous clot formation in nonflowing platelet-free plasma far away from fibroblasts via plasma lipid particles. Therefore, both the concentration and the distribution of TF determine relevance of a particular experimental system for the studies of rVIIa action. The results indicate that 300-1600 nmol/l (megadoses) of rVIIa may deliver coagulation outside of the TF-rich areas of blood vessel damage via the platelet-derived microparticles. Therefore, rate and extent of platelet-derived microparticles generation might be important with regard to rVIIa treatment safety.

Evaluation of liposomal dose in recombinant factor VIII reconstituted with pegylated liposomes for the treatment of patients with severe haemophilia A.

Patients with haemophilia A treated prophylactically require frequent factor VIII (FVIII) infusions for bleed protection. Recombinant sucrose-formulated FVIII (rFVIII-FS) with pegylated liposomes (PEGLip-rFVIII-FS) was previously shown to extend the bleed-free period after prophylactic infusion versus rFVIII-FS using two doses of FVIII with a fixed amount of liposomal diluent. This randomised, subject-blinded, four-way crossover study evaluated the efficacy and safety of PEGLip-rFVIII-FS using various quantities of pegylated liposomes with a fixed FVIII dose. Adults with severe haemophilia A were randomised to one of four treatment arms. Each arm had four treatment segments, with each segment consisting of a prophylactic infusion followed by on-demand infusions. The prophylactic infusions used a fixed dose of 35 IU/kg rFVIII-FS, but varied in the amount of pegylated liposomes used for reconstitution (4.2, 12.6, or 22.1 mg/kg of body weight or water as a control). On-demand infusions all used 35 IU/kg rFVIII-FS. After treatment of spontaneous bleeds and a wash-out, subjects crossed to another treatment segment (i.e. another PEGLip-rFVIII-FS dose or control). Sixteen subjects enrolled in and completed the study. Mean number of bleed-free days after prophylactic infusion increased from 7.8 days for control rFVIII-FS to 8.7, 10.8, and 10.9 days for PEGLip-rFVIII-FS reconstituted in 4.2, 12.6, and 22.1 mg/kg of pegylated liposomes, respectively. The difference in bleed-free days approached but did not achieve statistical significance in this small study population. No drug-related adverse events or inhibitors were reported. This study helps establish the optimal concentration of liposomes in PEGLip-rFVIII-FS to prolong the post-infusion bleed-free period.

Prolonged bleeding-free period following prophylactic infusion of recombinant factor VIII reconstituted with pegylated liposomes.

Prophylactic treatment for hemophilia A involves infusion of factor VIII (FVIII) concentrates every 2 to 3 days. Liposomes can be efficacious vehicles for medicines, and surface modification by PEGylation can prolong liposome circulation time. When reconstituted with PEGylated liposomes (PEGLip's), recombinant FVIII binds noncovalently but with high affinity to the external liposome surface. This preparation showed prolongation of FVIII half-life and increased protection from bleeding in preclinical models. Here we report a blinded, controlled, crossover, multicenter clinical study that evaluated the time free from bleeding episodes in patients with hemophilia A during prophylaxis with standard rFVIII (no liposomes) or PEGLip rFVIII (PEGLip reconstituted) at 25 and 35 IU/kg doses. Of 24 enrolled patients, 23 were eligible for efficacy analysis. Mean number of days without bleeds was 7.2 +/- 1.7 with standard rFVIII compared with 13.3 +/- 4.8 with PEGLip rFVIII at the 35 IU/kg dose and 5.9 +/- 1.7 with standard rFVIII versus 10.9 +/- 2.9 with PEGLip rFVIII at the 25 IU/kg dose (P < .05 between treatment groups for each dose). PEGLip rFVIII was well tolerated. These data suggest that reconstitution of rFVIII with PEGLip's may reduce the frequency of treatment during prophylaxis.

Effect of factor VIII on tissue factor-initiated spatial clot growth.

Using time-lapse videomicroscopy, we studied the role of coagulation factor VIII (fVIII) in tissue factor-initiated spatial clot growth on fibroblast monolayers in a thin layer of non-stirred recalcified plasma from healthy donors or patients with severe Haemophilia A. Analysis of temporal evolution of light-scattering profiles from a growing clot revealed existence of two phases in the clot growth-initiation phase in a narrow (0.2 mm) zone adjacent to activator surface and elongation phase in plasma volume. While the initiation phase did not differ in normal and haemophilic plasmas, the rate of clot growth in the elongation phase in haemophilic plasma constituted only 30% of that in normal plasma. Supplementation of haemophilic plasma with 0.05 U/ml fVIII restored the normal clot growth rate (44.9 +/- 2.5 microm/min) at high but not at low fibroblast density. Our results indicate that the functioning of the intrinsic tenase complex is critical for normal spatial clot growth.

Hemophilia A and B are associated with abnormal spatial dynamics of clot growth.

To gain greater insight into the nature of the bleeding tendency in hemophilia, we compared the spatial dynamics of clotting in platelet-free plasma from healthy donors and from patients with severe hemophilia A or B (factor VIII:C or IX:C<1%). Clotting was initiated via the intrinsic or extrinsic pathway in a thin layer of nonstirred plasma by bringing it in contact with the glass or fibroblast monolayer surface. The results suggest that clot growth is a process consisting of two distinct phases, initiation and elongation. The clotting events on the activator surface and the preceding period free of visible signs of clotting are the initiation phase. In experiments with and without stirring alike, this phase is prolonged in hemophilic plasma activated by the intrinsic, but not the extrinsic pathway. Strikingly, both hemophilia A and B are associated with a significant deterioration in the elongation phase (clot thickening), irrespective of the activation pathway. The rate of clot growth in hemophilic plasma is significantly lower than normal and declines quickly. The resulting clots are thin, which may account for the bleeding disorder.