Evidence that 'activated' prothrombin concentrates enhance platelet coagulant activity

Literature review of surgery management in inhibitor patients

A careful review of the available literature has shown with a moderate degree of evidence that surgery can be safely performed in haemophilic patients with either low-responding or high-responding inhibitors. Until recently, it was generally advised that surgery should be carried out in high responding haemophilic patients only if absolutely necessary, i.e. in emergency situations. Elective surgery was usually discouraged, with the exception of the placing of an indwelling catheter to carry out immune tolerance protocols. However, in reality elective surgery is often advisable in haemophilic patients with inhibitors because it may lead to an improved quality of life. At the present time, several options of treatment exist which appear to be sufficiently safe to assure haemostasis in different surgical procedures However, a more informed choice between different treatment options, based on estimation of advantages and risks linked to each treatment modality is desirable. Furthermore, the choice between different options has to be made taking into consideration costs of treatment. Ideally, randomised prospective studies are needed in order to provide a higher level of evidence of safety and efficacy of different treatment options and specifically comparing the two main bypassing options:activated prothrombin complex concentrates (aPCCs) and recombinant factor (rF)VIIa. In addition, such studies should be done with an adequate protocol, providing sufficient haematological data and an appropriate prolonged postoperative assessment.

Apparent intrinsic prothrombinase activity of human Factor X zymogen: identification with Factor VIII inhibitor bypassing activity (FEIBA)

Steady state kinetic studies have provided evidence for intrinsic prothrombinase activity of human factor X zymogen in a chromogenic assay system. Using a prothrombin substrate, kinetic parameters have been obtained for the action of factors X and Xa. The Km for prothrombin is of a different order of magnitude for the zymogen as compared with the active enzyme. Using a kinetic approach, we have obtained evidence for the binding of factor Xa zymogen to cofactors essential for the coagulant activity of factor Xa. Zymogen enzymatic activity is not inhibited by a specific serine proteinase inhibitor, (p-amidino-phenyl)methanesulfonyl fluoride (p-APMSF), a potent inhibitor of factor Xa. The apparent slow rate of zymogen inhibition by antithrombin III (AT III) as compared with the active enzyme suggests a different kind of zymogen-antithrombin interaction. Blood clotting studies paralleled the kinetic data. Factor X zymogen evidences factor VIII inhibitor bypassing activity (FEIBA) in an in vitro direct clotting system employing factor VIII deficient inhibitor plasma as substrate in both activated or nonactivated partial thromboplastin assay. Most significantly, zymogen coagulant is refractory to inhibition by p-APMSF or AT III. We conclude that a system consisting of factor X zymogen-phospholipid-factor Va can physiologically initiate blood clotting in the presence of inhibitors and may have a major role in the bypass mechanism of therapeutic prothrombin complex concentrate (PCC).

Human factor IX in animals: kinetics from isolated, radiolabelled protein and platelet destruction following crude concentrate infusions

Crude, concentrated and isolated, labelled human factor IX was prepared and infused into animals. Initially, two dogs with severe haemophilia B received both preparations and factor IX clotting activity and label survived well. Kinetic parameters fit a two-compartment open model. For 12 infusions of concentrates into normal baboons, clotting and antigen data again fit the model. Similar kinetics, with less variability, were found for nine infusions of [125I]factor IX. Radioactivity of the first post-infusion samples was > 90% precipitated in the double antibody system with an average of 84% of counts adsorbable to barium citrate. Compared to plasma, barium adsorbable counts gave a shorter t1/2 beta-elimination phase suggesting a small recirculating 125iodide pool. When [125I]factor IX was rechromatographed over heparin-agarose, variable amounts of altered, labelled protein were present; this species was more rapidly cleared from plasma post-infusion. Twelve infusions of crude concentrates into baboons were given to animals whose platelets had been labelled with 51Cr. Platelet counts as well as survival were then followed. For seven of these animals, mean counts decreased and the labelled platelets were destroyed, to an average of 21%, during the first 4 h post-infusion. Subsequent platelet survival was normal. Three infusions of gel-filtered, crude concentrates and two infusions of 'activated' commercial concentrates did not significantly alter platelet survival.

Early increase of a new platelet coagulant activity in rats fed a thrombogenic diet

A recently described platelet coagulant activity (factor X activating activity), whose pathophysiological significance is as yet unknown, was studied in rats fed a fat-rich (thrombogenic) diet for 1, 2 and 7 weeks and compared to rats fed normal laboratory chow. Whatever the duration of the special feeding period, a highly significant shortening of the clotting time, used for measuring this activity, was observed. When the platelet coagulant activity of individual "fat-fed"rats was quantitated by reference to that of individual control animals, we found a mean increase of 350% (n = 9) after one week and 267% (n = 3) after two weeks of dietary treatment. Partial thromboplastin time, thrombin time and soluble fibrin monomer complexes did not differ in control and treated animals. It seems that platelet coagulant activity, as measured in our test system, is one of the first laboratory parameters to be modified by fat-rich diets. These findings may be relevant to an understanding of the role of platelet coagulant activities other than platelet factor 3 in thrombotic phenomena.

A randomized study of factor VIII or prothrombin complex concentrate infusions in children with haemophilia and antibodies to factor VIII

In four children with haemophilia A and antibodies to factor VIII, 18 bleeding episodes were randomized for treatment with factor VIII concentrate (30 units/kg) and 18 for treatment with a prothrombin-complex concentrate (prothrombinex) given in a dose of 30 units of factor IX/kg. Treatment with prothrombinex was associated with a better clinical response, a significantly greater shortening of the kaolin partial thromboplastin time and significantly lower incidence of post-infusion increase of levels of factor VIII antibodies. Although treatment with factor VIII concentrate was clinically successful in 15 episodes, treatment failures occurred in three instances leading to parental request for withdrawal from study in two families.