Mechanisms of pro-hemostatic action of recombinant factor VIIa

Mechanisms of pro-hemostatic action of recombinant factor VIIa

Measurement of rFVIIa in plasma can be performed using a specific assay of the activated form of FVII using a recombinant variant of FT (Staclot ®VIIa-TF, Diagnostica Stago) [42]. The usual FVII measurement technique, derived from the Quick time, can also be applied to the measurement of rFVIIa because its principle consists of the quantitative conversion of the zymogen into its activated form which is then measured. The activity obtained represents the sum of rFVIIa and endogenous FVII, but this, close to 1 U / mL, is negligible compared to rFVIIa administered at therapeutic doses (20-30 U / mL immediately after the intravenous bolus of 90 mcg / kg). However, the results are highly dependent on the reagent used, human thromboplastins being able to give a result three times lower than that obtained with rabbit thromboplastins or recombinant [43].

Finally, FVIIa can be measured by an immunoassay technique (Imubind ®FVIIa, American Diagnostica). Comparison of the biological activity with the antigen indicates that the specific activity of rFVIIa decreases over time, probably as a result of partial degradation in vivo [43] Visit for more info about mechanism of action.
The rFVIIa level is a pharmacokinetic indicator but it is not predictive of the hemostatic response at the individual level. The usual laboratory tests (Quick – TQ – and Activated Partial Thromboplastin time – TCA -) can not be more. The time of Quick does not reproduce the physiological haemostasis because the concentration of FT used for its realization is from 10 4 to 10 5that estimated at the site of a vascular breach. Between 0 and 5 U / mL, rFVIIa progressively shortens the TQ from its baseline (10-12 s) to 7-8 s. For concentrations greater than 5 U / mL, the TQ no longer varies. RFVIIa shortens the TCA, a very long hemophiliac. It probably acts by the direct activation of FX, independent of FT, on the phospholipid surface of the assay [44].

tca mechanism of action
If rFVIIa only sub-optimally restores thrombin generation, any other element of variability in physiological haemostasis may be critical to treatment efficacy. Thus, the FV Leiden level modulates thrombin generation in a hemophilia A plasma model [45]. The level of FII [25], the number of platelets [7] and the pro-coagulant activity of these [46] have a major influence on thrombin generation. These elements are subject to a wide genetic and / or acquired variability and may explain some failures of rFVIIa treatments at the usual doses, observed in about 10% of patients. The detection of these would optimize their treatment.
The most relevant index of the pro-hemostatic effect of rFVIIa appears to be the total amount of thrombin produced in blood or platelet-rich plasma in response to very low FT concentrations. The approach developed by Hemker et al. [47] is probably suitable but has not yet been evaluated in this application. A different approach based on the measurement of whole blood clotting times after recalcification of plasma (ACT-LR, Hemocron ® , International Technidyne). [48] The coagulation time exceeds the measurement limits (400 s) in hemophiliacs because the test involves the so-called intrinsic pathway of coagulation. In vitro addition rFVIIa at a concentration above the usual pharmacological concentrations (200 nmol) reduces the time to a measurable value (order of 300 s, the normal value being 140-180 s). This partial correction is better in clinically responders (n = 4) than in those with experience of treatment failure (n = 4). Subject to confirmation with larger numbers, this method is the first to show a possible relationship with a clinical efficacy criterion.


In vitro reproduction Physiological coagulation is made difficult by the very nature of the studied system which is sensitive to contact with any surface other than the healthy vessel. Several models developed recently make it possible to understand the abnormalities responsible for the haemorrhagic or thrombotic risk in various clinical situations, in particular hemophilias. The TCA Mechanism of Action of rFVIIa has been studied using these models. At usual therapeutic doses, the efficacy of rFVIIa probably results from its ability to accelerate the generation of FXa, thus thrombin, in the initial phase of coagulation, thus normalizing the clot formation time and correcting the lack of activation of the clot. platelets. In contrast, the FX activation by rFVIIa on the activated platelet surface being much lower than that achieved by FIXa in the presence of FVIIIa, the thrombin generation at the coagulation amplification phase proceeds more slowly than in the normal subject and remains incomplete. RFVIIa only partially corrects the lack of clot stability observed in hemophiliacs. Significant individual variability in response to rFVIIain vitro is obvious but can not yet be identified by a simple laboratory test.
These elements make it possible to understand the efficiency and the limits of rFVIIa treatment as well as the interest of research in new directions. In all models, the effect is not saturated beyond the usual pharmacological concentrations, suggesting a potential benefit from using higher doses than currently recommended. Reciprocal relationships between thrombin and platelets legitimize the evaluation of rFVIIa in situations of quantitative or qualitative platelet deficiency or other intrinsic pathway deficits (eg FXI), when they are not accessible to substitution therapy. The significant apparent therapeutic margin of rFVIIa in