Preferential localization of recombinant factor VIIa to platelets activated with a combination of thrombin and a glycoprotein VI receptor agonist

Procoagulant Platelets: Mechanisms of Generation and Action

During the past decades, it has been increasingly recognized that the major function of accelerating membrane-dependent reactions of blood coagulation is predominantly implemented by a subset of activated platelets. These procoagulant platelets (also called collagen- and thrombin-activated or COAT, coated, necrotic, although there could be subtle differences between these definitions) are uniquely characterized by both procoagulant activity and, at the same time, inactivated integrins and profibrinolytic properties. The mechanisms of their generation both in vitro and in situ have been increasingly becoming clear, suggesting unique and multidirectional roles in hemostasis and thrombosis. In this mini-review, we shall highlight the existing concepts and challenges in this field.

Enhanced potency of recombinant factor VIIa with increased affinity to activated platelets

BACKGROUND

Recombinant factor VIIa (rFVIIa) enhances thrombin generation in a platelet-dependent manner; however, rFVIIa binds activated platelets with relatively low affinity. Triggering receptor expressed on myeloid cells (TREM)-like transcript (TLT)-1 is expressed exclusively on activated platelets.

OBJECTIVE

To enhance the potency of rFVIIa via binding TLT-1.

METHODS

Recombinant FVIIa was conjugated to a TLT-1 binding Fab. In vitro potency of this platelet-targeted rFVIIa (PT-rFVIIa) was evaluated using factor X activation assays and by measuring viscoelastic changes in whole blood. In vivo potency was evaluated using a tail vein transection model in F8-/- mice expressing human TLT-1.

RESULTS

PT-rFVIIa and rFVIIa had similar dissociation constant values for tissue factor binding and similar tissue factor-dependent factor X activation. However, PT-rFVIIa had increased catalytic efficiency on TLT-1-loaded vesicles and activated platelets. The in vitro potency in normal human blood with antibody-induced hemophilia A was dependent on assay conditions used; with maximally activated platelets, the half maximal effective concentration for clot time for PT-rFVIIa was 49-fold lower compared with rFVIIa. In the murine bleeding model, a 53-fold lower half maximal effective concentration was observed for blood loss for PT-rFVIIa, supporting the relevance of the assay conditions with maximally activated platelets. In vitro analysis of blood from subjects with hemophilia A confirmed the data obtained with normal blood.

CONCLUSIONS

Increasing the affinity of rFVIIa to activated platelets resulted in approximately 50-fold increased potency both in vitro and in the mouse model. The correlation of in vivo with in vitro data using maximally activated platelets supports that these assay conditions are relevant when evaluating platelet-targeted hemostatic concepts.

Coated platelets introduce significant delay in onset of peak thrombin production: Theoretical predictions

Platelets play a crucial role in the initiation, progress, termination as well as regulation of blood coagulation. Recent studies have confirmed that not all but only a small percentage of thrombin-activated platelets ("coated" platelets) exhibit procoagulant properties (namely the expression of phosphatidylserine binding sites) required for the acceleration and progress of coagulation. A mechanistic model is developed for in vitro coagulation whose key features are distinct equations for coated platelets, thrombin dose-dependence for coated platelets, and competitive binding of coagulation factors to platelet membrane. Model predictions show significant delay in the onset of peak Va production, and peak thrombin production when dose-dependence is incorporated instead of a fixed theoretical maximum percentage of coated platelets. Further, peak thrombin concentration is significantly overestimated when either fractional presence of coated platelets is ignored (by 299.4%) or when dose-dependence on thrombin is ignored (by 24.7%).

Human platelets express endothelial protein C receptor, which can be utilized to enhance localization of factor VIIa activity

Essentials Factor VIIa binds activated platelets to promote hemostasis in hemophilia patients with inhibitors. The interactions and sites responsible for platelet-FVIIa binding are not fully understood. Endothelial cell protein C receptor (EPCR) is expressed on activated human platelets. EPCR binding enhances the efficacy of a FVIIa variant and could impact design of new therapeutics.

SUMMARY

Background High-dose factor VIIa (FVIIa) is routinely used as an effective bypassing agent to treat hemophilia patients with inhibitory antibodies that compromise factor replacement. However, the mechanism by which FVIIa binds activated platelets to promote hemostasis is not fully understood. FVIIa-DVQ is an analog of FVIIa with enhanced tissue factor (TF)-independent activity and hemostatic efficacy relative to FVIIa. Our previous studies have shown that FVIIa-DVQ exhibits greater platelet binding, thereby suggesting that features in addition to lipid composition contribute to platelet-FVIIa interactions. Objectives Endothelial cell protein C receptor (EPCR) also functions as a receptor for FVIIa on endothelial cells. We therefore hypothesized that an interaction with EPCR might play a role in platelet-FVIIa binding. Methods/results In the present study, we used flow cytometric analyses to show that platelet binding of both FVIIa and FVIIa-DVQ is partially inhibited in the presence of excess protein C or an anti-EPCR antibody. This decreased binding results in a corresponding decrease in the activity of both molecules in FXa and thrombin generation assays. Enhanced binding to EPCR was sufficient to account for the increased platelet binding of FVIIa-DVQ compared with wild-type FVIIa. As EPCR protein expression has not previously been shown in platelets, we confirmed the presence of EPCR in platelets using immunofluorescence, flow cytometry, immunoprecipitation, and mass spectrometry. Conclusions This work represents the first demonstration that human platelets express EPCR and suggests that modulation of EPCR binding could be utilized to enhance the hemostatic efficacy of rationally designed FVIIa analogs.

Kinetic analysis and binding studies of a new recombinant human factor VIIa for treatment of haemophilia

INTRODUCTION/AIM

LR769 is a new second-generation recombinant human Factor VIIa (rhFVIIa) developed for haemophilia treatment. We determined enzymatic properties of LR769 and its interaction with antithrombin, tissue factor, platelets and endothelial protein C receptor (EPCR), compared with NovoSevenRT.

METHODS

Kinetic enzyme assays and active site titration were used for enzymatic studies. Surface Plasmon Resonance (SPR) was used for determination of binding constants. Cellular binding was determined for platelets and cultured human umbilical vein endothelial cells (HUVEC).

RESULTS

The dissociation constant (K_d ) for activated platelet binding was in the 1 μm range for both products. At saturation, more LR769 than NovoSevenRT was bound to the platelets. Binding to HUVEC was 25-50% higher for LR769 than for NovoSevenRT. Protein C, soluble EPCR, and anti-EPCR antibody all reduced the binding, indicating specificity for EPCR. LR769 was similar to NovoSevenRT in all kinetic assays. Active site titration demonstrated 0.7 mole of active site/mole of protein. The k_cat /K_m values for activation of FX and FIX with purified recombinant tissue factor and phospholipids were 10.5 s^-1 /0.32 μm and 3.3 s^-1 /0.44 μm respectively. The apparent second-order rate constant for inactivation by human plasma AT was 5.9 ± 0.4 × 10³ m^-1 s^-1 . The K_d values for binding of LR769 to soluble tissue factor and full-length tissue factor were 8.1 nm and 0.9 nm, respectively, and the K_d for binding to soluble EPCR was 41 nm.

CONCLUSION

Overall, LR769 exhibited characteristics similar to NovoSevenRT, but bound EPCR on HUVEC with somewhat higher affinity than NovoSevenRT.

The endothelial protein C receptor enhances hemostasis of FVIIa administration in hemophilic mice in vivo

Recombinant activated human factor VII (rhFVIIa) is an established hemostatic agent in hemophilia, but its mechanism of action remains unclear. Although tissue factor (TF) is its natural receptor, rhFVIIa also interacts with the endothelial protein C receptor (EPCR) through its γ-carboxyglutamic acid (Gla) domain, with unknown hemostatic consequences in vivo. Here, we study whether EPCR facilitates rhFVIIa hemostasis in hemophilia using a mouse model system. Mouse activated FVII (mFVIIa) is functionally homologous to rhFVIIa, but binds poorly to mouse EPCR (mEPCR). We modified mFVIIa to gain mEPCR binding using 3 amino acid changes in its Gla domain (L4F/L8M/W9R). The resulting molecule mFVIIa-FMR specifically bound mEPCR in vitro and in vivo and was identical to mFVIIa with respect to TF affinity and procoagulant functions. In macrovascular injury models, hemophilic mice administered mFVIIa-FMR exhibited superior hemostatic activity compared with mFVIIa. This was abolished by blocking mEPCR and was absent in ex vivo whole blood coagulation assays, implicating a specific mFVIIa-FMR and endothelial mEPCR interaction. Because mFVIIa-FMR models the TF-dependent and EPCR binding properties of rhFVIIa, our data unmask a novel contribution of EPCR on the action of rhFVIIa administration in hemophilia, prompting the rational design of improved and safer rhFVIIa therapeutics.

The kidneys play an important role in the clearance of rFVIIa in rats

INTRODUCTION

Previous distribution and histological studies have indicated that the kidneys and renal proximal tubular cells play a role in clearance of rFVIIa. However, the relative importance of the kidneys in clearance of rFVIIa has not previously been addressed. The objective of the present study was to evaluate the importance of the kidneys in the clearance process of rFVIIa after iv administration to rats using a nephrectomy model.

MATERIALS AND METHODS

A nephrectomized rat model was established and validated using inulin, a compound primarily cleared by the kidneys, as a test substance and several physiological parameters were monitored to ensure viability and robustness of the model. The model was then used for pharmacokinetic evaluation of renal clearance of rFVIIa. The pharmacokinetic parameters for rFVIIa were evaluated both by use of standard non-compartmental methods and by use of mixed effects methods, where a pharmacokinetic model was used to simultaneously model all data from healthy, sham operated, and nephrectomized rats.

RESULTS

Nephrectomized animals showed stable rectal temperature, SpO2 and pulse and as expected, clearance of inulin was essentially abolished compared to control animals (p<0.001). For rFVIIa, nephrectomy resulted in a clearance and terminal half-life of 34mL/h/kg and 2.8h compared to 68mL/h/kg and1.9h in rats exposed to sham surgery (p<0.0001 for both parameters).

CONCLUSION

The present data show that about 50% of the total clearance of rFVIIa from circulation in rats under isoflurane anaesthesia is due to renal clearance.

Decreased generation of procoagulant platelets detected by flow cytometric analysis in patients with bleeding diathesis

BACKGROUND

A clinically relevant bleeding diathesis is a frequent diagnostic challenge, which sometimes remains unexplained despite extensive investigations. The aim of our work was to evaluate the diagnostic utility of functional platelet testing by flow cytometry in this context.

METHODS

In case of negative results after standard laboratory workup, flow cytometric analysis (FCA) of platelet function was done. We performed analysis of surface glycoproteins Ibα, IIb, IIIa; P-selectin expression and PAC-1 binding after graded doses of ADP, collagen, and thrombin; content/secretion of dense granules; and ability to generate procoagulant platelets.

RESULTS

Of 437 patients investigated with standard tests between January 2007 and December 2011, we identified 67 (15.3%) with high bleeding scores and nondiagnostic standard laboratory workup including platelet aggregation studies. Among these patients, FCA revealed some potentially causative platelet defects: decreased dense granule content/secretion (n = 13); decreased α-granule secretion induced by ADP (n = 10), convulxin (n = 4), or thrombin (n = 3); decreased fibrinogen receptor activation induced by ADP (n = 11), convulxin (n = 11), or thrombin (n = 8); and decreased generation of COAT platelets, that is, highly procoagulant platelets induced by simultaneous activation with collagen and thrombin (n = 16).

CONCLUSION

Our work confirms that storage pool defects are frequent in patients with a bleeding diathesis and normal coagulation and platelet aggregations studies. Additionally, FCA is able to identify discrete platelet activation defects. In particular, we show for the first time that a relevant proportion of these patients has an isolated impaired ability to generate COAT platelets--a conceptually new defect in platelet procoagulant activity, which is missed by conventional laboratory workup.

Platelets do not express the oxidized or reduced forms of tissue factor

BACKGROUND

Expression of tissue factor (TF) antigen and activity in platelets is controversial and dependent upon the laboratory and reagents used. Two forms of TF were described: an oxidized functional form and a reduced nonfunctional form that is converted to the active form through the formation of an allosteric disulfide. This study tests the hypothesis that the discrepancies regarding platelet TF expression are due to differential expression of the two forms.

METHODS

Specific reagents that recognize both oxidized and reduced TF were used in flow cytometry of unactivated and activated platelets and western blotting of whole platelet lysates. TF-dependent activity measurements were used to confirm the results.

RESULTS

Western blotting analyses of placental TF demonstrated that, in contrast to anti-TF#5, which is directed against the oxidized form of TF, a sheep anti-human TF polyclonal antibody recognizes both the reduced and oxidized forms. Flow cytometric analyses demonstrated that the sheep antibody did not react with the surface of unactivated platelets or platelets activated with thrombin receptor agonist peptide, PAR-1. This observation was confirmed using biotinylated active site-blocked factor (F)VIIa: no binding was observed. Likewise, neither form of TF was detected by western blotting of whole platelet lysates with sheep anti-hTF. Consistent with these observations, no FXa or FIXa generation by FVIIa was detected at the surface of these platelets. Similarly, no TF-related activity was observed in whole blood using thromboelastography.

CONCLUSION AND SIGNIFICANCE

Platelets from healthy donors do not express either oxidized (functional) or reduced (nonfunctional) forms of TF.

Superactivated Platelets

Platelets contribute to hemostasis by forming the platelet plug and then contributing to coagulation by providing a catalytic surface where thrombin generation occurs efficiently. This catalytic activity, known as the platelet procoagulant response, is being recognized as a nuanced response. This review examines platelets’ response to strong stimuli, which results in the formation of a platelet subpopulation (superactivated platelets) with several unique properties, including enhanced procoagulant activity. These platelets contribute uniquely to thrombus architecture and seem to have thrombus regulatory activity. Superactivated platelets’ role in diseases of thrombosis and hemostasis, as either potentiating or mitigating factors, is not currently known, but may be an important pharmacological target.

Pharmacokinetics, pharmacodynamics and safety of recombinant canine FVIIa in a study dosing one haemophilia A and one haemostatically normal dog

Recombinant human FVIIa (rhFVIIa) corrects the coagulopathy in hemophilia A and B as well as FVII deficiency. This is also the case in dogs until canine anti-human FVIIa antibodies develop (~2 weeks). Recombinant canine factor VIIa (rcFVIIa), successfully over-expressed by gene transfer in haemophilia dogs, has provided long-term haemostasis (>2 years). However, pharmacokinetics (PK), pharmacodynamics (PD) and safety of rcFVIIa after pharmacological administration have not been reported. We therefore wanted to explore the safety, PK and PD of rcFVIIa in dogs. A pilot study was set up to evaluate the safety as well as PK and PD of rcFVIIa after a single intravenous dose of 270 μg kg(-1) to one HA and one haemostatically normal dog and to directly compare rcFVIIa with rhFVIIa in these two dogs. Single doses of rcFVIIa and rhFVIIa were well tolerated. No adverse events were observed. Pharmacokinetic characteristics including half-life (FVIIa activity: 1.2-1.8 h; FVIIa antigen 2.8-3.7 h) and clearance were comparable for rcFVIIa and rhFVIIa. Kaolin-activated thromboelastography approached normal in the HA dog with the improvement being most pronounced after rcFVIIa. This study provided the first evidence that administering rcFVIIa intravenously is feasible, safe, well tolerated and efficacious in correcting the haemophilic coagulopathy in canine HA and that rcFVIIa exhibits pharmacokinetic characteristics comparable to rhFVIIa in haemophilic and haemostatically competent dogs. This strengthens the hypothesis that rcFVIIa can be administered to dogs to mimic the administration of rhFVIIa to humans.

Monitoring rFVIIa 90 μg kg⁻¹ dosing in haemophiliacs: comparing laboratory response using various whole blood assays over 6 h

Recombinant FVIIa is a haemostatic agent administered to patients with severe FVIII or FIX deficiency with inhibitors. Although rFVIIa is effective at stopping bleeding, a reliable assay to monitor its effect is lacking. To characterize the pharmacokinetics and global coagulation effects of rFVIIa for 6 h following a IV dose of 90 μg kg⁻¹. Ten non-bleeding subjects with severe FVIII or FIX deficiency were infused with a single-dose of rFVIIa 90 μg k⁻¹ body weight and blood was collected before and at 0.5, 1, 2, 4 and 6 h postdose. Global haemostasis was characterized throughout the study utilizing whole blood analyses (Hemodyne HAS, TEG, ROTEM). The clearance and half-life of factor FVII:C was estimated as 39.0 ± 8.8 mL h⁻¹ kg⁻¹ and 2.1 ± 0.2 h respectively. There was good inter-assay agreement with respect to clot initiation parameters (R, CT and FOT) and these parameters all fell to a mean of approximately 9 min following rFVIIa dosing. The platelet contractile force (PCF) and clot elastic modulus (CEM) were positively correlated to FVII:C (P < 0.0001), and these parameters were dynamic throughout the 6-h period. The MA and MCF did not correlate to FVII:C nor did they significantly change during the study. Prothrombin F1 + 2 significantly increased following rFVIIa dosing (P < 0.001), but remained steady throughout the study. There was no change in D-dimer concentrations over time. The FOT, R and CT characterized clot initiation following rFVIIa dosing. The PCF and CEM were correlated to FVII:C and characterized the dynamics of platelet function and clot strength over the rFVIIa dosing interval. The clinical significance of these findings needs additional study.

Characterization of canine coagulation factor VII and its complex formation with tissue factor: canine-human cross-species compatibility

BACKGROUND

Canine models have been good predictors of efficacy of hemophilia treatments, including recombinant human coagulation factor (F)VIIa (hFVIIa). However, canine FVIIa and tissue factor (TF) have remained incompletely characterized.

OBJECTIVE

 To explore canine-human cross-species FVIIa-TF compatibility in order to strengthen the predictive value of canine models in research on FVIIa and TF.

METHODS

Canine FVIIa (cFVIIa) and canine TF((1-217)) [cTF((1-217))] were produced by recombinant techniques, and canine-human cross-species FVIIa-TF interactions were characterized in vitro.

RESULTS

 Recombinant cFVIIa and soluble cTF((1-217)) were produced and purified to homogeneity. hFVIIa and cFVIIa bound with comparably high affinities to cTF((1-217)) (K(D)=6.0±0.7 nm and K(D)=6.0±0.3 nm, respectively) and to cell surface-expressed cTF (K(D)=8.4±0.4 nm and K(D)=7.2±1.2 nm, for (125) I-labeled hFVIIa and cFVII, respectively). In contrast, cFVIIa bound to human TF (hTF) with decreased affinity, both in solution and on cell surfaces. The decreased binding resulted in reduced activity of cFVIIa in functional assays with hTF((1-209)) . In direct comparison, cFVIIa was more active than hFVIIa, both in the absence and the presence of cognate TF.

CONCLUSION

The present finding that hFVIIa binds to cTF essentially as it does to hTF substantiates the hypothesis that human FVIIa-TF biology can be reliably recapitulated in canine models on administration of hFVIIa to dogs.

Understanding the therapeutic action of recombinant factor VIIa in platelet disorders

Recombinant factor VIIa (rFVIIa) is a pro-haemostatic agent that is effective in the treatment of bleeding in haemophilia complicated by inhibitors and some other haemorrhagic disorders. The rFVIIa product promotes haemostasis in these settings by enabling the rapid generation of thrombin on the surface of activated platelets which increases fibrin deposition. rFVIIa also has therapeutic activity in bleeding in platelet function disorders such as Glanzmann's thrombasthenia and thrombocytopenia. This suggests that rFVIIa also has a pro-haemostatic effect on platelets and this may contribute to the overall therapeutic action of this agent. The effects of high-concentration FVIIa on platelets have been studied in detail in experiments using ex vivo model systems of haemostasis under static and flow conditions. These experiments show that FVIIa potentiates multiple functional markers of platelet activation and this effect requires direct binding of FVIIa to the activated platelet membrane and the generation of thrombin. The mechanism of this potentiating effect of FVIIa on platelet activation is complex and is likely to require multiple platelet activation receptors and signalling pathways. These include the GPIb-IX-V glycoprotein complex and the thrombin protease activated receptors. The proposed mechanism of action of high-concentration FVIIa has important implications for the therapeutic activity of rFVIIa in vivo. This review summarizes the clinical and experimental evidence supporting a potentiating effect of FVIIa on platelet activation and discusses the likely mechanism of action.

The glycoprotein Ib-IX-V complex contributes to tissue factor–independent thrombin generation by recombinant factor VIIa on the activated platelet surface

Several lines of evidence suggest that recombinant factor VIIa (rFVIIa) is able to activate factor X on an activated platelet, in a tissue factor-independent manner. We hypothesized that, besides the anionic surface, a receptor on the activated platelet surface is involved in this process. Here, we showed that, in an ELISA setup, a purified extracellular fragment of GPIbα bound to immobilized rFVIIa. Surface plasmon resonance established a affinity constant (Kd) of approximately 20 nM for this interaction. In addition, CHO cells transfected with the GPIb-IX-V complex could adhere to immobilized rFVIIa, whereas wild-type CHO cells could not. Furthermore, platelets sti-mulated with a combination of collagen and thrombin adhered to immobilized rFVIIa under static conditions. Platelet adhesion was inhibited by treatment with O-sialoglycoprotein endopeptidase, which specifically cleaves GPIbα from the platelet surface. In addition, rFVIIa-mediated thrombin generation on the activated platelet surface was inhibited by cleaving GPIbα from its surface. In summary, 3 lines of evidence showed that rFVIIa interacts with the GPIb-IX-V complex, and this interaction enhanced tissue factor-independent thrombin generation mediated by rFVIIa on the activated platelet surface. The rFVIIa-GPIbα interaction could contribute to cessation of bleeding after administration of rFVIIa to patients with bleeding disorders.

Recent advances in clot biology and assessment of clotting

Since the first description of the haemostatic process by Morawitz in 1904, knowledge about the haemostasis mechanism has undergone substantial modifications. Increasing knowledge of enzymology, purification and characterisation of coagulation proteins led to the introduction of the waterfall or cascade model of coagulation. However, these models were based on in vitro studies in the presence of artificial phospholipids and the absence of cells. Two pathways to achieve the formation of a haemostatic fibrin plug were identified, the so-called `extrinsic system' involving both factors present in the circulation and from the extravascular space, and the `intrinsic system' using only factors present in the circulation. However, with increasing knowledge about the interaction between factors from the two systems, the relevance of this model was questioned. The availability of recombinant FVIIa has made further research of the role of FVII/FVIIa and TF feasible, resulting in the current concept of haemostasis according to which the process principally occurs on two cell surfaces, the TF-bearing cell and the thrombin-activated platelet. A limited amount of thrombin is generated by the FVIIa-TF complex on the TF-bearing cell resulting in activation of platelets, FIX, FVIII and FV. The further and full thrombin generation then takes place on the activated platelet surface. The most frequently used assays for evaluation of the global haemostatic capacity are the prothrombin time (PT) and the activated partial thromboplastin time (APTT). The PT measures the formation of a fibrin clot in the presence of an abundance of TF thereby principally reflecting the initial thrombin generation dependent especially on FVII, FV and FX, while the APTT mimics the processes on the activated platelet surface involving FVIII, FIX, FXl, FV, FX and prothrombin. For more specific analyses, assay systems measuring the level of various coagulation factors are available. Platelet function is measured by platelet count and bleeding time. The platelet aggregation response to different agonists can be measured in special aggregometers. The usefulness of these techniques in evaluating a potential bleeding risk is, however, doubtful.

Further understanding of recombinant activated factor VII mode of action

Recombinant activated factor VII (rFVIIa) is being increasingly used to treat bleeding associated with a variety of non-hemophilic coagulopathic indications, and its mechanism of action in these areas is under active investigation. Numerous studies have shown that FVIIa binds with low affinity to activated platelets; rFVIIa can subsequently enhance platelet-surface thrombin generation by activating factor (F) X and by contributing additional FIXa to the hemostatic process. This FIXa can rapidly activate additional FX, which may explain why non-hemophilic coagulopathic bleeds respond to lower doses of rFVIIa than do hemophilic bleeds. However, the platelet surface may be able to process only a limited amount of FXa, accounting for the observation that some models of non-hemophilic coagulopathy show a plateau in the effect of rFVIIa.

Tissue factor and factor VIIa as therapeutic targets in disorders of hemostasis

For hemophilia patients with inhibitors against FVIII or FIX, the development of recombinant factor VIIa (rFVIIa) raises the possibility of a therapeutic alternative whose availability and convenience of treatment are comparable to those of FVIII or FIX. In support of this new concept for the treatment of bleeding episodes, pharmacological doses of FVIIa have been shown to induce hemostasis. Pharmacological doses of rFVIIa enhance thrombin generation on thrombin-activated platelets, thereby facilitating the formation of strong, well-structured fibrin plugs resistant to premature proteolysis. Modified rFVIIa molecules with a stronger hemostatic potential have been produced. Inhibition of the FVII-TF-dependent pathway (TFPI and rFVIIai) has been tried in attempts to prevent thrombosis, with promising results in animal models so far not confirmed in clinical trials.

Hemostatic and neuroprotective effects of human recombinant activated factor VII therapy after traumatic brain injury in pigs

Human recombinant activated factor-VII (rFVIIa) has been used successfully in the treatment of spontaneous intracerebral hemorrhage. In addition, there is increasing interest in its use to treat uncontrolled bleeding of other origins, including trauma. The aim of this study was to evaluate the safety and potential effectiveness of rFVIIa to mitigate bleeding using a clinically relevant model of traumatic brain injury (TBI) in the pig. A double injury model was chosen consisting of (1) an expanding cerebral contusion induced by the application of negative pressure to the exposed cortical surface and (2) a rapid rotational acceleration of the head to induce diffuse axonal injury (DAI). Injuries were performed on 10 anesthetized pigs. Five minutes after injury, 720 microg/kg rFVIIa (n=5) or vehicle control (n=5) was administered intravenously. Magnetic resonance imaging (MRI) studies were performed within 30 min and at 3 days post-TBI to determine the temporal expansion of the cerebral contusion. Euthanasia and histopathologic analysis were performed at day 3. This included observations for hippocampal neuronal degeneration, axonal pathology and microclot formation. The expansion of contusion volume over the 3 days post-injury period was reduced significantly in animals treated with rFVIIa compared to vehicle controls. Surprisingly, immunohistochemical analysis demonstrated that the number of dead/dying hippocampal neurons and axonal pathology was reduced substantially by rFVIIa treatment compared to vehicle. In addition, there was no difference in the extent of microthrombi between groups. rFVIIa treatment after TBI in the pig reduced expansion of hemorrhagic cerebral contusion volume without exacerbating the severity of microclot formation. Finally, rFVIIa treatment provided a surprising neuroprotective effect by reducing hippocampal neuron degeneration as well as the extent of DAI.

Impairment of the hemostatic potential of platelets during storage as evaluated by flow cytometry, thrombin generation, and thrombelastography under conditions promoting formation of coated platelets

BACKGROUND

The increasing demand for platelet (PLT) transfusions has focused attention on appropriate use. Coated PLTs are a subpopulation of highly procoagulant PLTs formed by simultaneous stimulation by the agonist's collagen and thrombin hypothesized to drive clot formation at the site of vascular injury. Prolonged storage of PLTs may reduce their ability to support optimal hemostasis upon transfusion.

STUDY DESIGN AND METHODS

PLT concentrates (PCs) stored for 1, 4, 6, and 8 days were costimulated with thrombin and the collagen glycoprotein VI (GPVI) receptor agonist convulxin, and their ability to form coated PLTs was determined by flow cytometry. Further, a plasma-based thrombin generation assay and thrombelastography were used to evaluate the aged PCs' capacity to support thrombin generation and clot formation, respectively. The stored PCs were additionally tested by standard quality control methods.

RESULTS

PLT quality as measured by standard analyses was acceptable according to current practice. The hemostatic potential, however, was impaired with increasing storage time. The formation of coated PLTs decreased significantly from approximately 85 to 55 percent with increasing storage time (p<0.05). The velocity of clot formation was significantly increased from Day 4 (p<0.05). The velocity of thrombin generation and resistance against fibrinolysis were significantly reduced on Day 8 compared to Day 1 of storage (p<0.05).

CONCLUSION

Data in the present study suggest that storage significantly reduced the stored PLTs' ability to respond to conditions expected to exist at the site of vascular injury and that storage-induced reduction in PLT activation sensitivity correlated with a loss of hemostatic potential.