Prospective observational study of the effect of dual antiplatelet therapy with tranexamic acid treatment on platelet function and bleeding after cardiac surgery.

BACKGROUND: The bleeding impact of dual antiplatelet therapy (DAPT), aspirin and clopidogrel, maintained until coronary artery bypass graft surgery (CABG), is still a matter of debate. The lack of preoperative antiplatelet activity measurement and heterogeneity of antifibrinolytic protocols in prior studies make the conclusions questionable. The aim of this prospective study was to determine, after preoperative antiplatelet activity measurement, if the maintenance of DAPT until CABG increases bleeding in patients treated with tranexamic acid (TA). METHODS: This observational study included 150 consecutive patients, 89 treated with aspirin and 61 treated with DAPT, undergoing a first-time planned on-pump CABG with TA treatment. Antiplatelet activity was measured with platelet aggregation tests and quantification of VASP phosphorylation. Postoperative bleeding at 24 h was recorded and propensity score analysis was performed. RESULTS: Based on VASP assay, 54% of patients showed high on-clopidogrel platelet activity inhibition. Postoperative bleeding at 24 h increased by 22% in the DAPT group, compared with the aspirin group (680 [95% CI: 360-1670] vs 558 [95%CI: 267-1270] ml, P < 0.01), consistent with increased blood transfusion (21% vs 7%, P = 0.01); a higher incidence of mediastinitis did not reach statistical significance (15% vs 4%, P = 0.05). Bleeding correlated with the extent of clopidogrel antiplatelet effect, with the best correlation for the VASP assay. CONCLUSIONS: Maintenance of DAPT until the day of CABG in patients treated with TA, increased postoperative bleeding at 24 h in parallel with preoperative antiplatelet activity induced by clopidogrel.

Increase in both angiogenic and angiostatic mediators in patients with idiopathic pulmonary fibrosis.

BACKGROUND: Idiopathic pulmonary fibrosis (IPF) is associated with a marked pulmonary vascular remodeling. The aim of this study was to investigate a potential imbalance between angiogenic and angiostatic factors in this disease. METHODS AND RESULTS: Sixty-four subjects with IPF and 10 healthy control subjects (60-70 years old) were prospectively included in this multicenter study. Plasma levels of vascular endothelial growth factor A (VEGF-A), thrombospondin-1 (TSP-1) and stem cell factor (SCF) were determined by Elisa. Comparisons between IPF and controls were made using the Mann-Whitney U test. We also analyzed these soluble mediators in relation with IPF severity (DLCO<40% or>40%) predicted or total lung capacity (TLC) and forced vital capacity (FVC) (both<55% or>55% predicted) using the same test. VEGF-A plasma levels were increased in IPF vs. controls (P=0.0008) as well as those of TSP-1 (P=0.008), irrespective of the severity of the disease as reflected by DLCO, TLC or FVC values. In contrast, SCF levels were similar in IPF and controls. CONCLUSIONS: Factors modulating angiogenic responses are dysregulated in patients with IPF with increases in VEGF-A and TSP-1. The serial assessment of VEGF-A and TSP-1 during the follow-up and the search for potential relationships with the outcome of the disease might give us hints to the clinical implication of these results.

Recombinant activated factor VII and prothrombin complex concentrates have different effects on bleeding and arterial thrombosis in the haemodiluted rabbit.

BACKGROUND: Recombinant activated factor VII (rFVIIa) is indicated in bleeding patients when a life-threatening haemorrhage occurs. Prothrombin complex concentrates (PCCs) are also used for this indication in several countries, without any evidence-based rationale. Our objective was to compare the efficacy and safety of PCC and rFVIIa in a model of bleeding and thrombosis in haemodiluted rabbits. METHODS: Forty-eight rabbits were randomly allocated into four groups: a control group and three treatment groups, in which animals were haemodiluted with hydroxyethyl starch 130/0.4 then administered either placebo, 160 µg kg(-1) rFVIIa, or 25 IU FIX kg(-1) PCC. The primary endpoint was hepatosplenic (HS) blood loss. Secondary endpoints were: (i) ear immersion bleeding time (IBT); (ii) thrombosis risk assessed by cyclic flow reductions (CFRs) of the carotid artery; and (iii) activated partial thromboplastin time (aPTT), and progress of thrombin activity. RESULTS: Haemodilution increased HS blood loss by 80% from 8 g (5-16) (control group) to 14 g (8-45) (placebo group) (P<0.01). HS blood loss was not different in animals receiving either rFVIIa [10 g (7-22)] or PCC [15 g (4-33)] (P<0.05) compared with the placebo group. Ear IBT was reduced with both rFVIIa and PCC. CFRs disappeared after haemodilution and were not restored with any treatment. Although PCC nearly doubled the total amount of thrombin generated, no significant change in the total amount of thrombin was seen in animals treated with rFVIIa. CONCLUSIONS: Neither rVIIa nor PCC reduced HS blood loss, whereas they both controlled the bleeding time, without increasing the thrombosis risk.

Distinct patterns of circulating endothelial cells in pulmonary hypertension.

The respective abundance of circulating endothelial cells and endothelial progenitor cells may reflect the balance between vascular injury and repair. As pulmonary arterial hypertension (PAH) and chronic thromboembolic pulmonary hypertension (CTEPH) can share features of pulmonary remodelling, we postulated that the two disorders might be associated with different types of pulmonary endothelial dysfunction. We studied 25 consecutive patients undergoing cardiac catheterisation for suspected pulmonary hypertension. Nine patients had PAH, nine had CTEPH, and seven had normal pulmonary arterial pressure and served as controls. Circulating endothelial cells were isolated with CD146-coated beads. CD34(+)CD133(+) cell and endothelial progenitor cell numbers were respectively determined by flow cytometry and cell culture, in peripheral vein and pulmonary artery blood. Plasma levels of soluble vascular endothelial growth factor (VEGF), soluble E-selectin and soluble vascular cell adhesion molecule (sVCAM) were measured by ELISA. No difference in progenitor counts or VEGF levels was found across the three groups. Compared to controls, circulating endothelial cell numbers were significantly increased in PAH but not in CTEPH, in keeping with the elevated soluble E-selectin and sVCAM levels found in PAH alone. In conclusion, PAH, in contrast to CTEPH, is associated with markers of vascular injury (circulating endothelial cells, soluble E-selectin and sVCAM) but not with markers of remodelling (endothelial progenitor cells, CD34(+)CD133(+) cells and VEGF).

Arterial and venous thrombosis is associated with different angiogenic cytokine patterns in patients with antiphospholipid syndrome.

Antiphospholipid syndrome (APS) is defined as a combination of antiphospholipid antibodies, arterial and/or venous thrombosis, and, in women, recurrent fetal loss. The mechanisms underlying this prothrombotic tendency are unclear. Here we determined plasma levels of the angiogenic growth factors vascular endothelial growth factor (VEGF), placental growth factor (PlGF) and stromal cell-derived factor-1 (SDF-1) in 34 patients with APS (median age 40 years) compared with 180 healthy controls and with 80 age-matched deep-venous thrombosis patients in whom the diagnosis of APS had been excluded. All of the patients met updated APS criteria and two-thirds of them were triply positive for antiphospholipid antibodies (lupus anticoagulant, anti-beta2-glycoprotein I and anti-cardiolipin antibodies). Angiogenic cytokines were quantified at least 6 months after an acute thrombotic event. VEGF levels were similar in the patients and controls, but were significantly higher in the patients with arterial thrombosis than in the patients with venous thrombosis. Plasma levels of SDF-1 and PlGF were significantly elevated in the patients, regardless of the arterial/venous nature of the thrombosis. Together, these results suggest that APS is associated with an angiogenic process, but that the angiogenic signal differs between patients with arterial and venous thrombosis. Lupus (2010) 19, 837-843.

[Antiplatelet agents and transfusion]. / Antiplaquettaires et transfusion.

Antiplatelet agents are at risk for bleeding complications, the management of which differs depending on the clinical situation and on the antiplatelet agent itself. Neutralization of antiplatelets is sometimes necessary, most often leading to platelet transfusion, although the benefit of this strategy is poorly documented. In addition, if platelet transfusion corrects the platelet inhibition induced by aspirin and probably by clopidogrel and prasugrel, it does not neutralize ticagrelor, as a consequence of its pharmacological properties. The clinical benefit of platelet transfusion is limited, and the most recent studies are challenging it. However, it is indicated on a perioperative basis for surgeries with high hemorrhagic risk and is discussed in severe hemorrhages. The neutralization of ticagrelor is a concern and the antidote currently under development may be a solution. In all cases, other therapeutic solutions may be considered, such as administration of desmopressin, tranexamic acid or activated factor VII.

Pulsed cavitational therapy using high-frequency ultrasound for the treatment of deep vein thrombosis in an in vitro model of human blood clot.

Post-thrombotic syndrome, a frequent complication of deep venous thrombosis, can be reduced with early vein recanalization. Pulsed cavitational therapy (PCT) using ultrasound is a recent non-invasive approach. We propose to test the efficacy and safety of high-frequency focused PCT for drug-free thrombolysis (thrombotripsy) in a realistic in vitro model of venous thrombosis. To reproduce venous thrombosis conditions, human whole blood was allowed to clot by stasis in silicone tubes (6 mm internal diameter) at a 30 cm H2O pressure, maintained during the whole experiment. We engineered an ultrasound device composed of dual 2.25 MHz transducers centered around a 6 MHz imaging probe. A therapeutic focus was generated at a 3.2 cm depth from the probe. Thrombotripsy was performed by longitudinally scanning the thrombus at three different speeds: 1 mm s-1 (n = 6); 2 mm s-1 (n = 6); 3 mm s-1 (n = 12). Restored outflow was measured every three passages. Filters were placed to evaluate the debris size. Twenty-four occlusive thrombi, of 2.5 cm mean length and 4.4 kPa mean stiffness, were studied. Flow restoration was systematically obtained by nine subsequent passages (4.5 min maximum). By varying the device's speed, we found an optimal speed of 1 mm s-1 to be efficient for effective recanalization with 90 s (three passages). Within 90 s, flow restoration was of 80, 62 and 74% at respectively 1, 2 and 3 mm s-1. For all groups, cavitation cloud drilled a 1.7 mm mean diameter channel throughout the clot. Debris analysis showed 92% of debris <10 µm, with no fragment > 200 µm.

[The role of extracorporeal removal of CO2 (ECCO2R) in the management of respiratory diseases]. / Place de l'épuration extracorporelle de CO2 (ECCO2R) dans la prise en charge des pathologies respiratoires.

INTRODUCTION: The aim of extracorporeal removal of CO2 (ECCO2R) is to ensure the removal of CO2 without any significant effect on oxygenation. ECCO2R makes use of low to moderate extracorporeal blood flow rates, whereas extracorporeal membrane oxygenation (ECMO) requires high blood flows. STATE OF THE ART: For each ECCO2R device it is important to consider not only performance in terms of CO2 removal, but also cost and safety, including the incidence of hemolysis and of hemorrhagic and thrombotic complications. In addition, it is possible that the benefits of such techniques may extend beyond simple removal of CO2. There have been preliminary reports of benefits in terms of reduced respiratory muscle workload. Mobilization of endothelial progenitor cells could also occur, in analogy to the data reported with ECMO, with a potential benefit in term of pulmonary repair. The most convincing clinical experience has been reported in the context of the acute respiratory distress syndrome (ARDS) and severe acute exacerbations of chronic obstructive pulmonary disease (COPD), especially in patients at high risk of failure of non-invasive ventilation. PERSPECTIVES: Preliminary results prompt the initiation of randomized controlled trials in these two main indications. Finally, the development of these technologies opens new perspectives in terms of long-term ventilatory support.

Interindividual variability in dabigatran and rivaroxaban exposure: contribution of ABCB1 genetic polymorphisms and interaction with clarithromycin.

Essentials Rivaroxaban and dabigatran are substrates of the P-glycoprotein (P-gp) encoded by the ABCB1 gene. We tested the effect of ABCB1 polymorphisms and of a P-gp inhibitor on both drugs' pharmacokinetics. The ABCB1 genotype was not a clinically relevant determinant of both drugs' pharmacokinetics. Administration of P-gp inhibitors with dabigatran or rivaroxaban should be exercised with caution. SUMMARY: Background The direct oral anticoagulants (DOACs) dabigatran and rivaroxaban are both substrates of the P-glycoprotein (P-gp) transporter, encoded by the ABCB1 gene. Rivaroxaban is metabolized by cytochrome P450 A4 (CYP3A4). Interindividual variability in DOAC exposure and frequent P-gp-associated drug-drug interactions have been described in patients. Objective To assess the influence of ABCB1 polymorphisms on the pharmacokinetics of dabigatran and rivaroxaban, associated or not with clarithromycin, a P-gp and CYP3A4 inhibitor. Methods Sixty healthy male volunteers, selected according to ABCB1 genotype (20 homozygous mutated, 20 heterozygous mutated, and 20 wild-type for haplotype 2677-3435), were included in this randomized, two-center, crossover study. All received sequentially a single dose of dabigatran etexilate (300 mg) and rivaroxaban (40 mg) associated or not with clarithromycin. Peak plasma concentration and area under the curve (AUC) were compared across the three ABCB1 genotypes. The effect of clarithromycin on dabigatran or rivaroxaban pharmacokinetics was assessed. Results Interindividual coefficients of variation for AUC were 77% for dabigatran and 51% for rivaroxaban. ABCB1 genotype did not significantly affect drug pharmacokinetics: AUC ratios between mutant-allele carriers and wild-type volunteers were 1.27 (95% confidence interval [CI] 0.84-1.92) and 1.20 (95% CI 0.96-1.51) for dabigatran and rivaroxaban, respectively. Clarithromycin coadministration led to a two-fold increase in both drugs' AUC, irrespective of ABCB1 genotype: ratios of geometric means were 2.0 (95% CI 1.15-3.60) and 1.94 (95% CI 1.42-2.63) for dabigatran and rivaroxaban, respectively. Conclusions ABCB1 genotype is not a significant determinant of interindividual variability in dabigatran and rivaroxaban pharmacokinetics. The levels of one drug did not predict the levels of the other. Coadministration of a P-gp/CYP3A4 inhibitor with dabigatran or rivaroxaban may warrant caution in patients at risk of overexposure.

The angiopoietin pathway is modulated by PAR-1 activation on human endothelial progenitor cells.

OBJECTIVES: The importance of protease-activated receptor-1 (PAR-1) in blood vessel development has been shown in knock-out mice. As endothelial progenitor cells (EPCs) express functional PAR-1, we examined whether PAR-1 stimulation by the peptide SFLLRN interfered with the angiopoietin pathway, that is EPC commitment, proliferation and migration. METHODS AND RESULTS: Given the strong PAR-1 expression on CD34+ cells, we tested the effect of SFLLRN 75 micromol L(-1) on the emergence of EPCs from cord blood. PAR-1 activation did not modify the number of colonies or the day of emergence, in keeping with the lack of induction of angiopoietin 1 gene expression. Conversely, SFLLRN treatment of EPCs induced angiopoietin 2 gene expression and protein synthesis. Experiments with polyclonal blocking antibodies showed that angiopoietin 2 was involved in the proliferative effect of PAR-1 activation. PAR-1 activation also enhanced migration toward angiopoietin 1 in a Boyden chamber assay. CONCLUSIONS: Our study demonstrates that PAR-1-induced proliferation of EPCs involves angiopoietin 2. PAR-1 also enhances EPC migration toward angiopoietin 1. These findings might explain the role of thrombin in neovascularization via the angiopoietin pathway.

[Acquired mutation of JAK2 tyrosine kinase and polycythaemia vera]. / Mutation acquise de la tyrosine kinase Jak2 et maladie de Vaquez.

Polycythaemia vera is an acquired myeloproliferative disorder characterised by a polycythaemia resulting of a clonal disorder arising in a multipotent hematopoietic stem cell. The increase of red cell mass exposes to a high risk of arterial or venous thrombosis and thus requires a cytoreductive treatment. An acquired genetic mutation in exon 12 of the JAK2 tyrosine kinase gene, leading to a substitution of a valine to a phenylalanine (V617F), has been described in most polycythaemia vera patients. This mutation increases the phosphorylation activity of JAK2, promotes the spontaneous cellular growth and induces erythrocytosis in a mouse model. Prevalence studies of V617F JAK2 mutation in different myeloproliferative disorders have found this genetic alteration in half of idiopathic myelofibrosis and in one third of essential thrombocythaemia. This finding is a huge progress in the understanding of polycythaemia vera physiopathology, it will be also an useful tool for the diagnosis of myeloproliferative disorders and it opens a new field for the development of targeted therapeutic approaches in these disorders.

The formation of complexes between human plasminogen activator inhibitor-1 (PAI-1) and sodium dodecyl sulfate: possible implication in the functional properties of PAI-1.

The effect of the anionic detergent sodium dodecyl sulfate (SDS) on human PAI-1 present in plasma, platelet extracts and endothelial cell cultures, was examined. Using the dye partitional extraction method of Mukerjee [1956) Anal. Chem. 28, 870-873) to quantitate ionic surfactants, and a discontinuous spectrophotometric assay for the titration of PAI-1 based on the measurement of residual active t-PA, we found (i) that SDS remains tightly bound to PAI-1 after equilibrium dialysis and (ii) that the activity of the latter was closely related to the amount of SDS carried over by the PAI-1 solution. The highest concentrations of SDS (ratio of SDS to protein greater than 0.1) were detected in the platelet-derived sources of PAI-1 which also showed the lowest residual t-PA activity. Moreover, it is demonstrated by SDS-PAGE and autoradiography that the tight binding of SDS to PAI-1 decreases its ability to form complexes with t-PA. Similar results were obtained with PAI-1 previously inactivated at 37 degrees C: the inability of PAI-1 to form complexes with t-PA was unchanged after SDS treatment. These observations suggest that the decrease in the residual activity of t-PA observed with the SDS-treated PAI-1 preparations is not related to an increase in the inhibitory activity of PAI-1. In fact, SDS was able to produce a decrease in both the binding of t-PA to fibrin and the activation of plasminogen by fibrin-bound t-PA. Bovine PAI-1 has been shown to exist in a latent SDS-activatable form. Our data indicate that such a form might not be present in the human sources of PAI-1 we have tested.

The specific thromboxane receptor antagonist S18886: pharmacokinetic and pharmacodynamic studies.

OBJECTIVES AND PATIENTS: We conducted a multicenter double-blind pharmacokinetic/pharmacodynamic (PK/PD) study of the new oral thromboxane receptor antagonist S18886 in 30 patients with peripheral artery disease, who were randomized to receive five different oral dosages of S18886 (1, 2.5, 5, 10 or 30 mg) for 12 weeks (83 days). Primary objective was to determine the effect of S18886 on platelet aggregation ex vivo. RESULTS: Pharmacokinetics of S18886 was linear, with peak plasma levels being reached between 30 min and 2 h and a terminal half-life of 5.8-10 h. No significant accumulation of S18886 in plasma was observed after repeated dosing. The relationship between the S18886 concentration and platelet inhibition was examined in terms of U46619-induced platelet aggregation. Over the range of doses studied, there was a predictable relation between the plasma drug concentration and the degree of platelet inhibition at each dose. Maximal inhibition of U46619-induced platelet aggregation was achieved within 1 h with all oral doses of S18886, and this effect was maintained for at least 12 h. The PK/PD relationship was direct, and U46619-induced platelet aggregation was strongly inhibited by S18886 plasma concentrations above 10 ng mL(-1). This concentration was thus the minimal effective antiplatelet level in this population, and was maintained only by the dosages of 10 and 30 mg. The safety profile of S18886 was excellent, whatever the unit dose, with no attributable adverse events. CONCLUSION: The results of this study, which included modeling and simulation, help identify the minimal effective plasma concentration of S18886 required for potent antiplatelet efficacy in patients with stable peripheral arterial disease.

[Prognostic factors in chronic lymphocytic leukaemia: contribution of recent biological markers]. / Facteurs pronostiques de la leucémie lymphoïde chronique: apport des marqueurs biologiques récents.

Chronic lymphocytic leukemia (CLL) is the most common lymphoid hemopathy in elderly. Diagnosis of CLL is easily made with a full blood count and immunophenotyping, but there is an heterogeneity in clinical evolution. Until now, scheduling of treatment is based on Rai or Binet staging systems. These staging systems can not distinguish patients with a rapid evolution and thus who will need an earlier treatment. In order to detect these patients, it is useful to have some relevant markers to predict disease evolution. This article reviews recent biologic markers that can be used to evaluate long term prognosis of CLL patients.

Multimodal assessment of non-specific hemostatic agents for apixaban reversal.

BACKGROUND: Non-specific hemostatic agents, namely activated prothrombin complex concentrate (aPCC), PCC and recombinant activated factor (F) VII (rFVIIa), can be used, off-label, to reverse the effects of FXa inhibitors in the rare cases of severe hemorrhages, as no approved specific antidote is available. We have evaluated the ability of aPCC, PCC and rFVIIa to reverse apixaban. METHODS: Healthy volunteer whole blood was spiked with therapeutic or supra-therapeutic apixaban concentrations and two doses of aPCC, PCC or rFVIIa. Tests performed included a turbidimetry assay for fibrin polymerization kinetics analysis, scanning electron microscopy for fibrin network structure observation, thrombin generation assay (TGA), thromboelastometry, prothrombin time and activated partial thromboplastin time. RESULTS: aPCC generated a dense clot constituting thin and branched fibers similar to those of a control without apixaban, increased fibrin polymerization velocity and improved quantitative (endogenous thrombin potential and peak height) as well as latency (clotting and lag times) parameters. Adding PCC also improved the fibrin and increased quantitative parameters, but fibrin polymerization kinetics and latency parameters were not corrected. Finally, rFVIIa improved latency parameters but failed to restore the fibrin network structure, fibrin polymerization velocity and quantitative parameters. CONCLUSION: aPCC was more effective than PCC or rFVIIa in reversing in vitro the effects of apixaban. aPCC rapidly triggered the development of an apparently normal fibrin network and corrected latency and quantitative parameters, whereas PCC or rFVIIa had only a partial effect.

Protein C and protein S deficiencies.

The protein C (PC) pathway, with its cofactor protein S (PS), is an important natural antithrombotic mechanism. Both PC and PS deficiencies have been implicated in thrombophilia. The molecular basis for hereditary PC and PS deficiencies is highly heterogeneous, with a large spectrum of mutations that have various effects on the expression of the relevant allele. A small subset of patients who are homozygous or compound heterozygous for a PC gene mutation have severe thrombotic complications at birth, whereas onset occurs later in the other cases. Patients heterozygous for a PC or PS gene abnormality may develop recurrent thrombosis during adulthood, with a probability of remaining free of thrombosis of about 50% at age 45. A PC or PS gene defect is associated with the factor V Arg 506 to Gln mutation in 10% to 30% of symptomatic patients, suggesting that clinical expression is controlled by several genes in heterozygous patients.

Amino acids 225-235** of the protein C serine-protease domain are important for the interaction with the thrombin-thrombomodulin complex.

Protein C (PC) is a vitamin K-dependent zymogen that inactivates factors Va and VIIIa after its activation by thrombin complexed to thrombomodulin. We characterized a monoclonal antibody (mAb) against PC, whose only influence on PC functions was to inhibit PC activation by the thrombin-thrombomodulin complex. It recognized an epitope in the PC heavy chain, the conformation of which is calcium-dependent. The mAb did not recognize a natural PC variant that was not activated by the thrombin-thrombomodulin complex (mutation R229Q) and did recognize a synthetic peptide corresponding to PC amino acids 225-235 in an Elisa assay. The peptide inhibited PC activation by the thrombin-thrombomodulin complex. These data confirm that the calcium-binding loop of the serine-protease domain is involved in the interaction of PC with the thrombin-thrombomodulin complex.

Human thrombin variable region 1, including E39, is involved in interactions with alpha 1-antitrypsin M358R and protein C.

We used an antithrombin autoantibody (IgG D), the epitope of which encompasses ABE1 and amino acids located within variable region 1, to study thrombin interactions with R358 alpha 1-AT and protein C. IgG D inhibited the thrombin interaction with R358 alpha 1-AT, while hirugen had no effect, indicating that the interaction of R358 alpha 1-AT with thrombin may involve the VR1 subsite. We also obtained evidence that VR1 may be involved in the activation of protein C by thrombin in the absence of thrombomodulin. Moreover, IgG D attenuated the inhibitory effect of calcium ions during protein C activation by thrombin, probably by masking E39 within the VR1 site.

A monoclonal antibody recognizing the activation domain of protein C in its calcium-free conformation.

A monoclonal antibody (mAb) binding to protein C (PC) heavy chain but not to activated PC was found to inhibit PC activation by free thrombin, suggesting that epitope involved the activation site. Using a set of overlapping synthetic peptides, we confirmed that this mAb recognizes the sequence encompassing the thrombin cleavage site (165QVDPRLI(171)). Surprisingly, epitope was only accessible in the absence of calcium, half-maximal inhibition of mAb binding occurring at 100 microM Ca2+. Thus, our antibody provides direct evidence that conformation and/or accessibility of the activation site differ between the apo and Ca2+-stabilized conformers of PC.

A new assay based on thrombin generation inhibition to detect both protein C and protein S deficiencies in plasma.

Activated protein C reduces thrombin generation by inactivating factors V and VIII in the presence of protein S. This prompted us to develop an assay which would allow specific exploration of this reaction. The total amount of thrombin formed in plasma after activation by tissue factor and phospholipids was reduced by adding thrombomodulin. This addition allowed protein C to be activated by endogenous thrombin. The inhibition of thrombin generation (ITG) due to protein C activation could be measured by comparing thrombin formation in the presence and in the absence of thrombomodulin. ITG increased with both protein C and protein S concentrations. Normal values of ITG expressed as a percentage were between 40 and 65% and were not influenced by age or sex. ITG increased in patients under heparin therapy, decreased in patients under oral anticoagulant therapy and was decreased in women using oral contraceptives. This method could be used for screening patients for protein C and protein S deficiencies.