Jovan P. Antovic

Atorvastatin reduces thrombin generation and expression of tissue factor, P-selectin and GPIIIa on platelet-derived microparticles in patients with peripheral arterial occlusive disease

We investigated the effects of statin treatment on platelet-derived microparticles (PMPs) and thrombin generation in atherothrombotic disease. Nineteen patients with peripheral arterial occlusive disease were randomised to eight weeks of treatment with atorvastatin or placebo in a cross-over fashion. Expression of GPIIIa (CD61), P-selectin (CD62P), tissue factor (TF, CD142) and phosphatidylserine (PS; annexin-V or lactadherin binding) was assessed on PMPs. Thrombin generation in vivo was assessed by measurement of prothrombin fragment 1+2 in plasma (F1+2) and ex vivo by using the calibrated automated thrombogram (CAT). During atorvastatin treatment, expression of TF, P-selectin and GPIIIa was significantly reduced vs. placebo (p<0.001 for all). No effect on annexin-V or lactadherin binding was seen. Thrombin generation was significantly reduced during atorvastatin as assessed by both the CAT assay (p<0.001) and by measurements of F1+2 (p<0.01). Subsequent in vitro experiments showed that when TF on microparticles (MPs) was blocked by antibodies, the initiation of thrombin generation was slightly but significantly delayed. Blocking PS on MPs using annexin-V or lactadherin resulted in almost complete inhibition of thrombin generation. In conclusion, atorvastatin reduces thrombin generation and expression of TF, GPIIIa and P-selectin on PMPs in patients with peripheral vascular disease. Microparticle-bound TF slightly enhances initiation of thrombin generation whereas negatively charged surfaces provided by MPs or lipoproteins could reinforce thrombin generation. Statins may inhibit initiation of thrombin generation partly through a microparticle dependent mechanism but the main effect is probably through reduction of lipoprotein levels.

Factor XI deficiency in animal models

Summary.  The blood coagulation system forms fibrin to limit blood loss from sites of injury, but also contributes to occlusive diseases such as deep vein thrombosis, myocardial infarction, and stroke. In the current model of a coagulation balance, normal hemostasis and thrombosis represent two sides of the same coin; however, data from coagulation factor XI‐deficient animal models have challenged this dogma. Gene targeting of factor XI, a serine protease of the intrinsic pathway of coagulation, severely impairs arterial thrombus formation but is not associated with excessive bleeding. Mechanistically, factor XI may be activated by factor XII following contact activation or by thrombin in a feedback activation loop. This review focuses on the role of factor XI, and its deficiency states as novel target for prevention of thrombosis with low bleeding risk in animal models.

On the monitoring of dabigatran treatment in “real life” patients with atrial fibrillation

INTRODUCTION The oral direct thrombin inhibitor dabigatran is increasingly used to prevent thromboembolic stroke in patients with atrial fibrillation (AF). Routine laboratory monitoring is currently not recommended, but measurements of dabigatran and/or its effect are desirable in certain situations. We studied dabigatran exposure and compared different tests for monitoring of dabigatran in a real-life cohort of AF patients. MATERIAL AND METHODS Ninety AF patients (68 ± 9 years, 67% men, mean CHADS2 score 1.5) were treated with dabigatran 150 (n=73) or 110 mg BID (n=17). Trough plasma concentrations of total and free dabigatran by liquid chromatography-tandem mass-spectrometry (LC-MS/MS) were compared to indirect measurements by Hemoclot thrombin inhibitors (HTI) and Ecarin clotting assay (ECA), as well as PT-INR and aPTT. RESULTS Total plasma dabigatran varied 20-fold (12-237 ng/mL with 150 mg BID) and correlated well with free dabigatran (r(2)=0.93). There were strong correlations between LC-MS/MS and HTI or ECA (p<0.001) but these assays were less accurate with dabigatran below 50 ng/mL. The aPTT assay was not dependable and PT-INR not useful at all. There were weak correlations between creatinine clearance (Cockcroft-Gault) and LC-MS/MS, HTI and ECA (p<0.001 for all). A high body weight with normal kidney function was associated with low dabigatran levels. CONCLUSIONS HTI and ECA reflect the intensity of dabigatran anticoagulation, but LC-MS/MS is required to quantify low levels or infer absence of dabigatran. Most real life patients with a normal creatinine clearance had low dabigatran levels suggesting a low risk of bleeding but possibly limited protection against stroke.

Total thrombin-activatable fibrinolysis inhibitor (TAFI) antigen and pro-TAFI in patients with haemophilia A.

Pro‐thrombin‐activatable fibrinolysis inhibitor (pro‐TAFI), also known as TAFI, procarboxypeptidase U, or procarboxypeptidase B, is a relatively recently described plasma glycoprotein synthesized in the liver. It can be catalysed into its active form, TAFI (TAFIa, carboxypeptidase U or B) by a complex of thrombin/thrombomodulin. TAFI can potentially inhibit fibrinolysis by removing carboxyterminal lysine residues from partially degraded fibrin, decreasing plasminogen binding on the surface of fibrin, which thereby results in a decrease of the fibrinolytic activity. As TAFI represents a connection between coagulation and fibrinolysis, it can be expected that TAFI levels are altered in different thrombotic and haemorrhagic diseases, such as haemophilia A. Total TAFI antigen (including pro‐TAFI, TAFI and the inactive form of TAFI [TAFIi]) and pro‐TAFI were determined in 17 patients with haemophilia A. Thirteen healthy age‐matched volunteers served as controls. No significant difference in levels of total TAFI antigen was observed between controls and patients with haemophilia, although it was slightly decreased in patients with haemophilia. Pro‐TAFI was significantly reduced in haemophilia patients compared to controls (P=0.0113). TAFI antigen levels similar to controls have already been described in different clinical conditions, including haemophilia A. Decrease of pro‐TAFI in haemophilia A can be an additional factor, together with decrease in thrombin generation, which induces impaired activation of pro‐TAFI to TAFI, and could cause accelerated fibrinolysis. This supports the validity of usage of antifibrinolytics in the treatment of haemophilia A. In this paper we use new nomenclature for TAFI, and we believe that this recommended terminology for different forms of TAFI can simplify further standardization in TAFI investigation.

Does recombinant factor VIIa, apart from overall hemostasis, regulate TAFI dependent fibrinolysis? In vitro analysis using overall hemostasis potential (OHP) assay

Using the recently developed overall hemostatic potential (OHP) assay, we investigated the effects of different concentrations of recombinant factor VIIa on overall hemostasis and on thrombin activatable fibrinolysis inhibitor (TAFI) dependent fibrinolysis in different factor deficient plasmas. rFVIIa increased OHP in FV, FVIII and FIX deficient plasmas but not up to the levels in normal pooled plasma (NPP). Maximal levels were found at 2.4 microg/mL of rFVIIa, while higher doses did not further increase OHP. In FXII deficient plasma, OHP is higher than in NPP and addition of rVIIa further increased it. Even very high concentrations of rFVIIa (9.6 microg/mL) did not induce a significant increase of OHP in NPP. Higher concentrations of rVIIa down-regulated fibrinolysis in FVIII, FIX and FXI deficient plasmas to values obtained in NPP. Using potato tuber carboxy-peptidase inhibitor (PTCI), a specific inhibitor of TAFI, it was found that TAFIs influence on fibrinolysis down-regulation in FVIII and FIX deficient plasmas increased after addition of higher concentrations of rFVIIa. From this in vitro study it seems that rFVIIa in a concentration of 2.4 microg/mL improves overall hemostasis in FV, FVIII and FIX deficient plasmas. rFVIIa, even at very high (supra-therapeutic) concentrations, does not induce hypercoagulability either in NPP or in deficient plasmas. Higher concentrations of rVIIa induce, at least partly, TAFI dependent down-regulation of fibrinolysis in FVIII and FIX deficient plasmas. These results, together with some previous ex vivo studies, point to OHP assay as a possible diagnostic tool for evaluating the hemostatic effects of rFVIIa.

Blood coagulation and fibrinolysis in acute ischaemic and haemorrhagic (intracerebral and subarachnoid haemorrhage) stroke: does decreased plasmin inhibitor indicate increased fibrinolysis in subarachnoid haemorrhage compared to other types of stroke?

Ischaemic and haemorrhagic stroke may cause haemostatic abnormalities, apart from concomitant brain damage. In this study, some blood coagulation and fibrinolysis parameters were investigated in 30 patients with ischaemic stroke (atherothrombotic) and 30 with haemorrhagic (20 with intracerebral and 10 with subarachnoid haemorrhage) stroke. The following parameters were determined within the first 24 h after stroke: prothrombin time (PT%), activated partial thromboplastin time (aPTT), fibrinogen, activity of FVII, antithrombin, plasmin inhibitor (PI) and fibrin D-dimer. Significant decreases in PT%, FVII activity and antithrombin as well as an increase in fibrinogen and D-dimer were noticed in ischaemic stroke and in both groups of patients with haemorrhagic stroke. PI levels were significantly lower in subarachnoid haemorrhage patients compared with those in controls and those in both the intracerebral haemorrhage and the ischaemic stroke patients. With the exception of this difference, there were no other differences between ischaemic stroke and the two types of haemorrhagic stroke. This could indicate that haemostatic abnormalities are a consequence of brain damage rather than primary haemostatic activation during thrombosis and/or bleeding in the acute phase of stroke. A decrease in the plasmin inhibitor could suggest excessive fibrinolysis in subarachnoid haemorrhage.

Thrombin-activatable fibrinolysis inhibitor antigen and TAFI activity in patients with APC resistance caused by factor V Leiden mutation

Thrombin-activatable fibrinolysis inhibitor (TAFI), also known as procarboxypeptidase U or plasma procarboxypeptidase B, is a relatively recently described plasma glycoprotein synthesised in the liver. It can be activated into active enzyme TAFIa (carboxypeptidase U or plasma carboxypeptidase B) by a complex of thrombin/thrombomodulin. TAFIa can potentially inhibit fibrinolysis by removing carboxyterminal lysine residues from partially degraded fibrin, decreasing plasminogen binding on the surface of fibrin, which thereby results in a decrease of the fibrinolytic activity. Since TAFI represents a connection between coagulation and fibrinolysis, it can be expected that TAFI levels are altered in different thrombotic and hemorrhagic diseases. Thrombin generation is increased in patients with activated protein C (APC) resistance, while it has been shown that APC has profibrinolytic effect. Therefore, changes in TAFI level should be found in patients with APC resistance due to factor V Leiden (FV Leiden) mutation. TAFI antigen (including TAFI, TAFIa and the inactive form TAFIai) and TAFI activity were determined in 17 female patients heterozygous for FV Leiden mutation while 13 healthy volunteers were controls. No statistically significant difference in levels of TAFI antigen was observed. TAFI activity was significantly reduced in APC resistance patients compared to control (P=.018). The nondifference in TAFI antigen, together with the decrease of TAFI activity level, can be explained by activation of TAFI to TAFIa and shifting of equilibrium towards an increase of the latter. This can be an indirect proof that TAFIa is increased in patients with APC resistance due to FV Leiden mutation, indicating that downregulation of fibrinolysis can be an additional risk factor for thrombosis in these patients.

Practical Viewpoints on the Diagnosis and Management of Heparin-Induced Thrombocytopenia

Heparin-induced thrombocytopenia (HIT, type II) is an immune-mediated disorder due to antibodies formed against heparin-platelet factor 4 complexes, usually appearing at days 5 to 14 after initiation of heparin. It is important to recognize HIT because heparin prophylaxis or treatment paradoxically associates with new venous and/or arterial thrombosis. Early clinical suspicion and diagnosis together with proper pharmacotherapy and close laboratory monitoring are the cornerstones for successful management. This includes monitoring of Thrombocytopenia, its Timing to heparin administration, appearance of new Thrombosis or resistance to treatment, and differential diagnosis by exclusion of o Ther causes (the 4Ts). Specific attention should be paid to the absence or presence of thrombosis and to tailoring thromboprophylaxis or anticoagulant therapy with a nonheparin alternative. Even in the absence of HIT-associated thrombosis, an active policy for prolonged thromboprophylaxis is demanded. Rapid and reliable assays should be developed for diagnosis and anticoagulation monitoring to secure safe management with nonheparins. Semiquantitative testing for on-call hours should be available and later confirmed as clinically needed. Alternative therapeutic options are available, but because their use is infrequent, experienced coagulation treatment centers should provide guidance in the treatment and in laboratory monitoring. Most of the evidence in HIT is grade IC, and thus the best evidence is provided by clinical experience. New anticoagulants and platelet inhibitors may offer future alternatives in the management of HIT, but the current treatment options provide the best experience and benefit. The joint clinical and laboratory guidelines provided in this article along with two practical case scenarios were prepared by a Nordic expert panel. They will be valuable for hematologists and colleagues who do not routinely encounter HIT.

Elevations in soluble CD40 ligand in patients with high platelet aggregability undergoing percutaneous coronary intervention.

High aggregatory responses despite antiplatelet treatment is associated with an increased risk of thrombotic complications following percutaneous coronary intervention (PCI). In the present study, we investigated the relationship between platelet aggregatory responses to ADP and the release of CD40L (sCD40L): an immunomodulatory compound involved in atherothrombosis – in patients undergoing PCI. ADP-induced platelet aggregation, sCD40L and soluble P-selectin (sP-selectin) were determined before and 24 h after PCI, in samples from 52 patients receiving aspirin and thienopyridines. Platelet aggregation to ADP above the median was defined as ‘high aggregation’, and aggregation below the median as ‘low aggregation’. Data below are medians and interquartile ranges. Patients with ‘high platelet aggregability’ had a significantly higher increase in both sCD40L (Δ-values: 0.78 (−0.19–3.18) vs. −0.65 (−2.10–0.00) ng/ml, P = 0.002) and sP-selectin (Δ-values: 8.0 (−2.00–16.00) vs. 4.50 (−13.00–0.50) ng/ml, P = 0.001) compared with patients with ‘low platelet aggregability’. In a multivariate linear regression analysis adjusted for clinical characteristics and type of preintervention therapy, the only independent predictors of sCD40L and sP-selectin were platelet aggregation to ADP before PCI (P < 0.001) and the combination of platelet aggregation to ADP before PCI and urgency of PCI (P < 0.001). Circulating CD40L is more markedly increased after PCI in patients with high ADP-induced platelet aggregation.

Overall haemostatic potential can be used for estimation of thrombin-activatable fibrinolysis inhibitor-dependent fibrinolysis in vivo and for possible follow-up of recombinant factor VIIa treatment in patients with inhibitors to factor VIII

Summary. Thrombin generation induced by recombinant factor VIIa (rFVIIa) in patients with haemophilia and/or inhibitors to factor VIII/IX could enhance generation of thrombin‐activatable fibrinolysis inhibitor (TAFI), a recently described link between coagulation and fibrinolysis. TAFI is unstable and it is not easy to measure its active form in vivo. Overall haemostatic potential (OHP) is a novel method for haemostasis estimation, based on determination of the fibrin aggregation curve in which tiny amounts of thrombin are used for activation of clotting. We measured OHP in six patients with inhibitors to factor VIII before injection of rFVIIa and 10 and 120 min thereafter. Overall fibrinolytic potential (OFP) and clot lysis time (CLT) analysed by this method could be used for indirect estimation of TAFI generation. We found no change in pro‐TAFI and total TAFI antigen before and after treatment with rFVIIa. OHP was almost undetectable before treatment but increased into the range of normal pooled plasma 10 and 120 min after rFVIIa treatment, as did CLT. However, after addition of potato tuber carboxypeptidase inhibitor, a specific inhibitor of TAFI, the shortening of CLT was lower than that in NPP. OFP was increased in patient plasma both 10 and 120 min after treatment compared with NPP. There was a strong positive correlation between pro‐TAFI concentration and shortening of CLT after PTCI addition and a negative correlation between pro‐TAFI concentration and OFP 10 min after rFVIIa injection. Thus, rFVIIa normalizes OHP and CLT 10 min after injection. While this improvement slightly decreases, but still exists after 2 hours, it suggests efficacy in bleeding prevention using a protocol based on rFVIIa administration every 2 hours.