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H Coenraad Hemker

Publications and source records attributed to H Coenraad Hemker.

16 recordsLinked to original sources
PubMed2006

Low molecular weight activated protein C inhibitors as a potential treatment for hemophilic disorders.

The synthesis and evaluation of inhibitors of activated protein C (aPC) are reported. This serine protease is partly responsible for the degradation of factor VIIIa, involved in the regulation of bleeding in hemophilia A. Benzamidine-containing derivatives were found to be potent aPC inhibitors, some of them showing selectivity against the procoagulant protease thrombin. Moreover, compound 1 significantly restored the generation of thrombin in hemophiliac plasma.

Benzamidines↗
PubMed2006

Thrombin generation, a function test of the haemostatic-thrombotic system.

By the use of a fluorogenic thrombin substrate and continuous calibration of each individual sample, it is now possible to obtain a thrombin generation (TG) curve (or thrombogram) in plasma, with or without platelets, in an easy routine procedure at high throughput and with an acceptable experimental error (<5%). Evidence is growing that the parameters of the thrombogram, and notably the area under the curve (endogenous thrombin potential, ETP), are useful in assessing bleeding- or thrombotic risk and its modification by antithrombotic- or haemostatic treatment. Available data strongly suggest that conditions (congenital, acquired, drug-induced) that increase TG all cause a thrombotic tendency and that conditions that decrease TG prevent thrombosis but, beyond a limit, cause bleeding. Diminution of TG is a common denominator of all antithrombotic treatment, including anti-platelet drugs. The thrombogram can also be used as a tool in the search for new antithrombotics and reflects the haemorrhagic or thrombotic side effects of other drugs (e.g. oral contraceptives). The thrombogram thus is a promising new approach to clinical management of bleeding and thrombotic disease as well as a tool in drug research and epidemiology. Our experience at this moment is insufficient, however, to already clearly define its limits.

Blood Coagulation Tests↗
PubMed2005

Evaluation of thrombin generating capacity in plasma from patients with haemophilia A and B.

In haemophilia patients, a relationship is usually observed between the clinical expression of the disease and plasmatic factor VIII/factor IX (FVIII/FIX) activity. However, it is known from clinical experience, that some haemophilia patients, despite similar FVIII/FIX plasma levels, could exhibit different bleeding phenotype. After determining preanalytical test conditions, we evaluated the thrombin generation capacity from haemophilia plasma samples in various conditions and the potential usefulness of thrombin generation test (TGT) in haemophilia patients. In a series of 46 haemophilia patients (34 haemophilia A and 12 haemophilia B patients), we found a significant correlation between plasmatic FVIII/FIX levels and endogenous thrombin potential (ETP), peak and time to peak obtained by thrombin generation measurement. In addition, a correlation was found between severe clinical bleeding phenotype and ETP. Our results suggest that TGT could be a promising tool to evaluate haemostasis capacity in patients with haemophilia. Our ex vivo results, obtained 24 hours after FVIII concentrate administration, showed that in patients presenting similar plasmatic FVIIII levels, thrombin generation capacity may be significantly different. These results suggest that in patients with haemophilia, TGT could be useful for individually tailoring prophylactic regimens as well as for adapting clotting factors infusions in surgical situations, in addition to FVIII/FIX plasma clotting activities.

Adult↗
PubMed2004

Factor XI-dependent reciprocal thrombin generation consolidates blood coagulation when tissue factor is not available.

OBJECTIVE: Feedback activation of factor XI by thrombin is a likely alternative for tissue factor-dependent propagation of thrombus formation. However, the hypothesis that thrombin can initiate and propagate its formation in a factor XI-dependent and platelet-dependent manner has not been tested in a plasma milieu. METHODS AND RESULTS: We investigated thrombin generation in recalcified platelet-rich plasma activated with varying amounts of thrombin or factor VIIa. Thrombin initiates and propagates dose-dependently thrombin generation only when platelets and plasma factor XI are present. Incubation of thrombin-activated platelets with a tissue factor neutralizing antibody had no effect on thrombin formation, indicating that platelet-associated tissue factor, if present at all, is not involved. In the absence of factor VIII, thrombin could not initiate its own formation, whereas factor VIIa-induced thrombin generation was reduced. Collagen strongly stimulated both thrombin-initiated and factor VIIa-initiated thrombin generation. CONCLUSIONS: These findings support the notion that platelet-localized feedback activation of factor XI by thrombin plays an important role in maintaining normal hemostasis as well as in sustaining thrombus formation when the TF pathway is inhibited by tissue factor pathway inhibitor.

Adult↗
PubMed2004

Phenotyping the haemostatic system by thrombography--potential for the estimation of thrombotic risk.

The aim of this paper is to review the thrombogram and its use for phenotyping the haemostatic system. The thrombogram can be readily obtained through Calibrated Automated Thrombography (CAT), using a commercially available fluorometer, dedicated software (Thrombinoscope) and a calibrator. Here we explore the possibility to use platelet-rich plasma (PRP) triggered with a low amount of recombinant human tissue factor (approximately 0.5 pM) and also explore the function of the protein C system by adding activated protein C (APC) or soluble recombinant thrombomodulin (TM). Examples are shown: inherited antithrombin (AT) and protein C deficiencies, and antiphospholipid antibodies.

Anticoagulants↗
PubMed2004

Thrombin generation assays: accruing clinical relevance.

PURPOSE OF REVIEW: After decades of near oblivion, thrombin generation is being revived as an overall function test of the plasmatic coagulation system in platelet-poor plasma (PPP). In platelet-rich plasma (PRP) it assesses platelet procoagulant functions as well. RECENT FINDINGS: The recently developed use of special fluorogenic thrombin substrates allows monitoring of thrombin concentration in clotting PPP and PRP on line in up to 24 parallel samples. Studies in model systems stress the importance of cell-bound thrombin generation such as measured in PRP. SUMMARY: The method can be profitably applied to various hitherto unyielding problems such as the control of (low-molecular-weight) heparin therapy, the detection of lupus anticoagulant, and various forms of thrombomodulin and activated protein C resistance (including the use of oral contraceptives) as well as monitoring the treatment of hemophiliacs by factor VIII bypassing therapy. In PRP it reflects the abnormalities encountered in von Willebrand disease and Glanzmann and Bernard-Soulier thrombopathy as well as the action of antiplatelet drugs.

Blood Coagulation Disorders↗
PubMed2004

Thrombin generation for the control of heparin treatment, comparison with the activated partial thromboplastin time.

Heparin can be quantified with antifactor Xa and IIa tests (aXa, aIIa) but the anticoagulant power of heparin depends upon plasma properties as well as upon heparin concentrations and thus differs between subjects. Measuring the effect, as with the activated partial thromboplastin time (APTT) therefore is clinically more relevant. Here we investigate the use of the endogenous thrombin potential (ETP) for this purpose. In 12 volunteers 9000 IU of four heparins of different mol. wt distributions were injected. Samples were taken at 11 time points between 0 and 24 h. With the exception of the 0 and 24-h time points, heparin could be demonstrated by its aIIa and aXa activity in virtually all samples. The APTT showed the effect of this heparin in 34% of the samples; the ETP in 80%. This is partly due to the wide margins of the normal values, caused by large interindividual variation [coefficient of variation (CV) approximately 12% for the APTT, approximately 17% for the ETP]. The intraindividual variation is much smaller (CV approximately 4% for the APTT, approximately 5% for the ETP). Relative to the baseline value of the individual, the heparin effect was recognized by the APTT in 55% of the cases and by the ETP in 98%. There were no large differences between the different types of heparin.

Adolescent↗
PubMed2003

Calibrated automated thrombin generation measurement in clotting plasma.

Calibrated automated thrombography displays the concentration of thrombin in clotting plasma with or without platelets (platelet-rich plasma/platelet-poor plasma, PRP/PPP) in up to 48 samples by monitoring the splitting of a fluorogenic substrate and comparing it to a constant known thrombin activity in a parallel, non-clotting sample. Thus, the non-linearity of the reaction rate with thrombin concentration is compensated for, and adding an excess of substrate can be avoided. Standard conditions were established at which acceptable experimental variation accompanies sensitivity to pathological changes. The coefficients of variation of the surface under the curve (endogenous thrombin potential) are: within experiment approximately 3%; intra-individual: <5% in PPP, <8% in PRP; interindividual 15% in PPP and 19% in PRP. In PPP, calibrated automated thrombography shows all clotting factor deficiencies (except factor XIII) and the effect of all anticoagulants [AVK, heparin(-likes), direct inhibitors]. In PRP, it is diminished in von Willebrand's disease, but it also shows the effect of platelet inhibitors (e.g. aspirin and abciximab). Addition of activated protein C (APC) or thrombomodulin inhibits thrombin generation and reflects disorders of the APC system (congenital and acquired resistance, deficiencies and lupus antibodies) independent of concomitant inhibition of the procoagulant pathway as for example by anticoagulants.

Anticoagulants↗
PubMed2003

Thrombinography shows acquired resistance to activated protein C in patients with lupus anticoagulants.

In patients with lupus anticoagulants (LA), acquired resistance to activated protein C (APC) is difficult to demonstrate with clot-based assays due to the presence of the anticoagulant. Via the conversion of a fluorogenic substrate (thrombinography), we monitored the complete process of thrombin formation and decay and its delimitation by the protein C system in eight consecutive LA-patients without anticoagulant therapy and non-carriers of the V Leiden polymorphism. Thrombin generation was triggered in platelet-poor and platelet-rich plasma by recalcification in the presence of a low concentration of tissue factor. In 7 out of 8 patients we observed a long lag-time before the thrombin burst (LA effect) together with a marked inability of APC to diminish the thrombin activity. The lag-phase was however prolonged to some degree by APC. The effects were more outspoken in the presence of phospholipids from patients' platelets than with added phospholipids. Thrombinography thus demonstrates APC resistance in LA-patients despite the occurrence of long lag-times (clotting times). The amount of thrombin activity generated in the presence of APC could be a better indicator of the thrombotic risk than the moment at which the thrombin burst starts.

Activated Protein C Resistance↗
PubMed2003

[The Choay domain -- the structure responsible for the anticoagulant action of heparins].

We describe the common structural basis for the anticoagulant action of the many different heparins available to the clinician. From different types of heparin we prepared fractions of virtually single molecular weight. We determined the molar concentration of material (HAM) containing the antithrombin (AT) binding pentasaccharide (A-domain), the specific catalytic activity in thrombin- and factor Xa inactivation and the capacity to inhibit thrombin generation (TG). We also calculated the molar concentration of A-domain with 12 sugar units at its non-reducing end, i.e. the structure that carries anti-thrombin activity (Choay- or C-domain). The anti-thrombin activity and the effects on TG are determined by the concentration of C-domain and independent of the source material or Mr. High Mr fractions (> 15,000) are less active, probably through interaction with non-AT plasma proteins. Anti-factor Xa activity is not indicative of anticoagulant potency but is a sensitive indicator of Mr and therefore predicts favourable pharmacokinetic properties: long half-life in the circulation and high bioavailability.

Anticoagulants↗
PubMed2002

Pharmacokinetic and pharmacodynamic characterization of a medium-molecular-weight heparin in comparison with UFH and LMWH.

Despite the well-established medical use of heparins, the question arises whether the efficacy-safety ratio of the available heparins can still be improved. Therefore, a medium-molecular-weight heparin (MMWH), a new heparin with an average molecular weight of 10.5 kDa and a narrow molecular weight range (9.5 to 11.5 kDa) was developed. Its in vitro activities amount to 174.9 anti-factor Xa (aXa) U/mg and 170.0 antithrombin (aIIa) U/mg. In the presented randomized, double-blind, cross-over study in healthy volunteers, the pharmacokinetics and pharmacodynamics of MMWH are compared with those of an unfractionated heparin (UFH) and a low-molecular-weight heparin (LWMH; enoxaparin). After subcutaneous administration of 9000 aXa-U of either heparin in 16 volunteers, the prolongation of the activated partial thromboplastin time (aPTT), the aXa activity, and the aIIa activities were determined at 11 time points spread over 24 hours after injection. The ex vivo analysis revealed striking pharmacodynamic and pharmacokinetic differences between the three heparins. UFH had the lowest bioavailability regarding the aPTT, aXa, and aIIa activities. Enoxaparin exhibited only low aIIa activity but the highest aXa activity. Unlike UFH and enoxaparin, MMWH showed a high recovery of aIIa activity, which suggests that it combines the high potency to inhibit thrombin that characterizes UFH with the high bioavailability of the LMWHs. Consequently, substantially lower doses are needed to bring about effects comparable to those of UFH and LMWH.

Adult↗
PubMed2002

The thrombogram in rare inherited coagulation disorders: its relation to clinical bleeding.

We investigated the relation between clotting factor concentration, the parameters of the thrombin generation curve (the thrombogram) and the severity of clinically observed bleeding in patients with congenital deficiency of prothrombin (n = 21), factor V (n = 22), factor VII (n = 22), factor X (n = 10), factor XI (n = 7) and factor XII (n = 6). The parameters used were: area under the curve (endogenous thrombin potential, ETP), peak concentration of thrombin attained and lag time before manifest formation. Peak height and ETP varied linearly with the concentration of prothrombin. For the other factors these parameters hyperbolically approached to the 100% limit with increasing clotting factor concentration. Half normal ETP was seen at about the following concentrations: prothrombin (50%), factor V (1%), factor VII (2%), factor X (5%) and factor XI (1%). As a rule, the peak height was some-what more sensitive to clotting factor decrease than the ETP was. In all the patients with severe bleeding symptoms the ETP was less than 20% of normal. Bleeding tendency was absent or mild in patients with an ETP of 30% or higher. This value (except for prothrombin) is already obtained at concentrations of clotting factor of 1%-2%, which corroborates the clinical observation that a severe bleeding tendency is only seen in severe clotting factor deficiencies (less than 1%). The one exception was a patient with factor VII deficiency and severe bleeding, who showed a normal ETP value, albeit with a decreased peak height and a prolonged lag-time.

Blood Coagulation Tests↗
PubMed2002

Thrombin-induced hyperactivity of platelets of young stroke patients: involvement of thrombin receptors in the subject-dependent variability in Ca2+ signal generation.

Activated platelets are implicated in the development of premature arterial vascular diseases, in particular ischemic stroke. Since elevated cytosolic [Ca(2+)](i) is an integrative marker of platelet activation, we determined the generation of Ca(2+) signal in stimulated platelets from 26 young patients recuperating from stroke, 20 patients with symptomatic peripheral arterial disease, and 56 healthy volunteers. Even in the presence of aspirin, the platelets from various individuals showed highly different thrombin-induced Ca(2+) responses. On average, the thrombin-induced Ca(2+) response was increased for platelets from either patient group in comparison to the controls (P <0.04). Relatively more stroke patients had high-responsive platelets (27%, 7/26) than patients with peripheral arterial disease (10%, 2/20) or healthy subjects (4%, 2/56). The average prothrombinase activities of platelets from patients and controls were similar, but 3 out of 6 patients with increased thrombin-induced Ca(2+) responses also exhibited high prothrombinase activity. In a follow-up study, the subject-dependent thrombin-induced Ca(2+) response was found to correlate strongly with the platelet response to protease-activated receptor 1 (PAR1) agonist (r = 0.91), but was not linked to the Pl(A1/2) polymorphism. It is concluded that a significant part of young patients with stroke have platelets with hyperactivity toward thrombin, which is not normalised by aspirin treatment. Furthermore, the subject-dependent variation in thrombin-induced signalling is likely to involve PAR1-mediated platelet activation.

Adult↗