Peter Bacher

Dose-finding and pharmacokinetic profiles of prophylactic doses of a low molecular weight heparin (reviparin–sodium) in pediatric patients

INTRODUCTION The low molecular weight heparin (LMWH), reviparin-sodium was studied in dose-finding and pharmacokinetic studies in children with central venous lines (CVLs). MATERIALS AND METHODS The dose-finding study was performed in 24 patients aged 3 days to 16 years. Dose adjustments were made using a nomogram based on anti-factor Xa levels (units (U)/ml) (target of 0.1-0.3 U/ml). The pharmacokinetic study was performed in 19 patients, 9 less than or equal to 5 kg (7 of whom were less than 3 months) and 10 greater than 5 kg (all more than 3 months). RESULTS The dose-finding study demonstrated that children over 5 kg required 30 International Units (IU)/kilogram (kg), subcutaneous (SC) twice daily (BID), and children less than or equal to 5 kg required 50 IU/kg, SC BID, to achieve target levels. The pharmacokinetic study demonstrated that 80% of anti-factor Xa levels were within the target range with both patient groups having similar peak (average=0.26 U/ml) and trough (average=0.13 U/ml) levels. CONCLUSIONS Peak anti-factor Xa levels (0.1-0.3 U/ml) using reviparin-sodium are achieved by administering 50 IU/kg in children greater than 3 months of age and 30 U/kg in children less than 3 months of age.

Differential effects of clivarin and heparin in patients undergoing hip and knee surgery for the generation of anti-heparin-platelet factor 4 antibodies

The pathophysiology of heparin-induced thrombocytopenia (HIT) syndrome is mediated via a heterogeneous group of heparin(s)-platelet factor 4 (H-PF4) complexes bound to their antibodies. These anti-H-PF4 (AHPF4) antibodies that are capable of binding to the FcgammaRIIA receptor [cluster of differentiation (CD) 32] on platelets, resulting in platelet activation, widely vary in their specific activities as platelet activation (functionality). Predisposing factors related to specific pathologic conditions may also contribute to the generation of these antibodies and their relative functionality during HIT syndrome. To understand this phenomenon, a sub-study was carried out in patients undergoing elective total hip and knee replacement surgery (ECHOS Study) and who were treated with unfractionated heparin (UFH) and a low-molecular-weight heparin (LMWH; Clivarin). Approximately 600 patients per arm [UFH=7,500 anti-Xa U twice a day (b.i.d.) subcutaneous (s.c.) and clivarin=4200 U once daily (o.d.) s.c.], age >40 years, received prophylactic treatment for a minimum of 11-14 days. Plasma samples were collected at pre-dose, days 2-4, days 11-14 and at follow-up 6-8 weeks after discharge and were analyzed for AHPF4 antibody titers. Functionality of the enzyme-linked immunosorbant assay (ELISA)-positive AHPF4 antibodies to cause platelet activation was tested by 14C-serotonin release assay (SRA). Both UFH and clivarin treatments in orthopedic surgical patients resulted in a progressive generation of AHPF4 antibodies. The relative prevalence/functionality of AHPF4 antibodies in clivarin arm was markedly lower (two- to threefold, p<0.001) as compared to UFH at each time point. Most of the samples in clivarin group were found to be SRA negative, suggesting the presence of AHPF4 antibodies that did not activate platelets (nonfunctional). Within the UFH arm, the relative prevalence/functionality of AHPF4 antibodies was much higher (p<0.002) in knee group compared to the corresponding hip group. This study, for the first time, reports on the elevated levels of AHPF4 antibodies generated by heparin associated with the pathogenesis of knee surgery. Clinical significance of the differential generation of HIT-associated antibodies remains unexplored at this time.

The thrombolytic potency of LMW-heparin compared to urokinase in a rabbit jugular vein clot lysis model.

Since the beginning of the clinical use of low molecular weight (LMW) heparins their thrombolytic or profibrinolytic potency has been a matter of controversial discussions. Regarding this problem, the aim of our study was to test a LMW-heparin (Sandoparin) in an in vivo model comparing its lytic activity to unfractionated heparin and urokinase at different doses. For this purpose a newly developed short-term rabbit jugular vein clot lysis model was developed. Urokinase infused at doses of 3300, 6600 and 10,000 U/kg to control animals for one hour showed a clear dose-dependent clot lysis. Test animals were injected with a bolus of 0.5 mg/kg of LMW-heparin followed by a constant infusion of either 0.5, 1.0 or 2.0 mg/kg for one hour. A similar dose-dependent effect was observed for LMW-heparin as for urokinase. Unfractionated heparin did not exhibit a dose-dependent lytic activity in this model. No lysis was found in rabbits treated with saline. These findings suggest that the LMW-heparin tested exhibits a dose-dependent in vivo lytic activity which can be compared to clinically effective doses of urokinase, and that this activity is not present with unfractionated heparin.

Pharmacological modulation of fibrinolysis by antithrombotic and cardiovascular drugs

vv HEREAS THE pharmacology of thrombolytic therapy is usually discussed in terms of agents capable of directly activating fibrinolysis, many nonthrombolytic drugs produce profibrinolytic effects that are mediated by indirect endogenous mechanisms. These profibrinolytic effects may involve the following: (1) release of endogenous tissue plasminogen activator (tPA), (2) reduction of plasminogen activator inhibitor (PAI) titres, (3) modification of serine proteases involved in the fibrinolytic process, and (4) modification of the fibrinogen molecule. Several drugs exert a stimulating effect on the endothelium, facilitating the release of plasminogen activators that may produce localized and/or systemic lytic effects. Initially, the indirect profibrinolytic effects of drugs were not detectable due to the limited sensitivity of conventional tests such as the fibrinogen level and euglobulin lysis time. However, with the development of newer ultrasensitive laboratory methods for the assessment of the fibrinolytic system, subclinical activation of the fibrinolytic system can now be readily measured. Table 1 lists some of the factors that contribute to the status of the fibrinolytic state. This physiological state can be affected by drugs and mechanical or physical manipulation. The mechanism by which drugs are capable of producing a fibrinolytic state are multiple. It is now well known that several inhibitors of fibrinolysis control this process. A decrease in the levels of these inhibitors or pharmacological inhibition of their function produces a profibrinolytic state. Inhibitors, such as PAIs, a,-antiplasmin (a,-AP), and histidine-rich glycoprotein (HRG), produce marked inhibition of the fibrinolytic process.4,s6,96 A decrease in these inhibitors r&sults in the facilitation of fibrinolysis. On the other hand, an increase in the levels of fibrinolytic activators, such as tPA, urokinase plasminogen activator (uPA), and related proteases may augment fibrinolysis. Many drugs are able to modify the fibrinogen molecule making it more susceptible to the action of plasmin.lo8 Similarly, enzymes such as plasmin, plasminogen activator, and urokinase can be modified by many pharmacological agents rendering these more efficient in potentiating the fibrinolytic process.147 Physical stimuli and pharmacological agents are both capable of releasing endogenous activators of fibrinolysis. This endogenous activation results in the net augmentation of fibrinolysis. Table 2 lists the cellular sites thought to be involved in modulation of the fibrinolytic system. Many cardiovascular drugs are capable of effecting these cellular sites where both the activators and inhibitors of fibrinolysis are produced. Individual drugs produce specific effects, with commonly used cardiovascular drugs causing both potentiation and inhibition of the fibrinolytic system. Endothelial cells and fibroblasts produce various modulators that contribute significantly to the regulation of the fibrinolytic process by drugs. Release of plasminogen activators and inhibitors from vascular storage sites due to drug action is now well documented. Thus, with the introduction of new cardiovascular drugs, their modulatory effects on the fibrinolytic system need to be considered. Some drugs that modulate the fibrinolytic process and the proposed mechanism by which this occurs are listed in Table 3. Different pharmacological agents produce variable effects on the fibrinolytic system. Glycosaminoglycans, such as heparin and dermatan, produce profibrinolytic effects. These drugs release endogenous tPA into the circulation and facilitate the generation of several other profibrinolytic mediators. The effects of glycosaminoglycans signifi-

Sustained Release of Tissue Factor Following Thrombosis of Lower Limb Trauma

This study was undertaken to provide evidence for the mechanism of venous thromboembolism (VTE) in healthy patients with minor lower limb injury (fracture; Achilles tendon rupture) that was medically managed with plaster cast/brace immobilization. The Plaster Cast clinical trial provided a unique opportunity to identify the natural history of VTE using placebo-controlled patients (n = 183) with validation of the mechanism using the low-molecular-weight heparin (LMWH; reviparin)-treated patients (n = 182). Confirmed VTE in this population was associated with a burst of tissue factor release (and a minor fibrinolytic deficit) leading to thrombin generation that was sustained at least 5 weeks, greater with fractures than with soft-tissue injuries and greater with surgery than with conservative treatment. The root cause likely involves platelet/leukocyte activation (inflammation) rather than endothelial cell injury. Thromboprophylaxis with a low dose of LMWH reduced thrombin generation, with patients undergoing surgery benefitting the most.

The antithrombotic and anticoagulant effects of a synthetic tripeptide and recombinant hirudin in various animal models.

The pharmacologic activities of two thrombin inhibitors (D-MePhe-Pro-Arg-H, recombinant-hirudin) were compared in two animal models. The antithrombotic effect was investigated in vivo in rabbits using a modified Wessler stasis thrombosis model. During these experiments, blood was drawn for ex vivo testing to determine the coagulation profile and to determine plasma concentrations using pre-constructed calibration curves. A dose-dependent antithrombotic effect was observed for both agents. On an equigravimetric basis (100 micrograms/kg i.v.), r-hirudin showed a stronger antithrombotic effect than the tripeptide, which correlated well with the ex vivo anticoagulant effect. No adverse reactions were observed during this study. In a rabbit ear bleeding model, a dose and time dependent hemorrhagic effect was observed for both agents. Only slight bleeding effects were observed at 1.0 mg/kg dosages. These studies show that the tripeptide D-MePhe-Pro-Arg-H and r-hirudin are specific thrombin inhibitors with potent antithrombotic effects and a high therapeutic (antithrombotic/hemorrhagic) index. Furthermore, the results of these two animal models and ex vivo analyses can be used to determine the therapeutic index of thrombin inhibitors.

Differential effects of unfractionated heparin and low-molecular-weight heparins on tissue thromboplastin inhibition test.

Circulating anticoagulants are endogenously pro duced substances that interfere with in vitro tests of coagulation like activated partial thromboplastin time (APTT), and cause prolongation of the clotting times. Evaluation of the abnormal APTT involving various factor assays and mixing studies may provide inconclusive and ambiguous results. Tissue thrombo plastin inhibition test (TTIT) is one of the screening assays for detection of circulating anticoagulants. However, this test is influenced by the presence of unfractionated heparin (UFH) from concentrations 0.2 U/mL and higher. Since low- molecular-weight heparins (LMWHs) are increasingly used for the prevention of thrombotic disorders and may replace UFH in the future, in this study the authors studied the influence of LMWHs on the performance of TTIT and compared the results with UFH. UFH and LMWHs showed a prolongation of TTIT in the concentration range of 0.25-1.0 U/mL. The marked pro longation of the TTIT with UFH and different LMWHs is in decreasing order of UFH > ardeparin > tinzaparin > dalteparin > enoxaparin. Patients with circulating anticoagulants who are given LMWHs may have false-positive results of TTIT and this influence should be kept in mind during patient management.

Advances in Heparins and Related Research. An Epilogue

The discovery of heparin in 1916 by Jay McLean, a medical student at Johns Hopkins University, not only provided a universal anticoagulant, but also laid the foundation for the discipline of hemostasis and thrombosis[...].

A simulated post-angioplasty low molecular weight heparin schedule in a non-human primate model

Due to a variety of pathophysiologic processes the long-term success rate of percutaneous transluminal angioplasty (PTCA) is only 50-70%. Acute restenosis also occurs in 30-40% of patients. Currently, studies are in progress to investigate the influence of low molecular weight heparin (LMWH) prophylaxis on the patency rate after PTCA. Our aim was to determine an optimal schedule to start the LMWH prophylaxis after the routinely performed heparinization. The alterations of the hemostatic parameters during the drug regimen change-over were evaluated. Non-human primates (Macaca mulatta) were divided into 6 treatment groups (n = 3/group). Three groups received heparin i.v. at 15 U/kg to mimic the end phase of therapeutic treatment infusion given 12-24 hrs. after PTCA. Three groups received full heparinization (250 U/kg i.v.) to mimic patients without the above interim phase therapy. Following both regimens, LMWH (Mono-Embolex) (1 mg/kg s.c.) was started at various intervals. The group initially treated with 15 U/kg heparin exhibited a continued anticoagulant effect when LMWH was started 30 min. after the heparin injection. Whereas, when LMWH was started after 2 hrs. the measurable anticoagulant effect was lost during 1 and 3 hrs. after the heparin injection. When LMWH was started 2 or 4 hrs. after the 250 U/kg dose, the anticoagulant response was sustained. Aside from the anti-IIa and the anti-Xa activity, there was no significant difference in other coagulation parameters between these two regimens. The fibrinolytic system was not altered in the therapeutic heparinization group. However, after the initial bolus of 250 mg/kg heparin, the monkeys treated with LMWH exhibited higher t-PA and D-dimer levels. Although our data shows definite differences between the two drug treatment schedules, further studies are warranted before an optimal drug regimen can be suggested for clinical use.