Studies on the Antithrombotic Effects and Pharmacokinetics of Heparin Fractions and Fragments

Fareed, Jawed; Walenga, Jeanine M.; Williamson, Karen; Emanuele, Rossella; Kumar, Arvind; Hoppensteadt, Debra

doi:10.1055/s-2007-1004360

Cited by 63 publications

(42 citation statements)

References 49 publications

(57 reference statements)

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“…It is important to consider that LMW heparins differ considerably from unfrac tionated heparin in both in vitro and in vivo testing systems [6][7][8][9][10][11][12][13][14][15][16][17], These differences are further magnified in vivo due to complex endogenous interactions. The route of ad ministration and the formulations of the heparins further add to these complications.…”

Section: Discussionmentioning

confidence: 99%

Comparative Study on the in vitro and in vivo Activities of Seven Low-Molecular-Weight Heparins

Fareed

Jm²,

Hoppensteadt³

et al. 1988

Pathophysiol Haemos Thromb

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Different low-molecular-weight (LMW) heparins are produced by fractionation, enzymatic and chemical methods. Although the manufacturer’s assigned molecular weights of these agents are similar (around 5,000 daltons), they exhibit considerable molecular structural heterogeneity due to the variations in the manufacturing process. In vitro standardization of these LMW heparins produces highly variable results due to the variability of assay specificity. In a comparative study with seven LMW heparins, differences were found in molecular weight distribution, antifactor Xa activity, antifactor Πa activity, Heptest® activity, USP potency, platelet interactions, protamine and platelet factor 4 neutralization and charge density ratios. Relative antithrombotic actions varied between the seven agents and the ratio of the intravenous:subcutaneous effects were inconsistent with expected results based on in vitro properties or observed ex vivo effects. These products were not bioequivalent in equigravimetric or equipharmacopeial dosages (antifactor Xa, USP). Although the intravenous bioavailability was proportional to the potency for an individual agent, the subcutaneous bioavailability of the same agent showed differences from that of the intravenous regimen. In addition, significant differences between the bleeding profiles of these agents were noted in both intravenous and subcutaneous routes. The bleeding profiles did not correlate well with the relative proportion of the antifactor Xa component. However, some relationship to the bleeding effect was observed with the antifactor Xa and activated-partial-thromboplastin-time activities. These LMW heparins produced different effects on platelets as studied in the heparin-induced thrombocytopenia and platelet activation systems. These observations suggest that the individual composition of each LMW heparin determines its in vivo behavior which may account for the different safety/efficacy ratios observed in clinical trials.

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Section: Discussionmentioning

confidence: 99%

Comparative Study on the in vitro and in vivo Activities of Seven Low-Molecular-Weight Heparins

Fareed

Jm²,

Hoppensteadt³

et al. 1988

Pathophysiol Haemos Thromb

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“…The metabolism of the products is dose-dependent with the clearance rate increasing as the dose is lowered (19). The degradation-product concentrations anticipated in these studies are much lower than 1 mg/ml and the products would be cleared at a faster rate.…”

mentioning

confidence: 88%

Ex vivo model of an immobilized-enzyme reactor.

Bernstein

Langer

1988

Proc. Natl. Acad. Sci. U.S.A.

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Immobilized-enzyme reactors are beginning to be studied for a variety of therapeutic applications. To facilitate the design of these devices for different clinical situations and a diverse patient population, mathematical models may be valuable. An immobilized-heparinase (EC 4.2.2.7) reactor was selected as a model system. The device removes heparin from blood that has been anticoagulated to prevent thrombus formation. Heparinase was immobilized to crosslinked agarose particles. A mathematical model was developed to describe the clearance of heparin by the reactor ex vivo and compared to experimental clearances measured in sheep. The model accounted for enzymatic degradation as well as the binding of heparin and its breakdown products to antithrombin. The device was modeled as a steady-state continuously stirred tank reactor. Molar conservation equations within the agarose particles accounted for simultaneous diffusion and chemical reaction. The model had no adjustable parameters and was able to predict the clearance of heparin within 5-25% for three different animals and 12 different perfusions.Therapeutic applications of immobilized enzymes have been proposed for a variety of clinical situations ranging from cancer therapy to the treatment of neonatal jaundice (1-3). In all cases an immobilized-enzyme reactor serves as a drug-or toxin-removal system. The clinical application and the patient requirements determine the reactor size necessary to clear the agent from the blood. This wide range of device specifications is an obstacle to their gaining widespread usage. Mathematical models of the performance of enzyme reactors can potentially eliminate this problem by facilitating the design of devices for different clinical situations.Previous modeling of enzyme reactors has been attempted in systems that have been studied with saline or buffer as the medium (4-6). Systems perfused with blood are more complex, since blood proteins can serve as carrier molecules for hydrophobic or ionic substrate molecules. Analysis of such systems may require treating the free and unbound forms of the substrate separately, since the substrate-protein complex can behave differently than the free substrate.As a first step to developing models of enzyme reactors that are applicable to operation in blood, a particular system, immobilized heparinase (EC 4.2.2.7), was selected for mathematical analysis. Heparinase degrades the drug heparin, which is required to fully anticoagulate patients undergoing extracorporeal therapy (e.g., hemodialysis or membrane oxygenation) to prevent clotting within the extracorporeal circuit. Heparin is cleaved to nontoxic low molecular weight fragments that have much less anticoagulant activity than the parent compound heparin both in vitro and ex vivo (7,8).Heparinase was covalently immobilized to agarose to prevent the direct in vivo introduction of the enzyme as well as to enhance its stability (9). The gel is placed in an arterial blood filter through which the blood is passed and the heparin is d...

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“…The underlying reasons for this have been exten sively addressed by our group [12][13][14][15][16], In contrast to unfractionated heparins, the LMWHs exhibited marked heterogeneity, both physicochemically and biologically.…”

Section: Validation Of the Low-molecular-weight Heparin Standardmentioning

confidence: 99%