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

Abstract

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.

A Standard for Low Molecular Weight Heparin?

“…Johnson and Fareed on the use of a standard preparation for the comparison of the activities of low molecu lar weight heparins. The discussion was pro voked by two articles from Dr. Fareed's lab oratory published in a supplement for Hae mostasis [1,2].…”

Effects of a Supersulfated Low Molecular Weight Heparin (IK-SSH) on Different Hemostatic Parameters

“…However, in contrast to the anticoagulant activity of other LMWHs [16,[26][27][28] after IK-SSH administration, TT, anti-FIIa or anti-FXa activity remained practically unchanged. In studies with different LMWHs in primates all substances prolonged Heptest and had anti-FIIa and stronger antiFXa activity [16]. Similar data were published by Hoppensteadt et al [26] with a LMWH concerning Heptest.…”

“…From in vitro studies [15,16] it was expected that supersulfated LMWHs (SSH) would have a stronger inhibiting effect on platelet adhesion than LMWH or UFH. In animal experiments some SSH exerted a much stronger antithrombotic effect than their parent compounds, while the anticoagulant effects are equal or slightly higher [17][18][19].…”

Rational design of low-molecular weight heparins with improved in vivo activity

“…In addition to their lack of significant anti-IIa activity, another drawback of LMWHs, like enoxaparin, is that they are not easily neutralized through the addition of polycationic peptides, like protamine (30,34). Thus, the potent anticoagulant activity of LMWHs cannot be easily reversed in the event of bleeding complications or for its safe use in other indications.…”