Comparative study on the in vitro and in vivo activities of seven low-molecular-weight heparins

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 IIa 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 IIa 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.