In vitro studies on the biochemistry and pharmacology of low molecular weight heparins.

Low molecular weight heparins (LMWHs) are obtained from unfractionated heparin by diverse chemical and enzymatic processes and findings with one LMWH cannot be extrapolated to another. Functional assays performed in vitro, evaluating antiprotease activity mediated via antithrombin III, heparin cofactor II interactions, antithrombin III binding, and plasma protein binding, showed wide variations between LMWHs, indicating that compositional differences among the LMWHs have a major impact on function. Evaluation in vitro showed varying potency in United States Pharmacopeia (USP) and anti-Xa assays. LMWHs tested at anti-Xa-adjusted concentrations exhibited varying potencies with anti-IIa, Heptest, and activated partial thromboplastin time (APTT) assays. Evaluation of these assays showed differences between LMWHs and a link with molecular weight. Each LMWH also varied in the in vitro neutralization by platelet factor 4, thrombin, and heparinase. LMWHs also varied in platelet interactions as assessed by whole blood clotting, thromboelastography and P-selectin expression, and in tissue factor pathway inhibitor release in cell culture. It was concluded that compositional variations in LMWHs give each product a unique biochemical profile. This profile, plus varying endogenous interactions and pharmacokinetic profiles may give rise to the clinical differences observed with LMWHs in specific indications.