Mechanisms for the Antithrombotic Activity in Man of Low Molecular Weight Heparins (LMWHs)
Authors: Samama M.M. , Bara L. , Gerotziafas G.T. 
Published: April 2009

Although LMWH have been widely used for several years, the mechanisms of their anti-thrombotic action are not well understood. This is due in part to the misconception that in vitro coagulation results can be extrapolated to the in vivo antithrombotic condition. Thus, the mode of action of heparins on thrombin generation in plasma after addition of increasing amounts of unfractionated heparin (UH) or LMWH has been investigated in several studies, but a distinction has to be made between the effects on the intrinsic and the extrinsic system. The same applies for experiments conducted with platelet-poor plasma (PPP) and platelet-rich plasma (PRP). These experiments show that inhibition of prothrombin activation in PPP is more pronounced in the intrinsic system than in the extrinsic system for both types of heparin. This inhibition has been attributed mainly to their antithrombin activity which reduces factor V and factor VIII activation and its related positive feedback on thrombin formation. The comparison of the effects of low doses of LMWH and UH in citrated PRP indicate that LMWH are more active than UH because they are more resistant to platelet neutralization by platelet factor 4. Moreover, in vitro study will neglect the role of pharmacokinetic parameters which are important determinants of the antithrombotic activity of heparins. Another important difference between in vitro and in vivo studies lies in the fact that sc injection of both heparins will release tissue factor pathway inhibitor (TFPI) in the blood. We have used native whole blood for in vitro and ex vivo experiments. This offers the advantage of studying heparin activity in the presence of platelets and calcium. In this condition the inhibition of pro-thrombin and factor VII activation during blood coagulation in a glass tube (intrinsic system) was judged by measuring residual prothrombin and factor Vila in serum obtained 2 to 4 hours after clotting. At prophylactic doses, LMWHs in contrast to UH significantly inhibit prothrombin activation. Although thrombin inhibition seems essential for the antithrombotic activity of both heparins, reduction of thrombosis is a global effect to which “both anti-IIa and anti-Xa activity contribute but to a different exent” (C. Hemker et al.). It should be noted that the clearance half-life of anti-Xa activity is significantly longer than that of anti-IIa. The efficacy of a single daily injection of LMWH in the prophylaxis of thrombosis is not logical in light of the short half-life of anti-IIa activity. We demonstate in a study where twelve volunteers received a prophylactic dose of enoxaparin and UH that although anti-IIa activity determined in vitro is lower in LMWH as compared to UH, 1300 and 5000 IU, respectively, comparable activities were found ex vivo. As expected anti-Xa activity was greater when volunteers received 4000 anti-Xa IU of enoxaparin than when they received 5000 anti-Xa IU of UH. These results illustrate the important differences existing between in vitro and in vivo studies. In conclusion, the anti-Xa/anti-IIa ratio has always been considered as an essential distinguishing characteristics of LMWH as compared to UH. We demonstrate that this concept established according to in vitro measurements of anti-Xa and anti-IIa activity is misleading when comparing the mode of action “in vivo” of both types of heparin. Finally, it is likely according to the literature and our data that both anti-Xa and anti-IIa activities contribute to the antithrombotic activity of LMWH.