MECHANISMS FOR THE ANTITHROMBOTIC ACTIVITY IN MAN OF LOW-MOLECULAR-WEIGHT HEPARINS (LMWHS)

Although LMWH have been widely used for several years, the mechanisms of their anti-thrombotic action are not well understood. This is due i n part to the misconception that in vitro coagulation results can be e xtrapolated 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 in vestigated in several studies, but a distinction has to be made betwee n the effects on the intrinsic and the extrinsic system. The same appl ies for experiments conducted with platelet-poor plasma (PPP) and plat elet-rich plasma (PRP). These experiments show that inhibition of prot hrombin activation in PPP is more pronounced in the intrinsic system t han in the extrinsic system for both types of heparin. This inhibition has been attributed mainly to their antithrombin activity which reduc es factor V and factor VIII activation and its related positive feedba ck on thrombin formation. The comparison of the effects of low doses o f LMWH and UH in citrated PRP indicate that LMWH are more active than UH because they are more resistant to platelet neutralization by plate let factor 4. Moreover, in vitro study will neglect the role of pharma cokinetic parameters which are important determinants of the antithrom botic activity of heparins. Another important difference between in vi tro and in vivo studies lies in the fact that sc injection of both hep arins 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 presen ce of platelets and calcium. In this condition the inhibition of pro-t hrombin and factor VII activation during blood coagulation in a glass tube (intrinsic system) was judged by measuring residual prothrombin a nd factor VIIa in serum obtained 2 to 4 hours after clotting. At proph ylactic doses, LMWHs in contrast to UH significantly inhibit prothromb in activation. Although thrombin inhibition seems essential for the an tithrombotic activity of both heparins, reduction of thrombosis is a g lobal 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 t han 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 demonstrate in a study where twelv e volunteers received a prophylactic dose of enoxaparin and UH that al though anti-IIa activity determined in vitro is lower in LMWH as compa red to UH, 1300 and 5000 IU, respectively, comparable activities were found ex vivo. As expected anti-Xa activity was greater when volunteer s received 4000 anti-Xa IU of enoxaparin than when they received 5000 anti-Xa IU of UH. These results illustrate the important differences e xisting between in vitro and in vivo studies. In conclusion, the anti- Xa/anti-IIa ratio has always been considered as an essential distingui shing characteristics of LMWH as compared to UH. We demonstrate that t his 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 activiti es contribute to the antithrombotic activity of LMWH.