Comparison of the antithrombotic and haemorrhagic effects of heparin and a new low molecular weight heparin in rats.

A new low molecular weight heparin (LMWH) and a conventional unfractionated heparin (H) were tested in rats for venous antithrombotic activity and bleeding tendency after intravenous administration. Both drugs showed antithrombotic activity, but LMWH at the low dosages tested (0.1 and 0.25 mg/kg) demonstrated significantly higher activity than H. In a rat bleeding time test (transection model) both heparins produced a prolonged bleeding time, but LMWH possessed significantly less potency than H at all the dosages tested. Ex vivo coagulation parameters (activated partial thromboplastin time and antiXa activity) were also evaluated: LMWH presented very low activity on the APTT test and a sustained antiXa activity, comparable to that of H.