Comparison of the platelet pro-aggregatory effect of conventional unfractionated heparins and a low molecular weight heparin fraction (CY 222).

Two unfractionated heparins (UH), a porcine intestinal mucosal heparin (PIM), a bovine lung heparin (BLH) and a low molecular weight heparin (LMWH), CY 222, were assessed for their capacity to enhance platelet aggregation in vitro. Their potential proaggregatory effect was investigated in four systems: (a) enhancement of submaximal platelet aggregation induced by conventional agonists in platelet rich plasma and (b) in whole blood; (c) reversal of inhibition of platelet aggregation induced by iloprost, a stable analogue of prostacyclin; (d) the direct aggregatory effect of these anticoagulants on hyperactive platelets prepared from patients with severe peripheral vascular disease or anorexia nervosa. Whereas BLH and PIM, at therapeutic concentrations, had a proaggregatory effect in all four systems, CY 222 had no significant effect when compared with the controls. BLH was more potent than PIM in three of the four systems studied. These observations confirm that conventional UH are more proaggregatory than LMWH, and thus the latter may be potentially safer. These observations are also consistent with the fact that BLH administration causes thrombocytopenia more frequently than PIM. The direct activation by UH of platelets from patients not previously exposed to heparin also challenges the hypothesis that heparin-induced platelet activation and thrombocytopenia is solely mediated by classical heparin-dependent immune mechanisms.