Significant Differences in Neutralization of Heparin and Its Low Molecular Fragments by Protamine Sulfate: An In Vivo Study.

Abstract In clinical practice, patients receiving low molecular weight heparin (LMWH) occasionally suffer bleeding. Protamine sulphate (PS) is often used to reverse the anticoagulant effect of LMWH in such cases. However, the optimal regimen of PS for complete neutralization of LMWH fragments has not been established. Results from our previous in vitro studies indicate that the ability of PS to neutralize LMWHs is inversely related to the charge of the low molecular weight heparin molecule; more heavily charged LMWHs (such as Tinzaparin) are more readily neutralized than less charged LMWHs (such as Enoxaparin). The aim of the current study was to confirm these findings using an in vivo model. Twenty minutes after administration of either saline, unfractionated heparin [UFH, 100U/kg], Tinzaparin [100U/kg] or enoxaparin [100U/kg], 50% of anesthetized rabbits received either saline or PS [1 mg/100 U of heparin or LMWH]. The efficacy of PS neutralization was assessed by serial measurements of anti-factor Xa heparin plasma levels. Results are presented as mean of the anti-factor Xa heparin activities normalized to the level at 10 minutes and summarized in the table below. As expected, PS completely neutralized the anti-factor Xa effect of UFH. However, PS was significantly less effective for neutralization of Tinzaparin (about 66%) and Enoxaparin (about 44%) at the dose tested. We conclude that when tested in an in vivo model LMWHs vary in their protamine neutralizability. More highly charged LMWHs (e.g. Tinazaparin) are more neutralizable than less highly charged products (e.g. Enoxaparin). Residual Anti-Xa heparin effect Time Enoxaparin Tinzaparin UFH Saline + PS Enoxaparin +PS Tinzaparin + PS UFH + PS 10 min 1.00 1.00 1.00 0 1.00 1.00 1.00 20 min 0.85 0.86 0.70 0 0.80 0.85 0.89 Protamine 25 min 0.71 0.75 0.69 0 0.45 0.29 0.01 35 min 0.68 0.64 0.48 0 0.40 0.28 0 50 min 0.48 0.32