NEUTRALASE(TM) REVERSES THE ANTI-COAGULANT BUT NOT THE ANTITHROMBOTICACTIVITY OF HEPARIN IN A RABBIT MODEL OF VENOUS THROMBOSIS

Neutralase(TM) (heparinase I; E.C. 4.2.2.7) is a heparin-degrading enz yme undergoing clinical evaluation as an alternative to protamine for reversing the anticoagulant effects of heparin in coronary bypass surg ery. The objective of this study was to assess the relative effects of Neutralase and protamine on reversal of heparin-dependent elevations in coagulation parameters and inhibition of clot formation in a rabbit vena caval stasis model, Rabbits were treated with saline or heparin (300 U/kg) for 10 minutes, followed by saline, protamine (2.6 mg/kg), or Neutralase (10 or 30 mu g/kg, representing 1.23 IU/kg and 3.69 IU/k g, respectively), Twenty minutes later, venous stasis was induced, and vena caval clots were excised, weighed, and characterized. Coagulatio n parameters [activated partial thromboplastin time (aPTT) and thrombi n clotting time (TCT)] and antiFactor IIa and Xa levels were measured throughout the protocol. Both protamine and Neutralase reversed hepari n-mediated increases in aPTT (>300 seconds to 26-35 seconds) and TCT ( >300 seconds to 29-56 seconds) to values that were not different from saline-treated, nonheparinized animals. Thrombus weight in the nonhepa rinized saline group was 62+/-7 mg; heparin-treated animals had no det ectable clots. Protamine reversal of heparin was associated with clot formation (89+/-20 mg) while Neutralase reversal was not (no clots). H eparin-induced increases in antiFactor IIa activity were reversed sim ilarly by protamine and Neutralase (from 4.3-8.8 U/ml to 0.2-0.3 U/ml) while antiFactor Xa activity was differentially reversed (from 3.9-5. 9 U/ml to 0.7-1.3 U/ml Neutralase; 5.5 U/ml to 0.02 U/ml protamine). T hese results are consistent with a hypothesis that Neutralase cleaves heparin into fragments, which are devoid of antiFactor IIa activity th at retain modest antiFactor Xa activity, resulting in reversal of anti coagulant, but not antithrombotic, heparin activity. This property of Neutralase may be beneficial in reducing post-surgical thrombotic even ts after reversal of heparin. (C) 1998 Elsevier Science Ltd.