Anti-Thrombotic Effect of a Synthetic Heparin Mimetic in an Arterial Model of Thrombosis in Rats: Comparison with Unfractionated Heparin and Nadroparin

Progress has been made recently in the synthesis of heparin oligosaccharides. It is now possible to obtain low molecular weight heparin mimetics composed of the pentasaccharidic sequence allowing the binding to antithrombin, of a neutral spacer and of the thrombin binding domain (1). The biological properties of such a compound (SanOrg 123781) have been recently reported in this journal (2, 3). This oligosaccharide has a comparable anti-Xa and anti-IIa activity, does not react with platelet factor 4 and exhibits a more potent antithrombotic effect than unfractionnated heparin (UH) when given by IV route, 5 min before the thrombogenic challenge. We report on the anti thrombotic activity of SanOrg 122429 A, another heparin mimetic having biological properties close to those of SanOrg 123781 A. This oligosaccharide is made of 17 saccharide units and has a MW of 5322 Da. The antithrombotic effect of SanOrg 122429 A was compared to those of UH and nadroparin in an arterio-arterial shunt placed between the 2 carotids of wistar rats (4, 5). The thrombogenic surface was a glass capillary (internal diameter 0.8 mm) coated with collagen, resulting in a platelet rich thrombus occluding the device in 301 ± 23 s (mean ± SD) in vehicle treated animals. The 3 compounds were given by sc route 2 h before the shunt was mounted on the basis of equivalent antifactor Xa units. For this purpose, UFH was calibrated against the 5 th international standard for UFH while nadroparin and SanOrg 122429 A were calibrated against the first international standard for LMWH in rat plasma. SanOrg 122429 A exhibited specific anti-Xa and anti-IIa activities of 551 and 586 U respectively. A citrated blood sample was taken to determine plasma anti-Xa and anti-IIa activities (chromogenic assays) at the beginning of the thrombosis model. For the 3 compounds, there were dose-dependent prolongations of the occlusion time of the shunt, but SanOrg 122429 A was more potent (p < 0.001) than UH and than nadroparin which were in turn equipotent (variance analysis with log-dose as covariant). Due to the non-linear pharmacokinetic properties of UH, the anti-Xa and anti-IIa activities were not correlated to the dose delivered. For nadroparin, these activities resulted from the high in vitro anti-Xa/antiII ratio of this LMWH and from the higher clearance of the anti-IIa activity after sub-cutaneous administration. For SanOrg 122429A, the 2 activities were roughly comparable but considerably higher than those generated by an equivalent dose of nadroparin. This results from the lower clearance of this heparin mimetic due to its high affinity for anti-thrombin. Thus, in addition to the other intrinsic properties of the molecule, including its lesser ability to be neutralized by platelet factor 4, this observation accounts for the higher antithrombotic effect of an equivalent dose of such oligosaccharide. Anti-Thrombotic Effect of a Synthetic Heparin Mimetic in an Arterial Model of Thrombosis in Rats: Comparison with Unfractionated Heparin and Nadroparin Thromb Haemost 2002; 88: 687–8