No abstract

Unwanted side effects of pharmacologically active compounds can usually be eliminated by structural modifications. But the complex heterogeneous structure of the polysaccharide heparin has limited this approach to fragmentation, leading to slightly better-tolerated heparin preparations of low molecular mass. Despite this improvement, heparin-induced thrombocytopaenia (HIT), related to an interaction with platelet factor 4 (PF4) and, to a lesser extent, haemorrhages, remain significant side effects of heparinotherapy. Breakthroughs in oligosaccharide chemistry made possible the total synthesis of the pentasaccharide antithrombin-binding site of heparin. This pentasaccharide represents a new family of potential antithrombotic drugs, devoid of thrombin inhibitory properties, and free of undesired interactions with blood and vessel components. To obtain more potent and well-tolerated antithrombotic drugs, we wished to synthesize heparin mimetics able to inhibit thrombin, that is, longer oligosaccharides. Like thrombin inhibition, undesired interactions are directly correlated to the charge and the size of the molecules, so we had to design structures that were able to discriminate between thrombin and other proteins, particularly PF4. Here we describe the use of multistep converging synthesis to obtain sulphated oligosaccharides that meet these requirements.

The importance of 3-O- and 6-O-sulfated glucosamine residues within the heparin octasaccharide iduronic acid(1)----N-acetylglucosamine 6-O-sulfate(2)----glucuronic acid(3)----N-sulfated glucosamine 3,6-di-O-sulfate(4)----iduronic acid 2-O-sulfate(5)----N-sulfated glucosamine 6-O-sulfate(6)----iduronic acid 2-O-sulfate(7)----anhydromannitol 6-O-sulfate(8) was determined by comparing with synthetic tetra- and penta-saccharides its ability to bind human antithrombin. The octasaccharide had an affinity for antithrombin of 1 X 10(-8) M (10.2 kcal/mol) measured by intrinsic fluorescence enhancement at 6 degrees C. The synthetic pentasaccharide, consisting of residues 2-6, had an affinity of 3 X 10(-8) M (9.6 kcal/mol). The same pentasaccharide, except lacking the 3-O-sulfate on residue 4, had an affinity of 5 X 10(-4) M (4.5 kcal/mol) measured by equilibrium dialysis. The tetrasaccharide, consisting of residues 2-5, bound antithrombin with an affinity of 5 X 10(-6) M (6.8 kcal/mol). The tetrasaccharide, consisting of residues 3-6, had an affinity of 5 X 10(-5) M (5.5 kcal/mol). Since the loss of either the 6-O-sulfated residue 2 or the 3-O-sulfate of residue 4 results in a 4-5 kcal/mol or a 40-50% loss in binding energy of the pentasaccharide, these two residues must be the major contributors to the binding and must be linked to the biologic activity of the octasaccharide.

SummaryThrombin generation (TG) initiated by diluted tissue-factor was investigated in whole human blood, in platelet-rich plasma (PRP), in platelet-poor plasma (PPP), and in PPP supplemented with red blood cells (RBCs). TG was characterized by the lag time preceding the thrombin burst and by the endogenous thrombin potential (ETP). RBCs at normal haematocrit were found to influence the lag time to the same extent as platelets. When TG was carried out in PRP or in PPP + RBCs, both the ETP and lag time were dependent on the platelet count or on the haematocrit, but the shapes of the dose-response curves were different. The inhibition of TG in PPP+ RBCs by two direct thrombin and factor Xa inhibitors: hirudin and DX 9065A, and two antithrombin III (AT)-dependent anticoagulants: heparin and SR 90107A was found to be similar to that previously described in PPP and in PRP: hirudin and DX 9065A only delayed TG whereas heparin and SR 90107A both delayed and decreased TG. FACscan analysis following labelling with FITC-annexin V or with phycoerythrin-labelled anti-glycophorin A of samples taken in the course of TG initiated in PPP + RBCs showed that no significant haemolysis occurred and revealed that 0.51 ± 0.075% (mean ± sem, n = 3) of RBCs steadily exposed procoagulant phospholipids on their outer surface throughout the TG course. Furthermore, incubation of factors Xa and Va with washed RBCs sampled during TG in PPP +RBCs resulted in a significant and constant prothrombinase activity.Taken together, these data show for the first time that normal RBCs may participate in the haemostatic process through exposure of procoagulant phospholipids.

The chemical composition and the 13C n.m.r. spectra of heparin oligosaccharides (essentially octasaccharides), having high affinity for antithrombin III and high anti-(Factor Xa) activity, prepared by three independent approaches (extraction, partial deaminative cleavage with HNO2 and partial depolymerization with bacterial heparinase), leading to different terminal residues, have been studied and compared with those of the corresponding inactive species. Combined with chemical data, the spectra of the active oligosaccharides and of their fragmentation products afforded information on composition and sequence. The three types of active oligosaccharides were shown to have the common hexasaccharide core