On designing non-saccharide, allosteric activators of antithrombin.

Antithrombin, a plasma glycoprotein serpin, requires conformational activation by heparin to induce an anticoagulant effect, which is mediated through accelerated factor Xa inhibition. Heparin, a highly charged polymer and an allosteric activator of the serpin, is associated with major adverse effects. To design better, but radically different activators of antithrombin from heparin, we utilized a pharmacophore-based approach. A tetrahydroisoquinoline-based scaffold was designed to mimic four critical anionic groups of the key trisaccharide DEF constituting the sequence-specific pentasaccharide DEFGH in heparin. Activator IAS(5) containing 5,6-disulfated tetrahydroisoquinoline and 3,4,5-trisulfated phenyl rings was found to bind antithrombin at pH 7.4 with an affinity comparable to the reference trisaccharide DEF. IAS(5) activated the inhibitor nearly 30-fold, nearly 2- to 3-fold higher than our first generation flavanoid-based designs. This work advances the concept of antithrombin activation through non-saccharide, organic molecules and pinpoints a direction for the design of more potent molecules.