Allosteric inhibition of factor XIa. Sulfated non-saccharide glycosaminoglycan mimetics as promising anticoagulants

Abstract

Recent development of sulfated non-saccharide glycosaminoglycan mimetics, especially sulfated pentagalloyl glucopyranoside (SPGG), as potent inhibitors of factor XIa (FXIa) (J. Med. Chem. 2013; 56:867–878 and J. Med. Chem. 2014; 57:4805–4818) has led to a strong possibility of developing a new line of factor XIa-based anticoagulants. In fact, SPGG represents the first synthetic, small molecule inhibitor that appears to bind in site remote from the active site. Considering that allosteric inhibition of FXIa is a new mechanism for developing a distinct line of anticoagulants, we have studied SPGG’s interaction with FXIa with a goal of evaluating its pre-clinical relevance. Comparative inhibition studies with several glycosaminoglycans revealed the importance of SPGG’s non-saccharide backbone. SPGG did not affect the activity of plasma kallikrein, activated protein C and factor XIIIa suggesting that SPGG-based anticoagulation is unlikely to affect other pathways connected with coagulation factors. SPGG’s effect on APTT of citrated human plasma was also not dependent on antithrombin or heparin cofactor II. Interestingly, SPGG’s anticoagulant potential was diminished by serum albumin as well as factor XI, while it could be reversed by protamine or polybrene, which implies possible avenues for developing antidote strategy. Studies with FXIa mutants indicated that SPGG engages Lys529, Arg530 and Arg532, but not Arg250, Lys252, Lys253 and Lys255. Finally, SPGG competes with unfractionated heparin, but not with polyphosphates and/or glycoprotein Ibα, for binding to FXIa. These studies enhance understanding on the first allosteric inhibitor of FXIa and highlight its value as a promising anticoagulant.

Medically-Induced Hemophilia C to Treat Thrombosis

“…In the present and previous investigations, SPGG was shown to be a relatively specific inhibitor of FXIa and not inhibiting FXIIa, plasma kallikrein, factor XIIIa, or any of the vitamin K-dependent proteins including activated protein C [5][6][7]. The IC 50 for factors XIIa and Xa were N256-fold higher concentration of SPGG than that for FXIa [7].…”

“…The IC 50 for factors XIIa and Xa were N256-fold higher concentration of SPGG than that for FXIa [7]. Likewise there also is little inhibition of plasma kallikrein, a structural form of enzyme most like FXIa [5]. SPGG appears to be a unique and potent glycosaminoglycan inhibitor of FXIa.…”

“…Although these characteristics are interesting and may recommend some features of SPGGs, it is not clear how useful these compounds will be in plasma. The concentration of the SPGG to double the activated partial thromboplastin time and prothrombin time is 45 ± 3 and 373 ± 26 μM, respectively [5]. These findings suggest that there will be a big step down in factor XIa inhibitory activity when added to plasma that may limit their usefulness as a clinical anticoagulant and antithrombotic.…”

“…In the present issue of Thrombosis Research, Al-Horani et al report that sulfated non-saccharide glycosaminoglycan mimetics, especially sulfated pentagalloyl glucopyranosides (SPGG), are potent inhibitors of factor XIa [5]. These compounds are unique because they do not interact with the FXIa active site; rather they bind the highly electropositive heparin-binding site of FXIa on the catalytic domain of factor XI.…”

“…1, the active site on the catalytic domain of factor XI is shown by the yellow amino acids from right to left: H413, D462, and S557. Facing the active site pocket are the amino acids K529, R530, and R532, the 3 amino acids whose mutagenesis to alanine blocked the inhibitory function of the SPGG compounds [5] (Fig. 1).…”