Factor XIa (fXIa) is being recognized as a prime target for developing safer anticoagulants. To discover synthetic, small, allosteric inhibitors of fXIa, we screened an in-house, unique library of 65 molecules displaying many distinct scaffolds and varying levels of sulfation. Of these, monosulfated benzofurans were the only group of molecules found to inhibit fXIa (∼100% efficacy) and led to the identification of monosulfated trimer >24 (IC50 0.82 μM) as the most potent inhibitor. Michaelis–Menten kinetics studies revealed a classic noncompetitive mechanism of action for >24. Although monosulfated, the inhibitors did not compete with unfractionated heparin alluding to a novel site of interaction. Fluorescence quenching studies indicated that trimer >24 induces major conformational changes in the active site of fXIa. Docking studies identified a site near Lys255 on the A3 domain of fXIa as the most probable site of binding for >24. Factor XIa devoid of the A3 domain displayed a major defect in the inhibition potency of >24 supporting the docking prediction. Our work presents the sulfated benzofuranscaffold as a promising framework to develop allosteric fXIa inhibitorsthat likely function through the A3 domain.
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