Design, synthesis and biological evaluation of novel FXIa inhibitors featuring five-membered heterocycles as P2' fragments

Thrombus formation within blood vessels poses a serious threat to human health and is closely associated with various cardiovascular disorders. Therefore, developing novel anticoagulants with reduced bleeding risk holds significant clinical importance. Factor XIa (FXIa), a key enzyme in the intrinsic coagulation pathway, has emerged as an attractive target for safer anticoagulant therapy. In this study, a series of novel small-molecule FXIa inhibitors were designed based on the scaffold of Asundexian through a bioisosteric replacement strategy. Starting from compound F22, structural optimization at the P2' region was conducted by replacing the amide with non-classical heterocycles, aiming to improve inhibitory potency, selectivity, and metabolic stability. Among the synthesized analogs, compound FE12 exhibited potent FXIa inhibition (IC₅₀ = 4.4 nM), high selectivity over PKal (SI = 60.3), and favorable metabolic stability (T_1/2 = 38.6 min in HLMs). Consistent with the in vitro enzyme assay, FE12 significantly prolonged aPTT in a dose-dependent manner, comparable to Asundexian. Molecular docking indicated that FE12 retains key hydrogen-bonding and water-mediated interactions within the FXIa active site, whereas its conformation in PKal shifts and loses these interactions, explaining its potent FXIa inhibitory activity and weak PKal inhibition. In the FeCl₃-induced rat thrombosis model, FE12 effectively inhibited thrombus formation comparable to Asundexian. In the mouse tail bleeding assay, FE12 did not cause a significant prolongation of bleeding time, indicating a minimal effect on hemostasis. Furthermore, acute toxicity evaluation demonstrated its good safety and tolerability. Overall, FE12 exhibits an excellent balance of potency, selectivity and safety, representing a promising lead compound for the development of small-molecule FXIa inhibitors.