Synthesis and Structural Characterization of Macrocyclic Plasmin Inhibitors

Two series of macrocyclic plasmin inhibitors with a C‐terminal benzylamine group were synthesized. The substitution of the N‐terminal phenylsulfonyl group of a previously described inhibitor provided two analogues with sub‐nanomolar inhibition constants. Both compounds possess a high selectivity against all other tested trypsin‐like serine proteases. Furthermore, a new approach was used to selectively introduce asymmetric linker segments. Two of these compounds inhibit plasmin with K i values close to 2 nM. For the first time, four crystal structures of these macrocyclic inhibitors could be determined in complex with a Ser195Ala microplasmin mutant. The macrocyclic core segment of the inhibitors binds to the open active site of plasmin without any steric hindrance. This binding mode is incompatible with other trypsin‐like serine proteases containing a sterically demanding 99‐hairpin loop. The crystal structures obtained experimentally explain the excellent selectivity of this inhibitor type as previously hypothesized.