Optimization of Cyclic Plasmin Inhibitors: From Benzamidines to Benzylamines

New macrocyclic plasmin
inhibitors based on our previously optimized
P2–P3 core segment have been developed. In the first series,
the P4 residue was modified, whereas the 4-amidinobenzylamide in P1
position was maintained. The originally used P4 benzylsulfonyl residue
could be replaced by various sulfonyl- or urethane-like protecting
groups. In the second series, the P1 benzamidine was modified and
a strong potency and excellent selectivity was retained by incorporation
of p-xylenediamine. Several analogues inhibit plasmin
in the subnanomolar range, and their potency against related trypsin-like
serine proteases including trypsin itself could be further reduced.
Selected derivatives have been tested in a plasma fibrinolysis assay
and are more effective than the reference inhibitor aprotinin. The
crystal structure of one inhibitor was determined in complex with
trypsin. The binding mode reveals a sterical clash of the inhibitor’s
linker segment with the 99-hairpin loop of trypsin, which is absent
in plasmin.