title: Novel low-molecular-weight inhibitor of PAI-1 (XR5118) promotes endogenous fibrinolysis and reduces postthrombolysis thrombus growth in rabbits.
abstract: BACKGROUND
  Elevated levels of plasminogen activator inhibitor 1 (PAI-1) have been associated with the occurrence of thrombotic disease, and inhibition of PAI-1 activity in vivo resulted in enhanced thrombolysis and a reduction in reocclusion. Besides monoclonal antibodies and peptides, no suitable agents that are able to block PAI-1 activity are available to date. The present study was designed to test the interaction between a nonantibody, nonpeptide, diketopiperazine-based inhibitor of PAI-1, XR5118, and PAI-1 and to assess the effect of XR5118 on PAI-1 activity in vitro and on in vivo thrombolysis and thrombus growth in an experimental thrombosis model in rabbits.
  
  
  RESULTS
  The binding site of XR5118 on the PAI-1 molecule was studied by competitive binding experiments with mapped anti-PAI-1 monoclonal antibodies by use of surface plasmon resonance experiments. XR5118 selectively and competitively inhibited binding of the PAl-1-inhibiting monoclonal antibody CLB-2C8, indicating that binding of XR5118 to PAI-1 takes place at the area between amino acids 110 and 145 of the PAI-1 molecule, which is known to be involved with the binding of PAI-1 to tissue plasminogen activator (TPA). Incubation of plasma or platelet releasate with XR5118 resulted in a dose-dependent inhibition of PAI-1 activity. Systemic infusion of XR5118 induced a significant reduction in plasma PAI-1 activity levels from 23.7+/-4.9 to 10.9+/-3.4 IU/mL. Administration of XR5118 resulted in a significant, twofold increase in endogenous thrombolysis compared with the control. Thrombus growth in rabbits receiving both XR5118 and rTPA was significantly attenuated compared with rabbits receiving rTPA alone (13.5+/-2.7% versus 19.9+/-3.8%, respectively).
  
  
  CONCLUSIONS
  XR5118 binds to PAI-1 and reduces plasma PAI-1 activity levels. Furthermore, XR5118 promotes endogenous thrombolysis and inhibits thrombus accretion and is the first nonpeptide compound with significant anti-PAI-1 activity in vivo in these models.
authors.affiliation: Center for Hemostasis, Thrombosis, Atherosclerosis, and Inflammation Research, Academic Medical Center, University of Amsterdam, Netherlands.
authors.forename: P W
authors.initials: PW
authors.lastname: Friederich
authors.forename: M
authors.initials: M
authors.lastname: Levi
authors.forename: B J
authors.initials: BJ
authors.lastname: Biemond
authors.forename: P
authors.initials: P
authors.lastname: Charlton
authors.forename: D
authors.initials: D
authors.lastname: Templeton
authors.forename: A J
authors.initials: AJ
authors.lastname: van Zonneveld
authors.forename: P
authors.initials: P
authors.lastname: Bevan
authors.forename: H
authors.initials: H
authors.lastname: Pannekoek
authors.forename: J W
authors.initials: JW
authors.lastname: ten Cate
mesh_terms: D000818:Animals; D001792:Blood Platelets / Q000378:metabolism; D005342:Fibrinolysis / Q000187:drug effects*; D006801:Humans; D007601:Jugular Veins; D008970:Molecular Weight; D010879:Piperazines / Q000737:chemistry / Q000378:metabolism / Q000494:pharmacology*; D010949:Plasma / Q000378:metabolism; D017395:Plasminogen Activator Inhibitor 1 / Q000097:blood* / Q000378:metabolism; D011817:Rabbits; D013917:Thrombin / Q000494:pharmacology*; D013924:Thrombophlebitis / Q000097:blood / Q000517:prevention & control*
chemical_list: D010879:Piperazines; D017395:Plasminogen Activator Inhibitor 1; C107761:XR 5118; D013917:Thrombin