Thrombin plays a central role in venous and arterial
thrombosis. We utilized two different rabbit models of in vivo
thrombosis to investigate the effect of inhibitors of
thrombin generation and
thrombin activity. The agents tested were specific inhibitors of
factor Xa (fXa) [N2-[(phenylmethyl)sulfonyl]-D-arginyl-N-[(1S)-4-[(aminoiminomethyl++ +)a mino]-1-(2-thiazolylcarbonyl)butyl]-
glycinamide (C921-78)] and
thrombin [D-phenylalanyl-N-[4-[(aminoiminomethyl)amino]-1-(chloroacetyl)but yl]-L-
prolinamide (
PPACK)], as well as drugs that affect both
thrombin and fXa, unfractionated and low molecular weight (
enoxaparin)
heparin. The agents administered as constant
intravenous infusion were evaluated for antithrombotic efficacy in anesthetized rabbits. All four agents were capable of dose dependent inhibition of
thrombosis in venous and arteriovenous
thrombosis models. However, due to the more aggressive nature of thrombotic stimulation in the arteriovenous shunt model, complete cessation of
thrombus growth was not achieved for any of the agents at the doses tested. Comparison between the agents focused on the differences in extension of coagulation parameters (activated partial thromboplastin time, prothrombin time,
thrombin clotting time), changes in hematological parameters, and extension of rabbit cuticle bleeding time at doses required to produce maximum inhibition in the
thrombosis models. In the
venous thrombosis model at the maximally effective dose,
C921-78 had minimal extension of ex vivo clotting parameters, while
enoxaparin and
unfractionated heparin demonstrated a two to sevenfold increase in activated partial
thromboplastin times, and
PPACK had a threefold extension of
thrombin clotting times. In addition, unlike the other three agents, which exhibited no significant changes in hematological parameters,
PPACK demonstrated dose dependent
thrombocytopenia. A standardized cuticle bleeding time was used as a measure of perturbation of hemostasis. The agents were evaluated for significant increases in bleeding time at doses up to eight times that needed to completely inhibit venous
thrombus formation.
Unfractionated heparin displayed a significant bleeding time effect at the dose required to inhibit
venous thrombosis (100 u/kg+2 u/kg/min).
Enoxaparin and
PPACK caused significant bleeding time extensions at four times the fully efficacious venous dose (800 u/kg+8 u/kg/min and 30 microg/kg/min). By contrast,
C921-78 did not significantly increase bleeding time even at eight times the maximally effective dose (240 microg/kg+7.2 microg/kg/min). Our results demonstrate that specific inhibition of fXa can be utilized to derive potent antithrombotic activity without disrupting extravascular hemostasis.