bivalirudin and Carotid-Artery-Thrombosis

The relation between the antithrombotic effect in vivo, and the inhibition constant (Ki) and the association rate constant (k(on)) in vitro was investigated for eight different thrombin inhibitors. The carotid arteries of anaesthetized rats were exposed to FeCl3 for 1 h, and the thrombus size was determined from the amount of incorporated 125I-fibrinogen. The thrombin inhibitors were given intravenously, and complete concentration- and/or dose-response curves were constructed. Despite a 50,000-fold difference between the Ki-values comparable plasma concentrations of hirudin and melagatran were needed (0.14 and 0.12 micromol l(-1), respectively) to obtain a 50% antithrombotic effect (IC50) in vivo. In contrast, there was a comparable in vitro (Ki-value) and in vivo (IC50) potency ratio for melagatran and inogatran, respectively. These results can be explained by the concentration of thrombin in the thrombus and improved inhibition by the low-molecular-weight compounds. For all eight thrombin inhibitors tested, there was an inverse relationship between k(on)-values in vitro and the slope of the dose response curves in vivo. Inhibitors with k(on)-values of < 1 x 10(7) M(-1) s(-1) gave steep dose response curves with a Hill coefficient > 1. The association time for inhibition of thrombin for slow-binding inhibitors will be too long to give effective antithrombotic effects at low plasma concentrations, but at increasing concentrations the association time will decrease, resulting in a steeper dose-response curve and thereby a more narrow therapeutic interval.

Topics: Amino Acid Chloromethyl Ketones; Animals; Anticoagulants; Arginine; Azetidines; Benzylamines; Carotid Artery Thrombosis; Dose-Response Relationship, Drug; Fibrinolytic Agents; Glycine; Hemodynamics; Heparin; Hirudin Therapy; Hirudins; Kinetics; Male; Oligopeptides; Partial Thromboplastin Time; Peptide Fragments; Pipecolic Acids; Piperidines; Rats; Rats, Sprague-Dawley; Recombinant Proteins; Sulfonamides; Thrombin; Thrombin Time

Adventitial heparin delivery has been shown to inhibit thrombosis and neointimal thickening in a rat carotid injury model. To determine whether sustained, local delivery of hirulog, a potent antithrombin agent, inhibits thrombus formation and neointimal thickening after arterial stenting, silicone polymers containing hirulog were formulated at a concentration of 5.8% by weight and were tested in vitro to determine the rate of drug release. An oversized metallic stent was implanted in the carotid artery of 18 juvenile farm pigs. Hirulog-impregnated silicone polymers were placed around the adventitial surface of one stented segment of each animal and a control polymer was placed contralaterally. Intravenous hirulog (4 mg/kg/hr) was infused for the duration of the procedure to maintain the activated clotting time of > 300 sec. Ex vivo testing estimated the release of hirulog to be 1.54 micrograms/mg matrix/day with no loss of anticoagulant activity of the released peptide. In four pigs killed on days 3 through 5, macroscopic thrombus was very faintly visible on the stent struts of one arterial segment treated with sustained-release hirulog but was readily evident in all control arteries. However, electron microscopy showed platelet adhesion and microscopic thrombus formation on each stent of both treated and untreated sides. Fourteen pigs were killed 32 +/- 4 days after stenting. Histologic analysis showed no difference between hirulog-treated and control sides in the volume of neointima (540 +/- 129 units vs 357 +/- 95 units, p = 0.27) or in the average intima to media ratio (0.44 +/- 0.12 vs 0.34 +/- 0.24, p = 0.47) over the length of the stented segment. Late thrombotic occlusion occurred in two hirulog-treated and two control arteries. In this model, local adventitial hirulog delivery at the dose and delivery rate used may reduce, but does not prevent, thrombus formation and does not reduce the severity of neointimal thickening after carotid stent implantation.

Topics: Animals; Antithrombins; Carotid Arteries; Carotid Artery Thrombosis; Drug Delivery Systems; Hirudin Therapy; Hirudins; In Vitro Techniques; Infusions, Intravenous; Microscopy, Electron, Scanning; Peptide Fragments; Recombinant Proteins; Silicone Elastomers; Stents; Swine; Time Factors; Tunica Intima