Early plus delayed hirudin reduces restenosis in the atherosclerotic rabbit more than early administration alone: potential implications for dosing of antithrombin agents

Evaluation of drug release from paclitaxel + hirudin-eluting balloons and the resulting vascular reactivity in healthy pigs

This study explored drug release from paclitaxel + hirudin-eluting balloons in a healthy pig coronary artery model and objectively evaluated the vascular reactivity after balloon intervention. A total of 12 healthy white pigs were used for the analysis of drug release from the experimental balloon. The observational time-points included immediately following implantation and day 7, 30, and 180 after surgery. At each time-point, two pigs were treated with the paclitaxel + hirudin-eluting balloon and one with the B. Braun paclitaxel-eluting balloon (control). Quantitative coronary angiography (QCA) of the target vessel was performed before and after balloon treatment. One pig died before the designated experimental endpoint, and no abnormal clinical signs or tissue lesions were observed in the other pigs. QCA showed different degrees of spasms after balloon treatment, all of which recovered shortly thereafter without intervention. The blood vessel lumens were all open and without dissection or angioma. Drug content determination showed that the experimental balloon performed better than the control balloon with regard to drug release, vascular absorption and expulsion rate. The paclitaxel + hirudin-eluting balloon catheter was easy to manipulate. The drug release from the experimental balloon was stable and resulted in good vascular reactivity. The safety and efficacy of the experimental balloon were not worse than the paclitaxel-eluting balloon that is currently on the market.

Assessment of Novel Anti-thrombotic Fusion Proteins for Inhibition of Stenosis in a Porcine Model of Arteriovenous Graft

BACKGROUND

Hemodialysis arteriovenous synthetic grafts (AVG) provide high volumetric blood flow rates shortly after surgical placement. However, stenosis often develops at the vein-graft anastomosis contributing to thrombosis and early graft failure. Two novel fusion proteins, ANV-6L15 and TAP-ANV, inhibit the tissue factor/factor VIIa coagulation complex and the factor Xa/factor Va complex, respectively. Each inhibitor domain is fused to an annexin V domain that targets the inhibitor activity to sites of vascular injury to locally inhibit thrombosis. This study's objective was to determine if these antithrombotic proteins are safe and effective in inhibiting AVG stenosis.

METHODS

A bolus of either TAP-ANV or ANV-6L15 fusion protein was administered intravenously immediately prior to surgical placement of a synthetic graft between the external jugular vein and common carotid artery in a porcine model. At surgery, the vein and artery were irrigated with the anti-thrombotic fusion protein. Control animals received intravenous heparin. At 4 weeks, MRI was performed to evaluate graft patency, the pigs were then euthanized and grafts and attached vessels were explanted for histomorphometric assessment of neointimal hyperplasia at the vein-graft anastomosis. Blood was collected at surgery, immediately after surgery and at euthanasia for serum metabolic panels and coagulation chemistries.

RESULTS

No acute thrombosis occurred in the control group or in either experimental group. No abnormal serum chemistries, activated clotting times or PT, PTT values were observed after treatment in experimental or control animals. However, at the vein-graft anastomosis, there was no difference between the control and experimental groups in cross-sectional lumen areas, as measured on MRI, and no difference in hyperplasia areas as determined by histomorphometry. These results suggest that local irrigation of TAP-ANV or ANV-6L15 intra-operatively was as effective in inhibiting acute graft thrombosis as intravenous administration of heparin, but failed to inhibit hyperplasia development and stenosis in AVG.

Inhibition of neointimal hyperplasia by a specific thrombin inhibitor

OBJECTIVE

Restenosis secondary to neointimal hyperplasia remains the major limiting factor after vascular interventions. Thrombin generated in high concentrations at the site of vascular injury plays a central role in thrombosis and hemostasis. Thrombin has also been implicated as a mitogen for smooth muscle cell proliferation that contributes to restenosis. This study was designed to determine the effects of a specific thrombin inhibitor on neointimal hyperplasia after balloon injury in a rat carotid artery model.

DESIGN

A total of 47 male Sprague-Dawley rats were divided into five groups. All groups underwent balloon injury of the left carotid artery. A specific thrombin inhibitor, inogatran, was given in four different regimens: low and high dose injections, short-term infusion for 3 h, and long-term infusion for 1 week. After 2 weeks the animals were killed and the carotid neointima/media area ratio and the luminal narrowing were calculated.

RESULTS

All treatments significantly reduced the neointimal hyperplasia. Inogatran given as a long-term infusion for 1 week had the lowest neointima/media ratio compared with the other groups. The percentage of lumen narrowing was also significantly lower in all treatment groups compared with the control group.

CONCLUSION

A specific direct thrombin inhibitor, inogatran, reduces neointimal hyperplasia after arterial injury in rats. A more prolonged administration of the thrombin inhibitor gave a further reduction of the neointimal hyperplasia. It seems that inhibition of thrombin activity is not only important early after injury, but also later. This could have clinical implications in the treatment of restenosis and needs to be further evaluated.

Beta-arrestins regulate atherosclerosis and neointimal hyperplasia by controlling smooth muscle cell proliferation and migration

Atherosclerosis and arterial injury-induced neointimal hyperplasia involve medial smooth muscle cell (SMC) proliferation and migration into the arterial intima. Because many 7-transmembrane and growth factor receptors promote atherosclerosis, we hypothesized that the multifunctional adaptor proteins beta-arrestin1 and -2 might regulate this pathological process. Deficiency of beta-arrestin2 in ldlr(-/-) mice reduced aortic atherosclerosis by 40% and decreased the prevalence of atheroma SMCs by 35%, suggesting that beta-arrestin2 promotes atherosclerosis through effects on SMCs. To test this potential atherogenic mechanism more specifically, we performed carotid endothelial denudation in congenic wild-type, beta-arrestin1(-/-), and beta-arrestin2(-/-) mice. Neointimal hyperplasia was enhanced in beta-arrestin1(-/-) mice, and diminished in beta-arrestin2(-/-) mice. Neointimal cells expressed SMC markers and did not derive from bone marrow progenitors, as demonstrated by bone marrow transplantation with green fluorescent protein-transgenic cells. Moreover, the reduction in neointimal hyperplasia seen in beta-arrestin2(-/-) mice was not altered by transplantation with either wild-type or beta-arrestin2(-/-) bone marrow cells. After carotid injury, medial SMC extracellular signal-regulated kinase activation and proliferation were increased in beta-arrestin1(-/-) and decreased in beta-arrestin2(-/-) mice. Concordantly, thymidine incorporation and extracellular signal-regulated kinase activation and migration evoked by 7-transmembrane receptors were greater than wild type in beta-arrestin1(-/-) SMCs and less in beta-arrestin2(-/-) SMCs. Proliferation was less than wild type in beta-arrestin2(-/-) SMCs but not in beta-arrestin2(-/-) endothelial cells. We conclude that beta-arrestin2 aggravates atherosclerosis through mechanisms involving SMC proliferation and migration and that these SMC activities are regulated reciprocally by beta-arrestin2 and beta-arrestin1. These findings identify inhibition of beta-arrestin2 as a novel therapeutic strategy for combating atherosclerosis and arterial restenosis after angioplasty.

The critical role of the intrinsic VSMC proliferation and death programs in injury-induced neointimal hyperplasia

Postangioplasty and in-stent restenosis remain ominous problems in percutaneous coronary intervention where good animal models of restenosis proneness and resistance are needed. We accidentally discovered that the carotid arteries (CAs) of the Harlan and Sasco substrains of Sprague-Dawley rats display drastically different restenosis phenotypes following balloon-induced endothelial denudation. When subjected to balloon injury, Sasco CAs exhibited significantly larger neointimal mass than did Harlan CAs at both days 14 and 32, as evidenced by a higher intima-to-media ratio and a greater number of intimal cells in Sasco CAs. This was due to a greater cell proliferation and to a less vigorous apoptosis of Sasco neointima, as assessed by 5-bromo-2'-deoxyuridine and terminal deoxynucleotidyl transferase-deoxyuridine nick-end labeling staining, respectively. At a cellular level, whereas vascular smooth muscle cells (VSMCs) isolated from Sasco and Harlan CAs were identical in morphology and in propensity to migrate, Sasco VSMCs proliferated more robustly and died far less, suggesting that under the exact same microenvironment, Sasco and Harlan VSMCs respond to growth and noxious stimuli in a drastically different fashion and that Sasco's significantly more robust neointimal proliferation after vascular injury in vivo can be accounted for by these intrinsic differences in VSMCs of these substrains in vitro. Sasco and Harlan Sprague-Dawley rats as well as VSMCs from these rats will prove to be powerful tools to study genes involved in the pathogenesis of restenosis.

Hirulog-like peptide reduces restenosis and expression of tissue factor and transforming growth factor-beta in carotid artery of atherosclerotic rabbits

Restenosis is responsible to approximately 30% of long-term failure following therapeutic vascular procedures. Thrombosis plays a key role in the development of restenosis. Thrombin-specific inhibitors have been considered as one type of candidates for the prevention of restenosis. Previous studies by our group demonstrated that a novel thrombin-specific inhibitor, hirulog-like peptide (HLP), reduced balloon catheter-induced neointima formation in rat carotid arteries. The present study examined the effect of HLP on angioplasty-induced restenosis in carotid arteries of atherosclerotic rabbits. New Zealand white rabbits were subject to air desiccation of the lumen of the right carotid arteries, then received high cholesterol/fat diet for 3 weeks. The rabbits were intravenously infused with HLP (1.6 mg/(kg/h)) or saline (n=7 per group) for 4 h started before angioplasty which dilated atherosclerotic lesions in right common carotid artery. Four weeks after the angioplasty, neointimal area, stenosis and neointima/media ratio in injured carotid arteries were reduced in atherosclerotic rabbits treated with HLP compared to saline controls by 62, 39 and 59% (P<0.05). The expression of tissue factor (TF) and transforming growth factor (TGF)-beta in the neointima of carotid arteries of rabbits treated with HLP was significantly weaker than saline controls (P<0.05 or <0.01). Activated partial thromboplastin time and bleeding time in HLP-treated rabbits were not significantly prolonged compared to controls. The results of the present study suggest that HLP may substantially reduce angioplasty-induced restenosis in atherosclerotic rabbits without increasing bleeding tendency. The inhibition on the expression of TF and TGF-beta in the neointima of the arterial wall may contribute to the preventive effect of HLP on restenosis in atherosclerotic rabbits.

Melagatran attenuates fibrin and platelet deposition in a porcine coronary artery over-stretch injury model

Melagatran is the active form of the oral direct thrombin inhibitor, ximelagatran. The purpose of this study was to compare the effects of different doses of melagatran with heparin or placebo on platelet deposition and relative fibrin content after coronary angioplasty in pigs. After 125I-labelled fibrinogen and autologous 111Indium-labelled platelets had been infused a balloon injury was performed in the left anterior descending and the right coronary arteries. Pigs were randomized to receive either heparin 200 IU/kg bolus plus 20 IU/kg per h infusion (n = 7); melagatran 1 mg/kg bolus plus 0.33 mg/kg per h infusion (n = 7); melagatran bolus 0.5 mg/kg plus 0.17 mg/kg per h infusion (n = 7); melagatran 0.15 mg/kg bolus plus 0.05 mg/kg per h infusion (n = 6) or saline (n = 4). Seventy-five minutes after the angioplasty, the pigs were euthanized and the injured vessel segments were measured in a gamma counter. Compared with placebo, platelet deposition and relative fibrin content were reduced after both heparin and melagatran, in the latter case with a dose-response relationship. Melagatran reduced platelet deposition and relative thrombus size in a dose-dependent manner when compared with placebo after coronary angioplasty in pigs. No statistically significant difference between melagatran and heparin was found.

Comparative Trial of Local Pharmacotherapy withl-Arginine, r-Hirudin, and Molsidomine to Reduce Restenosis after Balloon Angioplasty of Stenotic Rabbit Iliac Arteries

PURPOSE

To determine if local application of L-arginine, r-hirudin, or molsidomine significantly reduces restenosis after balloon angioplasty in stenotic rabbit iliac arteries.

MATERIALS AND METHODS

Thirty-one male cholesterol-fed New Zealand white rabbits underwent balloon dilation of both common iliac arteries to induce arterial stenosis. Four weeks later, one stenotic iliac artery was simultaneously dilated and received local application of L-arginine (210 mg/mL, n = 7), r-hirudin (0.5 mg/mL, n = 8), or molsidomine (0.2 mg/mL, n = 8) with a channeled balloon catheter. On the contralateral side, 0.9% saline was injected as a control. In eight sham animals, saline was applied to one iliac artery and balloon dilation to only the contralateral artery. Six weeks after local treatment, vessels were harvested, and computerized morphometric and immunohistologic analyses were performed.

RESULTS

Application of drugs resulted in a significant reduction of neointimal area as follows: 53% with L-arginine (1.01 mm(2) vs. 2.17 mm(2), P <.05), 43% with molsidomine (1.04 mm(2) vs. 1.89 mm(2), P <.05), and 20% with r-hirudin (1.79 mm(2) vs. 2.24 mm(2), P <.05). Infusion of saline led to a significant increase (50%, 1.21 mm(2) vs. 1.93 mm(2), P <.05) in neointimal area compared with balloon dilation alone. Immunohistologic findings showed a significant reduction of macrophages (5.0% vs. 10.2%, P <.05) and proliferating cells (6.2% vs. 10.6%, P <.05) in the neointima after local application of L-arginine.

CONCLUSION

Reduction of neointimal area was significant for L-arginine and molsidomine but not for r-hirudin. Saline infusion caused significant arterial trauma, resulting in additional neointimal proliferation.

There is no effect on remodeling of vascular wall 4 weeks after local delivery of antithrombin in a porcine model of coronary overstretching

BACKGROUND

Antithrombin III is an effective endogenous inhibitor of thrombin with antiproliferative and anti-inflammatory capabilities. Systematic administration of direct thrombin inhibitors as well as of antithrombin has proven effective at reducing formation of neointima after injury to vessel wall in various animal models. Local delivery of antithrombin attenuates deposition of platelets after balloon injury to porcine coronary vessels.

OBJECTIVE

To test our hypothesis that local delivery of antithrombin affects remodeling of vessel wall after balloon injury to the left anterior descending artery (LAD) in pigs.

METHODS

With a balloon:vessel diameter ratio of 1.5 deep vessel-wall injury was performed with conventional balloon angioplasty in the LAD in 16 pigs. After balloon injury the pigs were administered locally two doses of 2.5 ml antithrombin (250 U) or, as a control, two doses of 2.5 ml albumin (10 mg). All pigs were administered 200 U/kg bodyweight heparin before catheterization. The animals were then kept in stalls and fed normal grain. After approximately 4 weeks the animals were killed and lesions were assessed by computer-assisted image analysis. The areas of each vessel-wall layer and the percentage area stenosis were calculated. As a measure of injury, the length of rupture of the lamina elastica interna was measured.

RESULTS

The injury was found to be equally profound in pigs of these groups. There was no significant difference between the groups concerning the areas of vessel-wall layers. The percentage area stenosis was similar for pigs in these two groups (36.5 +/- 14.9% for pigs in the antithrombin group versus 35.4 +/- 16.2% for pigs in the control group, NS).

CONCLUSIONS

Local delivery of 250 U antithrombin to the LAD in pigs did not affect remodeling of the vessel wall 4 weeks after balloon injury.

The comparative pathobiology of atherosclerosis and restenosis

Percutaneous coronary interventions (PCIs) play an increasingly important role in the management of patients with coronary artery disease. However, these important procedures are complicated by restenosis in a sizeable number of patients. The pathobiology of atherosclerosis comprises a complex interaction among lipids, the endothelium, circulating and tissue inflammatory cells, platelets, and vascular smooth muscle cells. The superimposition of the mechanical and cellular consequences of PCIs on the abnormal substrate of atherosclerosis leads to a characteristic and distinct pathobiology that initiates and perpetuates restenosis. A clear understanding of the significant differences between atherosclerosis and restenosis will provide a rational basis for developing treatment plans that always address both problems. This article reviews and contrasts the pathobiology of atherosclerosis and restenosis and compares the mechanisms and time-course of these distinct entities.

Statin drugs and dietary isoprenoids as antithrombotic agents

Statin drugs and various isoprenoids from plant origins inhibit mevalonic acids, cholesterol, and other isoprenoid products. Among these, reduction of farnesyl and geranylgeranyl prenylated proteins impedes signal transduction at the cellular level. The authors envision that limiting such prenylated proteins downregulates thrombin-stimulated events, including decreasing the expression and availability of protease-activated receptor-1 mitigating thrombin stimulation of cells, tissue factor preventing additional thrombin generation, and plasminogen activator inhibitor-1 allowing thrombosis. Additional processes may enhance nitric oxide production and induce other processes. Downregulation of thrombin-stimulated events should promote hypothrombotic or quiescent conditions that reduce cardiovascular disease, thus contributing to longevity.

Statins induce hypothrombotic states?

Statins inhibit 3-hydroxy-3-methyl-glutaryl coenzyme A (HMGCoA) reductase, which synthesizes mevalonic acid in the isoprenoid pathways. These pathways lead to squalene and subsequently to cholesterol and related products (e.g., steroids, vitamin D, bile salts, lipopro teins) and have major branches producing cell regulatory substances (e.g., farnesyl- and geranylgeranyl conjugated proteins) (1,2). Although cholesterol reduc tion in blood has been widely believed to be beneficial (e.g., less available for accumulation by foam cells in atherosclerotic plaques), the ability of cholesterol reduc tion to mitigate the incidence and severity of cardiovas cular diseases has recently been questioned. Like others (3-10), we (11) believe that statins and other substances, for example, plant isoprenoids in the diet (12), have ben eficial antithrombotic properties arising through the in hibition of an isoprenoid product other than cholesterol. However, unlike others, we also believe that this isopren oid product has cell regulatory functions upregulated by thrombin stimulation. Moreover, through such cellular pathways, thrombin should upregulate its own genera tion, and statins and dietary isoprenoids should induce hypothrombotic states by downregulating thrombin gen eration (Fig. 1).