Effects of beta3-integrin blockade (c7E3) on the response to angioplasty and intra-arterial stenting in atherosclerotic nonhuman primates

Platelet adhesion on endothelium early after vein grafting mediates leukocyte recruitment and intimal hyperplasia in a murine model

Intimal hyperplasia (IH) is the substrate for accelerated atherosclerosis and limited patency of vein grafts. However, there is still no specific treatment targeting IH following graft surgery. In this study, we used a mouse model of vein grafting to investigate the potential for early intervention with platelet function for later development of graft IH. We transferred the inferior vena cava (IVC) from donor C57BL/6 mice to the carotid artery in recipients using a cuff technique. We found extensive endothelial injury and platelet adhesion one hour following grafting. Adhesion of leukocytes was distinct in areas of platelet adhesion. Platelet and leukocyte adhesion was strongly reduced in mice receiving a function-blocking antibody against the integrin αIIbβ3. This was followed by a reduction of IH one month following grafting. Depletion of platelets using antiserum also reduced IH at later time points. These findings indicate platelets as pivotal to leukocyte recruitment to the wall of vein grafts. In conclusion, the data also highlight early intervention of platelets and inflammation as potential treatment for later formation of IH and accelerated atherosclerosis following bypass surgery.

Integrin alpha(v)beta(3) as a target in the prevention of neointimal hyperplasia

Although major advances have been made in the prevention and treatment of restenosis following coronary and peripheral interventions, the persistent complications of thrombosis and reintervention remain a mainstay for repeat hospitalizations in this patient population. For many years, a ubiquitous cell surface receptor called alpha(v)beta(3) integrin was the target of investigators in the prevention of restenosis because its interaction with the extracellular matrix was believed to coordinate the migration of smooth muscle cells (SMCs) from the media to the intima, the seminal event in the formation of intimal occlusive lesion. After the publication of uniformly positive animal studies demonstrating that alpha(v)beta(3) integrin blockade led to a significant reduction in new intimal (neointimal) lesion formation, early clinical trials supported the association of avoidance of target lesion revascularization and the use of antagonists to the SMC integrin alpha(v)beta(3) and its related platelet integrin alpha(IIb)beta(3). However, a series of clinical trials subsequently demonstrated that these antagonists did not necessarily prevent revascularizations by inhibiting intimal hyperplasia per se. Additional animal studies subsequently showed that, indeed, in the setting of pre-existing SMCs in the intimal lesion (ie, atherosclerotic plaque, fatty streaks), inhibiting SMC migration by way of beta(3) integrin blockade was an ineffective approach in the prevention of intimal hyperplasia and restenosis. However, given the wealth of basic and clinical information on the alpha(v)beta(3) integrin and its antagonists, we discuss in this article our new approach to this old solution by targeting a new clinical problem of early failure arteriovenous access for hemodialysis. Given the uniqueness of arteriovenous access in that there are essentially no significant atherosclerotic lesions in the artery and vein prior to the anastomosis, the seminal event of the migration of SMCs from the media to the neointima could by targeted once again with beta(3) integrin antagonists.

Intraarterial reteplase and intravenous abciximab for treatment of acute ischemic stroke

We performed a preliminary feasibility and safety study using intravenous (IV) administration of a platelet glycoprotein IIb/IIIa inhibitor (abciximab) in conjunction with intraarterial (IA) administration of a thrombolytic agent (reteplase) in a primate model of intracranial thrombosis. We introduced thrombus through superselective catheterization of the intracranial segment of the internal carotid artery in 16 primates. The animals were randomly assigned to receive IA reteplase and IV abciximab ( n =4), IA reteplase and IV placebo ( n =4), IA placebo and IV abciximab ( n =4) or IA and IV placebo ( n =4). Recanalization was assessed by serial angiography during the 6-h period after initiation of treatment. Postmortem magnetic resonance (MR) imaging was performed to determine the presence of cerebral infarction or intracranial hemorrhage. Partial or complete recanalization at 6 h after initiation of treatment (decrease of two or more points in pre-treatment angiographic occlusion grade) was observed in two animals treated with IA reteplase and IV abciximab, three animals treated with IA reteplase alone and one animal treated with IV abciximab alone. No improvement in perfusion was observed in animals that received IV and IA placebo. Cerebral infarction was demonstrated on postmortem MR imaging in three animals that received IA and IV placebo and in one animal each from the groups that received IA reteplase and IV abciximab or IV abciximab alone. One animal that received IV abciximab alone had a small intracerebral hemorrhage on MR imaging. IA reteplase with or without abciximab appeared to be the most effective regimen for achieving recanalization in our model of intracranial thrombosis. Further studies are required in experimental models to determine the optimal dose, method of administration and efficacy of these medications in acute ischemic stroke.

Evolving Strategies for the Prevention and Treatment of Coronary Restenosis

Percutaneous coronary intervention has become an increasingly attractive alternative to medical therapy and surgical revascularization for the treatment of coronary artery disease. Over 1.5 million interventional procedures are performed annually worldwide, with the rate of procedures performed continuing to rise dramatically. Restenosis following percutaneous coronary intervention occurs at rates of 20% to 40% and has remained the Achilles heel of the procedure. Numerous early attempts at the prevention of restenosis with oral pharmacologic agents, such as antithrombotic therapies, lipid lowering agents, vasodilators, and growth factor inhibitors, failed to show benefit in clinical trials. The introduction of intracoronary stents resulted in a 30% reduction in restenosis rates by abolishing the early vessel recoil following angioplasty. However, as more complex lesions underwent percutaneous coronary intervention with stenting, rates of “in-stent” restenosis remained high (20% to 30%). With technologic advances and greater understanding of vascular pathobiology, novel therapeutic strategies, such as local delivery of ionizing radiation, immunosuppressive agents, and gene therapy, have been deployed to prevent coronary restenosis. In addition, a number of mechanical and radiation-based strategies have been used to treat those patients who develop restenosis. This review considers these emerging strategies for the prevention and treatment of restenosis.

Platelet and fibrinogen turnover at the exposed subendothelium measured over 1 year after a balloon catheter de-endothelializing injury to the rabbit aorta: thrombotic eruption at the late re-endothelialization stage

Balloon catheter de-endothelialization of the rabbit aorta in vivo causes a rapid release of thrombin and a consequent hemostatic response at the surface of the exposed subendothelium. Previously, we have compared the net fluxes of several hemostatic proteins from plasma into the exposed aorta subendothelium for up to 600 days after injury. We now report the turnover of platelets, compared to fibrinogen, at the de-endothelialized aorta for up to 390 days after injury. Anesthetized NZW rabbits received either a de-endothelializing or a sham injury (controls) to their aortas. At a predetermined time (either 10 min before or up to 390 days after injury), each rabbit was infused with known quantities of rabbit (51)Cr-platelets and rabbit (125)I-fibrinogen; the radiolabels were allowed to circulate for 10 min before the rabbit was rapidly exsanguinated. Radioactivity measurements and tissue analysis revealed that at 10 min after balloon injury, approximately 165,000 platelets/mm(2) were associated with the aorta surface, and platelet turnover was 840/min/mm(2). Turnover had decreased to <200/min/mm(2) at 10-21 days but, from 65 to 390 days, had increased to approximately 1500/min/mm(2). In comparison, approximately 17 pmol of fibrinogen/cm(2) saturated the ballooned surface by 10 min after injury. Fibrinogen turnover at the aorta surface at 10 min after injury amounted to 0.2 pmol/min/cm(2), increasing to 0.7 at 10 days but decreasing to 0.25 at 21 days. Between 65 and 390 days, fibrinogen turnover increased slowly to 1.3 pmol/min/cm(2). Fibrinogen turnover at the surface of the aorta paralleled that within the intima-media over 390 days. Platelet and fibrin(ogen) deposits within the aorta wall increased over the 21-390 days interval as shown by immunostaining. The results are consistent with the re-endothelializing aorta tending to support thrombosis and ulceration in the late healing stage.

Metalloproteinase inhibitor attenuates neointima formation and constrictive remodeling after angioplasty in rats: augmentative effect of alpha(v)beta(3) receptor blockade

Release of matrix metalloproteinases (MMP) from smooth muscle and foam cells following arterial injury facilitates cell migration, neointimal hyperplasia, and vessel wall remodeling. Inhibition of MMP activity using the hydroxamate, zinc-chelating mimicers of collagen, Batimastat and Marimastat, has shown efficacy in reducing constrictive vascular remodeling 6 weeks after experimental angioplasty but not intimal hyperplasia. Vitronectin receptor (alpha(v)beta(3)) blockade interferes with binding of this integrin to MMP-2 and proteolyzed collagen, thereby reducing cell invasion. This study tests the effect of MMP inhibition, with and without vitronectin receptor (alpha(v)beta(3)) blockade, on neointima formation and arterial remodeling in a long-term model (up to 212 months) of balloon injury in vivo. Male Sabra rats were treated with Batimastat (BB-94, British Biotech Pharmaceuticals Ltd., 30 mg/kg, intraperitoneally) and/or the alpha(v)beta(3) receptor inhibiting RGD peptide, G-Pen-GRGDSPCA (GIBCO BRL, 0.1 micromol), administered as a perivascular gel to the common carotid artery after balloon injury. Animals were sacrificed 3, 14, 25, and 75 days (n=21, 23, 22, and 21) after injury. Animals treated with BB-94, peptide, or both had markedly increased absolute luminal area with markedly reduced luminal cross-sectional-area narrowing by neointima and intima-to-media area ratio at all time points except for 3 days after balloon injury versus non-treated, ballooned animals. Combined treatment was significantly more effective than either one alone. Constrictive remodeling, most marked 212 months after balloon injury, was prevented at this time point in all treated animals. The pattern of reduction in luminal narrowing, neointimal formation, and constrictive remodeling across treatment groups correlated very significantly with the reduction in tissue MMP activity as determined by zymography at 3 days. Confirmation of the efficacy of this strategy in larger animals should be the next step toward testing the applicability of this novel approach to the interventional setting.

Platelet aggregation inhibitors for use in peripheral vascular interventions: what can we learn from the experience in the coronary arteries?

During the last decade, an enormous amount of information has been gathered about the function of the platelet and its impact on percutaneous vascular interventions. With the discovery of the GP IIb/IIIa receptor, which is responsible for platelet aggregation, new drug antagonists have been developed to prevent platelet aggregation that may result in arterial thrombosis or platelet microembolization. These drugs include the three GP IIb/IIIa receptor antagonists approved by the Food and Drug Administration: abciximab (ReoPro), eptifibatide (Integrilin), and tirofiban (Aggrastat). These drugs have been used in several large studies to improve the outcome of coronary interventions and in conjunction with plasminogen activators to accelerate thrombolysis. In addition, because no oral GP IIb/IIIa inhibitor exists, other oral regimens have been developed with use of the thienopyridines, ticlopidine (Ticlid) and clopidogrel (Plavix), in combination with aspirin to prevent platelet aggregation and thrombosis. Because the majority of investigations have been performed in patients undergoing coronary interventions, knowledge of these data is necessary to attempt to translate the use of these antiplatelet drugs to peripheral vascular interventions. The goal of this article is to review the use of these agents in the percutaneous treatment of coronary artery disease and give insight to their potential utility in noncoronary interventions.

Mechanisms of angioplasty and stent restenosis: implications for design of rational therapy

Restenosis after angioplasty or stenting remains the major limitation of both procedures. A vast array of drug therapies has been used to prevent restenosis, but they have proven to be predominantly unsuccessful. Recent trends in drug therapy have attempted to refine the molecular and biological targets of therapy, based on the assumption that a single biological process or molecule is critical to restenosis. In contrast, both stenting and brachytherapy, which are highly nonspecific, can successfully reduce restenosis after angioplasty or stenting, respectively. This review examines the biology of both angioplasty and stent stenosis, focussing on human studies. We also review the landmark human trials that have definitively proven successful therapies, such as stenting and brachytherapy. We suggest that the successful trials of stenting and brachytherapy and the failure of other treatments have highlighted the shortcomings of conventional animal models of arterial intervention, and gaps in our knowledge of human disease. In contrast to arguments advocating gene therapy, these studies suggest that the most likely successful drug therapy will have a wide therapeutic range, targeting as many of the components or biological processes contributing to restenosis as possible.

Platelets and restenosis

Restenosis is currently the major limitation of percutaneous transluminal coronary angioplasty (PTCA). Factors such as elastic recoil, migration of vascular smooth muscle cells from media to intima, neointimal proliferation and vascular remodeling underly the restenotic process. Presently there is no effective therapy available for restenosis. The role of platelets in the development of thrombosis and abrupt closure after PTCA is well recognized. However, the effects of platelets in PTCA extend well beyond the early phase. Although antiplatelet agents such as glycoprotein IIb/IIIa antagonists have been reported to reduce target vessel revascularization, major unresolved controversies still exist. This report reviews the potential role of platelets in restenosis. Various drugs, successfully tested in experimental studies and in a small number of human studies, that inhibit the effect of platelets on the restenotic process are also reviewed.

Decreased neointimal formation in Mac-1(-/-) mice reveals a role for inflammation in vascular repair after angioplasty

Inflammation plays an essential role in the initiation and progression of atherosclerosis, but its role in vascular repair after mechanical arterial injury (i.e., percutaneous transluminal coronary angioplasty, PTCA) is unknown. In animal models of vascular injury, leukocytes are recruited as a precursor to intimal thickening. Furthermore, markers of leukocyte activation - in particular, increased expression of the beta2-integrin Mac-1 (alphaMbeta2, or CD11b/CD18), which is responsible for firm leukocyte adhesion to platelets and fibrinogen on denuded vessels - predict restenosis after PTCA. To determine whether Mac-1-mediated leukocyte recruitment is causally related to neointimal formation, we subjected mice lacking Mac-1 to a novel form of mechanical carotid artery dilation and complete endothelial denudation. We now report that the selective absence of Mac-1 impairs transplatelet leukocyte migration into the vessel wall, reducing leukocyte accumulation over time. Diminished medial leukocyte accumulation was accompanied by markedly reduced neointimal thickening after vascular injury. These data establish a role for inflammation in neointimal thickening and suggest that leukocyte recruitment to mechanically injured arteries may prevent restenosis.

Regulation of smooth muscle cell migration and integrin expression by the Gax transcription factor

Homeobox transcription factors specify body plan by regulating differentiation, proliferation, and migration at a cellular level. The homeobox transcription factor Gax is expressed in quiescent vascular smooth muscle cells (VSMCs), and its expression is downregulated by vascular injury or other conditions that lead to VSMC proliferation. Previous investigations demonstrate that Gax may regulate VSMC proliferation by upregulating the cyclin-dependent kinase (cdk) inhibitor p21. Here we examined whether Gax influences VSMC migration, a key feature in the development of stenotic lesions after balloon injury. Transduction of a Gax cDNA inhibited the migratory response of VSMCs toward PDGF-BB, basic fibroblast growth factor, or hepatocyte growth factor/scatter factor. Gax expression also inhibited migration of NIH.3T3 fibroblasts and embryonic fibroblasts lacking p53. Gax was unable to inhibit the migration of fibroblasts lacking p21, but this effect could be restored in these cells by providing exogenous p21 or by overexpressing another cdk inhibitor, p16. Flow cytometric analysis implicated a Gax-mediated downregulation of alpha(v)beta(3) and alpha(v)beta(5) integrin expression in VSMCs as a potential cause for reduced cell motility. Gax specifically downregulated beta(3) and beta(5) in VSMCs in culture and after acute vascular injury in vivo. Repression of integrin expression was also found in NIH 3T3 cells and p53 knockout fibroblasts, but not in p21-knockout fibroblasts, unless these cells express exogenous p21 or p16. These data suggest that cycle progression, integrin expression, and cell migration can be regulated in VSMCs by the homeobox gene product Gax.