Coronary intimal proliferation after balloon injury and stenting in swine: an animal model of restenosis

Short- and long-term histopathologic evaluation of stenting using a self-expanding nitinol stent in pig carotid and iliac arteries

Stenting is increasingly being used to treat carotid artery disease. However, complications including distal embolization, stent thrombosis, stent collapse from external compression, the need for high-pressure inflation with increased neointimal response, or balloon rupture during stent expansion and stent loss are all potential problems and of concern. To address each of these specific concerns, a new stent was designed, which is self-expandable, made of nitinol, with temperature-dependent superelastic properties, and with high vessel wall surface coverage. Since this device has a number of novel characteristics, we aimed to assess the short- and long-term histopathologic response in pig carotid and iliac arteries. Single stents were deployed in pig carotid and iliac arteries after overstretch balloon injury. Angiograms were performed pre- and poststenting and prior to sacrifice. Intravascular ultrasound was used before implantation to determine vessel size. Vessels were examined histologically at 1 month (n = 6) and 6 months (n = 6) for morphometric analysis, hemorrhage and thrombus, endothelialization, and inflammatory and fibrotic responses. There was a 100% angiographic success rate at implantation. In one case, it was determined histologically that a single stent was implanted in a dissection plane of a pig's left iliac artery and was occluded by organized thrombus, with the true lumen being patent. At 6-month follow-up, this was the only evidence of a single stent occlusion, with flow adjacent to the stent in the true lumen. In the other vessels, the stents showed good vessel wall-stent apposition and the lumens were patent with a concentric and thin neointima. Inflammatory cells were rare and there were no mural thrombi. Coverage of the vessel wall by endothelial-like cells was complete at 1 month. The novel nitinol EndoStent appears to have favorable biocompatibility with minimal thrombus deposition or inflammatory response, and its use is feasible for clinical application in carotid and iliac arteries.

Placement of a flexible endovascular stent across the femoral joint: an in vivo study in the swine model

PURPOSE

To investigate the effects of joint motion on the structural integrity of periarticular stents and on the development of neointimal hyperplasia within these devices.

MATERIALS AND METHODS

In four juvenile farm swine, Wall-stents were implanted in the common femoral arteries and contralateral common femoral veins, centered at the point of maximal conformational change during passive hip flexion. Control stents were placed in the aortae and iliac veins. Angiography and transcatheter blood pressure measurements were obtained across each stent, with periarticular stents studied in flexion and extension. Two animals underwent repeated evaluation after 1 month, the others after 3 months. Findings were correlated with gross and histopathologic findings in the harvested stents.

RESULTS

No stent fractures occurred. One femoral vein was injured during stent placement and was occluded 1 month later at follow-up. Hemodynamically significant stenoses were identified in one arterial stent and one venous stent at 3 months. The amount of neointimal hyperplasia was greater in periarticular stents than in controls and greater in animals studied at 1 month than in those studied at 3 months. The pattern of neointimal hyperplasia within mobile arteries was circumferentially asymmetric and thicker at the distal ends of the stents. Venous neointimal hyperplasia was thicker and markedly different in character than that seen in arterial stents from the same animals.

CONCLUSIONS

Periarticular Wallstents and the underlying vascular anatomy remained structurally intact despite the stresses of repetitive motion during a 3-month period. Stents deployed across joints or in venous locations may be at greater risk for neointimal hyperplasia development and eventual occlusion than those deployed in immobile vessels and arteries. Neointimal hyperplasia may decrease in thickness after an initial period of exuberant development. Additional studies are necessary to determine long-term outcomes.

Dose and dose rate effects of beta-particle emitting radioactive stents in a porcine model of restenosis

BACKGROUND

Radioactive stents have been proposed as a means to prevent in-stent restenosis by inhibiting intimal proliferation with continuous low-dose irradiation.

OBJECTIVES

The purpose of this study is to determine the effects of cumulative dose and dose-rate delivery on neointimal formation using 32P and 90Y beta-particle emitting radioactive stents in a porcine coronary model of restenosis.

METHODS AND MATERIALS

We compared the late histologic results of 0.25 to 32.0 microCi 90Y (half-life 64 hours) (n = 64 stents) and 0.1 to 57.6 microCi 32P (half-life 14.3 days) (n = 55 stents) Beta-particle emitting radioactive stents with non-radioactive (n = 40) stents in a porcine coronary model of restenosis. A computer-based dosimetry modeling program was used to determine the 28 day cumulative dose and dose-rate delivery for the beta-particle emitting radioactive stents at a distance of 0.1 mm from the stent surface.

RESULTS

Continuous low dose-rate (1 to 5 cGy/hr) radiation delivery for > 2 weeks via a 0.1 to 0.5 microCi 32P radioactive stent effectively reduced in-stent neointimal hyperplasia at 90 days. Cumulative doses of > 55 Gy induced severe adventitial fibrosis, microvascular damage and promoted the formation of a matrix-rich neointima. Delayed vascular repair was evident at focal regions within the body of radioactive stents that delivered cumulative doses of > or = 140 Gy at 28 days and cumulative doses of 1,100 Gy at 90 days.

CONCLUSIONS

These data may be useful in predicting safe and effective dose and dose rate delivery for beta-particle emitting radioactive stents.

Prospective randomized trial of corticosteroids for the prevention of restenosis after intracoronary stent implantation

BACKGROUND

Inflammatory responses have been implicated as one of the major contributors to neointimal hyperplasia after coronary stenting. The aim of this study was to evaluate the effect of pretreatment with single-dose intravenous methylprednisolone on preventing in-stent restenosis.

METHODS

One hundred and forty consecutive patients for elective coronary stenting (focal, de novo lesion and reference diameter >/=3 mm) were randomly assigned to either a methylprednisolone or a placebo group. Either 1 g methylprednisolone or placebo was intravenously infused 6 to 12 hours before stenting with one of two types of stents. Follow-up angiography was performed at 6 months and clinical evaluation made at regular intervals.

RESULTS

Baseline characteristics were similar between both groups. Stenting was successful in all patients, and in-hospital events did not occur in any patients. Follow-up angiography was performed in 127 patients (follow-up rate of 91.4%). The minimal lumen diameter increased from 0.86 +/- 0.50 mm before intervention to 3.34 +/- 0.42 mm after intervention (P =.02). At follow-up, minimal lumen diameter decreased to 2.14 +/- 0.78 mm (P <. 01). Angiographic restenosis rate was 17.5% in the steroid group and 18.8% in the placebo group (P =.85), with no differences between the 2 types of stent. Clinical follow-up was available in all patients (10.3 +/- 2.5 months) and clinical events during the follow-up period were similar in both groups.

CONCLUSIONS

Single-dose pretreatment with intravenous methylprednisolone before coronary stenting had no effect on the change in minimal lumen diameter at a mean follow-up time.

Effects of a positron-emitting vanadium-48 nitinol stent on experimental restenosis in porcine coronary arteries: an injury-response study

BACKGROUND

The major limitation of coronary stenting is restenosis due to exaggerated neointimal thickening. We evaluated a positron-emitting V48 nitinol stent in a porcine coronary model of restenosis.

METHODS AND RESULTS

Pigs (n = 16) received a control nonradioactive and a V48 stent (1.5 or 10.6 muCi) randomized to the left anterior descending artery (LAD) and right coronary artery (RCA). Histology, morphometric variables, and strut injury scores were evaluated after 32 days. Peristrut fibrinoid deposits were greater in the high-dose group (p < 0.0001). Control stent area stenosis (AS) and mean neointimal thickness (NIT) correlated with injury (r = 0.81 and 0.79, respectively). Higher-dose stents reduced AS by 20% (0.57 +/- 0.13 vs. 0.71 +/- 0.16; p = 0.029) and mean NIT by 35% (0.44 +/- 0.16 vs. 0.71 +/- 0.24mm; p = 0.001) compared with controls. Lower-dose 1.5-muCi stents did not differ from controls. NIT over individual struts was reduced in the high-dose group compared with controls by 0.18 mm for grade 1 injury, 0.31 mm for grade 2, and 0.38 mm for grade 3 (p < 0.02 for all comparisons).

CONCLUSIONS

1.5-muCi V48 nitinol stents did not influence vessel histology or restenotic parameters in pig coronary arteries. In contrast, 10.6-muCi stents created a distinctive histological picture consisting of increased fibrinoid deposits on the neointimal-facing side of the struts without cellular organization. Higher dose radioactive stents significantly reduced AS and mean NIT. The reduction in neointimal thickening was greatest when the depth of strut penetration into the vascular wall was most severe.

Fibrocellular tissue responses to endovascular and external beam irradiation in the porcine model of restenosis

PURPOSE

Endovascular radiation has reduced postangioplasty restenosis in preclinical and early clinical studies. External radiation treatment may have advantages over endovascular therapy. We examined vascular and perivascular tissue responses to endovascular and external irradiation in pig coronary arteries.

METHODS AND MATERIALS

Ninety-one animals received endovascular or external radiation following balloon injury and were sacrificed at 14, 30, or 180 days. Injured segments of coronary vessels including perivascular and myocardial tissues were evaluated with histochemistry.

RESULTS

Endovascular radiation was associated with delayed arterial wound healing as late as 6 months, evidenced by paucity of smooth muscle alpha-actin in neointimal cells compared to control. External treatment was associated with increased collagen in neointima and adventitia, and focal interstitial necrosis in adjacent myocardium.

CONCLUSIONS

These investigations showed whole-heart 14 Gy external radiation treatment following coronary injury exacerbated certain aspects of arterial healing. In addition focal myocardial necrosis and fibrosis was observed following external but not endovascular irradiation. Endovascular radiation has some advantages over external irradiation; however the persistence of a synthetic smooth muscle cell phenotype in the neointima at 6 months suggests ionizing radiation in general may have profound effects on vessel architecture over the long term.

The effect of external electron beam on neointima in rat carotid artery injury model

PURPOSE

Endovascular irradiation with either a gamma or a beta source has shown to reduce neointimal proliferation. However, the effect of external-beam radiation on neointimal hyperplasia is controversial. The objective of this study was to determine the effect of external-beam irradiation with different doses on neointimal hyperplasia in the rat carotid artery injury model.

METHODS AND MATERIALS

Twenty-seven Sprague-Dawley rats underwent endothelial denudation injury by 2F Fogarty balloons on carotid artery. Immediately after the injury, rats were irradiated externally using 6-MeV electrons. Rats were grouped according to the radiation doses, 0 Gy as controls (n = 5), 5 Gy (n = 5), 10 Gy (n = 5), 15 Gy (n = 6), and 20 Gy (n = 6). Then, rats were sacrificed after 2 weeks and the carotid arteries were perfusion-fixed in paraformaldehyde. External elastic lamina (EEL) area, lumen area, maximal intimal thickness (MIT), and intimal area (IA) of the injured segments were measured on the basis of histomorphometry.

RESULTS

In EEL and lumen area, there was no statistically significant difference between the irradiated groups and the controls. In MIT and IA, low-dose radiation (5 Gy and 10 Gy) did not induce any significant reduction. High-dose radiation (15 Gy and 20 Gy), however, reduced MIT and IA significantly.

CONCLUSION

External electron beam reduced the intimal area, and the inhibition of neointimal proliferation was dependent upon radiation doses. This study suggests that the minimal effective dose for the inhibition of neointimal hyperplasia following denudation injury in the rat carotid model is between 10 Gy and 15 Gy.

Radiation therapy to prevent coronary artery restenosis

The problem of restenosis after coronary angioplasty remains a major limiting factor of the procedure. Intracoronary stenting has led to a modest reduction in the frequency of this event. In the early 1990s, the effectiveness of ionizing radiation combined with balloon angioplasty and stenting was first convincingly demonstrated in animal models of restenosis. Small feasibility studies and two randomized trials have, in general, supported the promise of these initial preclinical studies in the prevention of restenosis. Much remains to be learned about the application of radiation for this therapy. This article reviews the current status of preclinical and clinical investigation of this therapy.

The molecular and cellular biologic basis for the radiation treatment of benign proliferative diseases

Since its discovery, radiation has been used to treat numerous ailments, including many benign conditions. The most susceptible disorders have included keloids, heterotopic bone formation, and, most recently, vascular restenosis. These disorders are proliferative in nature and fall under the category of excessive wound healing or scar formation after trauma. In addition, radiation has been used for its immunosuppressive quality, eg, in organ transplantation to suppress graft rejection and in the treatment of autoimmune diseases. In this article, we have chosen keloids as an archetype for radiation use with benign conditions; the radiation inhibition of vascular restenosis will be used as a prototype to explore a paradigm for the molecular and cellular basis of radiation treatment for selected benign disorders. Vascular restenosis is currently one of the new frontiers of radiation therapy and offers opportunities to explore the role of inflammatory or immune cell responses in benign conditions that lead to excessive fibrogenesis and require treatment. The pathophysiology of surgical wound healing has not been avidly studied in the radiobiologic laboratory setting. However, the paradigm we propose for the effectiveness of radiation treatment for benign conditions is based on the model offered by Clark. He describes three phases of molecular and cellular events in which an inflammatory phase precedes the fibrogenic phase, occurs within hours of injury, and continues for weeks. We postulate that the radiosensitive targets within the vascular milieu are the monocyte/macrophages that would otherwise act as the trigger for the induced cytokine cascade, leading to the myofibroblast being recruited from a quiescent to a proliferative phase, resulting in fibrogenesis.

Coronary atherosclerosis and interventions: pathological sequences and restenosis

The primary cause of cardiac morbidity and mortality in developed countries is ischemic (coronary) heart disease. The incidence of this disease is virtually all due to atherosclerosis, and ischemic heart disease is also the most prevalent disease in the industrialized world, causing over 40% of all deaths in the United States and Western Europe. In Japan, the incidence of ischemic heart disease due to coronary atherosclerosis is gradually increasing as well. Compared with the classical nomenclature of atherosclerosis; that is, fatty streak, fibrous plaque and complicated lesions, the term Stary's classification has been universally accepted because it reflects the more recently acquired knowledge about the morphological and biochemical details of the processes in coronary atherosclerosis, which have been obtained by new strategies such as angioscopy, intravascular ultrasound and molecular biological methods. The term Stary's classification has been applied for the coronary atherosclerosis of patients with acute coronary syndrome at the National Cardiovascular Center, for the analysis of predisposing atherosclerosis of these patients. The recent findings regarding acute coronary syndrome resulting from a rupture of coronary atherosclerotic plaques indicate that this syndrome is probably the most important mechanism underlying the sudden onset. It has been found that the risk of plaque rupture may depend more on plaque composition than on plaque size. Plaques rich in soft extracellular lipids and macrophages are possibly more vulnerable to plaque rupture. Two of the goals of the present review are to clarify how plaque disruption occurs and to elucidate the relationship between plaque disruption and coronary risk factors in elderly Japanese patients with acute coronary syndrome. Coronary stents have been shown to be efficacious in the treatment of acute and threatened closure complicating percutaneous transluminal coronary angioplasty (PTCA) and have produced encouraging initial results in the prevention of restenosis. In the autopsy study of restenosis after PTCA, it was observed that dense caps of collagen fibers in the adventitia in the vicinity of the disrupted internal elastic laminae were present in all of the remodeling lesions. It is suggested that remodeling, which resulted in adventitial scarring, is one of the major causative factors of restenosis after PTCA. The long-term success of stenting, however, remains limited by the occurrence of late in-stent restenosis, with an incidence of 20-42% depending on the stent design and the patient population studied. Another aim of the present review is to describe the pathological mechanism of restenosis after PTCA and/or stent replacement and, consequently, the vascular remodeling that occurs around adventitial tissue after PTCA and intimal hyperplasia that is chronically irritated by a foreign body granulomatous reaction after stenting. Finally, the results of the investigation of the effect of a tissue factor pathway inhibitor on the prevention of interventional restenosis is described.

Endovascular brachytherapy for peripheral vascular disease

The most common cause of morbidity and mortality in the United States is vascular disease, which afflicts a wide spectrum of organs such as the heart (cardiovascular system), brain (cerebrovascular system), kidney (renal system), liver (hepatic system), and extremities (peripheral vascular system). The most common pathology of vascular diseases is occlusion. Percutaneous Transluminal Angioplasty (PTA) is currently the most common nonsurgical treatment for obstructive arteries. Unfortunately, the long-term effectiveness of PTA is limited by a high restenosis rate. Efforts to reduce post-PTA restenosis, including laser, mechanical atherectomy, intravascular stenting, and pharmacologic agents, have not been successful. With recent advances in the pathogenesis of restenosis, we have learned that the major problem is the intimal hyperplastic reaction in response to vessel injury. Encouraging animal data in the use of various radiotherapeutic approaches to prevent restenosis has led to a large number of multi-national, multicenter, randomized trials on coronary vascular systems. Because early results have been in favor of radiation therapy, and because the basic process of restenosis is similar for coronary and noncoronary vascular systems, many investigators extend the application of radiotherapy to the prevention of restenosis in peripheral vascular systems. However, the clinical scenarios are much different for peripheral vascular systems than for the coronary vascular system. This article discusses the current views of the pathophysiology of restenosis, major clinical trials, and perspectives on future studies. Experimental studies on animal models have documented the profound effects of endovascular brachytherapy in reducing restenosis caused by angioplasty and stenting. Early results of clinical trials are encouraging and confirm these positive results. Long-term follow-up data are needed to show that radiation does prevent, not merely delay, restenosis; Several areas of opportunity exist for both basic science research and clinical studies to enhance our knowledge of the pathophysiology. This would optimize the treatment strategy, maximizing the benefits and minimizing late complications.

Balloon-artery interactions during stent placement: a finite element analysis approach to pressure, compliance, and stent design as contributors to vascular injury

Endovascular stents expand the arterial lumen more than balloon angioplasty and reduce rates of restenosis after coronary angioplasty in selected patients. Understanding the factors involved in vascular injury imposed during stent deployment may allow optimization of stent design and stent-placement protocols so as to limit vascular injury and perhaps reduce restenosis. Addressing the hypothesis that a previously undescribed mechanism of vascular injury during stent deployment is balloon-artery interaction, we have used finite element analysis to model how balloon-artery contact stress and area depend on stent-strut geometry, balloon compliance, and inflation pressure. We also examined superficial injury during deployment of stents of varied design in vivo and in a phantom model ex vivo to show that balloon-induced damage can be modulated by altering stent design. Our results show that higher inflation pressures, wider stent-strut openings, and more compliant balloon materials cause markedly larger surface-contact areas and contact stresses between stent struts. Appreciating that the contact stress and contact area are functions of placement pressure, stent geometry, and balloon compliance may help direct development of novel stent designs and stent-deployment protocols so as to minimize vascular injury during stenting and perhaps to optimize long-term outcomes.

Endothelial implants inhibit intimal hyperplasia after porcine angioplasty

The perivascular implantation of tissue-engineered endothelial cells around injured arteries offers an opportunity to study fundamental vascular physiology as well as restore and improve tissue function. Cell source is an important issue because the ability to implant either xenogeneic or allogeneic cells would greatly enhance the clinical applications of tissue-engineered grafts. We investigated the biological and immunological responses to endothelial cell xenografts and allografts in pigs 4 weeks after angioplasty of the carotid arteries. Porcine or bovine aortic endothelial cells were cultured within Gelfoam matrices and implanted in the perivascular space of 42 injured arteries. Both porcine and bovine endothelial cell grafts reduced the restenosis index compared with control by 54% and 46%, respectively. Perivascular heparin release devices, formulated to release heparin at twice the rate of release of heparan sulfate proteoglycan from endothelial cell implants, produced no significant reduction in the restenosis index. Endothelial cell implants also reduced occlusive thrombosis compared with control and heparin release devices. Host immune responses to endothelial implants were investigated by immunohistochemical examination of explanted devices and by immunocytochemistry of serum samples. The bovine cell grafts displayed infiltration of leukocytes, consisting primarily of lymphocytes, and caused an increase in antibodies detected in serum samples. Reduced cellular infiltration and no humoral response were detected in animals that received allografts. Despite the difference in immune response, the biological effects of xenografts or allografts did not differ significantly.

PlA polymorphism of platelet glycoprotein IIIa and risk of restenosis after coronary stent placement

BACKGROUND

Platelets play a central role in the process of restenosis after percutaneous coronary interventions. A polymorphism of platelet glycoprotein IIIa (PlA) has been associated with a higher risk of coronary thrombosis. We designed this prospective study to test the hypothesis that PlA polymorphism of glycoprotein IIIa is associated with an increased risk for restenosis after coronary stent placement.

METHODS AND RESULTS

The study included 1150 consecutive patients with successful coronary stent placement and 6-month follow-up with coronary angiography. The end point of the study was the incidence of angiographic restenosis (>/=50% diameter stenosis) at follow-up. Of the 1150 patients, 72.5% were homozygous for PlA1, 24.7% were heterozygous (PlA1/A2), and 2.8% were homozygous for PlA2. Patients with the PlA2 allele demonstrated a significantly higher restenosis rate than did those without (47% versus 38%; OR, 1.42; 95% CI, 1.09 to 1.84). The risk was highest in homozygous carriers of PlA2 (53.1% restenosis rate). After adjustment for several clinical and angiographic characteristics, the presence of the PlA2 allele remained a significantly independent risk factor for restenosis (adjusted OR, 1.35; 95% CI, 1.07 to 1.70). The influence of the PlA2 allele on restenosis was stronger in women. Women with PlA2 had a restenosis rate of 52% compared with the 33% incidence among women homozygous for PlA1 (OR, 2.21; 95% CI, 1.27 to 3.85).

CONCLUSIONS

This study showed a significant association between the PlA polymorphism of glycoprotein IIIa and the risk of restenosis after coronary stent placement. The risk was more pronounced in patients homozygous for PlA2 allele and in female patients.

In-stent neointimal proliferation correlates with the amount of residual plaque burden outside the stent: an intravascular ultrasound study

BACKGROUND

The aim of this study was to evaluate the relationship between residual plaque burden after coronary stent implantation and the development of late in-stent neointimal proliferation.

METHODS AND RESULTS

Between January 1996 and May 1997, 50 patients underwent intravascular ultrasound (IVUS) interrogation at 6+/-1.2 months after coronary stent implantation in native coronary arteries. IVUS images were acquired with a motorized pullback, and cross-sectional measurements were performed within the stents at 1-mm intervals. The following measurements were obtained: (1) lumen area (LA), (2) stent area (SA), (3) area delimited by the external elastic membrane (EEMA), (4) percent neointimal area calculated as (SA-LA/SA)x100, and (5) percent residual plaque area calculated as (EEMA-SA)/EEMAx100. Volume measurements within the stented segments were calculated by applying Simpson's rule. In the pooled data analysis of 876 cross sections, linear regression showed a significant positive correlation between percent residual plaque area and percent neointimal area (r=0.50, y= 45.03+0.29x, P<0.01). There was significant incremental increase in mean percent neointimal area for stepwise increase in percent residual plaque area. Mean percent neointimal area was 16.3+/-10.3% for lesions with a percent residual plaque area of <50% and 27.7+/-11% for lesions with a percent residual plaque area of >/=50% (P<0.001). The volumetric analysis showed that the percent residual plaque volume was significantly greater in restenotic lesions compared with nonrestenotic lesions (58.7+/-4.3% versus 51.4+/-5.7%, respectively; P<0.01).

CONCLUSIONS

Late in-stent neointimal proliferation has a direct correlation with the amount of residual plaque burden after coronary stent implantation, supporting the hypothesis that plaque removal before stent implantation may reduce restenosis.

The degree of neointimal formation after stent placement in atherosclerotic rabbit iliac arteries is dependent on the underlying plaque

The purpose of this study was to determine the effects of stent placement on the underlying arterial morphology and the relations of stent-vessel wall interactions with subsequent neointimal formation in an atherosclerotic artery. Seven New Zealand White rabbits with experimentally induced atherosclerosis underwent balloon angioplasty (n = 7) and stent placement after balloon angioplasty (n = 7) in the iliac arteries. Histologic analysis of the treated arteries was performed at 28 days to assess device interactions with the artery and the pattern of the neointimal response. The area within the external elastic lamina of the stented vessels was 66% greater than the arteries with balloon angioplasty alone (p = 0.001) which contributed to a significantly greater late lumen area (3.33 +/- 0.51 mm2 versus 1.33 +/- 0.20 mm2, p = 0.0028). Neointimal thickness was measured at 220 stent wire sites from 21 sections of stented arteries of which 139 (63%) had underlying plaque and 81 (37%) were adjacent to normal media. Rupture of the internal elastic lamina (IEL) occurred at only 9 (11%) of the 81 stent wire sites over normal media. The mean neointimal thickness was 0.16 +/- 0.01 mm lor all stent wire sites. The neointimal thickness was greater at the stent wire sites with underlying plaque (0.23 +/- 0.01 min) than at the stent wire sites adjacent to normal media (0.08 +/- 0.01 mm) or at sites with rupture of the internal elastic lamina (0.16 +/- 0.02 mm, p = 0.0001). The degree of neointimal formation within the stents strongly correlated with the area of the underlying atherosclerotic plaque (r = 0.76, p = 0.0007) and the extent of plaque or medial compression by the struts (r = 0.90, p = 0.006). The present study characterizes stent interactions in a model commonly employed to evaluate novel therapies for the prevention of restenosis. The neointimal response was influenced by the local arterial morphology and correlated with the extent of plaque or medial compression by the stent. These data may be useful for future studies in this model and understanding the mechanism of in-stent restenosis.

Temporary Strecker stent for management of acute dissection in popliteal and crural arteries

Stent placement is a widely used bail-out treatment for dissection of peripheral arteries. Below the level of the superficial femoral artery permanent stenting is complicated by a high incidence of subacute thrombosis and restenosis. We present two cases of arterial occlusion due to acute iatrogenic dissection of the popliteal and distal fibular arteries. Successful treatment was achieved with a new bail-out procedure. Strecker stents were implanted to seal off the dissection flap. Stents were retrieved easily after 24 hr using a myocardial biopsy forceps. After stent retrieval the temporarily stented segments were patent and showed a larger lumen compared with segments treated by balloon dilatation alone. Temporary stenting is a simple and safe procedure and offers the advantage of tacking up dissection membranes and preventing recoil. Persistent presence of a metallic implant as a source of continued injury and stimulus for intimal proliferation is avoided.

Chimeric DNA-RNA hammerhead ribozyme to proliferating cell nuclear antigen reduces stent-induced stenosis in a porcine coronary model

BACKGROUND

Stent-induced coronary restenosis is a major clinical and public health problem. Proliferating cell nuclear antigen (PCNA) is an important regulator of cell division, and blocking of its expression after angioplasty may limit intimal proliferation.

METHODS AND RESULTS

We cloned the porcine PCNA gene and constructed a chimeric hammerhead ribozyme to a segment of the gene with human homology. In vitro studies with both cultured porcine and human vascular smooth muscle cells demonstrated uptake of ribozyme within the nucleus and significant inhibition of cellular proliferation. The ribozyme was then delivered locally into pig coronaries in a stent model. At 30 days, histomorphometric analysis showed neointimal thickness of 0.51+/-0.20 mm in the ribozyme group versus 0.71+/-0.27 and 0.66+/-0.25 mm in stent controls and scrambled ribozyme control, respectively (P=0.002, P=0.03). Quantitative angiographic analysis showed late loss of 1.4+/-0.5 mm for ribozyme versus 1.9+/-0.4 and 2.0+/-0.4 mm for the controls (P=0.05 and P=0. 02).

CONCLUSIONS

Chimeric hammerhead ribozyme to PCNA inhibits smooth muscle cell proliferation in vitro and reduces both histomorphometric and angiographic restenosis in the porcine coronary stent model when delivered locally.

A randomized comparison of trapidil (triazolopyrimidine), a platelet-derived growth factor antagonist, versus aspirin in prevention of angiographic restenosis after coronary artery Palmaz-Schatz stent implantation

The aim of the study is to assess the safety of trapidil in the setting of coronary stenting and to evaluate its efficacy in reducing angiographic in-stent restenosis. One hundred eighteen patients undergoing Palmaz-Schatz stent implantation were randomly assigned to receive antiplatelet therapy using either aspirin (325 mg/d) or trapidil (400 mg/d), in combination with ticlopidine (500 mg) for the first month. At entry, both groups were comparable with regard to clinical, angiographic, and procedural characteristics. At 6-month angiographic follow-up, >50% restenosis occurred in 15 of 52 lesions (28.8%) of the aspirin group and in 14 of 47 lesions (29.8%) of the trapidil group (P=not significant, NS). At 6-month clinical follow-up, there was no difference in the two groups in the rate of adverse events (2.0% vs. 2.1%, P=NS), medication side effects (4.0% vs. 4.2%, P=NS), and peripheral vascular complications (4.0% vs. 4.2%, P=NS). In conclusion, treatment with trapidil seems to be associated with a similar incidence of stent restenosis and adverse cardiac events as compared to aspirin and could be a valuable alternative to aspirin in the setting of coronary stenting.

The experimental animal models for assessing treatment of restenosis

Coronary restenosis after percutaneous interventions remains a major clinical problem. The assessment of therapies for the prevention of restenosis relies on the use of experimental models. This review describes the most frequently used animal models of coronary artery retenosis and the intraspecies differences among them, particularly in the extent and composition of the neointimal thickening. These differences in neointima formation should be considered in the interpretation of effective antiproliferative therapies before they are transferred into clinical trials.

The American Brachytherapy Society perspective on intravascular brachytherapy

BACKGROUND

Recent clinical studies indicate that intravascular brachytherapy (IVB) can reduce the rate of restenosis substantially after angioplasty procedures. However, no clinical guidelines exist for optimal therapy.

METHODS

The members of the IVB Subcommittee of the American Brachytherapy Society (ABS) identified the areas of consensus and controversies in IVB to issue the ABS perspective on IVB, based on analysis of published reports and the clinical experience of the members in brachytherapy.

RESULTS

IVB is still experimental. The long-term efficacy, toxicity, the target tissue, and dose required for IVB are not established. The ABS recommends that IVB procedures must be performed, with careful attention to radiation-related issues, in the context of controlled multidisciplinary clinical trials with the approval of the institutional review board, the Nuclear Regulatory Commission, the Food and Drug Administration, and under an Investigational Device Exemption. The therapeutic radiologist, with a qualified radiation physicist, is responsible for dose prescription and delivery and needs to be present during the IVB procedure as part of this multidisciplinary team. The long-term outcome from these studies should be reviewed critically and published in peer-reviewed journals. The ABS endorsed the dosimetric guidelines of the American Association of Physicists in Medicine Task Group 60 (AAPM TG-60) report. The ABS recommends that dose specification be defined clearly; to allow comparisons between studies, the dose should be prescribed at 2 mm from the source for intracoronary brachytherapy and at an average luminal radius of +2 mm for peripheral vascular brachytherapy. The prescription doses at the above point is generally in the 12-18 Gy range. Comprehensive procedures for quality assurance, radiation protection, and emergencies should be in place before initiating an IVB program. Higher energy beta sources, lower energy gamma sources, dose-volume histograms, and correlation of three-dimensional reconstructions of delivered dose with patterns of failure are areas for further research.

CONCLUSION

The ABS perspective on IVB is presented to assist the interventional team in developing protocols for the use of IVB in the prevention of restenosis. Long-term outcome data with a standardized reporting system are needed to establish the role of brachytherapy in preventing vascular restenosis. Endovascular brachytherapy is a new and evolving modality, and these recommendations are subject to modifications as new data become available.

Neointimal response following rotational atherectomy compared to balloon angioplasty in a porcine model of coronary in-stent restenosis

In-stent restenosis remains a clinical therapeutic challenge. Rotational atherectomy (RA) is an attractive treatment option as it may cause less vascular injury than balloon angioplasty (BA) and, therefore, limit further neointimal response. In an animal model of coronary in-stent restenosis, thermal injury and stenting created neointima (old NI). The treatment of in-stent restenosis with either BA (n = 9) or RA (n = 11) also generated neointima (new NI). The average areas (mm2) of old NI in the BA and RA groups were similar (3.77 +/- 0.40 vs. 3.67 +/- 0.53; P = 0.32). However, new NI formed after treatment of in-stent restenosis was significantly less in the RA as compared to the BA group (0.33 +/- 0.12 vs. 0.73 +/- 36, P < 0.01). In this porcine coronary artery model of in-stent restenosis, treatment with rotational atherectomy resulted in significantly less recurrent neointimal hyperplasia than balloon angioplasty. This animal study, thus, provides a rationale for the clinical use of rotablation in the treatment of in-stent restenosis.

Relationship between oversizing of self-expanding stents and late loss index in carotid stenting

Stenting of the internal carotid artery is facilitated by stenting across the carotid bifurcation and sizing the diameter of a self-expanding stent to the large common carotid segment. This usually results in marked oversizing of the self-expanding stent in the internal carotid segment. This study was done to determine the relationship between stent oversizing and late luminal loss index after stenting of the internal carotid artery. Between September 1995 and March 1997, there were 165 patients (189 vessels) who underwent successful carotid stenting with self-expanding stents. Fifty-nine patients (63 vessels) had six-month follow-up carotid angiograms and on-line quantitative angiographic analysis. The mean reference diameter of the internal carotid arteries was 4.93+/-1.31 mm. Nominal stent size was 5 mm in 4 patients, 6 mm in 6 patients, 8 mm in 106 patients, 10 mm in 77 patients, and 12 mm in 1 patient. The average stent/patient was 1.03+/-0.16. There were three patients who had more than 50% diameter renarrowing at follow-up. The mean late loss index was 0.25+/-0.41. By linear regression analysis, there was no clear linear relationship between stent oversizing and late loss index after stenting (correlation coefficient = -0.21, P = 0.09). When analysis of variance with linear contrast was used to analyze six groups of different stent/artery ratios (from 1.4 to > or = 2), late loss indexes are significantly lower in the groups of high stent/artery ratio than the groups of low stent/artery ratio (P = 0.01). The process of oversizing of self-expanding stents deployed in the internal carotid artery does not appear to be associated with late restenosis and high stent/artery ratio seems to be associated with low late loss index.

Accuracy of single-photon emission computed tomography myocardial perfusion imaging in patients with stents in native coronary arteries

Strategies to noninvasively evaluate patients after coronary stenting have not been evaluated. To determine the accuracy of single-photon emission computed tomography (SPECT) myocardial perfusion imaging in patients after coronary stenting, 209 patients who had undergone stenting followed by late stress SPECT myocardial perfusion imaging were evaluated. Quantitative coronary angiography was performed in 33 patients following SPECT imaging. SPECT restenosis was defined as a reversible or fixed defect within the stented vascular territory. Angiographic restenosis was examined using 2 definitions: total area narrowing > or =50% or > or =70% of the stent site or stented artery. The SPECT and angiographic findings were concordant in 22 of 33 stented vascular territories using the 50% definition of restenosis and in 29 of 33 stented territories using the 70% definition. Use of the 70% definition of restenosis resulted in improved accuracy of SPECT to detect a significant stenosis in the stented artery. Sensitivity, specificity, positive predictive value, negative predictive value, and accuracy of SPECT were 95%, 73%, 88%, 89%, and 88% respectively. In patients with positive SPECT scans, the most significant stenosis in the stented artery was outside the stent site in 50% of cases. SPECT imaging appears to be accurate to predict significant stenosis in the stented artery, although the most severe stenosis is frequently distant from the stent site.

Biocompatibility of polyurethane-coated stents: tissue and vascular aspects

To assess the arterial injury triggered by polyurethane-coated vs. uncoated stents, six polyurethane-coated and six bare nitinol stents were implanted in rabbit carotid arteries. All animals were sacrificed 4 wk after stent placement. Sections were evaluated by histology and morphometric analysis. At 4 wk, both the coated and uncoated stent struts were entirely endothelialized. The spaces between the struts showed a relatively mild proliferative response, with a few sections demonstrating neovascularization around the struts. Polyurethane coating was associated with an inflammatory tissue response consisting of lymphocytic infiltration and foreign-body reaction, with the appearance of multinucleated giant cells. Lumen, intimal, and medial cross-sectional areas varied little between coated and uncoated stented vessels (2.45+/-0.19 vs. 2.47+/-0.47 mm2, 1.17+/-0.52 vs. 0.78+/-0.30 mm2, and 0.66+/-0.18 vs. 0.58+/-0.27 mm2, respectively). In the rabbit carotid artery model, polyurethane coating does not affect the degree of neointimal proliferation after endovascular stenting compared with the conventional stenting approach. However, the inflammatory tissue response may indicate a low intrinsic biocompatibility of this stable polymer, so that it may not be an ideal material for coating intravascular devices.

Different response to balloon angioplasty of carotid and coronary arteries: effects on acute platelet deposition and intimal thickening

PTCA is a well-established intervention to reduce the severity of atherosclerotic coronary stenosis. Its primary success rate is seriously handicapped by the high incidence of late restenosis. Given the clinical and social importance of this proliferative process, new strategies are needed to prevent or reduce restenosis. Several animal models as well as different arteries have been used to study neointimal proliferation after arterial injury. A number of agents have shown to reduce neointimal proliferation after arterial injury in the carotids and iliac arteries of rodent models. Unfortunately, these results have not been replicated in humans. We have compared the acute and late response to vascular injury of the carotid and coronary arteries in the pig. Arterial injury was induced by performing balloon angioplasty of the carotid (elastic) and coronary (muscular) arteries in swine. Acute platelet-thrombus formation was evaluated by quantitation of Indium-labeled platelets deposited on the injured segments 1 h after procedure. Measurement of intimal area was performed by morphometry of the most stenotic cross-section at 28 days after balloon angioplasty. Platelet deposition after mild and severe injury in carotids (4 +/- 1 and 56 +/- 13 x 10(6) platelets/cm2, respectively) and coronaries (15 +/- 5 and 141 +/- 20 x 10(6) platelets/cm2, respectively) are significantly greater in deep, than in mild injury (P < 0.005), and significantly greater in coronary than in carotid arteries after deep injury (P < 0.05). Likewise, late neointima formation was significantly greater (P < 0.05) after mild and severe injury in coronary (17 +/- 0.5 and 56 +/- 2%, respectively) than in carotid arteries (5 +/- 0.5 and 12 +/- 1%, respectively). Acute platelet-thrombus formation and late neointimal thickening are modulated by the degree of injury induced during the interventions; and after disruption of the internal elastic lamina, coronary arteries always had significantly more acute thrombus and neointimal thickening. This study emphasizes the importance of the animal species, the type of injury and the artery chosen for studies on restenosis post interventions.

Irradiation for the treatment of intimal hyperplasia

Intimal hyperplasia represents a serious complication limiting the long-term benefits of vascular interventions such as balloon angioplasty and stent placement. Although pharmacological interventions have attempted to curtail restenosis, they have not been shown to be effective to date. Radiotherapy is one alternative that has shown promise as an inhibitor of intimal hyperplasia in several animal models. Irradiation causes cell death by producing irreparable damage to DNA. This is believed to be the mechanism of inhibition of VSMC proliferation. Delivery of irradiation can be either intraluminal via an angiographically directed catheter or extraluminal using an external radiation source such as an x-ray device. Intraluminal irradiation has generally utilized either gamma or beta-emitting sources. Both have been effective in producing a dose response, although some studies advocate the use of beta-type irradiation as a safer, more efficient means of delivery. Extraluminal irradiation also has been an effective inhibitor of intimal hyperplasia. Studies suggest that this form of irradiation provides a more even-dose distribution to vessel walls than an intravascular delivery system. The use of radiotherapy has more recently been extended to clinical trials, and initial studies have shown promising results. The success of irradiation must be balanced with its potential complications including radiation-induced arteritis, coronary artery stenosis, and secondary development of malignancy. Although these have been associated with irradiation, the dose used in these cases was often considerably higher than those used in the treatment of intimal hyperplasia. Finally, with the advent of gene therapy, irradiation may provide an additional means of supplementing this new type of therapy through radiation-inducible gene therapy.

Local adenovirus-mediated transfer of human endothelial nitric oxide synthase reduces luminal narrowing after coronary angioplasty in pigs

BACKGROUND

Nitric oxide, synthesized from L-arginine by nitric oxide synthase (NOS), is a vasodilator and inhibits vascular smooth muscle cell (SMC) proliferation and migration. The effects of local NOS gene transfer on restenosis after experimental balloon angioplasty were investigated.

METHODS AND RESULTS

Left anterior descending coronary artery angioplasty was performed in 25 pigs. Animals received an intramural injection of adenovirus (1.5 x 10(9) pfu) carrying either the NOS cDNA (AdCMVceNOS) or no cDNA (AdRR5) via the Infiltrator. Local gene transfer efficiency and bioactivity of recombinant protein were assessed after 4 days. Indices of restenosis were evaluated by computerized planimetry on coronary artery sections prepared 28 days after angioplasty. Adenoviral vectors permitted efficient gene delivery to medial SMCs and adventitial cells of coronary arteries. Vascular cGMP levels were depressed after angioplasty from 1.30+/-0.42 to 0.33+/-0.20 pmol/mg protein (P<0.05) but were restored after constitutive endothelial (ce) NOS gene transfer to 1.82+/-0.98 pmol/mg (P<0.05 versus injured group and P=NS versus control). The ratio of the neointimal area to the internal elastic lamina fracture length, maximal neointimal thickness, and percent stenosis were all reduced in AdCMVceNOS- versus AdRR5-transduced pigs (0.59+/-0.14 versus 0.80+/-0.19 mm, P=0.02; 0.75+/-0.21 versus 1.04+/-0.25 mm, P=0.019; and 53+/-15% versus 75+/-11%, P=0.006, respectively). Lumen area was significantly larger (0.70+/-0.35 mm2 in AdCMVceNOS versus 0.32+/-0.18 mm2 in AdRR5, P=0.007).

CONCLUSIONS

Percutaneous adenovirus-mediated NOS gene transfer resulted in efficient local overexpression of functional NOS after angioplasty in coronary arteries. Restored NO production in injured coronary arteries significantly reduced luminal narrowing, most likely through a combined effect on neointima formation and on vessel remodeling after angioplasty.

Selective ET(A) receptor antagonism reduces neointimal hyperplasia in a porcine coronary stent model

BACKGROUND

As endothelin binds to ET(A) receptors, it stimulates vascular smooth muscle cell proliferation and may thus be pivotally involved in the pathogenesis of restenosis. This study assessed the ability of a potent and selective ET(A) antagonist to reduce neointimal hyperplasia in a porcine coronary artery stented injury model.

METHODS AND RESULTS

Fifty-five pigs were randomized to receive placebo or the oral ET(A)-selective antagonist ABT147627 twice daily for 28 days in one of three doses: 0.75 mg/kg (low), 3.75 mg/kg (mid), and 10.0 mg/kg (high). Each underwent oversized stent deployment in two randomly assigned major epicardial coronary arteries. Three animals (5.5%) died as a consequence of stent thrombosis within 24 hours of the procedure. The remaining 52 animals (13 pigs per group) survived without complication until predetermined euthanasia at 28 days. In the placebo group, mean injury score was 1.73+/-0.80, with a mean neointimal response of 0.45+/-0.24 mm. By comparison, the low-dose group had a similar mean injury score of 1.79+/-0.75 with reduced neointimal response, 0.36+/-0.22 mm (P<0.01). Mean injury score in the mid-dose animals was significantly greater than in the placebo group (1.94+/-0.92; P<0.05). The neointimal hyperplasia associated with this injury was less than with placebo, although the difference did not reach statistical significance (0.40+/-0.25 mm; P=0.05). In the high-dose pigs, mean injury score was also significantly greater than in the placebo arm (1.93+/-0.73; P<0.05). Despite this, neointimal response was also significantly less (0.37+/-0.37 mm; P<0.01).

CONCLUSIONS

Oral, selective ET(A) receptor antagonism significantly reduced neointimal hyperplasia forming over porcine coronary stented injuries in the first 28 days. This strategy may have clinical potential for the limitation and treatment of coronary restenosis after percutaneous revascularization.

Remodeling of in-stent neointima, which became thinner and transparent over 3 years: serial angiographic and angioscopic follow-up

BACKGROUND

Recently, it has been reported that the luminal diameter shows phasic changes after stenting: the progression of luminal narrowing followed by its regression. To elucidate the mechanisms involved in the phasic changes in luminal diameter after stenting, we examined the changes in neointimal thickness and the appearance of neointima by a series of angiographic and angioscopic observations for 3 years after stent implantation.

METHODS AND RESULTS

In 12 patients who received a Wiktor coronary stent, serial angiographic and angioscopic examinations were performed immediately, 2 to 4 weeks, 3 months, 6 months, and 3 years after the stenting without repetition of angioplasty. Neointimal thickness was determined by angiography as the difference between stent and luminal diameters. The angioscopic appearance of neointima over the stent was classified as transparent or nontransparent according to the visibility of the majority of the stent. Neointimal thickness increased significantly at 3 months (0.75+/-0.32 mm) without further changes at 6 months (0.74+/-0.32 mm). Thereafter, however, it decreased significantly over 3 years (0.51+/-0.26 mm). The angioscopic appearance was classified as transparent in 8 patients (100) immediately after stenting, 6 patients (100%) at 2 to 4 weeks, 2 patients (17%) at 3 months, 2 patients (20%) at 6 months, and 7 patients (58%) at 3 years.

CONCLUSIONS

The neointima became thick and nontransparent until 6 months and then became thin and transparent by 3 years. We conclude that neointimal remodeling exists after stenting and plays a major role in the alteration of coronary luminal diameter after stenting.

Fibrin-film stenting in a porcine coronary injury model: efficacy and safety compared with uncoated stents

OBJECTIVES

This study was designed to test the efficacy and safety of a fibrin-film-covered stent compared with that of a bare metal stent in the porcine coronary injury model.

BACKGROUND

Biodegradable stents are a potential method of achieving total lesion coverage and delivering local, lesion-specific drug therapy.

METHODS

Two coronary arteries in each pig were randomly assigned to deployment of either a fibrin-film or a bare tantalum wire-coil stent. An oversized balloon injury, 1.15 to 1.30 times the reference vessel diameter, was induced in each coronary segment before stenting to simulate angioplasty injury. Thirty pigs were studied: group 1 for 28 days (15 pigs); group 2 for 90 days (5 pigs); group 3 for 6 months (5 pigs); group 4 for 1 year (5 pigs).

RESULTS

Two pigs died of occlusion of the bare stent and one of occlusion of the fibrin stent (p > 0.99). There were no significant differences between the fibrin-stented and bare-stented coronary segments with regard to arterial injury. In group 1 (28 days, 14 pigs), the mean neointimal thicknesses in the fibrin-stented and bare-stented groups were 0.57+/-0.31 and 0.57+/-0.27 mm, respectively (p=0.89). In groups 2 to 4 (90 days, four pigs; 6 months, four pigs; 1 year, five pigs), the mean neointimal thicknesses for fibrin- and bare-stented coronary segments at the times studied were 0.48+/-0.26 versus 0.50+/-0.22 mm at 90 days; 035+/-0.04 versus 0.35+/-0.16 mm at 6 months; and 0.33+/-0.14 versus 0.30+/-0.14 mm at 1 year (p=0.98).

CONCLUSIONS

Fibrin-film stents appear to be an excellent candidate for local drug delivery because they can completely and safely cover the stented coronary segment while degrading slowly over 1 to 3 months. This result is important when compared with the poor results of previous studies of synthetic polymer stents.

Pathobiologic responses to stenting

As the interventions available to treat obstructive coronary artery disease become increasingly complex, our understanding of the vascular response to these interventions requires greater sophistication. Molecular and cellular biologic tools have now propelled us into another phase of work with vascular devices. Initially, studies of endovascular implants such as stents focused on developing models of vascular injury. With these models, we could then examine the designs of individual stents and connect structure with performance. Now we are entering a third phase in which stent manufacturers are looking to science to dictate stent design.

Prolonged thrombin inhibition reduces restenosis after balloon angioplasty in porcine coronary arteries

BACKGROUND

Arterial injury after percutaneous transluminal coronary angioplasty (PTCA) triggers acute thrombus formation and thrombin generation. Hirudin, a potent and direct thrombin inhibitor, prevents thrombus formation after arterial injury. Two large clinical trials showed marked reduction in acute clinical events but no long-term benefits in reducing restenosis during short-term administration of thrombin inhibitors. Our hypothesis is that adequate, maintained thrombin inhibition, by inhibiting all the thrombin-dependent mechanisms, will reduce neointima formation after PTCA.

METHODS AND RESULTS

Thirty-six pigs received three different regimens of hirudin: bolus (1 mg/kg), short-term (bolus + 0.7 mg/kg per day for 2 days), and long-term (bolus + 0.7 mg/kg per day for 14 days). The results on neointima formation at 4 weeks after coronary angioplasty were compared with the control group (100 IU heparin/kg bolus). Hirudin was continuously administered for 2 weeks through an infusion pump. In vivo thrombin generation was persistently increased up to 2 weeks after angioplasty. Inhibition of thrombin activity for 14 days reduced luminal narrowing by 40% (58+/-3% versus 35+/-3%; P<.001). No differences were observed among the bolus and short-term hirudin groups and the control group.

CONCLUSIONS

Our results indicate that there is a continued, marked thrombin generation that lasts for at least 2 weeks after PTCA. Administration of r-hirudin for 2 weeks significantly reduces neointima formation after PTCA. This observation, if extrapolated to humans, could explain the lack of effect on restenosis observed in the clinical trials with antithrombin agents despite the clear benefits on reducing acute thrombotic complications after PTCA. Therefore an adequate and prolonged administration of thrombin inhibitors is needed to "passivate" the thrombogenic substrate (disrupted arterial wall) and achieve full benefit of this therapeutic approach.

In-stent restenosis: contributions of inflammatory responses and arterial injury to neointimal hyperplasia

OBJECTIVES

We examined the relative contributions of inflammation and arterial injury to neointimal formation in a porcine coronary overstretch restenosis model.

BACKGROUND

Previous studies established that stents cause neointimal proliferation proportional to injury. Although inflammation has been postulated to be a major contributor to restenosis after angioplasty, there is a paucity of data on the relation between inflammation and subsequent neointimal formation.

METHODS

Twenty-one pigs underwent balloon injury followed by implantation of oversized, tubular, slotted stents (stent/artery ratio 1.2:1) in the left anterior descending coronary artery. Morphometric analysis of the extent of injury (graded as injury score 0 to 3) and inflammation (graded as inflammation score 0 to 3) 1 month later was assessed and correlated with neointimal formation.

RESULTS

An inflammatory reaction was observed in 20 of 21 pigs, and significant positive correlations were found between the degree of arterial injury and the extent of the inflammatory reaction (r = 0.80, p < 0.01) and between the extent of inflammatory reaction and the neointimal thickness (r = 0.75, p < 0.01), neointimal area (r = 0.53, p = 0.01) and percent area stenosis (r = 0.66, p < 0.01) within the stents. Importantly, there were areas with inflammation only in the absence of injury, and vice versa, that were also associated with neointimal hyperplasia.

CONCLUSIONS

These data suggest that the inflammatory reaction plays an equally important role as arterial injury in neointimal formation after coronary stenting, and that anti-inflammatory approaches may be of value to reduce in-stent restenosis.

Vitamins C and E inhibit O2- production in the pig coronary artery

BACKGROUND

We previously found in a pig coronary balloon injury model that vitamins C and E as well as probucol had beneficial effects on the vessel response to injury measured by morphometry These effects correlated with an inhibition in the ability to oxidize LDLs ex vivo, suggesting that the morphological response was due to the antioxidant effect of the treatments.

METHODS AND RESULTS

In the present study, the production of O2- by vessels 14 days after balloon injury was determined and correlated with circulating and tissue levels of vitamins C and E. Twenty-five domestic pigs were divided into four groups: control (n=7), vitamin C (500 mg/d, group C, n=6), vitamin E (1000 IU/d, group E, n=6), and vitamins C and E (500 mg/d and 1000 IU/d, group C+E, n=6). Vitamins were administered 7 days before oversized balloon injury of the left anterior descending coronary artery (LAD) and continued for 14 days after injury. Vitamin C and E concentrations were determined in plasma and lymphocytes as an index for tissue levels. Vessels were harvested after animals were killed, and O2- production was measured by lucigenin chemiluminescence. O2- production by the injured LAD was 2.5-fold greater than O2- production by the uninjured LAD or right coronary artery (RCA). The increase in O2- was caused primarily by cells present in the media and neointima. All vitamin-treated groups showed significantly decreased O2- production in both the RCA and LAD (approximately 45% inhibition) relative to vessels in the control, untreated group. There was a significant correlation between LAD O2- production and lymphocyte vitamin E levels.

CONCLUSIONS

The present study is the first to show increased O2- production in injured vessels and to demonstrate that antioxidant vitamins reduce O2- production. These results suggest that beneficial effects of antioxidant vitamins in coronary artery disease are related, in part, to alterations in vessel redox state.

Apoptosis after stent implantation compared with balloon angioplasty in rabbits. Role of macrophages

Both cell proliferation and apoptosis (programmed cell death) are supposed to play a role in restenosis after angioplasty. We studied these processes in smooth muscle cells (SMCs) and macrophages 1, 4, and 12 weeks after balloon angioplasty or Palmaz-Schatz stent implantation in rabbit iliac arteries. Proliferating cells were visualized by immunostaining with antibodies directed against proliferating cell nuclear antigen. Apoptotic cells were detected using the TUNEL (terminal deoxynucleotidyl transferase-mediated dUTP nick and labeling) technique, propidium iodide staining, and transmission electron microscopy. At all time points, the neointimal cross-sectional area of the arteries was twofold to fourfold greater after stent implantation than after balloon angioplasty. The total number of neointimal cells was similar 1 and 12 weeks after both interventions. The neointimal cell density, however, decreased by 58% between the 1st and the 12th week after stent implantation compared with a 20% decrease after balloon angioplasty (P < .01). Stent implantation induced more cell proliferation but also more apoptosis in the media than balloon angioplasty after 1 and 4 weeks. In addition, stent implantation caused more macrophage accumulation and apoptosis in the neointima, but cell proliferation rates did not differ significantly in comparison with balloon angioplasty. The higher rate of apoptosis in the neointima 1 week after stent implantation compared with balloon angioplasty is due to an increased rate of SMC and macrophage death. Macrophage accumulation and apoptosis in the early phase after stent implantation appear to play a role in extracellular matrix secretion, which increases neointima formation after 4 and 12 weeks compared with balloon angioplasty in this model.

Local delivery of ethanol inhibits intimal hyperplasia in pig coronary arteries after balloon injury

BACKGROUND

Smooth muscle cell (SMC) hyperplasia is an important mechanism of restenosis after coronary angioplasty and the primary mechanism of restenosis within coronary stents. Ethanol has been shown to reduce the response of SMCs to local growth stimulants in vitro. This study was carried out to determine whether local delivery of ethanol solution could reduce intimal hyperplasia induced by balloon injury.

METHODS AND RESULTS

Three groups of juvenile domestic pigs underwent oversized balloon dilation injury of the left anterior descending and left circumflex coronary arteries. Immediately after the balloon injury, one of the arteries was randomized to local delivery of 15% ethanol with a local delivery balloon catheter, and the other received no further treatment. Histological and morphometric studies were carried out at 2 weeks in group 1 (n=16) and at 4 weeks in group 2 (n=10). In the third group (n=15), animals were killed at days 4, 8, and 14 after balloon injury, and coronary artery segments were studied by immunohistochemical staining against proliferating cell nuclear antigen (PCNA) and bromodeoxyuridine (BrdU). Histological injury scores were not different between the ethanol-treated and untreated arterial segments in either group 1 or 2. The neointimal areas were significantly smaller in the ethanol-treated arterial segments than in the untreated segments (0.25+/-0.08 versus 0.57+/-0.08 mm2, P=.004, at 2 weeks; 0.33+/-0.05 versus 0.54+/-0.07 mm2, P=.03, at 4 weeks). SMC proliferative activity was significantly lower in ethanol-treated arteries than in untreated arteries at 4 and 8 days after injury by BrdU and PCNA staining.

CONCLUSIONS

Local delivery of 15% ethanol solution to pig coronary arteries significantly decreased the SMC proliferative activity and neointimal formation induced by balloon dilation injury.

Effect of intravascular irradiation on cell proliferation, apoptosis, and vascular remodeling after balloon overstretch injury of porcine coronary arteries

BACKGROUND

Ionizing radiation has been shown to reduce vascular lesion formation after balloon overstretch injury of pig coronary arteries. The present series of experiments examines the mechanism by which this occurs.

METHODS AND RESULTS

Balloon injury was performed on porcine coronary arteries, followed immediately by ionizing radiation using either a source train of 90Sr/Y or 192Ir seeds designed to deliver 14 or 28 Gy at a depth of 2 mm from the source. The animals were killed 3, 7, or 14 days after injury. Bromodeoxyuridine was administered 24 hours before euthanasia to label proliferating cells. Cell proliferation was significantly reduced on day 3 in the adventitia and media of the irradiated vessels compared with controls. Two weeks after injury, there were fewer alpha-actin-positive myofibroblasts in the adventitia of the irradiated vessels than in nonirradiated controls, and morphometric analysis indicated that the vessel perimeter of the irradiated vessels was significantly larger than in controls. Together, these results suggest a positive effect of intravascular irradiation on vascular remodeling. Apoptosis was estimated by terminal transferase dUTP-biotin nick-end labeling (TUNEL) 3 and 7 days after injury. TUNEL-labeled cells were found primarily in the adventitia at the medial tear, but no differences were detected between irradiated and control vessels.

CONCLUSIONS

These studies suggest that intracoronary radiation primarily inhibits the first wave of cell proliferation in the vessel wall and demonstrates a favorable effect on late remodeling by preventing adventitial fibrosis at the injury site.

Intracoronary radiation decreases the second phase of intimal hyperplasia in a repeat balloon angioplasty model of restenosis

PURPOSE

Repeat balloon angioplasty is likely to induce intimal proliferation, which is associated with a higher restenosis rate. This study examined the effect of intracoronary ionizing radiation on restenotic lesions using repeat balloon injury in a normolipemic swine.

METHODS AND MATERIALS

Eight domestic normolipemic pigs underwent overstretch balloon angioplasty with a 3.5 mm balloon in the LAD and LCX, followed by repeat balloon injury at the same sites 4 weeks after the initial injury. At that time a high activity 192Iridium source was introduced immediately after the angioplasty by random assignment to deliver 14 Gy at 2 mm in eight of the injured coronary arteries (LAD and LCX). One month later the animals were killed and the coronary arteries pressure perfusion fixed. Serial sections were stained with H&E and VVG, then evaluated by histopathologic and morphometric techniques. Maximal intimal thickness (MIT), intimal area (IA), and intimal area corrected for the extent of injury (IA/FL) were measured in the irradiated and control arteries and were compared to control arteries with single injuries from previous studies.

RESULTS

Repeat balloon injury induced significant additional medial damage, which was associated with marked intimal hyperplasia in a concentric pattern. Intracoronary irradiation significantly decreased the total of neointima area formation (IA 93 + 0.35 mm2 compared to control 1.38 + 0.33 mm2 p < 0.01) and the MIT was also significantly reduced in the irradiated vessels (0.57 + 0.18 mm vs. 0.71 + 0.08 mm, p = 0.05).

CONCLUSIONS

Intracoronary irradiation immediately after the second balloon dilatation inhibits the intimal hyperplasia due to that injury. However, there was no effect on the existing neointima from the initial injury.

Postangioplasty restenosis: a practical model in the porcine carotid artery

Transluminal coronary angioplasty is a routine therapeutic intervention in coronary heart disease. Despite the high rate of primary success, restenosis continues to be its major limitation. Porcine models have been considered to be the most adequate experimental models for studying restenosis. One limitation of porcine models is the need for radiological guidance and the expenses involved. The objective of the present study was to adapt an experimental model of angioplasty in the porcine carotid artery that does not require radiological equipment. Eight animals were used to develop the technique of balloon injury to the common carotid artery by dissection without radiological guidance. This technique was then employed in six other animals. Under anesthesia, the left common carotid artery was dissected and incised at the carotid sinus for insertion of an over-the-wire angioplasty balloon towards the aorta. Overstretch injury of the carotid artery was performed under direct visualization. After 30 days, the arteries were excised and pressure-fixated. Uninjured carotid arteries from 3 additional animals were used as controls. A decreased luminal area associated with intimal hyperplasia and medial reaction was observed in all injured arteries. Immunohistochemistry identified the intimal hyperplastic cells as smooth muscle cells. Computerized morphometry of the ballooned segments revealed the following mean areas: lumen 2.12 mm2 (+/- 1.09), intima 0.22 mm2 (+/- 0.08), media 3.47 mm2 (+/- 0.67), and adventitia 1.11 mm2 (+/- 0.34). Our experimental model of porcine carotid angioplasty without radiological guidance induced a vascular wall reaction and permitted the quantification of this response. This porcine model may facilitate the study of vascular injury and its response to pharmacological interventions.

Histopathology of restenosis after stenting of narrowed coronary arteries after cardiac transplantation during the teenage years

This study describes the detailed histopathologic appearance of human coronary arteries at 3 weeks, and 3 and 7 months after stent implantation in a cardiac transplant recipient. There was modest arterial injury associated with stent implantation, and immunocytochemistry staining provided evidence that a proliferative response from the adventitia contributes to neointimal hyperplasia.

Coronary stenting with the half (disarticulated) Palmaz-Schatz stent: immediate results and six-month follow-up

Coronary stenting with the half disarticulated Palmaz-Schatz stent is particularly suitable for ostial stenoses, diaphragm stenoses, stenoses distal to tortuous segments or coronary bends and localized dissections after balloon angioplasty. Nevertheless very few data regarding the half stent exist and follow-up data are nonexistent. From January of 1994 to December of 1995 a total of 207 half stents were implanted in 175 patients. Most patients had stable or unstable angina and in the majority of cases the stent was implanted due to localized dissection or to suboptimal result. The procedural success rate was 98%. After stent implantation, 82 patients were treated with acetylsalicylic acid (ASA) and oral anticoagulant (group A), whereas 93 were treated with ASA and ticlopidine (group B). Seven patients had subacute thrombosis (5, group A; 2, group B), and six patients had major bleeding (5, group A; 1, group B). Overall, patients in group A had more cardiovascular complications than patients in group B (10, group A; 3, group B; p = 0.047). After 6-mo follow-up, 1 patient had died and 27 patients had symptoms of angina (16%). Thirteen patients underwent a second PTCA (7%) and four patients (2%) were referred for coronary artery bypass. In conclusion, coronary stenting with half Palmaz-Schatz stent appears to be a safe and effective procedure. In selected cases, the half Palmaz-Schatz stent is easier to handle than the complete stent, it is associated with a low rate of clinical restenosis, and it lowers procedural costs.