Comparison of the morphological changes of restenosis after the implantation of various types of stents in a swine model

Preclinical Models of Restenosis and Their Application in the Evaluation of Drug-Eluting Stent Systems

Coronary arterial disease (CAD) is the leading cause of death in the United States, the European Union, and Canada. Percutaneous coronary intervention (PCI) has revolutionized the treatment of CAD, and it is the advent of drug-eluting stent (DES) systems that has effectively allayed much of the challenge of restenosis that has plagued the success of PCI through its 30-year history. However, DES systems have not been a panacea: There yet remain the challenges associated with interventions involving bare metallic stents as well as newly arisen concerns related to the application of DES systems. To effectively address these novel and ongoing issues, animal models are relied on both to project the safety and efficacy of endovascular devices and to provide insight into the pathophysiology underlying the vascular response to injury and mechanisms of restenosis. In this review, preclinical models of restenosis are presented, and their application and limitation in the evaluation of device-based interventional technologies for the treatment of CAD are discussed.

Protease-resistant insulin-like growth factor (IGF)-binding protein-4 inhibits IGF-I actions and neointimal expansion in a porcine model of neointimal hyperplasia

IGF-I has been shown to play a role in the progression of atherosclerosis in experimental animal models. IGF-binding protein-4 (IGFBP-4) binds to IGF-I and prevents its association with receptors. Overexpression of a protease-resistant form of IGFBP-4 has been shown to inhibit the ability of IGF-I to stimulate normal smooth muscle cell growth in mice. Based on these observations, we prepared a protease-resistant form of IGFBP-4 and infused it into hypercholesterolemic pigs. Infusion of the protease-resistant mutant inhibited lesion development by 53.3 +/- 6.1% (n = 6; P < 0.01). Control vessels that received an equimolar concentration of IGF-I and the protease-resistant IGFBP-4 showed no reduction in lesion size compared with control lesions that were infused with vehicle. Infusion of a nonmutated form of IGFBP-4 did not significantly inhibit lesion development. Proliferating cell nuclear antigen analysis showed that the mutant IGFBP-4 appeared to inhibit cell proliferation. The area occupied by extracellular matrix was also reduced proportionally compared with total lesion area. Immunoblotting revealed that the mutant IGFBP-4 remained intact, whereas the wild-type IGFBP-4 that was infused was proteolytically cleaved. Further analysis of the lesions revealed that a marker protein, IGFBP-5, whose synthesis is stimulated by IGF-I, was decreased in the lesions that received the protease-resistant, IGFBP-4 mutant, whereas there was no change in lesions that received wild-type IGFBP-4 or the mutant protein plus IGF-I. These findings clearly illustrate that infusion of protease-resistant IGFBP-4 into the perilesion environment results in inhibition of cell proliferation and attenuation of the development of neointima. The findings support the hypothesis that inhibiting IGFBP-4 proteolysis in the lesion microenvironment could be an effective means for regulating neointimal expansion.

Coronary artery overexpansion increases neointimal hyperplasia after stent placement in a porcine model

OBJECTIVE

To determine if an aggressive approach to coronary revascularisation with oversized balloons is counterproductive, we studied the effect of increasing balloon-to-artery (B:A) ratio on neointimal hyperplasia following primary stent placement using a non-atherosclerotic porcine coronary overstretch model.

METHODS

60 vessels in 33 Yorkshire swine were randomly assigned to one of five B:A ratios between 1.0:1 and 1.4:1. Intravascular ultrasound (IVUS) imaging was performed before bare-metal stent placement to accurately determine vessel size, after stent placement, and at 28 days.

RESULTS

The mean prestent vessel diameter was 3.05 (0.31) (SD) mm. In-stent neointimal volume, in-stent volume stenosis and cross-sectional area stenosis at the stent minimum lumen diameter increased significantly with increasing achieved B:A ratio (multilevel regression test for slope, p<0.001, p = 0.002 and p<0.001, respectively) and were independent of vessel size. Even minor vessel overstretch at an achieved B:A ratio of 1.1:1 resulted in significant neointimal hyperplasia. Larger B:A ratios were also associated with more neointima beyond the stent edges (p = 0.008). For vessels from the same animal, neointimal response at a given B:A ratio was dependent upon the animal treated.

CONCLUSIONS

In a porcine model of IVUS-guided coronary primary stent placement, vessel overexpansion is counterproductive. Neointimal hyperplasia at 28 days is strongly associated with increasing B:A ratio. In addition, vessels do not respond independently of each other when multiple stents are placed within the same animal using a range of B:A ratios.

Rho-kinase inhibitor suppressed restenosis in porcine coronary balloon angioplasty

BACKGROUND

Constrictive remodeling is thought to be more important than neointimal formation in coronary restenosis after balloon angioplasty. The inhibition of Rho-kinase prevents neointimal proliferation, but now this inhibition that affects constrictive remodeling remains unknown. To explore this issue further, we investigated whether a specific Rho-kinase inhibitor, Y-27632, could suppress restenosis after coronary balloon angioplasty in a porcine model.

METHODS

Balloon angioplasty with local administration of Y-27632 (Y group) or vehicle (C group) was performed at 2 and 3 weeks after overstretch injury in a porcine coronary artery. Quantitative coronary angiography (QCA) and quantitative coronary ultrasound (QCU) were performed to assess the coronary lesion segment. A morphometrical analysis was performed in a histological study. Proliferative cells and p27(Kip1)-positive cells were evaluated in the arterial wall using immunohistochemistry.

RESULTS

QCA and QCU demonstrated that the minimal lumen diameter and minimal lumen area were greater, and % stenosis was less in the Y group than in the C group. The QCU analysis also revealed a significant inhibition in the increase of the intimal area and a prevention of constrictive remodeling by Y-27632. In the histological study, the intimal, adventitial and collagen areas were significantly smaller in the Y group than in the C group. The Y group also exhibited significantly less proliferative activity and a significantly higher percentage of cells expressing p27(Kip1) in the arterial wall.

CONCLUSION

Local delivery of Y-27632 suppressed constrictive remodeling as well as neointimal formation after coronary balloon angioplasty in pigs.

Eosinophilic infiltration in restenotic tissue following coronary stent implantation

OBJECTIVES

The aim of our study was to compare the histopathological features of restenotic tissue after balloon angioplasty and after stent placement. We emphasized on specific types of inflammatory cells to evaluate the type of tissue immune response in both situations.

METHODS

A total of 32 patients underwent elective directional coronary atherectomy; 16 patients had restenosis after balloon angioplasty, 16 patients had in-stent restenosis (ISR). Atherectomy specimens were stained with antibodies against T cells, eosinophils, smooth muscle cell actin, macrophages and with antibodies against T cell activation markers. Quantitative morphometric analysis was performed using image analysis software.

RESULTS

In-stent restenotic tissue contained more smooth muscle cells (P < 0.001), anti-CD3 positive T cells (P < 0.001) and eosinophils (P = 0.012). Anti-CD40L positive activated T cells were more numerous in ISR lesions (P = 0.003) and were frequently clustered around stent imprints in the tissue. Five ISR specimens contained grossly visible stent fragments amidst the restenotic tissue. In all cases of balloon restenosis, T cells and eosinophils (if present) were concentrated around lipid rich tissue.

CONCLUSIONS

Our study indicates involvement of inflammatory responses in both types of restenosis, with significantly more eosinophils encountered in case of in-stent restenosis. In contrast with clustering of inflammatory cells around stent struts after stent placement, the inflammatory cells in balloon restenosis were located in association with lipid rich tissue, suggesting different inflammatory triggers in balloon restenosis and in-stent restenosis.

The Mechanism of In-Stent Restenosis in Radius Stent-An Experimental Porcine Study-

BACKGROUND

We investigated the mechanism of in-stent restenosis in radius stents in comparison to balloon-expandable stent (NIR stent) in pigs, with a focus on extracellular matrix (ECM).

METHODS AND RESULTS

Radius (n = 4) or NIR (n = 4) stents were implanted in the left coronary arteries of miniature pigs. Quantitative coronary ultrasound (QCU) was performed before, immediately after, and at 1 and 4 weeks after the implantation. The stented-coronary arteries were harvested at 4 weeks after the implantation followed by immunohistochemical, histological, reverse transcription-polymerase chain reaction (RT-PCR) and real-time PCR studies. In QCU, mean luminal areas at 4 weeks did not differ between both groups, whereas the mean stent area and neointimal area were significantly greater in the radius (p < 0.01). The immunohistochemical study revealed a significantly decreased number of neointimal macrophages and neovascularizations (p < 0.05, p < 0.01, respectively), and a stronger expression of tenascin-C in the radius. The histological study showed a larger ECM area and less neointimal cell density in the radius than in the NIR. The RT-PCR and real-time PCR analysis revealed an enhanced expression of tanascin-C mRNA in the radius than in the NIR.

CONCLUSIONS

Increased production of ECM, especially tenascin-C, played a greater role in the neointimal formation in the radius stent than inflammation.

The influence of physical stent parameters upon restenosis

In this paper we examine whether the structure, geometry and dimensions of coronary stents influence the occurrence of restenosis. Whilst many consider these parameters to be less important since the advent of drug-eluting stents, this view reveals a poor appreciation of the technological development of stents over the last 18 years. Early 'slotted tube' stents were completely inflexible and posed major problems for delivery; and early 'coil' stents had poor radial strength, allowing considerable tissue prolapse. Nowadays, we are used to greatly improved physical stent parameters, which provide better deliverability, visibility, procedural success and scaffolding performance. Many of these physical parameters also impact upon restenosis, even in the current era of drug-eluting stent. In this paper we examine the contribution of mode of expansion (self vs. balloon-expandable), design (coil vs. tube), length and width to restenosis. We also consider the more subtle influence of advanced slotted tube vs. modular design, percent metal coverage, strut thickness, strut shape, surface smoothness and alloy composition.