Histopathologic analysis of in-stent neointimal regression in a porcine coronary model

Effects of a Matrix Metalloproteinase Inhibitor-Eluting Stent on In-Stent Restenosis

Background

The aim of this study was to compare changes in the extracellular matrix after implantation of a stent that elutes a matrix metalloproteinase (MMP) inhibitor (GM6001); and to determine the effects of the GM6001-eluting stent upon prevention of in-stent restenosis (ISR).

Material/Methods

We included 48 Guangxi Bama mini-pigs in this study. A GM6001-eluting stent was placed in one iliac artery and a stent that did not elute GM6001 was placed in the contralateral iliac artery. The iliac arteries were removed at 6 hours as well as 1, 7, 14, 56, 84, and 336 days after stent placement. Arteries were analyzed for morphometry, gelatinase content, different phenotypes of vascular smooth muscle cells (VSMCs), collagen content, apoptotic rate, and cell density.

Results

The vascular lumen areas of the GM6001 group were significantly increased and the neointimal areas were significantly reduced compared with the control group from the 7 days to the 336 days. In the 2 groups, expression of MMP-2 and MMP-9 peaked simultaneously, but GM6001-eluting stents inhibited expression of MMP-2 and MMP-9 in the vascular media and neointima (especially around the struts) significantly. In the GM6001 group, expression of tissue inhibitor of matrix metalloproteinase (TIMP)-1, TIMP-2, myosin heavy chain 10 (MYH-10, marker of the proliferative phenotype of VSMCs), collagen content, percentage of apoptotic cells, and cell density were also decreased significantly compared with those in the control group.

Conclusion

Use of GM6001-eluting stents resulted in persistent and potent inhibition of intimal hyperplasia, an increase in luminal area, and no obvious thrombosis in the arteries of the mini-pigs.

Apolipoprotein E deficient rats generated via zinc-finger nucleases exhibit pronounced in-stent restenosis

The long-term success of coronary stent implantation is limited by in-stent restenosis (ISR). In spite of a broad variety of animal models available, an ideal high-throughput model of ISR has been lacking. Apolipoprotein E (apoE) deficient rats enable the evaluation of human-sized coronary stents while at the same time providing an atherogenic phenotype. Whereas apoE deficient rats have been proposed as animal model of atherosclerosis, to date it is unknown whether they also develop pronounced ISR. We sought to assess ISR after abdominal aorta stent implantation in apoE deficient rats. A total of 42 rats (16 wildtype, 13 homozygous apoE^−/− and 13 heterozygous apoE^+/− rats) underwent abdominal aorta stent implantation. After 28 days blood samples were analyzed to characterize lipid profiles. ISR was assessed by histomorphometric means. Homozygous apoE^−/− rats exhibited significantly higher total cholesterol and low-density cholesterol levels than wildtype apoE^+/+ and heterozygous apoE^+/− rats. ISR was significantly pronounced in homozygous apoE^−/− rats as compared to wildtype apoE^+/+ (p = <0.0001) and heterozygous apoE^+/− rats (p = 0.0102) on western diet. Abdominal aorta stenting of apoE^−/− rats is a reliable model to investigate ISR after stent implantation and thus can be used for the evaluation of novel stent concepts. Apolipoprotein E (apoE) deficient rats have been proposed as animal model of atherosclerosis. We investigated the development of restenosis 28 days after stent implantation into the abdominal aorta of wildtype apoE^+/+, homozygous apoE^−/− and heterozygous apoE^+/− rats, respectively. Homozygous apoE^−/− rats exhibited significantly higher LDL and significantly lower HDL cholesterol levels compared to wildtype apoE^+/+ and heterozygous apoE^+/− rats. Restenosis after stent implantation was significantly pronounced in western-diet-fed homozygous apoE^−/− rats, accompanied by a significantly increased neointimal thickness. Thus, apoE knockout rats exhibit elevated restenosis and might provide a novel tool for testing of innovative stent concepts.

Altered processing enhances the efficacy of small-diameter silk fibroin vascular grafts

Current synthetic vascular grafts are not suitable for use in low-diameter applications. Silk fibroin is a promising natural graft material which may be an effective alternative. In this study, we compared two electrospun silk grafts with different manufacturing processes, using either water or hexafluoroisopropanol (HFIP) as solvent. This resulted in markedly different Young's modulus, ultimate tensile strength and burst pressure, with HFIP spun grafts observed to have thicker fibres, and greater stiffness and strength relative to water spun. Assessment in a rat abdominal aorta grafting model showed significantly faster endothelialisation of the HFIP spun graft relative to water spun. Neointimal hyperplasia in the HFIP graft also stabilised significantly earlier, correlated with an earlier SMC phenotype switch from synthetic to contractile, increasing extracellular matrix protein density. An initial examination of the macrophage response showed that HFIP spun conduits promoted an anti-inflammatory M2 phenotype at early timepoints while reducing the pro-inflammatory M1 phenotype relative to water spun grafts. These observations demonstrate the important role of the manufacturing process and physical graft properties in determining the physiological response. Our study is the first to comprehensively study these differences for silk in a long-term rodent model.

Location-Specific Comparison Between a 3D In-Stent Restenosis Model and Micro-CT and Histology Data from Porcine In Vivo Experiments

Background

Coronary artery restenosis is an important side effect of percutaneous coronary intervention. Computational models can be used to better understand this process. We report on an approach for validation of an in silico 3D model of in-stent restenosis in porcine coronary arteries and illustrate this approach by comparing the modelling results to in vivo data for 14 and 28 days post-stenting.

Methods

This multiscale model includes single-scale models for stent deployment, blood flow and tissue growth in the stented vessel, including smooth muscle cell (SMC) proliferation and extracellular matrix (ECM) production. The validation procedure uses data from porcine in vivo experiments, by simulating stent deployment using stent geometry obtained from micro computed tomography (micro-CT) of the stented vessel and directly comparing the simulation results of neointimal growth to histological sections taken at the same locations.

Results

Metrics for comparison are per-strut neointimal thickness and per-section neointimal area. The neointimal area predicted by the model demonstrates a good agreement with the detailed experimental data. For 14 days post-stenting the relative neointimal area, averaged over all vessel sections considered, was 20 ± 3% in vivo and 22 ± 4% in silico. For 28 days, the area was 42 ± 3% in vivo and 41 ± 3% in silico.

Conclusions

The approach presented here provides a very detailed, location-specific, validation methodology for in silico restenosis models. The model was able to closely match both histology datasets with a single set of parameters. Good agreement was obtained for both the overall amount of neointima produced and the local distribution. It should be noted that including vessel curvature and ECM production in the model was paramount to obtain a good agreement with the experimental data.

Electronic supplementary material

The online version of this article (10.1007/s13239-019-00431-4) contains supplementary material, which is available to authorized users.

The occurrence of neointimal hyperplasia after flow-diverter implantation is associated with cardiovascular risks factors and the stent design

Background

Neo-intimal hyperplasia (NIH) is frequently observed after flow-diverter stent (FDS) implantation. Although mostly asymptomatic, this vascular response can sometimes lead to delayed ischemic strokes. This study intended to evaluate the factors potentially influencing the rates of NIH following FDS treatment.

Material and Methods

All aneurysm treatments performed with a Pipeline embolization device (PED) or a SILK stent from May 2011 to May 2015 were collected in a prospectively maintained database. Patient demographics, clinical, and angiographic outcomes including both digital subtraction angiography and C-arm cone-beam CT were registered. Two blind reviewers rated the presence of NIH on a binary scale (present/absent).

Results

From 148 patients, 63 datasets were available for analysis. Inter-reader agreement was excellent (Kappa=0.88). NIH was positively correlated with smoking, dyslipidemia, and high blood pressure, but not with aneurysm characteristics. At early follow-up (<12 months), NIH was more frequently associated with the use of the SILK stent (68%) rather than the PED (38%): P<0.02. At long-term follow-up, the NIH rate in the total population dropped from 55% to 26% with no more significant difference between the two stents. The complete occlusion rate as seen in early follow-up was higher in the SILK group with 76% vs 65% but without statistical significance (P=0.4).

Conclusion

NIH is a dual-vessel reaction after FDS implant. When planning a treatment in locations at risk of ischemic complications if severe NIH would occur, then the stent design should be considered. However, minimal NIH might also be needed as it is involved in aneurysm healing. Before treatment patients should be recommended best medical management of their cardiovascular risks factors to prevent an excessive NIH reaction.

Histopathological comparison of biodegradable polymer and permanent polymer based sirolimus eluting stents in a porcine model of coronary stent implantation

Biodegradable stent coatings were recently introduced as a potential solution to overcome sustained inflammatory responses observed with permanent polymer-based drug-eluting stents. In a preliminary study, selected biodegradable or permanent polymer-based sirolimus-eluting stent (SES) formulations were screened for effectiveness in comparison to bare metal stents (BMS) at 28 days. Subsequently, the most favourable SES formulation was compared to commercially available SES (CypherTM) at 28,90 and 180 days to investigate the histopathologic response as well as tissue, blood and organ pharmacokinetics. Overlapping SES implantation was conducted to evaluate vascular healing at 28 days in this particular setting. SES with biodegradable poly (L-lactide) polymer (PLLA) or poly(lactide-co-glycolide) showed the most favourable outcome with regards to reductions in neointimal area in comparison to BMS at 28 days. The PLLA SES showed a similar reduction in neointimal area compared to CypherTM at 28 days, with significant greater reductions at 90 and 180 days (1.7 ± 0.7 mm2 vs. 3.1 ± 1.5 mm2, p=0.03 and 1.8 ± 1.2 mm2 vs. 3.0 ± 1.5 mm2, p=0.01, respectively). Sirolimus vascular tissue concentrations were detectable up to 90 days following implantation. Overlapping stented segments showed favourable histopathologic results with respect to fibrin deposition and endothelialisation at 28 days. In conclusion, the use of PLLA as drugeluting matrix resulted in mild inflammatory responses in the presence of effective sirolimus tissue concentrations. The greater efficacy observed at long-term follow-up in PLLA SES compared to CypherTM may be a multifactorial result of stent design, polymer biocompatibility and improved release kinetics.

Safety and Efficacy of New Biodegradable Polymer-based Sirolimus-Eluting Stents in a Preclinical Model

INTRODUCTION AND OBJECTIVES

New drug-eluting stents (DES) designed to overcome the limitations of existing devices should initially be tested in preclinical studies. Our objective was to analyze the safety and efficacy of new biodegradable polymer-based DES compared with bare-metal stents (BMS) and commercially available DES in a model of normal porcine coronary arteries.

METHODS

We randomly implanted 101 stents (BMS and biodegradable polymer-based sirolimus-eluting stents: 3 test stent iterations [BD1, BD2, and BD3], Orsiro, Biomime and Biomatrix) in the coronary arteries of 34 domestic pigs. Angiographic and histomorphometric studies were conducted 1 month (n = 83) and 3 months (n = 18) later.

RESULTS

The stents were implanted at a stent/artery ratio of 1.31 ± 0.21, with no significant differences between groups. At 1 month, the new test stents (BD1, BD2 and BD3) showed less late loss and angiographic restenosis, as well as lower histologic restenosis and neointimal area (P < .0005), than the BMS. There were no differences in endothelialization, vascular injury, or inflammation between the new test stents and BMS, although the new stents showed higher fibrin deposition (P = .0006). At 3 months, all these differences disappeared, except for a lower neointimal area with the new BD1 stent (P = .027). No differences at any time point were observed between the new test stents and commercially available controls.

CONCLUSIONS

In this preclinical model, the new biodegradable polymer-based DES studied showed less restenosis than BMS and no significant differences in safety or efficacy vs commercially available DES.

A Comparison of Fully-Coupled 3D In-Stent Restenosis Simulations to In-vivo Data

We describe our fully-coupled 3D multiscale model of in-stent restenosis, with blood flow simulations coupled to smooth muscle cell proliferation, and report results of numerical simulations performed with this model. This novel model is based on several previously reported 2D models. We study the effects of various parameters on the process of restenosis and compare with in vivo porcine data where we observe good qualitative agreement. We study the effects of stent deployment depth (and related injury score), reendothelization speed, and simulate the effect of stent width. Also we demonstrate that we are now capable to simulate restenosis in real-sized (18 mm long, 2.8 mm wide) vessel geometries.

Preclinical randomised safety, efficacy and physiologic study of the silicon dioxide inert-coated Axetis and bare metal stent: short-, mid- and long-term outcome

AIMS

To evaluate the short-, mid- and long-term safety, efficacy and vascular physiology of Axetis silicon dioxide (SiO2, abrading the micropores) inert-coated stent implantation in a randomised preclinical setting.

METHODS AND RESULTS

Coronary arteries of domestic pigs were randomised to receive either Axetis or BMS (same design) stents with one-, three- and six-month follow-up (FUP), controlled by coronary angiography, optical coherence tomography (OCT), intravascular ultrasound (IVUS) and histology (n=32). The time-dependent vasomotor reaction of coronary arteries to stenting was measured using modified myography (n=12). Complete endothelialisation of the Axetis stent was confirmed by OCT, IVUS and histology at one-month FUP. Histopathology revealed continuous healing of the vessel wall with a gradual reduction of inflammation and fibrin score during the six-month FUP in both stent types. Significantly smaller neointimal area and %area stenosis were measured in Axetis stents compared with BMS at each FUP time point. Vascular reactivity measurements showed significantly better endothelium-dependent vasodilation of stented arteries with Axetis implantation.

CONCLUSIONS

Implantation of the Axetis SiO2-coated stent resulted in a significantly better safety, efficacy and vessel physiology profile compared with BMS of the same design with a continuous decrease in vessel inflammation during the six-month FUP.

Ex vivo assessment of vascular response to coronary stents by optical frequency domain imaging

OBJECTIVES

This study sought to examine the capability of optical frequency domain imaging (OFDI) to characterize various morphological and histological responses to stents implanted in human coronary arteries.

BACKGROUND

A precise assessment of vascular responses to stents may help stratify the risk of future adverse events in patients who have been treated with coronary stents.

METHODS

Fourteen human stented coronary segments with implant duration ≥ 1 month from 10 hearts acquired at autopsy were interrogated ex vivo by OFDI and intravascular ultrasound (IVUS). Comparison with histology was assessed in 134 pairs of images where the endpoints were to investigate: 1) accuracy of morphological measurements; 2) detection of uncovered struts; and 3) characterization of neointima.

RESULTS

Although both OFDI and IVUS provided a good correlation of neointimal area with histology, the correlation of minimum neointimal thickness was inferior in IVUS (R(2) = 0.39) as compared with OFDI (R(2) = 0.67). Similarly, IVUS showed a weak correlation of the ratio of uncovered to total stent struts per section (RUTSS) (R(2) = 0.24), whereas OFDI maintained superiority (R(2) = 0.66). In a more detailed analysis by OFDI, identification of individual uncovered struts demonstrated a sensitivity of 77.9% and specificity of 96.4%. Other important morphological features such as fibrin accumulation, excessive inflammation (hypersensitivity), and in-stent atherosclerosis were characterized by OFDI; however, the similarly dark appearance of these tissues did not allow for direct visual discrimination. The quantitative analysis of OFDI signal reflections from various in-stent tissues demonstrated distinct features of organized thrombus and accumulation of foamy macrophages.

CONCLUSIONS

The results of the present study reinforce the potential of OFDI to detect vascular responses that may be important for the understanding of long-term stent performance, and indicate the capability of this technology to serve as a diagnostic indicator of clinical success.

Treatment of intracranial aneurysms using flow-diverting silk stents (BALT): a single centre experience

The Silk stent (Balt, Montmorency, France) is a retractable device designed to achieve curative reconstruction of the parent artery associated with an intracranial aneurysm. We present our initial experience with the Silk flow-diverting stent in the management and follow-up of 25 patients presenting with intracranial aneurysms.Twenty-five patients (age range, 34-81 years; 24 female) were treated with the Silk flow-diverting device. Aneurysms ranged in size from small (5), large (10) and giant (10) and included wide-necked aneurysms, multiple, nonsaccular, and recurrent intracranial aneurysms. Nine aneurysms were treated for headache, 14 for mass effect. None presented with haemorrhage. All patients were pretreated with dual antiplatelet medications for at least 72 hours before surgery and continued taking both agents for at least three months after treatment. A total of 25 Silk stents were used. Control MR angiography and/or CT angiography was typically performed prior to discharge and at one, three, six and 12 months post treatment. A follow-up digital subtraction angiogram was performed between six and 19 months post treatment.Complete angiographic occlusion or subtotal occlusion was achieved in 15 patients in a time frame from three days to 12 months. Three deaths and one major complication were encountered during the study period. Two patients, all with cavernous giant aneurysms, experienced transient exacerbations of preexisting cranial neuropathies and headache after the Silk treatment. Both were treated with corticosteroids, and symptoms resolved completely within a month.In our experience the Silk stent has proven to be a valuable tool in the endovascular treatment of intracranial giant partially thrombosed aneurysms and aneurysms of the internal carotid artery cavernous segment presenting with mass effect. The time of complete occlusion of the aneurysms and the risk of the bleeding is currently not predictable.

Influence of valsartan-eluting stent on neointima formation

OBJECTIVE

#ENTITYSTARTX02014; This study is to explore the effect of valsartan-eluting stents on neointima formation after stenting and to elucidate possible mechanisms how locally used valsartan prevents in-stent restenosis (ISR).

METHOD

valsartan- and carriereluting stents were manufactured by using multi-layer-coated technology. Bare stents, carrier-eluting stents and valsartan- eluting stents were implanted into the abdominal aortas of the rabbits respectively. Quantitative angiography (QA) before, immediately after and 3 months after stent implantation were compared between the groups of bare (n=8), carrier-eluting (n=8) and valsartan-eluting stent (n=10), which allows the comparison of vascular diameters of aortas as well as indices of vascular neointimal formation, i.e. luminal area (LA), neointimal area (NIA), inner elastic membrane luminal area (IELA) and the maximal inner-membrane thickness (MIT) in 15 rabbits. α-Actin protein expression were detected by Envision two-step immunohistochemistry. Mean positive indices (MPI) of the above protein were analyzed semi-quantatively by IMS(Information Management System) cell image analysis system. MPI=positive area×OD (optical density). Collagen deposition in neointima was observed through MASSON stain among the three groups.

RESULT

#ENTITYSTARTX02014; the mean aortic diameters were similar in the three groups:bare stents group(n=8), carrier-eluting stents group(n=8) and valsartan eluting stents group(n=10) measured by QA at different time. A larger luminal area and a less neointimal hyperplasia in valsartan eluting-stents group was found compared with the other two groups. The mean luminal areas were 4345548±125822um(2); 4302061±167952 um(2); 5016269±207934um(2) respectively. The mean neointimal areas were 1119635±163503um(2); 1135636±136555um(2); 441577±74099um(2) and the mean maximal inner-membrane thickness were 210±30um;192±21um; 116±12um respectively. α-Actin protein expression was significantly lower in neointima of valsartan eluting-stents group than the other two groups. Through MASSON stain we found that Collagen was much richer in neointima of bare stents group and carrier-eluting stents group than valsartan eluting-stents group.

CONCLUSION

#ENTITYSTARTX02014; Valsartan eluting-stents inhibited neointimal hyperplasia after stenting by decreasing collagen deposition and smooth muscle cell proliferation. Therefore it would be potentially effective in preventing in-stent restenosis.

ABBREVIATIONS

#ENTITYSTARTX02014; Quantitative angiography (QA), luminal area (LA), neointimal area (NIA), inner elastic membrane luminal area (IELA), the maximal inner-membrane thickness (MIT), Mean positive indices (MPI), optical density (OD), Drugeluting stents (DES), in-stent restenosis(ISR), percutaneous transluminal coronary angioplasty (PTCA), angiotensin α type 2 receptor (AT2).

Serial clinical and angiographic follow-up after phosphorylcholine-coated stent implantation

The efficacy of drug-eluting stents (DES) has been proven, but concerns about late complications after DES have been raised. Polymers that do not increase inflammatory or hypersensitivity reactions which may contribute to late complications are needed for new generation DES. To evaluate the safety and efficacy of phosphorylcholine-polymer coating, we investigated serial clinical and angiographic outcomes after phosphorylcholine-coated stent placement. Seventy-five consecutive patients treated with a BiodivYsio phosphorylcholine-coated stent for de novo lesions at our institute between October 2001 and August 2002 were enrolled. Six-month follow-up angiography was performed in 71 lesions (94.7%), and angiographic restenosis was found in 19 lesions (26.8%). Target lesion revascularization (TLR) was performed in 10 lesions (14.1%). Eighteen-month follow-up angiography was performed in 58 (95.1%) of the remaining 61 lesions (excluding TLR lesions), and angiographic restenosis was found in only 3 lesions. The cumulative MACE-free survival rate was 86.3%, 83.6%, and 78.6% at 6-month, 18-month, and 8-year follow-up, respectively. There were no episodes of stent thrombosis. Late loss decreased significantly from 0.74 ± 0.40 mm (6-months) to 0.51 ± 0.46 mm (18-months) (P < 0.0001). Phosphorylcholine-coated stent implantation was associated with acceptable clinical and angiographic results. Phosphorylcholine-coating may be an ideal polymer for new generation DESs.

A link between smooth muscle cell death and extracellular matrix degradation during vascular atrophy

OBJECTIVE

High blood flow induces neointimal atrophy in polytetrafluoroethylene (PTFE) aortoiliac grafts and a tight external PTFE wrap of the iliac artery induces medial atrophy. In both nonhuman primate models, atrophy with loss of smooth muscle cells and extracellular matrix (ECM) begins at ≤4 days. We hypothesized that matrix loss would be linked to cell death, but the factors and mechanisms involved are not known. The purpose of this study was to determine commonly regulated genes in these two models, which we hypothesized would be a small set of genes that might be key regulators of vascular atrophy.

METHODS

DNA microarray analysis (Sentrix Human Ref 8; Illumina, San Diego, Calif; ∼23,000 genes) was performed on arterial tissue from the wrap model (n = 9) and graft neointima from the graft model (n = 5) 1 day after wrapping or the switch to high flow, respectively. Quantitative reverse-transcription polymerase chain reaction (qRT-PCR) was also performed. Expression of this vascular atrophy gene set was also studied after Fas ligand-induced cell death in cultured smooth muscle cells and organ cultured arteries.

RESULTS

Microarray analysis showed 15 genes were regulated in the same direction in both atrophy models: 9 upregulated and 6 downregulated. Seven of nine upregulated genes were confirmed by qRT-PCR in both models. Upregulated genes included the ECM-degrading enzymes ADAMTS4, tissue plasminogen activator (PLAT), and hyaluronidase 2; possible growth regulatory factors, including chromosome 8 open reading frame 4 and leucine-rich repeat family containing 8; a differentiation regulatory factor (musculoskeletal embryonic nuclear protein 1); a dead cell removal factor (ficolin 3); and a prostaglandin transporter (solute carrier organic anion transporter family member 2A1). Five downregulated genes were confirmed but only in one or the other model. Of the seven upregulated genes, ADAMTS4, PLAT, hyaluronidase 2, solute carrier organic anion transporter family member 2A1, leucine-rich repeat family containing 8, and chromosome 8 open reading frame 4 were also upregulated in vitro in cultured smooth muscle cells or cultured iliac artery by treatment with FasL, which causes cell death. However, blockade of caspase activity with Z-VAD inhibited FasL-mediated cell death, but not gene induction.

CONCLUSION

Seven gene products were upregulated in two distinctly different in vivo nonhuman primate vascular atrophy models. Induction of cell death by FasL in vitro induced six of these genes, including the ECM-degrading factors ADAMTS4, hyaluronidase 2, and PLAT, suggesting a mechanism by which the program of tissue atrophy coordinately removes extracellular matrix as cells die. These genes may be key regulators of vascular atrophy.

Late progression after sirolimus-eluting stent implantation for de novo lesions--comparison with bare metal stent implantation

BACKGROUND

In previous studies, the minimal luminal diameter (MLD) of lesions treated with a bare metal stent (BMS) was shown to improve from 6 months to 3 years. However, the long-term response to a sirolimus-eluting stent (SES) implantation remains unclear.

METHODS AND RESULTS

To evaluate 6-month, 12-month and 3-year outcomes, clinical and angiographic follow-up data were analyzed for 367 consecutive patients (506 de novo lesions) who underwent successful SES implantation compared to follow-up data for 617 consecutive patients (802 de novo lesions) who underwent BMS implantation. Clinical follow-up information was obtained for 363 SES-treated patients (98.9%) and 581 BMS-treated patients (94.2%) at 1 year, and 334 SES-treated patients (91.0%) and 566 BMS-treated patients (91.7%) at 3 years. At 3 years, there were no significant differences in the cumulative cardiac death and myocardial infarction. Target lesion revascularization (TLR) rates were significantly higher in BMS-treated patients than in SES-treated patients. In BMS-treated patients, most TLR was performed within 450 days, however, after 450 days, the TLR rate was significantly lower than that for the SES-treated patients. In quantitative coronary angiographic data, among lesions that required no revascularization at the initial 12-month follow up, MLD increased significantly from the 12-month to the 3-year follow-up angiography in BMS-treated lesions. However, MLD decreased significantly in SES-treated lesions.

CONCLUSIONS

From a 12-month follow-up to a 3-year follow-up, stenosis in BMS-treated lesions regressed, but stenosis in SES-treated lesions progressed. And late TLR was more frequently required in the SES-treated patients.

A novel mouse model of in situ stenting

AIMS

Animal models of stenting are mostly limited to larger animals or involve substantial abdominal surgery in rodents. We aimed to develop a simple, direct model of murine stenting.

METHODS AND RESULTS

We designed a miniature, self-expanding, nitinol wire coil stent that was pre-loaded into a metal stent sheath. This was advanced into the abdominal aorta of the mouse, via femoral access, and the stent deployed. In-stent restenosis was investigated at 1, 3, 7, and 28 days post-stenting. The model was validated by investigation of neointima formation in mice deficient in signalling via the interleukin-1 receptor (IL-1R1), compared with other injury models. Ninety-two per cent of mice undergoing the procedure were successfully stented. All stented vessels were patent. Inflammatory cells were seen in the adventitia and around the stent strut up to 3 days post-stenting. At 3 days, an early neointima was present, building to a mature neointima at 28 days. In mice lacking IL-1R1, the neointima was 64% smaller than that in wild-type controls at the 28-day timepoint, in agreement with other models.

CONCLUSION

This is the first description of a successful model of murine in situ stenting, using a stent specifically tailored for use in small thin-walled arteries. The procedure can be undertaken by a single operator without the need for an advanced level of microsurgical skill and is reliable and reproducible. The utility of this model is demonstrated by a reduction in in-stent restenosis in IL-1R1-deficient mice.

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.

Effects of external wrapping and increased blood flow on atrophy of the baboon iliac artery

OBJECTIVE

Increased blood flow causes neointimal atrophy, whereas relief of wall tension with an external wrap causes arterial medial atrophy. To study the effects of blood flow and wall tension separately and together, we applied tight or loose wraps on high-flow or normal-flow iliac arteries in baboons.

METHOD

Baboon external iliac arteries were wrapped with loose-fitting and tight-fitting expanded polytetrafluoroethylene (ePTFE), leaving part unwrapped. A downstream arteriovenous fistula was constructed on one side to increase blood flow approximately twofold. The arteries were perfusion-fixed with 10% formalin after 4 (n = 5) and 28 days (n = 5).

RESULTS

At 4 days, compared with the unwrapped artery, the loosely and tightly wrapped normal-flow artery showed significant medial atrophy (23% and 30%, respectively; P < .05). The tightly wrapped artery showed a loss of cells (27%; P = .02) but no change in cell density. At 28 days, the medial cross-sectional area was decreased by the tight wrap and loose wrap under normal (45% and 28%, respectively; P < .05) and high (43% and 29%, respectively; P < .05) flow. High flow did not alter the effect of wrapping nor did it affect the unwrapped medial area. At 28 days, the normal and high flow tightly wrapped media showed an insignificant loss of cells but had increased cell density (47% and 30%, respectively; P < .05), suggesting preferential loss of extracellular matrix. Decorin was expressed at the late time only in the tightly wrapped normal and high-flow media and was associated with tight packing of the collagen, as detected by picrosirius red staining.

CONCLUSION

Loose-fitting and tight-fitting ePTFE wraps induced an inflammatory foreign body response that caused medial atrophy with loss of cells and extracellular matrix; the tight wrap was more effective. High blood flow did not prevent or augment medial atrophy.

CLINICAL RELEVANCE

Research in arterial restenosis has focused on the biologic mechanisms and pharmacologic approaches to the prevention of intimal hyperplasia. An alternative therapeutic approach might be to induce atrophy of established intimal hyperplasia. We have previously reported that high blood flow induces neointimal regression in expanded polytetrafluoroethylene grafts in baboons. Here we provide another model of vascular atrophy induced by external wrapping. The similarity between baboons and humans in their vascular systems and individual genetic heterogeneity makes these experiments of great relevance. Up- or down-regulated genes common to both models might be key regulators of vascular atrophy and therefore suitable therapeutic targets for pharmacologic treatment of established lesions.

Comparison of long-term angiographic follow-up after sirolimus-eluting stent and bare-metal stent implantation

BACKGROUND

The sirolimus-eluting stent (SES) has dramatically reduced the rate of restenosis in comparison to that with the bare-metal stent (BMS). In previous studies, the minimal luminal diameter (MLD) of lesions treated with a BMS was shown to improve from 6 months to 1 year.

METHODS

To evaluate 6-month and 1-year outcomes, angiographic follow-up data were analyzed for 285 patients (451 lesions) who underwent successful SES implantation compared to follow-up data for 2,561 patients (3,367 lesions) who underwent BMS implantation.

RESULTS

Angiographic follow-up was performed at 6 months for 396 SES-treated lesions and 2,628 BMS-treated lesions and at 1 year for 322 SES-treated lesions and 1,540 BMS-treated lesions. The 6-month angiographic restenosis rate was significantly lower for SES-treated lesions than for BMS-treated lesions (4.8% vs. 23.4%, p<0.01). From immediately after stent implantation to 6 months, quantitative coronary angiography revealed a significantly larger decrease in MLD of BMS-treated lesions than in MLD of SES-treated lesions (p<0.01). In BMS-treated lesions in which repeat revascularization was not performed at 6 months, MLD increased significantly from 2.08+/-0.63 mm at 6 months to 2.11+/-0.61 mm at 1 year (p<0.01). In SES-treated lesions, however, MLD decreased significantly from 2.55+/-0.56 mm at 6 months to 2.44+/-0.61 mm at 1 year (p<0.05).

CONCLUSIONS

From 6 months to 1 year, stenosis of BMS-treated lesions regressed, but stenosis of SES-treated lesions progressed.

Induction of vascular atrophy as a novel approach to treating restenosis. A review

Regardless of the type of arterial reconstruction, luminal narrowing (stenosis or restenosis) develops in approximately one third of the vessels. In the past, the focus of research has been on the mechanisms of stenosis (intimal hyperplasia, pathologic remodeling) and pharmacologic approaches to prevention. An alternative approach is to induce intimal atrophy after luminal narrowing has developed, thus limiting treatment to only those patients that develop a problem. This approach to treat established disease by reducing wall mass through induction of cell death and extracellular matrix removal would be particularly useful for treating stenosis in synthetic bypass grafts or stented vessels, in which intimal hyperplasia is the primary mechanism of stenosis. This approach may be applicable as well to other vascular proliferative disorders, such as pulmonary hypertension and chronic transplant arteriopathy. Proof of principle has been shown in experiments with antibodies to platelet-derived growth factor (PDGF) receptors that cause neointimal regression in baboon polytetrafluoroethylene (PTFE) grafts and with angiotensin-converting enzyme inhibitors that induce medial atrophy in hypertensive arteries. Possible molecular targets could include PDGF receptors, A20, and BMP4. Further studies are needed to determine the utility of such a therapeutic approach to vascular disease.

Increased In-Stent Stenosis in ApoE Knockout Mice

OBJECTIVE

We aimed to develop and validate a model of angioplasty and stenting in mice that would allow investigation of the response to stent injury using genetically modified mouse strains.

METHODS AND RESULTS

Aortic segments from either C57BL/6 wild-type or atherosclerotic ApoE-KO mice underwent balloon angioplasty alone or balloon angioplasty and stenting with a 1.25x2.5 mm stainless steel stent. Vessels were carotid-interposition grafted into genetically identical littermate recipients and harvested at 1, 7, 14, or 28 days. In wild-type mice, stenting generated an inflammatory vascular injury response between days 1 to 7, leading to the development of neointimal hyperplasia by day 14, which further increased in area by day 28 leading to the development of in-stent stenosis. Uninjured vessels and vessels injured by balloon angioplasty alone developed minimal neointimal hyperplasia. In stented ApoE-KO mice, neointimal area at 28 days was 30% greater compared with wild-type mice.

CONCLUSIONS

By reproducing important features of human stenting in atherosclerotic mice, we provide the potential to investigate molecular pathways and evaluate novel therapeutic targets for stent injury and restenosis.

Two-year follow-up intravascular ultrasound analysis after bare metal stent implantation in 120 lesions

The objective of this study was to examine long-term changes after bare metal stent implantation in a relatively large number of patients. There are few reports of intravascular ultrasound (IVUS) studies performed on stented and nonstented (reference) segments beyond 6 months after bare metal stenting. Using IVUS, we evaluated serial changes in stented and reference segments between 6 and 24 months after stent implantation in 110 patients with 120 lesions. Serial IVUS images were acquired at five equidistant intrastent sites and at two different reference segment sites. Measurements were made of the external elastic membrane (EEM), stent, lumen, and intimal hyperplasia (IH = stent - lumen) area. For the whole patient group, between 6 and 24 months, the mean IH area in stented segments decreased from 2.6 +/- 1.0 to 2.3 36+/- 0.9 mm2 (P < 0.001), and the mean lumen area increased from 6.2 +/- 2.0 to 6.5 +/- 1.9 mm2 (P < 0.001). The mean IH area decreased in 91 lesions (76%) and increased in 29 lesions (24%) between 6 and 24 months. There were no significant changes in EEM or lumen area in the reference segments. Late angiographic restenosis (diameter stenosis > or = 50%) occurred in three lesions between 6 and 24 months. A late target lesion revascularization was performed for one lesion. In the period of time between 6 and 24 months after stenting, IH regression occurred in most (76%) stent lesions, resulting in late lumen increase. However, IH progression was observed in 24% of in-stent lesions. No significant changes of EEM or lumen area occurred in the reference segments.

Accumulation and loss of extracellular matrix during shear stress-mediated intimal growth and regression in baboon vascular grafts

The composition of extracellular matrix during growth and regression of the neointima was analyzed during healing in a baboon aorto-iliac polytetrafluoroethylene graft. Graft neointimal thickening can be modulated by altering blood flow by construction of downstream arteriovenous fistulas. Normal flow with normal shear stress induces neointimal thickening, whereas high flow with high shear stress upstream of a fistula induces regression of established neointima. The neointima formed under normal shear stress is enriched in hyaluronan and proteoglycans, particularly versican. On the other hand, the neointima near the graft material is enriched in collagen and biglycan. Neointimal regression in response to high shear stress is associated with a loss of proteoglycans as detected by histochemical staining. Immunostaining with an antibody against an ADAMTS cleavage neoepitope of versican increases after switching to high flow, although immunostaining for versican core protein is not appreciably changed by high flow. The present data demonstrate that the graft neointima is enriched with proteoglycans, particularly versican and hyaluronan, as well as collagen, and there is a differential distribution of each. Neointimal atrophy occurs with an apparent loss of proteoglycans and evidence of versican degradation.

Accumulation and loss of extracellular matrix during shear stress-mediated intimal growth and regression in baboon vascular grafts

The composition of extracellular matrix during growth and regression of the neointima was analyzed during healing in a baboon aorto-iliac polytetrafluoroethylene graft. Graft neointimal thickening can be modulated by altering blood flow by construction of downstream arteriovenous fistulas. Normal flow with normal shear stress induces neointimal thickening, whereas high flow with high shear stress upstream of a fistula induces regression of established neointima. The neointima formed under normal shear stress is enriched in hyaluronan and proteoglycans, particularly versican. On the other hand, the neointima near the graft material is enriched in collagen and biglycan. Neointimal regression in response to high shear stress is associated with a loss of proteoglycans as detected by histochemical staining. Immunostaining with an antibody against an ADAMTS cleavage neoepitope of versican increases after switching to high flow, although immunostaining for versican core protein is not appreciably changed by high flow. The present data demonstrate that the graft neointima is enriched with proteoglycans, particularly versican and hyaluronan, as well as collagen, and there is a differential distribution of each. Neointimal atrophy occurs with an apparent loss of proteoglycans and evidence of versican degradation.

Dual role of matrix metalloproteinases (matrixins) in intimal thickening and atherosclerotic plaque rupture

Intimal thickening, the accumulation of cells and extracellular matrix within the inner vessel wall, is a physiological response to mechanical injury, increased wall stress, or chemical insult (e.g., atherosclerosis). If excessive, it can lead to the obstruction of blood flow and tissue ischemia. Together with expansive or constrictive remodeling, the extent of intimal expansion determines final lumen size and vessel wall thickness. Plaque rupture represents a failure of intimal remodeling, where the fibrous cap overlying an atheromatous core of lipid undergoes catastrophic mechanical breakdown. Plaque rupture promotes coronary thrombosis and myocardial infarction, the most prevalent cause of premature death in advanced societies. The matrix metalloproteinases (MMPs) can act together to degrade the major components of the vascular extracellular matrix. All cells present in the normal and diseased blood vessel wall upregulate and activate MMPs in a multistep fashion driven in part by soluble cytokines and cell-cell interactions. Activation of MMP proforms requires other MMPs or other classes of protease. MMP activation contributes to intimal growth and vessel wall remodeling in response to injury, most notably by promoting migration of vascular smooth muscle cells. A broader spectrum and/or higher level of MMP activation, especially associated with inflammation, could contribute to pathological matrix destruction and plaque rupture. Inhibiting the activity of specific MMPs or preventing their upregulation could ameliorate intimal thickening and prevent myocardial infarction.

Clinical and angiographic predictors of luminal changes beyond 6 months after implantation of thicker strut coronary stents

BACKGROUND

Late luminal changes beyond 6 months after thicker strut stent implantation have not been fully elucidated. The purpose of this study was to clarify the clinical and angiographic predictors of late changes in minimal lumen diameter (MLD) after 6-month follow-up of stenting.

METHODS AND RESULTS

Fifty-one lesions from 44 patients who underwent successfully S670/660 stent (Medtronic Vascular, Santa Rosa, CA, USA) implantations without target lesion revascularization were studied at 6-month follow-up and coronary angiography was repeated after the follow-up. Late luminal loss beyond 6 months after stenting significantly correlated with late loss (r=-0.42, p=0.0025) and MLD (r=0.28, p=0.047) at 6-month follow-up. On multivariate analysis, age (p=0.005), diabetes mellitus (p=0.002), hyperlipidemia (p=0.023), smoking (p=0.015), bifurcation lesion (p=0.018), small stent diameter (p=0.001) and MLD at 6-month follow-up (p<0.001) were identified as independent predictors of late luminal loss.

CONCLUSIONS

This study demonstrated that older age, diabetes mellitus, hyperlipidemia, smoking and small stent diameter (<3.0 mm) were associated with late luminal loss beyond 6 months after stenting, and that a bifurcation lesion and small lumen diameter at 6 months were associated with late luminal recovery.

Rat Abdominal Aorta Stenting: A New and Reliable Small Animal Model for In-Stent Restenosis

BACKGROUND

A high throughput animal model may enhance pathophysiological studies to mechanisms of in-stent restenosis (ISR). More and appropriate antibodies and transgenic and knockout strains are available in rats. Consequently, a model for ISR in the rat would be convenient for pathobiological studies. Here we present the full characteristics of a rat ISR model suitable for high throughput stent research.

METHODS

The abdominal aorta of rats was separated from surrounding tissue and a BeStenttrade mark 2 or a Cyphertrade mark sirolimus-eluting stent was locally inserted. After 1, 3, 7, 28 and 56 days, the aortas were harvested, fixed, embedded and cut. Morphometric analysis was performed and inflammation scored.

RESULTS

The neointimal area increased to a maximum after 28 days (0.55 +/- 0.08 mm(2)). Subsequently, the neointimal area slightly decreased. The injury score and the neointimal area were linearly correlated (r = 0.85, p < 0.01). Thrombus formation was present after 1 day. Leukocyte adherence was evident after 1 day, maximal after 3 days (93 +/- 21 cells/section) and decreased thereafter. The inflammation score increased after 3 days to a maximum after 7 days (1.37 +/- 0.06) and declined thereafter. After 28 days the Cypher sirolimus-eluting stent decreased the stenosis in comparison to the BeStent 2 (10.2 +/- 0.85 vs. 18.0 +/- 2.0%, respectively, p < 0.01).

CONCLUSIONS

Stent deployment in the rat abdominal aorta results in thrombus formation, inflammation and neointimal formation. Moreover, there is a linear correlation between the injury score and the neointimal area. These responses resemble ISR events as seen in other animal models. Moreover, a known anti-restenotic stent also reduces neointimal formation in this model. Rat abdominal aorta stenting is a promising animal model for ISR, it is suitable for testing commercially manufactured stents and studying the pathophysiology of ISR.

Preclinical restenosis models and drug-eluting stents: still important, still much to learn

Percutaneous coronary intervention continues to revolutionize the treatment of coronary atherosclerosis. Restenosis remains a significant problem but may at last be yielding to technologic advances. The examination of neointimal hyperplasia in injured animal artery models has helped in our understanding of angioplasty and stenting mechanisms, and as drug-eluting stent (DES) technologies have arrived, they too have been advanced through the study of animal models. These models are useful for predicting adverse clinical outcomes in patients with DESs because suboptimal animal model studies typically lead to problematic human trials. Similarly, stent thrombosis in animal models suggests stent thrombogenicity in human patients. Equivocal animal model results at six or nine months occasionally have been mirrored by excellent clinical outcomes in patients. The causes of such disparities are unclear but may result from differing methods, including less injury severity than originally described in the models. Ongoing research into animal models will reconcile apparent differences with clinical trials and advance our understanding of how to apply animal models to clinical stenting in the era of DESs.

Extracellular Matrix Changes in Stented Human Coronary Arteries

Background— Restenosis after stenting occurs secondary to the accumulation of smooth muscle cells (SMCs) and extracellular matrix (ECM), with the ECM accounting for >50% of the neointimal volume. The composition of the in-stent ECM has not been well characterized in humans.

Methods and Results— Postmortem human coronary arteries (n=45) containing stents underwent histological assessment of neointimal proteoglycans, hyaluronan, collagen (types I and III), SMCs, and CD44 (a cell surface receptor for hyaluronan). The mean duration of stent implantation was 18.7 months; stents in place ≥3 to <9 months (n=17) were assigned to group 1, stents ≥9 to <18 months old (n=19) to group 2, and stents ≥18 months old (n=9) to group 3. In groups 1 and 2, neointimal versican and hyaluronan staining was strongly positive, colocalized with α-actin-positive SMCs, and was greater in intensity compared with group 3. Conversely, decorin staining was greatest in group 3. The neointima of both group 1 and 2 stents was rich in type III collagen, with reduced staining in group 3. Type I collagen staining was weakest in group 1 stents, with progressively stronger staining in groups 2 and 3. SMC density and stent stenosis were significantly reduced in group 3 stents compared with groups 1 and 2. CD44 staining colocalized with macrophages and was associated with increased neointimal thickness.

Conclusions— The ECM within human coronary stents resembles a wound that is not fully healed until 18 months after deployment, followed by neointimal retraction. ECM contraction may be a target for therapies aimed at stent restenosis prevention.

Radioactive stents delay but do not prevent in-stent neointimal hyperplasia

BACKGROUND

Restenosis after conventional stenting is almost exclusively caused by neointimal hyperplasia. Beta-particle-emitting radioactive stents decrease in-stent neointimal hyperplasia at 6-month follow-up. The purpose of this study was to evaluate the 1-year outcome of (32)P radioactive stents with an initial activity of 6 to 12 microCi using serial quantitative coronary angiography and volumetric ECG-gated 3D intravascular ultrasound (IVUS).

METHODS AND RESULTS

Of 40 patients undergoing initial stent implantation, 26 were event-free after the 6-month follow-up period and 22 underwent repeat catheterization and IVUS at 1 year; they comprised half of the study population. Significant luminal deterioration was observed within the stents between 6 months and 1 year, as evidenced by a decrease in the angiographic minimum lumen diameter (-0.43+/-0.56 mm; P:=0.028) and in the mean lumen diameter in the stent (-0.55+/-0. 63 mm; P:=0.001); a significant increase in in-stent neointimal hyperplasia by IVUS (18.16+/-12.59 mm(3) at 6 months to 27.75+/-11. 99 mm(3) at 1 year; P:=0.001) was also observed. Target vessel revascularization was performed in 5 patients (23%). No patient experienced late occlusion, myocardial infarction, or death. By 1 year, 21 of the initial 40 patients (65%) remained event-free.

CONCLUSIONS

Neointimal proliferation is delayed rather than prevented by radioactive stent implantation. Clinical outcome 1 year after the implantation of stents with an initial activity of 6 to 12 microCi is not favorable when compared with conventional stenting.