Strut Position, Blood Flow, and Drug Deposition

Very long versus overlapping drug eluting stents for the management of long coronary artery lesions

BACKGROUND

The prevalence of long diffuse coronary artery disease (CAD) is increasing nowadays due to increase prevalence of multiple risk factors and population ageing. We aimed in our study to show the differences clinically or angiographically (guided by IVUS) between the use of single long stent versus overlapping stents in very long coronary lesions (≥40 mm) in patients presented with chronic coronary syndromes.

METHODS

550 patients presenting with chronic coronary syndromes were included: 320 treated with a single long stent (≥40 mm) and 230 patients with two or more overlapping stents. Angiographic follow-up (guided by IVUS) 6 months after PCI was performed only in 50 patients. We assessed the procedural characteristics and the occurrence of major adverse cardiovascular events (MACE) after a median follow-up of 24 months.

RESULTS

Total stent length was 56.16 ± 14.85 mm and mean diameter was 3.05 ± 0.36 mm. At the end of follow-up, MACE rate in the single long stent group was 4.1% vs. 7.8% in the overlapping stents group, with higher incidence in overlapping stents group but non-statistically significant (p value = 0.059). PCI using overlapping stents consumed more contrast volume (248 ± 85.36 vs 164.5 ± 70.43 ml, p < 0.001), and higher fluoroscopy time (23.65 ± 9.19 vs 19.72 ± 9.19 min, p < 0.001). Regarding IVUS subgroup follow-up, there was no significant difference between both groups regarding in-stent restenosis and MACE.

CONCLUSIONS

We can conclude that long or overlapping stents are both acceptable therapeutic choices for patients with long CAD. There was no difference between both strategies regarding angiographic follow-up guided by IVUS after 6 months.

Endovascular Drug Delivery

Drug-eluting stents (DES) and balloons revolutionize atherosclerosis treatment by targeting hyperplastic tissue responses through effective local drug delivery strategies. This review examines approved and emerging endovascular devices, discussing drug release mechanisms and their impacts on arterial drug distribution. It emphasizes the crucial role of drug delivery in modern cardiovascular care and highlights how device technologies influence vascular behavior based on lesion morphology. The future holds promise for lesion-specific treatments, particularly in the superficial femoral artery, with recent CE-marked devices showing encouraging results. Exciting strategies and new patents focus on local drug delivery to prevent restenosis, shaping the future of interventional outcomes. In summary, as we navigate the ever-evolving landscape of cardiovascular intervention, it becomes increasingly evident that the future lies in tailoring treatments to the specific characteristics of each lesion. By leveraging cutting-edge technologies and harnessing the potential of localized drug delivery, we stand poised to usher in a new era of precision medicine in vascular intervention.

Comparison of outcomes between single long stent and overlapping stents: a meta-analysis of the literature

OBJECTIVES

There is no consensus on whether to treat diffuse coronary artery lesions with a single long stent (SLS) or by overlapping two or more stents (OLS). The goal of this review was to compare the outcomes of these two approaches through a meta-analysis of the literature.

METHODS

We searched for relevant studies in MEDLINE, Scopus, EMBASE, Google Scholar, and the Cochrane Library. Our meta-analysis included 12 studies (n = 6414) that reported outcomes during the follow-up period.

RESULTS

Individuals who received OLS had a greater risk of cardiac mortality and target lesion revascularization (TLR) than those who received SLS (RR: 1.51, CI: 1.03-2.21, p = 0.03, I2 = 0% and RR: 1.64, CI: 1.02-2.65, p = 0.04, I2 = 38%, respectively). The fluoroscopy period in the OLS group was longer than in the SLS group (SMD: 0.35, CI: 0.25-0.46, p < 0.01, I2 = 0%). more contrast volume was sued for the OLS group; however, there was substantial variability in the pooled analysis (I2 = 95%). In terms of all outcomes, there were no differences between stent generation types.

CONCLUSION

In the first meta-analysis of mainly observational data comparing OLS vs. SLS for long coronary lesions, OLS had higher rates of cardiac mortality and TLR as well as longer fluoroscopy times compared to SLS.

Drug-Eluting Stent Restenosis: Modern Approach to a Classic Challenge

In-stent restenosis (ISR) is a recognized complication following percutaneous coronary intervention in which the luminal diameter is narrowed through neointimal hyperplasia and vessel remodeling. Although rates of ISR have decreased in most recent years owing to newer generation drug-eluting stents, thinner struts, and better intravascular imaging modalities, ISR remains a prevalent dilemma that proves to be challenging to manage. Several factors have been proposed to contribute to ISR formation, including mechanical stent characteristics, technical factors during the coronary intervention, and biological aspects of drug-eluting stents. Presentation of ISR can range from asymptomatic to late myocardial infarction and could be difficult to differentiate from acute thrombus formation. No definite guidelines are present on the management of ISR. In this review, we will discuss the mechanisms underlying ISR and provide insight into patient-related and procedural risk factors contributing to ISR, in addition to highlighting common treatment approaches utilized in the management of ISR.

An intricate interplay between stent drug dose and release rate dictates arterial restenosis

Since the introduction of percutaneous coronary intervention (PCI) for the treatment of obstructive coronary artery disease (CAD), patient outcomes have progressively improved. Drug eluting stents (DES) that employ anti-proliferative drugs to limit excess tissue growth following stent deployment have proved revolutionary. However, restenosis and a need for repeat revascularisation still occurs after DES use. Over the last few years, computational models have emerged that detail restenosis following the deployment of a bare metal stent (BMS), focusing primarily on contributions from mechanics and fluid dynamics. However, none of the existing models adequately account for spatiotemporal delivery of drug and the influence of this on the cellular processes that drive restenosis. In an attempt to fill this void, a novel continuum restenosis model coupled with spatiotemporal drug delivery is presented. Our results indicate that the severity and time-course of restenosis is critically dependent on the drug delivery strategy. Specifically, we uncover an intricate interplay between initial drug loading, drug release rate and restenosis, indicating that it is not sufficient to simply ramp-up the drug dose or prolong the time course of drug release to improve stent efficacy. Our model also shows that the level of stent over-expansion and stent design features, such as inter-strut spacing and strut thickness, influence restenosis development, in agreement with trends observed in experimental and clinical studies. Moreover, other critical aspects of the model which dictate restenosis, including the drug binding site density are investigated, where comparisons are made between approaches which assume this to be either constant or proportional to the number of smooth muscle cells (SMCs). Taken together, our results highlight the necessity of incorporating these aspects of drug delivery in the pursuit of optimal DES design.

Hemodynamic Impact of Stenting on Carotid Bifurcation: A Potential Role of the Stented Segment and External Carotid Artery

Carotid stenting near the bifurcation carina is associated with adverse events, especially in-stent restenosis, thrombosis, and side branch occlusion in clinical data. This study is aimed at determining the potential biomechanical mechanisms for these adverse events after carotid stenting. The patient-specific carotid models were constructed with different stenting scenarios to study the flow distribution and hemodynamic parameters, such as wall shear stress (WSS), flow velocity, relative residence time (RRT), and oscillating shear index (OSI) in the carotid bifurcation. The results suggested that the existing stents surely reduced blood flow to the external carotid artery (ECA) but enhanced local flow disturbance both in ECA and stented internal carotid artery (ICA), and the inner posterior wall of the stented ICA and the outer posterior wall of ECA might endure a relatively low level of WSS and remarkably elevated OSI and RRT. In addition, the implanted stent leads to more ECA adverse flow than ICA after stenting. While disturbed flow near the strut increased as stent length increased, blood flow and areas of local flow disturbance in ECA slightly decreased as stent length increased. In conclusion, the results revealed that ECA might be in relatively high levels of abnormal local hemodynamics after stenting, followed by stented ICA, leading to potential adverse events after intervention.

Specific and nonspecific binding of drug eluted from a half-embedded stent in presence of atherosclerotic plaque

This study is dealt with the two-phase binding (specific and nonspecific) of drug eluted from a half- embedded drug-eluting stent in presence of atherosclerotic plaque. The specific binding due to the interaction of drug molecules with specific receptors and nonspecific binding caused by the trapping of drug in the extra-cellular matrix have been paid due attention. An idealised wall consisting of a plaque and a healthy tissue region has been considered. Moreover, a Dirichlet release condition is imposed on the strut surface. In this investigation, a two-dimensional model governing drug transport and its two-phase binding in cylindrical polar coordinate system has been solved numerically by a finite-difference method. Our simulation predicts that plaque behaves like a physical barrier in two types of the binding process and there is an inverse relationship between bound drug concentration and plaque thickness. Simulations show that a single peak profile of drug is noted when the struts are situated one-strut radius apart and as the inter-strut distance increases, the peak concentration falls and distinct peak profiles over each strut are visualised. The model also reveals that in the region downstream of a strut, the concentration of both bound drug forms in the plaque and healthy regions increases, and eventually, the saturation length of binding sites increases. Predicted results show for smaller Damköhler number, the rapid saturation of binding sites takes place and the stent having thinner strut may perform well in terms of effectiveness as well as efficacy in the stent-based delivery.

Extended finite element method for fluid-structure interaction in wave membrane blood pump

Numerical simulations of cardiac blood pump systems are integral to the optimization of device design, hydraulic performance and hemocompatibility. In wave membrane blood pumps, blood propulsion arises from the wave propagation along an oscillating immersed membrane, which generates small pockets of fluid that are pushed towards the outlet against an adverse pressure gradient. We studied the Fluid-Structure Interaction between the oscillating membrane and the blood flow via three-dimensional simulations using the Extended Finite Element Method (XFEM), an unfitted numerical technique that avoids remeshing by using a fluid fixed mesh. Our three-dimensional numerical simulations in a realistic pump geometry highlighted, for the first time in this field of application, that XFEM is a reliable strategy to handle complex industrial problems. Moreover, they showed the role of the membrane deformation in promoting a blood flow towards the outlet despite an adverse pressure gradient. We also simulated the pump system at different pressure conditions and we validated the numerical results against in-vitro experimental data.

Mechanisms of drug-eluting stent restenosis

Drug-eluting stents (DES) were developed to overcome in-stent restenosis (ISR), which has long been considered the main complication limiting the long-term efficacy of coronary stenting. New-generation DES which composed of advanced stent design with and without specific biocompatible polymer contributes a reduction of the incidence of ISR to rate ranging from 5 to 10%. The precise reasons of DES restenosis are still controversial and not fully understood. Angiographic and coronary images at the index procedure, systemic status of patients, medications, and intracoronary imaging at ISR site are all considered to find the possible mechanisms of DES restenosis. Multiple biological, genetic, mechanical, and technical factors might intricately contribute to DES restenosis. Biological and genetic factors of ISR are not able to be sufficiently modified by the current medical approaches. Tailored treatments avoiding mechanical and technical factors of ISR are required to reduce DES restenosis. Elucidation of DES restenosis leads to further improvement in the current DES system and finds the optimal approach to treat DES restenosis. The possible mechanisms of DES restenosis are discussed in this review.

Long-term results of stenting of long coronary artery stenosis with consecutive implantation of stents of different types with overlapping edges in patients with acute coronary syndrome without ST-segment elevation and multivessel coronary artery disease

Introduction. In urgent interventional cardiology practice, combinations of drug-eluting stents and bare-metal stents are sometimes forced to treat extended stenosis in a clinic-dependent artery. Objective. A comparison of long-term results of treatment of patients with coronary heart disease and multivessel coronary lesions, which performed stenting of the clinic-dependent artery by two successive partially overlapping stents using stents of the 3rd generation with drug coating or a combination of the 3rd generation stent with drug coating and a bare-metal stent for acute coronary syndrome without St segment elevation and later — complete functional myocardial revascularization by endovascular method. Methods. The minimum overall length sentiremos section was made 55 mm. In main group included 32 patients for whom revascularization clinic-dependent artery performed endovascular intervention with the use of 2 stents 3-generation drug-coated sirolimus and biodegradable polymer implanted overlap. In 30 patients (control group), clinic-dependent artery revascularization was also performed by a combination of implanted overlap stents, one of which was a 3rd generation stent with sirolimus drug coating and biodegradable polymer, and the other was a bare-metal stent. There were no statistically significant differences between the groups in clinical, demographic and operational characteristics. Results. The analysis of the results revealed a significant difference between the groups in the frequency of repeated revascularization of the target artery, which were observed more often in the control group. Conclusion. When performing an extended stenting of the clinic-dependent artery in patients with acute coronary syndrome without ST segment elevation, overlapping of the drug-coated stent and the bare-metal stent should be avoided, since the antirestenotic effect of the drug-coated stent is leveled when overlapping with the bare-metal stent, but this strategy can be used in case of full coverage of the stenotic lesion or closure of the dissection.

Daily risk of adverse outcomes in patients undergoing complex lesions revascularization: A subgroup analysis from the RAIN-CARDIOGROUP VII study (veRy thin stents for patients with left mAIn or bifurcatioN in real life)

INTRODUCTION

Percutaneous coronary intervention (PCI) for complex lesions, including unprotected left main (ULM) and bifurcations, is gaining a relevant role in treating coronary artery disease with good outcomes, also thanks to new generation stents. The daily risk of adverse cardiovascular events and their temporal distribution after these procedures is not known.

METHODS

All consecutive patients presenting with a critical lesion of ULM or bifurcation treated with very thin struts stents, enrolled in the RAIN-Cardiogroup VII study, were analyzed. The daily risk of major acute cardiovascular events (MACE), target lesion revascularization (TLR) and stent thrombosis (ST) and their temporal distribution in the first year of follow-up was the primary endpoint. Differences among subgroups (ULM, patient presentation, kind of stent polymer) were the secondary endpoint.

RESULTS

2745 patients were included, mean age 68 ± 11 years, 33.3% diabetics, 54.5% had an acute coronary syndrome (ACS); 88.5% of treated lesions were bifurcations, 27.2% ULM. Average daily risk was 0.022% for MACE, 0.005% for TLR and 0.004% for ST, in the first year. Bimodal distribution of adverse events, especially TLR, with an early peak in the first 50 days and a late one after 150 days, was observed. Patients with ULM presented a significantly higher daily risk of events, and ACS patients presented higher MACE risk. No difference emerged according to the type of stent polymer.

CONCLUSIONS

The daily risk of adverse events in the first year after complex PCI in our study is acceptably low. PCI on ULM carries a higher risk of complications.

Feasibility of Implanting 50-60 mm-Tapered Drug Eluting Stents in Chronic Total Occlusions

BACKGROUND

Percutaneous coronary intervention (PCI) of chronic total occlusions (CTO) usually involves multiple overlapping stents implantation to cover long coronary segments. A higher rate of restenosis has been described with stent overlapping. Recently, new long tapered stents emerged as a potential tool for treating long coronary lesions. Feasibility of using these new devices for the CTO PCI has not been described. The aim of this work was to describe our initial experience with 50 and 60 mm-long tapered sirolimus-eluting stents (SES) in CTO PCI.

METHODS

We included 54 consecutive patients who underwent a CTO PCI and in whom an attempt to implant a 50 or 60 mm-long tapered SES was performed. Baseline clinical, angiographic, and procedural characteristics were recorded.

RESULTS

The median (IQR) age was 64 (58-73) years, and 45 (83.3%) patients were male. The tapered SES 50 and 60 mm-long was successfully implanted in 51 (94.4%) patients. In three patients, a 60 mm-long stent could not be implanted, and two or three overlapped shorter drug-eluting stents were deployed instead. An average of 1.4 ± 0.6 stents per patient was implanted. A single stent was deployed in 32 (59.3%) patients. During a median follow-up of 330 (149-551) days, repeat PCI in the target vessel was performed in two patients.

CONCLUSIONS

The use of the new BioMime Morph™ tapered SES for the treatment of CTO appears to be feasible in a high proportion of procedures. Further studies confirming the feasibility of this approach and its potential clinical advantages are needed.

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.

Intravascular fibrin molecular imaging improves the detection of unhealed stents assessed by optical coherence tomography in vivo

AIMS

Fibrin deposition and absent endothelium characterize unhealed stents that are at heightened risk of stent thrombosis. Optical coherence tomography (OCT) is increasingly used for assessing stent tissue coverage as a measure of healed stents, but cannot precisely identify whether overlying tissue represents physiological neointima. Here we assessed and compared fibrin deposition and persistence on bare metal stent (BMS) and drug-eluting stent (DES) using near-infrared fluorescence (NIRF) molecular imaging in vivo, in combination with simultaneous OCT stent coverage.

METHODS AND RESULTS

Rabbits underwent implantation of one BMS and one DES without overlap in the infrarenal aorta (N = 20 3.5 × 12 mm). At Days 7 and/or 28, intravascular NIRF-OCT was performed following the injection of fibrin-targeted NIRF molecular imaging agent FTP11-CyAm7. Intravascular NIRF-OCT enabled high-resolution imaging of fibrin overlying stent struts in vivo, as validated by histopathology. Compared with BMS, DES showed greater fibrin deposition and fibrin persistence at Days 7 and 28 (P < 0.01 vs. BMS). Notably, for edge stent struts identified as covered by OCT on Day 7, 92.8 ± 9.5% of DES and 55.8 ± 23.6% of BMS struts were NIRF fibrin positive (P < 0.001). At Day 28, 18.6 ± 10.6% (DES) and 5.1 ± 8.7% (BMS) of OCT-covered struts remained fibrin positive (P < 0.001).

CONCLUSION

Intravascular NIRF fibrin molecular imaging improves the detection of unhealed stents, using clinically translatable technology that complements OCT. A sizeable percentage of struts deemed covered by OCT are actually covered by fibrin, particularly in DES, and therefore such stents might remain prothrombotic. These findings have implications for the specificity of standalone clinical OCT assessments of stent healing.

Effects of Low Endothelial Shear Stress After Stent Implantation on Subsequent Neointimal Hyperplasia and Clinical Outcomes in Humans

Background

In‐stent hyperplasia (ISH) may develop in regions of low endothelial shear stress (ESS), but the relationship between the magnitude of low ESS, the extent of ISH, and subsequent clinical events has not been investigated.

Methods and Results

We assessed the association of poststent ESS with neointimal ISH and clinical outcomes in patients treated with percutaneous coronary interventions (PCI). Three‐dimensional coronary reconstruction was performed in 374 post‐PCI patients at baseline and 6 to 10 months follow‐up as part of the PREDICTION Study. Each vessel was divided into 1.5‐mm‐long segments, and we calculated the local ESS within each stented segment at baseline. At follow‐up, we assessed ISH and the occurrence of a clinically indicated repeat PCI for in‐stent restenosis. In 246 total stents (54 overlapping), 100 (40.7%) were bare‐metal stents (BMS), 104 (42.3%) sirolimus‐eluting stents, and 42 (17.1%) paclitaxel‐eluting stents. In BMS, low ESS post‐PCI at baseline was independently associated with ISH (β=1.47 mm² per 1‐Pa decrease; 95% CI, 0.38–2.56; P<0.01). ISH was minimal in drug‐eluting stents. During follow‐up, repeat PCI in BMS was performed in 21 stents (8.5%). There was no significant association between post‐PCI ESS and in‐stent restenosis requiring PCI.

Conclusions

Low ESS after BMS implantation is associated with subsequent ISH. ISH is strongly inhibited by drug‐eluting stents. Post‐PCI ESS is not associated with in‐stent restenosis requiring repeat PCI. ESS is an important determinant of ISH in BMS, but ISH of large magnitude to require PCI for in‐stent restenosis is likely attributed to factors other than ESS within the stent.

Very early tissue coverage after drug-eluting stent implantation: an optical coherence tomography study

The aim of this study was to evaluate neointimal coverage in the very early phase after second-generation drug-eluting stent (DES) implantation using optical coherence tomography (OCT). Patients who underwent staged percutaneous coronary intervention within 30 days after DES implantation were enrolled. OCT was performed to observe DES previously implanted. The median time interval from implantation to OCT examination was 21.5 days. A total of 10,625 struts of 54 stents (52 everolimus-eluting stents and 2 zotarolimus-eluting stents) in 42 lesions were analyzed. Strut tissue coverage was observed in 71.1 ± 19.2 % of the struts, malapposed struts in 2.56 ± 3.37 %, strut tissue coverage at the side branch orifice in 10.6 ± 17.2 %, and struts with protrusion in 0.95 ± 3.46 %. Mean tissue thickness on the covered struts was 39.8 ± 14.2 µm. The percentage of stent coverage was significantly lower in the overlapping segments than in the non-overlapping segments (48.4 ± 17.5 % vs. 74.4 ± 20.2 %, P < 0.05). Most of the stent struts were covered by tissue within 30 days after second-generation DES implantation. However, the percentage of strut coverage was lower in the overlapping segments than in the non-overlapping segments, suggesting that very early interruption of dual antiplatelet therapy might result in increased risk of stent thrombosis, even in second-generation DES.

Computational Model of Drug-Coated Balloon Delivery in a Patient-Specific Arterial Vessel with Heterogeneous Tissue Composition

Balloon angioplasty followed by local delivery of antiproliferative drugs to target tissue is increasingly being considered for the treatment of obstructive arterial disease, and yet there is much to appreciate regarding pharmacokinetics in arteries of non-uniform disease. We developed a computational model capable of simulating drug-coated balloon delivery to arteries of heterogeneous tissue composition comprising healthy tissue, as well as regions of fibrous, fibro-fatty, calcified and necrotic core lesions. Image processing using an unsupervised clustering technique was used to reconstruct an arterial geometry from a single, patient-specific color image obtained from intravascular ultrasound-derived virtual histology. Transport of free drug was modeled using a time-dependent reaction-diffusion model and the bound, immobilized drug using the time-dependent reaction equation. The governing equations representing the transport of free as well as bound drug along with a set of initial settings and boundary conditions were solved numerically using an explicit finite difference scheme that satisfied the Courant-Friedrichs-Lewy stability criterion. Our results support previous findings related to the transport and binding of drug in arteries where tissue retention is strongly dependent on local pharmacologic properties. Additionally, modeling results indicate that non-uniform disease composition leads to heterogeneous arterial drug distribution patterns, although further validation using animal studies is required to fully appreciate pharmacokinetics in disease-laden arteries.

Drug deposition in coronary arteries with overlapping drug-eluting stents

Drug-eluting stents are accepted as mainstream endovascular therapy, yet concerns for their safety may be under-appreciated. While failure from restenosis has dropped to below 5%, the risk of stent thrombosis and associated mortality remain relatively high. Further optimization of drug release is required to minimize thrombosis risk while maintaining therapeutic dose. The complex three-dimensional geometry of deployed stents together with the combination of diffusive and advective drug transport render an intuitive understanding of the situation exceedingly difficult. In situations such as this, computational modeling has proven essential, helping define the limits of efficacy, determine the mode and mechanism of drug release, and identify alternatives to avoid toxicity. A particularly challenging conformation is encountered in coronary arteries with overlapping stents. To study hemodynamics and drug deposition in such vessels we combined high-resolution, multi-scale ex vivo computed tomography with a flow and mass transfer computational model. This approach ensures high geometric fidelity and precise, simultaneous calculation of blood flow velocity, shear stress and drug distribution. Our calculations show that drug uptake by the arterial tissue is dependent both on the patterns of flow disruption near the wall, as well as on the relative positioning of drug-eluting struts. Overlapping stent struts lead to localized peaks of drug concentration that may increase the risk of thrombosis. Such peaks could be avoided by anisotropic stent structure or asymmetric drug release designed to yield homogeneous drug distribution along the coronary artery and, at the least, suggest that these issues need to remain in the forefront of consideration in clinical practice.

Endovascular Drug Delivery and Drug Elution Systems: First Principles

Endovascular drug delivery continues to revolutionize the treatment of atherosclerosis in coronary and peripheral vasculature. The key has been to identify biologic agents that can counter the hyperplastic tissue responses to device expansion/implantation and to develop effective local delivery strategies that can maintain efficacious drug levels across the artery wall over the course of device effects. This article reviews the evolution of endovascular drug delivery technology, explains the mechanisms they use for drug release, and provides a quantitative mechanistic framework for relating drug release mode to arterial drug distribution and effect.

Pathology of Endovascular Stents

Contemporary endovascular stents are the product of an iterative design and development process that leverages evolving concepts in vascular biology and engineering. This article reviews how insights into vascular pathophysiology, materials science, and design mechanics drive stent design and explain modes of stent failure. Current knowledge of pathologic processes is providing a more complete picture of the factors mediating stent failure. Further evolution of endovascular stents includes bioresorbable platforms tailored to treat plaques acutely and to then disappear after lesion pacification. Ongoing refinement of stent technology will continue to require insights from pathology to understand adverse events, refine clinical protocols, and drive innovation.

Numerical simulation on the effects of drug-eluting stents with different bending angles on hemodynamics and drug distribution

Implantation of drug-eluting stents in curved blood vessels may cause changes in hemodynamics and drug distribution, and even provoke in-stent restenosis. Due to the complexity of building three-dimensional (3-D) curved stent model, few studies have gone through such numerical simulations. In this study, three virtual stent models with different bending angles (0°, 30° and 90°) were developed to numerically investigate the distribution of wall shear stress (WSS) and drug concentration. The results showed that (1) the low-WSS regions in the inner bend of the stent models increased with the bending angles; (2) the drug concentration differed between the inner and outer bends of the stents but irrespective to the changes of bending angle; (3) the pattern of drug concentration for the curved stents found similar to that of the straight stents, and the phenomenon, 'proximal part low and distal part high' in the drug concentration showed in both the straight and curved stents. The increase in bending angles from 30° to 90° had little effect on the WSS and drug concentration; however, the largest effect of the curved stents was the remarkable difference of drug concentration between the inner and outer bends of the stents-about 20 %. Hence, it is feasible that quick analysis focused on the straight stents instead of the curved stents.

In-stent Restenosis

In-stent restenosis (ISR) is the narrowing of a stented coronary artery lesion. The mean time from percutaneous coronary intervention (PCI) to ISR was 12 months with drug-eluting stents (DES) and 6 months with bare metal stents (BMS). ISR typically presents as recurrent angina. The use of DES has significantly reduced the rate of ISR compared with BMS. Predictors of ISR include patient, lesion, and procedural characteristics. Intravascular ultrasound, optical coherence tomography, and fractional flow reserve are important tools for the anatomic and hemodynamic assessment of ISR. Treatment options for ISR include percutaneous coronary intervention with DES.

Drug diffusion and biological responses of arteries using a drug-eluting stent with nonuniform coating

The purpose of this study was to determine the effect of a nonuniform coating, abluminal-gradient coating (AGC), which leaves the abluminal surface of the curves and links parts of the stent free from the drug coating, on the diffusion direction of the drug and the biological responses of the artery to drug-eluting stent (DES) by comparing the AGC-sirolimus stent and the conventional full-surface coating (CFC) sirolimus stent. The study aimed to verify whether the AGC approach was appropriate for the development of a safer DES, minimizing the risks of stent thrombosis due to delayed endothelialization by the drug and distal embolization due to cracking of the coating layer on the hinge parts of the DES on stent expansion. In the in vitro local drug diffusion study, we used rhodamine B as a model drug, and rhodamine B released from the AGC stent diffused predominantly into the abluminal side of the alginate artery model. Conversely, rhodamine B released from the CFC stent quickly spread to the luminal side of the artery model, where endothelial cell regeneration is required. In the biological responses study, the luminal surface of the iliac artery implanted with the AGC-sirolimus stent in a rabbit iliac artery for 2 weeks was completely covered with endothelial-like cells. On the other hand, the luminal surface of the iliac artery implanted with the CFC-sirolimus stent for 2 weeks only showed partial coverage with endothelial-like cells. While thrombosis was observed in two of the three CFC-sirolimus stents, it was observed in only one of the three AGC-sirolimus stents. Taken together, these findings indicate that the designed nonuniform coating (AGC) is an appropriate approach to ensure a safer DES. However, the number of studies is limited and a larger study should be conducted to reach a statistically significant conclusion.

A NEW WAY TO OPTIMIZE DRUG RELEASE RATE OF DRUG ELUTING STENT (DES)

Local hemodynamic environment is a determinant factor in drug delivery from a drug eluting stent (DES) to the target arterial tissue. By using a simplified model of a DES, we demonstrated that if a DES had a drug release mode of uniform rate the drug released from the stent will distribute non-uniformly along the stent due to the flowing blood, with a significantly higher drug concentration at the distal part of the stent than that at the proximal one. This may explain why a DES could retard neointimal formation and vascular remodeling in downstream coronary segments. To solve this problem, we thereafter optimized the drug release mode of the DES as an exponential function. The simulation results showed that the optimized drug release mode could lead to a fairly uniform drug concentration distribution along the stent. Therefore, the present study suggested that to achieve a more effective result, optimization of drug eluting strategy (drug release mode) for the DES would be essential.

Role of Animal Models in Coronary Stenting

Coronary angioplasty initially employed balloon dilatation only. This technique revolutionized the treatment of coronary artery disease, although outcomes were compromised by acute vessel closure, late constrictive remodeling, and restenosis due to neointimal proliferation. These processes were studied in animal models, which contributed to understanding the biology of endovascular arterial injury. Coronary stents overcome acute recoil, with improvements in the design and metallurgy since then, leading to the development of drug-eluting stents and bioresorbable scaffolds. These devices now undergo computer modeling and benchtop and animal testing before evaluation in clinical trials. Animal models, including rabbit, sheep, dog and pig are available, all with individual benefits and limitations. In smaller mammals, such as mouse and rabbit, the target for stenting is generally the aorta; whereas in larger animals, such as the pig, it is generally the coronary artery. The pig coronary stenting model is a gold-standard for evaluating safety; but insights into biomechanical properties, the biology of stenting, and efficacy in controlling neointimal proliferation can also be gained. Intra-coronary imaging modalities such as intravascular ultrasound and optical coherence tomography allow precise serial evaluation in vivo, and recent developments in genetically modified animal models of atherosclerosis provide realistic test beds for future stents and scaffolds.

Safety and Efficacy of New Sirolimus-eluting Stent Models in a Preclinical Study

INTRODUCTION AND OBJECTIVES

Initial preclinical studies are required during the process of improving polymers, platforms, and drug-eluting systems for new coronary stent designs. Our objective was to analyze the efficacy and safety of new drug-eluting stent models compared with a conventional stent and commercialized drug-eluting stents in an experimental model with healthy porcine coronary arteries.

METHODS

Sixty stents (conventional stent, new sirolimus-eluting stents: drug-eluting stents 1, 2 and 3; Cypher(®) and Xience(®)) were randomly placed in the coronary arteries of 20 Large White domestic pigs. Angiographic and histomorphometric studies were done 28 days later.

RESULTS

The stents were implanted at a stent/artery ratio of 1.34±0.15, with no significant differences between groups. The new stents showed less late loss and angiographic restenosis than conventional stents (P=.006 and P<.001, respectively). Histologically, restenosis and neointimal area were lower with all the new platforms than with the conventional stents (P<.001 for each variable), and no differences were found vs the drug-eluting stents on the market. Safety data showed that endothelialization was lower with drug-eluting stents than with conventional stents, except for drug-eluting stent 3 (P=.084). Likewise, inflammation was lower with drug-eluting stent 3 than with other stents.

CONCLUSIONS

The new drug-eluting stent platforms studied are associated with less restenosis than conventional stents and showed no significant differences in safety or efficacy vs commercialized drug-eluting stents.

Influence of proximal drug eluting stent (DES) on distal bare metal stent (BMS) in multi-stent implantation strategies in coronary arteries

The aim of this study was to investigate the drug distribution in arteries treated with DES-BMS stenting strategy and to analyze the influence of proximal DES on distal segments of BMS. A straight artery model (Straight Model) and a branching artery model (Branching Model) were constructed in this study. In each model, the DES was implanted at the proximal position and the BMS was implanted distally. Hemodynamic environments, drug delivery and distribution features were simulated and analyzed in each model. The results showed that blood flow would contribute to non-uniform drug distribution in arteries. In the Straight Model the proximal DES would cause drug concentration in BMS segments. While in the Branching Model the DES in the main artery has slight influence on the BMS segments in the branch artery. In conclusion, due to the blood flow washing effect the uniformly released drug from DES would distribute focally and distally. The proximal DES would have greater influence on the distal BMS in straight artery than that in branching artery. This preliminary study would provide good reference for atherosclerosis treatment, especially for some complex cases, like coronary branching stenting.