External stenting reduces long-term medial and neointimal thickening and platelet derived growth factor expression in a pig model of arteriovenous bypass grafting

Periadventitial biomaterials to improve arteriovenous fistula and graft outcomes

Periadventitial biomaterials have been employed for nearly three decades to promote adaptive venous remodeling following hemodialysis vascular access creation in preclinical models and clinical trials. These systems are predicated on the combination of scaffolds, hydrogels, and/or particles with therapeutics (small molecules, proteins, genes, and cells) to prevent venous stenosis and subsequent maturation failure. Periadventitial biomaterial therapies have evolved from simple drug delivery vehicles for traditional drugs to more thoughtful designs tailored to the pathophysiology of access failure. The emergence of tissue engineering strategies and gene therapies are another exciting new direction. Despite favorable results in experimental and preclinical studies, no periadventitial therapy has been clinically approved to improve vascular access outcomes. After conducting an exhaustive review of the literature, we identify the seminal studies and clinical trials that utilize periadventitial biomaterials and discuss the key features of each biomaterial format and their respective shortcomings as they pertain to access maturation. This review provides a foundation from which clinicians, surgeons, biologists, and engineers can refer to and will hopefully inspire thoughtful, translatable treatments to finally address access failure.

Periadventitial β-aminopropionitrile-loaded nanofibers reduce fibrosis and improve arteriovenous fistula remodeling in rats

Background

Arteriovenous fistula (AVF) postoperative stenosis is a persistent healthcare problem for hemodialysis patients. We have previously demonstrated that fibrotic remodeling contributes to AVF non-maturation and lysyl oxidase (LOX) is upregulated in failed AVFs compared to matured. Herein, we developed a nanofiber scaffold for the periadventitial delivery of β-aminopropionitrile (BAPN) to determine whether unidirectional periadventitial LOX inhibition is a suitable strategy to promote adaptive AVF remodeling in a rat model of AVF remodeling.

Methods

Bilayer poly (lactic acid) ([PLA)-]- poly (lactic-co-glycolic acid) ([PLGA)] scaffolds were fabricated with using a two-step electrospinning process to confer directionality. BAPN-loaded and vehicle control scaffolds were wrapped around the venous limb of a rat femoral-epigastric AVF during surgery. AVF patency and lumen diameter were followed monitored using Doppler ultrasound surveillance and flow was measured before euthanasia. AVFs were harvested after 21 days for histomorphometry and immunohistochemistry. AVF compliance was measured using pressure myography. RNA from AVF veins was sequenced to analyze changes in gene expression due to LOX inhibition.

Results

Bilayer periadventitial nanofiber scaffolds extended BAPN release compared to the monolayer design (p < 0.005) and only released BAPN in one direction. Periadventitial LOX inhibition led to significant increases in AVF dilation and flow after 21 days. Histologically, BAPN trended toward increased lumen and significantly reduced fibrosis compared to control scaffolds (p < 0.01). Periadventitial BAPN reduced downregulated markers associated with myofibroblast differentiation including SMA, FSP-1, LOX, and TGF-β while increasing the contractile marker MYH11. RNA sequencing revealed differential expression of matrisome genes.

Conclusion

Periadventitial BAPN treatment reduces fibrosis and promotes AVF compliance. Interestingly, the inhibition of LOX leads to increased accumulation of contractile VSMC while reducing myofibroblast-like cells. Periadventitial LOX inhibition alters the matrisome to improve AVF vascular remodeling.

Biodegradable external wrapping promotes favorable adaptation in an ovine vein graft model

Vein grafts, the most commonly used conduits in multi-vessel coronary artery bypass grafting surgery, have high intermediate- and long-term failure rates. The abrupt and marked increase in hemodynamic loads on the vein graft is a known contributor to failure. Recent computational modeling suggests that veins can more successfully adapt to an increase in mechanical load if the rate of loading is gradual. Applying an external wrap or support at the time of surgery is one way to reduce the transmural load, and this approach has improved performance relative to an unsupported vein graft in several animal studies. Yet, a clinical trial in humans has shown benefits and drawbacks, and mechanisms by which an external wrap affects vein graft adaptation remain unknown. This study aims to elucidate such mechanisms using a multimodal experimental and computational data collection pipeline. We quantify morphometry using magnetic resonance imaging, mechanics using biaxial testing, hemodynamics using computational fluid dynamics, structure using histology, and transcriptional changes using bulk RNA-sequencing in an ovine carotid-jugular interposition vein graft model, without and with an external biodegradable wrap that allows loads to increase gradually. We show that a biodegradable external wrap promotes luminal uniformity, physiological wall shear stress, and a consistent vein graft phenotype, namely, it prevents over-distension, over-thickening, intimal hyperplasia, and inflammation, and it preserves mechanotransduction. These mechanobiological insights into vein graft adaptation in the presence of an external support can inform computational growth and remodeling models of external support and facilitate design and manufacturing of next-generation external wrapping devices. STATEMENT OF SIGNIFICANCE: External mechanical support is emerging as a promising technology to prevent vein graft failure following coronary bypass graft surgery. While variants of this technology are currently under investigation in clinical trials, the fundamental mechanisms of adaptation remain poorly understood. We employ an ovine carotid-jugular interposition vein graft model, with and without an external biodegradable wrap to provide mechanical support, and probe vein graft adaptation using a multimodal experimental and computational data collection pipeline. We quantify morphometry using magnetic resonance imaging, mechanics using biaxial testing, fluid flow using computational fluid dynamics, vascular composition and structure using histology, and transcriptional changes using bulk RNA sequencing. We show that the wrap mitigates vein graft failure by promoting multiple adaptive mechanisms (across biological scales).

Large animal model of vein grafts intimal hyperplasia: A systematic review

Coronary artery bypass grafting remains the treatment of choice for a large cohort of patients with significant coronary disease. Despite the increased use of arterial grafts, the long saphenous vein remains the most commonly used conduit. Long-term graft patency continues to be the Achilles heel of saphenous vein grafts. This is due to the development of intimal hyperplasia, a chronic inflammatory disease that results in the narrowing and occlusion of a significant number of vein grafts. Research models for intimal hyperplasia are essential for a better understanding of pathophysiological processes of this condition. Large animal models resemble human anatomical structures and have been used as a surrogate to study disease development and prevention over the years. In this paper, we systematically review all published studies that utilized large animal models of vein graft disease with a focus on the type of model and any therapeutic intervention, specifically the use of external stents/mesh.

Device Profile of the VEST for External Support of SVG Coronary Artery Bypass Grafting: Historical Development, Current Status and Future Directions

A search for strategies to address saphenous vein graft (SVG) failure - the main factor limiting the long-term success of coronary bypass grafting - has led to trialing of external stenting technologies.

AREAS COVERED

The manuscript covers historical development and current status of external scaffolding for the treatment of SVG intimal hyperplasia. Comprehensive literature review and personal communication with VGS leadership, the developer of the VEST device, served as the sources.

EXPERT OPINION

If the external scaffolding concept proves to be successful in mitigating the intimal hyperplasia inherent to arterialized saphenous vein conduits, it could have a dramatic impact on the recurrence of anginal symptoms, the need for repeat revascularization, and the incidence of myocardial infarction following CABG surgery. These laudable sequelae could ultimately convey significant public health repercussions by reducing healthcare resource use and improving the long-term survival and quality of life of CABG recipients.

External Stenting (Exostenting) to Correct Vascular Torsion and Angulation

Organ transplantation can be associated with vascular torsions and angulations of both recipient and donor vessels. Such kinks and/or torsions of vessels can compromise the vascular integrity, obstruct inflow and/or outflow, and result in loss of the organ and/or body parts. On many occasions, mild angulations and torsions can be successfully addressed by repositioning the organ. In cases where the abnormal findings persist, maneuvers such as placing a fat pad to create a smoother curve, or even opening the peritoneum (in the case of kidney transplants) to allow for a better positioning of the organ, are associated with successful outcomes. When such torsions/angulations persist despite these approaches, further innovative tactics are required. In the current report, we propose a technique that involves longitudinally opening of a synthetic graft that is rigid enough to maintain its shape, such as a ringed polytetrafluoroethylene graft, and placing it as an external stent around the angulated/torsioned vessel. This maneuver will correct the underlying vascular compromise without having to perform any further invasive interventions, such as reimplanting the organ or resecting part of the involved vessel. Although primarily illustrated for application by describing an instance in which exostenting was applied during kidney transplantation, our approach could be applied to any vessel under many circumstances where angulations/twists are encountered. In this report, we describe the use of an external stent, also called exostenting, to correct a severe torsion/angulation of the external iliac artery in a kidney transplant recipient where all other measures were unsuccessful.

Effect of pore size and spacing on neovascularization of a biodegradble shape memory polymer perivascular wrap

Neointimal hyperplasia (NH) is a main source of failures in arteriovenous fistulas and vascular grafts. Several studies have demonstrated the promise of perivascular wraps to reduce NH via promotion of adventitial neovascularization and providing mechanical support. Limited clinical success thus far may be due to inappropriate material selection (e.g., nondegradable, too stiff) and geometric design (e.g., pore size and spacing, diameter). The influence of pore size and spacing on implant neovascularization is investigated here for a new biodegradable, thermoresponsive shape memory polymer (SMP) perivascular wrap. Following an initial pilot, 21 mice were each implanted with six scaffolds: four candidate SMP macroporous designs (a-d), a nonporous SMP control (e), and microporous GORETEX (f). Mice were sacrificed after 4 (N = 5), 14 (N = 8), and 28 (N = 8) days. There was a statistically significant increase in neovascularization score between all macroporous groups compared to nonporous SMP (p < .023) and microporous GORETEX (p < .007) controls at Day 28. Wider-spaced, smaller-sized pore designs (223 μm-spaced, 640 μm-diameter Design c) induced the most robust angiogenic response, with greater microvessel number (p < .0114) and area (p < .0055) than nonporous SMPs and GORETEX at Day 28. This design also produced significantly greater microvessel density than nonporous SMPs (p = 0.0028) and a smaller-spaced, larger-sized pore (155 μm-spaced, 1,180 μm-sized Design b) design (p = .0013). Strong neovascularization is expected to reduce NH, motivating further investigation of this SMP wrap with controlled pore spacing and size in more advanced arteriovenous models.

A Soft, Conductive External Stent Inhibits Intimal Hyperplasia in Vein Grafts by Electroporation and Mechanical Restriction

Intimal hyperplasia (IH) in vein grafts (VGs) is a major issue in coronary artery bypass grafting (CABG) surgery. Although external stents can attenuate IH of VGs to some extent, none of the existing external stents have shown satisfactory clinical outcomes. Here we develop a flexible, biodegradable, and conductive external metal-polymer conductor stent (MPCS) that can electroporate the vessel wall and produce a protein that prevents IH. We designed the plasmid DNA encoding the tissue inhibitor of metalloproteinases-3 (TIMP-3) and lyophilized it on the inner surface of the MPCS to deliver into the adventitia and the middle layer of VGs for gene therapy. Coupled with its continuous mechanical support to prevent dilation after implanting, the MPCS can inhibit the IH of VGs significantly in the rabbit model. This proof-of-concept demonstration may aid the development of other implantable bioelectronics for electroporation gene therapy.

Inhibition of Lysyl Oxidase with β-aminopropionitrile Improves Venous Adaptation after Arteriovenous Fistula Creation

BACKGROUND

The arteriovenous fistula (AVF) is the preferred hemodialysis access for end-stage renal disease (ESRD) patients. Yet, establishment of a functional AVF presents a challenge, even for the most experienced surgeons, since postoperative stenosis frequently occludes the AVF. Stenosis results from the loss of compliance in fibrotic areas of the fistula which turns intimal hyperplasia into an occlusive feature. Fibrotic remodeling depends on deposition and crosslinking of collagen by lysyl oxidase (LOX), an enzyme that catalyzes the deamination of lysine and hydroxylysine residues, facilitating intra/intermolecular covalent bonds. We postulate that pharmacological inhibition of lysyl oxidase (LOX) increases postoperative venous compliance and prevents stenosis in a rat AVF model.

METHODS

LOX gene expression and vascular localization were assayed in rat AVFs and human pre-access veins, respectively. Collagen crosslinking was measured in humans AVFs that matured or failed, and in rat AVFs treated with β-aminopropionitrile (BAPN), an irreversible LOX inhibitor. BAPN was either injected systemically or delivered locally around rat AVFs using nanofiber scaffolds. The major endpoints were AVF blood flow, wall fibrosis, collagen crosslinking, and vascular distensibility.

RESULTS

Non-maturation of human AVFs was associated with higher LOX deposition in pre-access veins (N=20, P=0.029), and increased trivalent crosslinks (N=18, P=0.027) in human AVF tissues. Systemic and local inhibition of LOX increased AVF distensibility, while reducing wall fibrosis and collagen crosslinking in rat fistulas.

CONCLUSIONS

Our results demonstrate that BAPN-mediated inhibition of LOX significantly improves vascular remodeling in experimental fistulas.

Saphenous veins in coronary artery bypass grafting need external support

The saphenous vein is the most commonly used conduit for coronary artery bypass grafting. Arterial grafts are harvested with the outer pedicle intact whereas saphenous veins are harvested with the pedicle removed in the conventional graft harvesting technique. This conventional procedure causes considerable vascular damage. One strategy to improve vein graft patency has been to provide external support. Ongoing studies show that fitting a metal external support improves conventionally harvested saphenous vein graft patency. On the other hand, the no-touch technique of harvesting the saphenous vein provides an improved graft with long-term patency comparable to that of the internal mammary artery. This improvement is suggested to be due to preservation of vessel structures. Interestingly, many of the mechanisms proposed to be associated with the beneficial actions of an artificial external support on saphenous vein graft patency are similar to those underlying the beneficial effect of no-touch saphenous vein grafts where the intact outer layer acts as a natural support. Additional actions of external supports have been advocated, including promotion of angiogenesis, increased production of vascular-protective factors, and protection of endothelial cells. Using no-touch harvesting, normal vascular architecture is maintained, tissue and cell damage is minimized, and factors beneficial for graft patency are preserved. In this review, the significance of external support of saphenous vein grafts in coronary artery bypass grafting is discussed.

Vascular adaptation in the presence of external support - A modeling study

Vascular grafts have long been used to replace damaged or diseased vessels with considerable success, but a new approach is emerging where native vessels are merely supported, not replaced. Although external supports have been evaluated in diverse situations - ranging from aneurysmal disease to vein grafts or the Ross operation - optimal supports and procedures remain wanting. In this paper, we present a novel application of a growth and remodeling model well suited for parametrically exploring multiple designs of external supports while accounting for mechanobiological and immunobiological responses of the supported native vessel. These results suggest that a load bearing external support can reduce vessel thickening in response to pressure elevation. Results also suggest that the final adaptive state of the vessel depends on the structural stiffness of the support via a mechano-driven adaptation, although luminal encroachment may be a complication in the presence of chronic inflammation. Finally, the supported vessel can stiffen (structurally and materially) along circumferential and axial directions, which could have implications on overall hemodynamics and thus subsequent vascular remodeling. The proposed framework can provide valuable insights into vascular adaptation in the presence of external support, accelerate rational design, and aid translation of this emerging approach.

Role of smooth muscle cells in coronary artery bypass grafting failure

Atherosclerosis is the underlying pathology of many cardiovascular diseases. The formation and rupture of atherosclerotic plaques in the coronary arteries results in angina and myocardial infarction. Venous coronary artery bypass grafts are designed to reduce the consequences of atherosclerosis in the coronary arteries by diverting blood flow around the atherosclerotic plaques. However, vein grafts suffer a high failure rate due to intimal thickening that occurs as a result of vascular cell injury and activation and can act as 'a soil' for subsequent atherosclerotic plaque formation. A clinically-proven method for the reduction of vein graft intimal thickening and subsequent major adverse clinical events is currently not available. Consequently, a greater understanding of the underlying mechanisms of intimal thickening may be beneficial for the design of future therapies for vein graft failure. Vein grafting induces inflammation and endothelial cell damage and dysfunction, that promotes vascular smooth muscle cell (VSMC) migration, and proliferation. Injury to the wall of the vein as a result of grafting leads to the production of chemoattractants, remodelling of the extracellular matrix and cell-cell contacts; which all contribute to the induction of VSMC migration and proliferation. This review focuses on the role of altered behaviour of VSMCs in the vein graft and some of the factors which critically lead to intimal thickening that pre-disposes the vein graft to further atherosclerosis and re-occurrence of symptoms in the patient.

Biomechanics and Mechanobiology of Saphenous Vein Grafts

Within several weeks of use as coronary artery bypass grafts (CABG), saphenous veins (SV) exhibit significant intimal hyperplasia (IH). IH predisposes vessels to thrombosis and atherosclerosis, the two major modes of vein graft failure. The fact that SV do not develop significant IH in their native venous environment coupled with the rapidity with which they develop IH following grafting into the arterial circulation suggests that factors associated with the isolation and preparation of SV and/or differences between the venous and arterial environments contribute to disease progression. There is strong evidence suggesting that mechanical trauma associated with traditional techniques of SV preparation can significantly damage the vessel and might potentially reduce graft patency though modern surgical techniques reduces these injuries. In contrast, it seems possible that modern surgical technique, specifically endoscopic vein harvest, might introduce other mechanical trauma that could subtly injure the vein and perhaps contribute to the reduced patency observed in veins harvested using endoscopic techniques. Aspects of the arterial mechanical environment influence remodeling of SV grafted into the arterial circulation. Increased pressure likely leads to thickening of the medial wall but its role in IH is less clear. Changes in fluid flow, including increased average wall shear stress, may reduce IH while disturbed flow likely increase IH. Nonmechanical stimuli, such as exposure to arterial levels of oxygen, may also have a significant but not widely recognized role in IH. Several potentially promising approaches to alter the mechanical environment to improve graft patency are including extravascular supports or altered graft geometries are covered.

Vasa vasorum inside out/outside in communication: a potential role in the patency of saphenous vein coronary artery bypass grafts

The saphenous vein (SV) is the most commonly used conduit for revascularization in patients undergoing coronary artery bypass surgery (CABG). The patency rate of this vessel is inferior to the internal thoracic artery (ITA). In the majority of CABG procedures the ITA is removed with its outer pedicle intact whereas the (human) SV (hSV) is harvested with pedicle removed. The vasa vasorum, a microvessel network providing the adventitia and media with oxygen and nutrients, is more pronounced and penetrates deeper towards the lumen in veins than in arteries. When prepared in conventional CABG the vascular trauma caused when removing the hSV pedicle damages the vasa vasorum, a situation affecting transmural flow potentially impacting on graft performance. In patients, where the hSV is harvested with pedicle intact, the vasa vasorum is preserved and transmural blood flow restored at graft insertion and completion of CABG. By maintaining blood supply to the hSV wall, apart from oxygen and nutrients, the vasa vasorum may also transport factors potentially beneficial to graft performance. Studies, using either corrosion casts or India ink, have shown the course of vasa vasorum in animal SV as well as in hSV. In addition, there is some evidence that vasa vasorum of hSV terminate in the vessel lumen based on ex vivo perfusion, histological and ultrastructural studies. This review describes the preparation of the hSV as a bypass conduit in CABG and its performance compared with the ITA as well as how and why its patency might be improved by harvesting with minimal trauma in a way that preserves an intact vasa vasorum.

External Stenting with a New Polyester Mesh Reduces Neointimal Hyperplasia of Vein Grafts in a Sheep Model

Objective

External stents placed around vein grafts have demonstrated effectiveness in reducing neointimal hyperplasia by preventing distension of the thin-walled vein grafts when exposed to arterial pressure. However, the ideal stent material has yet to be defined. The following study investigates the short- and long-term effects of an innovative polyester mesh stent designed with optimized adaptation of circumferential compliance.

Methods

Following in vitro definition of the ideal macro-porous polyester stent material, a total of 12 sheep underwent implantation of bilateral carotid artery vein graft bypasses. In six sheep, the short-term outcome (four weeks of implantation) was investigated by comparing the newly-designed stent to native veins, micro-porous PTFE stent grafts and metallic Biocompound® stents (BCGs). Flow volume and graft diameter were measured prior to explantation. Grafts were evaluated histologically with respect to morphometry and immunoassaying. In the long-term group (6-month implantation time), the polyester stent was compared to native veins.

Results

All stents effectively prevented dilatation of the graft. Perfused vessel diameters of the polyester veins were 8.3±0.6 mm. BCG as well as PTFE veins showed diameter reduction to 7.4±0.7mm (p<0.05) and 7.8±0.4 mm (p<0.05), respectively. Both in the short and long terms, the new polyester stent led to significantly higher reduction of neointimal hyperplasia and luminal encroachment compared to the native vessel. It proved superior to PTFE stenting, while the Biocompound® material failed to prove efficacy.

Conclusions

It was demonstrated that the new macro-porous polyester mesh stent reduced neointimal hyperplasia more effectively than other commercially available stents.

Dysregulation of angiogenesis-specific signalling in adult testis results in xenograft degeneration

Ectopic xenografting of testis is a feasible option for preservation of male fertility and angiogenesis plays a pivotal role in xenograft survival and functionality. When compared to immature testis, the adult testis is unable to establish functional xenografts due to potentially lower efficiency to induce angiogenesis. The precise molecular mechanism, however, remains elusive. In the present study, we compared adult and immature testis xenografts for survival, maturation and germ cell differentiation. Further, we evaluated differential expression of angiogenesis signalling-specific proteins in adult and immature testis and their xenografts. Results showed that adult testis xenografts degenerated whereas immature testis xenografts survived and established spermatogenesis with the production of haploid germ cells. Protein expression analysis demonstrated that immature testis xenografts were able to establish angiogenesis either through eNOS activation via VEGF and PI3K/AKT or through EGFR-mediated STAT3 pathway. The role of ERK/MAPK pathway in xenograft angiogenesis was ruled out. The absence or reduced expression of angiogenesis-specific proteins in adult testis and its xenografts possibly resulted in poor angiogenesis and in their subsequent degeneration. This study provides insight into angiogenesis mechanism that can be utilized to augment testis xenografting efficiency.

High expression of ubiquitin-specific peptidase 39 is associated with the development of vascular remodeling

Vascular remodeling is the primary cause underlying the failure of angioplasty surgeries, including vascular stenting, transplant vasculopathy and vein grafts. Multiple restenosis‑associated proteins and genes have been identified to account for this. In the present study, the functions of ubiquitin‑specific peptidase 39 (USP39) were investigated in the context of two vascular remodeling models (a mouse common carotid artery ligation and a pig bilateral saphenous vein‑carotid artery interposition graft). USP39 has previously been observed to be upregulated in ligated arteries, and this result was confirmed in the pig vein graft model. In addition, Transwell assay results demonstrated that vascular smooth muscle cell (VSMC) migration was suppressed by lentiviral vector‑mediated downregulation of USP39 and enhanced by upregulation of USP39. Furthermore, knockdown of USP39 inhibited VSMC cell proliferation and the expression of cyclin D1 and cyclin‑dependent kinase 4, as analyzed via cell counting, MTT assay and western blotting. These results suggest that USP39 may represent a novel therapeutic target for treating vascular injury and preventing vein-graft failure.

Perivascular medical devices and drug delivery systems: Making the right choices

Perivascular medical devices and perivascular drug delivery systems are conceived for local application around a blood vessel during open vascular surgery. These systems provide mechanical support and/or pharmacological activity for the prevention of intimal hyperplasia following vessel injury. Despite abundant reports in the literature and numerous clinical trials, no efficient perivascular treatment is available. In this review, the existing perivascular medical devices and perivascular drug delivery systems, such as polymeric gels, meshes, sheaths, wraps, matrices, and metal meshes, are jointly evaluated. The key criteria for the design of an ideal perivascular system are identified. Perivascular treatments should have mechanical specifications that ensure system localization, prolonged retention and adequate vascular constriction. From the data gathered, it appears that a drug is necessary to increase the efficacy of these systems. As such, the release kinetics of pharmacological agents should match the development of the pathology. A successful perivascular system must combine these optimized pharmacological and mechanical properties to be efficient.

Arterial Blood Pressure Induces Transient C4b-Binding Protein in Human Saphenous Vein Grafts

BACKGROUND

Complement is an important mediator in arterial blood pressure-induced vein graft failure. Previously, we noted activation of cell protective mechanisms in human saphenous veins too. Here we have analyzed whether C4b-binding protein (C4bp), an endogenous complement inhibitor, is present in the vein wall.

METHODS

Human saphenous vein segments obtained from patients undergoing coronary artery bypass grafting (n = 55) were perfused in vitro at arterial blood pressure with either autologous blood for 1, 2, 4, or 6 hr or with autologous blood supplemented with reactive oxygen species scavenger N-acetylcysteine. The segments were subsequently analyzed quantitatively for presence of C4bp and complement activation product C3d using immunohistochemistry.

RESULTS

Perfusion induced deposition of C3d and C4bp within the media of the vessel wall, which increased reproducibly and significantly over a period of 4 hr up to 3.8% for C3d and 81% for C4bp of the total vessel area. Remarkably after 6 hr of perfusion, the C3d-positive area decreased significantly to 1.3% and the C4bp-positive area to 19% of the total area of the vein. The areas positive for both C4bp and C3d were increased in the presence of N-acetylcysteine.

CONCLUSIONS

Exposure to arterial blood pressure leads to a transient presence of C4bp in the vein wall. This may be part of a cell-protective mechanism to counteract arterial blood pressure-induced cellular stress and inflammation in grafted veins.

Neointima development in externally stented saphenous vein grafts

Introduction

The main limitation of coronary artery bypass grafting (CABG) is rapid neointimal hyperplasia leading to graft failure.

Aim

To assess plaque formation in saphenous vein grafts (SVG) covered by an external Dacron stent in comparison with the classical technique.

Material and methods

In the study group vein grafts covered by external stent mesh made of Dacron were implanted. An intravascular ultrasonography (IVUS) study was performed in 35 aorto-coronary SVG covered by an external Dacron stent and in 64 normal SVG during the first year after CABG. In each SVG 25 mm of good quality IVUS image, volumes of lumen, plaque (neointima), outer border of the vein graft (external SVG) and adventitia were calculated in three time periods: 0–130 days, 130–260 days and 260–390 days.

Results

Between the first and second time period, lumen volume (mm³) was reduced from 10.33 ±4.4, to 6.80 ±2.23 in the second period and 5.69 ±1.26 in the third one. This effect was much less marked in normal grafts. The corresponding lumen volume (mm³) was: 10.90 ±3.9, 9.15 ±2.94 and 8.92 ±2.93 in consecutive time periods. Plaque volume (mm³) did not change in control grafts during the course of the study, but it increased very significantly in stented grafts from 0.86 ±1.24 in the first period to 2.70 ±1.58 in the second and 3.29 ±2.66 in the third one.

Conclusions

The experimental technique of implanting SVG covered with an external elastic Dacron stent seems to be inferior to traditional ones. This is probably due to the more complicated process of vein implantation and higher micro-injury occurrence during the surgery.

Biomaterial-Based Approaches to Address Vein Graft and Hemodialysis Access Failures

Veins used as grafts in heart bypass or as access points in hemodialysis exhibit high failure rates, thereby causing significant morbidity and mortality for patients. Interventional or revisional surgeries required to correct these failures have been met with limited success and exorbitant costs, particularly for the US Centers for Medicare & Medicaid Services. Vein stenosis or occlusion leading to failure is primarily the result of neointimal hyperplasia. Systemic therapies have achieved little long-term success, indicating the need for more localized, sustained, biomaterial-based solutions. Numerous studies have demonstrated the ability of external stents to reduce neointimal hyperplasia. However, successful results from animal models have failed to translate to the clinic thus far, and no external stent is currently approved for use in the US to prevent vein graft or hemodialysis access failures. This review discusses current progress in the field, design considerations, and future perspectives for biomaterial-based external stents. More comparative studies iteratively modulating biomaterial and biomaterial-drug approaches are critical in addressing mechanistic knowledge gaps associated with external stent application to the arteriovenous environment. Addressing these gaps will ultimately lead to more viable solutions that prevent vein graft and hemodialysis access failures.

Photochemical Tissue Passivation Reduces Vein Graft Intimal Hyperplasia in a Swine Model of Arteriovenous Bypass Grafting

BACKGROUND

Bypass grafting remains the standard of care for coronary artery disease and severe lower extremity ischemia. Efficacy is limited by poor long-term venous graft patency secondary to intimal hyperplasia (IH) caused by venous injury upon exposure to arterial pressure. We investigate whether photochemical tissue passivation (PTP) treatment of vein grafts modulates smooth muscle cell (SMC) proliferation and migration, and inhibits development of IH.

METHODS AND RESULTS

PTP was performed at increasing fluences up to 120 J/cm(2) on porcine veins. Tensiometry performed to assess vessel elasticity/stiffness showed increased stiffness with increasing fluence until plateauing at 90 J/cm(2) (median, interquartile range [IQR]). At 90 J/cm(2), PTP-treated vessels had a 10-fold greater Young's modulus than untreated controls (954 [IQR, 2217] vs 99 kPa [IQR, 63]; P=0.03). Each pig received a PTP-treated and untreated carotid artery venous interposition graft. At 4-weeks, intimal/medial areas were assessed. PTP reduced the degree of IH by 66% and medial hypertrophy by 49%. Intimal area was 3.91 (IQR, 1.2) and 1.3 mm(2) (IQR, 0.97; P≤0.001) in untreated and PTP-treated grafts, respectively. Medial area was 9.2 (IQR, 3.2) and 4.7 mm(2) (IQR, 2.0; P≤0.001) in untreated and PTP-treated grafts, respectively. Immunohistochemistry was performed to assess alpha-smooth muscle actin (SMA) and proliferating cell nuclear antigen (PCNA). Objectively, there were less SMA-positive cells within the intima/media of PTP-treated vessels than controls. There was an increase in PCNA-positive cells within control vein grafts (18% [IQR, 5.3]) versus PTP-treated vein grafts (5% [IQR, 0.9]; P=0.02).

CONCLUSIONS

By strengthening vein grafts, PTP decreases SMC proliferation and migration, thereby reducing IH.

Administration of perivascular cyanoacrylate for the prevention of cellular damage in saphenous vein grafts: an experimental model

Objective:

The saphenous vein is the most commonly used graft in coronary artery bypass surgery, since no suitable arterial graft is available. However, the frequency of late graft failure is a cause for research into graft protection. The objective of this study was to investigate the effect of synthetic adhesive cyanoacrylate administration on the saphenous vein graft for preventing vascular damage due to internal pressure on the graft.

Methods:

In this study we enrolled 20 volunteer subjects who had undergone coronary artery bypass surgery and who had excess saphenous vein grafts. Perivascular cyanoacrylate was administered to one of two saphenous vein grafts explanted from each patient. The other saphenous vein graft from each patient was not treated and was used as the control. A model of the arterial system was created using a saphenous vein cardiopulmonary bypass system. Circulation was maintained at 120 mmHg for 45 minutes. Afterwards, the grafts were subjected to histopathological examination.

Results:

The cyanoacrylate group of grafts did not develop severe vascular damage compared with many instances of moderate and severe damage due to compression in the control group of grafts (p = 0.003).

Conclusion

Perivascular administration of cyanoacrylate appeared to be successful in the prevention of early saphenous vein graft injury. No in vivo study has been performed to date to assess endothelial damage in the saphenous vein, in order to demonstrate the long-term effect of cyanoacrylate. Further investigations are needed in this regard.

Upregulation of let-7a inhibits vascular smooth muscle cell proliferation in vitro and in vein graft intimal hyperplasia in rats

BACKGROUND

Proliferation of vascular smooth muscle cells (VSMCs) is a crucial event in the pathogenesis of intimal hyperplasia, which is the main cause of restenosis after vascular reconstruction. In this study, we assessed the impact of let-7a microRNA (miRNA) on the proliferation of VSMCs.

METHODS

Using miRNA microarrays analysis for miRNA expression in the vein graft model. Lentiviral vector-mediated let-7a was transfected into the vein grafts. In situ hybridization was performed to detect let-7a. Cultured rat VSMCs were transfected with let-7a mimics for different periods of time. Cell proliferation, migration and cell cycle activity were monitored following transfection of the let-7a mimics. Immunohistochemical and Western blotting analysis the expression levels of c-myc and K-ras.

RESULTS

We found that let-7a was the most downregulated miRNA in the vein graft model. In vivo proliferation of VSMCs was assessed in a rat model of venous graft intimal hyperplasia. Let-7a was found to localize mainly to the VSMCs. Let-7a miRNA expression was increased in VSMCs in the neointima of the let-7a treated group. Intimal hyperplasia was suppressed by upregulation of let-7a via lentiviral vector-mediated mimics. In cultured VSMCs, the expression of let-7a increased upon starving, and the upregulation of let-7a miRNA significantly decreased cell proliferation and migration. Immunohistochemical and Western blotting analysis demonstrated that treatment with let-7a mimics resulted in decreased expression levels of c-myc and K-ras.

CONCLUSIONS

The results indicate that let-7a miRNA is a novel regulator of VSMC proliferation in intimal hyperplasia. These findings suggest that let-7a miRNA is a promising therapeutic target for the prevention of intimal hyperplasia.

Saphenous vein graft failure after coronary artery bypass surgery: pathophysiology, management, and future directions

OBJECTIVE

To review our current understanding of the epidemiology and pathogenesis of vein graft failure (VGF), give an overview of current preventive and interventional measures, and explore strategies that may improve vein graft patency.

BACKGROUND

VGF and progression of native coronary artery disease limit the long-term efficacy of coronary artery bypass graft surgery.

METHODS

We reviewed the published literature on the pathophysiology, prevention, and/or treatment of VGF by searching the MEDLINE (January 1, 1966-January 1, 2012), EMBASE (January 1, 1980-January 1, 2012), and Cochrane (January 1, 1995-January 1, 2012) databases. In addition, we reviewed references from the selected articles for studies not identified in the initial search. Basic science and clinical studies were included; non-English language publications were excluded.

RESULTS

Acute thrombosis, neointimal hyperplasia, and accelerated atherosclerosis are the 3 mechanisms that lead to VGF. Preventive measures include matching and quality assessment of conduit and target vessel, lipid-lowering drugs, antithrombotic therapy, and cessation of smoking. Treatment of VGF includes medical therapy, percutaneous intervention, and redo coronary artery bypass graft surgery. In patients undergoing graft intervention, the use of drug-eluting stents, antiplatelet agents, and embolic protection devices may improve clinical outcomes.

CONCLUSIONS

Despite advances in management, VGF remains one of the leading causes of poor in-hospital and long-term outcomes after coronary artery bypass graft surgery. New developments in VGF prevention such as gene therapy, external graft support, fully tissue-engineered grafts, hybrid grafts, and synthetic conduits are promising but unproven. Future efforts to reduce VGF require a multidisciplinary approach with a primary focus on prevention.

Apolipoprotein(a) acts as a chemorepellent to human vascular smooth muscle cells via integrin αVβ3 and RhoA/ROCK-mediated mechanisms

Lipoprotein(a) (Lp(a)) is an independent risk factor for the development of cardiovascular disease. Vascular smooth muscle cell (SMC) motility and plasticity, functions that are influenced by environmental cues, are vital to adaptation and remodelling in vascular physiology and pathophysiology. Lp(a) is reportedly damaging to SMC function via unknown molecular mechanisms. Apolipoprotein(a) (apo(a)), a unique glycoprotein moiety of Lp(a), has been demonstrated as its active component. The aims of this study were to determine functional effects of recombinant apo(a) on human vascular SMC motility and explore the underlying mechanism(s). Exposure of SMC to apo(a) in migration assays induced a potent, concentration-dependent chemorepulsion that was RhoA and integrin αVβ3-dependent, but transforming growth factor β-independent. SMC manipulation through RhoA gene silencing, Rho kinase inhibition, statin pre-treatment, αVβ3 neutralising antibody and tyrosine kinase inhibition all markedly inhibited apo(a)-mediated SMC migration. Our data reveal unique and potent activities of apo(a) that may negatively influence SMC remodelling in cardiovascular disease. Circulating levels of Lp(a) are resistant to lipid-lowering strategies and hence a greater understanding of the mechanisms underlying its functional effects on SMC may provide alternative therapeutic targets.

Expandable external support device to improve Saphenous Vein Graft Patency after CABG

Objectives

Low patency rates of saphenous vein grafts remain a major predicament in surgical revascularization. We examined a novel expandable external support device designed to mitigate causative factors for early and late graft failure.

Methods

For this study, fourteen adult sheep underwent cardiac revascularization using two vein grafts for each; one to the LAD and the other to the obtuse marginal artery. One graft was supported with the device while the other served as a control. Target vessel was alternated between consecutive cases. The animals underwent immediate and late angiography and were then sacrificed for histopathologic evaluation.

Results

Of the fourteen animals studied, three died peri-operatively (unrelated to device implanted), and ten survived the follow-up period. Among surviving animals, three grafts were thrombosed and one was occluded, all in the control group (p = 0.043). Quantitative angiographic evaluation revealed no difference between groups in immediate level of graft uniformity, with a coefficient-of-variance (CV%) of 7.39 in control versus 5.07 in the supported grafts, p = 0.082. At 12 weeks, there was a significant non-uniformity in the control grafts versus the supported grafts (CV = 22.12 versus 3.01, p < 0.002). In histopathologic evaluation, mean intimal area of the supported grafts was significantly lower than in the control grafts (11.2 mm^² versus 23.1 mm^² p < 0.02).

Conclusions

The expandable SVG external support system was found to be efficacious in reducing SVG’s non-uniform dilatation and neointimal formation in an animal model early after CABG. This novel technology may have the potential to improve SVG patency rates after surgical myocardial revascularization.

External support in preventing vein graft failure

Saphenous vein remains a widely used conduit in coronary surgery. However, the long-term success of surgical myocardial revascularization is largely limited by the development of neointimal hyperplasia and superimposed atherosclerosis in vein grafts. Although strategies for preventing vein graft failure have been constantly explored, few therapeutic interventions to date have shown sustained benefits in the clinical setting. The application of external support has emerged as a promising strategy for modulating the overall biomechanical responses in venous wall. Nonetheless, clinical translation of this intervention has been formerly challenged, primarily due to several technique limitations. The purpose of the current review is to summarize the possible mechanisms involved in the external support strategy for preventing vein graft failure. Furthermore, several previously tested biomaterials and delivery techniques are also highlighted.

Saphenous vein graft disease: review of pathophysiology, prevention, and treatment

Saphenous vein graft (SVG) disease after coronary artery bypass grafting (CABG) occurs in three phases: thrombosis, intimal hyperplasia, and atherosclerosis. Within the first month, thrombosis plays a major role. From month 1 to month 12, intimal hyperplasia occurs. Beyond 12 months, atherosclerosis becomes the primary cause for late graft failure. Endothelial damage has been shown to be the major underlying pathophysiology of SVG disease. Many factors contribute to endothelial damage from the moment the vein is harvested to when the vein is grafted into an arterial environment. To address this disease process, various therapeutic modalities, from surgical methods to medical treatment, have been evaluated. Surgically, the technical method of harvesting the vein has been shown to affect SVG patency. From a pharmacologic perspective, only two guideline class I recommended medications, aspirin and statins, have been shown to improve short- and long-term SVG patency after CABG. Despite these surgical and medical advances, SVG disease remains a significant problem with 1-year patency rates of 89% dropping to 61% after 10 years. This review discusses the pathogenesis of SVG disease, predictors of SVG failure, and current surgical and pharmacologic therapies to address SVG disease, including possible future treatment.

'No-touch' saphenous vein harvesting improves graft performance in patients undergoing coronary artery bypass surgery: a journey from bedside to bench

The saphenous vein is the most commonly used conduit in patients undergoing coronary artery bypass surgery yet its patency is inferior to the internal thoracic artery. Vascular damage inflicted to the vein when using conventional harvesting techniques affects its structure. Endothelial denudation is associated with early vein graft failure while damage of the outermost vessel layers has adverse long-term effects on graft performance. While many in vitro and in vivo experimental studies aimed at improving vein graft patency have been performed to date no significant 'bench to bedside' advances have been made. Among experimental strategies employed is the use of pharmacological agents, gene targeting and external stents. A 'no-touch' technique, where the saphenous vein is removed with minimal trauma and normal architecture preserved, produces a superior graft with long term patency comparable to the internal thoracic artery. Interestingly, many experimental studies are aimed at repairing or replacing those regions of the saphenous vein damaged when harvesting conventionally. 'No-touch' harvesting is superior in coronary artery bypass patients with long-term data published 5years ago. Here we describe a 'bedside to bench' situation where the mechanisms underlying the improved performance of 'no touch' saphenous vein grafts in patients have been studied in the laboratory.

Vein graft failure: current clinical practice and potential for gene therapeutics

Autologous saphenous vein is commonly used as a conduit to bypass atherosclerotic lesions in coronary and femoral arteries. Despite the wide use of arterial conduits, which are less susceptible to complications and failure, as alternative conduits, the saphenous vein will continue to be used in coronary artery bypass grafting until acceptable alternative approaches are evaluated. Hence, preservation of vein graft patency is essential for the long-term success. Gene therapy is attractive in this setting as an ex-vivo technology to genetically manipulate the conduit before grafting. The use of safe and efficient vectors for delivery is a necessity as well as a strategy to improve patency in the long term. Here, we review the current clinical practice, the pathogenesis of bypass graft failure and adenovirus-mediated gene therapy strategies designed to improve late vein graft failure by modulation of smooth muscle cells in the vein wall.

Pathophysiology of saphenous vein graft failure: a brief overview of interventions

Coronary artery bypass graft surgery (CABG) is widely used for the treatment of atheromatous stenosis of coronary arteries. However, as many as 50% of grafts fail within 10 years after CABG due to neointima (NI) formation, a process involving the proliferation of vascular smooth muscle cells (VSMCs) and superimposed atherogenesis. To date no therapeutic intervention has proved successful in treating late vein graft failure. However, several diverse approaches aimed at preventing neointimal formation have been devised which have yielded promising results. In this review, therefore, we will summarise the pathophysiology of vein graft disease and then briefly consider interventional approaches to prevent late vein graft failure which include surgical technique, conventional pharmacology, external sheaths, cytostatic drugs and gene transfer.

Animal models for studying vein graft failure and therapeutic interventions

Vein grafts have been extensively used to bypass blockages in arteries, but are themselves subject to early closure by thrombosis or later obstruction by vein graft disease (neointimal hyperplasia and remodelling). Animal models are a crucial means of testing potential therapeutic and surgical interventions to prevent graft stenosis and occlusion. This review outlines many of the animal models of vein grafting. Recent studies include targeted gene therapy to prevent acute vein graft thrombosis and the use of folic acid to limit graft failure in diabetic pigs.

C1-esterase inhibitor protects against early vein graft remodeling under arterial blood pressure

OBJECTIVES

Arterial pressure induced vein graft injury can result in endothelial loss, accelerated atherosclerosis and vein graft failure. Inflammation, including complement activation, is assumed to play a pivotal role herein. Here, we analyzed the effects of C1-esterase inhibitor (C1inh) on early vein graft remodeling.

METHODS

Human saphenous vein graft segments (n=8) were perfused in vitro with autologous blood either supplemented or not with purified human C1inh at arterial pressure for 6h. The vein segments and perfusion blood were analyzed for cell damage and complement activation. In addition, the effect of purified C1inh on vein graft remodeling was analyzed in vivo in atherosclerotic C57Bl6/ApoE3 Leiden mice, wherein donor caval veins were interpositioned in the common carotid artery.

RESULTS

Application of C1inh in the in vitro perfusion model resulted in significantly higher blood levels and significantly more depositions of C1inh in the vein wall. This coincided with a significant reduction in endothelial loss and deposition of C3d and C4d in the vein wall, especially in the circular layer, compared to vein segments perfused without supplemented C1inh. Administration of purified C1inh significantly inhibited vein graft intimal thickening in vivo in atherosclerotic C57Bl6/ApoE3 Leiden mice, wherein donor caval veins were interpositioned in the common carotid artery.

CONCLUSION

C1inh significantly protects against early vein graft remodeling, including loss of endothelium and intimal thickening. These data suggest that it may be worth considering its use in patients undergoing coronary artery bypass grafting.

Highly flexible nitinol mesh to encase aortocoronary saphenous vein grafts: first clinical experiences and angiographic results nine months postoperatively

Saphenous vein graft patency is frequently limited by degeneration. Experimental studies have indicated that rigid external support of venous grafts by a flexible, tubular nitinol mesh may improve graft patency. The study presented was part of a prospective, randomized, multicenter first-in-man trial investigating the safety and effectiveness of nitinol-supported venous grafts in coronary artery bypass graft (CABG) surgery. From our clinic, 25 subjects with multivessel coronary artery disease requiring saphenous vein graft CABG of the right coronary artery (RCA) and the circumflex artery were entered into the trial. Subjects were randomized to receive a mesh-supported graft on one of these arteries; the other vessel received an untreated vein graft. Graft patency was assessed by coronary angiography nine months after surgery. The implantation of mesh grafts was simple and safe. In 10 cases, a nitinol mesh-supported venous graft was anastomosed to the circumflex artery and in 15 cases to the RCA. All patients survived the observation period. A total of 72% of the patients underwent control coronary angiography. The patency rate of mesh-supported grafts was 27.8% nine months postoperatively. Conventional vein grafts showed an 85.7% patency, and arterial grafts had a 100% patency. No complications directly related to the implantation of mesh-supported grafts were observed. The promising experimental results of mesh-supported venous grafts could not be reproduced in the study presented. A critical item seems to be correct selection of nitinol mesh diameter, the anastomotic method and fixation of the mesh tube to the venous graft.