Journal of Vascular Surgery
Volume 45, Issue 6, Supplement , Pages A74-A81, June 2007

Molecular engineering of vein bypass grafts

  • Michael S. Conte, MD

      Affiliations

    • Corresponding Author InformationReprint requests: Michael S. Conte, MD, Brigham and Women’s Hospital, Department of Vascular and Endovascular Surgery, 75 Francis St, Boston, MA 02115.

Department of Vascular and Endovascular Surgery, Brigham and Women’s Hospital, Boston, Mass.

Received 29 January 2007; accepted 11 February 2007.

Article Outline

Surgical bypass of arterial occlusions using autogenous vein provides an effective treatment for many patients with advanced coronary or peripheral atherosclerosis. However, the long-term benefit of bypass surgery is limited by the development of de novo occlusive lesions within the vein graft, which occurs in a significant percentage of patients over time. The pathophysiology of vein graft failure involves a complex interplay between an acute vascular injury response and the hemodynamic adaptation of the vein to arterial forces. Cell proliferation, inflammation, and matrix metabolism are critical components of postimplantation remodeling. Conventional pharmacotherapy has had limited impact on graft failure. Vein grafts present a unique and attractive opportunity for molecular engineering, which is defined for purposes of this review as the local application of genomic (eg, gene transfer or gene inhibition) or proteomic interventions designed to alter the healing response. The critical enabling technologies for these strategies are described, with a perspective on preclinical and clinical development for this indication. The recently completed clinical trials of edifoligide (E2F decoy oligodeoxynucleotide) provide important lessons for future studies. A better understanding of the remodeling response of vein grafts in humans is required to design effective molecular therapies and to define the appropriate target populations and surrogate markers for future clinical trials.

 

Despite an increasing application of endovascular techniques, surgical bypass grafting remains a mainstay of therapy for many patients with advanced coronary and peripheral atherosclerosis. Autogenous vein has proven to be an effective and versatile arterial substitute. In the lower extremity, however, vein graft stenosis or occlusion occurs in 30% to 50% of cases ≤5 years, and >50% of coronary grafts fail ≤10 years, leading to significant morbidity and mortality. For many patients with advanced peripheral arterial disease (PAD), failure of a bypass graft may lead directly to major limb amputation and diminished quality of life. Limited available autogenous conduit, the lack of a suitable small caliber arterial prosthetic, and the frequency of coexistent coronary and peripheral disease combine to elevate the importance of maintaining patency for each individual vein graft.1, 2

To date, conventional pharmacology has produced limited benefits for vein graft patients. Recognition of the significant unmet clinical need in this area has spurred interest in developing novel molecular approaches, including genetic modification strategies, to prevent bypass graft failure. Some of these therapeutic agents have reached the stage of advanced clinical trials.

Advances in genomics, proteomics, and drug delivery technology offer an increasing array of tools for modulating cellular function in vivo. Vein grafts present an attractive target for local molecular therapy because the target tissue is directly accessible and may be treated ex vivo (or in situ) at the time of implantation. Furthermore, because the surgical procedure effectively denotes the initiation of the pathophysiologic events within the vein, there is potential to re-engineer the cellular programs that mediate the healing response at “time zero.” With that concept as an underlying premise, this review will outline some of the approaches that have been under investigation and the current status of translational research in this area.

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Vein graft failure: Pathophysiologic considerations 

In the lower extremity, where vein graft surveillance is simplified because of anatomic position, three distinct phases of graft failure are recognized.3 Early graft occlusion (ie, ≤30 days), which occurs in 5% to 10% of cases, is generally ascribed to technical complications but also includes problems intrinsic to the conduit (eg, small diameter or pre-existing vein pathology) as well as extrinsic causes (eg, limited outflow, hypercoagulability). Mid-term (3 to 24 month) and late (>2 years) vein graft failures are most commonly ascribed to the development of fibrotic intimal hyperplasia (IH) and atherosclerotic degeneration, respectively. Rates of reintervention for lower extremity grafts are highest in the mid-term period, focusing attention on IH as a critical process for therapeutic targeting.

The cell and molecular biology of IH have been best characterized in the context of acute arterial injury such as balloon angioplasty. However, parallels to arterial IH have been suggested by experimental models of vein grafting in animals, particularly with regard to smooth muscle cell (SMC) activation, migration, and proliferation. Currently available animal models, including vein graft techniques in murine, rabbit, pig, and nonhuman primate species, have varying strengths and weaknesses for basic and preclinical investigations.4 For the most part, these models provide insight into the arterialization response of veins but have limited utility for the study of graft failure. The adaptation of vein grafts to the arterial environment has been studied to a far lesser extent in humans.

After implantation, vein grafts universally undergo structural changes characterized by the formation of a proliferative neointima and overall wall thickening, resulting in a reduction in wall tension. The view of vein graft stenosis as an exaggerated form of this adaptation process remains to be proven and, by analogy to postangioplasty arterial remodeling, may be an oversimplification. A broad review of the pathophysiology of vein graft disease is beyond the scope of this article, and the reader is referred to several excellent reports on this subject.5, 6, 7 The Table summarizes the major cellular processes that may be considered for therapeutic intervention. This list is by no means meant to be exhaustive, but rather provides an overview of the relevant spectrum, with appropriate examples of molecular targets for potential vein graft therapies.

Table. Major pathophysiologic processes relevant to vein graft disease, examples of molecular targets, and temporal window of treatment for which a local intervention would be likely to have a beneficial effect
1st process2nd processesMolecular targetsTemporal Rx window
EC functionEC regenerationVEGFTime 0—weeks
EC dysfunctionNOS, HO-1, SODTime 0—months
SMC growthSMC proliferationCell cycle genes, growth factors2-3 days—weeks
SMC apoptosisIAPs, proapoptotics2-3 days—weeks
Cell migration TIMPs, PAsTime 0—weeks
InflammationI/R, oxidative stressCytoprotectants (NOS, SOD, HO-1)Time 0—weeks
Leukocyte recruitmentAdhesion molecules, chemokinesTime 0—weeks
ThrombosisPlatelet activationProstacyclin, ADPases,Time 0—long term
Thrombin activationThrombin inhibitors, PAsTime 0—long term
Matrix/fibrosis Matrix genesWeeks—months

EC, Endothelial cell; SMC, smooth muscle cell; VEGF, vascular endothelial growth factor; NOS, nitrix oxide synthase(s); HO-1, heme oxygenase-1; SOD, superoxide dismutase(s); IAP, inhibitor of apoptosis protein; TIMP, tissue inhibitor of metalloprotease(s); PAs, plasminogen activators; I/R, ischemia/reperfusion; ADPases, adenosine diphosphohydrolase(s).

Among the cellular processes outlined in the Table, SMC growth has received the greatest attention as the most critical determinant of vein graft IH. Although recent observations suggest that this focus may be inappropriately myopic, most would agree that an excessive SMC proliferative response within the vein is deleterious. Fueled by an increased understanding of the fundamental processes of cell growth (cell cycle machinery) and death (apoptosis and its regulation), investigators have explored an array of targeted molecular interventions to control SMC growth in vascular injury settings.

A variety of cell cycle inhibitors and proapoptotic strategies have been studied in vein graft models. The transcription factor E2F, a critical regulator of cell cycle progression that coordinates the activation of several genes, was considered an attractive therapeutic target (see edifoligide trials below).

As another example, our recent work has focused on the intersection of cell survival and proliferation pathways. The inhibitor of apoptosis protein (IAP) survivin is unique in having dual critical functions regulating mitosis and preventing apoptosis under conditions of cellular stress. These functions have stimulated intense interest in survivin as a potential target for cancer therapy, and we have recently demonstrated the relevance of survivin to vascular injury and vein grafting in particular.8, 9, 10, 11, 12 Local knockdown of survivin using a dominant negative gene, delivered by periadventitial application of an adenoviral vector, dramatically altered the healing response of rabbit vein grafts. SMC proliferation was reduced and apoptosis correspondingly increased at 1 week; at 4 weeks, IH was significantly attenuated (Fig 1).10 Further studies are needed to determine if such combined antiproliferative/proapoptotic strategies may have useful application to surgical bypass grafting.

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  • Fig 1. 

    Regulation of intimal hyperplasia in rabbit vein grafts by local modulation of the inhibitor of apoptosis protein (IAP) survivin. Rabbit jugular vein carotid interposition grafts were treated at implantation with adenoviral vectors encoding a dominant-negative form of survivin (AdT34), wild-type survivin (AdWT), or an irrelevant gene (AdGFP). Panel A, Representative histology (I, Intima; M, media, A, adventitia) at 4 weeks. Panel B, morphologic analysis at 4 weeks. A significant (41%) reduction in wall thickness was observed in AdT34 treated grafts, and a corresponding increase (37%) was observed in AdWT treated grafts that overexpressed survivin.10

In a review of the Table and the growing literature on which it is based, several key features emerge that will ultimately determine the clinical success of interventions designed to improve vein graft patency. First, there is great redundancy in the molecular pathways that contribute to IH, similar in many ways to the problems of wound healing and fibrosis in general. Thus, strategies limited to a single gene or pathway are inherently risky unless that target has overlapping influence across several processes or is uniquely potent in determining the overall response. In addition, each pathway has a unique temporal window for intervention, though considerable overlap exists. The pharmacokinetics of the molecular intervention must match the temporal course of the process being targeted within the vein.

Our knowledge of these events is limited almost exclusively to animal studies of vein graft adaptation and is greatly hampered by an incomplete understanding of the process in humans (see subsequent section). However, this daunting complexity is tempered by the realization that clinical outcome may be significantly improved by even a moderate shift in one or more of the biologic processes that participate in graft healing. The observation that many vein grafts function well beyond 10 years attests to the fact that an autogenous vein has the potential to serve as an excellent long-term arterial replacement. A re-engineering of the healing response to moderate, rather than obliterate, cell growth (for example) may potentially yield a meaningful increase in long-term graft survival.

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Local delivery of biomolecules: Critical technologies 

For the purpose of this review, we will focus on the use of locally delivered biomolecules—defined as genes, oligonucleotides, proteins, and peptides—for therapeutic manipulation of the vein graft injury response. These may be broadly characterized into “genomic” (strategies designed to influence gene expression) or “proteomic” (strategies designed to influence protein function) approaches and are distinguished from conventional pharmaceuticals both by the nature of the agent and by the local delivery of these agents to the target tissue (vein) of interest.

Genomic strategies 

Genomic strategies may take several forms.

Gene replacement or augmentation 

These approaches involve the delivery of an intact gene that is either missing, present in a defective form or simply under-expressed relative to the level of protein product desired. Genes encoding proteins that are secreted or that generate diffusible mediators are attractive because delivery to a subpopulation of cells may yield a therapeutic result. Examples of this category of genes with vascular applications include the nitric oxide synthase (NOS) isoforms, which yield a readily diffusible product, nitric oxide (NO), thrombin inhibitors, or plasminogen activators. Other genes of interest may act exclusively in an intracellular fashion, altering the phenotype of the transfected cell for potential therapeutic purposes. Achieving gene transfer to most of the cells within a vein graft is a significant hurdle, however, particularly within the temporal constraints of an intraoperative protocol.

Genes are large, double-stranded DNA molecules that are inefficiently taken up by cells, and therefore, a specialized delivery system (“vector”) is required. Gene delivery vectors may be divided into two categories: viral and nonviral. For a more complete review of gene transfer vectors, the reader is referred to a number of excellent treatments of this topic.13, 14, 15, 16

Current available gene transfer vectors vary substantially in critical clinical attributes such as efficiency, stability of transgene expression, and host response. The ideal vector for a vein graft therapy depends on the nature of the gene being delivered as well as the temporal and spatial requirements of the molecular program being targeted. For example, endothelial cell (EC) targeting to promote graft thromboresistance (eg, by overexpression of a thrombin inhibitor) would have different requirements compared with suppressing proliferation within medial SMCs. Minimization of local or systemic inflammatory reactions to the vector is obviously also critical.

At the present time, adenoviral vectors are most commonly used in experimental cardiovascular studies and provide a useful compromise.17, 18 Adenoviruses infect vascular cells readily and can yield high levels of transgene expression for a short time period (1 to 2 weeks in vivo). However, a dose-dependent inflammatory response to the widely available first-generation and second-generation adenoviral vectors is well described and has been shown to exacerbate intimal hyperplasia in some models of vascular injury.19 Other viral vectors, including adeno-associated20, 21 and lentiviral systems,22, 23 offer unique potential advantages, but to date, have been limited primarily by low efficiency in vascular models.

For intraoperative vein graft therapy, safety and efficiency are paramount, and each unique gene-vector combination being considered will require careful characterization and optimization in appropriate preclinical models. These delivery obstacles have dampened enthusiasm for the clinical development of gene-based approaches, but the therapeutic potential of this strategy remains enormous.

Gene inhibition 

Another broad category of genetic manipulation is gene inhibition. A specific gene, or an entire cellular program (eg, cell cycle), may be inhibited using small nucleic acid molecules called oligonucleotides. These may function to block the translation of specific messenger RNAs, for example, antisense oligonucelotides, ribozymes, or small interfering RNAs (si)RNAs. They can also block the activity of regulatory proteins (transcription factors) that control gene expression.

Antisense oligodeoxynucleotides (AS-ODN) are designed to have a base sequence that is complementary to a segment of the target gene.24, 25 They are generally 15 to 20 bases in length, which confers specificity to the target messenger RNA (mRNA). This binding of ODN to mRNA either results in enzymatic degradation of the mRNA or prevents the translation of RNA into its protein product. A related form of gene blockade involves the use of ribozymes, segments of RNA that can act like enzymes to destroy specific sequences of target mRNA.26

Yet another type of gene inhibition involves the antagonism of transcription factors. Transcription factors regulate gene expression by binding to chromosomal DNA at specific promoter regions, and this binding turns on, or activates, an adjacent gene. Transcription factor decoys are double-stranded ODN designed to mimic the genomic binding sites of the target transcription factors. Once delivered to a cell, the decoy ODN binds to the available transcription factor, competitively inhibiting the transcription factor–promoter interaction and thereby preventing the subsequent activation of target genes.27

Recently, a powerful new approach to inhibit specific gene expression has been elucidated based on the intracellular effects of double-stranded (ds) RNA molecules.28 This phenomenon, termed RNA interference is mediated by short (21 to 23 nucleotide) dsRNA constructs known as siRNAs. Intracellular processing of siRNAs culminates in their incorporation into a multiprotein complex, called the RNA-induced silencing complex (RISC), which may then recognize and cleave specific mRNA molecules that have homology to the siRNA. This mechanism yields effective and specific silencing of the targeted gene, usually in a transient fashion.

Delivery of siRNAs to cells and tissues may be accomplished in a variety of ways, with interest increasing in the use of lentiviral vectors to achieve longer-term suppression of target genes by sustained intracellular production of the siRNA molecules. To date there have been limited studies examining the application of siRNA to blood vessels in vivo, and further investigation is needed to determine the clinical applicability of this approach.

In general, a major attraction of these gene inhibition strategies is that small synthetic ODN (typically 1/1000 the size of an entire gene) may be delivered more easily to cells and tissues with high efficiency and often do not require specific vectors. For example, the use of nondistending pressure has been shown to result in the rapid uptake of ODN by >80% of cells within the saphenous vein wall during a 10-minute exposure.29 Delivery efficiency and stability of target gene knockdown will be different for single-stranded AS-ODN vs double-stranded decoy-ODN, however, and siRNA molecules constitute an entirely distinct class in their pharmacologic attributes. Further studies are needed to determine if the exciting potential of these small molecule genetic approaches can be realized for complex in vivo applications such as vein bypass grafting or postangioplasty restenosis.

Proteomic strategies 

Proteomic strategies seek to directly modify the profile of functional protein species (proteome) within the target tissue to achieve a desirable effect. Examples would include the directed delivery of biologically active proteins or peptides, or the modification of post-translational processing (eg, glycosylation) steps that are critical for protein function. Although pharmacologic obstacles for systemic (eg, oral or parenteral) delivery of proteins and peptides are well known and substantial, the development of critical platform technologies for local delivery of these agents has accelerated. In particular, the discovery of cell permeability domains, also called peptide/protein transduction domains (PTDs), from the human immunodeficiency virus (HIV-1) TAT protein30, 31 and other sources (eg, antennapedia homeodomain protein,32 VP22 protein from herpes simplex virus)33 has allowed for the creation of fusion constructs capable of efficiently delivering peptides and small proteins directly into cells.34 In parallel, the development of high throughput techniques (eg, antibody-based microarrays, mass spectrometry) to profile proteins and peptides from normal and diseased tissues offers the potential to characterize complex diseases at the proteomic level. This field is in its infancy but can be expected to yield new insights and therapeutic opportunities for cardiovascular disorders including IH and vein graft disease.

Local delivery platform technologies 

Local delivery platform technologies have revolutionized the field of drug delivery during the last decade. The development of novel polymers and matrices for the modification of biomaterials (eg, intravascular stents) has been dramatic and offers the potential for programmable delivery of a variety of molecules directly to target tissues such as native vessels and vascular grafts. This technology has relevance for both genomic and proteomic approaches to modify vascular responses, as well as creating new opportunities for conventional drugs (eg, rapamycin) that may have favorable local effects if their systemic toxicity can be minimized.

There are broad implications of these local delivery platforms for cardiovascular surgery. For example, the hemostatic or tissue adhesive properties of a polymer carrier may be leveraged for simultaneous delivery of a drug or biomolecule to the site of a surgical anastomosis. For vein graft applications, molecular therapy may take a variety of forms, such as a preimplantation “soak” of a cell-permeable peptide construct, or periadventitial application of a polymer depot containing a protein, peptide, or genetic agent. The increased recognition of the role of the adventitia in modulating vein graft IH as led several investigators to use periadventitial gene delivery in animal models, with promising results. Several biotechnology companies are exploring approaches to modulate the hyperplastic response at vascular anastomoses using locally delivered biologic agents.

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Bench to bedside: The edifoligide story 

The principles described above were used to develop an ODN-decoy approach targeting SMC proliferation that was tested in animal models of arterial injury and vein bypass grafting.35, 36, 37 A double-stranded oligodeoxynucleotide (14 base-pairs) was designed to incorporate the binding site for the transcription factor E2F, which controls the expression of multiple genes that are responsible for cell cycle progression. In a rabbit model, vein grafts treated with the E2F decoy (edifoligide) in solution at the time of implantation demonstrated a marked reduction in intimal hyperplasia and resistance to graft atherosclerosis for up to 6 months. These exciting data, combined with the ease of delivery (10-minute exposure of the vein to edifoligide in solution) and presumed safety of this ex vivo approach, led rapidly to a clinical development program.

The E2F decoy strategy for preventing vein graft failure has now been examined in a series of clinical trials known as the Project of Ex Vivo Vein Graft Engineering via Transfection (PREVENT). PREVENT I was a single-institution pilot study in 41 patients undergoing lower extremity vein bypass.38 Intraoperatively, the veins were harvested, mounted on a cannula, and inserted into a device for pressure-mediated transfection with ODN (Fig 2). This small study demonstrated safety and feasibility, and suggested the possibility of biologic efficacy.

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  • Fig 2. 

    Device used for intraoperative, pressure-mediated transfection of vein grafts with ODN (edifoligide) in the PREVENT clinical trials. Top panel, Assembled device with vial for administering test article is shown. Bottom panel, Intraoperative photograph demonstrates treatment of a vein graft before implantation in PREVENT III.46

A corporate-sponsored (Corgentech, Inc, Palo Alto, Calif) phase II trial (PREVENT II) in patients undergoing coronary artery bypass graft surgery was completed in Germany.39 A total of 200 patients were randomized to treatment with E2F decoy or saline control. Follow-up included both clinical events and imaging (angiography and intravascular ultrasound) at 1 year. As in the PREVENT-I trial, no adverse events or complications were attributable to decoy ODN treatment. The angiographic analysis revealed a 30% relative reduction in critical stenosis (≥75%, P = .03). Analysis of intravascular ultrasound images revealed a statistically significant reduction in total wall volume (30%), suggesting a positive influence on remodeling throughout the lengths of the treated vessels. These studies led to United States Food and Drug Administration approval of a phase III trial for edifoligide.

Edifoligide was investigated in two parallel phase III trials involving lower extremity (PREVENT III) and coronary (PREVENT IV) bypass patients (cosponsored by Corgentech and Bristol Myers Squibb). Both studies were multicenter, randomized, double-blinded, and placebo-controlled. In PREVENT III,40 1404 patients requiring autogenous vein bypass for critical limb ischemia (CLI) were randomized to either E2F decoy or saline delivered with the aforementioned graft transfection apparatus. The study involved vascular surgeons from 83 sites across North America and was powered to detect a 30% reduction in the primary end point of graft failure at 1 year.

The PREVENT III trial design41 was broadly inclusive and therefore serves as an accurate representation of the current population of CLI patients undergoing revascularization procedures. Diabetes was present in 64% of the patients, 48% had a history of advanced coronary artery disease, 12% were on dialysis, and 28% had previously undergone an infrainguinal bypass procedure. Tissue loss was the presenting symptom in 75% of patients. High-risk conduits were used in 24%, including alternative vein in 20% (15% spliced, 5% nongreat saphenous vein) and small caliber (<3 mm diameter) grafts in 6%. Most grafts (65%) were placed to infrapopliteal targets.

Perioperative (30-day) mortality occurred in 2.7% of patients in PREVENT III. Major morbidity included myocardial infarction in 4.7% patients and early graft occlusion in 4.3%. Ex vivo treatment with edifoligide was well tolerated. Unfortunately, there was no significant difference between the treatment groups in the primary or secondary trial end points, primary graft patency, or limb salvage. For the overall cohort at 1 year, survival was 84%, primary patency was 60%, primary-assisted patency was 77%, secondary patency was 79%, and limb salvage was 88%. Of interest was that a significant improvement was observed in secondary graft patency at 1 year (81% edifoligide, 76% placebo, P = .0299).

The PREVENT IV investigators randomized 3014 patients at 107 sites.42 The primary end point was the incidence of critical graft stenosis (>75%) by coronary angiography at 12 months, which was performed in 2400 patients (80%). Paralleling the results of PREVENT III, these investigators found no difference in the occurrence of the primary trial end point between treatment groups. Other secondary end points, including the incidence of total graft occlusion, minimal graft lumen diameter, and major adverse cardiac events, were also not different with respect to edifoligide treatment.

The primary results of PREVENT III and IV demonstrate conclusively that a single, ex vivo treatment of vein grafts with edifoligide did not confer protection from graft failure. Although none of the prespecified study end points were met, a significant improvement in secondary patency was observed in PREVENT III. The risk reduction observed (4.7% absolute, 19.7% relative reduction in secondary patency events) was modest but not clinically irrelevant and appears to suggest some beneficial biologic effect that merits further inspection. Ongoing analyses from the PREVENT III database, including subgroup event rates and ultrasound findings, seek to better define the potential source of this secondary patency benefit.

Ultimately the reasons underlying the failure of edifoligide to meet the prespecified PREVENT trial end points remain unknown. Lacking imaging tools to noninvasively quantify IH within the grafts in these trials, it is unclear if the treatment had its intended effect on wall thickness and what role, if any, was played by remodeling.

Is SMC proliferation the correct target, and if so, was it adequately suppressed by the study drug administered in this fashion? Is the contribution of graft-extrinsic cells, as suggested by recent small-animal studies,43 an under-appreciated mechanism of graft IH? There are a number of fundamental questions and plausible explanations that cannot be addressed by the available data from these trials, indicating a critical need for more mechanistic clinical research on vein graft failure.

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Bedside to bench: Studying vein graft adaptation in humans 

Vein graft failure is a complex clinical entity with components derived from technical factors, patient-specific variables, hemodynamic influences, and biologic responses within the vein. Moreover, what accounts for the variability in outcome for a good quality saphenous vein conduit between two different patients and for the variability in lesion formation along the course of a single conduit remains largely unknown. Lessons from postangioplasty restenosis in arteries have led to an appreciation of vascular remodeling, in addition to IH, as a critical determinant of vessel patency. Little is known about the spectrum of remodeling of vein bypass grafts in humans, and the recent results of the PREVENT trials demonstrate our limited understanding of the process as well as the lack of adequate imaging tools or surrogate markers to monitor the effects of a molecular intervention on the healing response. The design of an effective intervention will depend on a more complete understanding of the nature of normal vs pathologic vein graft remodeling in patients.

Recently, we have undertaken a prospective investigation exploring the relationship between systemic inflammation and vein graft remodeling in patients undergoing lower extremity vein bypass. The goals of this project are:

1.to determine if increased inflammation, as measured by a panel of circulating markers, correlates with graft failure and clinical events;

2.to quantify the structural and biomechanical changes in arterialized veins using a combination of high resolution ultrasound and magnetic resonance imaging;

3.to determine if there is a relationship between systemic inflammation and unfavorable graft remodeling, and

4.to determine if a specific molecular signature can be defined as a surrogate marker (or predictor) for vein graft disease.

These ongoing studies have yielded some important new insights. First, we have defined distinct temporal phases of vein graft remodeling in the leg, with an early (first month) period of outward remodeling, followed by a more delayed process of increased wall stiffness during the first 6 months (Fig 3).44 The early outward remodeling of functioning vein bypass grafts is substantial (21% mean increase in lumen diameter), correlates with initial shear stress, and appears associated with subsequent clinical outcome. These preliminary results suggest that early outward remodeling of the vein is a critical determinant and that its mechanism requires further investigation. We hypothesize that variability in endothelial function in the early postimplantation period is a primary factor in this response.

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  • Fig 3. 

    Changes in lumen diameter and wall thickness in lower extremity vein grafts over time, assessed by ultrasound studies.44 A, Data demonstrate serial lumen diameter measurement of a defined region (index segment) of vein grafts in 90 subjects. B, Data show changes in wall stiffness assessed by graft pulse wave velocity in 83 subjects. Results are presented as mean ± standard deviation.

We have also demonstrated that systemic inflammation, as measured by high-sensitivity C-reactive protein assay, correlates with clinical outcomes after lower extremity vein bypass (Fig 4).45 Patients with elevated high-sensitivity C-reactive protein (>5 mg/L) had a 2.3-fold increased risk for adverse events after bypass surgery over a mean follow-up of 342 days, most of which were vein graft–related. This suggests that variability in the host inflammatory milieu may correlate with healing responses within the graft. Continuing studies seek to define a relationship between inflammatory markers and patterns of graft remodeling over time.

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  • Fig 4. 

    Relationship of preoperative high sensitivity C-reactive protein (CRP) level to adverse events during follow-up in 91 patients undergoing lower extremity vein bypass surgery.45

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Conclusions and future directions 

Armed with an expanding technology for implementing specific genomic and proteomic interventions, the era of in vivo tissue engineering has arrived. While the search for suitable small arterial substitutes continues, it is clear that an autogenous vein is the best substrate for designing a durable arterial replacement. Although it is intuitive that excessive SMC proliferation within the vein graft can lead to an occlusive lesion, other factors such as remodeling, endothelial function, and inflammatory cell recruitment may also play a crucial role and would define a new set of molecular targets and approaches. Translational research in vein graft disease thus needs to maintain focus in several key areas:

1.continued investigations of the cellular and molecular pathways of vein graft healing in suitable animal models;

2.better definition of patient-specific risk factors and genetic determinants;

3.quantitative characterization of the spectrum of vein graft remodeling in humans and the relationship between remodeling and clinical failure;

4.development of imaging tools and circulating markers to serve as surrogate end points in future clinical trials; and

5.optimization of local approaches for the delivery of potent biomolecules to the graft in a safe and efficient fashion.

The PREVENT studies have demonstrated conclusively that a molecular intervention can be applied to vein grafts in an intraoperative setting, and appropriately tested in well executed multi-center trials. Informed by the design and outcome of these studies, future translational efforts will have a greater likelihood to improve outcomes for patients requiring vein bypass surgery.

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 This work has also been supported by funds from the Public Health Service (HL75771) and the Department of Surgery, Brigham and Women’s Hospital. The PREVENT trials were sponsored by Corgentech Inc (currently Anesiva, Inc, South San Francisco, Calif) and Bristol-Myers Squibb (Princeton, NJ).Competition of interest: Dr Conte has served as a paid consultant to Corgentech, Inc, and Bristol-Myers Squibb.

PII: S0741-5214(07)00317-5

doi:10.1016/j.jvs.2007.02.031

Journal of Vascular Surgery
Volume 45, Issue 6, Supplement , Pages A74-A81, June 2007

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