DESIGN
Reassess the compounds
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Drug-eluting stents (DES) have dramatically reduced restenosis rates following percutaneous coronary interventions (PCI) and now represent the treatment of choice in many cases. However, there have been a number of reports of late-stent thrombosis with the use of DES. In many cases, a cessation of anti-platelet therapy has been associated with the event. Because this therapy is designed to prevent thrombus formation whilst the endothelium regrows, the association suggests that the endothelium has not regenerated sufficiently or is dysfunctional. This calls for an examination of what is known about the potential effects of the compounds used in existing DES on endothelial cell regrowth and function. This assessment will provide essential information for the design of the next generation of stents.
Endothelium
The endothelium is a monolayer of endothelial cells lining every artery in the body (Figure 1). It is responsible for maintaining vascular homeostasis and performs this role in various ways. Nitric oxide (NO) and prostacyclin (PGI2) are released by endothelial cells in response to shear stress at the arterial wall and cause relaxation of the underlying smooth muscle. They are also released into the blood to prevent platelet adhesion and are vital in preventing thrombosis within arteries.
The endothelium can also mediate inflammatory responses by altering adhesion molecule expression, antigen presentation and vascular permeability. Finally, it is responsible for controlling the angiogenesis of the vasculature by release of growth factors such as vascular endothelial growth factor (VEGF). The availability of NO is crucial in endothelial function, and endothelial dysfunction in cardiovascular disease states is often viewed as a reduced bioavailability of NO.
In PCI, the processes of balloon expansion and stent insertion remove the endothelial cell layer. Over time, the regeneration of the endothelium takes place by migration and proliferation of endothelial cells from undamaged endothelium on either side of the injury site. Recently, a second regeneration mechanism has been suggested whereby endothelial progenitor cells circulating in the blood can adhere to the damaged arterial wall (Figure 2). In either case, the regeneration of a functional endothelium is crucial for securing long term successful outcomes of DES.
Effect on regeneration and function
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The two leading DES used clinically, Cypher (Cordis Corp., www.cordis.com [7]) and Taxus (Boston Scientific Corp., www.bostonscientific.com [8]), use the anti-proliferative drugs sirolimus and paclitaxel, respectively. Although this approach reduces neointima formation by inhibiting smooth muscle cell proliferation and migration, there is evidence that endothelial cell regeneration may also be impaired. In vitro, both drugs are known to inhibit human endothelial cell proliferation at the concentrations required to inhibit human smooth muscle cell proliferation.1 Significantly, paclitaxel was found to inhibit endothelial cell migration, which is an important process in endothelial regeneration. Similarly, paclitaxel has been shown to impair endothelial cell proliferation on a coated stent in vitro.2 There is evidence that exposure to paclitaxel can impair endothelial cell function. In an in vitro study, pig coronary arteries preincubated in paclitaxel, produced impaired relaxation responses to the endothelium dependent relaxant calcimycin.3 Of more relevance clinically, are studies revealing that patients with an implanted cypher stent exhibit coronary artery constriction in response to exercise4 andacetylcholine.5 Finally, examination of 40 patient autopsies has revealed incomplete healing and reduced endothelialisation in DES versus bare-metal stents.6
Future trends
Because of the concerns outlined above, there is a need to examine other potentially therapeutic targets in the development of DES. A number of sirolimus analogues are at different stages of development and trials and it is not yet clear that this approach will represent a significant advance on existing treatments. Therefore, much attention is now focused on the development of strategies that inhibit neointima formation, while promoting endothelial cell recovery. Because the endothelium is crucial in modulating the response to injury and restoring normal arterial function in the long term, strategies designed to accelerate the regeneration of the endothelium may lead to improved outcomes. There are a number of approaches under development and these are discussed below.
Endogenous compounds
Endothelial derived substances such as prostacyclin and NO are known to inhibit platelet aggregation, smooth muscle cell proliferation and local inflammatory responses, and have known protective effects on cells. Therefore, they may act to protect the vessel against injury and promote regeneration of a functional endothelium. Because these compounds are produced naturally by the endothelium, they have potentially less deleterious side effects than existing approaches. Local stent-based release of the prostacyclin analogue, iloprost, in combination with the thrombin inhibitor hirudin, reduced neointima formation in pig and sheep coronary arteries.7 Stents coated with the NO donor, sodium nitroprusside, have had mixed results to date. One study failed to demonstrate a reduction in neointima in a pig coronary artery model.8 However, recently a reduction in neointima was achieved in a pig carotid artery model.9
Antioxidants
There is growing evidence that oxidative stress and the subsequent imbalance in the production of damaging reactive oxygen species play a significant role in the response to stent insertion, and ultimately in the process of neointima development. By reducing oxidative stress, antioxidants can protect the endothelium against dysfunction and encourage endothelialisation. A number of trials with oral delivery of antioxidants such as probucol and its analogue AGI-1067 have demonstrated beneficial effects in reducing constrictive remodelling of the vasculature following PCI.10 A phosphorylcholine coated stent releasing the antioxidant, carvedilol, reduced neointima formation by 42% in a pig model of in-stent restenosis.11
Oestradiol eluting stents
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Oestrogen is well known for its protective effects on the cardiovascular system. In pigs it inhibits smooth muscle cell proliferation and migration. However, it may represent a significant advance on existing DES because it also stimulates endothelial cell proliferation and migration.12 This prospect has led to the development of a phosphorylcholine-coated stent providing release of 17-beta-oestradiol. Clinical trials of this technology have demonstrated the safety of this device with no deaths or thromboses at six months.13 However, in a subsequent study, no beneficial effect on angio-graphic restenosis at six months was observed, compared with a phosphoryl-choline coated stent alone.14
Vascular endothelial growth factor
VEGF is released by endothelial cells to promote angiogenesis of the vasculature. Although its promotion of endothelial cell repair makes it an attractive option, it is also associated with increases in smooth muscle cell proliferation and migration. This potentially nonselective stimulation of angiogenesis may represent a drawback. In one clinical study, local catheter based delivery of a gene, which promotes the synthesis of VEGF, failed to demonstrate a reduction in restenosis,15 although the low overall restenosis rates in each arm of this study made differences difficult to observe. However, a phosphoryl-choline-coated stent releasing a VEGF gene has produced promising results in a rabbit iliac artery model. An increase in the rate of endothelialisation was observed in addition to an increase in the lumen diameter.16
Endothelial cell seeding
Seeding endothelial cells onto vascular prostheses is appealing because these cells would then be able to release various substances to limit neointima formation. Progress in this area has been limited to date because of the time required to grow a sufficient number of cells and the limited adherence of these cells to the artery surface during dynamic flow conditions. Endothelial cells have been grown within polymer matrices, which can be subsequently implanted to encourage vascular repair following injury.17 Surface coating with plasma protein such as fibronectin, which contains specific endothelial cell binding sites, is known to increase the rate of endothelial cell growth on artificial surfaces.18 More recently, neointima formation in rabbit arteries was reduced by the injection of fibrin glue that contained thrombin and endothelial cells into the artery immediately following angioplasty.19 However, significant obstacles remain such as the source and seeding time of endothelial cells before application in a clinical setting can be considered.
Getting endothelial progenitor cells
The discovery of endothelial progenitor cells and their potential role in re-endothelialisation following injury may provide an opportunity for the next generation DES. By releasing a specific peptide, which binds endothelial progenitor cells, from a polymer coated stent it has been shown that endothelialisation can be significantly improved. A consequent reduction in the neointimal area was also observed.20 A similar approach has been evaluated clinically, which has demonstrated the potential of this “prohealing” approach.21 In this study, a covalently coupled polysaccharide coating has been used in combination with a CD34 antibody for endothelial progenitor cell capture. No thrombosis was reported at six months despite cessation of clopidogrel anti-platelet therapy after one month, although no reduction in neointima formation was observed. Further improvements to this stent system are underway, including increasing the stability of the antibodies by producing the coating in a dry form, which removes the need for rinsing prior to use. Similarly, the bioactivity of the antibody coating has been enhanced by using a lower level of gamma irradiation (<15 kGy) in the sterilisation process. Taken together, these modifications have improved endothelial progenitor cell capture rate in vitro and may enhance the performance of the stent in subsequent trials.
Time for novel approaches
Existing drug-eluting stents have been successful in reducing restenosis rates by targeting smooth muscle cell proliferation and migration. As such, these devices have represented a significant advance in the treatment of coronary heart disease patients. However, the anti-proliferative approach is only one of many potential strategies that may be used to overcome restenosis. Several novel therapeutic targets and coating technologies have been identified as having the potential to inhibit restenosis and to promote regeneration of the endothelium. The present concern regarding late-stent thrombosis with today’s DES serves both as a rationale and catalyst for the development of these novel approaches.
References
1. T.J. Parry et al., “Drug-Eluting Stents: Sirolimus and Paclitaxel Differentially Affect Cultured Cells and Injured Arteries,” Eur.J.Pharmacol., 524,19–29 (2005).
2. C.K. Prasad et al., “Survival of Endothelial Cells In Vitro on Paclitaxel-Loaded Coronary Stents,” J.Biomater.Appl.,19,271–86 (2005).
3. S. Kennedy et al., “Effect of Antiproliferative Agents on Vascular Function in Normal and In Vitro Balloon-Injured Porcine Coronary Arteries,” Eur.J.Pharmacol.,481,101–7 (2003).
4. M. Togni et al., “Sirolimus-Eluting Stents Associated with Paradoxic Coronary Vasco-constriction,” J.Am.Coll.Cardiol.,46,231–6 (2005).
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6. M. Joner et al., “Pathology of Drug-Eluting Stents in Humans: Delayed Healing and Late Thrombotic Risk,” J.Am.Coll.Cardiol.,48,193–202 (2006).
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8. J.H. Yoon et al., “Local Delivery of Nitric Oxide from an Eluting Stent to Inhibit Neointimal Thickening in a Porcine Coronary Injury Model,” Yonsei Med.J.,43,242–51 (2002).
9. D. Hou et al., “Stent-Based Nitric Oxide Delivery Reducing Neointimal Proliferation in a Porcine Carotid Overstretch Injury Model,” Cardiovasc.Intervent.Radiol.,28,60–5 (2005).
10. J.C. Tardif et al., “Prevention of Restenosis with Antioxidants: Mechanisms and Implications,” Am.J.Cardiovasc.Drugs,2,323–34 (2002).
11. W. Kim et al., “Effect of Anti-Oxidant (Carvedilol and Probucol) Loaded Stents in a Porcine Coronary Restenosis Model,” Circ.J.,69,101–6 (2005).
12. P. Geraldes et al., “Specific Contribution of Estrogen Receptors on Mitogen-Activated Protein Kinase Pathways and Vascular Cell Activation,” Circ.Res.,93,399–405 (2003).
13. A. Abizaid et al., “First Human Experience with the 17-Beta-Estradiol-Eluting Stent: The Estrogen and Stents To Eliminate Restenosis (EASTER) Trial,” J.Am.Coll.Cardiol.,43,1118–21 (2004).
14. F. Airoldi et al., “17-Beta-Estradiol Eluting Stent Versus Phosphorylcholine-Coated Stent for the Treatment of Native Coronary Artery Disease,” Am. J. of Cardiol.,96,664–7 (2005).
15. M. Hedman et al., “Safety and Feasibility of Catheter-Based Local Intracoronary Vascular Endothelial Growth Factor Gene Transfer in the Prevention of Postangioplasty,” Circulation,107,2677–83 (2003).
16. D.H. Walter et al., “Local Gene Transfer of phVEGF-2 Plasmid by Gene-Eluting Stents: An Alternative Strategy for Inhibition of Restenosis,” Circulation,110,36–45 (2004).
17. H.M. Nugent, E.R. Edelman, “Endothelial Implants Provide Long-Term Control of Vascular Repair in a Porcine Model of Arterial Injury,” J. Surg. Res.,99,228–34 (2001).
18. N. Kipshidze et al., “Role of the Endothelium in Modulating Neointimal Formation: Vasculoprotective Approaches to Attenuate Restenosis after Percutaneous Coronary Interventions,” J.Am.Coll.Cardiol.,44,733–9 (2004).
19. N. Kipshidze et al., “Endoluminal Reconstruction of the Arterial Wall with Endothelial Cell/Glue Matrix Reduces Restenosis in an Atherosclerotic Rabbit,” J.Am.Coll.Cardiol.,36,1396–403 (2000).
20. R. Blindt et al., “A Novel Drug-Eluting Stent Coated with an Integrin-Binding Cyclic Arg-Gly-Asp Peptide Inhibits Neointimal Hyperplasia by Recruiting Endothelial Progenitor Cells,” J.Am.Coll.Cardiol.,47,1786–95 (2006).
21. J. Aoki et al., “Endothelial Progenitor Cell Capture by Stents Coated with Antibody Against CD34: the HEALING-FIM (Healthy Endothelial Accelerated Lining Inhibits Neointimal Growth-First In Man) Registry,” J.Am.Coll.Cardiol.,45,1574–9 (2005).
Chris McCormick is a final year doctoral student working jointly between the Strathclyde Institute of Pharmacy and Biomedical Sciences and the Strathclyde Institute of Medical Devices, University of Strathclyde, The John Arbuthnott Building, 27 Taylor Street, Glasgow G4 0NR, Scotland, UK, tel. +44 141 548 4110, e-mail: christopher.mccormick@strath.ac.uk [10]