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Summary

  1. Top of page
  2. Summary
  3. Review Criteria
  4. Message for the Clinic
  5. The pathogenesis of atherosclerosis
  6. Role of adhesion molecules in the pathogenesis of atherosclerosis
  7. The evidence supporting the role of VCAM-1 in the pathogenesis of atherosclerosis
  8. Succinobucol: an effective treatment for atherosclerosis?
  9. Conclusions
  10. References

Atherosclerosis is now well recognised as a chronic inflammatory process which may ultimately lead to myocardial infarction, stroke and peripheral vascular disease. The role of inflammation in the pathogenesis of atherosclerosis has lead to interest in developing therapies that target vascular inflammation. Leucocytes play a key role during atherosclerotic plaque development. Activated vascular endothelium expresses vascular cell adhesion cell molecule-1 (VCAM-1), a member of the adhesion molecule superfamily, to which monocytes and lymphocytes can bind. These inflammatory cells can then move through the endothelium by diapedesis and release cytokines and enzymes, important components in the progression of the lesion. Researchers have demonstrated that the extent of atherosclerotic lesions is significantly reduced in animal models with decreased VCAM-1 expression. VCAM-1 has therefore been identified as a potential anti-inflammatory therapeutic target, the hypothesis being that reduced expression of VCAM-1 will slow the development of atherosclerosis. Succinobucol (AGI-1067), an anti-oxidant compound also capable of inhibiting VCAM-1 gene expression, is an example of such an agent and is currently being investigated in a phase III cardiovascular end-point trial due to report in 2007. If the results are positive, further investigations should derive to what extent blockade of VCAM-1 by succinobucol, rather than its other effects, accounts for the reduction in vascular events.


Review Criteria

  1. Top of page
  2. Summary
  3. Review Criteria
  4. Message for the Clinic
  5. The pathogenesis of atherosclerosis
  6. Role of adhesion molecules in the pathogenesis of atherosclerosis
  7. The evidence supporting the role of VCAM-1 in the pathogenesis of atherosclerosis
  8. Succinobucol: an effective treatment for atherosclerosis?
  9. Conclusions
  10. References
  • This is not an exhaustive review but highlights key VCAM-1 research papers.
  • Medline and Pubmed searched for terms: VCAM-1, adhesion molecules, atherosclerosis, inflammation and succinobucol (combinations used and key papers selected).

Message for the Clinic

  1. Top of page
  2. Summary
  3. Review Criteria
  4. Message for the Clinic
  5. The pathogenesis of atherosclerosis
  6. Role of adhesion molecules in the pathogenesis of atherosclerosis
  7. The evidence supporting the role of VCAM-1 in the pathogenesis of atherosclerosis
  8. Succinobucol: an effective treatment for atherosclerosis?
  9. Conclusions
  10. References
  • Blocking the expression of VCAM-1 may represent a novel modality for the treatment of atherosclerosis.
  • This review is in advance of upcoming publication of Aggressive Reduction of Inflammation Stops Events (ARISE), a study of the impact of succinobucol (an agent which blocks VCAM-1 expression and has anti-oxidant effects) on clinical end-points.

The pathogenesis of atherosclerosis

  1. Top of page
  2. Summary
  3. Review Criteria
  4. Message for the Clinic
  5. The pathogenesis of atherosclerosis
  6. Role of adhesion molecules in the pathogenesis of atherosclerosis
  7. The evidence supporting the role of VCAM-1 in the pathogenesis of atherosclerosis
  8. Succinobucol: an effective treatment for atherosclerosis?
  9. Conclusions
  10. References

Under physiological conditions, the vascular endothelium separates the blood contents from outer layers of the arterial wall. Production of nitric oxide (NO) by endothelial cells counteracts pro-atherogenic factors. NO has numerous anti-atherogenic effects including vasodilation, inhibited leucocyte adhesion and platelet aggregation, antithrombotic and anti-inflammatory effects. Under suboptimal conditions such as hypertension, dyslipidaemia and smoking, however, endothelial dysfunction (i.e. diminished production or availability of NO) may occur, allowing the development of a pro-inflammatory state and atherosclerosis. Inflammation contributes to both the development of atheroma itself and, in susceptible individuals, also to eventual plaque rupture (1).

The development of an atherosclerotic plaque is a multistep process (see Figure 1A). Initially, endothelial dysfunction leads to a change in the normal homeostatic responses of the endothelium. Low-density lipoprotein (LDL) cholesterol accumulates in the artery wall and smooth muscle cell proliferation occurs. The LDL cholesterol is progressively oxidised. The next step involves the recruitment of leucocytes (monocytes and lymphocytes) to the region, transendothelial movement (diapedesis) into the developing lesion, and the formation of lipid-laden macrophages (foam cells). The accumulating macrophages also release cytokines and enzymes, including matrix metaloproteinases (MMP) responsible for degradation of the connective tissue matrix (see Figure 1B). The developing plaque, covered by a fibrous cap, gradually develops into an advanced and complex lesion. In some individuals, and for reasons not completely understood, ongoing inflammatory activity, particularly within the shoulder region of the plaque, gradually thins the fibrous cap creating an unstable plaque and potentially leading to the rupture of the plaque with thrombosis and clinical evidence of vascular occlusion.

image

Figure 1.  (A) The role of inflammation in the pathogenesis of atherosclerosis. Under conditions of endothelial dysfunction [such as exposure to excess low-density lipoprotein (LDL) cholesterol] a pro-inflammatory state develops. Vascular cell adhesion cell molecule-1 (VCAM-1), an adhesion molecule not expressed under physiological conditions, is expressed on the surface of the vascular endothelium. Recruited leucocytes (monocytes and lymphocytes) bind to the VCAM-1 and move between the endothelial cells, entering the developing atherosclerotic lesion and releasing their contents (cytokines and MMP). Macrophages derived from the monocytes internalise the oxidised LDL cholesterol, forming foam cells. Further inflammatory cells are attracted and the lesion slowly develops into a complex plaque. Blocking VCAM-1 expression may therefore interrupt this inflammatory process. NO, nitric oxide. (B) Cellular and molecular make-up of unstable plaques. The ongoing inflammatory process in the plaque leads to further accumulation of inflammatory cells, degradation of the extracellular matrix, apoptosis of the smooth muscle cells and thinning of the plaque's fibrous cap. Such a lesion is unstable and at risk of plaque rupture resulting in myocardial infarction, stroke and peripheral vascular disease. MMP, matrix metaloproteinases; SMC, smooth muscle cell.

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Transendothelial migration of inflammatory cells (leucocytes) is therefore a key early step in atherosclerosis and is mediated by the presence of cell adhesion molecules, including vascular cell adhesion molecule-1 (VCAM-1), on the vascular endothelium. As such, inhibiting adhesion molecule expression may be a novel therapeutic target.

Role of adhesion molecules in the pathogenesis of atherosclerosis

  1. Top of page
  2. Summary
  3. Review Criteria
  4. Message for the Clinic
  5. The pathogenesis of atherosclerosis
  6. Role of adhesion molecules in the pathogenesis of atherosclerosis
  7. The evidence supporting the role of VCAM-1 in the pathogenesis of atherosclerosis
  8. Succinobucol: an effective treatment for atherosclerosis?
  9. Conclusions
  10. References

Vascular cell adhesion molecule-1 is a member of the diverse cellular adhesion molecule family which includes integrins, selectins and the immunoglobulin superfamily (2) [e.g. VCAM-1 and intercellular adhesion molecules (ICAM)]. The immunoglobulin superfamily is crucial for the development of the embryo and for immune and inflammatory responses. These transmembrane glycoproteins, which are expressed on the cell surface, mediate cell interaction with, and adhesion to, other cells and the extracellular matrix. Structurally they are composed of a short cytoplasmic tail and an extracellular region containing immunoglobin-like repeats (3).

Multiple adhesion molecules play a role in leucocyte recruitment. The process of migration of a leucocyte through the vascular endothelium consists of the following steps: primary leucocyte–endothelium interaction (tethering and rolling); secondary leucocyte–endothelium interaction (firm adhesion) and transendothelial migration.

Selectins play a key role in the primary interaction between leucocyte and endothelium, namely tethering and rolling (4). Selectins include L-selectin expressed by leucocytes, E-selectin expressed by activated endothelial cells and P-selectin which is found on the cell surfaces of activated platelets and endothelial cells. The low-affinity nature of the interaction between selectins and target cell carbohydrate ligands allows leucocytes in the circulation to be tethered, to roll along the vascular endothelium and to be released if firm adhesion does not occur subsequently.

Firm leucocyte adhesion is dependent on the presence of activated leucocyte integrins which interact with adhesion molecules on the vascular endothelium (5). These include very late antigen-4 (VLA-4), an α4β1 integrin expressed on monocytes and lymphocytes but not neutrophils, which interacts with VCAM-1 on the activated vascular endothelium (6), and leucocyte function-associated antigen-1 which interacts with ICAM-1 and 2.

The flattened leucocyte adhering to the endothelium now enters the third stage, endothelial transmigration. Platelet-endothelial cell adhesion molecule-1, expressed by both endothelial cells and leucocytes, plays a vital role in the passage of the leucocyte between endothelial cells. The leucocytes appear to follow a chemotactic concentration gradient of cytokines across the endothelium. This gradient may be provided by the cytokines monocyte chemoattractant protein-1 (MCP-1) (7) and interleukin (IL)-8 (8) amongst others.

The evidence supporting the role of VCAM-1 in the pathogenesis of atherosclerosis

  1. Top of page
  2. Summary
  3. Review Criteria
  4. Message for the Clinic
  5. The pathogenesis of atherosclerosis
  6. Role of adhesion molecules in the pathogenesis of atherosclerosis
  7. The evidence supporting the role of VCAM-1 in the pathogenesis of atherosclerosis
  8. Succinobucol: an effective treatment for atherosclerosis?
  9. Conclusions
  10. References

During the early stages of the development of an atheroma, leucocytes are attracted to the site of inflammation. VCAM-1 is not routinely expressed under physiological conditions. However, under appropriate pro-inflammatory conditions where the endothelium is exposed to inflammatory cytokines such as tumour necrosis factor-α or IL-1β and becomes activated, gene expression is rapid leading to expression of VCAM-1 by the vascular endothelium (9). VCAM-1 is also expressed by smooth muscle cells in a developing atherosclerotic lesion (10) and in association with other inflammatory conditions. It is known to be expressed in fatty streaks, the earliest macroscopic atherogenic lesions. NO and l-arginine, the amino acid from which NO is derived, diminish the endothelial expression of VCAM-1 by cytokine-stimulated endothelium. VCAM-1 is eventually released from the endothelial cell surface by proteolytic cleavage allowing measurement of soluble VCAM-1. Thus far, however, circulating concentrations of soluble cellular adhesion molecules (CAM's), measured by enzyme-linked immunosorbent assay, have correlated poorly with other markers of endothelial activity (11) and, in addition, have been weakly associated with incident vascular events (12). The measurement of cellular expression of VCAM-1 is likely more indicative of true pathophysiology and has been achieved by immunohistochemistry.

Initial in vitro studies confirmed that VCAM-1 promoted firm adherence to α4β1 expressing leucocytes (13). Thereafter, it was demonstrated that cell adhesion to atherosclerotic lesions in the carotid arteries harvested from genetically modified mice [Apo E deficient (Apo E−/−)] could be greatly reduced by means of VLA-4 blocking antibodies (14). In vivo animal studies were limited by the finding that VCAM-1 knockout mice do not survive, apparently because α4β1: VCAM-1 interaction is necessary for fetal development. Studies designed to investigate atherogenesis in Apo E−/− and LDL receptor knockout mice (models of human hypercholesterolaemia) deficient in other adhesion molecules had already shown variable results but, with regards to VCAM-1, further progress was made when researchers developed mice with a D4D mutation. These mice lack one of the two VCAM-1 ligand binding sites but retain sufficient VCAM-1 expression, 2–8% of the wild type, for survival. When this genotype was crossed with the LDL receptor knockout genotype and the mice fed a pro-atherogenic diet, the extent of atherosclerotic lesions was greatly reduced (15). As such, mouse models of atherosclerosis have added support for a critical role of adhesion molecules in atherogenesis but extrapolation of the results to humans remains uncertain.

Observational studies have shown an association between endothelial expression of VCAM-1 by human cells and known cardiovascular risk factors such as hypercholesterolaemia (16) and long-term hyperglycaemia (17). Furthermore, VCAM-1 expression decreases during treatments with statins (18) and angiotensin II antagonists (19). Recently, the Canadian Anti-oxidant Restenosis Trials (CART) have tested the clinical effects of succinobucol (AGI-1067), a compound which, amongst other actions, inhibits VCAM-1 gene expression. The results have shown promise.

Succinobucol: an effective treatment for atherosclerosis?

  1. Top of page
  2. Summary
  3. Review Criteria
  4. Message for the Clinic
  5. The pathogenesis of atherosclerosis
  6. Role of adhesion molecules in the pathogenesis of atherosclerosis
  7. The evidence supporting the role of VCAM-1 in the pathogenesis of atherosclerosis
  8. Succinobucol: an effective treatment for atherosclerosis?
  9. Conclusions
  10. References

Various methods of treating atherosclerosis have been studied and may be loosely divided into three groups: conventional risk factors (statins, antihypertensives), anti-oxidants and anti-inflammatory treatments (see Table 1). Treatment of conventional risk factors has been moderately successful whilst treatment with pharmacological doses of naturally occurring anti-oxidant agents has shown no benefit.

Table 1.   Summary of conventional and novel methods to reduce vascular risk
MethodExamplesSuccess
  1. ARISE, Aggressive Reduction of Inflammation Stops Events; COX, cyclo-oxygenase; MMP, matrix metaloproteinases; VCAM-1: vascular cell adhesion molecule-1.

Conventional risk factorsStatins Antihypertensives Aspirin (antiplatelet)Proven reduced vascular risk Proven reduced vascular risk Proven reduced vascular risk
Naturally occurring anti-oxidants in high dosesVitamin E Vitamin C Folate β-CaroteneNot proven to reduce vascular risk; randomised control trials (all); and in some cases risk may even be enhanced
Anti-inflammatory agentsAspirin COX 2 inhibitors Cytokine suppression MMP inhibition Succinobucol (VCAM-1 expression blockade/anti-oxidative capacity)Proven reduced vascular risk Increase in cardiovascular events Prospective studies needed Prospective studies needed ARISE trial data awaited

Succinobucol is a metabolically stable analogue of probucol, a strong synthetic anti-oxidant which also lowers serum cholesterol. Probucol itself has yielded mixed results when studied. The Multivitamins and Probucol study demonstrated a reduction in coronary restenosis, following angioplasty, in participants on probucol monotherapy compared with placebo, from 38.9% to 20.7% (20). In the Probucol Quantitative Regression Swedish trial, however, probucol therapy for 3 years did not have a statistically significant impact on femoral atherosclerosis (21). This may be explained in part by the high-density lipoprotein (HDL)-cholesterol lowering effect of probucol (22,23). Long-term safety concerns were also raised given the effect of probucol in prolonging the QT interval (24). Succinobucol, a so-called vascular protectant drug, is a monosuccinic acid ester of probucol. Advantageously succinobucol appears to retain the anti-oxidant, anti-inflammatory and VCAM-1 gene blocking properties of probucol whilst having no effect on the QT interval. There is, however, a dose-dependent reduction in HDL on this therapy (23). Phase II trial data includes CART-1 and CART-2. In CART-1, treatment with succinobucol [280 mg daily from 2 weeks before until 4 weeks after percutaneous coronary intervention (PCI)] and probucol (500 mg daily from 2 weeks before until 4 weeks after PCI) showed a similar improvement in luminal dimensions at the site of intervention at 6 months, and a significant improvement compared with placebo (23). In CART-2, where participants were treated with succinobucol or placebo for 2 weeks before until 12 months after PCI, there was a significant reduction in plaque volume compared with baseline in the succinobucol treated arm only (25). However, this did not reach significance compared with placebo treatment at 12 months. Aggressive Reduction of Inflammation Stops Events (ARISE) is a phase III trial designed to investigate the impact of succinobucol on cardiovascular events in over 6000 participants with previous acute coronary syndrome. ARISE will assess the impact of treatment with succinobucol on rates of myocardial infarction, stroke, coronary revascularisation, unstable angina and death as a result of vascular disease, and also any additional benefit gained beyond treatment with current standard care for those with coronary heart disease. The effects of succinobucol on these vascular end-points will be reported together with safety data in March 2007 at the American College of Cardiology Scientific Sessions. More recently, CAM741, a derivative of a fungus-derived cyclopeptide which acts as a selective inhibitor of VCAM-1 synthesis in endothelial cells, has been identified and put forward as a potential therapeutic agent (26).

Apart from the blockade of VCAM-1 gene expression, succinobucol has other anti-inflammatory effects such as the inhibition of MCP-1 gene expression, and also strong anti-oxidant properties (27). Therefore even if ARISE demonstrates positive results, this will not necessarily confirm that blockade of VCAM-1 is cardioprotective as other effects of the drug may be more relevant. Further study using agents more specific to VCAM-1 expression (e.g. CAM741) will be required to confirm a specific benefit of VCAM-1 blockade. Nevertheless, a positive result in ARISE will stimulate considerable further interest in this area and promote relevant research.

Conclusions

  1. Top of page
  2. Summary
  3. Review Criteria
  4. Message for the Clinic
  5. The pathogenesis of atherosclerosis
  6. Role of adhesion molecules in the pathogenesis of atherosclerosis
  7. The evidence supporting the role of VCAM-1 in the pathogenesis of atherosclerosis
  8. Succinobucol: an effective treatment for atherosclerosis?
  9. Conclusions
  10. References

Adhesion molecules appear to be crucial role players in the development of atherosclerosis, allowing leucocytes to adhere to the vascular endothelium. Phase I and II trial data of medicines which in part target VCAM-1 expression have shown some promise with regards to improvements in surrogate markers of atherosclerosis. If the soon to be reported ARISE study confirms a vascular end-point benefit for succinobucol, then further research will be needed to determine the extent to which its effects on reducing VCAM-1 expression by vascular endothelial cells, as opposed to other potential succinobucol actions, explain such clinical benefit.

References

  1. Top of page
  2. Summary
  3. Review Criteria
  4. Message for the Clinic
  5. The pathogenesis of atherosclerosis
  6. Role of adhesion molecules in the pathogenesis of atherosclerosis
  7. The evidence supporting the role of VCAM-1 in the pathogenesis of atherosclerosis
  8. Succinobucol: an effective treatment for atherosclerosis?
  9. Conclusions
  10. References
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    Lasky LA. Selectins: interpreters of cell-specific carbohydrate information during inflammation. Science 1992; 258: 9649.
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    Adams DH, Shaw S. Leucocyte endothelial interactions and regulation of leucocyte migration. Lancet 1994; 343: 8316.
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    Tanaka Y, Adams DH, Hubscher S, Hirano H, Siebenlist U, Shaw S. T-cell adhesion induced by proteoglycan-immobilized cytokine MIP-1 beta. Nature 1993; 361: 7982.
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    Braun M, Pietsch P, Schror K, Baumann G, Felix SB. Cellular adhesion molecules on vascular smooth muscle cells. Cardiovasc Res 1999; 41: 395401.
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    Malik I, Danesh J, Whincup P et al. Soluble adhesion molecules and prediction of coronary heart disease: a prospective study and meta-analysis. Lancet 2001; 358: 9716.
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    Chen C, Mobley JL, Dwir O et al. High affinity very late antigen-4 subsets expressed on T cells are mandatory for spontaneous adhesion strengthening but not for rolling on VCAM-1 in shear flow. J Immunol 1999; 162: 108495.
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    Huo Y, Hafezi-Moghadam A, Ley K. Role of vascular cell adhesion molecule-1 and fibronectin connecting segment-1 in monocyte rolling and adhesion on early atherosclerotic lesions. Circ Res 2000; 87: 1539.
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    Cybulsky MI, Iiyama K, Li H et al. A major role for VCAM-1, but not ICAM-1, in early atherosclerosis. J Clin Invest 2001; 107: 125562.
  • 16
    Zhu Y, Liao HL, Lin JH, Verna L, Stemerman MB. Low-density lipoprotein augments interleukin-1 induced vascular adhesion molecule expression in human endothelial cells. Atherosclerosis 1999; 144: 35765.
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    Esposito C, Fasoli G, Plati AR et al. Longterm exposure to high glucose upregulates VCAM-induced endothelial cell adhesiveness to PBMC. Kidney Int 2001; 59: 18429.
  • 18
    Rasmussen LM, Hansen PR, Nabipour MT, Olesen P, Kristiansen MT, Ledet T. Diverse effects of inhibition of 3-hydroxy-3 methylglutaryl-CoA reductase on the expression of VCAM-1 and E-selectin in endothelial cells. Biochem J 2001; 360: 36370.
  • 19
    Pueyo ME, Gonzalez W, Nicoletti A, Savoie F, Arnal JF, Michel JB. Angiotensin II stimulates endothelial vascular cell adhesion molecule-1 via nuclear factor-kappa B activation induced by intracellular oxidative stress. Arterioscler Thromb Vasc Biol 2000; 20: 64551.
  • 20
    Tardif JC, Cote G, Lesperance J et al. Probucol and multivitamins in the prevention of restenosis after coronary angioplasty. N Engl J Med 1997; 337: 36572.
  • 21
    Walldius G, Erikson U, Olsson AG et al. The effect of probucol on femoral atherosclerosis: the Probucol Quantitative Regression Swedish Trial (PQRST). Am J Cardiol 1994; 74: 87583.
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    Rinninger F, Wang N, Ramakrishnan R et al. Probucol enhances selective uptake of HDL-associated cholesteryl esters in vitro by a scavenger receptor B-1 dependent mechanism. Arterioscler Thromb Vasc Biol 1999; 99: 305.
  • 23
    Tardif JC, Gregoire J, Schwartz L et al. Canadian Antioxidant Restenosis Trial (CART-1) Investigators. Effects of AGI-1067 and probucol after percutaneous coronary interventions. Circulation 2003; 107: 5528.
  • 24
    Reinoehl J, Frankovich D, Machado C et al. Probucol-associated tachyarrhythmic events and QT prolongation: importance of gender. Am Heart J 1996; 131: 118491.
  • 25
    Tardif JC, Gregoire J, L'allier PL et al. Effects of the antioxidant succinobucol (AGI-1067) on human atherosclerosis in a randomized clinical trial. Atherosclerosis 2007 (in press).
  • 26
    Besemer J, Harant H, Wang S et al. Selective inhibition of cotranslational translocation of vascular cell adhesion molecule 1. Nature 2005; 436: 2903.
  • 27
    Meng CQ, Somers PK, Rachita CL et al. Novel phenolic antioxidants as multifunctional inhibitors of inducible VCAM-1 expression for use in atherosclerosis. Bioorg Med Chem Lett 2002; 12: 25458.