Vulnerable plaque: from bench to bedside; local pacification versus systemic therapy.

Critical coronary stenoses accounts for a small proportion of acute coronary syndromes and sudden death. The majority are caused by coronary thromboses that arise from a nonangiographically obstructive atheroma. Recent developments in noninvasive imaging of so-called vulnerable plaques created opportunities to direct treatment to prevent morbidity and mortality associated with these high-risk lesions. This review covers therapy employed in the past, present, and potentially in the future as the natural history of plaque assessment unfolds.

INTRODUCTION

Atherosclerosis is the leading cause of mortality and comorbidity in Western countries. Progressive atherosclerotic lesion destabilization with subsequent rupture, acute luminal thrombosis, and coronary artery occlusion is the main mechanism in the pathogenesis of myocardial infarction (MI) and sudden coronary death.[12] Luminal thrombosis may occur from either plaque rupture, plaque erosion, or calcified nodules.[3] The term “vulnerable plaque” is often used to refer to lesions that are prone to rupture. They are characterized by increased inflammatory infiltrates composed of mainly monocytes, macrophages, some T cells, and neutrophils. Autopsy studies suggest that these vulnerable atherosclerotic lesions typically consist of a necrotic or lipid core, covered by a thin fibrous cap with severe infiltration of macrophages in the shoulder regions.[3]

Unfortunately, most of the vulnerable lesions show only a 30% luminal occlusion by coronary angiography, and detection by conventional imaging modalities of these rupture prone plaques has proved to be difficult.[4] The study of the vulnerable plaque phenotype and its detection has attracted increasing interest over the past decades. During this time, there has been a remarkable paradigm shift in methods to identify those patients at risk or needing treatment. Biologic imaging, using different uptake tracers or markers are used in modalities, such as positron emission tomography (PET), cardiac magnetic resonance imaging (MRI), and to a lesser extent cardiac computed tomography (CT). It has provided depth and insight to a myriad of possible inflammatory targets for future theranostic strategy.[5]

In this review article, we first describe the inflammatory process involved in the pathogenesis of vulnerable plaque to allow understanding into the specific therapies that are studied. We will also describe the defensive mechanism the endothelium employs once inflammation sets in and allude to future therapy.

The role of inflammation

Studies have shown that hypercholesterolemia causes endothelial activation in medium and large sized arteries. The infiltration of low-density lipoprotein (LDL) into the intimal layer initiates the inflammatory response at sites of hemodynamic strain with a subsequent increased expression of adhesion molecules.[6-8] Vascular adhesion molecule-1 (VCAM-1) is typically upregulated and cells with counter-receptors for VCAM-1, such as monocytes and lymphocytes, adhere to these sites with subsequent migration through the interendothelial junctions into the subendothelial space.[9] These monocytes then undergo further differentiation into macrophages as stimulated by the macrophage colony-stimulating factor,[10] a crucial step in atherosclerosis development. Upregulation of the scavenger receptors and Toll-like receptors on macrophages, as part of the innate immunity response, internalizes oxidized LDL particles, which ultimately lead to the formation of foam cell.[11] Macrophages then attempt to clear subendothelial lipid from the vessel wall, utilizing the vaso vasorum to stabilize atherosclerotic plaques.[12] In doing so, they set up an inflammatory cycle. They release proinflammatory cytokines, including interleukin-1, monocytes chemotactic protein-1 (MCP-1), and TNFα, attracting lymphocytes, mast cells, and neutrophils as well as inducing apoptosis in plaque-derived smooth muscle cells.[13] Macrophages may secrete enzymes capable of directly digesting the fibrous cap of the plaque, including several members of the matrix metalloproteinase (MMP) family that are able to degrade all types of extracellular matrix components and contribute to cap rupture as well as erosion.[14-16]

A balance is established between the proinflammatory actions of macrophages and infiltrating lymphocytes and the protective layer of smooth muscle cells separating the lipid core from the vessel lumen. Where the degree of inflammation is sufficient, the fibrous cap can rupture or erode, exposing the thrombogenic lipid core to the bloodstream. The degree of occlusion by the thrombosed plaque may depend on factors such as cap thickness, size of the lipid/necrotic core, inflammatory cell volume, and the thrombogenicity of the blood.[17]

Systemic factors that correlate with plaque rupture are altered blood rheology, increased coagulability, increased systemic inflammation, and recurrent infections. These unfavorable systemic changes often interact synergistically with risk factors of atherosclerosis and plaque rupture, such as hyperlipidemia, smoking, and diabetes.[18] Several plasma markers of inflammation may provide information of the patients’ susceptibility to plaque rupture. Elevated levels of the inflammatory markers, such as CRP, P-selectin,[19] soluble ICAM-1, soluble vascular cell adhesion molecule-1,[20] IL-6,[21] tumor necrosis factor-α,[22] IL-18,[23] and soluble CD40 have all been shown to predict future cardiovascular risk in a variety of clinical settings.

ROLE OF SYSTEMIC THERAPY IN TREATING INFLAMED VULNERABLE PLAQUES

Inflammation is an early defense mechanism byway the vasculature tries to promote self-healing. Self-healing is further assisted by 3 main processes, which include endothelial repair by endothelial progenitor cells (EPCs), plaque neovascularization, and reverse cholesterol transport.[24]

Endothelial progenitor cells

EPCs are a subgroup of peripheral blood monocytes that expresses stem cell-like antigenic determinants, including CD34+ and the vascular endothelial growth factor-2 receptor.[25] They are derived from the marrow, are activated once tissue trauma is detected[26] and have been implicated in neoangiogenesis.[27] Experiments in animals show that the systemic application or mobilization of stem cells and progenitor cells beneficially influences the repair of endothelial cells after injury and the progression of atherosclerosis.[28] A competent human bone marrow can translate injury into mobilization of EPCs and recruitment and therefore restoring endothelial function. However, as injury perpetuates, the bone marrow becomes less competent and the number of EPCs involved in vascular repair falls.[29] The number of circulating EPCs correlates well with the level of insult. Patients with diabetes mellitus, for example, have a lower count with worsening glycemic control and worsening severity of arteriopathy.[30] The greater number of circulating EPCs is associated with a reduced risk for cardiac death and other coronary events.[31] EPC modulation is therefore a promising therapeutic alternative in cardiovascular disease. EPC mobilization leads to a scenario of accelerated re-endothelialization, achieved by erythropoietin[32] and other factors, such as granulocyte-colony stimulating factor, stromal cell-derived factor-1, platelet-derived growth factor CC, brain-derived neurotrophic factor, and placental growth factor.[33]

Several pharmacologic pathways may mobilize and increase EPCs, and statin therapy is the most studied so far.[34-36] Other potential pharmacologic approaches to enhance EPCs include the peroxisome proliferator – activated receptor agonists and medications involved in the renin-angiotensin system.[37-39] In treating ruptured plaques, EPC capture stents are currently available. These stents are coated with antihuman CD34 antibodies and attract circulating EPC onto its stent surface with the aim of rapidly establishing the repair process and re-endothelialization.[40] Interim results are encouraging, with 1 year MACE of 12.2% and TLR of 4.1% in high-risk patients presenting with STEMI.[41] Larger, worldwide registry demonstrates a 12-month TLR of 5.7%.[42] Longer-term follow-up will determine the fate of this technology in terms of safety and efficacy in comparison with drug-eluting stents.

Plaque neovascularization

Neovascularization is pivotal in maintaining homeostasis and restoration of healthy tissue in wound healing, myocardial necrosis, chronic ischemia, and regeneration of heart muscle by stem cell therapy.[43] It serves as a passage for macrophages to exit and stabilize lipid-rich atherosclerotic plaques.[13] In the normal vessel wall, oxygen diffuses into the tunica intima directly from the lumen, whereas the tunica media and the adventitia are nurtured by vasa vasorum, removing waste products and thus maintaining metabolic homeostasis. Atherosclerosis results in a thicker intima, increasing the distance between the deep layers of the intima and the luminal surface which ultimately exceeds the oxygen diffusion threshold (250-500 μm). This results in local hypoxia and the induction of neovascularization.[44]

Progression of atherosclerosis is associated with a 10-fold increase in vessel wall flow through the vasa vasorum, resulting in reduction in lipid content along with very low levels of inflammatory cells, leading to plaque regression. This flow returns to normal with the cessation of the disease process.[45] However, plaque neovessels are fragile structures, with single-layer endothelial cells prone to leakage and rupture, allowing for extravasation of red blood cells, intraplaque hemorrhage and the accumulation of hemoglobin (Hb).[46] The heme iron component of Hb generates reactive oxygen species and activates the proinflammatory nuclear transcription factor-κB.[47] The haptoglobin (Hp) pathway promotes clearance of free Hb through the Hp-Hb complex, which is scavenged by the macrophage receptor CD163.[48] Reduced clearance of the macrophage-Hp-Hb complex favors iron deposition, oxidative stress, and further propagate macrophage accumulation.[49] This is seen particularly in diabetics who are homozygous for the Hp-2 allele, having a 4-5 times greater risk of cardiovascular events.[50] possibly as the accumulated macrophages secrete MMPs leading to cap digestion and potentiate plaque rupture.

Antioxidant therapy may antagonize the increased oxidative stress in diabetic individuals with the Hp 2-2 genotype.[51] A subgroup analysis of diabetic individuals with the Hp 2-2 genotype in the Heart Outcomes Prevention Evaluation (HOPE) study suggested a reduction in the primary composite events, despite vitamin E failing to prevent cardiovascular events in the overall population.[52] Modulation of neovascularization is still controversial. Proangiogenic therapy such as VEGF to promote neovascularization and enhance macrophage clearance may lead to neovessel growth in the retina as well as oncogenic regions, limiting its use.[53] Several studies have examined microvessels noninvasively through αvβ3-integrin-targeted gadolinium, using gadolinium-enhanced dynamic contrast MRI. The detection of intraplaque microvessels may help to determine the role of these neovessels in plaque vulnerability and their relevance in event prediction and future therapy.[54-56]

Reverse cholesterol transport

Reverse cholesterol transport is a term described by Glomset in 1968,[57] referring to the process by which extrahepatic (peripheral) cholesterol returns to the liver for excretion in the bile and feces. High-density lipoprotein (HDL) plays a major role for free cholesterol efflux out of macrophages through one of three processes, namely, passive diffusion, SR-BI receptor, or the more efficient ABCA-1 transporter.[58]

The Framingham study from the 1970s first revealed low HDL-cholesterol (HDL-C) levels to be associated with adverse cardiovascular events. In that pivotal study it was reported that an HDL-C level of <1.03 mmol/L (40 mg/dL) in men and <1.29 mmol/L (50 mg/dL) in women was associated with increased cardiovascular risk. Increasing the HDL-C level by 1 mg/dL may reduce the risk of cardiovascular disease by 2-3%.[59] Apart from cholesterol efflux, HDL also enhances NO production from endothelial nitric oxide synthase (eNOS) byinhibiting the uncoupling of eNOS by LDL-cholesterol (LDL-C).[60] HDL-C also enhances endothelial cell proliferation and migration, thereby promoting an intact endothelial cell monolayer and reducing opportunity for atherosclerosis formation. This effect is believed to be mediated through the activation of the SR-BI receptor and the activation of protein kinases, such as PI3K-Akt and MAPKs.[61] Other beneficial effect of HDL-C includes antiapoptotic, antithrombotic,[62] and anti-inflammatory[63] all of which contribute to its antiatherosclerotic properties.

Nicotinic acid is the most effective pharmacologic agent for raising HDL-C currently available. At clinical doses, it elevates HDL-C levels by 15-35%.[64] It elevates HDL levels by several mechanisms, including reducing the lipolysis of triacylglyerol, hence reducing levels of nonesterified fatty acids and decreasing hepatic triglyceride synthesis, resulting in lowering of very low-density lipoprotein particles, which attenuates cholesteryl ester transfer protein (CETP)-mediated depletion of HDL-C, it potentiates reverse cholesterol transport from macrophages via ABCA1 and lastly, it reduces uptake of HDL particles by the liver.[65] Both immediate and prolonged-release nicotinic acid have been administered in combination with statins and have been associated with elevations of HDL-C levels of 30%. Combination therapy has also demonstrated beneficial effects on clinical outcomes, with the HATS trial reporting that nicotinic acid and lovastatin reduced coronary stenosis grade by 0.4% and reduced coronary heart disease events by 90%.[66] Other drugs such as statins, and to a variable extent, fibrates, have the potential to increase HDL levels by up to 15% and is associated with regression in atherosclerosis on intravascular ultrasound.[67]

The effect on atherosclerotic regression is also seen with the infusion of Apo-AI Milano liposomes to patients with acute coronary syndromes.[68] Several peptides are being investigated, one of which an Apo-AI mimetic peptide, D-4F, an 18-amino acid peptide that is not degraded efficiently by gut peptidases and that can, be administered orally, albeit with low bioavailability.[69] Pharmacologic inhibition of the CETP was promising with the advent of the drug torcetrapib, which has been shown to be able to increase HDL levels by up to 106%.[70] However, serious safety concerns were raised in the ILLUMINATE study, whereby elevation in blood pressure was observed along with increased number of death to recipients of torcetrapib.[71] The lack of atherosclerotic regression on intravascular ultrasound imaging on subsequent studies compromises the drug further.[72] It is thought that the drug's failure is due to the intrinsic property of torcetrapib, as opposed to a class effect.[73] Dalcetrapib is a promising new agent with early data showing a reduction in HDL-C of 26.9% over a 2-year period associated with a reduction in carotid vessel wall inflammation as measured by 18FDG PET/CT at 6 months.[74]

Emerging experimental studies investigating the complex pathways of HDL metabolism have identified other new targets for raising HDL-C, including the inhibition of endolipase[75] and targeting the ABCA1 transporter in macrophage by activating the liver X receptor (LXR), which has been shown to induce plaque regression in mice.[76]

At present, systemic therapy with statin remains the viable therapeutic option in plaque stabilization. In animal models, statin therapy has been shown to reduce macrophages and collagen breakdown products in lipid pools.[77] In humans, intensive statin therapy has been shown to reduce coronary events in patients with both stable disease and acute coronary syndromes in the ASTEROID trial.[69] High-dose rosuvastatin (40 mg) is associated with marked decrease in LDL and this in turn is associated with an 11.1% reduction in coronary plaque volume as measured by IVUS. However, despite intensive statin therapy, we are still witnessing recurring events and marking out patients who are resistant to systemic therapy. This is shown in the PROVE IT trial, where the combination of ACE inhibitor, β-blockers, aspirin, and high-dose atorvastatin (80 mg) still yielded a 22% recurrent event rate at 2 years. This situation gives rise to the potential role of local preventive therapy as a supplement to systemic treatment in plaque stabilization.

LOCAL THERAPY FOR VULNERABLE PLAQUE

Any locally applied therapy would have to be able to prove that focal treatment of vulnerable plaques will lead to reduced coronary events in the future. Currently, randomized study to understand the natural history of detected vulnerable plaques are underway utilizing various vulnerable plaque detection methods, such as IVUS, palpography, thermography, and near infrared spectroscopy. Wang et al. have shown that 80% of acute MIs are due to ruptured plaques in the proximal coronary tree.[78] Despite treating these acute ruptures, up to 12% of patients would experience a recurring coronary event due to progression of moderate lesions seen on angiography independent of the initially treated segment.[7980] Identifying these proximal vulnerable plaques as well as those that are inflamed during angiography would allow the application of local plaque stabilization treatment.

Photodynamic therapy

Photodynamic therapy (PDT) is widely used in cancer and dermatologic patients. In atherosclerosis, Hayase et al., using a rabbit model of atherosclerosis by balloon injury, demonstrated that light activation of localized motexafin lutetium (Antrin) produced selective depletion of macrophages.[81] Similarly, Antrin was studied in a phase I safety trial in 75 patients undergoing coronary stenting and identified safe doses of the drug and light. Waksman et al. in 2006 published the results of using Miravant photosensitiser (MV0611) compound that destroyed macrophages and smooth muscle cells without damaging the structural integrity of the vessel by using green light with a shorter penetrating depth of 542 nm as opposed to red light with Antrin with a penetration depth of 732 nm[82] that is associated with damages to the media. The application of PDT is limited by the animal models used, which produce lesions that are similar to human pathologic intimal thickening and are not equivalent to the lipid core plaque that is associated with acute coronary syndromes. The absence of clinical data further prevents the application of this technology.

Plaque sealing

Meier et al. suggested in 1995 that identified vulnerable plaques may be intentionally ruptured with balloon inflation during intervention. The resulting plaque rupture will then heal and become fibrotic and reduce the risk of developing a coronary event.[83] This concept is termed plaque sealing and has fallen out of favor in the advent of coronary stenting and the lack of clinical data. It would be interesting to see in the future if this technique would return, especially with better identification and understanding of the history of so-called vulnerable plaques.

Drug-coated balloons have, in recent years, emerged as a viable therapeutic tool in the treatment of in stent restenosis as well as small vessel disease. In PEPCAD II, paclitaxel-coated balloon (SeQuent Please DCB) was compared with paclitaxel eluting stent (TAXUS Liberte) in 131 patients with in stent restenosis. The results are favorable with less target lesion revascularization in the drug-coated balloon arm at 12 months.[84] In the PICCOLETO study, a different paclitaxel-coated balloon was compared with TAXUS paclitaxel eluting stent in the treatment of coronary artery disease in vessel measuring <2.75 mm. This study was terminated early due to a superiority of Taxus stent in improving diameter stenosis (43.6% vs 24.3%, P = 0.029).[85] Whether this technology will be applicable for local therapy of vulnerable plaque remains to be seen.

Drug-eluting stents have been evaluated as a possible treatment for vulnerable plaques. Moreno et al. demonstrated that in the hypercholesterolemic rabbits with similar atherosclerotic plaques to humans, implantation of either a bare metal or drug-eluting stent in vulnerable plaques resulted in fibrous cap thickening at the cost of increased cap damage and peristrut healing patterns.[86] In the DEFER trial, stenting of intermediate lesions as assessed by fractional flow reserved (FFR) was compared with medical treatment. At 5-year follow-up, there was no difference in the low rate of cardiac death and MI between patients assigned to either intervention. In this low-risk group, it was concluded that coronary stenting is not justified as the risk of stent-related comorbidities is greater. It is notable, however, that this study excluded patients with acute MI and assessed only moderate flow limiting lesions by FFR and not of plaque inflammation. Future improvements in detection methods as well as better stent technology to reduce restenosis and stent thrombosis may tip the balance of stenting these lesions.

CONCLUSION

Our understanding of vascular biology is ever expanding. The field of vulnerable plaque and the vulnerable patient is getting even more exciting with the advent of molecular imaging, allowing greater insight into plaque biology. Despite the many possible targets for therapy, the best treatment option is still seen from the use of statins. At present, local therapy and plaque pacification has some ways to go before it becomes the default treatment in this very difficult scenario.