Peripheral arterial disease (PAD) is caused by the hardening of the arteries which, in turn, reduces blood flow to the limbs primarily to the legs. PAD is a widespread disease which is known as one of the leading causes of amputation to the lower limbs. Those affected with PAD often will have impaired motor performance in their limbs and limbs will exhibit abnormal hair and nail growth. A continuous decrease in blood flow to certain limbs increases pain levels and can result in wounds and incisions not healing properly and eventually, due to cell damage, skin ulcers may form. High levels of PAD are associated with severe pain in the affected limbs, poor and delayed wound healing, and an increased risk of cardiovascular events. Due to the diverse symptoms, patients should be closely observed to determine the severity of the disease and appropriate treatment. High levels of atherosclerosis can lead to critical limb ischaemia (CLI) which can potentially result in limb amputation due to the non-viable state of an ischemic limb. This is the worst-case scenario with PAD and yet it is still a frequent occurrence.
Illustration depicts how large series of replicated folds develop, ultimately morphologically affecting an artery lumen. Panel A exhibits the form of replicated folds regularly found in the elderly; folds in panel B, although exaggerated, are more characteristic of those found in patients with PAD. A fake colour atherosclerotic plaque photographed using (CD31) stain, through a microscope at 26.25x magnification. PAD greatly affects the lower limbs (legs and feet are common), thus the image is relevant as it is taken from an atherosclerotic plaque in a common iliac artery. Boot relevant. High levels of atherosclerosis can lead to critical limb ischaemia (CLI), which is common with PAD. This image illustrates, due to the growth of atherosclerotic plaque on arterial walls, the blockage of blood flow can occur and can potentially pose serious consequences. This thickly occluded femoral artery contributed to the patient’s left leg claudication (difficulty walking) and CLI. This partially contributes towards the many different symptoms pertaining to PAD due to the fact that symptoms can vary from person to person. These symptoms frequently manifest in elderly patients or those with impaired limb function, and thus need to be properly identified as such to avoid misdiagnosis and improper treatment.
Current Therapeutic Approaches
Cholesterol-lowering medications, specifically statins, have also been studied in patients with PAD. The Heart Protection Study compared simvastatin 40 mg to placebo in 20,536 patients with PAD and demonstrated a 20% proportional reduction in major vascular events per 1.0 mmol/L reduction in LDL cholesterol with no evidence of a threshold within the range studied. Despite the clear benefits of antiplatelet and cholesterol-lowering medications in patients with PAD, the use of these medications remains suboptimal. Control of blood pressure is another important aspect of PAD management. High blood pressure is a major risk factor for cardiovascular disease and medications aimed to control blood pressure have been shown to reduce vascular events in patients with PAD. ACE inhibitors are a type of blood pressure control medication that are thought to be particularly beneficial in patients with PAD due to their potential to improve walking distances in PAD patients. High doses of ramipril, a type of ACE inhibitor, improved the distance PAD patients could walk before the onset of pain in a six-minute walk test.
Pharmacological interventions are the most commonly used therapeutic modality in PAD and have been studied in numerous clinical trials. These interventions include: antiplatelet agents, cholesterol-lowering medications, blood pressure control medications, and medications aimed to alleviate the symptoms of intermittent claudication. Numerous clinical trials have demonstrated the beneficial effects of using antiplatelet agents in patients with PAD. Aspirin is an antiplatelet agent that has been studied in numerous clinical trials of PAD patients and has consistently been shown to reduce the risk of MI, stroke, and vascular death. Another antiplatelet agent that has been shown to be beneficial in PAD patients is clopidogrel. In a study of 6458 patients with symptomatic PAD, clopidogrel was associated with a 23% relative risk reduction in a composite end point of MI, stroke, or vascular death.
Pharmacological Interventions
We are on the doorstep of a new era in treatments for PAD because the recognition that walking impairment and PAD are important symptomatic markers for cardiovascular morbidity and mortality has been a major stimulus for research into effective, symptomatic treatments for PAD and claudication. Notable have been the numerous recent large-scale, multinational clinical trials in symptomatic PAD that were designed to capture relevant cardiovascular end points and have used walking impairment or claudication as part of a composite end point. In contrast to the traditional short-duration treadmill walking tests used in early phase clinical trials going back to the late 1970s, most of these modern trials have used well-validated, patient-centered, and relevant tests of walking endurance as primary or secondary outcome measures. An outstanding example is the newly completed “Ranger” Study in which 741 patients with symptomatic PAD were randomized to either 4 weeks of a treadmill exercise intervention plus cilostazol or to placebo in a double-blind design. The 6-month change in maximal treadmill walking duration (projected into 6-min walking distance) was the primary outcome variable and an anatomically relevant composite end point was a co-primary end point. An ancillary study to assess quality of life and functioning was performed in a subgroup of these patients. Cilostazol is a type III phosphodiesterase inhibitor that is specific for the enzyme phosphodiesterase type III. The drug has gained approval for the treatment of intermittent claudication in the USA, Canada, and Europe on the basis of positive effects on treadmill walking performance seen in several large, pivotal trials. Cilostazol increases cyclic adenosine monophosphate concentrations and has a variety of metabolic and hemorheological effects in addition to antiplatelet properties. Possible effects of the drug on cardiovascular mortality and morbidity are being studied in the “Cilostazol: Influence on PAD and Cardiovascular Endpoints” TRIPACER study. The drug has generated great interest because it is one of very few drugs demonstrated to have effects on walking impairment in PAD and for its novel and potentially disease-modifying mechanism of action.
Surgical and Endovascular Interventions
The alternative to endovascular therapy is surgical revascularization. This has been the traditional method for revascularization of lower limb arteries. Although it is more invasive than endovascular therapy and requires a general anesthetic, it has a better long-term success rate. It is more suitable for complex aorto-iliac and infra-inguinal lesions. Studies showed that for intermittent claudication or short distance claudication, surgical revascularization resulted in higher disease-free and limb salvage survival rates than angioplasty or conservative treatments. At present, there are two types of surgical revascularization techniques used: bypass and grafting. The type of surgery performed depends on the location and severity of the lesion. A bypass is where the diseased artery is bypassed by using a substitute graft to redirect blood flow. The graft can be synthetic or autologous using the patient’s own saphenous vein or, less frequently, the lesser saphenous vein. Bypasses are the most established and successful surgical revascularization technique. Due to the short- and long-term efficacy, it is the recommended treatment for severe limb ischemia. Grafting involves inserting a small blood vessel as a graft directly to the diseased artery to allow blood flow. This can be less effective than a bypass, and due to the complexity, it is not widely used. Both endovascular and surgical interventions have the potential to increase blood flow to the limb and relieve the symptoms of claudication or limb ischemia.
Endovascular therapy was first introduced in the 1960s and has rapidly evolved into a primary revascularization technique for PAD. This is a catheter-based technology that is delivered from a puncture site in the groin to the lesion. It is less invasive than surgery and is performed under local anesthesia. The different types of endovascular interventions include balloon angioplasty, atherectomy, and insertion of stents. This procedure can increase blood flow to the limb and relieve symptoms of claudication. However, it is not suitable for all patients. Balloon angioplasty is the simplest of the endovascular interventions, and there are two types: normal and percutaneous transluminal angioplasty (PTA). PTA is where a balloon is inserted from a catheter and inflated in the narrowed artery. Atherectomy involves removing the atheroma from the artery, and there are different techniques to do this, including using a rotary blade, laser, or directional pressure. Finally, stents are often inserted after other endovascular procedures. They act as a scaffold to prevent recoil and restenosis of the artery. Be it as they may, endovascular interventions also pose risks, including arterial dissection and distal embolization of atherosclerotic debris.
Surgical and endovascular interventions are treatments that are used in the more severe stages of peripheral artery disease. They both can be used to relieve intermittent claudication, ischemic rest pain, and/or to treat ulcers or gangrene on the leg or foot. The main goal of these interventions is to increase blood flow to the affected limb. However, they also pose risks and are not suitable for all patients.
Emerging Therapeutic Targets
Inflammation and Immune Modulation: Inflammation is now recognized as a key pathophysiological mechanism underlying atherosclerotic disease and its manifestations, including PAD. Of note, it has become increasingly apparent that the global burden of atherosclerosis is amplified by the rising prevalence of obesity and diabetes, which confer a pro-inflammatory state that accelerates atherosclerosis and increases cardiovascular risk. In view of this, therapies for PAD which target the inflammatory cascade represent an attractive strategy, particularly if they can be linked to modulation of the immune response that governs atherosclerotic lesion formation and stability. Despite the failure of several trials of anti-inflammatory agents intended for global cardiovascular risk reduction, the identification of specific inflammatory mediators and cytokines that contribute to atherosclerotic disease now offers the prospect of more precise targeted therapies for patients with PAD. An example of how this approach could bear fruit in the field of PAD is the research into 5-lipoxygenase, an enzyme which catalyzes the formation of leukotrienes from arachidonic acid, and is thought to play a detrimental role in atherosclerosis and vascular inflammation. Inhibition of 5-lipoxygenase has been shown to reduce lesion formation in a rodent model of atherosclerosis and subsequently, a pilot clinical trial exploring the effects of an oral 5-lipoxygenase inhibitor in patients with PAD demonstrated a reduction in plasma inflammatory markers and an improvement in walking distance, suggesting an effect on systemic and intra-limb ischemia.
Angiogenesis and Vascular Regeneration: The poor clinical outcomes associated with invasive therapeutic strategies for the treatment of PAD highlight a clear unmet need for novel therapies. Given the pivotal role of ischemia in determining the phenotype of patients with PAD and critical limb ischemia, strategies that could augment angiogenesis and collateral vessel formation in the ischemic limb are of particular interest. Current evidence in support of therapeutic angiogenesis for patients with PAD is limited, owing to the failure of numerous trials of angiogenic cytokines and growth factors to demonstrate clinical efficacy, or to gain regulatory approval. This has prompted a shift in research towards cell-based strategies and the more recent recognition of the potential for regenerative approaches to address the defective angiogenic response in patients with PAD. Studies of diverse cell types, including bone marrow derived cells, and more recently, those incorporating the vasculogenic and regenerative potential of endothelial and stem/progenitor cells have indicated that cell-based therapies can promote meaningful angiogenesis and arteriogenesis in ischemic tissues. Moreover, the vast and evolving field of research into mechanisms regulating the proliferation, migration, survival and differentiation of vascular cells, collectively termed vascular regeneration, represents a potential source of novel therapeutic targets for patients with PAD.
Angiogenesis and Vascular Regeneration
Although research into stimulating angiogenesis has shown promise, there are as yet no therapies that have translated into routine clinical practice for PAD.
An alternative approach to stimulating angiogenesis is to inhibit endogenous anti-angiogenic factors such as angiostatin and endostatin. In vivo studies using antibodies or gene knockout of these endogenous inhibitors have shown increased angiogenesis and improved tissue perfusion. Targeting matrix metalloproteinases (MMPs), which are enzymes involved in the degradation of the extracellular matrix and are essential for the migration and tube formation phases of angiogenesis, is another potential strategy for therapeutic angiogenesis. An MMP inhibitor has progressed to a phase II clinical trial for cancer therapy, and chronic administration of this or specific MMPs with selective substrates could be used in ischemic disease.
Therapies aimed at improving limb blood flow by stimulating angiogenesis or vascular regeneration have the potential to benefit a broad range of PAD patients, from those receiving medical therapy for intermittent claudication to those with CLI. Angiogenesis is the growth of new vessels from pre-existing vasculature. It is a complex process involving protease production, degradation of the basement membrane, migration and proliferation of endothelial (EC) and smooth muscle cells (SMC), tube formation, and finally, 3D structural remodeling in response to specific growth factors. Numerous growth factors are known to stimulate angiogenesis, including vascular endothelial growth factor (VEGF), fibroblast growth factor (FGF), and hepatocyte growth factor (HGF). Preclinical studies have shown that gene therapy using naked plasmid DNA or vectors expressing these growth factors can induce therapeutic angiogenesis and improve tissue perfusion in limb and other models of ischemia. A phase I/IIa clinical trial has recently been completed in which naked plasmid FGF DNA was injected at multiple sites into the ischemic limb of patients with CLI. Results suggest that this therapy is safe and may improve amputation-free survival and quality of life. A limitation of gene therapy is that its effects are short-term due to rapid degradation and dilution of the growth factor in vivo. Therefore, current research is focused on developing growth factor delivery systems or direct EC stimulation with the potential for sustained targeted angiogenic effects.
Peripheral arterial disease (PAD) is a manifestation of atherosclerosis that severely affects the quality of life in an increasing number of patients. The most severe form of PAD is critical limb ischemia (CLI), a condition that carries a 1-year mortality rate of 25-45%. Treatment for PAD aims to improve symptoms and prevent cardiovascular events. Therefore, patients with intermittent claudication (IC) are treated with pharmacological therapy, and a minority are offered revascularization procedures to relieve symptoms. Unfortunately, there are no effective medical therapies for patients with CLI. CLI patients suffer continual pain at rest, often leading to narcotic dependency and an inability to carry out activities of daily living.
Inflammation and Immune Modulation
Initial studies in mice with hind limb ischemia have yielded positive results using T cell modulation therapy. T cells have been shown to have a significant role in the pathogenesis of atherosclerosis and other inflammatory diseases. T cells can be divided into two functional subsets, and distinct lineages of CD4 Th cells have been identified. It is known that Th1 cells are proatherogenic and promote cellular immune responses by cooperating with macrophages, whereas Th2 cells are antiatherogenic and downregulate inflammatory responses. CD28 is a cell surface protein that provides co-stimulatory signals required for T cell activation. CD28 and B7 binding is a major co-stimulatory pathway in the induction of T-cell-mediated immune response. Ye et al. used a highly selective small peptide antagonist of the CD28 receptor (JJ316) in a hind limb ischemia model. They found that treatment with JJ316 significantly improved blood flow recovery and increased the density of small arteries in the ischemic limb. This was associated with a decrease in the number of CD4 and CD8 lymphocytes and increased CD4 T cell apoptosis at the site of muscle ischemia. JJ316-treated mice also had a significant decrease in macrophage infiltration and had less skeletal muscle degeneration. This study suggests that selective inhibition of the CD28 co-stimulatory pathway may be a potential therapy for PAD through the suppression of pathogenic T cell activity.
Inflammation is a complex biological response of vascular tissue to harmful stimuli, such as pathogens, damaged cells, or irritants. It is a protective attempt by the organism to remove the injurious stimuli and to initiate the healing process. Inflammation has been implicated in the pathogenesis of all stages of atherosclerosis, which PAD is a manifestation of. Different cellular and molecular inflammatory mechanisms have been associated with the development of atherosclerosis and inciting ischemic symptoms. It is now understood that immune and inflammatory responses in the arterial wall involve both specific (adaptive) and non-specific (innate) mechanisms. Inflammatory stimuli up-regulate the production of chemokines by a variety of cells in the arterial wall. These chemokines act on the leukocytes, which express corresponding receptors, causing the leukocytes to undergo change and “home” to the site of inflammation. Once at the site of inflammation, leukocytes and other inflammatory cells can release various enzymes, expressed cytokines, and growth factors that cause damage to the surrounding tissue and promote a prothrombotic phenotype. These immune and inflammatory mechanisms have become potential targets for therapeutic intervention in the treatment of atherosclerosis and PAD. A better understanding of the immune and inflammatory mechanisms of atherosclerosis has led to the development of immune-modulating therapies for PAD.
Metabolic and Lipid Modulation
Arguably, the most important risk factor for the development of PAD is hypercholesterolemia. Wood et al. reported in the Honolulu Heart Study that a ten percent increase in serum cholesterol was associated with a 1.3-fold increase in the risk of developing claudication. Hypercholesterolemia accelerates atherosclerosis at all levels of severity. Stepwise increases in LDL cholesterol are associated with plaque lipid content, and the resulting LDL can become oxidized, promoting endothelial injury and monocyte infiltration into the arterial intima. High cholesterol levels are also associated with a reduction in NO availability and an increase in superoxide release from endothelial cells. Therefore, there is a combined effect on vasoregulation and a proinflammatory state, which will accelerate plaque formation and instability. In experimental models, cholesterol feeding resulted in arterial lesions that had many similarities to human atherosclerotic lesions, including the presence of cholesterol clefts, foam cells, and fibrous tissue. An atherogenic diet in humans has been shown to increase AR and the progression of SFA stenosis. It is therefore clear that inhibition of cholesterol absorption and synthesis could be an effective method of preventing and treating PAD.
There is evidence that the promotion of a healthy lifestyle can reduce the incidence of CLI. Smoking cessation, exercise and dietary modification all play a role in primary prevention of peripheral vascular disease and CLI. With specific reference to nutrition, there is some evidence that a diet high in fruit and vegetables can reduce the incidence of PAD. These lifestyle changes impact favorably on traditional risk factors, particularly diabetes and the metabolic syndrome, which in turn will reduce the incidence of CLI. It is a slow process, but the implementation of an effective public health policy targeting these kinds of lifestyle changes may, in the long term, lead to a reduction in the overall burden of CLI. This is supported by the reduction in the incidence of cardiovascular events in the United States over the past 30 years, since the implementation of public health policy targeted at smoking cessation and control of hypertension. Unfortunately, the current trend of increasing obesity in developed countries will fuel the diabetic epidemic and predominantly Type 2-related PAD. This may offset any reduction in incidence related to lifestyle change, and therefore it is important to look at the effect of specific dietary components on the development of PAD and CLI.
Conclusion
In contrast, peripheral arterial disease remains relatively under-investigated at the basic science level, and limited progress has been made in understanding the mechanisms of disease progression. The increasing prevalence of diabetes has resulted in a rise in patients in whom critical limb ischemia is the final manifestation of their arterial disease. These patients have a particularly poor outcome with high levels of morbidity, mortality, and progression to amputation. Despite this, there are very few effective therapies for limb ischemia, and many patients are not suitable for revascularization procedures. As medical therapies for intermittent claudication and critical limb ischemia are a major focus in current clinical trials, it is likely that there will be a shift in the balance of research into atherosclerosis and arterial disease with more emphasis on the latter. Now is therefore an opportune time to assess the current understanding of the pathogenesis of peripheral arterial disease and propose future directions for research into this condition.
Arterial disease is a significant cause of morbidity and mortality throughout the world. Despite this, it is often a forgotten part of the cardiovascular disease spectrum. The reasons for this are complex but probably related to the lack of highly visible life-threatening events such as stroke and myocardial infarction. Over the past 10 years, there has been a revolution in our understanding of the molecular processes involved in atherosclerosis, and this has had a significant impact on the management of patients with arterial disease. Much of the work in atherosclerosis research has focused on the earliest stages of lesion formation and the terminal stages of plaque rupture and thrombosis.
