Atherosclerosis is defined as the thickening of the sub-endothelium of arteries by atheromatous plaque. This gradual deposition of atheroma has been found to be present even from the young age of 2 and the speed of progression increases as cardiovascular risk factors such as LDL of over 70mg/dL, hypertension, blood pressure above 120/80mmHg, obesity, low high density lipoprotein (HDL) and smoking increases. Studies indicated that atherosclerosis initiation is due to monocyte recruitment caused by endothelial cell activation as a "response-to-injury". This "response-to-injury" is said to be caused by lipoprotein retention, specifically LDL. Atherosclerosis is also a focal disease as it manifests mainly in areas of low laminar shear stress (LSS) like the arterial bifurcations and branch points. Normally, LSS increases the production of nitric oxide (NO) in endothelial cells. NO is important in the prevention of plaque formation as it has an antioxidant effect, relaxes smooth muscle cells (SMC), inhibits proliferation of SMC and decreases monocyte chemotactic protein-1(MCP-1) adhesion molecules. Thus, these prevent LDL from being retained in the tunica intima.
Proteoglycans, an integral part of the extracellular matrix, which are negatively-charged (as a result of sulphate groups) pull and trap apoprotein-B LDL in the sub-endothelium. LDL oxidation then occurs and further amplifies the "response-to-injury" and cause an inflammatory response. Activation of adhesion molecules, MCP-1 and NF-κB signalling promote monocyte migration and differentiation into macrophages. Macrophages enter the endothelium via scavenger receptors and ingest oxidised LDL, converting cholesterol to cholesterol ester and hence forming foam cells. Aggregation of foam cells creates a necrotic core of the atherosclerosis and hence causing a vicious cycle of inflammation and monocyte recruitment leading to plaque instability.
At the same time, platelet derived growth factor (PDGF) from macrophages promote migration of neointimal SMC to form the fibrous cap. Accumulation of plaque causes vascular stenosis but is otherwise relatively unhazardous and asymptomatic due to the slow progression of the disease. However, the major concern of atherosclerosis is plaque rupture leading to instant thrombotic occlusion of blood vessels causing acute ischemic heart diseases, particularly myocardial infarction. A summary of the progression of atherosclerosis is depicted in Figure 1.
Figure . Progression of atherosclerosis
The positively charged apoB-the tunica intima. LDL is slowly oxidised as vitamin E in it is being exhausted. Oxidised-LDL causes a "response-to-injury" that results in the release of signalling molecules (MCP-1 and NF-κB) and adhesion molecules that creates a chemical gradient to attract monocytes. Monocyte differentiates into macrophages (Mø) and enters the endothelium via scavenger receptors. Mø converts cholesterol to cholesterol esters and form foam cells. The necrotic core formed from foam cell aggregation causes an inflammatory cycle, recruiting more monocyte migration. Mø also releases PDGF that promotes SMC to migrate and thus, forming the fibrous cap.
III. ROLE OF STATINS
Lipid-lowering mechanism
As stated before, statin inhibits HMG-CoA reductase, leading to lower levels of plasma LDL. Hence, oxidation of LDL will also be inhibited. This will prevent the chain of events as stated above, such as the build-up of lipid, macrophage infiltration and T cell activation into the tunica media. This elevates the collagen content of atherosclerotic plaques since there is no secretion of interferon- γ from activated T cells. Ultimately, this will increase stability of the plaque. Vulnerable plaques that are more likely to rupture are those with thin fibrous caps, large lipid core, low collagen and fibrin content, few SMC and intra-plaque haemorrhages.
NO-dependant mechanism
Inhibition of LDL oxidation also increases transcription of endothelial nitric oxide synthase (eNOS) in endothelial cells which is responsible for the production of NO. Statins will also maintain eNOS mRNA stability; therefore eNOS generation will not be hindered. NO can then decrease hypoxia-mediated endothelial dysfunction. This can be seen in an experiment carried out by Brigham & Women's Hospital and Harvard Medical School to test the effects of simvastatin on hypoxic human saphenous vein endothelial cell cultures by exposing the cell to 3% of O2 for 2 hours. They found that treatment with 1µM of simvastatin not only countered the down regulation of eNOS as a result of hypoxia, but they also found that it resulted in a three-fold increase in eNOS activity over its basal activity of endothelial cell.
Moreover, statins can also contribute to a reduction of thrombin production and thrombus formation by decreasing expression of tissue factor (TF) by macrophages and platelet accumulation via eNOS upregulation. Mice subjected to experimental thrombosis were used to test this theory. It was found that only 25% of mice treated with atorvastatin for 14 days had thrombus formation. eNOS knockout mice (eNOS-/-) also underwent the same experiment and no antithrombotic effect was observed, indicating that eNOS mediated the effects of atorvastatin on platelet activity. The fibrinolytic imbalance of endothelial cells are also reversed by statins by increasing levels of tissue plasminogen activator (t-PA) and decreasing activity of plasminogen activator inhibitor-1 (PAI-1), resulting in a reduce risk of thrombus formation after plaque disruption. This insinuates that statin has an effect on cellular signalling by Rho proteins. Rho proteins will be discussed in detailed later.
Besides that, statins is also an activator of protein kinase Akt (aka protein kinase B) in endothelial cells. Protein kinase Akt serves many functions in the treatment and prevention of atherosclerosis but more importantly, it activates eNOS synthesis, inhibit endothelin-1 (a vasoconstrictor), inhibits apoptosis and promotes vasculature formation in vitro. Based on the observation of angiogenesis in ischemic limbs in rabbits with normal cholesterol that are treated with simvastatin, it implies that statins can also improve coronary blood flow.
Anti-inflammatory mechanism
Furthermore, statins also inactivate NF-κB, an activator of a cytokine transcription factor, which is up regulated in the presence of oxidised LDL. Therefore, pro-inflammatory cytokine expression will be lowered. Adhesion molecule expression and direct binding to integrin receptors are also inhibited by statins. A research was conducted by Timothy Stalker et al on normocholesterolaemic rats which are treated with rosuvastatin. Rosuvastatin inhibited the expression of P-selectin on the endothelial surface of rat mesenteric venules. This prevented leukocytes from rolling, adhering and transmigration through the endothelium, resulting in an anti-inflammatory effect. These anti-inflammatory attributes of statin are reflected by low plasma inflammatory markers such as C-reactive protein (CRP) and interleukin-6 (IL-6). Paul Ridker et al had even concluded that measuring CRP is a better and stronger predictor of cardiovascular events in contrast to LDL cholesterol levels based on the data they collected from following 27,939 healthy American women for a mean of eight years until the development of a cardiovascular event.
Inhibition of structural change
Farnesyl pyrophosphate (F-PP) and geranyl-geranyl pyrophosphates (GG-PP) are isoprenoid intermediates that arise from the mevalonate (MVA) pathway of the synthesis of cholesterol as shown in Figure 2. This isoprenylation process is a lipid modification that is essential for the covalent attachment to members of the small G protein superfamily and plays a role in protein-protein interactions. One of the best-recognised G protein family, the Rho protein family, regulates actin cytoskeletal reorganisation and gene expression, contributing to platelet activation, leukocyte migration, vascular SMC contraction and cellular adhesion. Therefore, the inhibition of isoprenylation by statins is vital in impeding the endothelial cell changes involved in atherosclerosis.
Degradation of the fibrous cap is most susceptible at macrophage-dense regions of the atherosclerotic plaque. Macrophages degrade the vascular extracellular matrix (ECM) via phagocytosis and the release of matrix metalloproteinases (MMPs).(33) MMPs are endogenous Zn2+ - dependant proteolytic enzymes capable of regulating ECM homeostasis in response to vascular wall injury. This means that MMPs are responsible for degradation of basement membrane, control of migration, proliferation, differentiation and apoptosis of vascular inflammatory cells.(34) Statin, specifically cerivastatin, was found to be able to decrease the accumulation of macrophages and its expression of MMP-1, MMP-3, MMP-9 and TF in the rabbit aortic atheroma in vivo.(35) In the same study, MMP-9 production was also decreased in cultured human monocyte. However, because MMPs promote apoptosis and the destruction of basement membrane, vascular SMCs are liberated and are able to migrate into the neointima, promoting plaque stability.(36) Thus, MMP actions on assisting versus impeding plaque stability are debatable.
Figure . The cholesterol synthesis pathway
Cholesterol synthesis causes a decrease in plasma LDL acquisition via LDLR because of negative feedback mechanism. Statin inhibits HMG-CoA reductase and thereby decreases cholesterol synthesis. This will lift the negative feedback mediated by cholesterol off the LDLR, allowing for a decrease in plasma LDL.
IV. COMPLICATION OF STATINS
Side effects
No drug is without its side effects. Statin has been recorded to induce muscle toxicity, cataracts, liver damage, kidney damage, gastrointestinal effects and many others. Amongst them, myotoxicity is most discussed in the literature and will be the focus of this section.
a. Myotoxicity
Myopathy is the general term given to all muscle disorders excluding those due to neuropathies. Myalgia is muscle pain or discomfort without a rise in creatine kinase (CK) as a result of over-stretching of muscles, viral infections, autoimmune disorders and metabolic defects.(37) Myositis is the inflammation of muscles with elevated CK. Rhabdomyolysis is muscle breakdown associated with very high CK (greater than 10 times the upper limit of normal) and is the main concern of statin-induced myotoxicity. Since the terminology of muscle toxicity is varied and inconsistent, in 2002(38), the American College of Cardiology/American Heart Association/National Heart, Lung and Blood Institute Clinical Advisory on the Use and Safety of Statins proposed the use of the concentration of enzyme Creatine Kinase (CK) in blood to describe the different types of myopathies. This classification is now widely used.
Prevalence of myotoxicity
A prospective cohort study involving 2,004,692 patients aged 30-84 was carried out to investigate the side effects of statins and their prevalence.(39) In women, the number needed to harm (NNH) of severe or moderate myopathy was 259 (95% CI 186 to 375) and in men, NNH was 91 (74 to 112). Statins caused a three-fold increase risk of myopathy independent of type of statins taken but only 1406 out of the 2 million patients (0.07%) reported to have moderate or serious myopathy. This exhibits the low prevalence of muscle toxicity with the use of statins which contradicted the PRIMO observational study which involved 7924 hyperlipidemic patients that has a 10.5% incidence of muscular symptoms.(40) This difference may be due to the failure of excluding patients with higher risk of myotoxicity and may give a false interpretation that statins contribute to myopathy. Hence, further studies are required to investigate these risk factors. An important risk factor worth stating is the use of gemfibrozil in combination with statins. Taking statins and gemfibrozil together increases the incidence of rhabdomyolysis by 12 times. This is because gemfibrozil inhibits specific isoenzymes competitively to decrease the clearance of statins.(41)
Pathophysiology of myotoxicity
The pathophysiology of statin associated myotoxicity has not been fully determined but there has been some possible hypothesises.
Firstly, as we have discussed before, statins inhibit isoprenylation and decrease isoprenoid intermediates such as F-PP and GG-PP. Depletion in isoprenoids induces apoptosis of many cell types, including skeletal muscle cells. This occurs because the lowering of isoprenoid intermediates causes an increase in cytosolic calcium, resulting in an activation of caspase-3 which plays an important role in the execution-phase of cell apoptosis. This mechanism is further proven with a research that found that introducing isoprenoid and mevalonate in human and rat reversed vascular SMC apoptosis by statins.(44)
Secondly, muscle related symptoms can also be induced by the lack of mitochondrial coenzyme Q10 (CoQ10), also known as ubiquinone, as a result of statins inhibiting mevalonate (a precursor of CoQ10) synthesis. CoQ10 serves as an electron carrier in the electron transport chain in all cells and is important in the synthesis of energy and protection against oxidative stress by free radicals.(45) Even though a suggested treatment for this is to take CoQ10 supplementation(46), it is still debatable whether CoQ10 levels have a significant decrease in muscle cells. Some studies showed a small reduction(47) while others were unsuccessful(48) to support this claim.
Another theory as to why statins cause myopathy is disturbance of calcium regulation in muscle. Calcium homeostasis is usually controlled by the electron transport chain and ryanodine receptors.(49) Patients with muscle problems caused by statins have been reported to have a rise in ryanodine receptor expression but it is inconclusive whether this rise was a risk factor or a consequence of the myotoxicity.(50)
Lastly, the fall in cholesterol levels due to statins has also contributed to the hypothesis of muscle toxicity. The proposed theory is that instability of myocyte cell membrane is more likely to occur as cholesterol concentration decreases causing myopathies. However, this is debatable as patients with low cholesterol because of inherited disorders of the distal cholesterol synthesis pathway do not develop myotoxicity.(51) Furthermore, inhibition of squalene synthase (refer to Figure 2) also does not lead to myotoxicity and might even be beneficial to the treatment of statin associated muscle disorders.
Diagnosis of myotoxicity
A good and thorough diagnosis is required to differentiate statin induced myopathy from other causes of muscle problems. A careful history of the patient must be taken to find risk factors such as vigorous physical activity, gemfibrozil usage, excessive alcohol intake and the use of cocaine. It is also important to note whether the symptoms persist or are elevated after cessation of statins. A physical examination will rule out other similar conditions such as osteoarthritis and muscle strains. Blood tests can also be carried out to measure CK level and to determine serious cases such as rhabdomyolysis. However, it is found that most patients with statin associated myotoxicity have CK level at the normal range.(54) Thyroid-Stimulating Hormone should also be measured to check for hyperthyroidism causing a rise in cholesterol and CK level. If muscle symptoms remained and CK level continues to increase even after discontinuation of statins, a muscle biopsy is required as a definitive test for rhabdomyolysis.
Management of myotoxicity
The National Lipid Association (NLA) recommends withdrawal of statin usage if the patient experiences intolerable muscle symptoms regardless of CK level until patient has recovered. Once the symptoms are gone, the same statin at a lower dose or another type of statin can be given to reproduce the symptoms. If symptoms reoccur, other cholesterol lowering drugs such as bile acid sequestrants, ezetimibie and fibrates should be prescribed. If the patient has tolerable muscle symptoms and mild elevation or normal CK level, it is possible to continue with the therapy or with a lower dose. If the patient has tolerable muscle symptoms but a moderate or severe CK level or the presence of clinical signs of rhabdomyolysis, discontinuation of the statin therapy is necessary. It is also important that patients must be careful with taking self-prescribed medication that is available over-the-counter, especially Chinese red rice fungus which contains statins and is best to avoid taking together with prescription medication.
b. Hepatotoxicity
Since the 1980's when statin trials were just beginning, it was found that 1-3% of patients had an elevated alanine aminotransferase test (ALT). However, raised ALT is not always indicative of liver damage and was actually first discovered in 1955 as a biomarker for acute myocardial infractions. This is further proven by the Scandinavian Simvastatin Survival Study (4S) which involved 4444 patients, in which 1.8% in the drug group and 1.4% in the placebo group experienced a three-fold rise above normal in ALT level in which both showed no sign of liver disease, showing that ALT elevation does not predict liver failure.(6) Even though ALT might increase during statin therapy, this increase tends to be asymptomatic and temporary.(57) The prevalence of reported hepatoxicity with statin use has also been poor. Only 30 cases of liver failure have been recorded by the Food and Drug Administration from 1987 to 2000 (about one case per million person years of use).(58) As of 2010, there has also no consistent liver biopsy related to statin-induced hepatoxicity(57) and in fact, has been shown to be safe when used in patients with hepatitis C.(59) Despite the lack of evidence of statin-associated liver damage, primary care physicians are still worried in prescribing statins to individuals with elevated ALT.(60) This fear might cause the underutilisation of statin medication and limiting therapeutic options.
c. Renal function impairment
There have been records of proteinuria related to statin therapy in opossums(61) and humans(62). Further research also showed that there is an increase in alpha-1 microglobulin excretion (a biomarker of proximal tubular function) when patients are on rosuvastatin treatment.(63) The mechanism proposed is that statins may disrupt the receptor-mediated endocytosis for albumin reabsorption. Sidaway et al(61) observed that adding 10μM of simvastatin into the opossum kidney cells inhibited albumin uptake. They then added 100μM of cholesterol and found that this had no effect on the receptor-mediated endocytosis but adding 10μM of mevalonate or 10μM of GG-PP prevented the statin-mediated inhibition. The implication of this is that mevalonate, GG-PP or their products are crucial in the regulation of protein reabsorption. Nevertheless, proteinuria due to statins has not yet been associated with any signs of damage on the proximal tubules cells. Conversely, there has also been reports of statins reducing protein excretion in type 2 diabetics (64) and improve renal function.
d. Cataracts
Cataracts has also been shown to be related to statins in animals.(68) This was backed up by a cohort study that showed that the NNH for cataracts with statin treatment was 33 (95% CI 28 to 38).(39) However, there are evidences that revealed that statins had no effect on cataract development and might even decrease the risk of nuclear cataracts by 50%.
e. Gastrointestinal symptoms
Nausea, diarrhoea, constipation and abdominal pain are the common symptoms associated with statin therapy that affect the gastrointestinal system. Nevertheless, these symptoms are rarely seen (0.2-0.3% in 9416 patients)(73) and are transient. Discontinuation of statin therapy for a brief period or prescribing a different type of statin will usually treat this.
Cost-effectiveness of statins
It has been shown many times that there is a positive correlation between statins and atherosclerotic plaque reduction while having weak evidences for major life-disabling side effects. Surely, this implies that everyone should be treated with statins as a means of primary prevention in lowering cholesterol to prevent atherosclerosis. However, that is not the case as the NHS has limited resources in funding the massive cost involved. Thus, by considering the cost effectiveness of statins, we can make informed decisions and choices as to which individuals should take statins or not. Such decisions are made by the use of quality-adjusted life year (QALY) as means of quantifying cost effectiveness.
QALY is the measure of the quantity and quality (such as risk of other diseases or permanent disability) of extra life years gained or improved due to a specific treatment. In each year, perfect health is assigned the value 1.0 and this value goes down to 0.0 for being dead. Cost effectiveness is expressed as the cost of treatment or drug (£/$) per QALY. According to the NICE guidelines, the treatment must not exceed £30,000 per QALY for it to be cost effective.(75) A similar threshold of about US$ 50,000 per QALY is also used in North America.
Relationship between cost-effectiveness and risk factors
Based on several health economic analysis and studies, statins will be more cost effective when the patient has a higher risk of atherosclerosis and when statins are used in secondary prevention as opposed to primary prevention. For example, this is the cost effectiveness of primary prevention in men using statins from a study conducted by Lisa Prosser et al(79):
Age (years)
Smoking status
Diastolic Blood Pressure (mmHg)
LDL Cholesterol Level (mmol/L)
HDL Cholesterol Level (mmol/L)
Cost Effectiveness ($/QALY)
35-44
Non-smoker
< 95
4.2-4.9
>1.3
420,000
75-84
Smoker
≥ 95
>4.9
<0.9
66,000
Table 1. Cost effectiveness of primary prevention in men using statins with different types of risk factors
Here, we can clearly see the correlation between higher risk factors such as being older, smoking, high blood pressure, high LDL cholesterol levels and low HDL cholesterol levels with better cost efficiency. In the same data gathered, it also shows us that using statin therapy in secondary prevention is more cost effective than primary prevention:
Primary prevention
$54,000 to $420,000/QALY in men
$62,000 to $1,400,000/QALY in female
Secondary prevention
<$50,000/QALY for all risk factor subgroups and about $10,000/QALY for most subgroups
Table 2. Comparison of cost effectiveness of statins in primary and secondary prevention
But by far, the Health Test Assessment review (HTA)(78) is the most useful UK health economic analysis because of the use of data from meta-analyses and randomised controlled trials from 1993 to 1997 and obtained costs directly from the drugs and health service. It concluded that the initial risks of coronary heart disease (CHD), as mentioned above, and cost of the specific statin played a large impact on the gross cost and hence the cost-effectiveness of statins. They found that the cost effectiveness can be further improved if statin therapy targeted people of 55 years and older with risks of CHD and that there is a 60% gross cost reduction when low cost statins were used instead.
Comparative costs between different types of statins
All statins generally work with the same mechanisms as discussed above but they have a slightly different chemical structure. Hence, different people may react differently to them. In the UK, simvastatin, pravastatin and atorvastatin are commonly prescribed because they currently have the strongest direct clinical evidence. Fluvastatin and rosuvastatin are prescribed less frequently because of their lesser known long term effects.(80) Since cost effectiveness depends on the cost of statin used, we have to consider the comparative costs between the different types of statins. This is done by relating the percentage of LDL cholesterol reduction with the costs of different types of statin and their dosages. For instance, a randomised controlled trial found that there is slightly more patients achieving the LDL-C target in those who are taking atorvastatin (89%) compared with the simvastatin group (80%).(81) Nonetheless, they found that the cost of maintaining the LDL-C goal was much lower in the simvastatin group and concluded that simvastatin was more cost effective in the secondary treatment of CHD. This research was supported by Merck & co, a manufacturer of simvastatin. Another randomised controlled trial involving 662 patients compared atorvastatin, simvastatin, lovastatin and fluvastatin by increasing their dosage over time until LDL-C target set by the National Cholesterol Education Program (NCEP) was achieved.(82) Mean total cost of care for each type of statin was also calculated. The end result was that atorvastatin was more effective in reaching LDL-C goals and has a lower mean total cost (US$1064) compared to simvastatin (US$1304), lovastatin (US$1972) and fluvastatin (US$1542). However, it is important to note that this study was sponsored by Parke-Davis Warner-Lambert Company, a manufacturer of atorvastatin. This seems to be the case for most studies conducted to compare effectiveness of different statins where the cheapest or most beneficial option is the statin from the sponsoring or supporting company. From the studies of statin, simvastatin and atorvastatin are the most extensively used and are generally more cost efficient compared to other statins.
'Should everyone above the age of 50 take statins?' The answer is yes, but despite the efforts of manufacturing cheaper statins, the cost of statin treatment for every patient with the risk of CHD is still likely to place a huge burden on NHS resources. In the analysis carried out by P Pharoah and W Hollingworth, they discussed that effective alternative interventions with a much lower cost exist.(88) These cheaper prevention treatment of CHD is much favoured by the NHS because of its cost effectiveness. These are dietary planning, antiplatelet treatment and lifestyle changes.
V. CONCLUSION
Besides inhibiting HMG-CoA reductase, new discoveries of the action of statins to combat atherosclerosis continue to grow since they were found back in 1973. With further research, statins may also have a place in the treatment of cancer(89), sepsis(90), acute ischemic stroke(91) and even in neurological disorders(92) such as Alzheimer's disease and Parkinson's disease. Even though, statins exhibit side effects, they are rare and can usually be relieved by cessation of statin therapy or changing to a different type of statin. Since statin has more benefits than harm, it is recommended that anyone with a high risk of atherosclerotic plaque rupture should take statin as means of prevention and treatment. The prospect of 'statins for all' is also a possibility as cheaper statins are being manufactured and more evidence of the advantages of statin therapy comes to light.
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