Cardiovascular disease (CVD) is the leading cause of morbidity and mortality in patients with diabetes mellitus (DM). DM is considered as a coronary artery disease equivalent for future risk of vascular events. There are 3 different classes of platelet-inhibiting drugs: cyclooxygenase-1 (COX-1) inhibitors (aspirin), ADP P2Y12 receptor antagonists (thienopyridines), and platelet glycoprotein (GP) IIb/IIIa inhibitors, and these platelet inhibitors are mostly used for the prevention and treatment of atherothrombotic disorders.
Aspirin inhibits the COX-1 enzyme and therefore blocks platelet thromboxane A2 synthesis. In 2007, the American Diabetes Association (ADA) and the AmericanHeart Association (AHA) jointly recommended primary prevention strategy in those with diabetes, and that was modified by The U.S. Preventive Services Task Force recently; they didnot differentiate their recommendations based on the presenceor absence of diabetes.ADA recommends the use of low-dose aspirin (75–162 mg/day) for secondary prevention of cerebrovascular and cardiovascular events in all diabetic patients.In this chapter we discuss the cardiovascular risk in diabetes, what aspirin resistance means, the mechanism of aspirin resistance in diabetes including platelet activity, methods that are useful to identify aspirin resistance, and methods and management of aspirin resistance.
2. Diabetes and cardiovascular risk
Prevalence of diabetes is increasing rapidly worldwide. Diabetes is projected to affect 300 million people around the world by 2025. Type 2 diabetes is the most common form of diabetes. The prevalence of type 2 diabetes increases with age. Type 2 DM creates a prothrombotic state thatis related to endothelial dysfunction, impaired fibrinolysis,increased levels of coagulation factors, and high platelet reactivity.(Carr 2001)Diabetes is considered as a coronary artery disease equivalent for future risk of vascular events http://www.nhlbi.nih.gov/guidelines/cholesterol/atglance.pdf. Despite a decline in mortality from CVD over the past decade, DM remains a key risk factor for CVD. Individuals with diabetes are at a 2- to 4-fold increased risk of cardiovascular events compared with age- and sex-matched individuals without diabetes. In diabetic patients over the age of 65 years, 68% of deaths are from coronary heart disease (CHD) and 16% are from stroke.(Pignone, Alberts et al. 2010) National Health and Nutrition Examination Survey data suggest thatdeclines in all-cause mortality have occurred among men withDM but not women. Mortality rates among individuals with DM remain approximately 2-fold higher compared to individuals without DM.(Preis, Hwang et al. 2009) Mechanisms leading to prothrombotic state are shown in the Figure 1.
Aspirin is one of the most important therapeutic agents used in the prevention of CVD (both primary and secondary) in patients with diabetes. Long-term aspirin administration in patients at high risk of occlusive vascular events reduced up to 34% of nonfatal myocardial infarction (MI), 25% of nonfatal stroke, and 18% of all-cause mortality. Low-dose aspirin (as low as 81 mg/day) irreversibly inhibits the COX-1 enzyme, by acetylating the serine residue at position 529, consequently impairing the transformation of arachidonic acid to prostaglandin (G2/H2), and TX A2, which is a potent mediator of platelet aggregation and activation. Aspirin's effect on COX-2 is minimal in doses <1200 mg per day.(Bucchi, Bodzenta et al. 1986; Frolich 1997) Equivalent doses of the enteric-coated aspirin are said to be as effective as plain aspirin.(Cox, Maree et al. 2006) Lower bioavailability of these preparations and poor absorption from the higher pH environment of the small intestine may result in inadequate platelet inhibition, particularly in heavier patients.(Cox, Maree et al. 2006)
3.1. Aspirin as a primary prevention strategy in diabetes mellitus
The Japanese Primary Prevention of Atherosclerosis With Aspirin for Diabetes (JPAD) trial (Ogawa, Nakayama et al. 2008) was the first prospectively designed trial to evaluate the use of aspirin (81 mg or 100 mg) in the primary prevention of cardiovascular events in patients with type 2 diabetes (
3.2. Aspirin as a secondary prevention strategy in diabetes mellitus
Two large meta-analyses of major secondary prevention trials by the Antithrombotic Trialists' Collaboration (ATC) showed oral aspirin to be protective in patients at high risk for CVD, including those with diabetes (1994; 2002). The meta-analyses included 287 secondary prevention trials involving 212,000 high-risk patients with acute or prior vascular disease or another condition that increased their risk of vascular disease. Of note, a low dose of aspirin (75–150 mg/day) was found to be at least as effective as higher daily doses. In more than 4,500 diabetic patients studied in the ATC, the incidence of vascular events was also reduced from 23.5% in the control group to 19.3% in the group treated with antiplatelet therapy (
3.3. Aspirin resistance
Aspirin resistance, defined as failure of suppression of thromboxane generation, increases the risk of cardiovascular events in a high-risk population.(Eikelboom, Hirsh et al. 2002)Causes of aspirin resistance include concurrent use of nonsteroidal anti-inflammatory drugs such as ibuprofen that may compete with aspirin at the COX-1 receptor site,(Catella-Lawson, Reilly et al. 2001)polymorphisms in the
3.4. Terminology (Ben-Dor, Kleiman et al. 2009)
The lack of agreement on a standardized definition for “aspirin resistance” has contributed to the disparity in reports of its incidence among different studies. Whereas some use the term “aspirin treatment failure,” others like to call it “aspirin non responsiveness.” The term
3.5. Diabetes and aspirin resistance
The benefit of aspirin in diabetic patients has been consistently documented in several trials. Aspirin is recommended for primary and secondary prevention in DM. Yet, in the meta-analysis of the ATC, the event rate of DM patients on treatment was similar to that of non-DM patients off treatment.(2002)In the Primary Prevention Project Trial, aspirin treatment reduced cardiovascular events and deaths in high-risk non-diabetic patients, but not in patients with type 2 DM (T2DM).Furthermore, in the recent Japanese Primary Prevention of Atherosclerosis with Aspirin for Diabetes Study,(Ogawa, Nakayama et al. 2008)a low dose of aspirin in primary prevention did not reduce the risk of cardiovascular events at 4 years in diabetic patients. Other studies in secondary prevention similarly suggested that aspirin might be less effective in T2DM, especially in patients with poor metabolic control, than in non-DM patients, the underlying mechanism being still debated. Since platelets play a key role in the development of atherothrombotic events, the dysfunctional status of platelets in DM patients may contribute to the enhanced atherothrombotic risk of these patients. It has been proposed that reduced sensitivity to aspirin in diabetic patients might be owing to accelerated thrombopoiesis, (Di Minno, Silver et al. 1986) or to reduced platelet permeability to aspirin caused by membrane glycosylation.(Winocour, Watala et al. 1992)
The mechanisms that lead to increased platelet reactivity observed in patients with DM can be grouped together into the following aetiopathogenic categories: a) hyperglycaemia, b) insulin deficiency and resistance, c) associated metabolic conditions, and d) other cellular abnormalities (as shown in Figure 2). Poor glucose control and body weight are also proposed to contribute to aspirin resistance. (Watala, Golanski et al. 2004; Singla, Antonino et al. 2009) Poorly controlled patients with diabetes have the greatest platelet reactivity. High platelet reactivity was defined as >46% for 5 micromol/L ADP-induced and >59% for 20 micromol/L ADP-induced platelet activity and may require alternative antiplatelet strategies, and further clinical investigations are warranted.(Singla, Antonino et al. 2009)
Platelets: Platelets play a key role in the development of atherothrombotic events. Platelets are essential for primary hemostasis and repair of the endothelium, but they also play a key role in the development of acute coronary syndromes and contribute to cerebrovascular events. Platelet adhesion is an essential function in response to vascular injury and is generally viewed as the first step during which single platelets bind through specific membrane receptors to cellular and extracellular matrix constituents of the vessel wall and tissues. Beyond acute activation as a consequence of vascular injury, circulating platelets are actively involved in all phases of the atherogenetic process, from atherosclerotic plaque formation to plaque inflammation and rupture. (Davi and Patrono 2007; Ruggeri and Mendolicchio 2007; Langer and Gawaz 2008)
Mechanism of aspirin resistance in diabetes: Increased platelet reactivity observed in patients with DM may be secondary to several factors. Acute hyperglycemia as well as poor control of diabetes is associated with increased platelet reactivity. Comparative studies of patients with good glycemic control show they have better response to aspirin compared to the patients with poor glycemic control. Although this might implicate that better glucose control leads to less incidence of aspirin non-responsiveness, the clinical significance of such findings should be carefully inspected, since in 2 of the largest trials assessing the role of aspirin on primary prevention of cardiovascular events in patients with type 2 diabetes, low-dose aspirin did not decrease the risk of cardiovascular events when compared to placebo. Insulin resistance, TNF-alfa and IL-6 are shown to affect platelet reactivity. The clinical determinants that help identify aspirin resistance in diabetes are suggested to be CVD, microalbuminuria, poor diabetes control, and increased waist circumference.(Yassine, Davis-Gorman et al. 2010)
Platelet abnormalities in Type 2 diabetes: The abnormalities described in patients with diabetes are listed here.
Increase in platelet-dependent thrombin generation.
Increased expression of platelet surface adhesion molecules such as CD31, CD49b, CD62P, and CD63, leading to increased platelet activation.
Increased platelet surface receptors such as P-selectin, GP Ib, and GP IIb/IIIa. (Gresele, Guglielmini et al. 2003)
Reduced vascular synthesis of the anti-aggregants PGI2 and NO, shifting balance towards aggregation and vasoconstriction.
Decreased platelet insulin receptor number and affinity and failure to reduce platelet responses to the agonists ADP, collagen, thrombin, arachidonate, and PAF.
Glycation of circulating LDL rendering platelets hypersensitive. Glycated LDL causes an increase in intracellular calcium concentration and platelet NO production, as well as inhibition of the platelet membrane Na+/K+-ATPase activity.
Key: GP = glycoprotein; PGI2 = prostacyclin; NO = nitric oxide; ADP = adenosine diphosphate; PAF = platelet-activating factor; LDL= low-density lipoprotein; Na+/K+-ATPase = Na+/K+-adenosine triphosphatase
An accelerated platelet turnover represented by the presence of a higher number of reticulated platelets has been observed in patients with DM.(Guthikonda, Alviar et al. 2008) The dysfunctional status of platelets in patients with DM may contribute to the enhanced atherothrombotic risk of these patients. Platelets obtained from diabetic patients show increased adhesiveness, hyperfunction both spontaneous as well as in response to agonists. These observed hyperfunctions are attributed to increased expression, activation or abundance of surface membrane receptors for agonists as well as cell matrix components, increased binding of fibrinogen, altered membrane fluidity, changes in activation mechanisms and signaling pathways. Changes in platelets in diabetes include enhanced GP receptor binding of agonists and adhesive proteins; decreased membrane fluidity; enhanced activation of the arachidonic acid pathway resulting in increased TxA2 formation; altered PI turnover leading to changes in diacylglycerol and inositol triphosphate production, calcium mobilization, and protein phosphorylation; impaired responses to antiaggregants resulting in decreased PGI2 receptor binding, cyclic nucleotide production and cyclic nucleotide–dependent protein phosphorylation; and reduced sensitivity to the inhibitory actions of insulin. These changes translate to impaired PGI2 stimulation of cAMP and blindness to the inhibitory actions of both PGI2 and NO. Platelet dysfunction coupled with decreased endothelial production of these antiaggregatory agents conspire to amplify the risk of CVD in patients with type 2 diabetes.(Vinik, Erbas et al. 2001)
Metabolic control and platelet reactivity
In the early 1960s Bridges
Insulin and platelet reactivity
The T2DM patients had platelet aggregation and shear-induced platelet function significantly increased compared to nondiabetic patients using all assays. Platelet aggregation was increased in ITDM (n = 68) compared with NITDM (n = 133) patients after P2Y12-specific stimuli. Insulin treatment was the strongest predictor of ADP-induced aggregation. Platelet function profiles were similar between ITDM and NITDM using assays non-specific to the P2Y12 pathway. Platelet dysfunction was independent of glycemic control and inflammatory status.(Angiolillo, Bernardo et al. 2006)
NF-kB is a transcription factor that stimulates numerous genes and activates inflammatory responses related to insulin resistance. Salicylates inhibit NFkB activation. This inhibition was shown to be associated with a significant decrease in IL-6 and TNF-α release, mediated through inhibition of IKKβ activity.
Platelet activity measures
A major urinary metabolite of thromboxane A2 synthesized from extra renal sources is 11-dehydro thromboxane B2. A major portion of this metabolite is believed to come from the platelet, but there are additional cellular sources. In the Heart Outcomes Prevention Evaluation (HOPE) trial, patients whose urinary 11-dehydro thromboxane B2 levels were in the highest quartile had an odds ratio of 2 for having a myocardial infarction and an odds ratio of 3.5 for a risk of having a cardiovascular-related death compared to those patients in the lowest quartile. Serum aspirin esterase (AE) activity may account for part of aspirin pharmacokinetics and has been proposed as one source of variation in aspirin effectiveness.(Adebayo, Williams et al. 2007)Elevated MPV values are associated with a shortened bleeding time and increased thromboxane B2 plasma levels. Thus, MPV could be considered an indicator of platelet function.(Vizioli, Muscari et al. 2009)Diabetic patients with coronary heart disease have significantly higher MPV values compared to control patients.(Tavil, Sen et al. 2010)
Methods that directly measure the capacity of platelets to synthesize TxA2 are certainly preferable. Of these, the urinary levels of the TxB2 metabolite, 11-dehydrothromboxane B2, represent a time-integrated index of TxA2 biosynthesis
In aspirin-treated patients, elevated urinary 11-dehydro thromboxane B2levels identify patients who are relatively resistant to aspirinand who may benefit from additional antiplatelet therapies ortreatments that more effectively block in vivo thromboxane productionor activity.(Eikelboom, Hirsh et al. 2002)
The clinical implication of aspirin resistance as measured
Factors that need to be considered in the approach to patients with suspected treatment failure include: compliance with aspirin use, ensure the optimal dose and drug form (avoid use of enteric-coated aspirin formulations), evaluate concomitant infections or inflammatory conditions, and assess possible drug–drug interactions. Several approaches have been evaluated for treatment failure and some of these approaches are based on laboratory testing for evidence of resistance. The role of testing in directing management is still controversial. Management strategies are currently limited to dosing alteration and introduction of other anti platelet agents. However, these measures have not met the expected efficacy or safety.
Another recommendation is that because saturated fat ingestion increases
Aspirin is recommended for primary prevention and secondary prevention of CVD in patients with type 2 diabetes. Aspirin resistance is common in patients with type 2 diabetes. Further studies are required to answer the question, will improving glycemic control in patients with poor glycemic control cause a change in responsiveness to aspirin? The relationship of the adipokines TNF-alfa and IL-6 to aspirin non-responsiveness needs further evaluation. Prospective randomized trials are needed to prove the clinical benefits of adapting the dosing of clopidogrel or switching to alternative compounds in high-risk patients with impaired antiplatelet effectiveness according to the result of platelet function assays.