The complications of atherosclerotic disease comprise the primary cause of mortality in the world today. Acute complications such as sudden cardiac death, myocardial infarction and stroke are typically due to the formation of a intra-arterial thrombus at the site of a rupture or erosion of an atherosclerotic lesion, with platelet activation and aggregation central to thrombus formation. Aspirin, first shown to have platelet-inhibitory properties over 40 years ago, has subsequently become the mainstay of preventative therapy in cardiovascular disease and is estimated to be used for this purpose by 20 to 30 million Americans every day.
The clinical benefit of aspirin in the acute and chronic treatment of patients with all manifestations of atherosclerotic disease has been convincingly demonstrated through the results of multiple placebo-controlled trials, involving literally hundreds of thousands of patients. A recent meta-analysis of all placebo-controlled aspirin trials, involving over 125,000 patients, found a 3% absolute reduction (RR = 22%) in ischemic events (cardiovascular death, myocardial infarction (MI) or stroke) associated with aspirin therapy. Additionally, in the community setting, retrospective analyses suggest that the widespread use of aspirin has been associated with a reduction in mortality among patients hospitalized with acute coronary syndrome (ACS). In other analyses it also appears that aspirin therapy is related to a therapeutic 'frame-shift' in which aspirin not only reduces the likelihood of experiencing an ischemic event, but also reduces the severity of breakthrough events.
The clinical benefits of aspirin are very likely related to its ability to inhibit platelet-derived prostaglandin G/H synthase, or cyclooxygenase 1 (COX-1), by acetylating the enzyme preventing substrate binding. COX-1 is central to the formation of ecosanoids, the first enzyme in the pathway that converts arachadonic acid into prostaglandin G and H and later thromboxane A2 (TXA2). TXA2 is a vasoconstrictor known to cause platelet aggregation. Aspirin is rapidly absorbed from the stomach and has a half-life of five to 15 minutes in the circulation. The interaction between platelets and aspirin occurs quickly within the portal circulation and a single dose of aspirin at 325mg will almost completely suppress thromboxane production within 15 to 30 minutes. Given the fact that platelets are anucleic, the effects of aspirin last for the approximately 10-day lifetime of the platelet. After a single dose of aspirin, COX activity is restored at the rate of normal platelet turnover, approximately 10% per day. Some studies have suggested that normal hemostasis can return when approximately 20% of the platelets have uninhibited COX activity.
Additional COX enzymes have been characterized.The gene for the COX-2 enzyme is typically not expressed constituitively but is very sensitive to induction by inflammatory stimuli and growth factors. Aspirin is a approximately 170 times less potent an inhibitor of the COX-2 enzyme. Until recently COX-2 activity had not been noted in platelets.Weber et al. have, however, discovered small amounts of COX-2 in human platelets perhaps originating in megakaryocytes where COX-2 activity functions as a regulator of hematopoesis. In situations with high platelet turnover this expression of COX-2 activity may be clinically relevant and there has been debate about the clinical importance of COX-2 inhibition in the setting of atherosclerosis.
Response to Aspirin is not Uniform
Emerging as an area of research and further debate with respect to chronic aspirin therapy is the concept of aspirin resistance. Aspirin resistance has been variably defined; in some instances including all patients who have experienced an ischemic event while taking aspirin (perhaps better referred to as treatment failures) and more often as those patients in which aspirin therapy fails to achieve an arbitrarily defined reduction in the measured level of platelet activation or aggregation. The response to aspirin has been characterized in a variety of ways including measurement of urinary metabolites of thromboxane, flow cytometric determination of platelet membrane molecule expression, or, most commonly, by employing one of multiple ex vivo platelet aggregation assays. It has been difficult to demonstrate the clinical relevance of the various assays of platelet function convincingly and no test has yet to emerge as a clear gold standard for identifying patients resistant to aspirin.
It is not surprising, therefore, that the reported incidence of aspirin resistance varies widely, from as low as 5% to as high as 75%, despite most studies involving relatively similar, stable populations.
The first study to demonstrate inter-patient variability in response to aspirin was published nearly 50 years ago. Since then, numerous trials evaluating responsiveness to aspirin, in a variety of different settings, have been undertaken. No trial to date has ever found a uniform response to aspirin despite using a wide range of techniques, including platelet aggregability, classical platelet aggregometry, flow cytometric analyses of markers of platelet activation, and bleeding time (see Table 1). More recently, several point-of-care assays of platelet function have been developed in order to overcome the limitations of the more technically demanding, laboratory-based assays initially utilized. These too have confirmed marked variability among all patients in response to aspirin.
The critically important question of whether a patient's responsiveness to aspirin might change in relation to clinical status or over time is not clear. Whereas some studies have found significant variability in the response to aspirin over time, the results are not uniform. In one study of over 300 patients with a history of ischemic stroke treated with between 325mg and 1,300mg of aspirin, complete inhibition was achieved in only 151 and this effect was durable over six months in only 104 (70%). In a more recent study during 24 months of aspirin treatment, patients exhibited a gradual decline in the level of inhibition achieved, and by the end of the study period the effects of aspirin appeared negligible.
Evidence of the Clinical Importance of Aspirin Resistance
There is a growing body of literature that establishes a link between the variability in response to aspirin and adverse clinical events. Grotemeyer et al., in a study of 180 patients admitted with stroke, used an in vitro measurement of platelet reactivity performed 12 hours following the administration of 500mg of aspirin and found that 33% of the patients had platelets that were reactive despite aspirin therapy. Patients were discharged on 500mg of aspirin three times daily. At two years of follow-up, patients who were initially non-responsive were more likely to suffer an adverse event including death, recurrent stroke and MI (4.4% versus 44%; p=0.0001). Mueller et al. evaluated the response to aspirin using whole blood platelet aggregometry in 100 patients with intermittent claudication treated with 100mg/day of aspirin. Again, these authors found that there was considerable variation in the response to aspirin over the course of year during which platelet aggregometry was performed four times. More importantly, patients found not fully responding to aspirin therapy had an increase in adverse events as manifested by reocclusion of vascular grafts.
Eikelboom et al. evaluated a subset of patients enrolled in the Heart Outcomes Prevention Evaluation (HOPE) trial for aspirin responsiveness by measuring levels of urinary 11-dehydro thromboxane B2, a stable urinary metabolite of TXA2. High urinary concentration among chronic aspirin users is thought to denote a lack of response to aspirin arising from either incomplete inhibition of platelet TXA2 production or from TXA2 production from sources other than platelets. In this case-control study baseline 11 dehydro thromboxane B2 levels in 488 patients who suffered an adverse event during the HOPE trial (defined as myocardial infarction, stroke or death) were compared with those of age and gender-matched event-free controls. They found that when compared to controls, patients suffering an adverse event had significantly elevated urinary levels of 11 dehyro thromboxane B2 and that increasing quartiles of urinary thromboxane metabolite were associated with increasing risk.
A more recent report supports these earlier studies but also points out the pitfalls associated with ex vivo measurements of platelet function. Gum et al. evaluated 325 patients with stable coronary disease treated chronically with 325mg of aspirin and found incomplete inhibition of platelet function by classical platelet aggregometry in 5.5% of the patients. An additional 23.8% were found to be semi-responders. These patients were also evaluated using a point-of-care platelet function analyzer (PFA-100, Dade Behring, Deerfield IL), which simulates primary hemostasis in whole blood samples. With this device, 9.5% of patients were found to be resistant to the effects of aspirin. There was no correlation between the methods as only 1.2% of the patients were found to be resistant to aspirin by both methods. Subsequently, the results of over two years of follow-up indicated that patients initially found to be resistant to aspirin by platelet aggregometry were at an increased risk of death, MI, or stroke. However, there was no relationship between aspirin resistance as measured by the PFA-100 and subsequent adverse events (12.9% aspirin sensitive, 15.1% aspirin resistant, P = 0.4).
A recent study utilizing a novel point-of-care device, the Ultegra Rapid Platelet Function Analyzer (Accumetrics Inc. San Diego CA), determined aspirin responsiveness among 151 patients scheduled for non-urgent percutaneous coronary intervention. Patients who were resistant to aspirin were much more likely to suffer post-procedure elevations in serum creatine kinase myocardial band levels than were patients responsive to aspirin (51.7% versus 24.6%; p = 0.006). This study is particularly interesting in light of the fact that these events occurred in the setting of dual antiplatelet therapy as all of the patients were pre-treated with clopidogrel >12 hours prior to their procedure.
Numerous mechanisms have been proposed to explain why some patients do not respond to aspirin and the subject has been intensively reviewed elsewhere. It is likely that more potential explanations will be discovered and that more than one mechanism will be operable in a given patient. As yet, no large clinical trial has been designed specifically to correlate clinical events and laboratory findings with respect to aspirin response.Given the lack of a clear ex vivo 'gold standard' assay for platelet activity, this trial would likely need to link clinical events with multiple assessments of platelet biochemistry and function. Until such a study is completed it will remain difficult to determine if the breakthrough events experienced by patients treated with aspirin represent definitive 'resistance' to aspirin or are related to more treatable issues such as aspirin dose, medication interactions, or medical non-compliance.