Anti-platelet Drug Resistance: New Independent Risk Factor?
Gundu H. R. Rao, PhD, Professor, Laboratory Medicine and Pathology, Lillehei Heart Institute, MMC 609, University of Minnesota, Minneapolis, Minnesota 55455 USA.
Take-home Message:
Large number of clinical trials has demonstrated that low to medium dose of aspirin (80-160mg) significantly reduces the risk of developing acute vascular events (1). Aspirin is the most cost-effective drug available in the market, for primary as well as secondary prophylaxis of vascular diseases (2). Millions of people take aspirin daily all over the world. In our studies over three decades we have not come across any healthy adult volunteer whose platelets were resistant to the action of aspirin. Based on the available data, it is difficult to explain the mechanism by which individuals develop resistance to the action of aspirin. Published results suggest that this phenomenon occurs mostly in patients with vascular disease and in those who are on long-term aspirin prophylactic therapy (3-15). Therefore it is essential to identify people who are non-responders to the action of aspirin, so that appropriate alternate prophylactic therapy could be administered. Difference in the definition of aspirin resistance, variations in the methods used to detect “non-responders”, intra-individual variations in the response of platelets to the action of different agonists, and lack of data from large clinical trials have hampered the advancement of knowledge in this area (13). There is a great need for the development of a simple, specific, rapid Point-of Care assay for monitoring aspirin therapy. Availability of a simple assay will facilitate better management of patients with vascular disease, whose platelets develop refractoriness to the action of aspirin (16).
Definition of the Problem:
Aspirin irreversibly inhibits platelet cyclooxygenase (Cox-1) enzymes and prevents the formation of pro-aggregatory prostaglandin (PG) endoperoxides and thromboxane (17). On the other hand, Clopidogrel, a newer anti-platelet drug, is converted to an active metabolite in vivo to a potent ADP receptor antagonist (18). Based on these facts, platelets of non-responders should generate PG endoperoxides and aggregate irreversibly in response to arachidonic acid stimulation. According to Cattaneo, “aspirin resistant” should be considered as a description for those individuals whom aspirin fails to inhibit platelet thromboxane A2 production, irrespective of the results of unspecific tests of platelet function (19). Platelets of patients (non-responders) who have ingested Clopidogrel, should respond with aggregation when stimulated with ADP as agonist. Aspirin and Clopidogrel prevent just two specific platelet activation pathways(18, 19). Blood platelets interact with a variety of soluble agonists such as epinephrine, ADP, thrombin, thromboxane and platelet activating factor (PAF); many cell matrix components including collagen, laminin fibronectin, and vonWillebrand factor (20). Therefore, no single anti-platelet drug offers full protection to the patients from developing risk for acute vascular events. None of the currently available platelet function testing methods is specific for monitoring aspirin resistance. Researchers have used a variety of assays to monitor aspirin resistance in patient populations (11, 16). Methods used to monitor platelet function include, platelet aggregometry, platelet aggregate ratio, platelet function analyzer (PFA 100), Accumetrics (RPFA-ASA); plasma, serum, urinary metabolite of thromboxanes and 11 dehydrothromboxane -2. Majority of these assays are labor intensive, time consuming, non-specific and expensive.
Importance of the Problem:
In spite of the concern and confusion created by this new finding, some recent studies provide evidence to show that non-responders to anti-platelet drugs have higher risk of developing acute vascular events compared to the risk of responders (3-16, 18, 19). These observations point out the fact that a large number of patients may be taking unnecessary risk, in the absence of a point-of-care assay to monitor the platelet sensitivity or lack of it, to the action of anti-platelet drugs. Indeed according to Dr, Michael Domanski, head of the clinical trials unit at the NIH, the non-responders may represent a huge number of patients (21). Therefore, identifying “non-responders” who are at higher risk is very important, so that they can be provided alternate therapies. It is also equally important to determine people with higher risk even in the population defined as “responders”. Because, aspirin used alone, does not offer full protection against developing risk for acute vascular events, especially in those, whose platelet/coagulation profiles are hypersensitive. Aspirin is used extensively for primary as well as secondary prophylaxis of vascular disease. Analysis of data from large number of clinical studies has demonstrated that low to medium dose of aspirin (80-160 mg) significantly reduces the risk of developing acute vascular events (1). If subjects taking aspirin for primary prophylaxis over the years develop resistance to the action of aspirin, it is important for them to recognize this condition, and seek alternate therapies. In addition studies in our laboratory have demonstrated that certain heme chelators and non-steroidal anti-inflammatory drugs antagonize the action of aspirin (22-24). There is a great need for education of the public as well as health care providers about some of these problems. This newly observed phenomenon of drug resistance is of significant public health importance and needs immediate attention. There is a great need for the development of a simple, specific point-of-care assay for monitoring platelet function. Large clinical trials are essential to establish the prevalence of anti-platelet drug resistance in normal population as well as patient population. There is also a need to understand the mechanism by which drug resistance develops in platelets of patients with vascular diseases.
Nature of the Scientific Advance:
Currently there are three major risk factors for the pathogenesis of atherosclerosis, thrombosis and stroke. They are smoking, blood lipid abnormalities, and altered pro-thrombotic state. Smoking is preventable by creating awareness of the problem and by altered life style. There is considerable progress in the treatment of lipid abnormalities by statins. When it comes to the management of pro-thrombotic state, we still are far behind and need further advances in research. There is couple of newer anti-platelet drugs available in the market (18). They are Clopidogrel, and GP11b/111a inhibitors. Story is similar when it comes to choice of drugs available for anti-coagulant therapy. Heparins and warfarin are the drugs of choice. We still do not have an effective anti-platelet drug for primary and secondary prophylaxis of vascular disease. There is considerable room for the development on newer anti-platelet drugs. Since large number of clinical studies has demonstrated aspirin to be an effective anti-platelet drug, there was less incentive for the drug companies to concentrate on this health problem. Recent findings that platelets of patients with vascular disease develop resistance, if proven right, will renew the interest of the drug industry, to do some research and development in this area (3-16).
Both aspirin and Clopidogrel have been shown to be effective anti-platelet drugs. However, aspirin, Clopidogrel or their combination, does not offer full protection against developing risk for acute vascular events. These drugs inhibit only one of the many platelet activation mechanisms. Studies from our laboratory have described a mechanism called “membrane modulation”, which restores the sensitivity of drug-mediated refractory platelets to the action of agonists (25-28). We demonstrated that alpha adrenergic receptor stimulation with epinephrine or nor-epinephrine followed by arachidonic acid, causes irreversible aggregation of aspirin treated platelets (29). Therefore, it is essential to understand that currently available anti-platelet drugs offer minimum protection against the risk of developing acute vascular events especially under stressful situations. Furthermore, we need to develop a battery of assays capable of monitoring people at risk, whether it is for primary or secondary prophylaxis of vascular disease.
Various methods have been developed to monitor platelet function including optical methods like platelet aggregometry, platelet adhesion assays, platelet aggregate ratio, platelet function analyzer (PFA 100), Accumetrics system etc (11, 16, 30, 31 ). However, majority of these methods are labor intensive, time consuming, non-specific and expensive. There is a need for the development of simple specific rapid assay for monitoring platelet function. Future advances in this area will provide assays that can monitor activation of platelets as well as coagulation cascade. These methodologies will build appropriate stimulatory mechanisms to improve the specificity of the assays. For instance, inclusion of arachidonic acid as a stimulant instead of propyl gallate in the Accumetrics System would provide specificity to monitor aspirin resistance. In future assays, stronger agonists such as collagen, PAF and Thrombin also may be included to monitor patients at risk for developing acute vascular events, even in so called responders. Future researchers will develop a battery of assays built into an automated rapid detection system capable of monitoring platelet aggregation, secretion, expression of activation markers, and clot formation in response to weak agonists, strong agonists and shear stress. Results of such point-of-care detection systems will provide the clinician the needed information on drug resistance, hyperactive state of platelets, as well as coagulation profile.
Importance of the Advance:
At present no point-of-care assays are available to monitor platelet function at a clinical laboratory setting, acute care facility or at doctor’s office (32). Availability of such a detection system, which provides the information on the sensitivity or lack of it, to the action of inhibitory drugs, will be very valuable. For instance a hand-held monitor, which can provide results on arachidonic acid stimulated activation of platelets in less than five minutes from a drop of blood obtained from finger stick, will be very valuable at any doctor’s office or even at home, for self monitoring of aspirin therapy for primary prophylaxis. Similar detection system in a cardiac “cath” laboratory will be useful to monitor platelet function during the interventional procedures, so that the dose of GP 11b/111a antagonists could be adjusted appropriately and prevent life threatening bleeding episodes. A system capable of monitoring both hyperactive states of platelets and coagulation profiles will be of great help, to screen individuals at risk for heart attack and stroke. People at risk could be administered minimal anti-platelet and anti-coagulant therapies for primary prophylaxis.
Expected Future Directions:
Now that anti-platelet drug resistance has surfaced out and created concern in the clinical community, there will be renewed interest in the devise development industry, for improving the available technologies for monitoring platelet and coagulation pathways (31-16, 32-36). Chronolog Corporation, one of the leaders in developing optical aggregometers, has already come up with a less expensive whole-blood aggregometer for monitoring aspirin resistance. Accumetrics System has introduced recently arachidonic acid as a stimulant instead of propyl gallate, for monitoring aspirin resistance. Similarly research and development work is going to improve PAF 100 as well, for monitoring aspirin resistance. Currently available data on the use of PFA 100 to monitor aspirin resistance is inconclusive. A large study with 1000 patients is in progress. Results from this study may shed some light on the usefulness of this detection system (30). All these methodologies require relatively large amounts of fresh blood. Platelet function assay is labor intensive, time consuming and expensive. Future directions will focus on the development of rapid, specific point-of-care assays using minimal amounts of blood. Studies at the University of Minnesota have focused on the development of devises that need only a small drop of blood from a finger stick, for monitoring platelet function or coagulation profile. These assay systems will incorporate appropriate stimulants to build needed specificity of the reactions monitored. Work is in progress to incorporate weak agonists, stronger agonists as well as shear stress, as stimulants for achieving platelet activation, to monitor populations at risk for developing acute vascular events.
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