Relation between inhibition of platelet aggregation and clinical outcomes.
Despite a clinical trials experience in excess of 30,000 randomized patients, there is no definitive answer or agreement as to the relation between the level of inhibition of platelet aggregation and clinical outcome. Work with abciximab in the setting of percutaneous intervention would suggest that a level of platelet inhibition in excess of 80% provides the most favorable clinical benefit. Whether this applies to the small molecule inhibitors such as eptifibatide, tirofiban, and lamifiban is unknown and will require testing of the hypothesis in large-scale clinical trials. In the acute coronary syndromes, only the peptide and nonpeptide inhibitors have been studied independent of a need for percutaneous intervention. The level of platelet inhibition required to achieve maximal clinical benefit is unknown and awaits data from the large-scale unstable angina and myocardial infarction trials. Issues of combining heparin with GP IIb/IIIa inhibition and the platelet-altering effects of thrombolysis must be considered when examining what might be the optimal level of platelet inhibition that provides maximal efficacy while maintaining safety. Long-term therapy with the oral platelet inhibitors poses a particular challenge in determining an optimal level of treatment that renders clinical effectiveness while preserving safety. Large, ongoing clinical trials with these agents should provide insight into this question. Finally, given the complexity of using such potent therapies in any individual patient, there is a need for improved monitoring of the platelet inhibitory effect. Standard platelet aggregometry has its problems and limitations, as described, and is not a test that has wide-scale applicability. Whole blood aggregometry and other methods of rapidly assessing platelet function at the point of care have promise in this arena. In a clinical trial, such technology might allow better delineation of the population's pharmacodynamic response to an agent because it could be more widely applied and therefore more patients would be studied. In clinical practice, patients could be dosed individually so that a range of platelet inhibition could be achieved that was believed to provide optimal benefit and safety.
Harrington, RA; Kleiman, NS; Granger, CB; Ohman, EM; Berkowitz, SD
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