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Relationship of Arachidonic Acid Concentration to Cyclooxygenase-Dependent Human Platelet Aggregation

From the Division of Clinical Pharmacology, Department of Medicine, Thomas Jefferson University, Philadelphia, Pennsylvania (Ms. Burke, Dr. Kraft, Dr. Greenberg, Ms. Gleave, Dr. Pitari, Ms. VanBuren, Dr. Waldman) and Merck Research Laboratories, Blue Bell, Pennsylvania and Rahway, New Jersey (Dr. Wagner). Ms. Burke's current affiliation is Merck & Co., Inc., Blue Bell, Pennsylvania.

Inhibition of ex vivo arachidonic acid (AA)-induced aggregation is a biomarker for the isotype selectivity of cyclooxygenase (COX) inhibitors since platelets express COX-1 but not COX-2. At low concentrations, there is broad inter- and intrasubject variability in AA-induced aggregation of platelets ex vivo. This study defined a concentration that reliably induces aggregation without overcoming inhibition by therapeutic aspirin therapy (ASA, 81-mg) treatment. Logistic regression analysis of ex vivo aggregation, induced with increasing concentrations of AA in platelet-rich plasma (PRP), estimated that platelets from 90% of subjects would aggregate at 1.5 mM AA (95% confidence interval [CI], 1.1, 2.1). A concentration of 1.6 mM AA failed to aggregate platelets from 26 healthy volunteers, who had previously aggregated at this concentration, following six daily oral doses of 81 mg of ASA. These data demonstrate that 1.6 mM AA reproducibly induces platelet aggregation in PRP from healthy volunteers without overcoming the antiplatelet effect of daily low-dose aspirin therapy.

Key Words: Arachidonic acid • pharmacokinetics • pharmacodynamics • cyclooxygenase inhibitors • low-dose aspirin therapy • platelet aggregation

Address for reprints: Joanne Burke, Merck & Co., Inc., 5 West Sentry Parkway, BLB-33, Blue Bell, PA 19422.

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