Abstract 13963: Redefining the Assessment of “Therapeutic” Platelet Response to Acetylsalicylic Acid

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Background: Despite extensive investigations, the optimal assessment of the “therapeutic response” to acetylsalicylic acid (ASA) remains unclear. Limited information is available on the relation between serum thromboxane (Tx)B 2 ; arachidonic acid (AA)-induced platelet aggregation (PA) by light transmittance aggregometry (LTA); aspirin reaction units (ARU) by VerifyNow aspirin test (VN), and pharmacokinetics (PK). Methods: Serial PD and PK analyses were performed immediately after a single 162 or 650 mg dose of chewed and swallowed ASA in ten healthy volunteers. ASA response was defined as >95% inhibition of serum TxB 2, <550 ARU by VN test and ≤20% AA-induced PA. Correlation analyses between PK and PD measurements and receiver operating characteristic (ROC) curve analyses were performed. Results: ASA Response measured by VN, and AA-induced PA was achieved within 30 minutes of ASA administration. A good correlation was observed between VN ARU and 1 mM AA-induced maximum PA (r=0.80, p<0.001), serum TxB 2 (r=0.76, p<0.001), and inhibition of serum TxB 2 (r=0.87, p<0.001). The relation between platelet aggregation and TxB 2 inhibition appears to be dichotomous, with only modest inhibition of TxB 2 (~50% inhibition) associated with marked inhibition of platelet aggregation. ROC curve analyses indicated that ≤558 ARU and ≤7% AA-induced PA were associated with >95% inhibition of TxB 2 . A plasma ASA 686 ng/mL cut-off was associated with >95% inhibition of serum TxB 2 , ≤7% 1 mM AA-induced PA by LTA, and ≤585 ARU. Conclusion: Our study demonstrated an important relationship between PD and PK parameters measured immediately following oral ASA and cutoff values for VN assay and LTA that is associated with >95% inhibition of serum TxB 2 . We demonstrated that ~50% inhibition of TxB 2 is required to reach high levels of platelet function inhibition.