Defining platelet response to acetylsalicylic acid: the relation between inhibition of serum thromboxane B and agonist-induced platelet aggregation.

Arachidonic acid (AA)-induced platelet aggregation (PA) and serum thromboxane B (TxB) inhibition are widely used to indicate cyclooxygenase-1 activity and the antiplatelet effect of acetylsalicylic acid (ASA). Despite decades of investigations, the relation between these measurements remains unclear. We sought to evaluate the relation between AA-PA and serum TxB inhibition. We serially measured AA-PA (conventional aggregation), serum TxB plasma ASA and salicylic acid (SA) (liquid chromatography-mass spectrometry), and urinary 11-dehydro thromboxane B (u11-dh TxB) (enzyme-linked immunosorbent assay) levels at 10 times over 24 hours in seventeen healthy volunteers receiving a single dose of 162 mg chewed and swallowed ASA (n = 6), 50 mg inhaled ASA (n = 6), or 100 mg inhaled ASA (n = 5) (ClinicalTrials.gov Identifier: NCT04328883, April 1, 2020). Baseline variability was more pronounced with serum TxB (31-680 ng/mL) as compared to maximal AA-PA (65-81%) and u11-dh TxB (1556-4440 pg/mg creatinine). The relation between serum TxB inhibition and AA-PA was stepwise; after 30-40% inhibition of serum TxB, AA-PA fell to < 5%. By receiver operating characteristic curve analysis using AA-PA < 5% to define aspirin responsiveness, serum TxB2 inhibition > 49% and u11-dh TxB2 < 1520 pg/mg creatinine met the definition. Our study demonstrates a non-linear relation between serum TxB inhibition and AA-PA. Aggregation was nil once TxB inhibition reached > 49%. Moreover, these results suggest that the definition of > 95% inhibition of serum TxB to indicate the level of platelet COX-1 inhibition needed for clinical efficacy may be overestimated and should be re-considered in future translational research investigations that attempt to link the clinical efficacy of ASA with a laboratory measurement cutoff.