TXA2 synthesis and COX1-independent platelet reactivity in aspirin-treated patients soon after acute cerebral stroke or transient ischaemic attack

Abstract Introduction The pharmacological target of aspirin is the inhibition of cyclooxygenase-1 (COX1) and thromboxane-A2 (TX) synthesis. Very few data are available on TX assessment in patients with stroke. We studied platelet TX synthesis, COX1-independent platelet reactivity, the influence of platelet–erythrocyte interactions and the potential association between platelet responses and the severity of stroke, evaluated with a clinical score (NIHSS). Material and Methods We examined 157 aspirin-treated patients with acute stroke or TIA, 128 aspirin-free and 15 aspirin-treated healthy subjects (HS). Collagen-induced TX, platelet recruitment in whole blood and platelets ± erythrocytes (haematocrit 40%) were assessed in patients on daily-aspirin within three days from onset. Arachidonic-acid-, ADP-, thrombin-receptor activating peptide TRAP-, and collagen-induced aggregation were also evaluated. Results Partial TX inhibition (< 95% inhibition vs aspirin-free controls) was observed in 13% of patients. This was associated with marked increases in COX1-dependent responses (arachidonic-acid- and collagen-induced aggregation and platelet recruitment; P < 0.0001) but not with differences in ADP- or TRAP-induced aggregation. Partial TX inhibition was independently associated with severe stroke (NIHSS ≥ 12) at both admission ( P < 0.05) and discharge ( P < 0.05). Among patients with fully blocked TX, those with elevated COX1-independent platelet reactivity (mean + 2SD of aspirin-treated HS) were most likely to suffer severe stroke ( P < 0.05). Platelet–erythrocyte interactions enhanced platelet reactivity in these patients by COX1-dependent and -independent mechanisms ( P < 0.0001). Conclusions TX inhibition by aspirin varied across patients. Partial TX inhibition and COX1-independent platelet hyperfunction were associated with more-severe stroke.