PTGS1 and neoplasm: Although aspirin is a potent inactivator of cyclooxygenase-1 (COX-1), thus an inhibitor of platelet function, its failure to demonstrate an anti-metastatic role in some studies may be due to the ability of the tumour to activate platelets efficiently without COX-1-dependent synthesis of thromboxane A2 (TxA2), a hormone responsible for promoting platelet activation and aggregation.