Protocatechualdehyde synergizes with aspirin at the platelet cyclooxygenase-1 level.

Polyphenol-aspirin interactions were recently identified; however, the interaction mode and underlying mechanisms remained elusive. Here, we quantitatively assessed the potential interactions among two important polyphenolic compounds, caffeic acid (CA) and protocatechualdehyde (Pro), and aspirin in the AA-induced platelet aggregation model by applying the isobologram and universal response surface approach (URSA) methods. A molecular docking approach and an originally developed platelet-associated aspirin clearance approach (PAACA) were then applied to explore the potential interaction mechanisms. Although Pro and CA themselves exhibited weak inhibitory effect on arachidonic acid (AA)-induced platelet aggregation and the production of thromboxane B₂ (TXB₂), both Pro and CA potentiated aspirin action in a synergistic manner. The most prominent synergism was found between Pro and aspirin. Pro formed a stable complex into the cyclooxygenase-1 (COX-1) channel by in silico docking and significantly promoted the platelet-associated aspirin clearance, suggesting that the Pro interaction with COX-1 was favorable to the binding of aspirin with COX-1. Taken together, our findings suggest that the capacity of Pro and potentially other structurally similar polyphenolic compounds on promoting the binding of aspirin on platelet COX-1 might be the main mechanism of their synergism with aspirin.