Improving the therapeutic window

Antiplatelet therapies are commonly used to lower the risk of heart attack and stroke caused by the formation of blood clots containing aggregated platelets over ruptured or denuded atherosclerotic plaques (atherothrombosis). However, such therapies also impair normal platelet function, and this can cause prolonged or even fatal bleeding. Now, a ligand-based design strategy targeting a G protein-coupled receptor (GPCR) on platelets has been used to identify a novel antiplatelet agent that does not prolong bleeding and which might be used in combination with existing agents to provide more effective antiplatelet therapy.

Platelet aggregation is regulated by multiple GPCRs, ligands for which include thromboxane A₂ (TXA₂) and ADP. Currently, the most widely used antiplatelet therapies are aspirin, which inhibits the production of TXA₂, and clopidogrel, which antagonizes the P₂Y₁₂ ADP receptor. Another platelet GPCR — the EP₃ receptor for the inflammatory mediator prostaglandin E₂ (PGE₂) — has also recently been identified as having a key role in atherothrombosis by Fabre and colleagues (see Further reading). PGE₂ production is increased in inflamed atherosclerotic plaques, but the healthy arterial wall produces negligible amounts of PGE₂, suggesting that antagonizing the platelet EP₃ system might not have detrimental effects on normal platelet function, unlike aspirin and P₂Y₁₂ antagonists.

To investigate this possibility, Singh et al. used a ligand-based design strategy to synthesize EP₃ receptor antagonists. Following detailed structure–activity relationship studies, one compound (DG-041) was selected for further evaluation. DG-041 showed >1,000-fold selectivity for EP₃ receptors in radioligand displacement assays against other PGE₂ receptors, whereas it was inactive against a range of other GPCRs. Furthermore, it blocked the PGE₂-mediated inhibition of cAMP production and inhibited the platelet aggregation response in both rat and human plasma.

The authors then compared the in vivo effects of DG-041 on platelet function and bleeding time with those of the P₂Y₁₂ antagonist clopidogrel. DG-041 completely inhibited platelet aggregation with no concurrent increase in bleeding time (even at high doses), indicating that EP₃ blockade has a negligible effect on bleeding. Conversely, the minimum effective dose of clopidogrel required to inhibit platelet aggregation significantly increased bleeding time. Furthermore, PGE₂ restored the aggregation response, indicating that the efficacy of P₂Y₁₂ antagonists is limited in the context of inflamed plaque.

Finally, as initial clinical trials of DG-041 are likely to require co-administration with current antiplatelet drugs, the effects of combining EP₃ and P₂Y₁₂ antagonists were investigated. Adding DG-041 suppressed the PGE₂ facilitation of platelet aggregation seen with clopidogrel alone and did not increase bleeding time, suggesting that co-administration of EP₃ and P₂Y₁₂ antagonists could represent a superior antiplatelet therapy.