Platelet-Aggregation Inhibitory Activity of Oleanolic Acid, Ursolic Acid, Betulinic Acid, and Maslinic Acid

Abstract

Platelet aggregation is the process by which platelets adhere to each other at sites of vascular injury. This process has long been recognized as critical for hemostatic plug formation and thrombosis. Until relatively recently, platelet aggregation was considered a straightforward process involving the non-covalent bridging of integrin I±IIbI²3 receptors on the platelet surface by the dimeric adhesive protein fibrinogen. Naturally occurring triterpenes are endowed with a broad range of useful pharmacological properties. In our search for new and potent ethnopharmaceuticals, oleanolic acid (OA) (1), ursolic acid (UA) (2) betulinic acid (BA) (3), and maslinic acid (MA) (4) isolated from Syzygium aromaticum, Eucalyptus grandis, Callistemon viminalis, and Syzygium aromaticum, respectively, were evaluated in vitro for anti-platelet aggregation activity on thrombin, adenosine diphosphate (ADP), and epinephrine-induced rat platelet aggregation. The triterpenes exhibited a dose dependent inhibitory activity on platelet aggregation induced by the three platelet agonists. The compounds exhibited more potency mostly at the highest concentration (10.0 mg/ml) except for UA (2) on thrombin-induced platelet aggregation. The percentage inhibitory activity of UA (2) on throbine-induced platelet aggregation was found to decrease with increase in concentration (86.8±1.23, 53.0±0.43, 46.2±0.23) 1-10.0 mg/ml, this invariably suggests an optimal concentration (≤ 1.0 mg/ml). The IC50s of the compounds are remarkably better than that of heparin (IC50 of 2.80mg/ml). The highest activity by OA (1) (IC50 of 0.84 mg/ml) and mixture of BA/OA (IC50 of 2.61 mg/ml) was observed on thrombin-induced platelet aggregation. BA/OA (IC50 of 2.57 mg/ml) also showed a significant platelet aggregation inhibitory activity on epinephrine-induced platelet aggregation. These preliminary findings show that these pentacyclic triterpenoic acids possess pharmacological activity and could be potential templates for development of new anti-platelet agents.