A mixture of triterpenes named a-and b-amyrin (AMI), isolated from the Brazilian medicinal herb Protium hetaphyllum (Aubl) March (Burseraceae), was evaluated for the ability to inhibit aggregation of human platelets induced by adenosine 5 0 -diphosphate (ADP, 1.5 and 3 mM), collagen, and arachidonic acid (AA) in vitro. The results showed that AMI significantly inhibited platelet aggregation (40, 64, and 60%) in the assay carried out with ADP (3 mM) as agonist, at the doses of 100, 150, and 200 mM, respectively. In the presence of a lower ADP concentration (1.5 mM), a 3-time higher percentage of inhibition (32%) was observed with AMI 50 mM, as compared to that seen with 3.0 mM ADP. In the test using collagen (10 mg=mL) as agonist, AMI (50, 100, and 150 mM) inhibited aggregation by 26, 47, and 39%, respectively, while in the presence of the arachidonic acid (150 mM) at the doses of 50, 100, 150, and 200 mM, it inhibited platelet aggregation by 20, 21, 25, and 27%, respectively. The lowest IC 50 value for the AMI inhibitory effect was observed with collagen (90.0 mM), followed by ADP (117.9 mM) and arachidonic acid (181.4 mM). With ADP as agonist, the anti-aggregant effect of the acetylsalicylic acid (ASA) was potentiated by AMI but not by dipyridamole. No potentiation was observed after the combination of ASA and AMI with collagen or arachidonic acid as agonists. Our results indicated that AMI possesses a platelet anti-aggregant activity in a concentration-dependent manner and probably acts on a biochemical pathway common to all the agonists tested.
