Abstract P066: Purinergic Receptor Function In The Middle Cerebral Artery From Clopidogrel-treated Rabbits

Arterial thrombosis is the leading cause of death in the developed world and hypertensive patients are at an increased risk. Dual antiplatelet therapy, consisting of low-dose aspirin in combination with a P2Y 12 antagonist, is the most common preventative treatment for arterial thrombosis. Clopidogrel, a P2Y 12 antagonist, is associated with cerebral microbleeds and intracerebral hemorrhages. We have previously reported in rabbits that clopidogrel (3mg/kg) induces bleeding at lower doses than required to inhibit platelet aggregation, making it unlikely that the clopidogrel-mediated bleeding events are the result of platelet inhibition. I hypothesized that clopidogrel was acting on other purinergic receptors to impair artery function due to the structural similarity of the metabolites of clopidogrel.

Purinergic receptors, P2Y 1 , P2Y 2 , P2Y 4 and P2Y 6 , are expressed in the endothelium of cerebral vessels. The purpose of this study was to determine the effects of clopidogrel on vascular function in response to activation of P2Y 1 , P2Y 2 , P2Y 4 and P2Y 6 . The middle cerebral artery (MCA) was used as it is the site most common for cerebrovascular events. New Zealand white rabbits (n=3) were treated for 3 days with vehicle or clopidogrel (3mg/kg/day) prior to MCA isolation. Myogenic tone was allowed to develop before assessing the response of the MCA to increasing concentrations (10 -9 -10 -5 M) of specific purinergic receptor agonists P2Y 1 : MRS 2365; P2Y 2 : MRS 2768; P2Y 4 : MRS 4062; P2Y 6 : MRS 2693. If myogenic tone was not generated the MCA was preconstricted with 10 -6 M serotonin. Response was analyzed by two-way ANOVA and compared to vehicle using Dunnett’s post-test. In all cases, p < 0.05 denote statistical significance. Constriction was calculated as a percent change from baseline. Stimulation with the P2Y 4 agonist resulted in a mean maximal constriction of 28.95% (28.95 of 100) for vehicle and 52.72% (52.72 of 100) for 3mg/kg clopidogrel. There was no significant difference upon stimulation with the P2Y 1 , P2Y 2 , or P2Y 6 agonists compared to vehicle. These data suggest that clopidogrel has vascular effects on the P2Y 4 receptor expressed in the MCA and may explain the cerebral bleeding side effects observed.

DT-678 inhibits platelet activation with lower tendency for bleeding compared to existing P2Y12 antagonists

The novel clopidogrel conjugate, DT-678, is an effective inhibitor of platelets and thrombosis in preclinical studies. However, a comparison of the bleeding risk with DT-678 and currently approved P2Y12 antagonists has yet to be determined. The objective of this study was to evaluate the bleeding tendency of animals treated with clopidogrel, ticagrelor, and DT-678. Ninety-one New Zealand white rabbits were randomized to one of 13 treatment groups (n = 7). Platelet activation was assessed by flow cytometry and light transmission aggregometry before and after the administration of various doses of DT-678, clopidogrel, and ticagrelor. Tongue template bleeding times were also measured before and after drug treatment. Treatment with P2Y12 receptor antagonists caused a dose-dependent reduction in markers of platelet activation (P-selectin and integrin αIIbβ3) and aggregation in response to adenosine diphosphate stimulation. At the same doses required for platelet inhibition, clopidogrel and ticagrelor significantly prolonged bleeding times, while DT-678 did not. DT-678 and the FDA-approved P2Y12 antagonists clopidogrel and ticagrelor are effective inhibitors of platelet activation and aggregation. However, unlike clopidogrel and ticagrelor, DT-678 did not prolong bleeding times at equally effective antiplatelet doses. The results suggest a more favorable benefit/risk ratio for DT-678 and potential utility as part of a dual antiplatelet therapy regimen.