Comparative Pharmacokinetics and Pharmacodynamics of Platelet Adenosine Diphosphate Receptor Antagonists and their Clinical Implications

Over the last two decades or more, anti-platelet therapy has become established as a cornerstone in the treatment of patients with ischaemic cardiovascular disease, since such drugs effectively reduce arterial thrombotic events. The original agent used in this context was aspirin (acetylsalicylic acid) but, with the advent of adenosine diphosphate (ADP) receptor antagonists, the use of dual anti-platelet therapy has resulted in further improvement in cardiovascular outcomes when compared with aspirin alone. The first group of platelet ADP receptor antagonists to be developed was the thienopyridine class, which comprise inactive pro-drugs that require in vivo metabolism to their active metabolites before exerting their inhibitory effect on the P2Y₁₂ receptor. Clopidogrel has been the principal ADP receptor antagonist in use over the past decade, but is limited by variability in its in vivo inhibition of platelet aggregation (IPA). The pharmacokinetics of clopidogrel are unpredictable due to their vulnerability to multiple independent factors including genetic polymorphisms. Expression of the 3435T/T genetic variant encoding the MDR1 gene for the P-glycoprotein efflux transporter results in a significantly reduced maximum drug concentration and area under the plasma concentration-time curve as intestinal absorption of clopidogrel is reduced; and the expression of the mutant *2 allele of CYP2C19 results in similar pharmacokinetic effects as the two cytochrome P450 (CYP)-mediated steps required for the production of the active metabolite of clopidogrel are impaired. These variable pharmacokinetics lead to erratic pharmacodynamics and cannot reliably be overcome with increased dosing. Both prasugrel, a third-generation thienopyridine, and ticagrelor, a cyto-pentyl-triazolo-pyrimidine, have more predictable pharmacokinetics and enhanced pharmacodynamics than clopidogrel. Neither appears to be affected by the same genetic polymorphisms as clopidogrel; prasugrel requires only a single CYP-mediated step to produce its active metabolite and ticagrelor is not a pro-drug. Enhanced IPA by both prasugrel and ticagrelor is achieved at the expense of increased major bleeding, although this is partially mitigated in the case of ticagrelor due to its reversible IPA. However, the reversible binding of ticagrelor to the P2Y₁₂ receptor requires a twice-daily dosing regimen. Due to limited data from clinical studies, the use of prasugrel is currently restricted to individuals undergoing percutaneous coronary intervention who are ≤75 years old and have a body weight ≥60 kg. The clinical data for ticagrelor are more comprehensive and this drug therefore has a place in the management of patients with acute coronary syndrome at moderate-to-high risk of ischaemic events, irrespective of treatment strategy. Here we review in detail the pharmacokinetics and pharmacodynamics of clopidogrel, prasugrel and ticagrelor, and explore the implications of the differences in these parameters for their clinical use.