Platelet aggregation and activation are serious concerns for patients who undergo percutaneous coronary intervention and stent placement. The underlying mechanism of platelet aggregation is mediated through two G protein-coupled P2 receptors, P2Y 1 and P2Y 12 (Gachet, 2001). P2Y 1 activation leads to a transient aggregation, whereas P2Y 12 activation maintains a sustained aggregation. To reduce platelet aggregation, the development of P2Y 12 -selective inhibitors has yielded the thienopyridine prodrugs, which include ticlopidine, clopidogrel (structures available in Farid et al., 2008), and prasugrel ( Fig. 1), a novel thienopyridine currently in clinical development.Although the active metabolites for prasugrel and clopidogrel have equipotency at the P2Y 12 receptor in vitro (Sugidachi et al., 2007), p.o. administered prasugrel is 10 and 100 times more effective on an equal-dose basis in inhibiting platelet aggregation than clopidogrel and ticlopidine, respectively (Niitsu et al., 2005). Clopidogrel and prasugrel differ markedly in the biotransformation pathways leading to their activation. Prasugrel (Fig. 1) has a single dominant metabolic pathway leading to the active metabolite (Farid et al., 2007a). However, clopidogrel has two competing metabolic pathways for the parent compound, with the major pathway leading to the formation of an inactive metabolite, clopidogrel carboxylic acid derivative (Caplain et al., 1999). The clopidogrel carboxylic acid derivative is formed through ester hydrolysis by the human carboxylesterase (hCE) 1 (Tang et al., 2006). The minor pathway in clopidogrel metabolism yielding the active metabolite requires two sequential steps of cytochrome P450 (P450) biotransformation (Kurihara et al., 2005), whereas prasugrel bioactivation requires the hydrolysis of the ester and then oxidation of the formed thiolactone, R-95913 (Farid et al., 2007a) (Fig. 1), to form the active metabolite of prasugrel. The oxidation of R-95913 has been shown to be mediated by several P450 enzymes but primarily by CYP3A and CYP2B6 (Rehmel et al., 2006).The carboxylesterases are a multigene family that hydrolyze compounds containing an ester, amide, or thioester linkage. Carboxylesterases are broadly expressed throughout the body with two major Article, publication date, and citation information can be found at