Clopidogrel "resistance": pre- vs post-receptor determinants

The clinical efficacy of the P2Y12 receptor antagonist clopidogrel as an agent to prevent thrombotic events predominantly reflects its anti-aggregatory effects. Stent thrombosis in particular is more likely to occur in patients in whom clopidogrel effect is limited. "Resistance" to clopidogrel in general should theoretically arise either because of a reduction in plasma concentration of the active metabolite and/or of the downstream intracellular biochemical changes underlying antiplatelet effects. We therefore postulate that "resistance" to clopidogrel arises via a combination of pharmacogenetic, pharmacokinetic and intracellular biochemical mechanisms. Considerable attention has been so far directed to the finding that stent thrombosis occurs more frequently in patients with loss-of-function mutations of CYP2C19, thus limiting clopidogrel bioactivation. Furthermore, a number of drug-drug interactions may marginally impair responsiveness to clopidogrel, largely via impairment of bioactivation. However, population data also suggest that clopidogrel "resistance" occurs more frequently in patients with acute coronary syndromes than in normal subjects, and that "resistance" is particularly common in obese subjects and with diabetes. Here we critically review available literature and speculate on the possibility that non-genetic causes of clopidogrel "resistance" may arise from impairments of the intracellular signaling cascade initiated by P2Y12 receptor inhibition. In such cases, "resistance" to clopidogrel may also theoretically occur with other P2Y12 receptor antagonists, irrespective of the need for bioactivation. Delineation of this non-genetic component of "resistance" to P2Y12 inhibitors may facilitate the development of optimal therapeutic strategies for high-risk cardiovascular patients.