The transition from clopidogrel to ticagrelor is the only switch between P2Y12 inhibitors that has been investigated in a trial powered for clinical endpoint, even if the study was not specifically designed to assess the safety and efficacy of the transition from clopidogrel to ticagrelor. In PLATO, nearly 50% of patients randomly allocated to receive ticagrelor had been pre-treated with clopidogrel, mostly given as a 300–600 mg loading dose.20 The efficacy and safety of ticagrelor were not affected by previous clopidogrel exposure.88 On the other hand, the TRITON-TIMI 38 trial mandated that previous exposure of patients to a P2Y12 receptor inhibitor should be an exclusion criterion for study entry.23 While registry data provide reassuring information with respect to the safety profile of switching from clopidogrel to prasugrel,89–91 no randomized data exist in the setting of studies powered for clinical endpoint. Similarly, all other switching possibilities, including between prasugrel and ticagrelor or from ticagrelor/prasugrel to clopidogrel, have not been investigated with outcome data.92–94 This practice is therefore discouraged due to a lack of safety/efficacy data. As the need to switch between P2Y12 inhibitors may arise for clinical reasons (i.e. side effects or drug intolerance), and registry data indicate that switching is not infrequent in practice, switching algorithms based on pharmacodynamic studies are provided (Figure 2).
