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Herein, we aimed to evaluate the recently proposed risk assessment strategies of a cytochrome P450 (CYP) mediated drug-drug interaction (DDI) according to the European Medicines Evaluation Agency (EMEA) draft guideline, and discuss the differences between this guideline and the Food and Drug Administration (FDA) draft guidance. A retrospective study on reported 35 clinical DDI cases revealed that the EMEA assessment successfully predicts moderate-to-strong DDIs, i.e. drugs that cause more than 2-fold increase in the area under the curve in the presence and absence of CYP inhibitor (AUC(i)/AUC); however, EMEA tends to overlook weak DDIs with AUC(i)/AUC ≤ 2 to > 1.25. For CYP3A4 inhibitors, even clinically insignificant DDIs were overemphasized if the intestinal DDI is considered. The differences between unbound fraction in plasma and microsomes account for the discrepancies in DDI risk assessment results between EMEA and FDA assessments. Comparing two assessment results for CYP2D6 and CYP2C9 inhibitors, the FDA assessment suggested potential DDI risks for sulphinpyrazone and amitriptyline, while the EMEA assessment indicated no potential risk for these drugs. Through a retrospective study, we showed practical differences in the DDI assessment strategies of EMEA and FDA and suggested improvements in their current strategies.

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