**5. Concluding Remarks**

Evaluations of DDIs are a crucial issue for drug e fficacy and safety. Although methodologies to evaluate DDIs and information about mechanisms of PK-based DDIs have greatly advanced, the incidences and severity of DDIs still remain high in clinical cases. Moreover, previously unknown DDIs have been revealed and their mechanisms have been suggested in clinical DDI studies, but most of them have then been confirmed by, for example, in vitro studies through a reverse translation process. Since clinical DDI studies may not cover the numerous combinations and various factors that cause these outcomes, it is necessary to interpret the PK and PD data to provide a comprehensive understanding of DDIs with clinical relevance. Although there is currently no optimal way to study DDIs, its evaluation needs to be based on interpretation of the available data about PK-based DDIs, which can ensure the safety and maximal usefulness of DDI studies.

**Funding:** This research was funded by National Research Foundation of Korea (NRF) grants funded by the Korea governmen<sup>t</sup> (MSIT) (NRF-2016R1C1B2010849 and NRF-2018R15A2023217).

**Conflicts of Interest:** The authors declare no conflict of interest.
