While in vitro assays showed compound X to be a substrate for human BCRP and not P-gp, in vivo studies indicated a synergistic effect between rodent efflux transporters. However, this only accounted for ~50% of restricted BBB-transport, suggesting involvement of other efflux transporters. Considering Kp, uu as a key criterion for assessing the technical quality of CNS candidates before progression into clinical development, it is important to identify relevant screening assays for a better understanding of low Kp, uu and brain distribution in pre-clinical models for translation to humans.