Correlation between Membrane Protein Expression Levels and Transcellular Transport Activity for Breast Cancer Resistance Protein.

Emerging evidence indicates an important role for the breast cancer resistance protein (BCRP) in limiting brain penetration of substrate drugs. While in vitro transwell assays can provide an indication of BCRP substrate potential, the predictability of these assays in relation to in vivo brain penetration is still under debate. The present study examined the correlation of BCRP membrane protein expression level and transcellular transport activity across Madin-Darby canine kidney (MDCK) II monolayers. We expressed human BCRP or murine BCRP1 in MDCKII wild-type cells using BacMam2 virus transduction. The selective P-glycoprotein (P-gp) inhibitor LY335979 (1 μM) was included in the transport medium to measure BCRP-mediated transcellular transport for P-gp and BCRP cosubstrates. The BCRP levels in membrane extracts from MDCKII-BCRP or MDCKII-Bcrp1 cells were quantified by liquid chromatography-tandem mass spectrometry. The results are summarized as follows: 1) the membrane protein expression levels correlate with the corrected efflux ratios of substrates for human BCRP and murine BCRP1 within the efflux ratios investigated; 2) we demonstrate good concordance in rank order between the BCRP and BCRP1-mediated efflux ratios for 12 drugs; and 3) we propose an approach to contextualize in vitro BCRP transport data of discovery compounds by comparing them to the in vitro and in vivo transport data of the reference drug dantrolene and taking into account interbatch variation in BCRP expression. This approach correctly predicted compromised brain penetration for 25 discovery compounds in rodents, which were BCRP substrates but not P-gp or weak P-gp substrates. These results suggest that BCRP-expressing MDCKII cells are useful in predicting the in vivo role of BCRP in brain penetration.