Kinetic Analysis of the Cooperation of P-Glycoprotein (P-gp/Abcb1) and Breast Cancer Resistance Protein (Bcrp/Abcg2) in Limiting the Brain and Testis Penetration of Erlotinib, Flavopiridol, and Mitoxantrone

breast cancer resistance protein (Bcrp)/Abcg2 was reported to limit the brain penetration of their common substrates. This study investigated this based on pharmacokinetics using Mdr1a/1b(-/-), Bcrp(-/-), and Mdr1a/1b(-/-)/Bcrp(-/-) mice. Comparison of the brain- and testis-to-plasma ratios (C-brain/C-plasma and C-testis/C-plasma, respectively) of the reference compounds quinidine and dantrolene for P-gp and Bcrp, respectively, indicates that impairment of either P-gp and Bcrp did not cause any change in the efflux activities of Bcrp or P-gp, respectively, at both the blood-brain barrier (BBB) and blood-testis barrier (BTB). The C-brain/C-plasma and C-testis/C-plasma of the common substrates erlotinib, flavopiridol, and mitoxantrone were markedly increased in Mdr1a/1b(-/-)/Bcrp(-/-) mice even compared with Mdr1a/1b(-/-) and Bcrp(-/-) mice. Efflux activities by P-gp and Bcrp relative to passive diffusion at the BBB and BTB were separately evaluated based on the C-brain/C-plasma and C-testis/C-plasma in the knockout strains to the wild-type strain. P-gp made a larger contribution than Bcrp to the net efflux of the common substrates, but Bcrp activities were also significantly larger than passive diffusion. These parameters could reasonably account for the marked increase in C-brain/C-plasma and C-testis/C-plasma in the Mdr1a/1b(-/-)/Bcrp(-/-) mice. In conclusion, the synergistic effect of P-gp and Bcrp on C-brain/C-plasma and C-testis/C-plasma can be explained by their contribution to the net efflux at the BBB and BTB without any interaction between P-gp and Bcrp.