Abstract

The study objectives were 1) to test the hypothesis that the lack of P-glycoprotein (P-gp) and the inhibition of breast cancer resistance protein (Bcrp) at the blood-brain barrier after cassette dosing of potent P-gp and Bcrp inhibitors were due to low plasma concentrations of those inhibitors and 2) to examine the effects of P-gp on the unbound brain (C_u,brain) and cerebrospinal fluid (CSF) concentrations (C_u,CSF) of P-gp substrates in rats. In vitro inhibition of 11 compounds (amprenavir, citalopram, digoxin, elacridar, imatinib, Ko143 [(3S,6S,12aS)-1,2,3,4,6,7,12,12a-octahydro-9-methoxy-6-(2-methylpropyl)-1,4-dioxopyrazino[1′,2′:1,6]pyrido[3,4-b]indole-3-propanoic acid 1,1-dimethylethyl ester], loperamide, prazosin, quinidine, sulfasalazine, and verapamil) on P-gp and Bcrp was examined in P-gp– and Bcrp-expressing Madin-Darby canine kidney cells, respectively. An in vivo study was conducted in wild-type and Mdr1a(−/−) rats after subcutaneous cassette dosing of the 11 compounds at 1–3 mg/kg, and the brain, CSF, and plasma concentrations of these compounds were determined. At the maximal unbound concentrations observed in rats at 1–3 mg/kg, P-gp and Bcrp were not inhibited by a cassette of the 11 compounds. For non–P-gp/Bcrp substrates, similar C_u,brain, C_u,CSF, and unbound plasma concentrations (C_u,plasma) were observed in wild-type and P-gp knockout rats. For P-gp/Bcrp substrates, C_u,brain ≤ C_u,CSF ≤ C_u,plasma in wild-type rats, but C_u,brain and C_u,CSF increased in the P-gp knockout rats and were within 3-fold of C_u,plasma for six of the seven P-gp substrates. These results indicate that P-gp and Bcrp inhibition at the blood-brain barrier is unlikely in cassette dosing and also suggest that P-gp and Bcrp activity at the blood–CSF barrier is functionally not important in determination of the CSF concentration for their substrates.