LC-MS/MS mediated absolute quantification and comparison of bile salt export pump and breast cancer resistance protein in livers and hepatocytes across species.

In the present study, capillary liquid chromatography (LC) nano electrospray ionization quadruple time-of-flight (nano-ESI-Q-TOF) mass spectrometry was utilized to identify the unique proteotypic peptides for liquid chromatography-tandem mass spectrometry (LC-MS/MS) mediated breast cancer resistance protein (BCRP/ABCG2) and bile salt export pump (BSEP/ABCG11) quantification, using insect membrane vesicles overexpressing the proteins. The lower limit of quantification was established to be 31.25 pM and 125 nM for BCRP/ABCG2 and BSEP/ABCG11, respectively. The linearity of standard curves was up to 5000 pM. The accuracy and precision of the LC-MS/MS method were evaluated by adding the known amount of synthetic proteotypic peptide or synthetic surrogate peptide substrates in the membrane protein extracts of livers or hepatocytes. The overall relative error (RE) and coefficient of variation (CV) were below 15.9% and 14.2% for BCRP/ABCG2 quantification or below 15.6% and 6.4% for BSEP/ABCG11, respectively. The absolute differences of BCRP/Bcrp and BSEP/Bsep proteins were determined in livers and isolated hepatocytes across species by the newly developed LC-MS/MS methods, with ranking order of dog > rat > monkey approximately = human and rat approximately = monkey > dog approximately = human, respectively (where the uppercase letters identify the human protein, i.e., BSEP and BCRP, and lowercase letters indicate that the transporter derives from a preclinical species, i.e., Bsep and Bcrp). The freshly isolated and cryopreserved hepatocytes conserved the protein levels of BSEP/Bsep and BCRP/Bcrp similarly to those found in liver tissue. We report, for the first time, an absolution quantification method for BCRP/Bcrp and BSEP/Bsep and the differences of the protein expressions across species. The results could serve as supportive information for extrapolation of hepatobiliary elimination from preclinical species to human.