PGP and prostate cancer: As a positive control we chose AT3B-1 prostate cancer cells, as opposed to Caco-2 or MDCK-II lines routinely used for measuring efflux of small molecules [52], because AT3B-1 cells possessed the previously well-characterized Dox-derived MDR phenotype [53] and have subsequently been used in studies involving membrane transport through P-gp [54].