MDCKII and LLC-PK1 cells transfected with the human MDR1 gene, which encodes the Pgp protein, were used in concentration equilibrium transport assays (CETA) to determine the substrate status of ZNS, PGB, and RFM. For each drug, an equal concentration was added to apical and basal chambers, and the concentration in both chambers was measured up to 4 h.
RFM, ZNS, and PGB were not transported by MDR1-transfected cells from basolateral to apical sides in CETA.
ZNS, RFM, and PGB are not substrates of human Pgp. These data suggest that resistance to these drugs may not be attributed to increased Pgp activity in resistant patients.