Kinetic characterization of P-glycoprotein-mediated efflux of rhodamine 6G in the intact rabbit lung.

P-Glycoprotein (P-gp) is an ATP-dependent drug efflux transporter involved in multidrug resistance and drug disposition in many organ systems. A majority of P-gp substrates are lipophilic amine drugs which also exhibit rapid extensive accumulation in lung tissue. P-gp is expressed in lung tissue, and the very nature of this drug efflux mechanism suggests a moderating role in pulmonary drug disposition. Little is known about P-gp-mediated efflux out of lung tissue or its kinetic characteristics as they may relate to the impact of P-gp on pulmonary drug accumulation. The present study develops an experimental and kinetic model to characterize the kinetics of P-gp-mediated efflux of rhodamine 6G dye (R6G) out of the intact rabbit lung. The perfusate concentration of R6G with time during recirculation through an isolated perfused rabbit lung was measured, and 66.6 +/- 2.6% (S.E.) of the perfusate R6G was taken up by the lung. In the presence of P-gp inhibitors, R6G uptake increased significantly to 87.5 +/- 1.1% (P < 0.002), indicating a functional pulmonary P-gp efflux transporter. Fractional lung accumulation of R6G increased with increasing R6G perfusate concentration, a result consistent with saturation of an efflux transporter. A parsimonious three-compartment kinetic model of R6G pulmonary disposition was used to interpret data sets from experiments with different perfusion variables and to estimate parameters descriptive of the dominant kinetic processes involved in R6G pulmonary accumulation. The estimated value of the kinetic parameter, k(pgp), rate constant for P-gp-mediated R6G efflux, indicates that this transporter plays a significant role in moderating R6G pulmonary disposition.