The role of P-glycoprotein in intestinal transport versus the BBB transport of tetraphenylphosphonium

Tetraphenylphosphonium (TPP), a phosphonium cation, is a promising means for tumor imaging. A major contributor to the pharmacokinetics of phosphonium cations is the efflux transporter P-glycoprotein (P-gp). For this application it is important to ascertain the influence of the multidrug resistance system on TPP. Therefore, our aim was to characterize the interaction of TPP with P-gp, in vitro and in in vivo models. P-gpmediated transport of [³H]-TPP was assessed in Caco-2 cells and ex vivo in rat intestinal wall by the use of a diffusion cell system. The distribution of [³H]-TPP across the blood-brain barrier (BBB) was studied in rats and mice treated with P-gp modulators and in Mdr-1a/b(^-/-) knockout mice. The in vitro permeability coefficient of basolateral-to-apical transfer (PappB-A) of [ ³H]-TPP was 8-fold greater than apical-to-basolateral (PappA-B) coefficient, indicative of net mucosal secretion. A concentration dependent decrease of this secretion was obtained by the P-gp substrate verapamil, while no effect was evident by the MRP2 inhibitor MK-571 and the BCRP inhibitor FTC. [³H]-TPP transfer across rat jejunum wall was directional and concentration-dependent. 2,4-Dinitrophenol, cyclosporin A (CsA), verapamil and PSC-833 enhanced A-B transport of TPP 3.6-fold, 4-fold, 4.6-fold and 5.3-fold respectively. Likewise, PappA-B of [³H]-TPP was 5-fold greater in P-gp knockout mice than in controls. In vivo, PSC-833, P-gp inhibitor, significantly increased the uptake of [³H]-TPP in the liver, heart, small intestine and the lungs but not the brain. Similar results were obtained in P-gp knockout mice. Our study demonstrates that P-gp mediates TPP efflux in vitro and in vivo; however, the consistently poor BBB permeation of TPP in all in vivo studies including P-gp knockout animals indicates that it is most likely mediated by other mechanisms. These findings are important for optimized clinical application of TPP as an imaging agent in cancer.