Effects of MDR1 and MDR3 P-glycoproteins, MRP1, and BCRP/MXR/ABCP on the transport of Tc-99m-Tetrofosmin

Multidrug resistance (MDR1) P-glycoprotein (Pgp), multidrug resistance-asso ciated protein (MRP1), and breast cancer resistance protein (BCRP/MXR/ABCP) are members of the ATP-binding-cassette (ABC) superfamily of membrane tran sporters and are thought to function as energy-dependent efflux pumps of a variety of structurally diverse chemotherapeutic agents. We herein report t he characterization of Tc-99m-Tetrofosmin, a candidate radiopharmaceutical substrate of ABC transporters. Tc-99m-Tetrofosmin showed high membrane pote ntial-dependent accumulation in drug-sensitive KB 3-1 cells and low antagon ist-reversible accumulation in MDR KB 8-5 and KB 8-5-11 cells in proportion to levels of MDR1 Pgp expression. In KB 8-5 cells, EC50 values of the pote nt MDR antagonists N-(4-[2-(1,2,3,4-tetrahydro-6,7-dimethoxy-2-isoquinoliny l)- ethyl]-phenyl)-9,10-dihydro-5-methoxy-9-oxo-4-acridine carboxamide (GF1 20918), (2R)-anti-5-{3-[4-(10,11-difluoromethanodibenzo-suber-5-yl)piperazi n-1-yl]-2-hydroxypropoxy}quinoline trihydrochloride (LY335979), and (3'-ket o-Bmt')-[Val(2)]-cyclosporin A (PSC 833) were 40, 66, and 986 nM, respectiv ely. Furthermore, only baculoviruses carrying human MDRI, but not MDR3, con ferred both a decrease in accumulation of Tc-99m-Tetrofosmin in host Spodop tera frugiperda (Sf9) cells and a GF120918-induced enhancement. Transport s tudies with a variety of stably transfected and drug-selected tumor cell li nes were performed with Tc-99m-Tetrofosmin and compared with Tc-99m-Sestami bi, a previously validated MDR imaging agent. MDR1 Pgp readily transported each agent. To a lesser extent, MRP1 also transported each agent, likely as co-transport substrates with GSH; neither agent was a substrate for the BC RP/MXR/ABCP half-transporter. In mdr1a(-/-) and mdr1a/1b(-/-) mice, Tc-99m- Tetrofosmin showed similar to 3.5-fold greater brain uptake and retention c ompared with wild-type, with no net change in blood pharmacokinetics, consi stent with transport in vivo by Pgp expressed at the capillary blood-brain barrier. Molecular imaging of the functional transport activity of ABC tran sporters in vivo with Tc-99m-Tetrofosmin and related radiopharmaceuticals m ay enable non-invasive monitoring of chemotherapeutic and MDR gene therapy protocols. BIOCHEM PHARMACOL 60;3:413-426, 2000. (C) 2000 Elsevier Science Inc.