Multidrug resistance (MDR1) P-glycoprotein (Pgp), multidrug resistance-associated protein (MRP1), and breast cancer resistance protein (BCRP/MXR/ABCP) are members of the ATP-binding-cassette (ABC) superfamily of membrane transporters and are thought to function as energy-dependent efflux pumps of a variety of structurally diverse chemotherapeutic agents. We herein report the characterization of Tc-99m-Tetrofosmin, a candidate radiopharmaceutical substrate of ABC transporters. Tc-99m-Tetrofosmin showed high membrane potential-dependent accumulation in drug-sensitive KB 3-1 cells and low antagonist-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 potent MDR antagonists N-(4-[2-(1,2,3,4-tetrahydro-6,7-dimethoxy-2-isoquinolinyl)- ethyl]-phenyl)-9,10-dihydro-5-methoxy-9-oxo-4-acridine carboxamide (GF120918), (2R)-anti-5-{3-[4-(10,11-difluoromethanodibenzo-suber-5-yl)piperazin-1-yl]-2-hydroxypropoxy}quinoline trihydrochloride (LY335979), and (3'-keto-Bmt')-[Val(2)]-cyclosporin A (PSC 833) were 40, 66, and 986 nM, respectively. Furthermore, only baculoviruses carrying human MDRI, but not MDR3, conferred both a decrease in accumulation of Tc-99m-Tetrofosmin in host Spodoptera frugiperda (Sf9) cells and a GF120918-induced enhancement. Transport studies with a variety of stably transfected and drug-selected tumor cell lines were performed with Tc-99m-Tetrofosmin and compared with Tc-99m-Sestamibi, 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 BCRP/MXR/ABCP half-transporter. In mdr1a(-/-) and mdr1a/1b(-/-) mice, Tc-99m-Tetrofosmin showed similar to 3.5-fold greater brain uptake and retention compared with wild-type, with no net change in blood pharmacokinetics, consistent with transport in vivo by Pgp expressed at the capillary blood-brain barrier. Molecular imaging of the functional transport activity of ABC transporters in vivo with Tc-99m-Tetrofosmin and related radiopharmaceuticals may enable non-invasive monitoring of chemotherapeutic and MDR gene therapy protocols. BIOCHEM PHARMACOL 60;3:413-426, 2000. (C) 2000 Elsevier Science Inc.
