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

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 (99m)Tc-Tetrofosmin, a candidate radiopharmaceutical substrate of ABC transporters. (99m)Tc-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, EC(50) 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 MDR1, but not MDR3, conferred both a decrease in accumulation of (99m)Tc-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 (99m)Tc-Tetrofosmin and compared with (99m)Tc-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, (99m)Tc-Tetrofosmin showed approximately 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 (99m)Tc-Tetrofosmin and related radiopharmaceuticals may enable non-invasive monitoring of chemotherapeutic and MDR gene therapy protocols.