Assessment of the in vitro and in vivo properties of a 99mTc-labeled inhibitor of the multidrug resistant gene product P-glycoprotein

Abstract

Overexpression of P-glycoprotein (Pgp), which is present in the plasma membrane of various tumor cells and in several normal cell types, contributes to the multidrug resistance (MDR) phenotype of many human cancers. As a prerequisite for therapy, the expression of Pgp must be studied. The available clinical radiopharmaceuticals for studying the expression of Pgp include the lipophilic 99mTc cations (sestamibi, tetrofosmin) as well as [ 99mTc]Q57, [ 99mTc]Q58, and [ 99mTc]Q63. Here we describe the in vitro and in vivo properties of the structurally different complex (3-thiapentane-1,5-dithiolato)[[N-(3-phenylpropyl)-N-2(3-quinazoline-2,4-dionyl)-ethyl]amino-ethylthiolato} oxotechnetium(V) ( 99/99mTc1) as a potential inhibitor of Pgp. 99Tc1 enhances the net cell accumulation of Pgp substrates [ 3H]vinblastine, [ 3H]vincristine, [ 3H]colchicine, [ 99mTc]sestamibi, and [ 99mTc]tetrofosmin in rat brain endothelial cells (RBE4), an immortalized endothelial cell line that expresses Pgp. In addition, the cell accumulation of 99mTc1 could be increased by verapamil and reserpine, which are known Pgp inhibitors. A multitracer approach was used to study the side effects of 99Tc1 on cell metabolism. The cells were simultaneously incubated with [ 99mTc]sestamibi, 2-[ 18F]fluoro-2-deoxyglucose ([ 18F]FDG), and various 3H-labeled tracers. Two-dimensional scatter plots of [ 99mTc]sestamibi uptake/[ 18F]FDG uptake show typical changes of known Pgp inhibitors including 99Tc1. The effects of 99Tc1 on the in vivo distribution of [ 99mTc]sestamibi and [ 18F]FDG in rats also are comparable with the effects of verapamil, an established Pgp inhibitor and calcium channel blocker. We conclude that 99/99mTc1 is a transport substrate and a potential inhibitor of Pgp. Our approach may be useful in the design of further radiotracers with specificity to Pgp.