Evaluation of the in vitro and in vivo properties of a potential Tc-labelled inhibitor of the MDR gene product P-glycoprotein

Human multiple drug resistance (MDR) gene transfer and expression in bone marrow cells followed by autologous bone marrow transplantation in patients with advanced cancer is seen as a strategy to minimize the risks of high dose chemotherapy. Monitoring of the successful gene transfer may be a prerequisite for the therapy, i.e. it is required to study the expression of the MDR genes products, P-glycoprotein (Pgp) and the multidrug resistance-related protein (MRP) after gene transfer. Up to now the available clinical radiopharmaceuticals to study the expression of the PgP and MRP the lipophilic 99mTc-cations (sestamibi, tetrofosmin) as well as 99mTc-Q57, 99mTc-Q58, and 99mTc-Q63. All these complexes are transported by the Pgp. Here we describe the in vitro and in vivo properties of the structurally different (3-thiapentane-1,5-dithiolato){[N-(3-phenylpropyl)-N-2(3-chinazoline-2,4-dionyl)-ethyl]amino-ethylthiolato)-oxotechnetium(V) (P398) complex as a potential inhibitor of the Pgp. P398 and its rhenium congener enhance the net cell accumulation of the Pgp substrates 3H-vinblastine, 3H-vincristine, 3H-colchicine, 99mTc-sestamibi, 99mTc-tetrafosmine in RBE4 cells, an immortalized endothelial cell line which expresses Pgp and MRP. However, the cell accumulation of the n.c.a. P398 could not be influenced by quinidine and reserpine as known Pgp inhibitors. A multitracer approach was used to study also the side effects of P398 on cell metabolism. The cells were simultaneously incubated with 99mTc-sestamibi, 18F-FDG and various 3H-labelled tracers. Two-dimensional scatter plots of 99mTc-sestamibi uptake / 18F-FDG uptake show typical changes of known Pgp inhibitors including the P398. Also the effects of the P398 on the in vivo distribution of 99mTc-sestamibi and 18F-FDG in rats are comparable with the effects of verapamil, an established Pgp inhibitor and Ca-channel blocker.
Taken together, our data show that the multitracer approach can be used in vitro and in vivo to measure the functional characteristics of the Pgp and to study the interaction of potential radioligands with the Pgp. We conclude that P398 is not a transport substrate but a potential inhibitor of the Pgp and/or verapamil-sensitive Ca-channel. Our approach may be useful in the design of new radiotracers with specificity to the Pgp.