Functional Imaging of Multidrug-resistant P-Glycoprotein with an Organotechnetium Complex

Abstract

The multidrug-resistant P-glycoprotein (Pgp), a M_r 170,000 plasma membrane protein encoded by the mammalian multidrug resistance gene (MDRI), appears to function as an energy-dependent efflux pump. Many of the drugs that interact with Pgp are lipophilic and cationic at physiological pH. We tested the hypothesis that the synthetic γ-emitting organotechnetium complex, hexakis(2-methoxyisobutylisonitrile)technetium(I) ([^99mTc]SESTAMIBI), a lipophilic cationic radiopharmaceutical, could be a suitable Pgp transport substrate capable of functional imaging of the MDR phenotype. The cellular pharmacological profile of [^99mTc]SESTAMIBI transport was examined in Chinese hamster V79 lung fibroblasts and the 77A and LZ derivative cell lines which express modestly low, intermediate, and very high levels of Pgp, respectively. Steady-state contents of [^99mTc]SESTAMIBI in V79, 77A, and LZ cells were 10.0 ± 0.5 (SEM) (n = 9), 3.6 ± 0.5 (n = 8), and 0.4 ± 0.02 (n = 9) fmol·(mg protein)^-1 (nm_o)^-1, respectively, consistent with enhanced extrusion of the imaging agent by Pgp-enriched cells. Maximal doses (>100 µm) of the multidrug-resistant reversal agents verapamil and cyclosporin A enhanced [^99mTc]SESTAMIBI accumulation in V79, 77A, and LZ cells by approximately 10-, 25-, and 200-fold, respectively. The median effective concentration values for tracer accumulation in the presence of verapamil in V79, 77A, and LZ cells were 4, 100, and 200 µm, and those for cyclosporin A were 0.9, 3, and >25 µm, respectively. Pgp-mediated [^99mTc]SESTAMIBI transport occurred against its electrochemical gradient and was found to be ATP dependent displaying an apparent K_m of 50 µm. Carrier-added [⁹⁹Tc]SESTAMIBI was 11- to 13-fold less toxic in multidrug-resistant cells, and inhibited photolabeling of Pgp by [¹²⁵I]iodoaryl azidoprazosin in a concentration-dependent manner; half-maximal displacement was observed at approximately 100- to 1000-fold molar excess [⁹⁹Tc]SESTAMIBI. Exploiting the favorable γ emission properties of ^99mTc, functional expression of Pgp was successfully imaged in human tumor xenographs in nude mice with pharmacologically inert tracer quantities of [^99mTc]SESTAMIBI. Functional imaging with these organotechnetium complexes may provide a novel mechanism to rapidly characterize Pgp expression in human tumors in vivo, target reversal agents in vivo, and ultimately provide a means to direct patients to specific cancer therapies.