Characterization of Multidrug Resistance P-Glycoprotein Transport Function with an Organotechnetium Cation

Multidrug resistance (MDR) in mammalian cells and tumors is associated with overexpression of an ~170 kDa integral membrane efflux transporter, the MDR1 P-glycoprotein. Hexakis(2- methoxyisobutyl isonitrile)technetium(I) (Tc-SESTAMIBI), a γ-emitting lipophilic cationic metallopharmaceutical, has recently been shown to be a P-glycoprotein transport substrate. Exploiting the negligible lipid membrane adsorption properties of this organometallic substrate, we studied the transport kinetics, pharmacology, drug binding, and modulation of P-glycoprotein in cell preparations derived from a variety of species and selection strategies, including SW-1573, V79, Alex, and CHO drug-sensitive cells and in IT A, LZ-8, and Alex/A.5 MDR cells. Rapid cell accumulation (t_1/2 ≈ 6 min) of the agent to a steady state was observed which was inversely proportional to immunodetectable levels of P-glycoprotein. Many MDR cytotoxic agents inhibited P-glycoprotein-mediated Tc-SESTAMIBI efflux, thereby enhancing organometallic cation accumulation. Median effective concentrations (EC50; wM) were as follows: vinblastine, 13; daunomycin, 55; idarubicin, 65; actinomycin D, 235; colchicine, minimal inhibition; adriamycin, no effect. P-glycoprotein modulators generally demonstrated significantly greater potency (EC50; M): SDZ PSC 833, 0.08; cyclosporin A, 1.3; verapamil, 4.1; quinidine, 6.4; prazosin, >300. Modulator-induced enhancement up to 100-fold was observed with Hill coefficients %1, consistent with simple Michaelis-Menten kinetics. Vanadate was an efficacious transport inhibitor, while agents usually not included in the MDR phenotype were without effect. Scatchard analysis showed quinidine to be a noncompetitive inhibitor of P-glycoprotein-mediated Tc-SESTAMIBI transport, indicating allosteric effector sites on P-glycoprotein. The lipid bilayer adsorbing agents tetraphenyl borate and phloretin induced large increases in final Tc-SESTAMIBI accumulation, showing maximal accumulations 2-fold greater than classic MDR modulators and Hill coefficients » 2. In V79 and 77A cells, modulators of PKC activity altered Tc-SESTAMIBI accumulation, while there was no indication of modulation of P-glycoproteinmediated Tc-SESTAMIBI transport by hypotonic buffer, extracellular ATP, Cl^-, or K⁺ (membrane potential). While recognized and avidly transported by the P-glycoprotein at buffer concentrations as low as 7 pM, Tc-SESTAMIBI at up to 100 uM only minimally modulated the cytotoxic action of colchicine, doxorubicin, or vinblastine in MDR cells. In conclusion, transport analysis with Tc-SESTAMIBI is a sensitive assay for detecting functional expression of low levels of P-glycoprotein and for the quantitative characterization of transporter modulation and regulation. The biochemical data favor a high Km, high capacity allosterically modulated translocation mechanism for P-glycoprotein-mediated transport of this organometallic cation. In addition, the known physicochemical properties of Tc-SESTAMIBI combined with effects of the membrane adsorbing agents indicate that lipid-protein interactions are critical for transport of this metallopharmaceutical and would suggest that this drug gains access to protein transport domains from the lipid phase with rapid time constants.