INVOLVEMENT OF GLUTATHIONE IN LOSS OF TECHNETIUM-99M-MIBI ACCUMULATION RELATED TO MEMBRANE MDR PROTEIN EXPRESSION IN TUMOR-CELLS

It was reported recently that Tc-99m-hexakis-2-methoxyisobutyl isonitr ile (MIBI) uptake is drastically reduced in cancer cells that express the multidrug resistance (MDR) product, Pgp 170 kDa (Pgp), suggesting that Tc-99m-MIBI is a transport substrate for this transmembrane glyco protein. In our study, we explored if another pump, a multidrug resist ance-associated protein (MRP), could affect Tc-99m-MIBI uptake. In add ition, we studied the involvement of intracellular glutathione (GSH) a s a modulator of Tc-99m-MIBI uptake by both Pgp and MRP proteins. Meth ods: MDR, and MRP gene expression in seven human tumor cell lines was determined on a transcriptional level by reverse transcriptase polymer ase chain reaction and on a protein level using immunocytochemistry. T echnetium-99m-MIBI uptake was quantified by measuring radioactivity re tained in the cells incubated at 37 degrees C in the presence or absen ce of buthionine sulfoximine (BSO), which depletes cellular GSH. The c ellular GSH content was determined with Ellman's reagent. Results: Cel l lines were classified according to their phenotypic characteristics: 1/MRP-/Pgp-: breast cancer cells (MCF7), lung carcinoma cells (H69S) and mouth epidermoid tumor cells (KB 3.1), 2/MRP-/Pgp+: MCF7 mdr+, KBA .1; and 3/MRP+/Pgp-: small-cell lung carcinoma (H69 AR and A 549). Tec hnetium-99m-MIBI uptake was significantly lower in cells expressing MR P as well as Pgp compared to MRP/Pgp cells. Depletion of GSH by BSO re sulted in an increase of (TC)-T-99m-MIBI uptake in multidrug resistant cells overexpressing MRP but not expressing Pgp. Conclusion: Techneti um-99m-MIBI is extruded by both Pgp and MRP efflux pumps. However, MRP action is indirect and involves intracellular GSH for a presumed inte raction with the 99mTc-MIBI before its efflux. Technetium-99m-MIBI see ms to be a good candidate for a noninvasive marker to diagnose MDR1, r elated to Pgp and MRP expression in tumors of different origin.