Technetium-99m-tetrofosmin would be a substrate for multidrug resistance-associated protein (MRP): comparison between a leukemia cell line with high MRP gene expression and its parental cell line

Study of Monoglutathionyl Conjugates TC-99MSestamibi and TC-99M-Tetrofosmin Transport Mediated by the Multidrug Resistance–Associated Protein Isoform 1 in Glioma Cells

The emergence of multidrug resistance (MDR) is a major obstacle to successful chemotherapy of malignant glioma tumors. Overexpression of the multidrug resistance-associated protein isoform 1 (MRP1), associated with a high level of intracellular glutathione (GSH), is a well-characterized mechanism of MDR in glioma cells. Previously, we have investigated the role of GSH and MRP1 in the accumulation of two radiopharmaceuticals classically used in nuclear medicine: (99m)Tc-sestamibi (MIBI) and (99m)Tc-tetrofosmin (TFOS), in a model of glioma cell lines. Although the involvement of GSH in MRP1-mediated transport of the two radiopharmaceuticals has been demonstrated, the exact transport mechanisms involving phase II (conjugation) and phase III (efflux) detoxification of these lipophilic cations has not been fully elucidated. To clarify the difference of release kinetics observed between MIBI and TFOS, we have studied the efficiency of formation of monogluthationyl conjugates mediated by glutathione S-transferses (GSTs). Our results clearly demonstrate that, in our model, the main efflux mechanism for radiopharmaceuticals is on monoglutathionyl-conjugates of MIBI (MIBI-SG) and TFOS (TFOS-SG). These mechanisms involving MRP1, and the phase II of detoxification is not efficient for TFOS in resistant glioma cells. A relatively slower catalytic efficiency of formation of TFOS-SG conjugate (0.006%.s(-1)) prevents its expulsion, contrary to MIBI (0.133%.s(-1)), suggesting that TFOS should be interesting in the detection and management of patients with high-grade glioma.

Influence of Glutathione Depletion on Plasma Membrane Cholesterol Esterification and on Tc-99m-Sestamibi and Tc-99m-Tetrofosmin Uptakes: A Comparative Study in Sensitive U-87-MG and Multidrug-Resistant MRP1 Human Glioma Cells

In our previous studies, we demonstrated a possible effect of cellular glutathione (GSH) depletion on plasma-membrane permeability and fluidity in glioma-cell lines. We therefore investigated the effect of GSH modulation on accumulation of two radiotracers, Tc-99m-sestamibi (MIBI) and Tc-99m-tetrofosmin (TFOS), and on plasma-membrane cholesterol content in sensitive U-87-MG and resistant U-87-MG-CIS and U-87-MG-MEL (MRP1 positive) human glioma-cell lines. GSH depletion was mediated by BSO pretreatment and addition of N-acetylcysteine reversed the effect. MIBI and TFOS uptakes, total cholesterol, and cholesteryl-ester contents were evaluated under each condition. In contrast with TFOS, MIBI accumulation was inversely proportional to the cell multidrug resistance phenotype. Similar cholesterol contents were observed in all cell lines, demonstrating that MRP1 did not modify lipid membrane composition. A decrease of intracellular GSH allows an increase of plasma-membrane cholesterol and a decrease of cholesteryl-ester content, which in turn results in spectacular TFOS uptake. The GSH status of the cells plays an important role in the plasma membrane cholesterol composition and TFOS uptake, which appears to be particularly sensitive to this modification. In contrast with MIBI, TFOS is not an MRP1 probe in glioma cells, and therefore appears to be a suitable tracer in this indication.

Limitations of (99m)Tc tetrofosmin in assessing reversal effects of verapamil on the function of multi-drug resistance associated protein 1

BACKGROUND

Previous reports have demonstrated the feasibility of scintigraphic assessment of the multi-drug resistance (MDR) of tumours caused by ATP binding cassette (ABC) transporters by using Tc cationic tracers such as Tc tetrofosmin (TF). Furthermore, the potential of these tracers for evaluating the effects of reversal agents for MDR has been documented. However, most reversal agents simultaneously affect cationic ion transporters related to tracer accumulation in tumours.

METHODS

The uptake of Tc-TF was examined in the MCF7/WT cell line, a wild-type breast cancer cell line that does not exhibit MDR, and its subclonal etoposide resistant cell line MCF7/VP, which expresses high levels of MRP1, one of the multi-drug resistance associated proteins (MRPs), in the presence of increasing concentrations of verapamil, a classical MDR modulator. In the absence of verapamil, MCF7/VP cells showed significantly lower Tc-TF uptake than did MCF7/WT cells, indicating that Tc-TF is a substrate for MRP1. The presence of verapamil enhanced the uptake of Tc-TF in MCF7/VP cells. On the other hand, verapamil also increased tracer uptake in MCF7/WT cells, which was readily appreciated when the uptake values were corrected by viable cell numbers: an approximately 100% increase of Tc-TF uptake was observed in comparison with that in the absence of verapamil in viable MCF7/WT cells whereas a 100-200% increase occurred in viable MCF7/VP cells. In addition, verapamil prolonged the retention of radioactivity in both MCF7/WT cells and MCF7/VP cells.

CONCLUSION

These results suggest that cellular functions other than MRP1 function, probably cationic ion transporters, are simultaneously and significantly involved in the verapamil induced changes of cellular uptake of Tc-TF. Tc-TF scintigraphy may overestimate the reversal effects of modulators on chemoresistance caused by MRP1 when the modulators simultaneously affect ion transporters.