Evaluation of the relationship between [18F]FDG and P-glycoprotein expression: an experimental study
Introduction
The resistance of malignant tumors to chemotherapy is a major cause of treatment failure. One important mechanism in this resistance is the overexpression of adenosine triphosphate (ATP)-binding cassette transporters. The best-characterized member of this superfamily of transporters is P-glycoprotein (P-gp), which is significantly correlated with protein overexpression and chemoresistance in neuroblastoma, lymphoma, osteosarcoma and hepatocellular carcinoma (HCC) [1], [2], [3], [4]. P-gp is a 170-kDa, 1280-amino-acid product of the multidrug resistant (MDR) 1 gene found on the long arm of chromosome 7. It acts as an energy-dependent efflux pump that transports various chemically and functionally unrelated substances out of tumor cells, including, but not limited to, chemotherapeutic agents.
Positron emission tomography (PET) and PET/computed tomography (CT) are minimally invasive diagnostic tools that are used to detect and stage malignant tumors and to monitor the response of chemotherapeutics. The most commonly used PET radiotracer for in vivo tumor diagnostics is 2-[18F]fluoro-2-deoxy-d-glucose ([18F]FDG), which reflects changes in the glucose metabolic rate in tumor tissues. Several reports have shown that [18F]FDG uptake was inversely correlated with the expression level of P-gp in lung cancer, HCC and intrahepatic cholangiocarcinoma [4], [5], [6]. Lorke et al. [7] reported that [18F]FDG uptake was reduced in P-gp-positive tumor cells compared with P-gp-negative tumor cells in an animal study. Seo et al. [8] showed that [18F]FDG accumulation in vitro was reduced in MDR1 cell lines. However, Marian et al. [9] reported that MDR1-resistant tumors cells showed significantly higher [18F]FDG accumulation than MDR1-sensitive cells in vitro and in vivo studies. These contradictory results may have been due to the use of different tumor cell lines. We therefore chose the human breast cancer cell lines Bcap37 and Bcap37/MDR1, transfected with the human MDR1 (hMDR1) gene, to study the relationship of P-gp expression and [18F]FDG uptake in vitro and in vivo.
We report the results of [18F]FDG and 99mTc-methoxyisobutylisonitrile ([99mTc]MIBI) imaging in the presence and absence of verapamil (VER) in Bcap37 and Bcap37/MDR1 tumors and cell lines.
Section snippets
Cell culture
The human breast carcinoma cell lines Bcap37 and Bcap37/MDR1 were obtained from the Zhejiang University College of Pharmaceutical Sciences. The cells were cultivated in Roswell Park Memorial Institute (RPMI)-1640 medium (Gibco, Karlsruhe, Germany) under standard conditions (37°C, 100% relative humidity, 5% CO2/95% air) and in McCoy's 5A medium (Gibco/Life Technologies, Karlsruhe, Germany) supplemented with 10% heat-inactivated fetal calf serum.
Uptake of R123 in vitro
Rhodamine 123 (R123) is a cationic compound that
R123 uptake
R123 is taken up and retained by nonresistant cells but pumped rapidly out of the cytoplasm by the P-gp transporter protein in MDR cells. The Bcap37 and Bcap37/MDR1 cells were incubated in the presence of R123. All Bcap37 cells exhibited a bright fluorescent staining indicating the presence of R123 (Fig. 1A). In contrast, the Bcap37/MDR1 cells showed only a very weak fluorescence (Fig. 1B). Additionally, inhibition of the P-gp function by adding VER to the R123 incubation medium resulted in a
Discussion
This study was designed to assess the relationship between P-gp expression and [18F]FDG uptake in vitro and in vivo. For that purpose, the MDR breast cancer Bcap37/MDR1 cell line and the Bcap37 nonresistant tumor cell line were used. The P-gp function of the Bcap37/MDR1 cells was assessed by R123-uptake experiments. The P-gp drug transporter activity was assessed by measuring [99mTc]MIBI accumulation [10].
R123 is a cationic compound that excites at 488 nm and emits at 515–575 nm (green
Conclusion
[18F]FDG combined with VER may be an effective noninvasive method for the diagnosis of P-gp expression in tumors. Furthermore, [18F]FDG-PET or PET/CT using the ListMode acquisition mode may have potential clinical value for detecting P-gp in tumors.
Acknowledgments
The authors would like to thank Jiangsu Institute of Nuclear Medicine for the help in the mice data acquisition by micro-PET.
The study was financially supported by the Key Laboratory of Nuclear Medicine Open Subject, Ministry of Health, Jiangsu Institute of Nuclear Medicine, P.R. China.
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