The Synthesis and Evaluations of the 68Ga-Lissamine Rhodamine B (LRB) as a New Radiotracer for Imaging Tumors by Positron Emission Tomography

Purpose. The aim of this study is to synthesize and evaluate 68Ga-labeled Lissamine Rhodamine B (LRB) as a new radiotracer for imaging MDA-MB-231 and MCF-7 cells induced tumor mice by positron emission tomography (PET). Methods. Firstly, we performed the radio synthesis and microPET imaging of 68Ga(DOTA-LRB) in athymic nude mice bearing MDA-MB-231 and MCF-7 human breast cancer xenografts. Additionally, the evaluations of 18F-fluorodeoxyglucose (FDG), as a glucose metabolism radiotracer for imaging tumors in the same xenografts, have been conducted as a comparison. Results. The radiochemical purity of 68Ga(DOTA-LRB) was >95%. MicroPET dynamic imaging revealed that the uptake of 68Ga(DOTA-LRB) was mainly in normal organs, such as kidney, heart, liver, and brain and mainly excreted from kidney. The MDA-MB-231 and MCF-7 tumors were not clearly visible in PET images at 5, 15, 30, 40, 50, and 60 min after injection of 68Ga(DOTA-LRB). The tumor uptake values of 18F-FDG were 3.79 ± 0.57 and 1.93 ± 0.48%ID/g in MDA-MB-231 and MCF-7 tumor xenografts, respectively. Conclusions. 68Ga(DOTA-LRB) can be easily synthesized with high radiochemical purity and stability; however, it may be not an ideal PET radiotracer for imaging of MDR-positive tumors.


Introduction
Tumor growth depends on the energy metabolism of the supply, and the biological energy of tumor has received much attention in recent years [1,2]. A metabolic shift from oxidative phosphorylation in the mitochondria to glycolysis in cancer was first described about 80 years ago by Warburg [3]. Increased glucose metabolism is an important feature of cancer [4]. Active glucose uptake by cancer cells constitutes the basis for 18 F-fluorodeoxyglucose-positron emission tomography ( 18 F-FDG PET), an imaging technology commonly used in cancer diagnosis. However, the reverse Warburg effect was recently found in a human breast cancer model [5][6][7]. The researchers found that breast cancer cells showed a significant increase activity in mitochondria [8]. However, the development of molecular imaging probes targeting tumor mitochondria is very limited.
It has been reported that the mitochondrial potential in carcinoma cells is significantly higher than that in normal epithelial cells [9,10], and mitochondrial potential is negative; many organic cations are driven through these cell membranes and able to localize in the mitochondria of tumor cells [11][12][13]. Several studies proposed to use the 64 Cu(DO3Axy-TPEP) and 18 F-labeled phosphonium cations as PET radiotracers for tumor mitochondria, but they had high background in normal organs [14,15]. Lissamine Rhodamine B (LRB) is a derivative of rhodamine, which has been used as probe for mitochondrial potentials. 64 Cu-LRB, a radiotracer targeting tumor mitochondria for U87MG human glioma xenografts, has low radioactivity accumulation in the brain, and 64 Cu requires high energy cyclotron for production, both of which limit the clinical application in the tumor [16]. 68 Ga is a generator-produced radionuclide, and its half-life is 67.6 min, which is produced by 68 Ge/ 68 Ga generator; the production of 68 Ga is not dependent on the cyclotron.
The objective of our study is to synthesize and evaluate 68 Ga-labeled Lissamine Rhodamine B (LRB) (Figure 1  a new radiotracer for imaging MDA-MB-231 and MCF-7 cells induced tumor mice by positron emission tomography (PET). Additionally, 18 F-FDG, as a glucose metabolism radiotracer for imaging tumors in the same xenografts, was further evaluated as a comparison.

HPLC Methods.
The semiprep HPLC method used a Waters 2545+BIOSCAN Flowcount system equipped with a UV/Vis detector ( = 254 nm) and CHROM-MATRIX C-18 semiprep column (10 mm × 250 mm). The flow rate was 3 mL/min. The mobile phase was isocratic with 70% A (0.1% TFA in water) and 30% B (0.1% TFA in methanol) at 0-5 min, followed by a gradient mobile phase going from 30% B at 5 min to 80% B at 20 min, followed by a gradient mobile phase going from 80% B at 20 min to 30% B at 25 min. The radio-HPLC analysis method used a system (Waters, Inc., USA) consisting of Agilent TC-18 Chromatographic column (4.6 × 250 mm, 5 m), Perkinzimer online radioactivity detector, and a UV detector ( = 254 nm). The flow rate was 1 mL/min. The mobile phase was isocratic with 60% A (0.1% TFA in water) and 40% B (0.1% TFA in methanol) at 0-1 min, followed by a gradient mobile phase going from 40% B at 1 min to 90% B at 40 min, followed by a gradient mobile phase going from 90% B at 40 min to 98% B at 45 min. 68 Ga was obtained from a 68 Ge/ 68 Ga generator (Garching GmbH, Germany) eluted with 0.1 N HCl. Fresh 68 Ga was loaded into an ion exchange column. By using a mixture of 400 L 97.6% acetone and 0.05 M hydrochloric acid, 68 Ga was eluted from the exchange column and added to the solution containing 10 g DOTA-LRB in 400 L 0.25 M HEPES (pH 4.0); the reaction mixture was then heated at 100 ∘ C for 20 min. All experiments were performed using 6-week-old female athymic nude mice purchased from Shanghai Silaike Experimental Animal Co. Ltd. Athymic nude mice derived are in compliance with regulations of our institution. All animal experiments were approved by the China Medical University Animal Care and Use Committee.

68 Ga Radiolabeling.
Subcutaneous injection of 5 × 10 6 tumor cells into the breast fat pad of female athymic nude mice generated the tumor model. When the tumor volume was 100∼300 mm 3 (about 3∼4 weeks after inoculation), the mice underwent small animal PET imaging studies.

Statistical Analysis.
Quantitative data is expressed as mean ± SD. Means were compared using Student's -test. < 0.05 was considered statistically significant.  MicroPET dynamic imaging revealed the uptake of 68 Ga(DOTA-LRB) in normal organs (kidney, heart, and liver) and the excretion from the kidney. It had very low 68 Ga(DOTA-LRB) radioactivity accumulation in the brain.

Discussion
Increase of mitochondrial transmembrane potential (ΔΨ ) is an important characteristic of cancer [18][19][20]. Molecular imaging probes based on mitochondrial transmembrane potential have attracted intensive research attention in recent years. Although many radiolabeled cationic tracers have been reported, they all need to be produced by the cyclotron. 68 Ga is produced by 68 Ge-68 Ga generator. 68 Ga is the short half-life radionuclide, which is difficult for commercial distribution. The major advantage of the generator is that it can produce continuous source of 68 Ga independent of the cyclotron; 68 Ga-labeled biomolecules have great advantages in clinical application [21][22][23].
This is the first synthesis study for 68 Ga(DOTA-LRB), which was easily labeled with 68 Ga and the radiochemical purity of 68 Ga(DOTA-LRB) could reach more than 95% with HPLC purification. The HPLC retention time was 9.8 min. The experiments in vitro demonstrated that 68 Ga(DOTA-LRB) was stable in PBS at 37 ∘ C for 2 h.
MicroPET dynamic imaging revealed that normal organs (kidney, heart, and liver) had 68 Ga(DOTA-LRB) uptake and mainly excreted from the kidney. It had very low 68 Ga(DOTA-LRB) radioactivity accumulation in the normal brain tissue. The distribution of 68 Ga(DOTA-LRB) in normal tissues was consistent with that of 64 Cu(DOTA-LRB) [16]. 68 Ga(DOTA-LRB) was very low accumulation in the normal brain; it is probably because this compound is not able to cross the blood brain barrier (BBB). 68 Ga(DOTA-LRB) showed better biodistribution in normal organs in this study, compared with another report using 64 Cu-labeled acridinium cation, which is high and prolonged liver uptake [24].
The previous study showed that the uptake of 64 Cu(DOTA-LRB) was positive in U87MG human glioma xenografts [16], whereas our study showed 68 Ga(DOTA-LRB) uptake in MDA-MB-231 and MCF-7 breast cancer cells was negative. We attributed the difference to different cell lines. The study by Dr. Liu's group with 64 Cu(DOTA-LRB) used the U87MG human glioma cell, which is negative expression of multidrug resistance (MDR) protein tumor cell [16], whereas our study used the MDA-MB-231 and MCF-7 breast cancer cell lines, which are not MRP-negative cancer cell. It was reported that the MDR had positive expression in MDA-MB-231 and MCF-7 breast cancer cells [25]. Because some cations are the substrate for MDR protein, cationic radiotracers have been clinically used for noninvasive monitoring of the multidrug resistance transport function in tumors [26,27]. Lissamine Rhodamine B (LRB) is a member of rhodamine derivatives, which is also the substrate for MDR protein. Therefore, lower 68 Ga(DOTA-LRB) tumor uptake in the two breast cancer cells may be associated with    MDR. 68 Ga(DOTA-LRB) may enter the tumor cells but pump out of the tumor cells as a substrate for MDR. These results suggested that the 68 Ga(DOTA-LRB) molecular probe may be used to measure the MDR of tumor.
We also found that the uptake of MDA-MB-231 and MCF-7 was positive by 18 F-FDG microPET imaging, and the uptake of MDA-MB-231 in the high invasive 18 F-FDG tumor was slightly higher than that in the low invasive MCF-7 tumor, but without statistical significance. Previous group has demonstrated that some types of aggressive breast cancers are associated with a high uptake for 18 F-FDG, while more indolent breast cancers are characterized by low 18 F-FDG uptake [28,29].
In non-MDR negative tumors, the uptake of 68 Ga(DOTA-LRB) was low in MDA-MB-231 xenografts and MCF-7 xenografts, but it was very easy to synthesize. In the future study, we will perform a study of 68 Ga(DOTA-LRB) in MDR negative tumors.

Conclusions
68 Ga(DOTA-LRB) can be easily synthesized with high radiochemical purity and stability. 68 Ga(DOTA-LRB) may be not an ideal PET radiotracer for tumor imaging of non-MDRnegative tumors.