title: 99mTc-labeled estradiol as an estrogen receptor probe: Preparation and preclinical evaluation.
abstract: BACKGROUND
  Most breast cancers express estrogen receptors (ERs). Noninvasive imaging of ER expression may be helpful for planning therapy of ER+ tumors. We developed a new ER- binding probe, (99m)Tc-labeled estradiol, with diethylenetriaminepentaacetic acid (DTPA) as a chelating ligand, and assessed its targeting ability in vitro and in vivo.
  
  
  METHODS
  3-Aminoethyl estradiol was synthesized in two steps from estrone, followed by (99m)Tc labeling. Western blotting and immunofluorescence staining were used to detect ER expression in MCF-7 and MDA-MB-231 breast cancer cells. Saturation binding and specific binding were performed by incubating MCF-7 cells with increasing concentrations of (99m)Tc-DTPA-estradiol. Cell uptake, efflux, and blocking assays were also performed. To test (99m)Tc-DTPA-estradiol in vivo, nude mice bearing either MCF-7- (high ER expression) or MDA-MB-231- derived tumors (low ER expression) were injected with (99m)Tc-DTPA-estradiol, and underwent single-photon emission-computed tomography (SPECT). Mice injected with excess unlabeled DTPA-estradiol were used as controls. Ex vivo gamma-counting of tissues from normal and tumor-bearing mice was used to evaluate (99m)Tc-DTPA-estradiol biodistribution.
  
  
  RESULTS
  The radiochemical purity of (99m)Tc-DTPA-estradiol was 98.3%±1.3% with a specific activity of 33.1±1.5 MBq/μmol (n=3). Western blotting and immunofluorescence staining confirmed extensive expression of ERs by the MCF-7 cells, and less extensive expression by MDA-MB-231 cells. There was high binding affinity of (99m)Tc-DTPA-estradiol to MCF-7 cells with a>45% specific rate of total cell uptake. SPECT images and the biodistribution study results showed significantly higher uptake by MCF-7 tumors (6.06±0.38 %ID/g) than by MDA-MB-231 tumors (1.57±0.28 %ID/g). Pre-injection of MCF-7 tumor-bearing nude mice with excess unlabeled DTPA-estradiol significantly reduced tumor uptake of (99m)Tc-DTPA-estradiol (2.24±0.28 %ID/g), suggesting that (99m)Tc-DTPA-estradiol specifically targets ERs in tumors.
  
  
  CONCLUSIONS
  (99m)Tc-DTPA-estradiol can be synthesized with satisfactory labeling efficiency and stability. (99m)Tc-DTPA-estradiol specifically targeted ERs in vitro and in vivo with favorable pharmacokinetics, allowing ER receptor expression assessment with SPECT imaging.
authors.affiliation: Department of Nuclear Medicine, Union Hospital, Tongji Medical College, Huazhong University of Science and Technology, Hubei Province Key Laboratory of Molecular Imaging, Wuhan 430022, China.
authors.forename: Xiaotian
authors.initials: X
authors.lastname: Xia
authors.affiliation: Department of Nuclear Medicine, Union Hospital, Tongji Medical College, Huazhong University of Science and Technology, Hubei Province Key Laboratory of Molecular Imaging, Wuhan 430022, China.
authors.forename: Hongyan
authors.initials: H
authors.lastname: Feng
authors.affiliation: Department of Nuclear Medicine, Union Hospital, Tongji Medical College, Huazhong University of Science and Technology, Hubei Province Key Laboratory of Molecular Imaging, Wuhan 430022, China.
authors.forename: Chongjiao
authors.initials: C
authors.lastname: Li
authors.affiliation: Department of Nuclear Medicine, Union Hospital, Tongji Medical College, Huazhong University of Science and Technology, Hubei Province Key Laboratory of Molecular Imaging, Wuhan 430022, China.
authors.forename: Chunxia
authors.initials: C
authors.lastname: Qin
authors.affiliation: Department of Nuclear Medicine, Union Hospital, Tongji Medical College, Huazhong University of Science and Technology, Hubei Province Key Laboratory of Molecular Imaging, Wuhan 430022, China.
authors.forename: Yiling
authors.initials: Y
authors.lastname: Song
authors.affiliation: Department of Nuclear Medicine, Union Hospital, Tongji Medical College, Huazhong University of Science and Technology, Hubei Province Key Laboratory of Molecular Imaging, Wuhan 430022, China. Electronic address: zhyx1229@163.com.
authors.forename: Yongxue
authors.initials: Y
authors.lastname: Zhang
authors.affiliation: Department of Nuclear Medicine, Union Hospital, Tongji Medical College, Huazhong University of Science and Technology, Hubei Province Key Laboratory of Molecular Imaging, Wuhan 430022, China. Electronic address: LXL730724@hotmail.com.
authors.forename: Xiaoli
authors.initials: X
authors.lastname: Lan
mesh_terms: D000818:Animals; D004355:Drug Stability; D004958:Estradiol / Q000138:chemical synthesis* / Q000737:chemistry / Q000378:metabolism*; D004970:Estrone / Q000737:chemistry; D005260:Female; D006801:Humans; D007553:Isotope Labeling; D061986:MCF-7 Cells; D051379:Mice; D015335:Molecular Probes / Q000138:chemical synthesis* / Q000737:chemistry / Q000378:metabolism*; D004369:Pentetic Acid / Q000737:chemistry; D011960:Receptors, Estrogen / Q000378:metabolism*; D013667:Technetium / Q000737:chemistry*; D015899:Tomography, Emission-Computed, Single-Photon
chemical_list: D015335:Molecular Probes; D011960:Receptors, Estrogen; D004970:Estrone; D004958:Estradiol; D013667:Technetium; D004369:Pentetic Acid
keywords: Breast cancer; Estradiol; Estrogen receptors; Single-photon emission-computed tomography