Synthesis and evaluation of 7α-(3-[18F]fluoropropyl) estradiol
Introduction
Determination of the estrogen receptor (ER) status (positive or negative) is of prime importance in therapeutic management of breast cancer patients [1], [2], [3]. While histopathological analysis is still the gold standard for evaluating ER status, in vivo imaging of ER status by positron emission tomography (PET) is an attractive alternative to determine in situ ER status [4], [5], [6]. First, noninvasive ER imaging allows the simultaneous visualization of primary and metastatic tumor sites. Second, ER imaging is not subject to intrinsic heterogeneity of ER expression within a tumor, or to the possible discordance between primary and metastatic tumors. Finally, serial ER imaging can evaluate tumor responsiveness to hormonal therapy.
Over the past 30 years, several derivatives of fluorine 18 (18F)-labeled 17β-estradiol have been synthesized and evaluated [7]. The most successful ligand advanced to date is 16α-[18F]fluoro-17β-estradiol (16α-[18F]FES), developed at Washington University [8]. Several reports of clinical studies of 16α-[18F]FES showed the feasibility and potential of PET for ER density examination [9], [10], [11]. However, 16α-[18F]FES is not an optimal radioligand for ER imaging due to its rapid conversion to circulating radiometabolites, which prevents optimal localization of ER-binding sites [12].
In another approach, several lines of evidence suggest that C-7α-substituted estradiol derivatives are well tolerated by the ER [13], [14], [15], [16], [17]. In line with this hypothesis, we are interested in the design and synthesis of C-7α-substituted estrogens as molecular probes to visualize ER function. Recently, we successfully synthesized a boron-dipyrromethene derivative of estradiol and visualized in situ hormone–receptor interactions in the nuclei of uterine epithelial cells [18]. Furthermore, 7α-(5-[18F]fluoropentyl) estradiol (C5-7α-[18F]FES) was synthesized and its biodistribution was studied in immature rats [13]. Although blocking studies showed selective uptake in target tissues, the levels of nontarget tissue uptake, especially in fatty tissue and the blood, were high. This low selectivity may be due to the increased lipophilicity of the additional five-carbon chain. These findings prompted us to develop shorter alkyl chain length derivatives of 18F-labeled C-7α-substituted estradiol.
In the present study, we synthesized three-carbon derivative of 18F-labeled C-7α-substituted estradiol (C3-7α-[18F]FES) and characterized its in vitro binding, in vivo distribution, and performed blocking studies in mature female mice. We also analyzed in vivo metabolites in the plasma and uterus. Furthermore, in vivo ER-selective uptake was confirmed using ER-positive T-47D and ER-negative MDA-MB-231 tumor-bearing mice. We also compared the in vivo biodistribution of C3-7α-[18F]FES with 16α-[18F]FES to discuss the contribution of shorter side chain length of C-7α-substituted estradiol.
Section snippets
General
3-O-(Methoxymethyl)-16,17-O-sulfuryl-16-epistriol (MMSE) and 16α-Fluoro-17β-estradiol (16α-FES) were purchased from ABX GmbH (Radeberg, Germany). [2,4,6,7-3H(N)]-Estradiol ([3H]estradiol; 3300 GBq/mmol, 37 MBq/mL) was purchased from PerkinElmer (Boston, MA). Human recombinant estrogen receptor α-subtype (ERα) was purchased from Sigma-Aldrich Co. (St. Louis, MO). Anti-estrogen receptor α antibody (mouse monoclonal 1D5), anti-estrogen receptor β antibody (rabbit monoclonal EPR3778), and anti-actin
Chemistry
The precursor 5 was synthesized in 3 steps from 7α-allyl-estradiol as shown in Fig. 1. The overall yield starting from 7α-allyl-estradiol was 43%. C3-7α-FES was prepared by fluorination of 5 with tetrabutylammonium fluoride, followed by acid deprotection of the methoxymethyl group.
Radiosynthesis
18F-Fluorination of the tosylate 5 was carried out using the 18F-Kryptofix222-potassium carbonate system in acetonitrile at 100 °C for 15 minutes and deprotected with 0.5 N HCl in 50% acetonitrile at 100 °C for 2 minutes
Discussion
French et al. synthesized C5-7α-[18F]FES and evaluated its biodistribution in immature rats [13]. They found selective uptake to a certain degree in target tissues. However, the levels of nontarget tissue uptake were high, possibly due to the increased lipophilicity o the additional five-carbon chain. In the present study, we further investigated the shorter three-carbon derivative of 18F-labeled C-7α-substituted estradiol (C3-7α-[18F]FES) and characterized its biological properties.
C3-7α-[18
Acknowledgments
We thank Mr. Kunpei Hayashi (SHI Accelerator Service Co. Ltd) for his technical support with the cyclotron operation and radiosynthesis, and Dr. Seijiro Hosokawa (Waseda University) and Dr. Kazuo Nagasawa (Tokyo University of Agriculture and Technology) for their valuable advice. This work was supported in part by a Grant-in Aid for Scientific Research (B) 25293271 from Japan Society for the Promotion of Science (JSPS) and a Grant-in-Aid for the Global COE Program “Practical Chemical Wisdom”
References (28)
- et al.
Beneficial effect of adjuvant tamoxifen therapy in primary breast cancer patients with high oestrogen receptor values
Lancet
(1985) - et al.
18F-labeleing of A-ring substituted 16α-fluoro-estradiols as potential imaging agents for PET imaging
Steroids
(2009) - et al.
The estradiol pharmacophore: ligand structure-estrogen receptor binding affinity relationships and a model for the receptor binding site
Steroids
(1997) - et al.
Analysis of blood clearance and labeled metabolites for the estrogen tracer [F-18]-16α-fluoroestradiol (FES)
Nucl Med Biol
(1997) - et al.
A synthesis of 7α-substituted estradiols: synthesis and biological evaluation of a 7α-pentyl-substituted BODIPY fluorescent conjugate and a fluorine-18-labeled 7α-pentylestradiol analog
Steroids
(1993) - et al.
Novel steroidal pure antiestrogens
Steroids
(1989) - et al.
Synthesis and bioassay of a boron-dipyrromethene derivatives of estradiol for fluorescence imaging in vivo
Steroids
(2012) - et al.
18F-labeled difluoroestradiols: preparation and preclinical evaluation as estrogen receptor-binding radiopharmaceuticals
Steroids
(2002) - et al.
Estrogen receptor binding tolerance of 16α-substituted estradiol derivatives
Steroids
(1988) - et al.
Assessment of a robust model protocol with accelerated throughput for a human recombinant full length estrogen receptor-α binding assay: protocol optimization and intralaboratory assay performance as initial steps toward validation
Reprod Toxicol
(2010)
Titration of the in vivo uptake of 16α-[18F]fluoroestradiol by target tissues in the rat: competition by tamoxifen, and implications for quantitating estrogen receptors in vivo and the use of animal models in receptor-binding radiopharmaceutical development
Nucl Med Biol
The pharmacology and clinical uses of tamoxifen
Pharm Ther
Survival from first recurrence: relative importance of prognostic factors in 1,015 breast cancer patients
J Clin Oncol
Breast cancers: estrogen and progesterone receptor status as a predictor of in vitro chemotherapeutic response
J Surg Res
Cited by (7)
Cell-selective breast cancer targeting through estrogen receptors
2022, Targeted Nanomedicine for Breast Cancer TherapyResearch progress of <sup>18</sup>F labeled small molecule positron emission tomography (PET) imaging agents
2020, European Journal of Medicinal ChemistryCitation Excerpt :As shown in Fig. 2, a wide range of 18F labeled radiopharmaceuticals (1-16) have been successfully synthesized through aliphatic nucleophilic fluorination and the details of synthesis and application were described below. In 2015, Okamoto al. reported the synthesis of C-7ɑ-substituted estradiol derivative C3-7ɑ-[18F]FES (1, Fig. 2) by using the nucleophilic fluorination method [55]. Tracer 1 was produced for the location of primary and metastatic breast cancer and the evaluation of the effect of hormones on tumor treatment.
Optimization of the alkyl side chain length of fluorine-18-labeled 7α-alkyl-fluoroestradiol
2016, Nuclear Medicine and BiologyCitation Excerpt :In contrast, di-methoxymethyl-protected groups of labeling precursors of Cn-7α-[18F]FES derivatives may be removed more quickly and have acceptable times required for radiosynthesis. In this study, we further synthesized fluoromethyl (22; C1-7α-[18F]FES) to fluorohexyl (25; C6-7α-[18F]FES) derivatives of Cn-7α-[18F]FES, except fluoropropyl (C3-7α-[18F]FES) and fluoropentyl derivatives (C5-7α-[18F]FES), which were synthesized previously [5,15]. We characterized the in vitro binding and in vivo distribution of these derivatives in mature female mice compared to the previously published data for C3-7α-[18F]FES and 16α-[18F]FES, and we discuss the optimization of the alkyl side chain length.
Kit-like <sup>18</sup>F-labeling of an estradiol derivative as a potential PET imaging agent for estrogen receptor-positive breast cancer
2017, Journal of Radioanalytical and Nuclear ChemistryPrototypic <sup>18</sup>F-Labeled Argininamide-Type Neuropeptide Y Y<inf>1</inf>R Antagonists as Tracers for PET Imaging of Mammary Carcinoma
2017, ACS Medicinal Chemistry Letters