Estrogen receptor PET imaging


Estrogens (oesterogens) are lipophilic steroid hormones which, by binding estrogen receptors, regulate gene expression, metabolism, cell growth, proliferation, and anti-apoptosis.

Estradiol (E2)

Human estrogens include estrone (E1), 17β-estradiol (estradiol, E2), estriol (E3), and estetrol (E4), of which estradiol is most relevant physiologically due to its wide distribution and functions in multiple organs and tissues. E1 is the predominant estrogen in women undergoing menopause. E3 and E4 are mainly secreted by the placenta during pregnancy.

Estrogens in blood stream are bound to sex hormone binding globulin (SHBG, SBP) and albumin. SHBG binding protects estrogens from metabolism in the liver, where steroid hormones are conjugated to glucuronides. SHBG binding also helps in hormone transport into target cells through membrane receptors for SHBG (Noé et al., 1992; Hammes et al., 2005). The protein binding of steroid hormone analogue PET radioligands can be measured individually (Tewson et al., 1999; Peterson et al., 2011).


Estrogen is produced in organs that contain aromatase, mainly in the ovaries in ovalatory women and testes in men. Synthesis starts with cholesterol that is converted to pregnenolone, then to dehydroepiandrosterone (DHEA), further to androstenedione, and to testosterone (Lefebvre-Lacœuille et al., 2015). Aromatase enzyme complex (estrogen synthase, CYP19) catalyses the final step of estrogen biosynthesis, converting testosterone to estradiol, and androstenedione to estrone. Aromatase is found in most tissues, including skin, WAT, bone, muscle, and brain. In women in postmenopausal period the major source of estrogens is adipose tissue and skin. In ovalatory women, E2 is secreted in a cyclic manner as controlled by the hypothalamic-hypophyseal axis. The relatively low serum levels of estrogens in males may not accurately reflect their local regulative importance because of the wide tissue distribution of aromatase.

Aromatase availability in the brain can be assessed in vivo using [11C]vorozole and [11C]cetrozole (Biegon et al., 2010, 2015 and 2016; Logan et al., 2014; Takahashi et al., 2014 and 2018; Jonasson et al., 2020). [11C]Vorozole PET can also be used to measure aromatase expression in primary breast cancer and metastatic lesions (Biegon et al., 2020).

There is no aromatase activity in healthy uterine tissue, but in uterine diseases, such as endometriosis, leiomyoma, and endometrial cancer, aromatase activity has been observed (Lefebvre-Lacœuille et al., 2015).

Estrogen receptors

There are two cytoplasmic estrogen receptors, ERα and ERβ, and a membrane-bound G protein-coupled estrogen receptor (GPER1 or GPER, originally called GPR30). Estrogens can readily pass cell membranes and also BBB, and when bound to ERα or ERβ, receptor dimerizes and dissociates from inhibitory heat shock proteins and is translocated to the nucleus where it acts as transcription factor. GPER1 and cytosolic ERα and ERβ splice variants can propagate kinase signalling cascades, affecting cellular functions without direct effect on transcription. Growth factor-stimulated kinases can also activate ERα, without estrogens. Biological effects of activation of ERα and ERβ differ or are even opposite.

Endocrine therapy can alter the distribution and kinetics of ER radioligands in blood and healthy tissues, which may affect the quantification and interpretation of PET data (Iqbal et al., 2022).


ERα is primarily expressed in the reproductive tissues (endometrium, ovarian stromal cells), bone, white adipose tissue, kidney, liver, mammary glands, prostate, hypothalamus, and pituitary gland. More than 70% of breast cancers express ERα. Activation of ERα upregulates progesterone receptor (Horwitz & McGuire, 1978), and progesterone receptor imaging can be used to assess ERα signalling pathway.



The estrogen-based radiopharmaceutical 16α-[18F]fluoro-17β-estradiol ([18F]FES) has high binding affinity for estrogen receptor, and is selective for ERα over the ERβ subtype (Kiesewetter et al., 1984; Yoo et al., 2005). The assessment of ERα expression with [18F]FES PET in human breast cancer patients has been validated by immunohistologic and biochemical receptor assays (Mintun et al., 1988; Dehdashti et al., 1995; Peterson et al., 2008; Liao et al., 2016). Antiestrogen therapy reduces [18F]FES uptake in tumours (McGuire et al., 1991). Pretreatment [18F]FES PET predicts positive response to endocrine therapy and aromatase inhibitors (Liao et al., 2016).

In rats and humans, about 95% of [18F]FES is bound to plasma proteins, initially to albumin, and then rapidly also to SHBG (Mathias et al., 1987; Mankoff et al., 1997; Tewson et al., 1999). In rats, [18F]FES is freely diffusible between RBCs and plasma, and RBC-to-plasma ratio increased to 0.37±0.07 at 5 min, and then stayed on that level (Moresco et al., 1995). [18F]FES is highly extracted and glucuronide- and sulphate-conjugated in the liver, and then eliminated by the kidneys, while the metabolites excreted into the bile are reabsorbed in the small intestine via enterohepatic circulation (Lefebvre-Lacœuille et al., 2015; Liao et al., 2016). SHBG binding may protect [18F]FES from metabolism in the liver (Jonson et al., 1999). 20 min after administration the fraction of non-metabolized [18F]FES in plasma is 20%, and the radioactive metabolites contribute to background activity in PET images (Mathias et al., 1987; Mankoff et al., 1997).

Visual and semi-quantitative (tumour-to-background ratio) evaluation has shown high interobserver agreement (Mammatas et al., 2020). Recommendations for the use of [18F]FES PET, including the indications, correct patient preparation, scan acquisition, and analysis of the images, have been published (Venema et al., 2016; Kurland et al., 2020). Logan plot with metabolite-corrected plasma curve as input function has been used to study drug occupancy in the brain (Conlan et al., 2020). In rat brain, VT has been calculated from 2TCM with fixed VB (Khayum et al., 2014). In breast cancer, data has been analysed with FUR, using metabolite-corrected venous plasma curve as input function (Peterson et al., 2008). Metabolite-corrected plasma curve can be obtain from image-derived blood curve by using plasma-to-blood ratios and metabolite fractions from venous blood samples (Iqbal et al., 2022).

In rats, striatum has been used as reference tissue to calculate binding potential from Logan plot derived VTs, and Bmax and KD from tissue concentrations during equilibrium at different ER saturation levels (Moresco et al., 1995).

SUV is sufficient to assess ER expression in breast cancer; pre-menopausal estradiol levels, and differences in [18F]FES metabolism do not interfere with [18F]FES uptake, but SHBG and BMI influences SUV; SUVLBM may be better when comparing SUV between patients (Peterson et al., 2011).

Rapid metabolism and blood clearance of [18F]FES has led to development of numerous FES derivatives, with the aim to improve the metabolic stability and affinity to ER (Xu et al., 2018).


4-fluoro-11β-methoxy-16α-[18F]-fluoroestradiol ([18F]4FMFES) is an estradiol analogue and [18F]FES derivative. Effective dose is well within acceptable limits, but the absorbed dose to the gallbladder is relatively high, because of high hepatobiliary excretion (Beauregard et al., 2009). In breast cancer, [18F]4FMFES offers better tumour contrast than [18F]FES (Paquette et al., 2018). [18F]4FMFES allows visualization of ER expression in the pituitary with better contrast than [18F]FES in humans, rats, and mice (Paquette et al., 2020).


ERβ is expressed in ovarian granulosa cells, male reproductive organs, central nervous system (CNS), cardiovascular system, lung, immune system, gut, kidney and prostate. Adenocarcinomas of lung origin usually express ERβ.


2-[18F]fluoro-6-(6-hydroxynaphthalen-2-yl)pyridin-3-ol ([18F]FHNP) binds selectively to ERβ (Antunes et al., 2017a). In rats, [18F]FHNP uptake in studied tumours has been been low compared with [18F]FES uptake, reflecting the lower expression of ERβ to ERα in these tumour models. [18F]FHNP is metabolized rapidly, so that 5 min after administration the fraction of intact tracer in plasma is only 6%. Metabolite corrected plasma curve indicated a rapid monoexponential clearance with half-life of 0.29±0.06 min (Antunes et al., 2017b). Corresponding Ki values are obtained from Patlak plot and irreversible 2TCM, and semi-quantitative SUV correlates well with Ki (Antunes et al., 2017b).


GPER1 is widely expressed, for example in the skeletal muscle, neurons, vascular endothelium, various immune cells, effector organs of autonomic nervous system, and in several tumour types.

See also:


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Updated at: 2023-01-08
Created at: 2023-01-02
Written by: Vesa Oikonen