Recommendations and Technical Aspects of 16α-[18F]Fluoro-17β-Estradiol PET to Image the Estrogen Receptor In Vivo: The Groningen Experience

Design, Synthesis, and Biological Evaluation of Novel Nonsteroidal 18F-Labeled PET Probes Specifically Targeting Estrogen Receptor α

The estrogen receptor α positive (ERα+) subtype represents nearly 70% of all breast cancers (BCs), which seriously threaten women's health. Positron emission computed tomography (PET) characterizes its superiority in detecting the recurrence and metastasis of BC. In this article, an array of novel PET probes ([18F]R-1, [18F]R-2, [18F]R-3, and [18F]R-4) targeting ERα based on the tetrahydropyridinyl indole scaffold were developed. Among them, [18F]R-3 and [18F]R-4 showed good target specificity toward ERα and could distinguish MCF-7 (ERα+) and MDA-MB-231 (ERα-) tumors efficiently. Especially, [18F]R-3 could differentiate the ERα positive/negative tumors successfully with a higher tumor-to-muscle uptake ratio (T/M) than that of [18F]R-4. The radioactivity of [18F]R-3 in the MCF-7 tumor was 5.24 ± 0.84%ID/mL and its T/M ratio was 2.49 ± 0.62 at 25 min postinjection, which might be the optimal imaging time point in PET scanning. On the contrary, [18F]R-3 did not accumulate in the MDA-MB-231 tumor at all. The autoradiography analysis of [18F]R-3 on the MCF-7 tumor-bearing mice model was consistent with the PET imaging results. [18F]R-3 exhibited the pharmacokinetic property of rapid distribution and slow clearance, making it suitable for use as a diagnostic PET probe. Overall, [18F]R-3 was capable of serving as a PET radiotracer to delineate the ERα+ tumor and was worthy of further exploitation.

Clinical and economic outcomes of adding [18F]FES PET/CT in estrogen receptor status identification in metastatic and recurrent breast cancer in the US

BACKGROUND AND OBJECTIVES

Correct identification of estrogen receptor (ER) status in breast cancer (BC) is crucial to optimize treatment; however, standard of care, involving biopsy and immunohistochemistry (IHC), and other diagnostic tools such as 2-deoxy-2-[18F]fluoro-D-glucose or 2-[18F]fluoro-2-deoxy-D-glucose ([18F]FDG), can yield inconclusive results. 16α-[18F]fluoro-17β-fluoroestradiol ([18F]FES) can be a powerful tool, providing high diagnostic accuracy of ER-positive disease. The aim of this study was to estimate the budget impact and cost-effectiveness of adding [18F]FES PET/CT to biopsy/IHC in the determination of ER-positive status in metastatic (mBC) and recurrent breast cancer (rBC) in the United States (US).

METHODS

An Excel-based decision tree, combined with a Markov model, was developed to estimate the economic consequences of adding [18F]FES PET/CT to biopsy/IHC for determining ER-positive status in mBC and rBC over 5 years. Scenario A, where the determination of ER-positive status is carried out solely through biopsy/IHC, was compared to scenario B, where [18F]FES PET/CT is used in addition to biopsy/IHC.

RESULTS

The proportion of true positive and true negative test results increased by 0.2 to 8.0 percent points in scenario B compared to scenario A, while re-biopsies were reduced by 94% to 100%. Scenario B resulted in cost savings up to 142 million dollars.

CONCLUSIONS

Adding [18F]FES PET/CT to biopsy/IHC may increase the diagnostic accuracy of the ER status, especially when a tumor sample cannot be obtained, or the risk of a biopsy-related complication is high. Therefore, adding [18F]FES PET/CT to biopsy/IHC would have a positive impact on US clinical and economic outcomes.

Dynamic whole-body [18F]FES PET/CT increases lesion visibility in patients with metastatic breast cancer

BACKGROUND

Correct classification of estrogen receptor (ER) status is essential for prognosis and treatment planning in patients with breast cancer (BC). Therefore, it is recommended to sample tumor tissue from an accessible metastasis. However, ER expression can show intra- and intertumoral heterogeneity. 16α-[18F]fluoroestradiol ([18F]FES) Positron Emission Tomography/Computed Tomography (PET/CT) allows noninvasive whole-body (WB) identification of ER distribution and is usually performed as a single static image 60 min after radiotracer injection. Using dynamic whole-body (D-WB) PET imaging, we examine [18F]FES kinetics and explore whether Patlak parametric images ( K i ) are quantitative and improve lesion visibility.

RESULTS

This prospective study included eight patients with metastatic ER-positive BC scanned using a D-WB PET acquisition protocol. The kinetics of [18F]FES were best characterized by the irreversible two-tissue compartment model in tumor lesions and in the majority of organ tissues. K i values from Patlak parametric images correlated with K i values from the full kinetic analysis, r2 = 0.77, and with the semiquantitative mean standardized uptake value (SUVmean), r2 = 0.91. Furthermore, parametric K i images had the highest target-to-background ratio (TBR) in 162/164 metastatic lesions and the highest contrast-to-noise ratio (CNR) in 99/164 lesions compared to conventional SUV images. TBR was 2.45 (95% confidence interval (CI): 2.25-2.68) and CNR 1.17 (95% CI: 1.08-1.26) times higher in K i images compared to SUV images. These quantitative differences were seen as reduced background activity in the K i images.

CONCLUSION

[18F]FES uptake is best described by an irreversible two-tissue compartment model. D-WB [18F]FES PET/CT scans can be used for direct reconstruction of parametric K i images, with superior lesion visibility and K i values comparable to K i values found from full kinetic analyses. This may aid correct ER classification and treatment decisions. Trial registration ClinicalTrials.gov: NCT04150731, https://clinicaltrials.gov/study/NCT04150731.

Head-to-head comparison of 18F-FDG and 18F-FES PET/CT for initial staging of ER-positive breast cancer patients

PURPOSE

To compare the diagnostic performance of 18F-fluorodeoxyglucose (18F-FDG) and 18F-fluoroestradiol (18F-FES) positron emission tomography/computed tomography (PET/CT) for initial staging of estrogen receptor (ER) positive breast cancer.

METHODS

Twenty-eight patients with ER-positive breast cancer underwent 18F-FDG and 18F-FES PET/CT for initial staging. Diagnostic performance and concordance rates were analyzed for both radiotracers. Semiquantitative parameters of maximum standardized uptake value (SUVmax) and tumor-to-normal ratio (T/N ratio) were compared using Wilcoxon signed-rank test. Factors potentially affecting the degree of radiotracer uptake were analyzed by multi-level linear regression analysis.

RESULTS

The overall diagnostic performance of 18F-FES was comparable to 18F-FDG, except for higher specificity and NPV, with sensitivity, specificity, PPV, NPV, and accuracy of 87.56%, 100%, 100%, 35.14%, and 88.35%, respectively, for 18F-FES and 83.94%, 30.77%, 94.74%, 11.43%, and 95.37%, respectively, for 18F-FDG. Diagnostic performance of strong ER expression was better in 18F-FES but worse for 18F-FDG. There was a correlation of mucinous cell type and Allred score 7-8 with 18F-FES uptake, with correlation coefficients of 26.65 (19.28, 34.02), 5.90 (- 0.005, 11.81), and p-value of < 0.001, 0.05, respectively. Meanwhile, luminal B and Ki-67 were related to 18F-FDG uptake, with correlation coefficients of 2.76 (1.10, 0.20), 0.11 (0.01, 0.2), and p-value of 0.018, 0.025, respectively.

CONCLUSION

Diagnostic performance of 18F-FES is comparable to 18F-FDG, but better for strongly ER-positive breast cancer. Combination of 18F-FES and 18F-FDG would potentially overcome the limitations of each tracer with more accurate staging.

Diffuse bilateral 18F-Fluoroestradiol pulmonary uptake in patients with metastatic estrogen receptor positive breast cancer: A case report

INTRODUCTION

Breast cancer is the most commonly diagnosed cancer in women and hormonal therapy is an established treatment for estrogen receptor (ER) positive breast cancers. 18F-Fluoroestradiol (FES) is an emerging radiotracer used to determine hormone status in patients with ER positive breast cancer as FES specifically binds to the alpha subtype of estrogen receptors. As with all radiotracers, FES demonstrates background uptake within various tissues and organs besides the targeted breast cancer and metastatic disease. To date, FES has mostly been shown to demonstrate uptake within the lungs from metastatic disease or in a more focal region after radiation therapy.

CASE AND OUTCOMES

We present two patients with stage IV ER positive breast cancer who underwent FES positron emission tomography and computed tomography (PET/CT) scans to evaluate for metastatic disease; both of which demonstrated diffuse bilateral mild-moderate pulmonary uptake. The first patient had a severe lung injury which was improving but still present at the time of her FES PET/CT. The second patient had a remote history of prior right breast radiation therapy for a prior breast cancer as well as emphysema and mild interstitial disease.

DISCUSSION

To date, FES uptake within the lungs has been shown to be secondary to fibrotic changes secondary to prior radiation therapy and the uptake is localized to a focal region within the lung corresponding to the localized region around the tumor needing radiation therapy. We present two FES PET/CT scans that demonstrate diffuse bilateral mild-moderate uptake. We believe the first patient's bilateral FES uptake was secondary to inflammatory changes from her acute lung injury. Our second patient has a remote history of right breast radiation therapy that would not account for her diffuse lung uptake but does have emphysema and mild interstitial disease which can account for the FES uptake seen in her FES PET/CT.

CONCLUSION

18F-Fluoroestradiol is an emerging radiotracer that binds to estrogen receptors and is being used to determine hormone receptor status in women with ER positive breast cancers, which is the most commonly diagnosed cancer in women. Therefore, it's important to understand where it might demonstrate uptake and why. We highlighted two unique cases of mild-moderate pulmonary uptake of FES to provide further information about FES. Overall, we conclude that diffuse bilateral mild-moderate FES uptake within the lungs is likely secondary to inflammation, interstitial disease, or a combination thereof.

Estrogen Receptor-Targeted and Progesterone Receptor-Targeted PET for Patients with Breast Cancer

Estrogen receptor (ER)-targeted imaging with 16α-18F-fluoro-17β-fluoroestradiol (18F-FES) has multiple proven clinical applications for patients with ER-positive breast cancer, including helping to select optimal patients for endocrine therapies, assessing ER status in lesions that are difficult to biopsy, and evaluating lesions with inconclusive results on other imaging tests. This has led to US Food and Drug Administration approval of 18F-FES PET for patients with ER-positive breast cancer. Newer progesterone receptor-targeted imaging agents are in clinical trials.

Dynomics: A Novel and Promising Approach for Improved Breast Cancer Prognosis Prediction

Traditional imaging techniques for breast cancer (BC) diagnosis and prediction, such as X-rays and magnetic resonance imaging (MRI), demonstrate varying sensitivity and specificity due to clinical and technological factors. Consequently, positron emission tomography (PET), capable of detecting abnormal metabolic activity, has emerged as a more effective tool, providing critical quantitative and qualitative tumor-related metabolic information. This study leverages a public clinical dataset of dynamic ¹⁸F-Fluorothymidine (FLT) PET scans from BC patients, extending conventional static radiomics methods to the time domain-termed as 'Dynomics'. Radiomic features were extracted from both static and dynamic PET images on lesion and reference tissue masks. The extracted features were used to train an XGBoost model for classifying tumor versus reference tissue and complete versus partial responders to neoadjuvant chemotherapy. The results underscored the superiority of dynamic and static radiomics over standard PET imaging, achieving accuracy of 94% in tumor tissue classification. Notably, in predicting BC prognosis, dynomics delivered the highest performance, achieving accuracy of 86%, thereby outperforming both static radiomics and standard PET data. This study illustrates the enhanced clinical utility of dynomics in yielding more precise and reliable information for BC diagnosis and prognosis, paving the way for improved treatment strategies.

Increased 18 F-Fluoroestradiol Uptake of Radiation Pneumonitis in a Patient With Metastatic Breast Cancer

ABSTRACT

A 42-year-old woman diagnosed with de-novo stage IV breast cancer underwent 18 F-fluoroestradiol (FES) PET/CT to evaluate the estrogen receptor status of metastatic lesions. The largest pulmonary nodule showed obvious FES uptake, consistent with pulmonary metastases from breast cancer. Interestingly, the images revealed a striking accumulation of FES in ground-glass attenuation in the left lobe of lung, suggestive of radiation pneumonitis.

Breast PET/MRI Hybrid Imaging and Targeted Tracers

The recent introduction of hybrid positron emission tomography/magnetic resonance imaging (PET/MRI) as a promising imaging modality for breast cancer assessment has prompted fervent research activity on its clinical applications. The current knowledge regarding the possible clinical applications of hybrid PET/MRI is constantly evolving, thanks to the development and clinical availability of hybrid scanners, the development of new PET tracers and the rise of artificial intelligence (AI) techniques. In this state-of-the-art review on the use of hybrid breast PET/MRI, the most promising advanced MRI techniques (diffusion-weighted imaging, dynamic contrast-enhanced MRI, magnetic resonance spectroscopy, and chemical exchange saturation transfer) are discussed. Current and experimental PET tracers (¹⁸ F-FDG, ¹⁸ F-NaF, choline, ¹⁸ F-FES, ¹⁸ F-FES, ⁸⁹ Zr-trastuzumab, choline derivatives, ¹⁸ F-FLT, and ⁶⁸ Ga-FAPI-46) are described in order to provide an overview on their molecular mechanisms of action and corresponding clinical applications. New perspectives represented by the use of radiomics and AI techniques are discussed. Furthermore, the current strengths and limitations of hybrid PET/MRI in the real world are highlighted. EVIDENCE LEVEL: 2 TECHNICAL EFFICACY: Stage 2.

Nuclear Receptor Imaging In Vivo-Clinical and Research Advances

Nuclear receptors are transcription factors that function in normal physiology and play important roles in diseases such as cancer, inflammation, and diabetes. Noninvasive imaging of nuclear receptors can be achieved using radiolabeled ligands and positron emission tomography (PET). This quantitative imaging approach can be viewed as an in vivo equivalent of the classic radioligand binding assay. A main clinical application of nuclear receptor imaging in oncology is to identify metastatic sites expressing nuclear receptors that are targets for approved drug therapies and are capable of binding ligands to improve treatment decision-making. Research applications of nuclear receptor imaging include novel synthetic ligand and drug development by quantifying target drug engagement with the receptor for optimal therapeutic drug dosing and for fundamental research into nuclear receptor function in cells and animal models. This mini-review provides an overview of PET imaging of nuclear receptors with a focus on radioligands for estrogen receptor, progesterone receptor, and androgen receptor and their use in breast and prostate cancer.

Case report: 18F-FES PET/CT predicted treatment responses of second-line and third-line CDK4/6 inhibitors after disease progression on first-line CDK4/6 inhibitor in a HR+/HER2- metastatic breast cancer patient

Background

Cyclin-dependent kinase 4/6 inhibitor (CDK4/6i) has become the commonest first-line treatment of hormonal receptor positive and human epidermal growth factor receptor 2 negative (HR+/HER2-) metastatic breast cancer (MBC). However, therapy is quite individualized after progression of disease (PD) when CDK4/6i fails. Estrogen receptor (ER) status of metastatic lesions of bone, lung or liver might be different from the primary tumor and biopsy of metastatic lesions was invasive and not always available. Prediction of treatment response after PD of CDK4/6i remains unsolved. ¹⁸F-fluoroestradiol (FES) PET/CT could non-invasively reveal ER expression both in primary and metastatic breast cancer and recognize heterogeneity of ER status.

Case presentation

A 70-year-old woman with Parkinson's disease, osteoporosis and cardiovascular co-morbidity was diagnosed with HR+/HER2- breast cancer (pT2N2M0, stage IIIa). Three years later, she developed metastases in right lung and pleura with pleural effusion and received palbociclib + letrozole. After 8 months the disease progressed, and ¹⁸F-FES PET/CT revealed multiple ER-positive pleural lesions and ER-negative pulmonary nodules after PD and the progression-free survival (PFS) of first-line CDK4/6i was 8 months. Since most of the metastatic lesions were ER-positive, abemaciclib + fulvestrant were chosen as the second-line CDK4/6i treatment and the PFS was 15 months. Another ¹⁸F-FES PET/CT showed a new ER-positive pleural mass with multiple ER-negative pulmonary nodules. Since ¹⁸F-FES PET/CT revealed that the dominant lesions were still ER-positive, dalpiciclib + exemestane + fulvestrant were prescribed as the third-line CDK4/6i treatment. Currently the patient's disease had been stable for 2 months.

Conclusion

This case demonstrated that ¹⁸F-FES PET/CT could show ER heterogeneity non-invasively and reveal the treatment responses a predictive imaging tool of serial second- and third-line of CDK4/6i treatments when first-line CDK4/6i failed in HR+/HER2- MBC. So long as the dominant or newly-developed metastatic lesion was ER-positive on ¹⁸F-FES PET after first-line CDK4/6i, the patient might show certain therapeutic response towards endocrine-based treatment including second- and third-line of CDK4/6i, and thus increased the time to chemotherapy (TTC).

Clinical Validity of 16α-[18F]Fluoro-17β-Estradiol Positron Emission Tomography/Computed Tomography to Assess Estrogen Receptor Status in Newly Diagnosed Metastatic Breast Cancer

PURPOSE

Determining the estrogen receptor (ER) status is essential in metastatic breast cancer (MBC) management. Whole-body ER imaging with 16α-[¹⁸F]fluoro-17β-estradiol positron emission tomography ([¹⁸F]FES-PET) is increasingly used for this purpose. To establish the clinical validity of the [¹⁸F]FES-PET, we studied the diagnostic accuracy of qualitative and quantitative [¹⁸F]FES-PET assessment to predict ER expression by immunohistochemistry in a metastasis.

METHODS

In a prospective multicenter trial, 200 patients with newly diagnosed MBC underwent extensive workup including molecular imaging. For this subanalysis, ER expression in the biopsied metastasis was related to qualitative whole-body [¹⁸F]FES-PET evaluation and quantitative [¹⁸F]FES uptake in the corresponding metastasis. A review and meta-analysis regarding [¹⁸F]FES-PET diagnostic performance were performed.

RESULTS

Whole-body [¹⁸F]FES-PET assessment predicted ER expression in the biopsied metastasis with good accuracy: a sensitivity of 95% (95% CI, 89 to 97), a specificity of 80% (66 to 89), a positive predictive value (PPV) of 93% (87 to 96), and a negative predictive value (NPV) of 85% (72 to 92) in 181 of 200 evaluable patients. Quantitative [¹⁸F]FES uptake predicted ER immunohistochemistry in the corresponding metastasis with a sensitivity/specificity of 91%/69% and a PPV/NPV of 90%/71% in 156 of 200 evaluable patients. For bone metastases, PPV/NPV was 92%/81%. Meta-analysis with addition of our data has increased diagnostic performance and narrowed the 95% CIs compared with previous studies with a sensitivity/specificity of both 86% (81 to 90 and 73 to 93, respectively).

CONCLUSION

In this largest prospective series so far, we established the clinical validity of [¹⁸F]FES-PET to determine tumor ER status in MBC. In view of the high diagnostic accuracy of qualitatively assessed whole-body [¹⁸F]FES-PET, this noninvasive imaging modality can be considered a valid alternative to a biopsy of a metastasis to determine ER status in newly MBC (ClinicalTrials.gov identifier: NCT01957332).

PET Imaging of Estrogen Receptors Using 18F-Based Radioligands

In vivo molecular imaging of estrogen receptor alpha (ER) can be performed via positron emission tomography (PET) using ER-specific radioligands, such as 16α-[¹⁸F]fluoro-17β-estradiol (¹⁸F-FES). ¹⁸F-FES is a radiopharmaceutical recently approved by the United States Food and Drug Administration for use with PET imaging to detect ER+ lesions in patients with recurrent or metastatic breast cancer as an adjunct to biopsy. ¹⁸F-FES PET imaging has been used in clinical studies and preclinical research to assess whole-body ER protein expression and ligand binding function across multiple metastatic sites, to demonstrate inter-tumoral and temporal heterogeneity of ER expression, to quantify the pharmacodynamic effects of ER antagonist treatment, and to predict endocrine therapy response. ¹⁸F-FES PET has also been studied for imaging ER in endometrial and ovarian cancer. This chapter details the experimental protocol for ¹⁸F-FES PET imaging of ER in preclinical tumor xenograft models. Consistent adherence to key methodologic details will facilitate obtaining meaningful and reproducible ¹⁸F-FES PET preclinical imaging results, which could yield additional insight for clinical trials regarding imaging biomarkers and oncologic therapy.

Identification of the estrogen receptor beta as a possible new tamoxifen-sensitive target in diffuse large B-cell lymphoma

Diffuse large B-cell lymphoma (DLBCL) is the most common lymphoma subtype. Despite the proven efficacy of combined immunochemotherapy (R-CHOP) in the majority of patients, ~40% of DLBCL patients do not respond or will relapse and consequently have a very poor prognosis. The development of targeted therapies has not improved patient survival, underscoring the need for new treatment approaches. Using an unbiased genome-wide CD20 guilt-by-association approach in more than 1800 DLBCL patients, we previously identified the estrogen receptor beta (ERβ) as a new target in DLBCL. Here, we demonstrate that ERβ is expressed at significantly higher levels in DLBCL compared to normal B cells, and ERβ plays a role in the protection against apoptosis in DLBCL. Targeting of the ERβ with the selective estrogen receptor modulator tamoxifen reduces cell viability in all tested DLBCL cell lines. Tamoxifen-induced cell death was significantly decreased in an ERβ knock-out cell line. The activity of tamoxifen was confirmed in a xenograft human lymphoma model, as tumor growth decreased, and survival significantly improved. Finally, tamoxifen-treated breast cancer (BC) patients showed a significantly reduced risk of 38% for DLBCL compared to BC patients who did not receive tamoxifen. Our findings provide a rationale to investigate tamoxifen, a hormonal drug with a good safety profile, in DLBCL patients.

Determination of the Estrogen Receptor Status of Leptomeningeal Metastasis in Patients with Metastatic Breast Cancer Using [18F]-FES PET/CT: a Case Report

Accurate determination of the hormone receptor status in patients with breast cancer is necessary to provide better palliative treatment because of the frequent discordant status of hormone receptors between primary breast cancer and metastatic lesions. In particular, the invasive nature and small number of cells recovered from the cerebrospinal fluid collected through lumbar puncture limit the detailed immunocytochemistry investigation of leptomeningeal metastasis. Moreover, none of the conventional imaging modalities provide any information on the hormone receptor status of leptomeningeal metastasis in patients with breast cancer. Herein, we report estrogen receptor-positive leptomeningeal metastasis clearly demonstrated by 16α-[¹⁸F]-fluoro-17β-estradiol ([¹⁸F]-FES) positron emission tomography/computed tomography (PET/CT) in a patient with metastatic breast cancer.

Non-conventional and Investigational PET Radiotracers for Breast Cancer: A Systematic Review

Breast cancer is one of the most common malignancies in women, with high morbidity and mortality rates. In breast cancer, the use of novel radiopharmaceuticals in nuclear medicine can improve the accuracy of diagnosis and staging, refine surveillance strategies and accuracy in choosing personalized treatment approaches, including radioligand therapy. Nuclear medicine thus shows great promise for improving the quality of life of breast cancer patients by allowing non-invasive assessment of the diverse and complex biological processes underlying the development of breast cancer and its evolution under therapy. This review aims to describe molecular probes currently in clinical use as well as those under investigation holding great promise for personalized medicine and precision oncology in breast cancer.

Analyzing the Estrogen Receptor Status of Liver Metastases with [18F]-FES-PET in Patients with Breast Cancer

Background: Positron emission tomography (PET) with 16α-[¹⁸F]-fluoro-17β-estradiol ([¹⁸F]-FES) can visualize estrogen receptor (ER) expression, but it is challenging to determine the ER status of liver metastases, due to high physiological [¹⁸F]-FES uptake. We evaluated whether [¹⁸F]-FES-PET can be used to determine the ER status of liver metastases, using corresponding liver biopsies as the gold standard. Methods: Patients with metastatic breast cancer (n = 23) were included if they had undergone a [¹⁸F]-FES-PET, liver metastasis biopsy, CT-scan, and [¹⁸F]-FDG-PET. [¹⁸F]-FES-PET scans were assessed by visual and quantitative analysis, tracer uptake was correlated with ER expression measured by immunohistochemical staining and the effects of region-of-interest size and background correction were determined. Results: Visual analysis allowed ER assessment of liver metastases with 100% specificity and 18% sensitivity. Quantitative analysis improved the sensitivity. Reduction of the region-of-interest size did not further improve the results, but background correction improved ER assessment, resulting in 83% specificity and 77% sensitivity. Using separate thresholds for ER+ and ER− metastases, positive and negative predictive values of 100% and 75%, respectively, could be obtained, although 30% of metastases remained inconclusive. Conclusion: In the majority of liver metastases, ER status can be determined with [¹⁸F]-FES-PET if background correction and separate thresholds are applied.

Value of 18F-FES PET in Solving Clinical Dilemmas in Breast Cancer Patients: A Retrospective Study

Breast cancer (BC) is a heterogeneous disease in which estrogen receptor (ER) expression plays an important role in most tumors. A clinical dilemma may arise when a metastasis biopsy to determine the ER status cannot be performed safely or when ER heterogeneity is suspected between tumor lesions. Whole-body ER imaging, such as 16α-¹⁸F-fluoro-17β-estradiol (¹⁸F-FES) PET, may have added value in these situations. However, the role of this imaging technique in routine clinical practice remains to be further determined. Therefore, we assessed whether the physician's remaining clinical dilemma after the standard workup was solved by the ¹⁸F-FES PET scan. Methods: This retrospective study included ¹⁸F-FES PET scans of patients who had (or were suspected to have) ER-positive metastatic BC and for whom a clinical dilemma remained after the standard workup. The scans were performed at the University Medical Center of Groningen between November 2009 and January 2019. We investigated whether the physician's clinical dilemma was solved, defined either as solving the clinical dilemma through the ¹⁸F-FES PET results or as basing a treatment decision directly on the ¹⁸F-FES PET results. In addition, the category of the clinical dilemma was reported, as well as the rate of ¹⁸F-FES-positive or -negative PET scans, and any correlation to the frequency of solved dilemmas was determined. Results: One hundred ¹⁸F-FES PET scans were performed on 83 patients. The clinical dilemma categories were inability to determine the extent of metastatic disease or suspected metastatic disease with the standard workup (n = 52), unclear ER status of the tumor (n = 31), and inability to determine which primary tumor caused the metastases (n = 17). The dilemmas were solved by ¹⁸F-FES PET in 87 of 100 scans (87%). In 81 of 87 scans, a treatment decision was based directly on ¹⁸F-FES PET results (treatment change, 51 scans; continuance, 30 scans). The frequency of solved dilemmas was not related to the clinical dilemma category (P = 0.334). However, the frequency of solved dilemmas was related to whether scans were ¹⁸F-FES-positive (n = 63) or ¹⁸F-FES-negative (n = 37; P < 0.001). Conclusion: For various indications, the ¹⁸F-FES PET scan can help to solve most clinical dilemmas that may remain after the standard workup. Therefore, the ¹⁸F-FES PET scan has added value in BC patients who present the physician with a clinical dilemma.

Detection of Dural Metastases Before the Onset of Clinical Symptoms by 16α-[18F]Fluoro-17β-Estradiol PET in a Patient With Estrogen Receptor-Positive Breast Cancer

ABSTRACT

We offer an illustrative case about estrogen receptor (ER) imaging (also known as 16α-[18F]fluoro-17β-estradiol ([18F]-FES) PET) and the detection of metastatic lesions in the dural region. We present a case of a woman with ER-positive metastatic breast cancer and high [18F]-FES uptake in the dural region on PET imaging, without associated clinical symptoms. These lesions were missed on [18F]-FDG PET because of physiological [18F]-FDG uptake in the brain. This case highlighted some difficulties in the interpretation of imaging of brain metastases and demonstrated the added value of [18F]-FES PET imaging. [18F]-FES PET could be used to prove the presence of ER-positive metastases in the brain.

The quest for improving the management of breast cancer by functional imaging: The discovery and development of 16α-[18F]fluoroestradiol (FES), a PET radiotracer for the estrogen receptor, a historical review

INTRODUCTION

16α-[18F]Fluoroestradiol (FES), a PET radiotracer for the estrogen receptor (ER) in breast cancer, was the first receptor-targeted PET radiotracer for oncology and is continuing to prove its value in clinical research, antiestrogen development, and breast cancer care. The story of its conception, design, evaluation and use in clinical studies parallels the evolution of the whole field of receptor-targeted radiotracers, one greatly influenced by the research and intellectual contributions of William C. Eckelman.

METHODS AND RESULTS

The development of methods for efficient production of fluorine-18, for conversion of [18F]fluoride ion into chemically reactive form, and for its rapid and efficient incorporation into suitable estrogen precursor molecules at high molar activity, were all methodological underpinnings required for the preparation of FES. FES binds to ER with very high affinity, and its in vivo uptake by ER-dependent target tissues in animal models was efficient and selective, findings that preceded its use for PET imaging in patients with breast cancer.

ADVANCES IN KNOWLEDGE AND IMPLICATIONS FOR PATIENT CARE

Comparisons between ER levels measured by FES-PET imaging of breast tumors with tissue-specimen ER quantification by IHC and other methods show that imaging provided improved prediction of benefit from endocrine therapies. Serial imaging of ER by FES-PET, before and after dosing patients with antiestrogens, is used to determine the efficacious dose for established antiestrogens and to facilitate clinical development of new ER antagonists. Beyond FES imaging, PET-based hormone challenge tests, which evaluate the functional status of ER by monitoring rapid changes in tumor metabolic or transcriptional activity after a brief estrogen challenge, provide highly sensitive and selective predictions of whether or not there will be a favorable response to endocrine therapies. There is sufficient interest in the clinical applications of FES that FDA approval is being sought for its wider use in breast cancer.

CONCLUSIONS

FES was the first PET probe for a receptor in cancer, and its development and clinical applications in breast cancer parallel the conceptual evolution of the whole field of receptor-binding radiotracers.

Image Quality and Interpretation of [18F]-FES-PET: Is There any Effect of Food Intake?

Background: High physiological 16α-[¹⁸F]-fluoro-17β-estradiol ([¹⁸F]-FES) uptake in the abdomen is a limitation of this positron emission tomography (PET) tracer. Therefore, we investigated the effect of food intake prior to PET acquisition on abdominal background activity in [¹⁸F]-FES-PET scans. Methods: Breast cancer patients referred for [¹⁸F]-FES-PET were included. Three groups were designed: (1) patients who consumed a chocolate bar (fatty meal) between tracer injection and imaging (n = 20), (2) patients who fasted before imaging (n = 20), and (3) patients without diet restrictions (control group, n = 20). We compared the physiological [¹⁸F]-FES uptake, expressed as mean standardized uptake value (SUV_mean), in the abdomen between groups. Results: A significant difference in [¹⁸F]-FES uptake in the gall bladder and stomach lumen was observed between groups, with the lowest values for the chocolate group and highest for the fasting group (p = 0.015 and p = 0.011, respectively). Post hoc analysis showed significant differences in the SUVmean of these organs between the chocolate and fasting groups, but not between the chocolate and control groups. Conclusion: This exploratory study showed that, compared to fasting, eating chocolate decreases physiological gall bladder and stomach [¹⁸F]-FES uptake; further reduction through a normal diet was not seen. A prospective study is warranted to confirm this finding.

A phase 1b study evaluating the effect of elacestrant treatment on estrogen receptor availability and estradiol binding to the estrogen receptor in metastatic breast cancer lesions using 18F-FES PET/CT imaging

BACKGROUND

Elacestrant is an oral selective estrogen receptor (ER) degrader. This phase 1b open-label, non-randomized study (RAD1901-106) was initiated to determine the effect of elacestrant on the availability of ER in lesions from postmenopausal women with ER+ advanced breast cancer (ABC) using 16α-¹⁸F-fluoro-17β-estradiol positron emission tomography with low-dose computed tomography (FES-PET/CT).

METHODS

Eligible patients were postmenopausal women with ER+, HER2- ABC; tumor progression after ≥ 6 months of 1-3 lines of endocrine treatment for ABC; and measurable or evaluable disease. Two 8-patient cohorts were enrolled: one treated with 400 mg elacestrant once daily (QD) and one treated with 200 mg elacestrant QD with dose escalation to 400 mg QD after 14 days. Elacestrant was dosed continuously until progressive disease, toxicity, or withdrawal. FES-PET/CT was performed pre-dose at baseline and 4 h post-dose on day 14. The primary endpoint was the percentage difference in FES uptake in tumor lesions (maximum 20) after 14 days of treatment compared to baseline. Overall response was investigator-assessed by Response Evaluation Criteria in Solid Tumors [RECIST] version 1.1.

RESULTS

Patients (n = 16; median age, 53.5 years) had ABC with a median 2.5 prior lines of endocrine therapy. Median reduction in tumor FES uptake from baseline to day 14 was 89.1% (Q1, Q3: 75.1%, 94.1%) and was similar in both cohorts (89.1% [Q1, Q3: 67.4%, 94.2%], 200/400 mg and 88.7% [Q1, Q3: 79.5%, 94.1%], 400 mg). Residual ER availability (> 25% persistence in FES uptake) on day 14 was observed in 3 patients receiving 200/400 mg (3/78, 37.5%) and 1 patient receiving 400 mg (1/8, 12.5%). The overall response rate (ORR) was 11.1% (1 partial response), and clinical benefit rate (CBR) was 30.8%. Median percentage change in FES uptake did not correlate with ORR or CBR. Adverse events occurring in > 20% of the patients were nausea (68.8%), fatigue (50.0%), dyspepsia (43.8%), vomiting (37.5%), and decreased appetite, dysphagia, and hot flush (31.3% each). Most events were grade 2 in severity.

CONCLUSION

Elacestrant 200 mg and 400 mg QD greatly reduced ER availability measured by FES-PET/CT. In a heavily pretreated population, elacestrant was associated with antitumor activity.

TRIAL REGISTRATION

ClinicalTrials.gov, NCT02650817 . Registered on 08 January 2016.

PET Imaging Agents (FES, FFNP, and FDHT) for Estrogen, Androgen, and Progesterone Receptors to Improve Management of Breast and Prostate Cancers by Functional Imaging

Many breast and prostate cancers are driven by the action of steroid hormones on their cognate receptors in primary tumors and in metastases, and endocrine therapies that inhibit hormone production or block the action of these receptors provide clinical benefit to many but not all of these cancer patients. Because it is difficult to predict which individuals will be helped by endocrine therapies and which will not, positron emission tomography (PET) imaging of estrogen receptor (ER) and progesterone receptor (PgR) in breast cancer, and androgen receptor (AR) in prostate cancer can provide useful, often functional, information on the likelihood of endocrine therapy response in individual patients. This review covers our development of three PET imaging agents, 16α-[18F]fluoroestradiol (FES) for ER, 21-[18F]fluoro-furanyl-nor-progesterone (FFNP) for PgR, and 16β-[18F]fluoro-5α-dihydrotestosterone (FDHT) for AR, and the evolution of their clinical use. For these agents, the pathway from concept through development tracks with an emerging understanding of critical performance criteria that is needed for successful PET imaging of these low-abundance receptor targets. Progress in the ongoing evaluation of what they can add to the clinical management of breast and prostate cancers reflects our increased understanding of these diseases and of optimal strategies for predicting the success of clinical endocrine therapies.

Application of PET Tracers in Molecular Imaging for Breast Cancer

PURPOSE OF REVIEW

Molecular imaging with positron emission tomography (PET) is a powerful tool to visualize breast cancer characteristics. Nonetheless, implementation of PET imaging into cancer care is challenging, and essential steps have been outlined in the international "imaging biomarker roadmap." In this review, we identify hurdles and provide recommendations for implementation of PET biomarkers in breast cancer care, focusing on the PET tracers 2-[18F]-fluoro-2-deoxyglucose ([18F]-FDG), sodium [18F]-fluoride ([18F]-NaF), 16α-[18F]-fluoroestradiol ([18F]-FES), and [89Zr]-trastuzumab.

RECENT FINDINGS

Technical validity of [18F]-FDG, [18F]-NaF, and [18F]-FES is established and supported by international guidelines. However, support for clinical validity and utility is still pending for these PET tracers in breast cancer, due to variable endpoints and procedures in clinical studies. Assessment of clinical validity and utility is essential towards implementation; however, these steps are still lacking for PET biomarkers in breast cancer. This could be solved by adding PET biomarkers to randomized trials, development of imaging data warehouses, and harmonization of endpoints and procedures.

18F-Fluoroestradiol PET Imaging in a Phase II Trial of Vorinostat to Restore Endocrine Sensitivity in ER+/HER2- Metastatic Breast Cancer

Histone deacetylase inhibitors (HDACIs) may overcome endocrine resistance in estrogen receptor-positive (ER+) metastatic breast cancer. We tested whether ¹⁸F-fluoroestradiol PET imaging would elucidate the pharmacodynamics of combination HDACIs and endocrine therapy. Methods: Patients with ER+/human epidermal growth factor receptor 2 (HER2)-negative metastatic breast cancer with prior clinical benefit from endocrine therapy but later progression on aromatase inhibitor (AI) therapy were given vorinostat (400 mg daily) sequentially or simultaneously with AI. ¹⁸F-fluoroestradiol PET and ¹⁸F-FDG PET scans were performed at baseline, week 2, and week 8. Results: Eight patients were treated sequentially, and then 15 simultaneously. Eight patients had stable disease at week 8, and 6 of these 8 patients had more than 6 mo of stable disease. Higher baseline ¹⁸F-fluoroestradiol uptake was associated with longer progression-free survival. ¹⁸F-fluoroestradiol uptake did not systematically increase with vorinostat exposure, indicating no change in regional ER estradiol binding, and ¹⁸F-FDG uptake did not show a significant decrease, as would have been expected with tumor regression. Conclusion: Simultaneous HDACIs and AI dosing in patients with cancer resistant to AI alone showed clinical benefit (6 or more months without progression) in 4 of 10 evaluable patients. Higher ¹⁸F-fluoroestradiol PET uptake identified patients likely to benefit from combination therapy, but vorinostat did not change ER expression at the level of detection of ¹⁸F-fluoroestradiol PET.

Whole-Body Characterization of Estrogen Receptor Status in Metastatic Breast Cancer with 16α-18F-Fluoro-17β-Estradiol Positron Emission Tomography: Meta-Analysis and Recommendations for Integration into Clinical Applications

Estrogen receptor (ER) status by immunohistochemistry (IHC) of cancer tissue is currently used to direct endocrine therapy in breast cancer. Positron emission tomography (PET) with 16α-18F-fluoro-17β-estradiol (¹⁸ F-FES) noninvasively characterizes ER ligand-binding function of breast cancer lesions. Concordance of imaging and tissue assays should be established for ¹⁸ F-FES PET to be an alternative or complement to tissue biopsy for metastatic lesions. We conducted a meta-analysis of published results comparing ¹⁸ F-FES PET and tissue assays of ER status in patients with breast cancer. PubMed and EMBASE were searched for English-language manuscripts with at least 10 patients and low overall risk of bias. Thresholds for imaging and tissue classification could differ between studies but had to be clearly stated. We used hierarchical summary receiver-operating characteristic curve models for the meta-analysis. The primary analysis included 113 nonbreast lesions from 4 studies; an expanded analysis included 327 total lesions from 11 studies. Treating IHC results as the reference standard, sensitivity was 0.78 (95% confidence region 0.65-0.88) and specificity 0.98 (0.65-1.00) for the primary analysis of nonbreast lesions. In the expanded analysis including non-IHC tissue assays and all lesion sites, sensitivity was 0.81 (0.73-0.87) and specificity 0.86 (0.68-0.94). These results suggest that ¹⁸ F-FES PET is useful for characterization of ER status of metastatic breast cancer lesions. We also review current best practices for conducting ¹⁸ F-FES PET scans. This imaging assay has potential to improve clinically relevant outcomes for patients with (historically) ER-positive metastatic breast cancer, including those with brain metastases and/or lobular histology. IMPLICATIONS FOR PRACTICE: 16α-18F-fluoro-17β-estradiol positron emission tomography (¹⁸ F-FES PET) imaging assesses estrogen receptor status in breast cancer in vivo. This work reviews the sensitivity and specificity of ¹⁸ F-FES PET in a meta-analysis with reference tissue assays and discusses best practices for use of the tracer as an imaging biomarker. ¹⁸ F-FES PET could enhance breast cancer diagnosis and staging as well as aid in therapy selection for patients with metastatic disease. Tissue sampling limitations, intrapatient heterogeneity, and temporal changes in molecular markers make it likely that ¹⁸ F-FES PET will complement existing assays when clinically available in the near future.

Radiopharmaceuticals for Breast Cancer and Neuroendocrine Tumors: Two Examples of How Tissue Characterization May Influence the Choice of Therapy

Molecular medicine has gained clinical relevance for the detection and staging of oncological diseases, to guide therapy decision making and for therapy follow-up due to the availability of new highly sensitive hybrid imaging camera systems and the development of new tailored radiopharmaceuticals that target specific molecules. The knowledge of the expression of different receptors on the primary tumor and on metastases is important for both therapeutic and prognostic purposes and several approaches are available aiming to achieve personalized medicine in different oncological diseases. In this review, we describe the use of specific radiopharmaceuticals to image and predict therapy response in breast cancer and neuroendocrine tumors since they represent a paradigmatic example of the importance of tumoral characterization of hormonal receptors in order to plan a tailored treatment. The most attractive radiopharmaceuticals for breast cancer are 16α-[¹⁸F]-fluoro-17β-estradiol for PET assessment of the estrogen expression, radiolabeled monoclonal antibody trastuzumab to image the human epidermal growth factor receptor 2, but also the imaging of androgen receptors with [¹⁸F]-fluorodihydrotestosterone.

Non-FDG PET/CT in Diagnostic Oncology: a pictorial review

Positron emission tomography/computed tomography (PET/CT) is currently one of the main imaging modalities for cancer patients worldwide. Fluorodeoxyglucose (FDG) PET/CT has earned its global recognition in the modern management of cancer patients and is rapidly becoming an important imaging modality for patients with cardiac, neurological, and infectious/inflammatory conditions.

Despite its proven benefits, FDG has limitations in the assessment of several relevant tumours such as prostate cancer. Therefore, there has been a pressing need for the development and clinical application of different PET radiopharmaceuticals that could image these tumours more precisely. Accordingly, several non-FDG PET radiopharmaceuticals have been introduced into the clinical arena for management of cancer. This trend will undoubtedly continue to spread internationally. The use of PET/CT with different PET radiopharmaceuticals specific to tumour type and biological process being assessed is part of the personalised precision medicine approach.

The objective of this publication is to provide a case-based method of understanding normal biodistribution, variants, and pitfalls, including several examples of different imaging appearances for the main oncological indications for each of the new non-FDG PET radiopharmaceuticals. This should facilitate the interpretation and recognition of common variants and pitfalls to ensure that, in clinical practice, the official report is accurate and helpful.

Some of these radiopharmaceuticals are already commercially available in many countries (e.g. ⁶⁸Ga-DOTATATE and DOTATOC), others are in the process of becoming available (e.g. ⁶⁸Ga-PSMA), and some are still being researched. However, this list is subject to change as some radiopharmaceuticals are increasingly utilised, while others gradually decrease in use.

Recent Advances in Imaging Steroid Hormone Receptors in Breast Cancer

Estrogen receptor (ER) and progesterone receptor (PR) are important prognostic and predictive biomarkers in breast cancer. PET using ER- and PR-specific radioligands enables a whole-body, noninvasive assessment of receptor expression. Recent investigations of ER imaging with ¹⁸F-fluoroestradiol have focused on diagnosing ER-positive metastatic disease, optimizing ER-targeted drug dosage, and predicting endocrine therapy benefit. Studies of PR imaging with ¹⁸F-fluorofuranyl norprogesterone have investigated how imaging changes in PR expression as a downstream target of ER activation may reflect an early response to ER-targeted therapy. This focused review highlights recent achievements in preclinical and clinical imaging of ER and PR in breast cancer.

Enhanced pulmonary uptake on 18F-FES-PET/CT scans after irradiation of the thoracic area: related to fibrosis?

Rationale

The use of 16α-[¹⁸F]fluoro-17β-estradiol (FES) positron emission tomography (PET) in clinical dilemmas and for therapy decision-making in lesions expressing estrogen receptors is growing. However, on a considerable number of FES PET scans, previously performed in a research and clinical setting in our institution, FES uptake was noticed in the lungs without an oncologic substrate. We hypothesized that this uptake was related to pulmonary fibrosis as a result of radiation therapy. This descriptive study therefore aimed to investigate whether radiation therapy in the thoracic area is possibly related to enhanced pulmonary, non-tumor FES uptake.

Methods

All FES-PET/CT scans performed in our institution from 2008 to 2017 were retrospectively analyzed. Scans from patients who had received irradiation in the thoracic area prior to the scan were compared to scans of patients who had never received irradiation in the thoracic area. The primary outcome was the presence of enhanced non-tumor FES uptake in the lungs, defined as visually increased FES uptake in the absence of an oncologic substrate on the concordant (contrast-enhanced) CT scan. All CT scans were evaluated for the presence of fibrosis or oncologic substrates.

Results

A total of 108 scans were analyzed: 70 scans of patients with previous irradiation in the thoracic area and 38 of patients without. Enhanced non-tumor FES uptake in the lungs was observed in 39/70 irradiated patients (56%), versus in 9/38 (24%) of non-irradiated patients. Fibrosis was present in 37 of the 48 patients with enhanced non-tumor FES uptake (77%), versus in 15 out of 60 (25%) patients without enhanced non-tumor uptake, irrespective of radiotherapy (p < 0.001).

Conclusion

After irradiation of the thorax, enhanced non-tumor uptake on FES-PET can be observed in the radiation field in a significant proportion of patients. This seems to be related to fibrosis. When observing enhanced FES uptake in the lungs, this should not be interpreted as metastases. Information on recent radiation therapy or history of pulmonary fibrosis should therefore be taken into consideration.

18F-Fluoroestradiol PET Imaging of Activating Estrogen Receptor-α Mutations in Breast Cancer

The purpose of this study was to determine the effect of estrogen receptor-α gene (ESR1) mutations at the tyrosine (Y) 537 amino acid residue within the ligand binding domain on ¹⁸F-fluoroestradiol (¹⁸F-FES) binding and in vivo tumor uptake compared with wild-type (WT)-estrogen receptor α (ER). Methods: ER-negative MDA-MB-231 breast cancer cells were used to generate stable cell lines that express WT-ER, Y537S, or Y537C mutant ER. Receptor expression and localization were confirmed by Western blot and immunofluorescence, respectively. ER transcriptional function was measured using an estrogen response element-luciferase reporter gene assay and quantitative polymerase chain reaction analysis of ER-regulated endogenous target genes. Saturation binding and competition assays were performed to determine equilibrium dissociation constant (K_d) and half maximal inhibitory concentration (IC50) values. ¹⁸F-FES uptake was measured in tumor xenografts grown in female athymic nude mice by small-animal PET/CT imaging and tissue biodistribution using 5.55 MBq (150 μCi) of ¹⁸F-FES. A 10-fold-lower injected dose of 0.555 MBq (15 μCi) of ¹⁸F-FES was also used for tissue biodistribution. Statistical significance was determined using ANOVA. Results: Y537S and Y537C mutations resulted in increased ER transcriptional activity in the absence of estrogen compared with WT-ER (11.48 ± 2.42 fold; P = 0.0002, and 5.89 ± 0.94 fold; P = 0.04, respectively). Constitutive ER activation of two target genes (PGR and TFF1) in the absence of estrogen was also observed in Y537S- and Y537C-ER cells compared with WT-ER. K_d values for ¹⁸F-FES were 0.98 ± 0.54 nM for Y537S-ER (P = 0.27) and 0.24 ± 0.03 nM for Y537C-ER (P = 0.95) compared with 0.07 ± 0.03 nM for WT-ER. IC50 values were 0.22 ± 0.09 nM for Y537S-ER (P = 0.97), 0.18 ± 0.09 nM for Y537C-ER (P = 0.99), and 0.19 ± 0.11 nM for WT-ER. Tumor xenografts expressing Y537S-ER (mean percentage injected dose per gram, 1.45 ± 0.06; P = 0.77) and Y537C-ER (2.09 ± 0.20; P = 0.21) had similar ¹⁸F-FES uptake compared with WT-ER (1.68 ± 0.12). Comparable ¹⁸F-FES uptake between Y537S-, Y537C-, and WT-ER xenografts was also observed using a 10-fold-lower injected dose with the tissue biodistribution assay. Conclusion: Since tumoral uptake of ¹⁸F-FES is not significantly impacted by Y537S-ER or Y537C-ER mutations, the potential diagnostic utility of ¹⁸F-FES PET imaging is expected to be equally valid for patients with or without these activating ESR1 mutations.

18F-16α-17β-Fluoroestradiol Binding Specificity in Estrogen Receptor-Positive Breast Cancer

Purpose To determine the binding specificity of ¹⁸F-16α-17β-fluoroestradiol (FES) in estrogen receptor (ER) α-positive breast cancer cells and tumor xenografts. Materials and Methods Protocols were approved by the office of biologic safety and institutional animal care and use committee. By using ER-negative MDA-MB-231 breast cancer cells, clonal lines were created that expressed either wild-type (WT; 231 WT ER) or G521R mutant ERα (231 G521R ER), which is defective in estradiol binding. ERα protein levels, subcellular localization, and transcriptional function were confirmed. FES binding was measured by using an in vitro cell uptake assay. In vivo FES uptake was measured in tumor xenografts by using small-animal positron emission tomographic/computed tomographic imaging of 24 mice (17 WT ER tumors, nine mutant G521R ER tumors, eight MDA-MB-231 tumors, and four MCF-7 ER-positive tumors). Statistical significance was determined by using Mann-Whitney (Wilcoxon rank sum) test. Results ERα transcriptional function was abolished in the mutated 231 G521R ER cells despite appropriate receptor protein expression and nuclear localization. In vitro FES binding in the 231 G521R ER cells was reduced to that observed in the parental cells. Similarly, there was no significant FES uptake in the 231 G521R ER xenografts (percent injected dose [ID] per gram, 0.49 ± 0.042), which was similar to the negative control MDA-MB-231 xenografts (percent ID per gram, 0.42 ± 0.051; P = .20) and nonspecific muscle uptake (percent ID per gram, 0.41 ± 0.0095; P = .06). Conclusion This study showed that FES retention in ER-positive breast cancer is strictly dependent on an intact receptor ligand-binding pocket and that FES binds to ERα with high specificity. These results support the utility of FES imaging for assessing tumor heterogeneity by localizing immunohistochemically ER-positive metastases that lack receptor-binding functionality. ^© RSNA, 2017 Online supplemental material is available for this article.