Molecular Imaging of Steroid Receptors in Breast Cancer

Abstract

Steroid receptors regulate gene expression for many important physiologic functions and pathologic processes. Receptors for estrogen, progesterone, and androgen have been extensively studied in breast cancer and their expression provides prognostic information as well as targets for therapy. Noninvasive imaging utilizing positron emission tomography and radiolabeled ligands targeting these receptors can provide valuable insight into predicting treatment efficacy, staging whole-body disease burden, and identifying heterogeneity in receptor expression across different metastatic sites. This review provides an overview of steroid receptor imaging with a focus on breast cancer and radioligands for estrogen, progesterone, and androgen receptors.

Introduction

Steroid hormone receptors are part of a larger group of proteins in the nuclear receptor superfamily.^1,2 Nuclear receptors primarily function as transcription factors responsible for cell signaling, differentiation, proliferation, metabolism, and development.^1,2 Abnormal receptor expression or function is a hallmark of numerous diseases, including cancer.^1,2

Steroid receptors generally act upon binding to a hormone ligand.³ Steroidal ligands diffuse or are transported across the cell membrane and bind to intracellular receptors leading to gene regulation.³ Despite the wide diversity of steroidal ligands, there are 48 known human nuclear receptors which are structurally similar with five functional protein domains.^1,2 The DNA-binding and ligand-binding domains are the most highly conserved.³ Steroid receptors in subgroup 3 of the nuclear receptor superfamily include estrogen, progesterone, androgen, mineralocorticoid, and glucocorticoid receptors.¹

The understanding of steroid receptor structure and function has grown significantly through research and technological advancements and has laid the groundwork for the development of synthetic ligands, therapeutics, and targeted imaging agents.⁴ Two members of this subfamily, estrogen and progesterone receptors, are key prognostic and predictive biomarkers used clinically in breast cancer and are the focus of this review.⁵ More recently, the androgen receptor (AR) has been gaining recognition, particularly in triple-negative breast cancer as another potential therapeutic target with longstanding evidence of the benefit of AR targeting in prostate cancer.^5–7 This review highlights the research and clinical advances of molecular imaging for breast cancer with radioligands for estrogen, progesterone, and androgen receptors.

Breast cancer is the most common cancer type in females (excluding non-melanoma skin cancer) and is second in cancer-related deaths for females.⁸ Since 2000, the incidence of breast cancer in females has grown annually by approximately 0.5%.⁸ Advancements in screening mammography have allowed for earlier detection and treatment of breast cancer.⁹ In certain subsets of patients with breast cancer, positron emission tomography/computed tomography (PET/CT), CT, and/or magnetic resonance imaging (MRI) has the potential to improve clinical management.^10–13

2-Deoxy-2-[¹⁸F]fluoro-D-glucose (FDG) is the most common radiopharmaceutical used for PET imaging in advanced stage breast cancer.¹² This modality can be used for staging in patients with known stage IIIA or higher disease, locally advanced disease, or inflammatory breast cancer as well as for monitoring response to therapy and detecting recurrence.^14,15 FDG is a glucose analogue that enters the cell via facilitated diffusion and is trapped after phosphorylation by hexokinase.¹⁶ Many cancers, including breast, upregulate glucose metabolism which is useful for molecular imaging with FDG.^17,18 FDG PET imaging is not without limitations; for example, false positives can be seen in infectious and inflammatory processes and false negatives can be seen with small lesions (less than 1 cm) and slow growing or well differentiated tumors (such as ductal carcinoma in situ and invasive lobular carcinomas).^19,20 In addition, FDG PET is unable to identify the hormone receptor status of the tumor.^21,22

Estrogen Receptor (ER)-Targeted Imaging

Estrogen receptor (ER) is highly expressed in most breast cancers (~70%), is routinely assessed by tissue immunohistochemistry, and is used to determine whether endocrine therapy targeting ER function is indicated.^23–25 There is evidence of a correlation between ER positivity and clinical benefit of endocrine therapy.^26,27 Evaluating whole-body ER positivity in breast cancer metastases, especially in cases that are heterogenous, may help clinicians determine the best candidates for endocrine therapy. Patients with disease that is strongly and homogenously ER-positive may respond better to endocrine therapy compared with those who have heterogeneous ER expression.^28–31

16α-[¹⁸F]fluoro-17β-estradiol (FES) was the first steroid imaging agent approved by the U.S. Food and Drug administration (FDA) in 2020 under the brand name Cerianna (GE Healthcare, Chicago, IL, USA).³² FES is an estrogen analogue which binds to the ER.^33,34 FES was approved for use with PET imaging for detecting ER-positive lesions in patients with recurrent or metastatic breast cancer.³² Since approval, clinical use of FES has been increasing but is not widespread.^35,36

FES is 17β-estradiol radiolabeled with a Fluorine-18 radioisotope and has a high binding affinity for ER.^33,37 Of the two receptor subtypes, FES has a 6.3-fold greater affinity for ERα compared to ERβ.³⁸ The ER must have a functional ligand-binding domain for FES uptake, therefore imaging with this radiopharmaceutical demonstrates both ER protein expression level and binding ability.³⁹ FES PET allows for whole body evaluation of ER-positive breast cancer with reported ranges of sensitivity from 78% to 86% and specificity from 85% to 98%.^40,41

Appropriate Use Criteria for FES PET Imaging

In January 2023, the Society of Nuclear Medicine and Molecular Imaging (SNMMI) released appropriate use criteria for FES PET imaging.⁴² The expert working group evaluated the appropriateness of FES PET for patients with ER-positive breast cancer in 14 specific clinical scenarios (Table 1).⁴² Four of the 14 scenarios were deemed appropriate.⁴² Two of the 4 appropriate scenarios for clinical use of FES PET are for “problem solving” such as for detecting ER status when other imaging tests are equivocal or when patients have more than one type of primary malignancy and the metastatic origin is unclear.^43–46 In the retrospective study by Boers et al, FES PET solved 87% (87/100) of these types of clinical dilemmas with treatment decisions directly based on the FES PET results in 81 of 87 cases.⁴³ FES PET was also deemed appropriate for assessing ER status in lesions that are difficult to biopsy or when biopsy is nondiagnostic.⁴² Either the lesion is in a location that is difficult to access (Figure 1) or imposes significant risk to the patient for biopsy, thus a noninvasive means of obtaining ER status would be preferred.^40,47–49

Table 1:

SNMMI Appropriate Use Criteria for FES PET Imaging

Appropriate	Maybe Appropriate	Rarely Appropriate
Assessing ER status in lesions that are difficult to biopsy, or when biopsy is nondiagnostic Detecting ER status when other imaging tests are equivocal or suspicious After progression of metastatic disease, for considering second line of endocrine therapy At initial diagnosis of metastatic disease, for considering endocrine therapy	Diagnosing malignancy of unknown primary when a biopsy is not feasible or is nondiagnostic Routine staging of extra-axillary nodes and distant metastases Staging invasive lobular carcinoma and low-grade invasive ductal carcinoma Detecting lesions in patients with suspected/known recurrent or metastatic breast cancer	Diagnosing primary breast cancer Routine staging of the primary tumor (T staging) Routine staging of the primary tumor (T staging) At initial diagnosis of primary breast cancer, for considering endocrine therapy Measuring response to therapy

Adapted from⁴²

Figure 1:

62-year-old female with history of bilateral invasive breast cancer (ER+/PR+/HER2-) treated with neoadjuvant chemotherapy, bilateral mastectomy and axillary dissection, adjuvant radiation therapy, and adjuvant endocrine therapy presented with neurologic symptoms and brain MRI lesions suspicious for metastases. Restaging CT of the chest, abdomen, and pelvis and whole-body bone scan were negative for metastases. FES PET/CT was done for problem solving to support the diagnosis of ER-positive intracranial breast cancer metastases given the difficult location for tissue biopsy and also to localize any other potential metastatic sites for biopsy. Left quadrigeminal plate (arrows) and right cerebellum (arrowhead) lesions demonstrate abnormal FES uptake on the maximum intensity projection image (A). The left quadrigeminal plate lesion is also shown on midbrain axial FES PET (B), noncontrast CT (C), fused FES PET/CT (D), and postcontrast T1-weighted MRI (E). Results of the FES PET were used to confirm the treatment plan for first-line endocrine therapy combined with a CDK4/6 inhibitor.

The other two appropriate clinical scenarios for FES PET involve treatment selection for patients with metastatic ER-positive breast cancer. There are multiple therapy options available with differing mechanisms of action; however, resistance to endocrine therapy remains problematic.^15,26,50 FES PET was deemed appropriate when selecting first-line endocrine therapy at the initial diagnosis of metastatic disease and when considering second-line endocrine therapy after progression of metastatic disease (Figure 2).⁴² Multiple studies have shown that FES PET may be a superior biomarker than ER immunohistochemistry for predicting response to endocrine therapies.^51–65 Systematic reviews and meta-analyses of these studies have reported 78% to 88% negative predictive value of FES PET for treatment response and 45% to 65% positive predictive value based on a threshold SUV_max of 1.5 or 2.0.^48,66,67 The majority of these studies included patients previously treated with multiple lines of therapy. The negative predictive value of FES PET for clinical benefit of first-line endocrine therapy for metastatic ER-positive breast cancer was the focus of the recently completed multi-center phase 2 study (NCT 02398773) which will lend further evidence for this clinical scenario.

Figure 2:

60-year-old female with history of ER/PR-positive invasive ductal carcinoma of the right breast post lumpectomy, radiation, and endocrine therapy. She developed liver recurrence, initiated therapy with palbociclib and letrozole, and demonstrated partial treatment response on imaging. She then developed nausea and early satiety, underwent endoscopic ultrasound with biopsies, and was found to have metastatic disease to the gastric fundus. FDG PET/CT was performed for re-staging and demonstrated FDG-avid diffuse gastric fundal thickening reflecting biopsy-proven metastasis (SUV_max 5.8, arrow) seen on coronal FDG PET (A), axial FDG PET (B), and axial fused FDG PET/CT (C). Additionally, there was mild uptake in a hepatic metastasis (SUV_max 4, not shown) reflecting partially treated disease. In the setting of metastatic ER-positive breast cancer with progression on first-line endocrine therapy, FES PET/CT was performed. Of note, FES PET/CT can be performed in patients on aromatase inhibitors (such as this patient) without a washout period which is required for patients on SERMs and SERDs. The FES PET/CT demonstrated abnormal FES uptake in the thickened gastric fundus (SUV_max 5.9, arrow) seen on coronal FES PET (D), axial FES PET (E), and axial fused FES PET/CT (F). FES positivity indicated that the patient was a good candidate for second-line endocrine therapy and she was switched to fulvestrant and ribociclib. The hepatic metastasis could not be evaluated on FES PET due to significant liver metabolism of FES, a known limitation of FES PET/CT.

FES PET was categorized as “may be appropriate” for staging invasive lobular carcinoma (ILC) and low-grade invasive ductal carcinoma (IDC).⁴² ILC is a distinct disease process from IDC and makes up approximately 15% of breast malignancies.³⁵ Compared to the more common IDC, ILC has differing genetic, molecular, and pathologic features.⁶⁸ Due to lack of E-cadherin, ILC grows in a noncohesive single-file linear pattern, which can be difficult to image with mammography, ultrasound, MRI, or FDG PET.⁶⁹ Both ILC and low-grade IDC are less likely to show distant disease sites with FDG avidity (Figure 3)^24,69,70; however, ILC is almost always ER-positive.^68,71 Data from small studies support potential utility of FES PET imaging for invasive lobular breast cancer, but more research is needed.^36,72,73

Figure 3:

65-year-old female with history of ER-positive invasive lobular carcinoma of the right breast status post modified mastectomy and axillary dissection followed by adjuvant chemotherapy, radiation, and anastrozole endocrine therapy. A skin punch biopsy was performed for diffuse rash which developed while on endocrine therapy and which did not respond to topical therapy, revealing metastatic lobular breast cancer. Restaging FDG PET/CT demonstrated peritoneal thickening with minimal diffuse FDG uptake with one region demonstrating mild focal FDG uptake (SUV_max 4.9, black arrow, A, FDG PET MIP, B, axial PET with corresponding, C, axial CT), suspicious for carcinomatosis. There was also mild radiotracer uptake in the anterior abdominal wall reflecting biopsy-proven metastases (white arrow, SUV_max 1.9, D, axial FDG PET with corresponding, E, axial CT). FES PET/CT was subsequently performed to clarify the extent of disease and demonstrated marked FES-positive diffuse peritoneal carcinomatosis throughout the abdomen and pelvis (SUV_max 14, black arrow, F, coronal FES PET, G, axial FES PET, and corresponding, H, axial CT). Mild radiotracer uptake was again seen in the anterior abdominal wall (white arrow, I, axial FES PET with corresponding, J, axial CT). Additionally, mild FES-avid disease was seen in the bilateral orbits (not shown, initially seen on interval MRI). In the setting of widespread ER-positive metastatic disease which progressed on endocrine therapy, the patient was transitioned to chemotherapy. She unfortunately continued to progress and expired five months later.

Four additional clinical scenarios were assessed as “may be appropriate” for FES PET imaging.⁴² These include diagnosing malignancy of unknown primary when biopsy is not feasible or is nondiagnostic, assessing ER status in lieu of biopsy in lesions that are easily accessible for biopsy, detecting lesions in patients with suspected or known recurrent or metastatic breast cancer, and routine staging of extraaxillary nodes and distant metastases,.⁴² Regarding staging, a recent systematic review and meta-analysis consisting of 7 studies and 171 patients was performed to compare the sensitivity of FES and FDG PET/CT for detecting ER-positive breast cancer metastases.⁷⁴ The authors found similar high pooled sensitivity at the patient level (97% for FDG; 94% for FES).⁷⁴ FES had slightly better sensitivity at the lesion level (95% for FES; 85% for FDG) and in the subset of studies performed for restaging (98% for FES; 81% for FDG). In a subsequently published study of FDG and FES PET/CT for initial staging of 28 patients with ER-positive breast cancer, overall diagnostic performance was similar with the exception of higher specificity and negative predictive value observed with FES.⁷⁵ In a subset analysis of strongly ER-positive breast cancer, the accuracy of FES increased from 88% to 95% while the accuracy of FDG decreased from 81% to 70%.⁷⁵ The authors propose that more accurate staging could be achieved through combined imaging by overcoming the limitations of each tracer.⁷⁵

Recognizing clinical scenarios for which FES PET was rated “rarely appropriate” is also important as these questions may arise during multidisciplinary care conferences. FES PET is not recommended for diagnosing and routine staging of primary breast cancer (T staging), or routine staging of axillary lymph nodes.⁴² These situations are best addressed using conventional breast imaging (i.e., mammography, ultrasound, MRI) and tissue biopsy (i.e., percutaneous breast or axillary lymph node biopsy, sentinel lymph node biopsy).

FES PET imaging was also categorized as “rarely appropriate” in two scenarios related to therapy.⁴² FES PET is appropriate to assess ER binding activity of metastatic lesions but should not be used to assess therapy response as a replacement for FDG or conventional anatomic imaging. This is particularly relevant when patients are being treated with ER-blocking medications since ER-positive lesions may still be present and viable but may not be detectable due to competition between the therapy agent and FES for binding ER. Lastly, there is insufficient evidence to recommend using FES PET to predict response to neoadjuvant therapy, unlike in the situation of metastatic or recurrent ER-positive breast cancer. While small studies have suggested that FES uptake may be a potential biomarker for response to neoadjuvant therapy, more research is needed.^76,77

Procedure Standard/Practice Guideline for FES PET Imaging

In June 2023, SNNMI and the European Association of Nuclear Medicine (EANM) published a joint procedure standard/practice guideline to promote standardization in FES PET imaging procedures for patients with breast cancer.⁷⁸ FES is administered intravenously by slow infusion, preferably on the side contralateral to the primary breast cancer, due to its propensity for venous uptake. The recommended injected activity ranges from 3 to 7.6 mCi.⁷⁹ The typical uptake time is 60 minutes after injection (range 20 to 80 minutes), with no restrictive activities for the patient during this time.⁷⁹ FES demonstrates rapid uptake in the liver in keeping with usual steroid metabolism.^80,81 Hepatic uptake significantly limits evaluation of ER-positive metastases in the liver.^40,81,82 FES is then excreted into the biliary system and enters into the small bowel with little activity observed in the colon due to reabsorption and renal excretion.^80,81 The patient receives approximately 0.022 mSv/MBq (80 mrem/mCi) effective dose equivalent with the liver receiving the highest dose (0.13 mGy/MBq, 380 mrad/mCi) followed by the gallbladder and urinary bladder.⁸¹ These dose levels are considered safe and comparable to other PET imaging agents.⁸¹ Unlike with FDG, fasting, dietary restrictions, avoidance of strenuous exercise, and blood glucose measurements are not required prior to FES PET.⁸³

The SNMMI/EANM Procedure Standard/Practice Guideline Interpretation also provides key details on how to appropriately interpret and report the results of FES PET.⁷⁸ A qualitative visual analysis is typically sufficient for defining FES-positive lesions by identifying increased focal uptake greater than local background that is not related to physiologic uptake or clearance. Thus, it is important to be familiar with the normal biodistribution of FES and recognize expected sites of normal tissue ER expression such as the uterus, ovaries, and pituitary gland.^81,84,85 Quantitative analysis of suspected or known sites of disease based on correlative imaging can be performed when visual inspection does not show uptake above local background. A commonly used threshold for defining FES positivity based on correlation with ER immunohistochemistry is an SUV_max greater than or equal to 1.5 for lesions measuring at least 10 mm in size.^29,47,86 Thresholds between 1.5 and 2.0 have been used to define FES positivity for predicting response to endocrine therapy.⁶⁶ Results from the recently completed multi-center phase 2 study (NCT 02398773) will provide further evidence for defining this threshold. Lastly, the guidelines emphasize that correlative imaging with FDG PET, CT, MRI, or bone scintigraphy for defining active sites of disease is critical for interpretation of FES PET to identify ER-negative lesions and evaluate ER heterogeneity that impact prognosis and therapy response.^{30,59,60,62,87}

There are some limitations to be aware of when considering FES PET imaging. Normal circulating estrogens in premenopausal nonpregnant and postmenopausal females do not impact FES distribution; however, medications that interact with the ER binding site may cause a false-negative scan.^57,88 For example, tamoxifen, a selective ER modulator (SERM), and fulvestrant, an injectable selective ER degrader (SERD), compete with FES for binding ER, and a washout period of 8 weeks for tamoxifen or 28 weeks for fulvestrant is indicated by FDA guidelines prior to imaging with FES which significantly limits feasibility in clinical practice.⁷⁹ Oral SERDs, such as elacestrant and rintodestrant, have a much shorter half-life than fulvestrant but more research is needed to establish an appropriate washout period.^65,89 Aromatase inhibitors, CDK 4/6 inhibitors, mTOR inhibitors, and oral contraceptives do not impact FES studies.^57,78

While FES PET cannot be used clinically in the setting of ER blockers such as SERMs and SERDs, the ability for FES PET to assess ER binding blockade has been used successfully in research.^{57,65,88,90–94} FES PET before and during fulvestrant determined that standard dosing led to incomplete ER blockade in 38% of patients and that this incomplete blockade was associated with early progression.⁸⁸ Building on this approach to assess ER availability, FES PET has been used in phase I dose escalation trials to identify the optimal dose of novel SERMs and oral SERDs.

Another limitation is the potential for false-negative and false-positive results which are important to consider during FES PET interpretation. False-negatives can occur in lesions with low-levels of ER expression, lesions in the liver, small bowel, or peritoneal carcinomatosis due to high physiologic clearance, with pleural effusions and ascites, with lesions that are small size (typically less than 1 cm) due to partial volume effects, and with pharmacological treatment with ER-binding medications as discussed above.^{40,57,82,83,95,96} False-positive findings have been reported with radiation fibrosis/pneumonitis, fibrous dysplasia, insufficiency fractures, meningiomas, and uterine leiomyomas.^29,96–103 Despite these limitations, FES PET is a powerful new tool for patients with advanced ER-positive breast cancer.

FES PET has also been used to characterize endometrial carcinoma.^104–106 Combined evaluation with both FES and FDG PET has proven the most useful in this patient population – with high glucose metabolism (high FDG uptake) and low ER expression (low FES uptake) seen in aggressive endometrial malignancies and the opposite pattern seen in less aggressive or benign disease.¹⁰⁶ In a subsequent study, low FES uptake was an independent prognostic factor associated with poor progression-free and overall survival in patients with endometrial carcinoma.¹⁰⁵

PR-Targeted Imaging

The progesterone receptor (PR) is a steroid receptor and tumor biomarker that is routinely assessed in breast cancer using immunohistochemistry.²⁵ PR positivity is observed in about two-thirds of breast cancers, the majority of which are also ER positive.¹⁰⁷ Together, ER and PR positivity are predictors of favorable response to endocrine therapy.^108–110 ER controls transcription of the gene encoding PR, PGR, as evidenced by increased expression of PR in response to estrogen stimulation and thus, implies a functionally intact ER response.^111–113 Loss of PR may predict resistance to therapy with tamoxifen; however, it may not predict resistance to aromatase inhibitors.¹¹⁴ Mounting research suggests a further bi-directional link between ER and PR in cell signaling (i.e., crosstalk)^115–117 and PR-targeted therapies have had renewed interest in clinical trials.^118,119 Therefore, accurate determination of PR status is clinically significant and may help when selecting an optimal treatment regimen for patients with breast cancer.^114,120,121

Multiple PR-targeted positron-emitting radiopharmaceuticals have been developed for imaging since the late 1980s, but translation to clinical use has proved challenging.^5,122–130 For example, 21-[¹⁸F]fluoro-16α-ethyl-19-norprogesterone (FENP) had promising in vitro and in vivo results during initial preclinical trials.^5,122,127 Despite early success, FENP imaging characteristics in patients with breast cancer were problematic due to high levels of nonspecific uptake, noncorrelative tumor uptake compared to tissue PR expression, suboptimal target-to-background uptake ratio, and rapid metabolism.^5,124,125 21-[¹⁸F]Fluorofuranylnorprogesterone (FFNP) has emerged as the most suitable PR-targeted radiopharmaceutical with high affinity and high selectivity for the receptor in both preclinical and clinical research.^{5,128,129,131–138}

Preclinical studies with FFNP have demonstrated real-time changes in PR expression as a function of ER signaling and predictor of endocrine sensitivity.^{133,134,136,137} For example, PR expression increased in endocrine-sensitive ER-positive, PR-positive T47D human breast cancer cells and tumor xenografts after estradiol stimulation for a few days.¹³⁶ Conversely when estrogen was decreased in mice after ovariectomy or with fulvestrant treatment, PR expression quickly decreased as demonstrated by less FFNP uptake in STAT1-deficient mouse mammary tumors.^133,137 These dynamic changes in FFNP uptake occurred in endocrine-sensitive ER-positive, PR-positive STAT1-deficient tumors, but not in endocrine-resistant tumors, and occurred prior to changes in tumor size, supporting a role for FFNP PET as an approach for assessing early tumor responsiveness to endocrine therapy.^133,137 Another study demonstrated the ability of FFNP PET to predict endocrine resistance in tumor xenografts with activating mutations in the ESR1 gene. Overall, these preclinical studies have demonstrated that imaging early molecular changes in PR expression using FFNP PET can predict endocrine sensitivity.

Results in clinical trials have aligned with the preclinical studies, and FFNP has been shown to be safe for use in humans.^131,132,138 In 2012, the “first-in-human” trial of FFNP PET/CT with 20 patients with breast cancer reported no adverse events and a radiation exposure comparable to current PET radiopharmaceuticals being used in clinical practice.¹³¹ The study showed peak FFNP uptake in the tumor within a few minutes after injection that was sustained through 60 minutes without significant washout.¹³¹ Higher FFNP uptake was observed in PR-positive tumors compared to PR-negative tumors when uptake was corrected for normal breast tissue.¹³¹ Preliminary results from a separate ongoing research study also indicate correlation of FFNP uptake measured using breast PET/MRI and PR expression by immunohistochemistry in newly diagnosed primary invasive ductal carcinoma.¹³⁸ Investigation of FFNP PET/CT as an early biomarker for endocrine therapy response was performed as a single-center, phase 2 study of 43 postmenopausal women with advanced and metastatic ER-positive breast cancer.¹³² This study found that an increase in tumor FFNP uptake by at least 7% after one day administration of 6 mg oral estradiol predicted clinical benefit from endocrine therapy for at least 6 months with 100% specificity and sensitivity (Figures 4 and 5).¹³² With a median follow-up of 27 months, subjects who experienced an increase in tumor FFNP uptake by at least 7% after the estradiol-challenge had longer overall survival compared to those with tumor SUV change less than 7%.¹³² Thus, FFNP PET has been demonstrated to be a safe, noninvasive method for determining tumor PR expression and that dynamic quantitative changes in FFNP uptake can predict endocrine therapy benefit and survival in patients with advanced ER-positive breast cancer.

Figure 4:

A woman with newly diagnosed ER+/PR+/HER2- invasive ductal carcinoma with metastatic disease at diagnosis in left axillary lymph nodes. She was treated with an aromatase inhibitor and palbociclib after an estradiol challenge test¹³² and had marked improvement of all lesions. Selected fused transaxial 21-[¹⁸F]fluorofuranylnorprogesterone [FFNP]-PET/CT (A) and CT (B) images at baseline show moderate FFNP uptake (SUV_max 6.8) in a left axillary lymph node metastasis (arrows). (C, D) One day after estradiol, the tumor FFNP uptake (SUV_max 8.8) (arrows) was increased by 29% from baseline. Clinical response was observed at six months.

Figure 5:

A woman with ER+/PR-/HER2- invasive ductal carcinoma status post neoadjuvant endocrine therapy and breast conserving surgery, who developed progression of osseous metastatic disease. She had been previously treated with an aromatase inhibitor, tamoxifen, palbociclib, and mTOR kinase inhibitors. She was treated with an aromatase inhibitor and palbociclib after the estradiol challenge test¹³² but developed progressive disease. Selected fused transaxial FFNP-PET/CT (A) and CT (B) images at baseline show mild FFNP uptake (SUV_max 4.2) in the sacral metastasis (arrow). (C, D) There was no increase in tumor FFNP uptake (SUV_max 2.3) (arrow) one day after estradiol. Clinical non-response was observed at six months.

Further research is needed before FFNP is ready for clinical practice. FFNP is not yet approved by the FDA and is not commercially available, thus must be produced locally.^139,140 Like FES, FFNP undergoes normal hepatic metabolism that results in intense background activity making identification and evaluation of liver metastases challenging. Non-steroidal PR-targeted radiopharmaceuticals may help address this issue, however no “first-in-human” data is available.¹⁴¹ Other ¹⁸F-radiolabeled progestins have been developed and tested in preclinical models with variable success but these agents have not yet been evaluated in human subjects.^141–145

AR-Targeted Imaging

The androgen receptor (AR) is a steroid receptor expressed in various tissues in males and females, including the breasts, and has important implications in prostate and breast cancer.^146–148 AR is abundantly expressed in breast cancer -- approximately 70% of all primary breast cancers, 75% to 95% in ER-positive disease, and 10% to 35% in triple-negative (ER-negative, PR-negative, and HER2-neagative) disease demonstrate AR positivity.^{146,149–152} Patients with AR-positive breast cancer have a better prognosis compared to AR-negative breast cancer.¹⁵¹ However, AR status is not a helpful biomarker for selection of conventional endocrine therapy targeting ER.¹⁵³ AR expression is heterogenous and can change over time especially if on hormone therapy, much like ER.¹⁵⁴ Immunohistochemical testing for AR expression in breast cancer is not routinely performed in clinical practice and there are currently no standardized guidelines from the College of American Pathologists. Furthermore, a cut-off value for AR positivity by immunohistochemistry has not been standardized.^6,151,155 Unlike prostate cancer, the clinical utility of AR as a therapeutic biomarker for breast cancer is less clear with AR antagonists remaining in clinical trials.

Noninvasive PET imaging of AR expression has shown some utility in identifying breast cancer subtype, disease extent, tumor heterogeneity, and potential for patient benefit from AR-targeted therapy, but this approach remains investigational.^154,156 Testosterone, an androgen, is converted to 17β-estradiol or dihydrotestosterone (DHT) in breast tissue.¹⁵¹ For AR imaging, 16β−¹⁸F-fluoro-5α-dihydrotestosterone (FDHT), a radiolabeled analogue of 5α-dihydrotestosterone (DHT), has shown some encouraging results with a few drawbacks.^5,120,148 FDHT has been more widely studied in prostate cancer focusing on metastatic staging and response to antiandrogen therapy in patients with metastatic castration-resistant prostate cancer (mCRPC).^157–159 This research demonstrated that FDHT detected 78% to 86% of known prostate cancer metastases.^158,159 FDHT is rapidly metabolized, and its metabolites persist due to an affinity for blood proteins, resulting in high background uptake.^5,160 Despite this limitation, some studies have shown high repeatability and interobserver reproducibility of FDHT PET imaging for patients with mCRPC.¹⁶¹

In breast tissue, 17β-estradiol activates cell proliferation while DHT inhibits it. AR stimulation can inhibit ER-positive breast cancer tumor growth but can stimulate ER-negative tumor growth.^{151,156,162,163} Therefore, it was anticipated that FDHT would be able to identify patients with metastatic breast cancer who would respond to AR-targeted therapy; however, clinical trial results have been variable thus far.^164,165 A pilot study of 11 women with ER-positive metastatic breast cancer showed that FDHT PET may be a potential biomarker for response to GTx-024, a novel oral nonsteroidal selective AR modulator.¹⁶⁵ In contrast, a feasibility study evaluating 21 patients with AR-positive metastatic breast cancer imaged with serial FDHT PET failed to predict responders from nonresponders on antiandrogen bicalutamide therapy.¹⁶⁴ This shortcoming is likely due to a similar issue found in FDHT imaging of prostate cancer; it is hypothesized that AR therapy is blocking bioavailable AR and a decrease on imaging does not necessarily correlate with treatment response.^120,158

Additional FDHT studies in patients with breast cancer have failed to show strong reader agreement. A prospective study with ten patients with ER-positive metastatic breast cancer imaged with both FES PET/CT and FDHT PET/CT showed high interobserver agreement for visual evaluation of FES and relatively low interobserver agreement for FDHT based on visual analysis due to low tumor-to-background ratios.¹⁶² However, both radiopharmaceuticals showed good quantitative agreement between readers.¹⁶²

Other ligands, such as enzalutamide, an AR antagonist used in prostate cancer hormone therapy, are being considered for radiolabeling to improve metabolic stability and binding affinity.¹⁶⁶ There may be potential for ¹⁸F-enzalutamide to enhance signal-to-noise on imaging, but more data is needed.

Summary

Through diligent research and advancements in technology, knowledge has substantially increased regarding breast cancer-related steroid receptor structure and function, pathophysiology, targeting with endocrine therapy, and molecular imaging with radiopharmaceuticals. FES is currently the most clinically validated ER-targeted radioligand and is FDA approved for imaging patients with ER-positive breast cancer. The SNNMI recently released appropriate use criteria and practice guidelines for recommending, ordering, and interpreting FES PET. Significant progress has also been made for imaging PR expression with FFNP in the last decade and clinical trial results have been encouraging. FFNP is considered clinically safe and changes in PR dynamics, as detected on imaging, are highly predictive of endocrine therapy success and overall survival in patients with advanced ER-positive breast cancer. FFNP is not currently FDA approved for clinical use. There is limited data on FDHT for AR imaging in breast cancer. FDHT is also not currently FDA approved and availability of both FFNP and FDHT is limited. Finally, there currently are no companion targeted radioligand therapies available for ER, PR, or AR. Continued innovations in steroid receptor imaging could lead to therapy breakthroughs via development of novel targeted endocrine therapeutics and theranostics to improve clinical care for patients with breast cancer.