18F-Fluoroestradiol PET/CT Measurement of Estrogen Receptor Suppression during a Phase I Trial of the Novel Estrogen Receptor-Targeted Therapeutic GDC-0810: Using an Imaging Biomarker to Guide Drug Dosage in Subsequent Trials

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.

Navigating Breast Cancer Oligometastasis and Oligoprogression: Current Landscape and Future Directions

PURPOSE

We examine the potential for curative approaches among metastatic breast cancer (MBC) patients by exploring the recent literature on local ablative therapies like surgery and stereotactic body radiation therapy (SBRT) in patients with oligometastatic (OM) breast cancer. We also cover therapies for MBC patients with oligoprogressive (OP) disease.

KEY FINDINGS

Surgery and SBRT have been studied for OM and OP breast cancer, mainly in retrospective or non-randomized trials. While many studies demonstrated favorable results, a cooperative study and single-institution trial found no support for surgery/SBRT in OM and OP cases, respectively.

CONCLUSION

While there is interest in applying local therapies to OM and OP breast cancer, the current randomized data does not back the routine use of surgery or SBRT, particularly when considering the potential for treatment-related toxicities. Future research should refine patient selection through advanced imaging and possibly explore these therapies specifically in patients with hormone receptor-positive or HER2-positive disease.

Quarter-Century Transformation of Oncology: Positron Emission Tomography for Patients with Breast Cancer

PET radiotracers have become indispensable in the care of patients with breast cancer. 18F-fluorodeoxyglucose has become the preferred method of many oncologists for systemic staging of breast cancer at initial diagnosis, detecting recurrent disease, and for measuring treatment response after therapy. 18F-Sodium Fluoride is valuable for detection of osseous metastases. 18F-fluoroestradiol is now FDA-approved with multiple appropriate clinical uses. There are multiple PET radiotracers in clinical trials, which may add utility of PET imaging for patients with breast cancer in the future. This article will describe the advances during the last quarter century in PET for patients with breast cancer.

Women's Health Update: Growing Role of PET for Patients with Breast Cancer

Positron Emission Tomography (PET) has been growing in usage for patients with breast cancer, due to an increased number of FDA-approved PET radiotracers pertinent to patients with breast cancer as well as increased prospective evidence for the value of these agents. The leading PET radiotracer for patients with breast cancer is 18F-fluorodeoxyglucose (18F-FDG), which measures glucose metabolism. There is prospective evidence for the use of 18F-FDG PET in systemic staging of newly diagnosed locally advanced breast cancer (stages IIB-IIIC), monitoring breast cancer treatment response, and detecting breast cancer recurrence, particularly in no special type (NST) breast cancer. 16α-18F-fluoro-17β-Fluoroestradiol (18F-FES) is a radiolabeled estrogen which evaluates estrogen receptor (ER) accessible for estrogen binding. There is prospective evidence supporting 18F-FES PET as a predictive biomarker for selecting patients with metastatic breast cancer for endocrine therapies. 18F-FES PET has also been shown to be valuable in the evaluation of ER status of lesions which are difficult to biopsy, for evaluation of ER status in lesions that are equivocal on other imaging modalities, and for selecting optimal dosage of novel ER-targeted systemic therapies in early clinical trials. Multiple investigators have suggested 18F-FES PET will have an increasing role for patients with invasive lobular breast cancer (ILC), which is less optimally evaluated by 18F-FDG PET. Sodium 18F-Fluoride (18F-NaF) evaluates bone turnover and has been effective in evaluation of malignancies which commonly metastasize to bone. In patients with metastatic breast cancer, 18F-NaF PET/CT has demonstrated superior sensitivity for osseous metastases than 99mTc-MDP or CT. In addition to these three FDA-approved PET radiotracers, there are multiple novel radiotracers currently in clinical trials with potential to further increase PET usage for patients with breast cancer.

Fluorine-18-Labeled Fluoroestradiol PET/CT: Current Status, Gaps in Knowledge, and Controversies-AJR Expert Panel Narrative Review

PET/CT using 16α-[18F]-fluoro-17β-estradiol (FES) noninvasively images tissues expressing estrogen receptors (ERs). FES has undergone extensive clinicopathologic validation for ER+ breast cancer and received FDA approval in 2020 for clinical use as an adjunct to biopsy in patients with recurrent or metastatic ER+ breast cancer. Clinical use of FES PET/CT is increasing, but is not widespread in the United States. This AJR Expert Panel Narrative Review explores the present status and future directions of FES PET/CT, including image interpretation, existing and emerging uses, knowledge gaps, and current controversies. Specific controversies discussed include whether both FES PET/CT and FDG PET/CT are warranted in certain scenarios, whether further workup is required after negative FES PET/CT results, whether FES PET/CT findings should inform endocrine therapy selection, and whether immunohistochemistry should remain the standalone reference standard for determining ER status for all breast cancers. Consensus opinions from the panel include agreement with the appropriate clinical uses of FES PET/CT published by a multidisciplinary expert workgroup in 2023; anticipated expanded clinical use of FES PET/CT for staging ER-positive invasive lobular carcinomas and low-grade invasive ductal carcinomas pending ongoing clinical trial results; and the need for further research regarding use of FES PET/CT for ER-expressing nonbreast malignancies.

A genetically encoded protein tag for control and quantitative imaging of CAR T cell therapy

Chimeric antigen receptor (CAR) T cell therapy has been successful for hematological malignancies. Still, a lack of efficacy and potential toxicities have slowed its application for other indications. Furthermore, CAR T cells undergo dynamic expansion and contraction in vivo that cannot be easily predicted or controlled. Therefore, the safety and utility of such therapies could be enhanced by engineered mechanisms that engender reversible control and quantitative monitoring. Here, we use a genetic tag based on the enzyme Escherichia coli dihydrofolate reductase (eDHFR), and derivatives of trimethoprim (TMP) to modulate and monitor CAR expression and T cell activity. We fused eDHFR to the CAR C terminus, allowing regulation with TMP-based proteolysis-targeting chimeric small molecules (PROTACs). Fusion of eDHFR to the CAR does not interfere with cell signaling or its cytotoxic function, and the addition of TMP-based PROTACs results in a reversible and dose-dependent inhibition of CAR activity via the proteosome. We show the regulation of CAR expression in vivo and demonstrate imaging of the cells with TMP radiotracers. In vitro immunogenicity assays using primary human immune cells and overlapping peptide fragments of eDHFR showed no memory immune repertoire for eDHFR. Overall, this translationally-orientied approach allows for temporal monitoring and image-guided control of cell-based therapies.

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.

Research progress on tamoxifen and its analogs associated with nuclear receptors

Tamoxifen, a triphenylethylene-based selective estrogen-receptor modulator, is a landmark drug for the treatment of breast cancer and is also used for treating liver cancer and osteoporosis. Structural studies of tamoxifen have led to the synthesis of more than 20 novel tamoxifen analogs as receptor modulators, including 16 ERα modulators 2-17, an ERRβ inverse agonist 19 and six ERRγ inverse agonists 20-25. This paper summarizes the research progress and structure-activity relationships of tamoxifen analogs modulating these three nuclear receptors reported in the literature, and introduces the relationship between these three nuclear receptor-mediated diseases and tamoxifen analogs to guide the research of novel tamoxifen analogs.

An Open-label Phase I Study of GDC-0927 in Postmenopausal Women with Locally Advanced or Metastatic Estrogen Receptor-Positive Breast Cancer

PURPOSE

GDC-0927 is a novel, potent, nonsteroidal, orally bioavailable, selective estrogen receptor (ER) degrader that induces tumor regression in ER+ breast cancer xenograft models.

PATIENTS AND METHODS

This phase I dose-escalation multicenter study enrolled postmenopausal women with ER+/HER2- metastatic breast cancer to determine the safety, pharmacokinetics, and recommended phase II dose of GDC-0927. Pharmacodynamics was assessed with [18F]-fluoroestradiol (FES) PET scans.

RESULTS

Forty-two patients received GDC-0927 once daily. The MTD was not reached. The most common adverse events (AE) regardless of causality were nausea, constipation, diarrhea, arthralgia, fatigue, hot flush, back pain, and vomiting. There were no deaths, grade 4/5 AEs, or treatment-related serious AEs. Two patients experienced grade 2 AEs of special interest of deep vein thrombosis and jugular vein thrombosis, both considered unrelated to GDC-0927. Following dosing, approximately 1.6-fold accumulation was observed, consistent with the observed half-life and dosing frequency. There were no complete or partial responses. Pharmacodynamics was supported by >90% reduction in FES uptake and an approximately 40% reduction in ER expression, suggesting ER degradation is not the mechanistic driver of ER antagonism. Twelve patients (29%) achieved clinical benefit; 17 patients (41%) showed a confirmed best overall response of stable disease. Baseline levels of ER and progesterone receptor protein and mutant ESR1 circulating tumor DNA did not correlate with clinical benefit.

CONCLUSIONS

GDC-0927 appeared to be well tolerated with pharmacokinetics supporting once-daily dosing. There was evidence of target engagement and preliminary evidence of antitumor activity in heavily pretreated patients with advanced/metastatic ER+/HER2- breast cancer with and without ESR1 mutations.

Summary: Appropriate Use Criteria for Estrogen Receptor-Targeted PET Imaging with 16α-18F-Fluoro-17β-Fluoroestradiol

PET imaging with 16α-¹⁸F-fluoro-17β-fluoroestradiol (¹⁸F-FES), a radiolabeled form of estradiol, allows whole-body, noninvasive evaluation of estrogen receptor (ER). ¹⁸F-FES is approved by the U.S. Food and Drug Administration as a diagnostic agent "for the detection of ER-positive lesions as an adjunct to biopsy in patients with recurrent or metastatic breast cancer." The Society of Nuclear Medicine and Molecular Imaging (SNMMI) convened an expert work group to comprehensively review the published literature for ¹⁸F-FES PET in patients with ER-positive breast cancer and to establish appropriate use criteria (AUC). The findings and discussions of the SNMMI ¹⁸F-FES work group, including example clinical scenarios, were published in full in 2022 and are available at https://www.snmmi.org/auc Of the clinical scenarios evaluated, the work group concluded that the most appropriate uses of ¹⁸F-FES PET are to assess ER functionality when endocrine therapy is considered either at initial diagnosis of metastatic breast cancer or after progression of disease on endocrine therapy, the ER status of lesions that are difficult or dangerous to biopsy, and the ER status of lesions when other tests are inconclusive. These AUC are intended to enable appropriate clinical use of ¹⁸F-FES PET, more efficient approval of FES use by payers, and promotion of investigation into areas requiring further research. This summary includes the rationale, methodology, and main findings of the work group and refers the reader to the complete AUC document.

18F-Fluoroestradiol: Current Applications and Future Directions

In the United States, breast cancer is the second leading cause of cancer death in all women and the leading cause of cancer death in Black women. The breast cancer receptor profile, assessed with immunohistochemical staining of tissue samples, allows prediction of outcomes and direction of patient treatment. Approximately 80% of newly diagnosed breast cancers are hormone receptor (HR) positive, which is defined as estrogen receptor (ER) and/or progesterone receptor (PR) positive. Patients with ER-positive disease can be treated with therapies targeting the ER; however, the assessment of ER expression with immunohistochemical staining of biopsy specimens has several limitations including sampling error, false-negative results, challenging or inaccessible biopsy sites, and the inability to synchronously and serially assess all metastatic sites to identify spatial and/or temporal ER heterogeneity. In May 2020, after decades of research, the U.S. Food and Drug Administration approved the PET radiotracer fluorine 18 (¹⁸F) fluoroestradiol (FES) for clinical use in patients with ER-positive recurrent or metastatic breast cancer as an adjunct to biopsy. FES binds to the ER in the nucleus of ER-expressing cells, enabling whole-body in vivo assessment of ER expression. This article is focused on the approved uses of FES in the United States, including identification of a target lesion for confirmatory biopsy, in vivo assessment of biopsy-proven ER-positive disease, and evaluation of spatial and temporal ER heterogeneity. FES is an example of precision medicine that has been leveraged to optimize the care of patients with breast cancer. ^© RSNA, 2023 See the invited commentary by Fowler in this issue. Quiz questions for this article are available through the Online Learning Center.

18F-Fluciclovine PET Imaging of Glutaminase Inhibition in Breast Cancer Models

Aggressive cancers such as triple-negative breast cancer (TNBC) avidly metabolize glutamine as a feature of their malignant phenotype. The conversion of glutamine to glutamate by the glutaminase enzyme represents the first and rate-limiting step of this pathway and a target for drug development. Indeed, a novel glutaminase inhibitor (GLSi) has been developed and tested in clinical trials but with limited success, suggesting the potential for a biomarker to select patients who could benefit from this novel therapy. Here, we studied a nonmetabolized amino acid analog, ¹⁸F-fluciclovine, as a PET imaging biomarker for detecting the pharmacodynamic response to GLSi. Methods: Uptake of ¹⁸F-fluciclovine into human breast cancer cells was studied in the presence and absence of inhibitors of glutamine transporters and GLSi. To allow ¹⁸F-fluciclovine PET to be performed on mice, citrate in the tracer formulation is replaced by phosphate-buffered saline. Mice bearing triple-negative breast cancer (TNBC) xenografts (HCC38, HCC1806, and MBA-MD-231) and estrogen receptor-positive breast cancer xenografts (MCF-7) were imaged with dynamic PET at baseline and after a 2-d treatment of GLSi (CB839) or vehicle. Kinetic analysis suggested reversible uptake of the tracer, and the distribution volume (V_D) of ¹⁸F-fluciclovine was estimated by Logan plot analysis. Results: Our data showed that cellular uptake of ¹⁸F-fluciclovine is mediated by glutamine transporters. A significant increase in V_D was observed after CB839 treatment in TNBC models exhibiting high glutaminase activity (HCC38 and HCC1806) but not in TNBC or MCF-7 exhibiting low glutaminase. Changes in V_D were corroborated with changes in GLS activity measured in tumors treated with CB839 versus vehicle, as well as with changes in V_D of ¹⁸F-(2S,R4)-fluoroglutamine, which we previously validated as a measure of cellular glutamine pool size. A moderate, albeit significant, decrease in ¹⁸F-FDG PET signal was observed in HCC1806 tumors after CB839 treatment. Conclusion: ¹⁸F-fluciclovine PET has potential to serve as a clinically translatable pharmacodynamic biomarker of GLSi.

Evaluation of tumour heterogeneity by 18F-fluoroestradiol PET as a predictive measure in breast cancer patients receiving palbociclib combined with endocrine treatment

Background

Predictive biomarkers are needed to identify oestrogen receptor-positive, human epidermal growth factor receptor 2-negative (ER + /HER2-) metastatic breast cancer (MBC) patients who would likely benefit from cyclin-dependent kinase 4 and 6 inhibitors combined with endocrine therapy. Therefore, we performed an exploratory study to evaluate the tumour heterogeneity parameters based on 16α-¹⁸F-fluoro-17β-oestradiol (¹⁸F-FES)-PET imaging as a potential marker to predict progression-free survival (PFS) in MBC patients receiving palbociclib combined with endocrine therapy.

Methods

Fifty-six ER + MBC patients underwent ¹⁸F-FES-PET/CT before the initiation of palbociclib. ¹⁸F-FES uptake was quantified and expressed as the standardized uptake value (SUV). Interlesional heterogeneity was qualitatively identified according to the presence or absence of ¹⁸F-FES-negative lesions. Intralesional heterogeneity was measured by the SUV-based heterogeneity index (HI = SUVmax/SUVmean). Association with survival was evaluated using the Cox proportional hazards model.

Results

A total of 551 metastatic lesions were found in 56 patients: 507 lesions were identified as ¹⁸F-FES-positive, 38 lesions were distributed across 10 patients without ¹⁸F-FES uptake, and the remaining 6 were liver lesions. Forty-three patients obtained a clinical benefit, and 13 developed progressive disease (PD) within 24 weeks. Nine out of 10 patients with an ¹⁸F-FES-negative site developed PD, and the median PFS was only 2.4 months. Among 46 patients with only ¹⁸F-FES-positive lesions, only four patients had PD, and the median PFS was 23.6 months. There were statistically significant differences between the two groups (P < 0.001). For the subgroup of patients with only ¹⁸F-FES-positive lesions, low FES-HI patients experienced substantially longer PFS times than those with high FES-HI (26.5 months vs. 16.5 months, P = 0.004).

Conclusions

¹⁸F-FES-PET may provide a promising method for identifying and selecting candidate ER + /HER2- MBC patients who would most likely benefit from palbociclib combined with endocrine treatment and could serve as a predictive marker for treatment response.

Trial registration NCT04992156, Date of registration: August 5, 2021 (retrospectively registered).

AMEERA-1 phase 1/2 study of amcenestrant, SAR439859, in postmenopausal women with ER-positive/HER2-negative advanced breast cancer

AMEERA-1 is a Phase 1/2 open-label single-arm study evaluating once-daily (QD) amcenestrant, an orally bioavailable selective estrogen receptor (ER) degrader, in postmenopausal women with ER+/HER2− advanced breast cancer (NCT03284957), who were mostly heavily pretreated (including targeted therapies and fulvestrant). In the dose escalation phase (Part A: n = 16), patients received amcenestrant 20-600 mg QD. Based on absence of dose-limiting toxicities, paired functional ¹⁸F-fluoroestradiol positron emission tomography, and pharmacokinetics, 400 mg QD was selected as recommended Phase 2 dose (RP2D) for the dose expansion phase (Part B: n = 49). No Grade ≥3 treatment-related adverse events or clinically significant cardiac/eye toxicities were reported. The Part B primary endpoint, confirmed objective response rate (ORR) was 3/45 at the interim analysis and 5/46 (10.9%) at the final analysis. The overall clinical benefit rate (CBR) was 13/46 (28.3%). CBRs among patients with baseline wild-type and mutated ESR1 were 9/26 (34.6%) and 4/19 (21.1%), respectively. Paired tumor biopsy and cell-free DNA analyses revealed ER inhibition and degradation, and a reduction in detectable ESR1 mutations, including Y537S. In conclusion, amcenestrant at RP2D of 400 mg QD for monotherapy is well-tolerated with no dose-limiting toxicities, and demonstrates preliminary antitumor activity irrespective of baseline ESR1 mutation status.

There is a need for potent and non-toxic estrogen receptor (ER) antagonists to overcome the limitations of existing endocrine therapies. Here the authors report the results from Arm 1 of the Phase 1/2 study (AMEERA-1) among postmenopausal women with ER+/HER2− advanced breast cancer, which evaluates the safety, antitumor activity, pharmacokinetics, and pharmacodynamics of amcenestrant administered as monotherapy.

Novel applications of molecular imaging to guide breast cancer therapy

The goals of precision oncology are to provide targeted drug therapy based on each individual’s specific tumor biology, and to enable the prediction and early assessment of treatment response to allow treatment modification when necessary. Thus, precision oncology aims to maximize treatment success while minimizing the side effects of inadequate or suboptimal therapies. Molecular imaging, through noninvasive assessment of clinically relevant tumor biomarkers across the entire disease burden, has the potential to revolutionize clinical oncology, including breast oncology. In this article, we review breast cancer positron emission tomography (PET) imaging biomarkers for providing early response assessment and predicting treatment outcomes. For 2-¹⁸fluoro-2-deoxy-D-glucose (FDG), a marker of cellular glucose metabolism that is well established for staging multiple types of malignancies including breast cancer, we highlight novel applications for early response assessment. We then review current and future applications of novel PET biomarkers for imaging the steroid receptors, including the estrogen and progesterone receptors, the HER2 receptor, cellular proliferation, and amino acid metabolism.

Use of Radionuclide-Based Imaging Methods in Breast Cancer

Breast cancer is one of the most commonly occurring cancers in women globally and is the primary cause of cancer mortality in females. Thus, early and effective breast cancer diagnosis is crucial for enhancing the survival rate. Current standard diagnostic techniques to assess the hormone receptor status in biopsies include immunohistochemistry and fluorescence in situ hybridization. However, in recent years, there has been an increase in research on noninvasive techniques for molecular imaging of hormone receptors. These methods offer many advantages over conventional imaging, as repeated measurements can be used to capture heterogeneous tumor expression throughout the body, as well as transformations in receptor status during disease progression. Thus, the noninvasive method, as an adjunct to conventional imaging, offers the potential to improve patient selection, optimize dose and schedule, and streamline the assessment of response.

Latest generation estrogen receptor degraders for the treatment of hormone receptor-positive breast cancer

INTRODUCTION

The selective estrogen receptor degrader (SERD) and full receptor antagonist provides an important therapeutic option for hormone receptor (HR)-positive breast cancer. Endocrine therapies include tamoxifen, a selective estrogen receptor modulator (SERM), that exhibits receptor agonist and antagonist activity, and aromatase inhibitors that block estrogen biosynthesis but which demonstrate acquired resistance. Fulvestrant, the only currently approved SERD, is limited by poor drug-like properties. A key focus for improving disease management has been development of oral SERDs with optimized target occupancy and potency and superior clinical efficacy.

AREAS COVERED

Using PubMed, clinicaltrials.gov, and congress websites, this review explored the preclinical development and clinical pharmacokinetics from early phase clinical studies (2015 or later) of novel oral SERDs, including giredestrant, amcenestrant, camizestrant, elacestrant, and rintodestrant.

EXPERT OPINION

Numerous oral SERDs are in clinical development, aiming to form the core endocrine therapy for HR-positive breast cancer. Through property- and structure-based drug design of estrogen receptor-binding, antagonism, degradation, anti-proliferation, and pharmacokinetic properties, these SERDs have distinct profiles which impact clinical dosing, efficacy, and safety. Assuming preliminary safety and activity data are confirmed in phase 3 trials, these promising agents could further improve the management, outcomes, and quality of life in HR-positive breast cancer.

Building the Bridge: Molecular Imaging Biomarkers for 21st Century Cancer Therapies

Precision medicine, where the molecular underpinnings of the disease are assessed for tailored therapies, has greatly impacted cancer care. In parallel, a new pillar of therapeutics has emerged with profound success, including immunotherapies such as checkpoint inhibitors and cell-based therapies. Nonetheless, it remains essential to develop paradigms to predict and monitor for therapeutic response. Molecular imaging has the potential to add substantially to all phases of cancer patient care: predicative, companion diagnostics can illuminate therapeutic target density within a tumor, and pharmacodynamic imaging biomarkers can complement traditional modalities to judge a favorable treatment response. This "Focus on Molecular Imaging" article discusses the current role of molecular imaging in oncology and highlights an additional step in clinical paradigm termed a "therapeutic biomarker," which serves to assess whether next generation drugs reach their target to elicit a favorable clinical response.

Leveraging Antiprogestins in the Treatment of Metastatic Breast Cancer

Although incurable, the prognosis for patients with metastatic breast cancer (MBC) has considerably improved with the approvals of multiple targeted and cytotoxic therapies. For hormone receptor-positive (HR+), ie, estrogen receptor and progesterone receptor positive (ER+/PgR+) and human epidermal growth factor receptor-2 negative (ie, ERBB2 gene nonamplified or HER2-) MBC, current approved treatment options include palliative endocrine therapy (ET), cyclin-dependent kinase (CDK 4/6) inhibitors, mTOR inhibitors, and PI3 kinase inhibitors. Most treatments target ER+ disease regardless of PgR status. Although the presence of PgR is crucial for ER+ cell proliferation in both normal and malignant mammary tissue, currently, there are no approved treatments that specifically target PgR. Recent literature has demonstrated the potential of antiprogestins in the treatment of MBC both in preclinical and clinical studies. Antiprogestins, including selective PgR modulators (SPRMs) that act as PgR antagonists, are a promising class of therapeutics for overcoming endocrine resistance in patients who develop activating estrogen receptor 1 (ESR1) and phosphatidylinositol 3-kinase (PI3K) gene mutations after prior endocrine therapy. Herein, we summarize the role of PgR and antiprogestins in the treatment of MBC. Other aspects on the use of functional imaging, clinical trials incorporating novel antiprogestins, and potential treatment combinations to overcome endocrine resistance will be briefly discussed.

Pitfalls on PET/CT Due to Artifacts and Instrumentation

PET/CT has become a preferred imaging modality over PET-only scanners in clinical practice. However, along with the significant improvement in diagnostic accuracy and patient throughput, pitfalls on PET/CT are reported as well. This review provides a general overview on the potential influence of the limitations with respect to PET/CT instrumentation and artifacts associated with the modality integration on the image appearance and quantitative accuracy of PET. Approaches proposed in literature to address the limitations or minimize the artifacts are discussed as well as their current challenges for clinical applications. Although the CT component can play an important role in assisting clinical diagnosis, we concentrate on the imaging scenarios where CT is used to provide auxiliary information for attenuation compensation and scatter correction in PET.

Targeted Protein Degradation: An Emerging Therapeutic Strategy in Cancer

Drug discovery in the scope of cancer therapy has been focused on conventional agents that nonselectively induce DNA damage or selectively inhibit the activity of key oncogenic molecules without affecting their protein levels. An emerging therapeutic strategy that garnered attention in recent years is the induction of Targeted Protein Degradation (TPD) of cellular targets by hijacking the intracellular proteolysis machinery. This novel approach offers several advantages over conventional inhibitors and introduces a paradigm shift in several pharmacological aspects of drug therapy. While TPD has been found to be the major mode of action of clinically approved anticancer agents such as fulvestrant and thalidomide, recent years have witnessed systematic endeavors to expand the repertoire of proteins amenable to therapeutic ablation by TPD. Such endeavors have led to three major classes of agents that induce protein degradation, including molecular glues, Proteolysis Targeting Chimeras (PROTACs) and Hydrophobic Tag (HyT)-based degraders. Here, we briefly highlight agents in these classes and key advances made in the field with a focus on clinical translation in cancer therapy.

Assessment of ultrasmall nanocluster for early and accurate detection of atherosclerosis using positron emission tomography/computed tomography

The development of atherosclerosis therapy is hampered by the lack of molecular imaging tools to identify the relevant biomarkers and determine the dynamic variation in vivo. Here, we show that a chemokine receptor 2 (CCR2) targeted gold nanocluster conjugated with extracellular loop 1 inverso peptide (AuNC-ECL1i) determines the initiation, progression and regression of atherosclerosis in apolipoprotein E knock-out (ApoE^-/-) mouse models. The CCR2 targeted ⁶⁴Cu-AuNC-ECL1i reveals sensitive detection of early atherosclerotic lesions and progression of plaques in ApoE^-/- mice. CCR2 targeting specificity was confirmed by the competitive receptor blocking studies. In a mouse model of aortic arch transplantation, ⁶⁴Cu-AuNC-ECL1i accurately detects the regression of plaques. Human atherosclerotic tissues show high expression of CCR2 related to the status of the disease. This study confirms CCR2 as a useful marker for atherosclerosis and points to the potential of ⁶⁴Cu-AuNC-ECL1i as a targeted molecular imaging probe for future clinical translation.

Targeting Nuclear Receptors for Cancer Therapy: Premises, Promises, and Challenges

Nuclear receptors are a family of transcription factors localized in cell nuclei, sensing specific ligands and fine-tuning a variety of cell physiological events. They have been intensively investigated in cancer biology. With their excellent properties of druggability and actionability, nuclear receptors have demonstrated much promise as novel therapeutic targets for different cancer types. Accumulating evidence has highlighted the essential roles of certain nuclear receptors in tumor immunology, suggesting the possibility for them to serve as cancer immunotherapeutic targets. Here, we summarize the roles of nuclear receptors in cancer biology and tumor immunology, and underscore the current advances of clinical trials for nuclear receptor-based cancer therapeutics.

In Vivo Receptor Visualization and Evaluation of Receptor Occupancy with Positron Emission Tomography

Positron emission tomography (PET) is useful for noninvasive in vivo visualization of disease-related receptors, for evaluation of receptor occupancy to determine an appropriate drug dosage, and for proof-of-concept of drug candidates in translational research. For these purposes, the specificity of the PET tracer for the target receptor is critical. Here, we review work in this area, focusing on the chemical structures of reported PET tracers, their K_i/K_d values, and the physical properties relevant to target receptor selectivity. Among these physical properties, such as cLogP, cLogD, molecular weight, topological polar surface area, number of hydrogen bond donors, and pK_a, we focus especially on LogD and LogP as important physical properties that can be easily compared across a range of studies. We discuss the success of PET tracers in evaluating receptor occupancy and consider likely future developments in the field.

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.

Hormone Receptor Loss in Breast Cancer: Molecular Mechanisms, Clinical Settings, and Therapeutic Implications

Hormone receptor-positive breast cancer (HR+ BC) accounts for approximately 75% of new BC diagnoses. Despite the undisputable progresses obtained in the treatment of HR+ BC in recent years, primary or acquired resistance to endocrine therapies still represents a clinically relevant issue, and is largely responsible for disease recurrence after curative surgery, as well as for disease progression in the metastatic setting. Among the mechanisms causing primary or acquired resistance to endocrine therapies is the loss of estrogen/progesterone receptor expression, which could make BC cells independent of estrogen stimulation and, consequently, resistant to estrogen deprivation or the pharmacological inhibition of estrogen receptors. This review aims at discussing the molecular mechanisms and the clinical implications of HR loss as a result of the therapies used in the neoadjuvant setting or for the treatment of advanced disease in HR+ BC patients.

Automated synthesis of the 16α-[18F]fluoroestradiol ([18F]FES): minimization of precursor amount and resulting benefits

The 16α-[18F]Fluoroestradiol ([18F]FES) is an established PET radiotracer for estrogen positive (ER+) breast cancer. Although the radiosynthesis is well-described, the majority of the published methods suffer from modest or irreproducible yields and time-intensive purification procedures. In view of the considerable clinical applications, development of a more efficient and faster synthesis of [18F]FES still remains a task of a significant practical importance. [18F]FES was produced by a direct nucleophilic radiofluorination of 3-O-methoxymethyl-16,17-O-sulfuryl-16-epiestriol (MMSE), followed by acidic hydrolysis using HCl/CH3CN. [18F]Fluoride retained on a QMA carb cartridge (46 mg) was eluted by solution of 1.2 mg of tetrabutylammonium tosylate (TBAOTs) in EtOH. After fluorination reaction (0.3 mg MMSE, 1 ml of CH3CN/100 °C, 5 min) [18F]FES was isolated by single-cartridge SPE purification using OASIS WAX 3cc, elution accomplished with aqueous ethanol of different concentrations. On а GE TRACERlab FX N Pro automated module [18F]FES (formulated in normal saline with 5% EtOH) was obtained in 33 ± 3% yield (n = 5, non-decay corrected) within 32 min. Reduction of precursor amount, exclusion of azeotropic drying step and simplification of purification make the suggested method readily adaptable to various automated synthesizers and offers significant cost decrease.

Update on Quantitative Imaging for Predicting and Assessing Response in Oncology

Molecular imaging has revolutionized clinical oncology by imaging-specific facets of cancer biology. Through noninvasive measurements of tumor physiology, targeted radiotracers can serve as biomarkers for disease characterization, prognosis, response assessment, and predicting long-term response/survival. In turn, these imaging biomarkers can be utilized to tailor therapeutic regimens to tumor biology. In this article, we review biomarker applications for response assessment and predicting long-term outcomes. ¹⁸F-fluorodeoxyglucose (FDG), a measure of cellular glucose metabolism, is discussed in the context of lymphoma and breast and lung cancer. FDG has gained widespread clinical acceptance and has been integrated into the routine clinical care of several malignancies, most notably lymphoma. The novel radiotracers 16α-¹⁸F-fluoro-17β-estradiol and ¹⁸F-fluorothymidine are reviewed in application to the early prediction of response assessment of breast cancer. Through illustrative examples, we explore current and future applications of molecular imaging biomarkers in the advancement of precision medicine.

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.

Cross-Species Physiological Assessment of Brain Estrogen Receptor Expression Using 18F-FES and 18F-4FMFES PET Imaging

PURPOSE

A retrospective analysis was performed of preclinical and clinical data acquired during the evaluation of the estrogen receptor (ER) PET tracer 4-fluoro-11β-methoxy-16α-[¹⁸F]-fluoroestradiol (4FMFES) and its comparison with 16α-[¹⁸F]-fluoroestradiol (FES) in mice, rats, and humans with a focus on the brain uptake.

PROCEDURES

Breast cancer tumor-bearing female BALB/c mice from a previous study and female Sprague-Dawley rats (control and ovariectomized) were imaged by 4FMFES or FES-PET imaging. Immediately after, low-dose CT was performed in the same bed position. Semi-quantitative analysis was conducted to extract %ID/g data. Small cohorts of mice and rats were imaged with 4FMFES in an ultra-high-resolution small animal PET scanner prototype (LabPET II). Rat brains were dissected and imaged separately with both PET and autoradiography. In parallel, 31 breast cancer patients were enrolled in a clinical phase II study to compare 4FMFES with FES for oncological assessment. Since the head was included in the field of view, brain uptake of discernable foci was measured and reported as SUV_Max.

RESULTS

Regardless of the species studied, 4FMFES and FES uptake were relatively uniform in most regions of the brain, except for bilateral foci at the base of the skull, at the midsection of the brain. Anatomical localization of the PET signal using CT image fusion indicates that the signal origins from the pituitary in all studied species. 4FMFES yielded lower pituitary uptake than FES in patients, but an inverse trend was observed in rodents. 4FMFES pituitary contrast was higher than FES in all assessed groups. High-resolution small animal imaging of the brain of rats and mice revealed a supplemental signal anterior to the pituitary, which is likely to be the medial preoptic area. Dissection data further confirmed those findings and revealed additional signals corresponding to the arcuate and ventromedial nuclei, along with the medial and cortical amygdala.

CONCLUSION

4FMFES allowed visualization of ER expression in the pituitary in humans and two different rodent species with better contrast than FES. Improvement in clinical spatial resolution might allow visualization and analysis of other ER-rich brain areas in humans. Further work is now possible to link 4FMFES pituitary uptake to cognitive functions.

Oncology-Inspired Treatment Options for COVID-19

CR3022 is a human antibody that binds to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Here, we explore the use of CR3022 as a molecularly targeted radiotherapeutic. Methods: CR3022 was labeled with ¹³¹I and purified, yielding ¹³¹I-CR3022. Using a magnetic bead assay and a recombinant SARS-CoV-2 spike protein fragment, we tested binding of ¹³¹I-CR3022 in the presence and absence of CR3022. Results: We conjugated the antibody CR3022 with a purity of more than 98% and a specific activity of more than 292 MBq/mg. Using a bead-based assay, we confirmed that binding of ¹³¹I-CR3022 is selective and is significantly reduced in the presence of unlabeled antibody (3.14% ± 0.14% specific uptake and 0.10% ± 0.01% specific uptake, respectively; P < 0.0001). Conclusion: Our results confirm the potential of CR3022 as a molecularly targeted probe for SARS-CoV-2. A labeled version of CR3022 could potentially be used for Auger radiotherapy or noninvasive imaging.

Head-to-Head Evaluation of 18F-FES and 18F-FDG PET/CT in Metastatic Invasive Lobular Breast Cancer

Invasive lobular carcinoma (ILC) demonstrates lower conspicuity on ¹⁸F-FDG PET than the more common invasive ductal carcinoma. Other molecular imaging methods may be needed for evaluation of this malignancy. As ILC is nearly always (95%) estrogen receptor (ER)-positive, ER-targeting PET tracers such as 16α-¹⁸F-fluoroestradiol (¹⁸F-FES) may have value. We reviewed prospective trials at Memorial Sloan Kettering Cancer Center using ¹⁸F-FES PET/CT to evaluate metastatic ILC patients with synchronous ¹⁸F-FDG and ¹⁸F-FES PET/CT imaging, which allowed a head-to-head comparison of these 2 PET tracers. Methods: Six prospective clinical trials using ¹⁸F-FES PET/CT in patients with metastatic breast cancer were performed at Memorial Sloan Kettering Cancer Center from 2008 to 2019. These trials included 92 patients, of whom 14 (15%) were of ILC histology. Seven of 14 patients with ILC had ¹⁸F-FDG PET/CT performed within 5 wk of the research ¹⁸F-FES PET/CT and no intervening change in management. For these 7 patients, the ¹⁸F-FES and ¹⁸F-FDG PET/CT studies were analyzed to determine the total number of tracer-avid lesions, organ systems of involvement, and SUV_max of each organ system for both tracers. Results: In the 7 comparable pairs of scans, there were a total of 254 ¹⁸F-FES-avid lesions (SUV_max, 2.6-17.9) and 111 ¹⁸F-FDG-avid lesions (SUV_max, 3.3-9.9) suggestive of malignancy. For 5 of 7 (71%) ILC patients, ¹⁸F-FES PET/CT detected more metastatic lesions than ¹⁸F-FDG PET/CT. In the same 5 of 7 patients, the SUV_max of ¹⁸F-FES-avid lesions was greater than the SUV_max of ¹⁸F-FDG-avid lesions. One patient had ¹⁸F-FES-avid metastases with no corresponding ¹⁸F-FDG-avid metastases. There were no patients with ¹⁸F-FDG-avid distant metastases without ¹⁸F-FES-avid distant metastases, although in one patient liver metastases were evident on ¹⁸F-FDG but not on ¹⁸F-FES PET. Conclusion: ¹⁸F-FES PET/CT compared favorably with ¹⁸F-FDG PET/CT for detection of metastases in patients with metastatic ILC. Larger prospective trials of ¹⁸F-FES PET/CT in ILC should be considered to evaluate ER-targeted imaging for clinical value in patients with this histology of breast cancer.

Translational strategy using multiple nuclear imaging biomarkers to evaluate target engagement and early therapeutic efficacy of SAR439859, a novel selective estrogen receptor degrader

PURPOSE

Preclinical in vivo nuclear imaging of mice offers an enabling perspective to evaluate drug efficacy at optimal dose and schedule. In this study, we interrogated sufficient estrogen receptor occupancy and degradation for the selective estrogen receptor degrader (SERD) compound SAR439859 using molecular imaging and histological techniques.

MATERIAL AND METHODS

[¹⁸F]FluoroEstradiol positron emission tomography (FES-PET), [¹⁸F]FluoroDeoxyGlucose (FDG) PET, and [¹⁸F]FluoroThymidine (FLT) PET were investigated as early pharmacodynamic, tumor metabolism, and tumor proliferation imaging biomarkers, respectively, in mice bearing subcutaneous MCF7-Y537S mutant ERα+ breast cancer model treated with the SERD agent SAR439859. ER expression and proliferation index Ki-67 were assessed by immunohistochemistry (IHC). The combination of palbociclib CDK 4/6 inhibitor with SAR439859 was tested for its potential synergistic effect on anti-tumor activity.

RESULTS

After repeated SAR439859 oral administration over 4 days, FES tumoral uptake (SUVmean) decreases compared to baseline by 35, 57, and 55% for the 25 mg/kg qd, 12.5 mg/kg bid and 5 mg/kg bid treatment groups, respectively. FES tumor uptake following SAR439859 treatment at different doses correlates with immunohistochemical scoring for ERα expression. No significant difference in FDG uptake is observed after SAR439859 treatments over 3 days. FLT accumulation in tumor is significantly decreased when palbociclib is combined to SAR439859 (- 64%) but not different from the group dosed with palbociclib alone (- 46%). The impact on proliferation is corroborated by Ki-67 IHC data for both groups of treatment.

CONCLUSIONS

In our preclinical studies, dose-dependent inhibition of FES tumoral uptake confirmed target engagement of SAR439859 to ERα. FES-PET thus appears as a relevant imaging biomarker for measuring non-invasively the impact of SAR439859 on tumor estrogen receptor occupancy. This study further validates the use of FLT-PET to directly visualize the anti-proliferative tumor effect of the palbociclib CDK 4/6 inhibitor alone and in combination with SAR439859.

Molecular imaging in lymphoma beyond 18F-FDG-PET: understanding the biology and its implications for diagnostics and therapy

Mature lymphoproliferative diseases are a heterogeneous group of neoplasms arising from different stages of B-cell and T-cell development. With improved understanding of the molecular processes in lymphoma and novel treatment options, arises a growing need for the molecular characterisation of tumours. Molecular imaging with single-photon-emission CT and PET using specific radionuclide tracers can provide whole-body information to investigate cancer biology, to evaluate phenotypic heterogeneity, to identify resistance to targeted therapy, and to assess the biodistribution of drugs in patients. In this Review, we evaluate the existing literature on molecular imaging in lymphoma, other than ¹⁸F-fluordeoxyglucose molecular imaging. The aim is to examine the contribution of molecular imaging to the understanding of the biology of lymphoma and to discuss potential implications for the diagnostics and therapy of this disease. Finally, we discuss possible applications for molecular imaging of patients with lymphoma in the clinical context.

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.

The Predictive Value of Early Changes in 18 F-Fluoroestradiol Positron Emission Tomography/Computed Tomography During Fulvestrant 500 mg Therapy in Patients with Estrogen Receptor-Positive Metastatic Breast Cancer

BACKGROUND

The aim of this study was to investigate the predictive value of early changes in ¹⁸ F-fluoroestradiol (FES) positron emission tomography (PET)/computed tomography (CT) during fulvestrant 500 mg therapy in patients with estrogen receptor (ER)-positive metastatic breast cancer.

MATERIALS AND METHODS

Patients underwent ¹⁸ F-FES PET/CT scans at both baseline (scan 1) and day 28 (scan 2). The maximum standardized uptake value (SUVmax) of all metastatic sites was determined in each scan, and the percentage reduction in SUVmax (ΔSUVmax) was calculated as [(SUVmax on scan 1-SUVmax on scan 2)/ SUVmax on scan 1] * 100%.

RESULTS

In total, 294 ¹⁸ F-FES-positive lesions from 36 patients were identified. The ¹⁸ F-FES SUVmax varied widely among lesions (median 5.7; range 1.8-32.4) and patients (median 5.1; range 2.5-13.2). After treatment, the median SUVmax among lesions and patients was 2.1 and 2.1, respectively. The ΔSUVmax ranged from -5.1% to 100%, with a median reduction of 61.3%. Using receiver operating characteristic analysis, the optimal cutoff point to discriminate patients who could derive clinical benefit from fulvestrant was determined to be 38.0%. Patients with a median ΔSUVmax ≥38.0% experienced significantly longer progression-free survival (PFS) than those with ΔSUVmax <38.0% (28.0 months vs. 3.5 months, p = .003). Multivariate analysis demonstrated that ΔSUVmax ≥38.0% was an independent predictor of PFS benefit in patients receiving fulvestrant therapy.

CONCLUSION

Changes in SUVmax measured by serial imaging of ¹⁸ F-FES PET/CT could be used early to predict PFS benefit in patients receiving fulvestrant therapy.

IMPLICATIONS FOR PRACTICE

The aim of this study was to evaluate the role of ¹⁸ F-fluoroestradiol (FES) positron emission tomography (PET)/computed tomography (CT) in predicting response to fulvestrant 500 mg therapy in patients with hormone receptor-positive/human epidermal growth receptor 2-negative metastatic breast cancer. This study highlights the utility of FES PET/CT as a predictive factor to discriminate patients who might benefit from fulvestrant. Moreover, these findings showed that this molecular imaging technique might be a potential tool for physicians to make individualized treatment strategies.

GLL398, an oral selective estrogen receptor degrader (SERD), blocks tumor growth in xenograft breast cancer models

PURPOSE

Selective estrogen receptor degrader (SERD) has proven clinically effective in treating advanced or metastatic breast cancer since the approval of fulvestrant by FDA in 2002. Recent expansion of indications as a first line monotherapy and as combination therapy with CDK4/6 inhibitors further extends its clinical utility as an efficacious breast cancer endocrine regimen. However, the poor pharmacokinetic properties of fulvestrant and its injection-only administration route has driven continued efforts to develop orally bioavailability SERD that could potentially improve clinical response to SERD treatment. GLL398, a boron-modified GW5638 analog, showed superior oral bioavailability, while retaining both antiestrogenic activity and ER degrading efficacy at a potency level comparable to the more active metabolite of GW5638, GW7604.

METHODS

Here we used molecular modeling, ER (Y537S) binding assay, MCF-7 Xenograft tumor, and patient-derived xenograft (PDX) tumor model to conduct further studies on the pharmacology and metabolism of GLL398.

RESULTS

Consistent with GLL398's robust activities in breast cancer cells that either are tamoxifen resistant or express constitutively active, mutant ESR1 (Y537S), it was found to bind the mutant ER^Y537S with high affinity. Molecular modeling of the binding mode of GLL398 to ER also found its molecular interactions consistent with the experimentally determined high binding affinity towards WT ER and ER^Y537S. To test the in vivo efficacy of GLL398, mice bearing MCF-7-derived xenograft breast tumors and patient-derived xenograft tumors harboring ER^Y537S were treated with GLL398 which potently inhibited tumor growth in mice.

CONCLUSIONS

This study demonstrates GLL398 is an oral SERD that has therapeutic efficacy in clinically relevant breast tumor models.

Positron emission tomography imaging of vascular endothelial growth factor with 64Cu-labeled bevacizumab for non-invasive diagnosis of endometriosis

Introduction:

Non-invasive diagnosis of endometriosis remains challenging. A promising approach for diagnosing endometriosis is the molecular imaging of vascular endothelial growth factor because angiogenesis plays a role in the establishment of endometriosis. This study aimed to evaluate the potential of copper-64-labeled bevacizumab, an anti–vascular endothelial growth factor antibody, for endometriosis imaging.

Methods:

Mouse endometriosis model was prepared by autologous transplantation. The vascular endothelial growth factor expression was evaluated by immunohistochemical staining. Biodistribution study and positron emission tomography imaging were performed at 1, 24, and 48 h after the injection of radiolabeled bevacizumab.

Results:

The immunohistochemical staining revealed that vascular endothelial growth factor is expressed around the stroma and glandular epithelial cells in the endometriosis lesion. The biodistribution study showed a high uptake of indium-111 bevacizumab in the endometriosis lesion. Positron emission tomography imaging with copper-64-labeled bevacizumab clearly visualized the endometriosis lesions at 24 and 48 h after injection.

Conclusion:

These results indicate the potential usefulness of copper-64-labeled bevacizumab for endometriosis imaging.

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.

Recent Advances in Nuclear Imaging of Receptor Expression to Guide Targeted Therapies in Breast Cancer

Breast cancer remains the most frequent cancer in women with different patterns of disease progression and response to treatments. The identification of specific biomarkers for different breast cancer subtypes has allowed the development of novel targeting agents for imaging and therapy. To date, patient management depends on immunohistochemistry analysis of receptor status on bioptic samples. This approach is too invasive, and in some cases, not entirely representative of the disease. Nuclear imaging using receptor tracers may provide whole-body information and detect any changes of receptor expression during disease progression. Therefore, imaging is useful to guide clinicians to select the best treatments for each patient and to evaluate early response thus reducing unnecessary therapies. In this review, we focused on the development of novel tracers that are ongoing in preclinical and/or clinical studies as promising tools to lead treatment decisions for breast cancer management.

Tumor Drug Penetration Measurements Could Be the Neglected Piece of the Personalized Cancer Treatment Puzzle

Precision medicine aims to use patient genomic, epigenomic, specific drug dose, and other data to define disease patterns that may potentially lead to an improved treatment outcome. Personalized dosing regimens based on tumor drug penetration can play a critical role in this approach. State-of-the-art techniques to measure tumor drug penetration focus on systemic exposure, tissue penetration, cellular or molecular engagement, and expression of pharmacological activity. Using in silico methods, this information can be integrated to bridge the gap between the therapeutic regimen and the pharmacological link with clinical outcome. These methodologies are described, and challenges ahead are discussed. Supported by many examples, this review shows how the combination of these techniques provides enhanced patient-specific information on drug accessibility at the tumor tissue level, target binding, and downstream pharmacology. Our vision of how to apply tumor drug penetration measurements offers a roadmap for the clinical implementation of precision dosing.

Accelerating anticancer drug development - opportunities and trade-offs

The traditional approach to drug development in oncology, with discrete phases of clinical testing, is becoming untenable owing to expansion of the precision medicine paradigm, whereby patients are stratified into multiple subgroups according to the underlying cancer biology. Seamless approaches to drug development in oncology hold great promise of accelerating the accessibility of novel therapeutic agents to the public but are also accompanied by important trade-offs, including the limited availability of information on the clinical benefit and safety of novel agents at the time of market entry. In this Perspectives article, we describe several opportunities, in the form of novel trial designs or modelling strategies, to improve the efficiency of drug development in oncology, as well as new mechanisms to obtain information about anticancer therapies throughout their life cycle, such as innovative functional imaging techniques or the use of real-world clinical data.