Preclinical development of a neutral, estrogen receptor-targeted, tridentate 99mTc(I)-estradiol-pyridin-2-yl hydrazine derivative for imaging of breast and endometrial cancers

Radiosynthesis and Preclinical Evaluation of 18F-Labeled Estradiol Derivatives with Different Lipophilicity for PET Imaging of Breast Cancer

About 75% of breast tumors show an overexpression of the estradiol receptor (ER), making it a valuable target for tumor diagnosis and therapy. To date, 16α-[18F]fluoroestradiol (FES) is the only FDA-approved imaging probe for the positron emission tomography (PET) imaging of ER-positive (ER+) breast cancer. However, FES has the drawback of a high retention in the liver. Therefore, the aim of this study was the development and preclinical evaluation of estradiol (E2) derivatives with different lipophilicity. Three 18F-labeled prosthetic groups (two glycosyl and one PEG azide) were chosen for conjugation with ethinyl estradiol (EE) by 18F-CuAAC (Cu-catalyzed azide-alkyne cycloaddition). The cellular uptake in ER+ MCF-7 tumor cells was highest for the less hydrophilic derivative (18F-TA-Glyco-EE). In nude mice bearing different breast tumors (ER+ MCF-7 and T47D versus ER- MDA-MB-231), 18F-TA-Glyco-EE revealed a high uptake in the liver (13%ID/g, 30 min p.i.), which decreased over 90 min to 1.2%ID/g, indicating fast hepatobiliary clearance. The statistically significant difference of 18F-TA-Glyco-EE uptake in T47D compared to MDA-MB-231 tumors at 60-90 min p.i. indicated ER-specific uptake, whereas in vivo PET imaging did not provide evidence for specific uptake of 18F-TA-Glyco-EE in MCF-7 tumors, probably due to ER occupation by E2 after E2-dependent MCF-7 tumor growth in mice. However, in vitro autoradiography revealed a high specific binding of 18F-TA-Glyco-EE to ER+ tumor slices. We conclude that 18F-TA-Glyco-EE, with its increased hydrophilicity after deacetylation in the blood and thus rapid washout from non-target tissues, may be a viable alternative to FES for the PET imaging of breast cancer.

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.

Radioiodinated Ethinylestradiol Derivatives for Estrogen Receptor Targeting Breast Cancer Imaging

Two novel PEGylated ethinylestradiol (PEG = poly(ethylene glycol)) estrogen receptor (ER) targeting probes [¹³¹I]EITE and [¹³¹I]MITE were synthesized and evaluated. Both probes had a nanomolar binding affinity to the ER receptor (36.47 nM for [¹³¹I]EITE and 61.83 nM for [¹³¹I]MITE). They showed high uptake in ER-positive MCF-7 cells and tumors, which could be significantly blocked by a coinjection of excess estradiol. Their ER specificities were further demonstrated by the low uptake in ER-negative MDA-MB-231 cells and tumors. The maximum tumor-to-muscle (T/M) ratios reach to 6.59 for [¹³¹I]EITE at 1 h postinjection (p.i.) and to 3.69 for [¹³¹I]MITE at 2 h p.i. in MCF-7 tumors. Among these two probes, [¹³¹I]EITE showed a faster tumor accumulation and a higher T/M ratio indicating it could be a better candidate for the potential diagnosis of ER-positive breast cancers.

Radionuclide-Based Imaging of Breast Cancer: State of the Art

Simple Summary

Breast cancer is one of the most commonly diagnosed malignant tumors, possessing high incidence and mortality rates that threaten women’s health. Thus, early and effective breast cancer diagnosis is crucial for enhancing the survival rate. Radionuclide molecular imaging displays its advantages for detecting breast cancer from a functional perspective. Noninvasive visualization of biological processes with radionuclide-labeled small metabolic compounds helps elucidate the metabolic state of breast cancer, while radionuclide-labeled ligands/antibodies for receptor-targeted radionuclide molecular imaging is sensitive and specific for visualization of the overexpressed molecular markers in breast cancer. This review focuses on the most recent developments of novel radiotracers as promising tools for early breast cancer diagnosis.

Abstract

Breast cancer is a malignant tumor that can affect women worldwide and endanger their health and wellbeing. Early detection of breast cancer can significantly improve the prognosis and survival rate of patients, but with traditional anatomical imagine methods, it is difficult to detect lesions before morphological changes occur. Radionuclide-based molecular imaging based on positron emission tomography (PET) and single-photon emission computed tomography (SPECT) displays its advantages for detecting breast cancer from a functional perspective. Radionuclide labeling of small metabolic compounds can be used for imaging biological processes, while radionuclide labeling of ligands/antibodies can be used for imaging receptors. Noninvasive visualization of biological processes helps elucidate the metabolic state of breast cancer, while receptor-targeted radionuclide molecular imaging is sensitive and specific for visualization of the overexpressed molecular markers in breast cancer, contributing to early diagnosis and better management of cancer patients. The rapid development of radionuclide probes aids the diagnosis of breast cancer in various aspects. These probes target metabolism, amino acid transporters, cell proliferation, hypoxia, estrogen receptor (ER), progesterone receptor (PR), human epidermal growth factor receptor 2 (HER2), gastrin-releasing peptide receptor (GRPR) and so on. This article provides an overview of the development of radionuclide molecular imaging techniques present in preclinical or clinical studies, which are used as tools for early breast cancer diagnosis.

Synthesis of Hydrazinylpyridines via Nucleophilic Aromatic Substitution and Further Transformation to Bicyclo[2.2.2]octenes Fused with Two N-Aminosuccinimide Moieties

Efficient and reliable synthesis of substituted hydrazinylpyridines in thick-wall ACE tubes via nucleophilic substitution of a chlorine substituent in different chloropyridines is presented. Hydrazine hydrate and alkylhydrazines were used as nucleophiles and simple alcohols and diethyl ether were the only organic solvents necessary, making the process environmentally and user friendly, potentially reaching 100% atomic efficiency. In the next step, transformations of succinic anhydride moieties fused to the bicyclo[2.2.2]octene framework into succinimide moieties via nucleophilic substitution of oxygens were conducted. As nucleophiles two of the synthesized hydrazinylpyridines (2-hydrazinyl-3-nitropyridine and 2-hydrazinyl-5-nitropyridine) and also hydrazine hydrate, phenylhydrazine, and 4-nitrophenylhydrazine were used. Reactions were again carried out in ACE tubes and only simple alcohols, diethyl ether, and acetone were needed as solvents. One of the prepared bicyclo[2.2.2]octene adducts displayed water solubility thus being a promising candidate for future studies as a novel bidentate ligand for various metal cations in aqueous solutions or acting as an unprecedented halogen bond acceptor.

Gynaecological Cancer Diagnostics: 99mTc-Cisplatin Complex as a Future Approach for Early, Prompt and Efficient Diagnosis of Gynaecological Cancer

BACKGROUND

Gynaecological cancers (GCCa) are common and have a significant mortality rate all over the world. Early diagnosis of cancer can play a key role in the treatment and survival of a patient. Identification, staging, treatment, and monitoring of gynaecological malignancies is being done successfully by nuclear medicines.

DISCUSSION

Currently, single-photon emission computed tomography (SPECT) and positron emission tomography (PET) centered imaging techniques are being developed for use in patients with GCCa as a diagnostic tool. The present work elucidates several clinical studies on the use of radiopharmaceuticals, based on their effectiveness, in the early detection and management of GCCa. It also highlights the importance of reconsidering the biology for nuclear imaging as a future modality for early, rapid and efficient diagnosis of gynecological cancers. This comprehensive review is a part of our study designed to detect gynaecological cancers at an early stage using radionuclide complex, 99m Tc-Cisplatin.

CONCLUSION

This article summarizes the significance of radioscintigraphy such as single-photon emission computed tomography (SPECT) and PET for identification of GCCa in the experimental humans and animals.

Radioiodinated estradiol dimer for estrogen receptor targeted breast cancer imaging

The aim of this study was to develop a 1-(2-(2-(2-(1,2,3-triazol)ethoxy)ethoxy)ethyl)-5-[^125/131 I]iodo-1,2,3-triazole-diestradiol ([^125/131 I]ITE2), for estrogen receptor (ER)-expressing breast cancer imaging with single-photon emission computed tomography (SPECT). [^125/131 I]ITE2 was prepared in good radiochemical yield (94.4 ± 0.4%) with high radiochemical purity (>99%). [^125/131 I]ITE2 had good stability in vitro and moderate molar activity (0.3 ± 0.2 GBq/µmol). Higher uptake in ER-positive MCF-7 cells than that of ER-negative MDA-MB-231 cells was observed at all time points. Rats biodistribution showed that [¹³¹ I]ITE2 had high uptake in ER-abundant uterine and ovarian (5.7 ± 0.4 and 10.1 ± 1.4%ID/g at 1 hr postinjection) and could be blocked by co-injection of estradiol (2.7 ± 0.1 and 5.5 ± 0.4%ID/g) obviously. In the SPECT/CT imaging study, [¹²⁵ I]ITE2 showed significant higher uptake in MCF-7 tumor (3.1 ± 0.4%ID/g) than that of MDA-MB-231 (0.9 ± 0.1%ID/g). Furthermore, the specific uptake of [¹²⁵ I]ITE2 in ER-positive MCF-7 tumor could be blocked effectively by preadministration of fulvestrant (1.2 ± 0.4%ID/g). A novel radioiodinated dimeric estrogen was designed and synthesized with promising ER targeting ability and specificity. It is worthy of further investigation to validate the advantages of the dimer in ER-positive breast cancer diagnosis.

Development and evaluation of a 99mTc(V)-nitrido complex derived from estradiol for breast cancer imaging

Estrogen receptors are overexpressed in 70% of breast cancer and identification of their presence is important to select the appropriate treatment. This work proposes the preparation and evaluation of an estradiol derived as potential ER imaging agent. Ethinylestradiol was derivatized to introduce a dithiocarbamate function for Tc coordination. Labeling was achieved through the formation of a symmetric Tc(V)-nitrido complex with a radiochemical purity (RCP) > 95%. Physicochemical evaluation, cell uptake, biodistribution in normal animals and in nude mice bearing induced ER + breast tumors showed promising results.

Review of fluorescent steroidal ligands for the estrogen receptor 1995-2018

The development of fluorescent ligands for the estrogen receptor (ER) continues to be of interest. Over the past 20 years, most efforts have focused on appending an expanding variety of fluorophores to the B-, C- and D-rings of the steroidal scaffold. This review highlights the synthesis and evaluation of derivatives substituted primarily at the 6-, 7α- and 17α-positions, culminating with our recent work on 11β-substituted estradiols, and proposes an approach to new fluorescent imaging agents that retain high ER affinity.

Less Exploited GPCRs in Precision Medicine: Targets for Molecular Imaging and Theranostics

Precision medicine relies on individually tailored therapeutic intervention taking into account individual variability. It is strongly dependent on the availability of target-specific drugs and/or imaging agents that recognize molecular targets and patient-specific disease mechanisms. The most sensitive molecular imaging modalities, Single Photon Emission Computed Tomography (SPECT) and Positron Emission Tomography (PET), rely on the interaction between an imaging radioprobe and a target. Moreover, the use of target-specific molecular tools for both diagnostics and therapy, theranostic agents, represent an established methodology in nuclear medicine that is assuming an increasingly important role in precision medicine. The design of innovative imaging and/or theranostic agents is key for further accomplishments in the field. G-protein-coupled receptors (GPCRs), apart from being highly relevant drug targets, have also been largely exploited as molecular targets for non-invasive imaging and/or systemic radiotherapy of various diseases. Herein, we will discuss recent efforts towards the development of innovative imaging and/or theranostic agents targeting selected emergent GPCRs, namely the Frizzled receptor (FZD), Ghrelin receptor (GHSR-1a), G protein-coupled estrogen receptor (GPER), and Sphingosine-1-phosphate receptor (S1PR). The pharmacological and clinical relevance will be highlighted, giving particular attention to the studies on the synthesis and characterization of targeted molecular imaging agents, biological evaluation, and potential clinical applications in oncology and non-oncology diseases. Whenever relevant, supporting computational studies will be also discussed.

18F-labeled estradiol derivative for targeting estrogen receptor-expressing breast cancer

INTRODUCTION

A novel radiotracer 1‑(2‑(2‑(2‑[¹⁸F]fluoroethoxy)ethoxy)ethyl)‑1H‑1,2,3‑triazole‑estradiol ([¹⁸F]FETE) was successfully synthesized, characterized and evaluated in mice for estrogen receptor (ER)-positive breast cancer targeting with positron emission tomography (PET) imaging.

METHODS

The tosylate precursor 3 was radiolabeled with ¹⁸F and then reacted with 17α‑ethinyl‑estradiol to produce the final [¹⁸F]FETE. The physicochemical properties of [¹⁸F]FETE were tested in vitro, including determination of the octanol/water partition coefficient, stability and cellular uptake in MCF-7 (ER-positive) and MDA-MB-231 (ER-negative) cells. An ex vivo biodistribution study was performed in normal Sprague Dawley rats, and in vivo microPET imaging was performed on MCF-7 and MDA-MB-231 tumor-bearing mice. The results of biodistribution and PET imaging of [¹⁸F]FETE were compared with that of known 16α‑[¹⁸F]fuoro‑17β‑estradiol ([¹⁸F]FES). Radiation dose estimates for [¹⁸F]FETE were also analyzed.

RESULTS

[¹⁸F]FETE was obtained in high radiochemical yield (46.59 ± 8.06%) with high radiochemical purity (>99%) after HPLC purification and high molar activity (15.45 ± 3.15 GBq/μmol). [¹⁸F]FETE is a moderate lipophilic compound with good in vitro stability and the total synthesis time was 55 to 65 min. In biodistribution studies, [¹⁸F]FETE showed high uptake in the ER-abundant uterine tissue of normal immature SD rats (8.55 ± 1.21 and 6.83 ± 1.70%ID/g at 1 h after intravenous and intraperitoneal injection, respectively), and could be blocked with estradiol effectively (the uterus uptake was decreased to 0.63 ± 0.35%ID/g at 1 h after iv injection). MicroPET imaging of tumor-bearing mice with [¹⁸F]FETE at 1 h after iv injection revealed considerable uptake in ER-positive MCF-7 tumors (4.63 ± 0.73%ID/g) that could be inhibited (1.47 ± 0.29%ID/g) and low uptake in ER-negative MDA-MB-231 tumors (1.97 ± 0.36%ID/g). [¹⁸F]FES has relatively low uptake in ER-positive tumor (0.24 ± 0.19%ID/g) when compared with [¹⁸F]FETE. The adult female effective radiation dose of [¹⁸F]FETE in mice was estimated as 0.0022 mSv/MBq.

CONCLUSIONS

A novel 17α‑ethinyl‑estradiol-based ER probe [¹⁸F]FETE was developed with high molar activity and good in vitro stability. Based on the results of bio-evaluation in normal immature rats and tumor-bearing mice, it might be a promising candidate for specific PET imaging of ER-positive breast cancer.

Twenty years of the G protein-coupled estrogen receptor GPER: Historical and personal perspectives

Estrogens play a critical role in many aspects of physiology, particularly female reproductive function, but also in pathophysiology, and are associated with protection from numerous diseases in premenopausal women. Steroids and the effects of estrogen have been known for ∼90 years, with the first evidence for a receptor for estrogen presented ∼50 years ago. The original ancestral steroid receptor, extending back into evolution more than 500 million years, was likely an estrogen receptor, whereas G protein-coupled receptors (GPCRs) trace their origins back into history more than one billion years. The classical estrogen receptors (ERα and ERβ) are ligand-activated transcription factors that confer estrogen sensitivity upon many genes. It was soon apparent that these, or novel receptors may also be responsible for the "rapid"/"non-genomic" membrane-associated effects of estrogen. The identification of an orphan GPCR (GPR30, published in 1996) opened a new field of research with the description in 2000 that GPR30 expression is required for rapid estrogen signaling. In 2005-2006, the field was greatly stimulated by two studies that described the binding of estrogen to GPR30-expressing cell membranes, followed by the identification of a GPR30-selective agonist (that lacked binding and activity towards ERα and ERβ). Renamed GPER (G protein-coupled estrogen receptor) by IUPHAR in 2007, the total number of articles in PubMed related to this receptor recently surpassed 1000. In this article, the authors present personal perspectives on how they became involved in the discovery and/or advancement of GPER research. These areas include non-genomic effects on vascular tone, receptor cloning, molecular and cellular biology, signal transduction mechanisms and pharmacology of GPER, highlighting the roles of GPER and GPER-selective compounds in diseases such as obesity, diabetes, and cancer and the obligatory role of GPER in propagating cardiovascular aging, arterial hypertension and heart failure through the stimulation of Nox expression.

GPER modulators: Opportunity Nox on the heels of a class Akt

The (patho)physiology of estrogen and its receptors is complex. It is therefore not surprising that therapeutic approaches targeting this hormone include stimulation of its activity through supplementation with either the hormone itself or natural or synthetic agonists, inhibition of its activity through the use of antagonists or inhibitors of its synthesis, and tissue-selective modulation of its activity with biased ligands. The physiology of this hormone is further complicated by the existence of at least three receptors, the classical nuclear estrogen receptors α and β (ERα and ERβ), and the 7-transmembrane G protein-coupled estrogen receptor (GPER/GPR30), with overlapping but distinct pharmacologic profiles, particularly of anti-estrogenic ligands. GPER-selective ligands, as well as GPER knockout mice, have greatly aided our understanding of the physiological roles of GPER. Such ligands have revealed that GPER activation mediates many of the rapid cellular signaling events (including Ca2+ mobilization, ERK and PI3K/Akt activation) associated with estrogen activity, as opposed to the nuclear ERs that are traditionally described to function as ligand-induced transcriptional factors. Many of the salutary effects of estrogen throughout the body are reproduced by the GPER-selective agonist G-1, which, owing to its minimal effects on reproductive tissues, can be considered a non-feminizing estrogenic compound, and thus of potential therapeutic use in both women and men. On the contrary, until recently GPER-selective antagonists had predominantly found preclinical application in cancer models where estrogen stimulates cell growth and survival. This viewpoint changed recently with the discovery that GPER is associated with aging, particularly that of the cardiovascular system, where the GPER antagonist G36 reduced hypertension and GPER deficiency prevented cardiac fibrosis and vascular dysfunction with age, through the downregulation of Nox1 and as a consequence superoxide production. Thus, similar to the classical ERs, both agonists and antagonists of GPER may be of therapeutic benefit depending on the disease or condition to be treated.

Molecular imaging using PET and SPECT for identification of breast cancer subtypes

Breast cancer is a major disease with high morbidity and mortality in women. As a highly heterogeneous tumor, it contains different molecular subtypes: luminal A, luminal B, human epidermal growth factor 2-positive, and triple-negative subtypes. As each subtype has unique features, it may not be universal to the optimal treatment and expected response for individual patients. Therefore, it is critical to identify different breast cancer subtypes. Targeting subcellular levels, molecular imaging, especially PET and single photon emission computed tomography, has become a promising means to identify breast cancer subtypes and monitor treatment. Different biological processes between various subtypes, including changes correlated with receptor expression, cell proliferation, or glucose metabolism, have the potential for imaging with PET and single photon emission computed tomography radiopharmaceuticals. Receptor imaging, with radiopharmaceuticals targeting estrogen receptor, progesterone receptor, or human epidermal growth factor 2, is available to distinguish receptor-positive tumors from receptor-negative ones. Cell proliferation imaging with fluorine-18 fluorothymidine PET aids identification of luminal A and B subtypes on the basis of the correlation with the immunohistochemical biomarker Ki-67. Glucose metabolism imaging with fluorine-18 fluorodeoxyglucose PET may have potential to discriminate triple-negative subtypes from others. With increasing numbers of novel radiopharmaceuticals, noninvasive molecular imaging will be applied widely for the identification of different subtypes and provide more in-vivo information on individualized management of breast cancer patients.

99mTc-labeled estradiol as an estrogen receptor probe: Preparation and preclinical evaluation

INTRODUCTION

Most breast cancers express estrogen receptors (ERs). Noninvasive imaging of ER expression may be helpful for planning therapy of ER+ tumors. We developed a new ER- binding probe, (99m)Tc-labeled estradiol, with diethylenetriaminepentaacetic acid (DTPA) as a chelating ligand, and assessed its targeting ability in vitro and in vivo.

METHODS

3-Aminoethyl estradiol was synthesized in two steps from estrone, followed by (99m)Tc labeling. Western blotting and immunofluorescence staining were used to detect ER expression in MCF-7 and MDA-MB-231 breast cancer cells. Saturation binding and specific binding were performed by incubating MCF-7 cells with increasing concentrations of (99m)Tc-DTPA-estradiol. Cell uptake, efflux, and blocking assays were also performed. To test (99m)Tc-DTPA-estradiol in vivo, nude mice bearing either MCF-7- (high ER expression) or MDA-MB-231- derived tumors (low ER expression) were injected with (99m)Tc-DTPA-estradiol, and underwent single-photon emission-computed tomography (SPECT). Mice injected with excess unlabeled DTPA-estradiol were used as controls. Ex vivo gamma-counting of tissues from normal and tumor-bearing mice was used to evaluate (99m)Tc-DTPA-estradiol biodistribution.

RESULTS

The radiochemical purity of (99m)Tc-DTPA-estradiol was 98.3%±1.3% with a specific activity of 33.1±1.5 MBq/μmol (n=3). Western blotting and immunofluorescence staining confirmed extensive expression of ERs by the MCF-7 cells, and less extensive expression by MDA-MB-231 cells. There was high binding affinity of (99m)Tc-DTPA-estradiol to MCF-7 cells with a>45% specific rate of total cell uptake. SPECT images and the biodistribution study results showed significantly higher uptake by MCF-7 tumors (6.06±0.38 %ID/g) than by MDA-MB-231 tumors (1.57±0.28 %ID/g). Pre-injection of MCF-7 tumor-bearing nude mice with excess unlabeled DTPA-estradiol significantly reduced tumor uptake of (99m)Tc-DTPA-estradiol (2.24±0.28 %ID/g), suggesting that (99m)Tc-DTPA-estradiol specifically targets ERs in tumors.

CONCLUSIONS

(99m)Tc-DTPA-estradiol can be synthesized with satisfactory labeling efficiency and stability. (99m)Tc-DTPA-estradiol specifically targeted ERs in vitro and in vivo with favorable pharmacokinetics, allowing ER receptor expression assessment with SPECT imaging.

International Union of Basic and Clinical Pharmacology. XCVII. G Protein-Coupled Estrogen Receptor and Its Pharmacologic Modulators

Estrogens are critical mediators of multiple and diverse physiologic effects throughout the body in both sexes, including the reproductive, cardiovascular, endocrine, nervous, and immune systems. As such, alterations in estrogen function play important roles in many diseases and pathophysiological conditions (including cancer), exemplified by the lower prevalence of many diseases in premenopausal women. Estrogens mediate their effects through multiple cellular receptors, including the nuclear receptor family (ERα and ERβ) and the G protein-coupled receptor (GPCR) family (GPR30/G protein-coupled estrogen receptor [GPER]). Although both receptor families can initiate rapid cell signaling and transcriptional regulation, the nuclear receptors are traditionally associated with regulating gene expression, whereas GPCRs are recognized as mediating rapid cellular signaling. Estrogen-activated pathways are not only the target of multiple therapeutic agents (e.g., tamoxifen, fulvestrant, raloxifene, and aromatase inhibitors) but are also affected by a plethora of phyto- and xeno-estrogens (e.g., genistein, coumestrol, bisphenol A, dichlorodiphenyltrichloroethane). Because of the existence of multiple estrogen receptors with overlapping ligand specificities, expression patterns, and signaling pathways, the roles of the individual receptors with respect to the diverse array of endogenous and exogenous ligands have been challenging to ascertain. The identification of GPER-selective ligands however has led to a much greater understanding of the roles of this receptor in normal physiology and disease as well as its interactions with the classic estrogen receptors ERα and ERβ and their signaling pathways. In this review, we describe the history and characterization of GPER over the past 15 years focusing on the pharmacology of steroidal and nonsteroidal compounds that have been employed to unravel the biology of this most recently recognized estrogen receptor.

GPER-targeted, 99mTc-labeled, nonsteroidal ligands demonstrate selective tumor imaging and in vivo estrogen binding

Our understanding of estrogen (17β-estradiol, E2) receptor biology has evolved in recent years with the discovery and characterization of a 7-transmembrane-spanning G protein-coupled estrogen receptor (GPER/GPR30) and the development of GPER-selective functional chemical probes. GPER is highly expressed in certain breast, endometrial, and ovarian cancers, establishing the importance of noninvasive methods to evaluate GPER expression in vivo. Here, we developed (99m)Tc-labeled GPER ligands to demonstrate the in vivo status of GPER as an estrogen receptor (ER) and for GPER visualization in whole animals. A series of (99m)Tc(I)-labeled nonsteroidal tetrahydro-3H-cyclopenta[c]quinolone derivatives was synthesized utilizing pyridin-2-yl hydrazine and picolylamine chelates. Radioligand receptor binding studies revealed binding affinities in the 10 to 30 nmol/L range. Cell signaling assays previously demonstrated that derivatives retaining a ketone functionality displayed agonist properties, whereas those lacking such a hydrogen bond acceptor were antagonists. In vivo biodistribution and imaging studies performed on mice bearing human endometrial and breast cancer cell xenografts yielded significant tumor uptake (0.4-1.1%ID/g). Blocking studies revealed specific uptake in multiple organs (adrenals, uterus, and mammary tissue), as well as tumor uptake with similar levels of competition by E2 and G-1, a GPER-selective agonist. In conclusion, we synthesized and evaluated a series of first-generation (99m)Tc-labeled GPER-specific radioligands, demonstrating GPER as an estrogen-binding receptor for the first time in vivo using competitive binding principles, and establishing the utility of such ligands as tumor imaging agents. These results warrant further investigation into the role of GPER in estrogen-mediated carcinogenesis and as a target for diagnostic/therapeutic/image-guided drug delivery.

IMPLICATIONS

These studies provide a molecular basis to evaluate GPER expression and function as an ER through in vivo imaging.

The role of preclinical SPECT in oncological and neurological research in combination with either CT or MRI

Preclinical imaging with SPECT combined with CT or MRI is used more and more frequently and has proven to be very useful in translational research. In this article, an overview of current preclinical research applications and trends of SPECT combined with CT or MRI, mainly in tumour imaging and neuroscience imaging, is given and the advantages and disadvantages of the different approaches are described. Today SPECT and CT systems are often integrated into a single device (commonly called a SPECT/CT system), whereas at present combined SPECT and MRI is almost always carried out with separate systems and fiducial markers to combine the separately acquired images. While preclinical SPECT/CT is most widely applied in oncology research, SPECT combined with MRI (SPECT/MRI when integrated in one system) offers the potential for both neuroscience applications and oncological applications. Today CT and MRI are still mainly used to localize radiotracer binding and to improve SPECT quantification, although both CT and MRI have additional potential. Future technology developments may include fast sequential or simultaneous acquisition of (dynamic) multimodality data, spectroscopy, fMRI along with high-resolution anatomic MRI, advanced CT procedures, and combinations of more than two modalities such as combinations of SPECT, PET, MRI and CT all together. This will all strongly depend on new technologies. With further advances in biology and chemistry for imaging molecular targets and (patho)physiological processes in vivo, the introduction of new imaging procedures and promising new radiopharmaceuticals in clinical practice may be accelerated.

Molecular imaging probes for diagnosis and therapy evaluation of breast cancer

Breast cancer is a major cause of cancer death in women where early detection and accurate assessment of therapy response can improve clinical outcomes. Molecular imaging, which includes PET, SPECT, MRI, and optical modalities, provides noninvasive means of detecting biological processes and molecular events in vivo. Molecular imaging has the potential to enhance our understanding of breast cancer biology and effects of drug action during both preclinical and clinical phases of drug development. This has led to the identification of many molecular imaging probes for key processes in breast cancer. Hormone receptors, growth factor receptor, and angiogenic factors, such as ER, PR, HER2, and VEGFR, have been adopted as imaging targets to detect and stage the breast cancer and to monitor the treatment efficacy. Receptor imaging probes are usually composed of targeting moiety attached to a signaling component such as a radionuclide that can be detected using dedicated instruments. Current molecular imaging probes involved in breast cancer diagnosis and therapy evaluation are reviewed, and future of molecular imaging for the preclinical and clinical is explained.

Synthesis and characterization of tricarbonyl-Re/Tc(I) chelate probes targeting the G protein-coupled estrogen receptor GPER/GPR30

The discovery of the G protein-coupled estrogen receptor GPER (also GPR30) and the resulting development of selective chemical probes have revealed new aspects of estrogen receptor biology. The potential clinical relevance of this receptor has been suggested from numerous studies that have identified GPER expression in breast, endometrial, ovarian and other cancers. Thus GPER can be considered a candidate biomarker and target for non-invasive imaging and therapy. We have designed and synthesized a series of organometallic tricarbonyl-rhenium complexes conjugated to a GPER-selective small molecule derived from tetrahydro-3H-cyclopenta[c]quinoline. The activity and selectivity of these chelates in GPER-mediated signaling pathways were evaluated. These results demonstrate that GPER targeting characteristics depend strongly on the structure of the chelate and linkage. Ethanone conjugates functioned as agonists, a 1,2,3-triazole spacer yielded an antagonist, and derivatives with increased steric volume exhibited decreased activities. Promising GPER selectivity was observed, as none of the complexes interacted with the nuclear estrogen receptors. Radiolabeling with technetium-99m in aqueous media was efficient and gave radioligands with high radiochemical yields and purity. These chelates have favorable physicochemical properties, show excellent stability in biologically relevant media, exhibit receptor specificity and are promising candidates for continuing development as diagnostic imaging agents targeting GPER expression in cancer.

Novel 7α-alkoxy-17α-(4'-halophenylethynyl)estradiols as potential SPECT/PET imaging agents for estrogen receptor expressing tumours: synthesis and binding affinity evaluation

In order to develop potential radiolabelled probes for imaging estrogen receptor (ER) positive tumours, we have synthesized and characterized a series of novel 7α-alkoxy-17α-(4'-iodophenylethynyl)estra-1,3,5(10)-triene-3,17β-diols and 7α-alkoxy-17α-(4'-fluorophenylethynyl)estra-1,3,5(10)-triene-3,17β-diols. The fluoro-substituted compounds showed a higher ER binding affinity than the corresponding iodo-derivatives, where 7α-methoxy- and 17α-(4'-fluorophenylethynyl)estra-1,3,5(10)-triene-3,17β-diol showed the highest ER binding affinities (RBA=80.9% and 78.9%, respectively), among the halophenylethynyl compounds studied and should be further explored as potential PET biomarkers for imaging of ER expressing tumours.

Influence of ligand denticity on the properties of novel ⁹⁹mTc(I)-carbonyl complexes. Application to the development of radiopharmaceuticals for imaging hypoxic tissue

An important issue in the development of metal-based radiopharmaceuticals is the selection of the labelling strategy in order to couple the metal to the pharmacophore without losing the biological activity. With the aim to evaluate the correlation between ligand denticity and biological behaviour of the corresponding (99m)Tc complexes, we designed a tridentate and a bidentate 5-nitroimidazole derivatives suitable for (99m)Tc(I) tricarbonyl complexation and with potential use as radiopharmaceuticals towards hypoxic tissue diagnosis. Ligands were synthesized using metronidazol, a pharmaceutical containing the bioreductive pharmacophore as starting material. The chelating units were connected to the pharmacophore using the click reaction of Huisgen. Both (99m)Tc complexes were obtained in high yield and were hydrophilic and stable in labelling milieu. The complex obtained from the tridentate ligand exhibited high stability in human plasma, low protein binding and a favourable biodistribution characterized by low blood and liver uptake, fast elimination and negligible uptake in other organs or tissues. Selective uptake and retention in tumour together with favourable tumour/muscle ratio makes this (99m)Tc-complex a promising candidate for further evaluation as potential hypoxia imaging agent in tumours. The bidentate ligand, on the other hand, yielded a less stable (99m)Tc-complex that experimented hydrolysis in vitro and decomposition in human plasma and showed high protein binding, high blood and liver uptake and moderate excretion. Although selective uptake and retention in tumour was also observed physicochemical and biological behaviour are inadequate for in vivo use, demonstrating that denticity of the ligand is particularly important and that tridentate ligands are preferable in order to prepare (99m)Tc-tricarbonyl complexes for Nuclear Medicine imaging.

Design and synthesis of a new class of membrane-permeable triazaborolopyridinium fluorescent probes

A new class of fluorescent triazaborolopyridinium compounds was synthesized from hydrazones of 2-hydrazinylpyridine (HPY) and evaluated as potential dyes for live-cell imaging applications. The HPY dyes are small, their absorption/emission properties are tunable through variation of pyridyl or hydrazone substituents, and they offer favorable photophysical characteristics featuring large Stokes shifts and general insensitivity to solvent or pH. The stability, neutral charge, cell membrane permeability, and favorable relative influences on the water solubility of HPY conjugates are complementary to existing fluorescent dyes and offer advantages for the development of receptor-targeted small-molecule probes. This potential was assessed through the development of a new class of cysteine-derived HPY-conjugate imaging agents for the kinesin spindle protein (KSP) that is expressed in the cytoplasm during mitosis and is a promising chemotherapeutic target. Conjugates possessing the neutral HPY or charged Alexa Fluor dyes that function as potent, selective allosteric inhibitors of the KSP motor were compared using biochemical and cell-based phenotypic assays and live-cell imaging. These results demonstrate the effectiveness of the HPY dye moiety as a component of probes for an intracellular protein target and highlight the importance of dye structure in determining the pathway of cell entry and the overall performance of small-molecule conjugates as imaging agents.

Influence of charge on cell permeability and tumor imaging of GPR30-targeted 111in-labeled nonsteroidal imaging agents

Recent clinical studies implicate the role of G protein-coupled estrogen receptor, GPR30, in aggressive forms of breast, ovarian, and endometrial cancers. However, the functional role of GPR30 at cellular and molecular levels remains less clear and controversial, particularly its subcellular location. The primary objective of this study was to develop radiolabeled neutral and charged GPR30-targeted nonsteroidal analogues to understand the influence of ligand charge on cell binding, cellular permeability, and in vivo tumor imaging. Therefore, we developed a series of GPR30-targeted (111/113)In(III)-labeled analogues using macrocyclic and acyclic polyamino-polycarboxylate chelate designs that would render either a net negative or neutral charge. In vitro biological evaluations were performed to determine the role of negatively charged analogues on receptor binding and activation using calcium mobilization and phosphoinositide 3-kinase assays. In vivo evaluations were performed on GPR30-expressing human endometrial Hec50 tumor-bearing mice to characterize the biodistribution and potential application of GPR30-targeted imaging agents for translational research. In vitro functional assays revealed an effect of charge, such that only the neutral analogue activated GPR30-mediated rapid signaling pathways. These observations are consistent with expectations for initial rates of membrane permeability and suggest an intracellular rather than the cell surface location of functional receptor. In vivo studies revealed receptor-mediated uptake of the radiotracer in target organs and tumors; however, further structural modifications will be required for the development of future generations of GPR30-targeted imaging agents with enhanced metabolic properties and decreased nonspecific localization to the intestines.

Synthesis and characterization of iodinated tetrahydroquinolines targeting the G protein-coupled estrogen receptor GPR30

A series of iodo-substituted tetrahydro-3H-cyclopenta[c]quinolines was synthesized as potential targeted imaging agents for the G protein-coupled estrogen receptor GPR30. The affinity and specificity of binding to GPR30 versus the classical estrogen receptors ER alpha/beta and functional responses associated with ligand-binding were determined. Selected iodo-substituted tetrahydro-3H-cyclopenta[c]quinolines exhibited IC(50) values lower than 20 nM in competitive binding studies with GPR30-expressing human endometrial cancer cells. These compounds functioned as antagonists of GPR30 and blocked estrogen-induced PI3K activation and calcium mobilization. The tributylstannyl precursors of selected compounds were radiolabeled with (125)I using the iodogen method. In vivo biodistribution studies in female ovariectomized athymic (NCr) nu/nu mice bearing GPR30-expressing human endometrial tumors revealed GPR30-mediated uptake of the radiotracer ligands in tumor, adrenal, and reproductive organs. Biodistribution and quantitative SPECT/CT studies revealed structurally related differences in the pharmacokinetic profiles, target tissue uptake, and metabolism of the radiolabeled compounds as well as differences in susceptibility to deiodination. The high lipophilicity of the compounds adversely affects the in vivo biodistribution and clearance of these radioligands and suggests that further optimization of this parameter may lead to improved targeting characteristics.