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

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.

Development of Preladenant-Based Radiotracers for Imaging A2AR in Tumors

Activation of the adenosine 2A receptor (A2AR) can lead to tumor immunosuppression, which results in poor prognosis of immunotherapy. The aim of this study was to design novel 18F-labeled probes ([18F]F-PFP2 and [18F]F-PFP4) to visualize A2AR in the tumor. The uptake of radioprobes in A2AR-negative 4T1 breast tumor was lower than that of A2AR-positive B16F10 melanoma at 1 h p.i. (1.22 ± 0.36% ID/g vs 2.80 ± 0.72% ID/g), 2 h p.i. (1.09 ± 0.20% ID/g vs 2.93 ± 0.76% ID/g) and 3 h p.i. (0.89 ± 0.27% ID/g vs 2.73 ± 0.58% ID/g), respectively. B16F10 lung metastasis models were employed to expand the application scenarios, observing significantly higher uptake of [18F]F-PFP2 in metastatic lesions compared to normal lung tissue (5.55 ± 2.18% ID/g vs 1.89 ± 0.65% ID/g, tumor/lung ratio ∼3). It is given that [18F]F-PFP2 might lay the foundation for establishing an A2AR-targeted imaging evaluation system for tumors, which will provide more precise guidance for personalized treatment.

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

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

PET Imaging of Estrogen Receptors Using 18F-Based Radioligands

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

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.

Radioiodinated 4-(p-Iodophenyl) Butanoic Acid-Modified Estradiol Derivative for ER Targeting SPECT Imaging

Overexpression of estrogen receptors (ERs) is one of the important characteristics of most breast cancers. We aim to develop a new type of ER-specific radioiodine-labeled estrogen derivative ([¹³¹I]IPBA-EE), which was modified with an albumin-specific ligand 4-(p-iodophenyl) butyric acid (IPBA) to improve the metabolic stability and enhance the ER-targeting ability of estrogen. [¹³¹I]IPBA-EE can effectively bind to albumin in vitro, and its dissociation constant (K_d = 0.31 μM) is similar to IPBA (K_d = 0.30 μM). The uptake of [¹³¹I]IPBA-EE in ER-positive MCF-7 cells (41.81 ± 3.41%) was significantly higher than that in ER-negative MDA-MB-231 cells (8.78 ± 2.37%, ***P < 0.0005) and could be significantly blocked (3.92 ± 0.35%, ***P < 0.0005). The uptakes of [¹³¹I]IPBA-EE in rat uterus and ovaries were 5.66 ± 0.34% ID/g and 5.71 ± 2.77% ID/g, respectively, at 1 h p.i., and these uptakes could be blocked by estradiol (uterus: 2.81 ± 0.41% ID/g, *P < 0.05; ovarian: 3.02 ± 0.08% ID/g, *P < 0.05). SPECT/CT imaging showed that ER-positive MCF-7 tumor uptake of [¹³¹I]IPBA-EE reached to 6.07 ± 0.20% ID/g at 7 h p.i., which was significantly higher than that of ER-negative MDA-MB-231 tumor (0.87 ± 0.08% ID/g, **P < 0.005) and could be blocked obviously with fulvestrant (1.65 ± 1.56% ID/g, *P < 0.05). In conclusion, a novel radioiodinated estradiol derivative, [¹³¹I]IPBA-EE with albumin-binding property and good metabolic stability, was developed to image the ER in breast cancer. This promising ER-targeted probe has the potential to warrant further preclinical investigations.

Research progress of 18F labeled small molecule positron emission tomography (PET) imaging agents

With the development of positron emission tomography (PET) technology, a variety of PET imaging agents labeled with radionuclide ¹⁸F have been developed and widely used in the diagnosis and treatment of various clinical diseases in recent years. For example, they have showed a great value of study in the field of tumor detection, tumor treatment and evaluation of tumor therapy in a non-invasive, qualitative and quantitative way. In this review, we highlight the recent development in chemical synthesis, structure and characterization, imaging characterization, and potential applications of these ¹⁸F labeled small molecule PET imaging agents for the past five years. The development and application of ¹⁸F labeled small molecules will expand our knowledge of the function and distribution of diseases-related molecular targets and shed light on the diagnosis and treatment of various diseases including tumors.

18F-labeled ethisterone derivative for progesterone receptor targeted PET imaging of breast cancer

PURPOSE

A novel radiolabeled probe 1‑(17‑[¹⁸F]fluoro‑3,6,9,12,15‑pentaoxaheptadecyl‑1H‑1,2,3‑triazole testosterone ([¹⁸F]FPTT) was synthesized and evaluated for PET imaging of progesterone receptor (PR)-positive breast cancer.

METHODS

The ethinyl group of ethisterone, a PR targeting pharmacophore, was coupled with azide modified PEG-OTs by click chemistry to obtain the labeling precursor. The final [¹⁸F]FPTT was synthesized by a one-step nucleophilic substitution reaction with ¹⁸F. The in vitro stabilities of [¹⁸F]FPTT in saline or rat serum were determined after 2 h incubation. Then the in vitro cell binding, ex vivo biodistribution and in vivo imaging of [¹⁸F]FPTT were further investigated to evaluate the PR targeting ability and feasibility for the diagnosis of PR-positive breast cancer with PET imaging.

RESULTS

[¹⁸F]FPTT was obtained in high decay-corrected radiochemical yield (78 ± 9%) at the end of synthesis. It had high radiochemical purity (>98%) after HPLC purification and good in vitro stability. The molar activity of [¹⁸F]FPTT was calculated as 17 GBq/μmol. The microPET imaging of [¹⁸F]FPTT in tumor-bearing mice showed much higher tumor uptake in PR-positive MCF-7 tumor (3.9 ± 0.20%ID/g) than that of PR-negative MDA-MB-231 tumor (1.3 ± 0.08%ID/g). The high MCF-7 tumor uptake could be specifically inhibited by blocking with ethisterone (1.3 ± 0.11%ID/g) or [¹⁹F]FPTT (2.20 ± 0.17%ID/g), respectively. The biodistribution in estrogen-primed female SD rats of [¹⁸F]FPTT showed high uterus and ovary uptakes (8.31 ± 1.74%ID/g and 3.79 ± 0.82%ID/g at 1 h post-injection). The specific uptakes of uterus and ovary in normal rats were 3.52 ± 0.29%ID/g and 3.22 ± 0.50%ID/g respectively and could be inhibited by co-injecting of ethisterone.

CONCLUSION

A novel [¹⁸F]FPTT probe based on ethisterone modification could be a potential diagnostic agent for PR-positive breast cancer.