Effect of administration route on FES uptake into MCF-7 tumors

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.

Kisspeptin system in ovariectomized mice: Estradiol and progesterone regulation

The kisspeptin system is clustered in two main groups of cell bodies (the periventricular region, RP3V and the arcuate nucleus, ARC) that send fibers mainly to the GnRH neurons and in a few other locations, including the paraventricular nucleus, PVN. In physiological conditions, gonadal hormones modulate the kisspeptin system with expression changes according to different phases of the estrous cycle: the highest being in estrus phase in RP3V and PVN (positive feedback), and in ARC during the diestrus phase (negative feedback). In this work we wanted to study these hormonal fluctuations during the estrous cycle, investigating the role played by progesterone (P) or estradiol (E₂), alone or together, on the kisspeptin system. Gonadectomized CD1 female mice were treated with P, E₂ or both (E₂ + P), following a timing of administration that emulates the different phases of estrous cycle, for two cycles of 4 days. As expected, the two cell groups were differentially affected by E₂; the RP3V group was positively influenced by E₂ (alone or with the P), whereas in the ARC the administration of E₂ did not affect the system. However P (alone) induced a rise in the kisspeptin immunoreactivity. All the treatments significantly affected the kisspeptin innervation of the PVN, with regional differences, suggesting that these fibers arrive from both RP3V and ARC nuclei.

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.

Small-animal PET of steroid hormone receptors predicts tumor response to endocrine therapy using a preclinical model of breast cancer

Estrogen receptor-α (ERα) and progesterone receptor (PR) are expressed in most human breast cancers and are important predictive factors for directing therapy. Because of de novo and acquired resistance to endocrine therapy, there remains a need to identify which ERα-positive (ERα(+))/PR-positive (PR(+)) tumors are most likely to respond. The purpose of this study was to use estrogen- and progestin-based radiopharmaceuticals to image ERα and PR in mouse mammary tumors at baseline and after hormonal therapy and to determine whether changes in these imaging biomarkers can serve as an early predictive indicator of therapeutic response.

METHODS

Mammary adenocarcinomas that spontaneously develop in aged female mice deficient in signal transducer and activator of transcription-1 (STAT1) were used. Imaging of ERα and PR in primary tumor-bearing mice and mice implanted with mammary cell lines (SSM1, SSM2, and SSM3) derived from primary STAT1-deficient (STAT1(-/-)) tumors was performed. Hormonal treatments consisted of estradiol, an ER agonist; letrozole, an aromatase inhibitor; and fulvestrant, a pure ER antagonist. Small-animal PET/CT was performed using (18)F-fluoroestradiol ((18)F-FES) for ER, (18)F-fluoro furanyl norprogesterone ((18)F-FFNP) for PR, and (18)F-FDG for glucose uptake. Tracer uptake in the tumor was quantified and compared with receptor concentration determined by in vitro assays of resected tumors.

RESULTS

Primary STAT1(-/-) mammary tumors and implanted SSM2 and SSM3 tumors showed high (18)F-FES and (18)F-FFNP uptake and were confirmed to be ERα(+)/PR(+). Classic estrogen-induced regulation of the progesterone receptor gene was demonstrated by increased (18)F-FFNP uptake of estradiol-treated SSM3 tumors. Treatment with fulvestrant decreased (18)F-FFNP, (18)F-FES, and (18)F-FDG uptake and inhibited growth of SSM3 tumors but decreased only (18)F-FES uptake in SSM2 tumors, with no effect on growth, despite both tumors being ERα(+)/PR(+). Decreased (18)F-FFNP uptake by SSM3 tumors occurred early after initiation of treatment, before measurable tumor growth inhibition.

CONCLUSION

Using small-animal PET, a profile was identified that distinguished fulvestrant-sensitive from fulvestrant-resistant ERα(+)/PR(+) tumors before changes in tumor size. This work demonstrates that imaging baseline tumoral (18)F-FES uptake and initial changes in (18)F-FFNP uptake in a noninvasive manner is a potentially useful strategy to identify responders and nonresponders to endocrine therapy at an early stage.