Synthesis of an estrogen receptor beta-selective radioligand: 5-[18F]fluoro-(2R,3S)-2,3-bis(4-hydroxyphenyl)pentanenitrile and comparison of in vivo distribution with 16alpha-[18F]fluoro-17beta-estradiol

Synthesis and evaluation of aryl-substituted diarylpropionitriles, selective ligands for estrogen receptor beta, as positron-emission tomographic imaging agents

We have investigated halogen-substituted non-steroidal estrogens with selective binding affinity for the estrogen receptor beta (ERbeta that might be used for imaging the levels of this ER-subtype in breast tumors by positron emission tomography (PET). Based on diarylpropionitrile (DPN, 1a), a compound previously reported that has a 72-fold binding selectivity for ERbeta, we developed a series of DPN analogs having methyl-, hydroxyl-, and halogen substituents, including fluoroethyl and fluoropropyl groups. In competitive radiometric binding assays with [(3)H]estradiol, all of these DPN analogs showed high ERbeta/ERalpha selectivity; while the selectivity varied, in some cases it reached nearly 300-fold (RBA: ERalpha, 0.023%; ERbeta, 6.25%). The absolute ERbeta binding affinities, however, were not sufficient to merit further consideration for developing these ligands as PET imaging agents.

Small molecule receptors as imaging targets

The aberrant expression and function of certain receptors in tumours and other diseased tissues make them preferable targets for molecular imaging. PET and SPECT radionuclides can be used to label specific ligands with high affinity for the target receptors. The functional information obtained from imaging these receptors can be used to better understand the systems under investigation and for diagnostic and therapeutic applications. This review discusses some of the aspects of receptor imaging with small molecule tracers by PET and SPECT and reviews some of the tracers for the receptor imaging of tumours and brain, heart and lung disorders.

The effect of the [F]-PEG group on tracer qualification of [4-(phenylamino)-quinazoline-6-YL]-amide moiety—An EGFR putative irreversible inhibitor

Previous reports have designated the labeled derivatives of [4-(phenylamino)-quinazoline-6-yl]-amide group as the most promising EGFR-PET imaging agent candidates. To further improve tracer qualifications and increase stability and solubility, additional derivatives of this group substituted at the 7-position with various lengths of fluoro-polyethyleneglycol (F-PEG) chains were synthesized. These novel derivatives inhibited EGFR autophosphorylation with IC(50) values of 5-40 nM. The compounds were successfully labeled with fluorine-18 at the PEG chain via a three-step radiosynthesis route. The labeled final products were obtained with a 13-32% decay corrected radiochemical yield, 99% radiochemical purity, and high specific activity.

Synthesis and biodistribution of fluorine-18-labeled fluorocyclofenils for imaging the estrogen receptor

C4-[18F]Fluorocyclofenil ([18F]FCF, 6) and C3-[18F]fluoroethylcyclofenil ([18F]FECF, 9), two high-affinity nonsteroidal estrogens, were prepared and investigated as potential agents for imaging estrogen receptors (ERs) in breast tumors. Both of these compounds could be prepared conveniently from alkyl methanesulfonate precursors (5,8) by fluoride displacement reactions, and they were obtained in high radiochemical purity and radiochemical yields, with effective specific activities sufficient for in vivo biodistribution studies. While the biodistribution of [18F]FCF (6) in immature female rats showed no selective target tissue uptake, the biodistribution of [18F]FECF (9) showed selective uptake by the uterus, but this uptake could not be blocked by excess estradiol. The poor in vivo biodistribution of these otherwise high-affinity ligands arouses curiosity, and together with recent results on the biodistribution of other nonsteroidal ligands suggests that factors other than receptor binding affinity are important for in vivo imaging of estrogen target tissues and ER-positive breast tumors.

In vivo imaging of estrogen receptor concentration in the endometrium and myometrium using FES PET — influence of menstrual cycle and endogenous estrogen level

PURPOSE

The goals of this study were to measure estrogen receptor (ER) concentration in the endometrium and myometrium using 16alpha-[(18)F]fluoro-17beta-estradiol (FES) positron emission tomography (PET) and to investigate the relationship between changes in these parameters with the menstrual cycle and endogenous estrogen levels.

METHODS

Sixteen female healthy volunteers were included in this study. After blood sampling to measure endogenous estrogen level, FES PET image was acquired 60 min postinjection of FES. After whole-body imaging of FES PET, averaged standardized uptake values (SUVs) in the endometrium and myometrium were measured, and the relationship between FES uptake and menstrual cycle or endogenous estrogen level was evaluated.

RESULTS

Endometrial SUV was significantly higher in the proliferative phase than in the secretory phase (6.03+/-1.05 vs. 3.97+/-1.29, P=.022). In contrast, there was no significant difference in myometrial SUV when the proliferative and secretory phases were compared (P=.23). Further, there was no correlation between SUV and endogenous estrogen level in the proliferative phase.

CONCLUSIONS

The change of ER concentration relative to menstrual cycle as characterized by FES PET was consistent with those from previous reports that used an immunohistochemical technique. These data suggest that FES PET is a feasible, noninvasive method for characterizing changes in ER concentration.

Microwaving in F-18 chemistry: quirks and tweaks

Since the late 1980s, microwave dielectric heating has been used to speed up chemical transformations, also in radiolabeling tracers for positron emission tomography. In addition to shorter reaction times, higher yields, cleaner product mixtures and improved reproducibility have also been obtained for reactions involving polar components that require heating at elevated temperatures. The conditions used in microwave chemistry can differ considerably from those in conventional heating. Understanding the factors that influence the interaction of the electromagnetic field with the sample is critical for the successful implementation of microwave heating. These parameters are discussed here and exemplified with radiolabelings with fluorine-18.

Synthesis, radiolabeling, and in vivo evaluation of an 18F-labeled isatin analog for imaging caspase-3 activation in apoptosis

A non-peptide-based isatin sulfonamide analog, WC-II-89, was synthesized and its inhibition toward recombinant human caspase-3 and other caspases was determined. This compound showed high potency for inhibiting caspase-3 and -7, and high selectivity against caspases-1, -6, and -8. [(18)F]WC-II-89 was synthesized via a nucleophilic substitution of the corresponding mesylate precursor in high yield and radiochemical purity. Biodistribution studies using [(18)F]WC-II-89 revealed higher uptake in liver and spleen of cycloheximide-treated rats, an animal model of apoptosis, relative to control animals. Western blot analysis confirmed the presence of activated caspase-3 in the liver and spleen of cycloheximide-treated animals. MicroPET imaging studies revealed a high uptake of the radiotracer in the liver of a cycloheximide-treated rat relative to the untreated control. These data suggest that [(18)F]WC-II-89 is a potential radiotracer for imaging caspase-3 activation in tissues undergoing apoptosis.

Fluorine-substituted cyclofenil derivatives as estrogen receptor ligands: synthesis and structure-affinity relationship study of potential positron emission tomography agents for imaging estrogen receptors in breast cancer

In a search for estrogen receptor (ER) ligands to be radiolabeled with fluorine-18 for imaging of ER-positive breast tumors with positron emission tomography (PET), we investigated cyclofenil analogues substituted at the C3 or C4 position of the cyclohexyl group. McMurry coupling of 4,4'-dihydroxybenzophenone with various ketones produced key cyclofenil intermediates, from which C3 and C4 substituents containing alkyl and various oxygen or fluorine-substituted alkyl groups were elaborated. Binding assays to both ERalpha and ERbeta revealed that the C3 site is more tolerant of steric bulk and polar groups than the C4 site, consistent with a computational model of the ERalpha ligand binding pocket. Fluorine substitution is tolerated very well at some sites, giving some compounds having affinities comparable to or higher than that of estradiol. These fluoro and fluoroalkyl cyclofenils merit further consideration as fluorine-18 labeled ER ligands for PET imaging of ERs in breast tumors.

Progestin receptor expression in the developing rat brain depends upon activation of estrogen receptor alpha and not estrogen receptor beta

Perinatal 17beta-estradiol (E2) rapidly and markedly affects the morphological and neurochemical organization of the vertebrate brain. For instance, the sex difference in perinatal progestin receptor (PR) immunoreactivity in the medial preoptic nucleus (MPN) of the rat brain is due to the intracellular conversion of testosterone into E2 in males. Neonatal alpha-fetoprotein prevents circulating estrogens from accessing the brain, therefore, to overcome alpha-fetoprotein sequestration of E2, estrogen replacement studies during development have used natural and synthetic estrogen dosages in the milligram to microgram range. These levels could be considered as supraphysiological. Moreover, it is not clear through which ER subtype E2 acts to induce PR expression in the neonatal rat MPN because E2 binds similarly to estrogen receptor (ER)alpha and ERbeta. Consequently, we investigated whether nanogram levels of E2 affected PR protein and mRNA levels in the neonatal MPN. Furthermore, propylpyrazole-triol (PPT), a highly selective agonist for ERalpha, and diarylpropionitrile (DPN), a highly selective agonist for ERbeta, were used to determine if E2-dependent PR expression in the neonatal rat is mediated through ERalpha and/or ERbeta. Immunocytochemistry and quantitative real-time RT-PCR determined that as little as 100 ng E2 significantly induced PR protein and mRNA in the female and neonatally castrated male MPN on PN 4, indicating that the neonatal rat brain is highly sensitive to circulating estrogens. PPT, but not DPN, induced PR expression in the neonatal MPN and arcuate nucleus (Arc), demonstrating that PR expression in the neonatal rat brain depends solely on E2 activated ERalpha. In the lateral bed nucleus of the stria terminalis (BSTL), neither PPT nor DPN affected PR expression, suggesting the presence of a gonadal hormone-independent PR regulatory mechanism.