Synthesis, receptor binding and biodistribution of the gem-21-chloro-21-iodovinylestradiol derivatives

Correlating molecular character of NIR imaging agents with tissue-specific uptake

Near-infrared (NIR) fluorescent contrast agents are emerging in optical imaging as sensitive, cost-effective, and nonharmful alternatives to current agents that emit harmful ionizing radiation. Developing spectrally distinct NIR fluorophores to visualize sensitive vital tissues to selectively avoid them during surgical resection of diseased tissue is of great significance. Herein, we report the synthetic variation of pentamethine cyanine fluorophores with modifications of physicochemical properties toward prompting tissue-specific uptake into sensitive tissues (i.e., endocrine glands). Tissue-specific targeting and biodistribution studies revealed localization of contrast agents in the adrenal and pituitary glands, pancreas, and lymph nodes with dependence on molecular characteristics. Incorporation of hydrophobic heterocyclic rings, alkyl groups, and halogens allowed a fine-tuning capability to the hydrophobic character and dipole moment for observing perturbation in biological activity in response to minor structural alterations. These NIR contrast agents have potential for clinical translation for intraoperative imaging in the delineation of delicate glands.

(17alpha,20E/Z)-iodovinyl- and 16alpha-iodP618-homoestradiol derivatives: synthesis and evaluation for estrogen receptor imaging

Three new 125I-radioiodinated estrogens featuring a 13beta-ethyl instead of the natural 13beta-methyl group, i.e. 18-homoestradiols, were synthesized and evaluated as potential estrogen receptor imaging agents. The 16alpha-iodo-18-methylestradiol and the 125I-labeled analog were synthesized from the corresponding 16beta-bromo analog by the halogen-exchange method. The cis-bromohydrin precursor was obtained by bromination of an estrone enolacetate, followed by epimerization and reduction. The isomeric (17alpha,20E/Z)-iodovinyl-18-methylestradiols were prepared via the vinyltin intermediates. Treatment of 18-methyl-17alpha-ethynylestradiol with tri-n-butyltin hydride, in the presence of azobisisobutyronitrile as catalyst and heating at 90-100 degrees C afforded the (17alpha,20E)-tri-n-butylstannyl isomer as the major product. Changing the catalyst for triethyl borane, at room temperature, mainly gave the 20Z-isomer. The nca 125I-labeled analogs were obtained from their corresponding tin intermediates upon treatment with [125I]NaI in the presence of H2O2. The 16alpha-[125I]iodo- and isomeric (17alpha,20E/Z)-[125I]iodovinyl-18-methylestradiols were evaluated for estrogen receptor-mediated uterine uptake in immature female rats. Homologation of the C13-methyl group did improve the uterine uptake of the iodovinyl derivatives, but also increased blood retention, resulting in lower target uptake ratios. In the case of the 16alpha-iodo analog uterine retention decreased upon C13-homologation.

The estradiol pharmacophore: ligand structure-estrogen receptor binding affinity relationships and a model for the receptor binding site

The accumulated knowledge on the binding of estradiol (E2) and its analogs and the results of affinity-labeling studies have been reviewed and are used herein to derive a binding site model for the estrogen receptor (ER). Estradiol is nonpolar and hydrophobic, except at its molecular termini. Most of its skeletal flexibility resides in the B-ring, and it probably binds in a low-energy conformation. The phenolic OH group in the A-ring contributes about 1.9 kcal/mol to the binding free energy and probably acts primarily as a hydrogen bond donor. The 17 beta-hydroxyl group in the D-ring contributes approximately 0.6 kcal/mol to the binding and probably acts as a hydrogen bond acceptor, either directly or via a water molecule. There also seems to be a degree of flexibility in the region of the receptor that encompasses the D-ring. The aromatic ring contributes about 1.5 kcal/mol, probably through weak polar interactions with receptor residues that contact the beta-face of the steroid. The receptor seems to surround the ligand, so that all four rings contribute significantly to binding. Small hydrophobic substituents enhance binding affinity at positions 4, 12 beta, 14, and 16 alpha; whereas, larger hydrophobic substituents are tolerated at positions 7 alpha, 11 beta, and 17 alpha. In general, the ER is intolerant of polar substituents. Based on E2 analogs bearing affinity-labeling groups, cysteine residues might be present in the binding site in the area of C-4, C-17 alpha, and C-17 beta, and a lysine residue might be located near C-16. Models that represent the limits of deformability of the ligand binding site, the position of preformed pockets, and space occupied by the receptor are presented. The various elements in this model for the binding of steroidal estrogens by the estrogen receptor are consistent with evidence emerging from the crystal structures of related nuclear hormone receptor ligand complexes.

Stereochemical probes for the estrogen receptor: synthesis and receptor binding of (17 alpha,20E/Z)-21-phenyl-19-norpregna-1,3,5(10), 20-tetraene-3,17 beta-diols

Previous studies from our laboratory using 17 alpha-E- and 17 alpha-Z-halovinyl and phenylthiovinyl estradiols demonstrated a marked preference for the Z stereochemistry and a significant steric tolerance for the Z-vinyl substituent. To further explore the extent of that stereochemical preference and steric tolerance we have prepared stereoselectively the 17 alpha-E- and 17 alpha-Z-phenylvinyl estradiols (E- and Z-styrylestradiols). The results, in addition to demonstrating a facile preparation of the target compounds, supported the previously observed stereochemical and steric effects. The relative binding affinities for the Z isomer were 3-4 fold greater than the E isomer at both 4 degrees C and 25 degrees C, and only one-half to one-fourth those of estradiol under similar conditions. The developing model for ligand-accessible space within the estrogen receptor suggests that Z-phenylvinyl estradiols may provide interesting and useful probes for mapping the receptor.