Estrogen platinum-diamine complexes: preparation of a non-steroidal estrogen platinum-diamine complex labeled with platinum-191 and a study of its binding to the estrogen receptor in vitro and its tissue distribution in vivo

Design, synthesis, cytocidal activity and estrogen receptor α affinity of doxorubicin conjugates at 16α-position of estrogen for site-specific treatment of estrogen receptor positive breast cancer

Doxorubicin (DOX) is an important medicine for the treatment of breast cancer, which is the most frequently diagnosed and the most lethal cancer in women worldwide. However, the clinical use of DOX is impeded by serious toxic effects such as cardiomyopathy and congestive heart failure. Covalently linking DOX to estrogen to selectively deliver the drug to estrogen receptor-positive (ER(+)) cancer tissues is one of the strategies under investigation for improving the efficacy and decreasing the cardiac toxicity of DOX. However, conjugation of drug performed until now was at 3- or 17-position of estrogen, which is not ideal since the hydroxyl groups at this position are important for receptor binding affinity. In this study, we designed, prepared and evaluated in vitro the first estrogen-doxorubicin conjugates at 16α-position of estradiol termed E-DOXs (8a-d). DOX was conjugated using a 3-9 carbon atoms alkylamide linking arm. E-DOXs were prepared from estrone using a seven-step procedure to afford the desired conjugates in low to moderate yields. The antiproliferative activities of the E-DOX 8a conjugate through a 3-carbon spacer chain on ER(+) MCF7 and HT-29 are in the micromolar range while inactive on M21 and the ER(-) MDA-MB-231 cells (>50 μM). Compound 8a exhibits a selectivity ratio (ER(+)/ER(-) cell lines) of >3.5. Compounds 8b-8d bearing alkylamide linking arms ranging from 5 to 9 carbon atoms were inactive at the concentrations tested (>50 μM). Interestingly, compounds 8a-8c exhibited affinity for the estrogen receptor α (ERα) in the nanomolar range (72-100 nM) whereas compound 8d exhibited no affinity at concentrations up to 215 nM. These results indicate that a short alkylamide spacer is required to maintain both antiproliferative activity toward ER(+) MCF7 and affinity for the ERα of the E-DOX conjugates. Compound 8a is potentially a promising conjugate to target ER(+) breast cancer and might be useful also for the design of more potent E-DOX conjugates.

Synthesis of 17 alpha-substituted ethynylestradiols: potential ligands for drug vectors

17alpha-substituted ethynylestradiols, derived from estrone, were converted to their corresponding 17 alpha-(bromo- or iodo-propargyl)estrone intermediates. Nucleophilic substitution onto these moieties with malonate diester followed by hydrolysis and complexation with cis-Pt(Me(2)en)I(2) (Me(2)en=N,N-dimethylethylenediamine) gave cis-Pt(Me(2)en)(2-(3-(17beta-estradiol-17 alpha-yl)-prop-2-ynyl)malonato) 7, thus demonstrating that these estrogen-derived compounds can be used to synthesize stable Pt(II) complexes. The 3-(17beta-estradiol-17 alpha-yl)-prop-2-ynyl-1-sulfanylethylthiol 23 was also prepared.

An Estrogen–Platinum Terpyridine Conjugate: DNA and Protein Binding and Cellular Delivery

A platinum metal complex in which terpyridine joins estradiol (via an ethynyl link) to a platinum with a labile ligand (chloride) has been designed, synthesised and its X-ray crystal structure determined. The aim of this work was to link a targeting motif (in this case estrogen) to a metal-based biomolecule recognition unit (the platinum moiety). The target molecule: 17alpha-[4'-ethynyl-2,2':6',2'-terpyridine]-17beta-estradiol platinum(II) chloride (PtEEtpy) has been shown to bind to both human and bovine serum albumin (SA) and to DNA. FTICR mass spectrometry shows that the bimolecular units are in each case linked through coordination to the platinum with displacement of the chloride ligand. Circular dichroism indicates that a termolecular entity involving PtEEtpy, SA and DNA is formed. A range of electrospray mass spectrometry experiments showed that the PtEEtpy complex breaks and forms coordination bonds relatively easily. A whole cell estrogen receptor assay in an estrogen receptor positive cell (MCF-7) confirms binding of both EEtpy and PtEEtpy to the estrogen receptor in cells. The work demonstrates the concept of linking a targeting moiety (in this case estrogen) to a DNA binding agent.

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.

Chemical stability, biological activity and cellular uptake of a cisplatin analogue having a 1,2-diarylethyleneamine ligand in cultures of human breast cancer cells

The platinum(II) complex PtCl2(meso-6), which has the estrogenic ligand meso-1,2-bis(2,6-dichloro-4- hydroxyphenyl)ethylenediamine (meso-6), has been reported to be an effective antitumor drug for estrogen-receptor(ER)-positive tumors in animal experiments. The goal of this study was to investigate whether the observed biological effects could be ascribed to the intact PtCl2(meso-6). Cultures of the ER-positive human breast cancer cell line MCF-7 were used as the in vitro test system. In culture medium containing 10% fetal calf serum, PtCl2(meso-6) had a half-life of about 2 h, as determined by HPLC analysis, and no PtCl2(meso-6) was detectable after 10 h. The Pt complex bound irreversibly to serum protein. After 30 min, the diamine ligand was found released, with a maximum conversion of about 35% at 24 h. At this time the culture medium still had estrogenic activity, i.e. it induced ER processing in the MCF-7 cells. This indicates that the estrogenic effect was elicited by the released diamine ligand. In contrast, the growth-inhibitory activity of the medium preincubated with PtCl2(meso-6) was lost at a rate similar to the rate of loss of PtCl2(meso-6) from the medium. This accords with the platinum complex being the main cytotoxic entity. When MCF-7 cells were incubated with PtCl2([3H]meso-6), no free Pt complex could be identified in cellular extracts, and most of the cell-associated radioactivity coeluted with meso-6 in HPLC analysis. After 12 h, only 1.4% of the total cellular platinum was bound to DNA, but no tritium label could be detected. In conclusion, diamine ligand is released from the Pt(II) complex and can account for the estrogenic effects so far ascribed to PtCl2(meso-6).

Titration of the in vivo uptake of 16 alpha-[18F]fluoroestradiol by target tissues in the rat: competition by tamoxifen, and implications for quantitating estrogen receptors in vivo and the use of animal models in receptor-binding radiopharmaceutical development

We have measured in vivo the uptake of 16 alpha-[18F]estradiol (FES) by target tissues in the immature rat at increasing dose levels (obtained by dilution of [18F]FES with unlabeled estradiol). This was done to examine the binding capacity of target tissues in vivo and to determine whether the uptake in receptor-rich tissues was flow limited, as this has implications concerning the appropriateness of using receptor-rich tissues in experimental animals as models for FES uptake by receptor-poor breast tumors in humans. We also wanted to establish the dose level of the anti-estrogen tamoxifen required to block target tissue uptake of FES. We found that in untreated rats, specific uptake in the uterus saturated at c. 180 pmol/g, in the ovary at c. 54 pmol/g and in the muscle at c. 2 pmol/g. At an intermediate dose of tamoxifen (570 micrograms/kg), uptake saturated at somewhat lower levels, and at a high tamoxifen dose (1710 micrograms/kg), yet lower specific uptake was evident. In the FES titrations at low dose levels of FES, both the uterus and the ovaries, but not the muscle, showed characteristics of flow-limited uptake, i.e. the uptake-to-dose ratio reached a maximum level. This flow limitation suggests that only when receptor levels are sufficiently low will the FES uptake be related to receptor concentration. While receptor-rich tissues such as the rat uterus will show this flow limitation, the receptor concentration in most primary and metastatic human breast tumors is sufficiently low, so that the uptake should parallel receptor content. In in vivo distribution studies, target tissues (or tumors) with low receptor content will be more fully saturated and ligand more readily displaced. Also, uptake by secondary target tissues (i.e. those with a lower content of estrogen receptor, such as muscle, thymus and kidney) may be better models for assessing the effectiveness of new breast tumor imaging agents than uptake by receptor-rich tissues.