Synthesis, toxicity and antitumor activity of cobalt carbonyl complexes targeting hepatocellular carcinoma

Based on our previous research, a series of targeting hepatocellular carcinoma complexes, [R-Glycyrrhetinic acid-CH₂C₂H-[Co₂(CO)₆] (R = H, 1; R = NSAIDs-COOH, 2-4; R = Aromatic acid, 5-7; R = Amino acid, 8-10), were synthesized. The test showed they are slow CO releasers. Using HeLa, A549, HT-29, SMMC7721 and HepG2 cells as models, their activities against tumor cell proliferation were firstly evaluated. The resulting data show all the complexes displayed a good anti-proliferation activity against the HepG2 and SMMC-7721 liver cancer cells, and their IC₅₀ values were in the range of 10.07-66.06 µM; compared with cis-platin (DDP), their activities were comparable or even better under the same condition. Among them, complexes 3, 4, 6 and 9 exhibited higher anti-proliferation activities against HepG2 and SMMC-7721 cell lines than the other cell lines. To confirm further these complexes have selectivity to the liver cells, the uptakes of complexes 3, 4, 6 and 9 by HepG2, HT-29, A549 and SMMC7721 cell lines were studied. The results show the cell uptake rates of the complexes by HepG2 cells and SMMC7721 cells were much greater than by other cells under the same condition. In following tests, the tested complexes displayed higher activities in inhibiting NF-kB, COX-2 and iNOS; and they induced HepG2 cells apoptosis by mitochondrial pathway, which assessed by staining with different fluorescent reagent DAPI, PI, Mito-Tracker Green and DCFH-DA. Meanwhile, the tested complexes up-regulated the expression levels of caspase-3 and Bax, down-regulated the Bcl-2 expression. In addition, they had no effect on zebrafish embryo survival, embryo hatching, embryonic movement, zebrafish malformation and zebrafish movement at below 0.5 µM. This suggests the complexes are potential candidates to be used in clinic for liver cancers.

Syntheses and anti-cancer activity of CO-releasing molecules with targeting galactose receptors

CO-releasing molecules (CORMs) containing cobalt have many bioactivities, but most of them do not dissolve in water and have no selectivity to tissue and organs. On the basis of the specific recognition of galactose or sialic acid by a receptor, a series of CORMs based on carbohydrates were synthesized and evaluated. The test results show that all the complexes displayed anticancer activity. Among them, the effects of the complexes of galactose (1), GalNAc (8) and sialic acid (10) were very distinct. Complex 1 displayed higher activity against HeLa, HePG2, MCF-7 and HT-29 cell proliferation than cis-platin (DDP), and its selectivity was far much better than DDP compared with normal cell W138. Furthermore, the uptakes of complexes 1, 8 and 10 by HePG2, HT-29, A549 and RAW264.7 cell lines were studied. The uptake ratio of each cell line for complex 1 was different, and the order of uptake ratio in the four cell lines was HePG2 > HT-29 > RAW264.7 > A549. The HePG2 cells absorbed complex 1 beyond 60% after incubation for 8 h, while A549 absorbed only 27.8%. For complex 8, the uptake trend was similar to that of complex 1 with it being absorbed by all the four cancer cells, but the uptake rate was lower. However, differently, complex 10 was absorbed heavily by macrophage RAW264.7, followed by HePG2; after 8 h incubation, the uptake ratio of RAW264.7 was over 50%. In addition, the mechanism of action was explored, and the results showed that the complexes inhibited cell cycle arrest at the G2/M phase; complex 1 up-regulated the expression levels of caspase-3 and Bax, and down-regulated the Bcl-2 expression, giving rise to HePG2 cell apoptosis.

Toxicity, bio-distribution and metabolism of CO-releasing molecules based on cobalt

CO-releasing molecules (CORMs) containing [Co2(CO)6] moiety show many bioactivities, such as anti-inflammatory and antitumor cell proliferation. However, so far, no one knows their properties in vivo. So, here, we evaluated some these kind CORMs from drug-like properties including cytotoxicity, toxicity in vivo, distribution and metabolism. The results show all the tested complexes displayed antiproliferative activity to HeLa cell and HepG2 cell lines, and their IC50 values were 36-110µM against HeLa cells and 39-140µM against HepG2 cells. Toxicity tests of mice, we used oral acute toxic class method and got their LD50 values; among them, LD50 of complex 1 and complex 4 were in 2500-5000mgkg(-1) and complex 7 over 5000mgkg(-1). The developmental toxicities of the complexes were investigated in embryonic zebrafish. The mortality, hatch rate, malformation, heart rate, spontaneous movement, and larval behavior were examined, and we found both complexes 4 and 7 have not toxicity at low concentration (<1.0μM) but have higher toxicity at high concentration (>5.0μM). After several consecutive i.p administrations, tested complexes severely damaged rat liver and kidney in both functional and morphological aspects. Through metal ion measurement using ICP-AES, we found the tested complexes were unevenly distributed in tissues and organs; complex 4 has a big prone to collect in liver, whereas complex 7 easily enters to kidney. After administration 480min later, most of complex 7 excreted from kidney and entered urine, while complex 4 needed 9h at least. This results show cobalt did not accumulate, and could excrete with the urine. In vivo, Co(0) in complexes was oxidised to Co(II). In addition, the substituents significantly affected the rate of CO-release, cytotoxicity and their bio-distribution. In the view of these aspects, the CORMs based cobalt has a potential property to be a medicine.

Synthesis, characterization and binding affinities of rhenium(I) thiosemicarbazone complexes for the estrogen receptor (α/β)

The binding affinities towards estrogen receptors (ERs) α and β of a set of thiosemicarbazone ligands (HL(n)) and their rhenium(I) carbonyl complexes [ReX(HL(n))(CO)3] (X=Cl, Br) were determined by a competitive standard radiometric assay with [(3)H]-estradiol. The ability of the coordinated thiosemicarbazone ligands to undergo deprotonation and the lability of the ReX bond were used as a synthetic strategy to obtain [Re(hpy)(L(n))(CO)3] (hpy=3- or 4-hydroxypyridine). The inclusion of the additional hpy ligand endows the new thiosemicarbazonate complexes with an improved affinity towards the estrogen receptors and, consequently, the values of the inhibition constant (Ki) could be determined for some of them. In general, the values of Ki for both ER subtypes suggest an appreciable selectivity towards ERα.

7α- and 17α-Substituted estrogens containing tridentate tricarbonyl rhenium/Technetium complexes: synthesis of estrogen receptor imaging agents and evaluation using microPET with technetium-94m

To develop technetium and rhenium-labeled imaging agents for estrogen receptor (ER) positive breast tumors, we have prepared tridentate metal tricarbonyl chelates substituted at the 7alpha- and 17alpha-positions of estradiol. Some of the Re(CO)(3) conjugates have high binding for the ER in vitro. The in vivo biodistribution of the highest affinity of these novel metal tricarbonyl conjugates, prepared as the (94m)Tc labeled analogue, was evaluated by tissue dissection and microPET imaging. Although target tissue-selective uptake was not apparent, it is notable that microPET imaging identified the stomach as a major site of activity deposition, a site that might have been missed by standard tissue distribution studies.

Aryl cyclopentadienyl tricarbonyl rhenium complexes: novel ligands for the estrogen receptor with potential use as estrogen radiopharmaceuticals

The need for imaging agents for estrogen receptor positive (ER+) tumors that are both cost effective and widely available, as well as the need for novel radiotherapeutic agents for the treatment of breast cancer, has prompted us to investigate cyclopentadienyl tricarbonyl metal [CpMet(CO)(3), Met=Re, Tc-99m] complexes that bind well to the ER. Thus, we have prepared a series of p-hydroxyphenyl-substituted CpRe(CO)(3) complexes and evaluated them (and, in some cases, their cyclopentadiene precursors) for binding to ER. These compounds constitute a new class of structurally integrated organometallic ligands for ER in which the CpMet(CO)(3 )organometallic unit forms the very structural core of these molecules and thus is necessarily intimately involved in their interaction with the receptor. The CpRe(CO)(3) compounds were prepared by reaction of the lithium salt of the arene-substituted cyclopentadiene with a suitable Re(CO)(3)(+) precursor, followed by deprotection of the methyl ether. The X-ray crystal structure of one of these analogues shows that it has the classical 'piano stool'-like geometry, with the alkyl groups directed upward, away from the tripodyl metal carbonyl base. The aryl-substituted CpRe(CO)(3) complexes that we have prepared all bind to the ER, some with affinity as great as 20% that of the native ligand, estradiol. In general, at least two p-hydroxyphenyl substituents and one to two alkyl groups attached to the organometallic cyclopentadienyl core are needed for high ER affinity. Where we have been able to make comparisons, the metal complexes bind to ER with an affinity greater than their cyclopentadiene precursors. The high affinity of some of these complexes indicates that the bulky Re(CO)(3) unit is able to exploit the considerable volume in the center of the ER ligand binding pocket that is not occupied by most ligands, a consideration that is supported by molecular modeling. The preparation of the best of these agents in technetium-99m labeled form is currently being investigated.

Radiochemical synthesis and tissue distribution of Tc-99m-labeled 7alpha-substituted estradiol complexes

The diagnosis and staging of breast cancer could be improved by the development of radiopharmaceutical imaging agents that provide a noninvasive determination of the estrogen receptor (ER) status of tumor cells. Agents labeled with (99m)Tc would be especially valuable in this regard. In attempting to achieve this goal, we synthesized four (99m)Tc-labeled 7alpha-substituted estradiol complexes. One complex utilizes the "3+1" mixed ligand design to introduce the Tc metal, whereas the other three took advantage of the cyclopentadienyltricarbonylmetal (CpTM) design. The Tc moieties were attached to the 7alpha position of estradiol with a hexyl tether, a monoether tether, or a polyether tether. The corresponding rhenium compounds have binding affinities for the ER of 20-45% compared with estradiol. Radiochemical yields of the (99m)Tc-labeled compounds ranged from approximately 15% for the CpT-Tc complexes to 95% for the 3 + 1 inorganic complex. Tissue distribution studies in immature female rats showed low nonreceptor-mediated uptake in the target organs and high uptake in nontarget organs such as the liver and fat. These complexes represent the first time that estradiol has been labeled at the 7alpha position with (99m)Tc and provide a further refinement of our understanding of ligand structure-binding affinity correlations for the ER.

Synthesis and Binding Affinities of Novel Re-Containing 7alpha-Substituted Estradiol Complexes: Models for Breast Cancer Imaging Agents

The diagnosis and staging of breast cancer could be improved by the development of imaging radiopharmaceuticals that provide a noninvasive determination of the estrogen receptor status in the tumor cells. Toward this goal, we have synthesized a number of novel Re-containing 7alpha-substituted estradiol complexes. The introduction of the 7alpha side chain involves the alkylation of tetrahydropyranyloxy-protected 6-keto estradiol. The methods used to introduce the rhenium metal involve "3 + 1" and "4 + 1" mixed ligand complexes (2a-c and 5, respectively), tricarbonyl dithioether complexes (3), and the cyclopentadienyltricarbonylmetal organometallic system (4ab, 6, 7). These complexes showed binding affinities for the estrogen receptor (as high as 45% for the "3 + 1" complex 2c) when compared to the native ligand estradiol. The polarity of some complexes (4ab) was modified to improve biodistribution properties by introducing (poly)ether linkages into the 7alpha side chain (6, 7). These complexes provide a further refinement of our understanding of ligand structure-binding affinity correlations for the estrogen receptor, and they furnish the synthetic groundwork for the synthesis of the analogous Tc-99m complexes for evaluation as breast tumor imaging agents.

Synthesis and binding affinities of new 17 alpha-substituted estradiol-rhenium "n + 1" mixed-ligand and thioether-carbonyl complexes

The development of technetium and rhenium-based radiotracers for the steroid receptor system requires the use of suitable donor groups on the steroid to provide stable binding sites for the metal. Previous approaches have mainly exploited methods involving various N- and S-coordinating chelate systems or organometallic complexes. In this work, we have prepared several novel chelate systems attached to a series of 17 alpha-substituted estradiol derivatives and examined their binding to the estrogen receptor (ER). The neutral "n + 1" mixed-ligand and dithioether-carbonyl complexes that we prepared contain the metal in three oxidation states, +5, +3 or +1, attached to a 17 alpha-substituted estradiol derivative through a thiol group, an isocyanide group, or a dithioether unit, respectively. In our preliminary investigations, we used rhenium as a nonradioactive analog of the radionuclide technetium. All complexes synthesized were evaluated in a competitive radiometric receptor binding assay at 0 degree C and 25 degrees C to determine their relative binding affinities (RBA) to the ER (relative to 3,17 beta-estradiol, RBA = 100%). The complexes show binding affinities up to 23.4% at 0 degree C and 14.1% at 25 degrees C.

Technetium-99m-labeled receptor-specific small-molecule radiopharmaceuticals: recent developments and encouraging results

The development of technetium-99m-labeled small-molecule radiopharmaceuticals directed at specific high-affinity binding sites, as are found in receptors for hormones and neurotransmitters, transport systems, and certain enzymes, is a natural outgrowth from the successful development of technetium radiopharmaceuticals for imaging flow and metabolism. Although many receptor-specific radiopharmaceuticals labeled with PET and other SPECT isotopes already exist, the low cost and widespread availability of technetium-99m would make their 99mTc-labeled counterparts much more accessible to the medical community. This review has four goals: (a) To survey and analyze critically the results of a flurry of research activity in this area in recent years, which has led to the preparation of a number of novel technetium-labeled radiopharmaceuticals targeted at high-affinity sites, a few of which appear to be very promising; (b) to provide a conceptual analysis of how these agents are being designed; (c) to provide a context in terms of binding and uptake behavior by which these agents should be judged; and (d) to highlight emerging knowledge on the structure of receptors and related high-affinity binding biomolecules and their distribution, which may serve as reference points for understanding the results that have been obtained so far, and may be useful guides for future design.

Improved synthesis of a protected 11-oxoestrone

An improved synthesis of 11-oxoestrone-3-acetate-17-ethyleneketal is reported. Adjustments are proposed for the oxidation of estrone by 2,3-dichloro-5,6-dicyano-1,4-benzoquinone into 9(11)-dehydroestrone. A complete hydroboration-oxidation of the resulting ketal, by means of borane-methylsulfide complex, gives the corresponding 11-hydroxy derivative. This latter compound is then acetylated for successful oxidation with pyridinium chlorochromate on alumina. The overall yield is 30%.

Radiolabeling with technetium-99m to study high-capacity and low-capacity biochemical systems

After a brief review of the history of the development of technetium-99m-labeled radiopharmaceuticals, the use of technetium chelates in high-capacity systems is discussed. The latter are used in the study of five organ systems, the kidneys, liver, bone, brain, and heart. The chemical characterization of 99mTc complexes is also reviewed, followed by discussion of the various approaches to the labeling of proteins with direct labeling, the preformed chelate approach, and the antibody chelator conjugate approach. Thereafter, the labeling of biochemicals with 99mTc for use with easily saturated sites, e.g., receptors and enzymes, is considered. Finally, attention is given to factors that affect the preparation of high specific activity, high affinity 99mTc-labeled biochemicals.