A solid support-based synthetic strategy for the site-selective functionalization of peptides with organometallic half-sandwich moieties

The number of donor atoms available on peptides that can competitively coordinate to metal centers renders the site-selective generation of advanced metal-peptide conjugates in high purity a challenging venture. Herein, we present a transmetalation-based synthetic approach on solid support in which an imidazolium proligand can be used to selectively anchor a range of transition metal half-sandwich complexes onto peptides in the presence of multiple coordinative motifs. Amenable to solid support, a range of N-terminus and/or lysine conjugated metal-peptide conjugates were obtained in high purity after cleavage from the resin. The metalated peptides were evaluated for their anticancer properties against human cancer cell lines. While no cytotoxic activity was observed, this platform has the potential to i) provide a pathway to site-selective peptide labelling, ii) be explored as a biorthogonal handle and/or iii) generate a new strategy for ligand design in transition metal catalysts.

On the interaction of N-heterocyclic carbene Ir+I complexes with His and Cys containing peptides

In the interaction of an [Ir(+i)(COD)(NHC)Cl] complex with model peptides a chelating motif with a particularly interesting bimetallic peptide-bridged Ir(+iii)–NHC motif was identified with loss of the COD and Cl ligands and oxidation of the metal.

A Gold(III) Pincer Ligand Scaffold for the Synthesis of Binuclear and Bioconjugated Complexes: Synthesis and Anticancer Potential

Cyclometalated (C^N^C)Au^III complexes bearing functionalized N-heterocyclic carbene (NHC) ligands provide a high-yielding, modular route to bioconjugated and binuclear complexes. This methodology has been applied to the synthesis of bioconjugated complexes presenting biotin and 17α-ethynylestradiol vectors, as well as to the synthesis of bimetallic Au^III /Au^I complexes. The in vitro antiproliferative activities of these compounds against various cancer cells lines depend on the linker length, with the longer linker being the most potent. The estradiol conjugate AuC₆ Estra proved to be more toxic against the estrogen receptor positive (ER+) cancer cells than against the ER- cancer cells and non-cancer cells. The bimetallic complex AuC₆ Au was more selective for breast cancer cells with respect to a healthy cell standard than the monometallic complex AuNHC. The metal uptake study on cells expressing or not biotin and estrogen receptors revealed an improved and targeted delivery of gold for both the bioconjugated complexes AuC₆ Biot and AuC₆ Estra compared to the non-vectorised analogue AuNHC. The investigations of the interaction of the bioconjugates and bimetallic complexes with human telomeric G-quadruplex DNA using FRET-melting techniques revealed a reduced ability to stabilize this DNA structure with respect to the non-vectorised analogue AuNHC.

(C^Npz^C)AuIII complexes of acyclic carbene ligands: synthesis and anticancer properties

Synthesis of amino ester conjugated (C^N^pz^C)Au acyclic carbene complexes with low micromolar cytotoxicity on human cancer cells.

N-Heterocyclic Carbene-Gold(I) Complexes Conjugated to a Leukemia-Specific DNA Aptamer for Targeted Drug Delivery

This report describes the synthesis and characterization of novel N-heterocyclic carbene (NHC)-gold(I) complexes and their bioconjugation to the CCRF-CEM-leukemia-specific aptamer sgc8c. Successful bioconjugation was confirmed by the use of fluorescent tags on both the NHC-Au(I) complex and the aptamer. Cell-viability assays indicated that the NHC-Au(I) -aptamer conjugate was more cytotoxic than the NHC-gold complex alone. A combination of flow cytometry, confocal microscopy, and cell-viability assays provided clear evidence that the NHC-Au(I) -aptamer conjugate was selective for targeted CCRF-CEM leukemia cells.

Synthesis and Applications of (ONO Pincer)Ruthenium-Complex-Bound Norvalines

Two (ONO pincer)ruthenium-complex-bound norvalines, Boc-[Ru(pydc)(terpy)]Nva-OMe (1; Boc=tert-butyloxycarbonyl, terpy=terpyridyl, Nva=norvaline) and Boc-[Ru(pydc)(tBu-terpy)]Nva-OMe (5), were successfully synthesized and their molecular structures and absolute configurations were unequivocally determined by single-crystal X-ray diffraction. The robustness of the pincer Ru complexes and norvaline scaffolds against acidic/basic, oxidizing, and high-temperature conditions enabled us to perform selective transformations of the N-Boc and C-OMe termini into various functional groups, such as alkyl amide, alkyl urea, and polyether groups, without the loss of the Ru center or enantiomeric purity. The resulting dialkylated Ru-bound norvaline, n-C11 H23 CO-l-[Ru(pydc)(terpy)]Nva-NH-n-C11 H23 (l-4) was found to have excellent self-assembly properties in organic solvents, thereby affording the corresponding supramolecular gels. Ru-bound norvaline l-1 exhibited a higher catalytic activity for the oxidation of alcohols by H2 O2 than parent complex [Ru(pydc)(terpy)] (11 a).

Facile rhenium-peptide conjugate synthesis using a one-pot derived Re(CO)3 reagent

We have synthesized two Re(CO)3-modified lysine complexes (1 and 2), where the metal is attached to the amino acid at the Nε position, via a one-pot Schiff base formation reaction. These compounds can be used in the solid phase synthesis of peptides, and to date we have produced four conjugate systems incorporating neurotensin, bombesin, leutenizing hormone releasing hormone, and a nuclear localization sequence. We observed uptake into human umbilical vascular endothelial cells as well as differential uptake depending on peptide sequence identity, as characterized by fluorescence and rhenium elemental analysis.

Lipophilicity-dependent ruthenium N-heterocyclic carbene complexes as potential anticancer agents

Five novel Ru(ii)–N-heterocyclic carbenes (NHC) were synthesized and biologically evaluated, one of which showed superior activity against PC-3 cell lines and 6 fold more activity than cisplatin.

Rhenium and technetium tricarbonyl complexes of N-heterocyclic carbene ligands

A strategy for the conjugation of N-heterocyclic carbene (NHC) ligands to biomolecules via amide bond formation is described. Both 1-(2-pyridyl)imidazolium or 1-(2-pyridyl)benzimidazolium salts functionalized with a pendant carboxylic acid group were prepared and coupled to glycine benzyl ester using 1-ethyl-3-(3-(dimethylamino)propyl)carbodiimide. A series of 10 rhenium(I) tricarbonyl complexes of the form [ReX(CO)3(ĈN)] (ĈN is a bidentate NHC ligand, and X is a monodentate anionic ligand: Cl(-), RCO2(-)) were synthesized via a Ag2O transmetalation protocol from the Re(I) precursor compound Re(CO)5Cl. The synthesized azolium salts and Re(I) complexes were characterized by elemental analysis and by (1)H and (13)C NMR spectroscopy, and the molecular structures for one imidazolium salt and seven Re(I) complexes were determined by single-crystal X-ray diffraction. (1)H NMR and mass spectrometry studies for an acetonitrile-d3 solution of [ReCl(CO)3(1-(2-pyridyl)-3-methylimidazolylidene)] show that the monodentate chloride ligand is labile and exchanges with this solvent yielding a cationic acetonitrile adduct. For the first time the labeling of an NHC ligand with technetium-99m is reported. Rapid Tc-99m labeling was achieved by heating the imidazolium salt 1-(2-pyridyl)-3-methylimidazolium iodide and Ag2O in methanol, followed by the addition of fac-[(99m)Tc(OH2)3(CO)3](+). To confirm the structure of the (99m)Tc-labeled complex, the equivalent (99)Tc complex was prepared, and mass spectrometric studies showed that the formed Tc complexes are of the form [(99m/99)Tc(CH3CN)(CO)3(1-(2-pyridyl)-3-methylimidazolylidene)](+) with an acetonitrile molecule coordinated to the metal center.

Gold(I) N-heterocyclic carbene complexes with naphthalimide ligands as combined thioredoxin reductase inhibitors and DNA intercalators

Organometallic conjugates consisting of a gold(I) N-heterocyclic carbene (NHC) moiety and a naphthalimide were prepared and investigated as cytotoxic agents that interact with both DNA and the disulfide reductase enzyme thioredoxin reductase (TrxR). The complexes were potent DNA intercalators related to their naphthalimide partial structure, inhibited TrxR as a consequence of the incorporation of the gold(I) moiety, and triggered efficient cytotoxic effects in MCF-7 breast and HT-29 colon adenocarcinoma cells. Strong effects on tumor cell metabolism were noted for the most cytotoxic complex, chlorido[1-(3'-(4''-ethylthio-1'',8''-naphthalimid-N''-yl))-propyl-3-methyl-imidazol-2-ylidene]gold(I) (4 d). In conclusion, the conjugation of naphthalimides with gold(I) NHC moieties provided a useful strategy for the design of bioorganometallic anticancer agents with multiple modes of action.

Radioiodinated and astatinated NHC rhodium complexes: synthesis

INTRODUCTION

The clinical development of radioimmunotherapy with astatine-211 is limited by the lack of a stable radiolabeling method for antibody fragments. An astatinated N-heterocyclic carbene (NHC) Rhodium complex was assessed for the improvement of radiolabeling methodologies with astatine.

METHODS

Wet harvested astatine-211 in diisopropyl ether was used. Astatine was first reduced with cysteine then was reacted with a chlorinated Rh-NHC precursor to allow the formation of the astatinated analogue. Reaction conditions have been optimized. Astatine and iodine reactivity were also compared. Serum stability of the astatinated complex has been evaluated.

RESULTS

Quantitative formation of astatide was observed when cysteine amounts higher than 46.2 nmol/μl of astatine solution were added. Nucleophilic substitution kinetics showed that high radiolabeling yields were obtained within 15 min at 60°C (88%) or within 5 min at 100°C (95%). Chromatographic characteristics of this new astatinated compound have been correlated with the cold iodinated analog ones. The radioiodinated complex was also synthesized from the same precursor (5 min. at 100°C, up to 85%) using [(125)I]NaI as a radiotracer. In vitro stability of the astatinated complex was controlled after 15 h incubation in human serum at 4°C and 37°C. No degradation was observed, indicating the good chemical and enzymatic stability.

CONCLUSION

The astatinated complex was obtained in good yield and exhibited good chemical and enzymatic stability. These preliminary results demonstrate the interest of this new radiolabeling methodology, and further functionalizations should open new possibilities in astatine chemistry.

ADVANCES IN KNOWLEDGE AND IMPLICATIONS FOR PATIENT CARE

Although there are many steps and pitfalls before clinical use for a new prosthetic group from the family of NHC complexes, this work may open a new path for astatine-211 targeting.

Peptide-tethered monodentate and chelating histidylidene metal complexes: synthesis and application in catalytic hydrosilylation

The Nδ,Nε-dimethylated histidinium salt (His*) was tethered to oligopeptides and metallated to form Ir(III) and Rh(I) NHC complexes. Peptide-based histidylidene complexes containing only alanine, Ala-Ala-His*-[M] and Ala-Ala-Ala-His*-[M] were synthesised ([M] = Rh(cod)Cl, Ir(Cp*)Cl2), as well as oligopeptide complexes featuring a potentially chelating methionine and tyrosine residue, Met-Ala-Ala-His*-Rh(cod)Cl and Tyr-Ala-Ala-His*-Rh(cod)Cl. Chelation of the methionine-containing histidylidene ligand was induced by halide abstraction from the rhodium centre, while tyrosine remained non-coordinating under identical conditions. High catalytic activities in hydrosilylation were achieved with all peptide-based rhodium complexes. The cationic S(Met),C(His*)-bidentate peptide rhodium catalyst outperformed the monodentate neutral peptide complexes and constitutes one of the most efficient rhodium carbene catalysts for hydrosilylation, providing new opportunities for the use of peptides as N-heterocyclic carbene ligands in catalysis.

Evaluation of arene ruthenium(II) N-heterocyclic carbene complexes as organometallics interacting with thiol and selenol containing biomolecules

Metal complexes with N-heterocyclic carbene (NHC) ligands have been widely used in catalytic chemistry and are now increasingly considered for the development of new chemical tools and metal based drugs. Ruthenium complexes of the type (p-cymene)(NHC)RuCl(2) interacted with biologically relevant thiols and selenols, which resulted in the inhibition of enzymes such as thioredoxin reductase or cathepsin B. Pronounced antiproliferative effects could be obtained provided that an appropriate cellular uptake was achieved. Inhibition of tumor cell growth was accompanied by a perturbation of metabolic parameters such as cellular respiration.

Stereospecific synthesis and catalytic activity of L-histidylidene metal complexes

We report on the synthesis, metal coordination, and catalytic impact of histidylidene, a histidine-derived N-heterocyclic carbene (NHC) ligand. The histidinium salt 3, comprising methyl substituents at both heterocyclic nitrogens and protected at the C- and N-terminus of the amino acid, was rhodated and iridated by a transmetallation protocol using Ag(2)O. Ambient temperature and short reaction times were pivotal for full retention of configuration at the α-carbon. The stereospecificity of the reaction was conveniently probed by (31)P NMR spectroscopy after transmetallation with rhodium(I) and coordination of enantiopure (S)-Ph-binepine. The histidylidene rhodium complexes are highly efficient catalysts for the mild hydrosilylation of ketones. For the cationic complexes [Rh(cod)(histidylidene)(phosphine)](+), lowering the temperature shifted the rate-limiting step of the catalytic reaction to an earlier stage that is not enantioselective. Hence the asymmetric induction-which is governed by the chiral phosphine-did not improve at low temperature.

Preparation of mononuclear, homodinuclear, and heterotrinuclear complexes by salicylaldiminato-functionalized imidazolium salt: approach to multifunctional catalysts

A salicylaldiminato imidazolium salt that bears both a Schiff base and imidazolium salt moiety was used to synthesize heterometallic compounds that could serve as multifunctional catalysts in certain reactions. The successful preparation of seven mononuclear compounds with a variety of transition metals (Pd, Ir, Ru, Zn, Ni) illustrated the high versatility of this class of ligands, which is crucial for the design of catalysts. Synthesis of homodinuclear compounds and heterotrinuclear compounds provided practical methods to connect multiple metal fragments through these ligands. The heterotrinuclear complex (Ni/Ir) was employed as a catalyst in the reaction of dehalogenation/transfer hydrogenation of halo-acetophenones. The preliminary catalytic study showed that this heterometallic species is more active than a combination of the corresponding monometallic species.

Synthesis, structural characterisation and anti-proliferative activity of NHC gold amino acid and peptide conjugates

We report the synthesis of new NHC gold(I) and NHC gold(III) halide, amino acid and dipeptide complexes. Transmetallation of the N-phenylalanine-substituted NHC silver complex 3 with Me2SAuCl yields the phenylalanine-NHC gold(I) conjugate 4a. Halide exchange with LiBr and oxidation of 4a with Br2 in CH2Cl2 yields the phenylalanine-NHC Au(I) and Au(III) bromides 4b and 4c, respectively. Reaction of N-Boc protected cysteine methyl ester (Boc-Cys-OMe) or the dipeptide N-Boc-Leu-Cys-OMe with the NHC gold chloride 6a yields the (NHC)Au-S complexed amino acid and dipeptide derivatives 8 and 9. The NHC gold(III) complexes 4c and 6c were characterised by single crystal X-ray analysis. All of the tested gold carbene complexes showed significant anti-tumor activity on the HeLa, HepG2 and HT-29 cancer cell lines. The best compounds show activity comparable to the well-known anti-cancer drug cisplatin. There seems to be no clear cut structure-activity relationship in the compounds tested, nor did we observe a dependence on the metal oxidation state or the different halide substituents. Given the ease of preparation, stability and high activity of the compounds described herein, it may be possible to design tumor-specific anti-cancer agents based on NHC gold amino acid conjugates in the future.

Metal-NHC complexes: a survey of anti-cancer properties

New weapons to fight cancer are constantly needed. Among chemotherapeutics, anti-cancer metal-drugs have enjoyed a long and successful history since the discovery of the benchmark cisplatin. Advances in metal-drug discovery have motivated chemists to build plethora of complex structures. Among them, a novel area is emerging. This article presents a survey of the metal-N-Heterocyclic Carbenes (Ag(I), Au(I), Pd(II) and Cu(I)-NHCs) as potential anti-cancer agents. Most of the metal-NHCs considered display higher cytotoxicities than the reference metallo-drug cisplatin. Some of them are even selective for particular cell lines. Their mechanisms of action at the cellular level are further discussed, showing that the nature of the metal is of great importance. All these promising results demonstrate that this approach deserves more attention and work.

Synthesis and characterization of dicobalthexacarbonyl-alkyne derivatives of amino acids, peptides, and peptide nucleic acid (PNA) monomers

The reaction of Co(2)(CO)(8) with alkyne-containing amino acids [1a: phenylalanine (Phe) and 1b: methionine (Met)], two suitably alkyne-functionalized derivatives of the neuropeptide enkephalin (Enk) [3: Ac-Enk-Prop and 5: Ac-Enk(Pgl)-NH(2) (Ac--Acetyl; Pgl--propargylglycine; Prop--propargylamine)], a thymine Peptide Nucleic Acid (T-PNA) monomer (7), and a PNA-like monomer (9) derivative gave the respective dicobalthexacarbonyl bioconjugates in very good yields. Two different sites for labeling of the biomolecules were successfully used: The organometallic moiety was reacted with the C-terminus of alkyne-containing amino acids, peptide or PNA thymine monomers, and alternatively the organometallic compound was complexed to an internal site in the peptide or PNA. To this end, a simple glycine was replaced by propargylglycine in peptides, and a new alkyne-containing PNA-like monomer, in which an alkyne chain replaces the nucleobase, was used for PNA chemistry. For the synthesis of the two alkyne-containing enkephalin derivatives 3 and 5, two different resins, namely sulfamylbutyryl and Rink amid, were used as they allow to selectively insert, on the solid phase, an alkyne moiety at the C-terminus and on a side-chain of a peptide sequence, respectively. The identity and constitution of all cobalt complexes were confirmed by different analytical methods (IR, FAB, ESI-MS, and NMR). Most notably, IR spectroscopy shows intensive bands in the 2100-2000 cm(-1) region because of the Co(2)(CO)(6) moiety. In both (1)H NMR spectra of the dicobalthexacarbonyl PNA monomer derivatives 8 and 10, all signals are doubled because of the cis-trans isomerism about the central amide bond. The X-ray structure of a dicobalthexacarbonyl phenylalanine derivative (2a) confirms the proposed composition of the bioconjugates and shows that, as anticipated, the alkyne group of 2a is no longer linear upon complexation in comparison to the alkyne group of the bioconjugate precursor 1a, as indicated by a C-C[triple bond]C angle of about 143 degrees in 2a. Moreover, the C[triple bond]C bond of 1a was elongated by about 0.15 A upon Co(2) coordination.