Nils Metzler-Nolte

Organometallic Anticancer Compounds

The quest for alternative drugs to the well-known cisplatin and its derivatives, which are still used in more than 50% of the treatment regimes for patients suffering from cancer, is highly needed.1,2 Despite their tremendous success, these platinum compounds suffer from two main disadvantages: they are inefficient against platinum-resistant tumors, and they have severe side effects such as nephrotoxicity. The latter drawback is the consequence of the fact that the ultimate target of these drugs is ubiquitous: It is generally accepted that Pt anticancer drugs target DNA, which is present in all cells.3,4 Furthermore, as a consequence of its particular chemical structure, cisplatin in particular offers little possibility for rational improvements to increase its tumor specificity and thereby reduce undesired side effects. In this context, organometallic compounds, which are defined as metal complexes containing at least one direct, covalent metal−carbon bond, have recently been found to be promising anticancer drug candidates. Organometallics have a great structural variety (ranging from linear to octahedral and even beyond), have far more diverse stereochemistry than organic compounds (for an octahedral complex with six different ligands, 30 stereoisomers exist!), and by rational ligand design, provide control over key kinetic properties (such as hydrolysis rate of ligands). Furthermore, they are kinetically stable, usually uncharged, and relatively lipophilic and their metal atom is in a low oxidation state. Because of these fundamental differences compared to “classical coordination metal complexes”, organometallics offer ample opportunities in the design of novel classes of medicinal compounds, potentially with new metal-specific modes of action. Interestingly, all the typical classes of organometallics such as metallocenes, half-sandwich, carbene-, CO-, or π-ligands, which have been widely used for catalysis or biosensing purposes, have now also found application in medicinal chemistry (see Figure ​Figure11 for an overview of these typical classes of organometallics). Figure 1 Summary of the typical classes of organometallic compounds used in medicinal chemistry. In this Perspective, we report on the recent advances in the discovery of organometallics with proven antiproliferative activity. We are emphasizing those compounds where efforts have been made to identify their molecular target and mode of action by biochemical or cell biology studies. This Perspective covers more classes of compounds and in more detail than a recent tutorial review by Hartinger and Dyson.(5) Furthermore, whereas recent reviews and book contributions attest to the rapid development of bioorganometallic chemistry in general,6,7 this Perspective focuses on their potential application as anticancer chemotherapeutics. Another very recent review article categorizes inorganic anticancer drug candidates by their modes of action.(8) It should be mentioned that a full description of all currently investigated types of compounds is hardly possible anymore in a concise review. For example, a particularly promising class of organometallic anticancer compounds, namely, radiolabeled organometallics, has been omitted for space limitations. Recent developments of such compounds have been reviewed in detail by Alberto.(9)

The potential of organometallic complexes in medicinal chemistry

Organometallic complexes have unique physico-chemical properties, which have been widely used in homogenous catalysis, for example, for the synthesis of lead compounds and drug candidates. Over the past two decades, a few scientists from all over the world have extended the use of the specific characteristics of these compounds (e.g. structural diversity, possibility of ligand exchange, redox and catalytic properties) for medicinal purposes. The results are stunning. A few organometallic compounds have already entered clinical trials and it can be anticipated that several more will follow in coming years. In this short review, we present the specific advantages that organometallic metal complexes have over purely organic and also coordination compounds. Furthermore, using specific examples, we illustrate how these particular properties can be put to good use in medicinal chemistry. The examples we present have an emphasis on, but are not restricted to, anti-cancer activity.

Systematizing structural motifs and nomenclature in 1,n′-disubstituted ferrocene peptides

Ferrocene peptide conjugates display an array of structural features including helical ferrocene based chirality and a number of different intramolecular hydrogen bonding patterns. In this tutorial review we present a rigorous nomenclature for these systems, followed by a section that summarises and categorises the structures known to date. The issues discussed herein are of general relevance for all metallocene-based chiral transition metal catalysts and peptide turn mimetics.

Label‐Free Imaging of Metal–Carbonyl Complexes in Live Cells by Raman Microspectroscopy

The search for novel metal complexes with therapeutic activity, in particular against cancer and infectious diseases, is an active and important area of research in medicinal inorganic chemistry. In addition to the well-studied platinumand ruthenium-based coordination complexes, organometallic compounds have gained considerable importance in recent years. Of those, metal–carbonyl compounds are steadily increasing in interest, with some exhibiting remarkable antitumor activity. The most prominent example is probably the use of such compounds as “solid storage forms” for carbon monoxide. These CO-releasing molecules (CORMs) allow the biological action of this important small molecule messenger to be investigated. To elucidate the biological mode of action of any drug candidate it is mandatory to obtain a detailed picture of the intracellular distribution of such substances and how it evolves over time. Until now, the localization of metal complexes inside cells has been studied using X-ray fluorescence (XRF) 14] and atomic absorption spectroscopy (AAS). While AAS offers high sensitivity but almost no spatial resolution, XRF requires intense X-ray sources such as synchrotrons which will cause damage to biological tissue and is also not routinely available as an analytical technique. Most cellular studies therefore use fluorescence microscopy. Furthermore, this technique requires the additional attachment of a fluorescent label, which might be difficult. Optical excitation can also cause additional problems such as the onset of photochemical reactions or photobleaching. Moreover, the label can alter the biodistribution and properties of the molecule of interest, as recently shown for ruthenium–bipyridyl complexes. Efforts have been made to overcome these limitations by identifying biologically active metal complexes which show inherent fluorescence in vivo, but this has only been possible for a small number of metal–ligand combinations. Thus, it is highly desirable to develop innovative and generally applicable imaging techniques for the study of the uptake and distribution of bioactive metal complexes which do not require any labeling or special photophysical properties but instead use the intrinsic spectroscopic signature of the compound of interest. Raman microspectroscopy is emerging as a powerful noninvasive method to assess and image cellular compartments and processes without further sample preparation or labeling. Since Puppels et al. first showed the feasibility of confocal Raman microspectroscopy for imaging cells, its ability to study whole cells and subcellular organelles such as the nucleus and chromatin, mitochondria, and lipid bodies has been demonstrated by various research groups. Apart from imaging subcellular features, Raman imaging has been used to follow the uptake of molecules by cells. So far, however, these investigations have been restricted to the incorporation of deuterated building blocks as sensitive and specific markers into bio(macro)molecules. Herein, we investigate the uptake and cellular distribution of the new manganese-based CORM [Mn(tpm)(CO)3]Cl (tpm = tris(1-pyrazolyl)methane), which has photoinduceable cytotoxic activity against cancer cells. Metal–carbonyl complexes such as [Mn(tpm)(CO)3]Cl show strong C O stretching vibrations between 1800 and 2200 cm , a region where vibrational signals from the constituents of the cell are negligible. We show that the C O vibrations of this compound can be used as an ideal marker for imaging these complexes in living cancer cells. Although the spectroscopic signature of metal–carbonyl compounds has already been used in bioanalytical techniques such as the carbonyl–metal immunoassay (CMIA), their use in cellular imaging is so far unprecedented, except for an investigation of osmium–carbonyl clusters in dried cells by using infrared microscopy. The IR and Raman spectra of solid [Mn(tpm)(CO)3]Cl show strong C O stretching vibrations at about 1944 and 2050 cm , as expected for local C3v symmetry (Figures S1 and S2 A in the Supporting Information). The different relative intensities of the two peaks can be explained by the distinct selection rules for Raman and IR spectroscopy. The O H stretching vibration localized at about 3400 cm 1 dominates the spectrum of a 2 mm aqueous solution of [Mn(tpm)[*] K. Meister, Dr. D. A. Schmidt, Prof. Dr. M. Havenith Lehrstuhl f r Physikalische Chemie II, Ruhr-Universit t Bochum Universit tsstrasse 150, 44801 Bochum (Germany) E-mail: martina.havenith@rub.de Homepage: www.rub.de/pc2

Small organometallic compounds as antibacterial agents.

The emergence of bacterial resistance to commercial antibiotics is an issue of global importance. During the last two decades, the number of antibacterial agents that have been discovered and introduced into the market has steadily declined and failed to meet the challenges posed by rapidly increasing resistance of the pathogens against common antibacterial drugs. The development of new classes of compounds to control the virulence of the pathogens is therefore urgently required. This perspective describes the historical development in brief and recent advances on the preparation of small organometallic compounds as new classes of antibacterial agents with potential for clinical development.

Structural Complexity in Metal–Organic Frameworks: Simultaneous Modification of Open Metal Sites and Hierarchical Porosity by Systematic Doping with Defective Linkers

A series of defect-engineered metal-organic frameworks (DEMOFs) derived from parent microporous MOFs was obtained by systematic doping with defective linkers during synthesis, leading to the simultaneous and controllable modification of coordinatively unsaturated metal sites (CUS) and introduction of functionalized mesopores. These materials were investigated via temperature-dependent adsorption/desorption of CO monitored by FTIR spectroscopy under ultra-high-vacuum conditions. Accurate structural models for the generated point defects at CUS were deduced by matching experimental data with theoretical simulation. The results reveal multivariate diversity of electronic and steric properties at CUS, demonstrating the MOF defect structure modulation at two length scales in a single step to overcome restricted active site specificity and confined coordination space at CUS. Moreover, the DEMOFs exhibit promising modified physical properties, including band gap, magnetism, and porosity, with hierarchical micro/mesopore structures correlated with the nature and the degree of defective linker incorporation into the framework.

Solid‐Phase Synthesis, Characterization, and Antibacterial Activities of Metallocene–Peptide Bioconjugates

This work shows how the introduction of an organometallic group enhances and modifies the specificity of biologically active peptides. Ferrocene was chosen as an organometallic group because it has been shown to alter the pharmacodynamic profile of bioactive compounds. A comparison with the isosteric cobaltocenium group allows one to explore the influence of charge and redox potential on the biological activity of the conjugates. Arginine and tryptophan containing peptides H‐WRWRWR‐NH2 and H‐RWRWRW‐NH2 and the metallocene peptide bioconjugates [M]‐C(O)‐RWRWR‐NH2 and [M]‐C(O)‐WRWRW‐NH2, where [M]=[Co(Cp)(C5H4)]+, [Fe(Cp)(C5H4)] were prepared by solid‐phase peptide synthesis (SPPS). They were purified by HPLC, characterized by ESIMS and NMR spectroscopy, and tested for antibacterial properties against Escherichia coli, Pseudomonas aeruginosa, and Staphylococcus aureus using the minimum inhibitory concentration (MIC) test. In most cases, no metal‐specific activity could be observed. However, the conjugate [Fe(Cp)(C5H4)‐C(O)‐WRWRW‐NH2] 6 was found to be particularly effective against the Gram‐positive S. aureus. The activity of this metallocene–pentapeptide conjugate (7.1 μM) was even better than the 20 amino acid naturally occurring pilosulin 2, which was used as a positive control. Unlike all other compounds tested, which were most active against the Gram‐negative E. coli strain, the ferrocene conjugate 6 was the only compound in this series that was most active against Gram‐positive bacteria. Given the health concerns resulting from multidrug resistant S. aureus strains, the incorporation of metallocenes may provide a novel line of attack.

Chemistry of SURMOFs: layer-selective installation of functional groups and post-synthetic covalent modification probed by fluorescence microscopy.

Layer-selective installation of functional groups at SURMOFs (surface-attached metal-organic framework multilayers) is reported. Multilayers of [Cu(ndc)(dabco)(0.5)] grown in [001] orientation on pyridine-terminated organic self-assembled monolayers on Au substrates were functionalized with amino groups by step-by-step liquid-phase epitaxy. The method allows the growth of samples exhibiting one monolayer of functional groups at the external thin-film surface. In situ quartz crystal microbalance monitoring confirmed the presence of amino groups by turning the multilayer film from a non-reactive to a reactive material for covalent binding of fluoresceinisothiocyanate, and fluorescence microscopy displays the expected luminous property.

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.

A spontaneous gold(I)-azide alkyne cycloaddition reaction yields gold-peptide bioconjugates which overcome cisplatin resistance in a p53-mutant cancer cell line

Solid-phase peptide synthesis (SPPS) is a versatile technique for the assembly of small to medium size peptides, that can help in the delivery of bound metal complexes to certain cellular compartments, for example in cancer cells. This work shows a new route to gold-peptide bioconjugates via a non-catalyzed [3 + 2] cycloaddition reaction of gold azides with alkynyl peptides. Gold(I) tetrapeptide conjugates with a mitochondria-targeting sequence were synthesized and display prolonged stability in the presence of thiol-containing biological media. Their antiproliferative potency against selected cancer cells (2–50 μM) corresponds to the lipophilicity of the conjugates. The cellular uptake of Au, determined by atomic absorption spectroscopy (AAS), shows that high initial uptake equals strong cytotoxicity. Respiration and acidification rates react immediately upon treatment with the Au-peptide conjugates, and a terminal breakdown of essential cellular functions is complete within ca. 12 h at most, as observed by online monitoring of the cancer cell metabolism in a microfluidic biosensor device (Bionas sensorchip system). The mode of action of these Au-peptide bioconjugates was elucidated by a variety of biochemical and cell biological experiments. First, a strong selective inhibition of the enzyme thioredoxin reductase (TrxR), a regulator of cellular redox processes, was found. In this context, elevated levels of reactive oxygen species (ROS) and strong effects on the respiration of isolated mouse liver mitochondria were found. These finally lead to cell death via apoptotic pathways, as indicated by flow cytometry, low mitochondrial membrane potential (MMP) and DNA fragmentation. Intriguingly, cisplatin-resistance in p53-mutant MDA-MB231 breast cancer cells could be overcome by the Au-peptide conjugates presented herein.