Tridentate 3-Substituted Naphthoquinone Ruthenium Arene Complexes: Synthesis, Characterization, Aqueous Behavior, and Theoretical and Biological Studies.

A series of nine RuII arene complexes bearing tridentate naphthoquinone-based N,O,O-ligands was synthesized and characterized. Aqueous stability and their hydrolysis mechanism were investigated via UV/vis photometry, HPLC-MS, and density functional theory calculations. Substituents with a positive inductive effect improved their stability at physiological pH (7.4) intensely, whereas substituents such as halogens accelerated hydrolysis and formation of dimeric pyrazolate and hydroxido bridged dimers. The observed cytotoxic profile is unusual, as complexes exhibited much higher cytotoxicity in SW480 colon cancer cells than in the broadly chemo- (incl. platinum-) sensitive CH1/PA-1 teratocarcinoma cells. This activity pattern as well as reduced or slightly enhanced ROS generation and the lack of DNA interactions indicate a mode of action different from established or previously investigated classes of metallodrugs.

Fast and Highly Efficient Affinity Enrichment of Azide-A-DSBSO Cross-Linked Peptides.

Cross-linking mass spectrometry is an increasingly used, powerful technique to study protein-protein interactions or to provide structural information. Due to substochiometric reaction efficiencies, cross-linked peptides are usually low abundance. This results in challenging data evaluation and the need for an effective enrichment. Here we describe an improved, easy to implement, one-step method to enrich azide-tagged, acid-cleavable disuccinimidyl bis-sulfoxide (DSBSO) cross-linked peptides using dibenzocyclooctyne (DBCO) coupled Sepharose beads. We probed this method using recombinant Cas9 and E. coli ribosome. For Cas9, the number of detectable cross-links was increased from ∼100 before enrichment to 580 cross-links after enrichment. To mimic a cellular lysate, E. coli ribosome was spiked into a tryptic HEK background at a ratio of 1:2-1:100. The number of detectable unique cross-links was maintained high at ∼100. The estimated enrichment efficiency was improved by a factor of 4-5 (based on XL numbers) compared to enrichment via biotin and streptavidin. We were still able to detect cross-links from 0.25 µg cross-linked E. coli ribosomes in a background of 100 µg tryptic HEK peptides, indicating a high enrichment sensitivity. In contrast to conventional enrichment techniques, like SEC, the time needed for preparation and MS measurement is significantly reduced. This robust, fast, and selective enrichment method for azide-tagged linkers will contribute to mapping protein-protein interactions, investigating protein architectures in more depth, and helping to understand complex biological processes.

Biological evaluation of novel thiomaltol-based organometallic complexes as topoisomerase IIα inhibitors.

Topoisomerase IIα (topo2α) is an essential nuclear enzyme involved in DNA replication, transcription, recombination, chromosome condensation, and highly expressed in many tumors. Thus, topo2α-targeting has become a very efficient and well-established anticancer strategy. Herein, we investigate the cytotoxic and DNA-damaging activity of thiomaltol-containing ruthenium-, osmium-, rhodium- and iridium-based organometallic complexes in human mammary carcinoma cell lines by means of several biological assays, including knockdown of topo2α expression levels by RNA interference. Results suggest that inhibition of topo2α is a key process in the cytotoxic mechanism for some of the compounds, whereas direct induction of DNA double-strand breaks or other DNA damage is mostly rather minor. In addition, molecular modeling studies performed for two of the compounds (with Ru(II) as the metal center) evinces that these complexes are able to access the DNA-binding pocket of the enzyme, where the hydrophilic environment favors the interaction with highly polar complexes. These findings substantiate the potential of these compounds for application as antitumor metallopharmaceuticals.

Synthesis, Modification, and Biological Evaluation of a Library of Novel Water-Soluble Thiopyridone-Based Organometallic Complexes and Their Unexpected (Biological) Behavior.

A series of 16 dinuclear thiopyridone-based organometallics with excellent water solubility, increased stability and remarkable cytotoxicity were synthesized and characterized. The complexes of this work formed dimeric species featuring a double positive charge in polar protic solvents, accounting for their outstanding solubility in aqueous solution. Most of them displayed higher antiproliferative activity than their parental thiomaltol complex, with unexpected cytotoxicity trends depending on the employed metal center, ligand modification, and cell line. Insights into their behavior in biological systems were gathered by means of amino-acid interaction studies, cytotoxicity tests in 3D spheroid models, laser ablation, cellular accumulation measurements, as well as cell cycle experiments.

The First Anticancer Tris(pyrazolyl)borate Molybdenum(IV) Complexes: Tested in Vitro and in Vivo-A Comparison of O,O-, S,O-, and N,N-Chelate Effects.

The synthesis, characterization and biological activity of molybdenum(IV) complexes containing Trofimenko's scorpionato ligand, hydrotris(3-isopropylpyrazolyl)borate (TpiPr ), in addition to varying biologically active as well as other conventional ligands is described. Ligands employed include (O,O-) (S,O-) (N,N-) donors that have been successfully coordinated to the molybdenum center by means of oxygen-atom transfer (OAT) reactions from the known MoVI starting material, TpiPr MoO2 Cl. The synthesized complexes were characterized by standard analytical methods and where possible by X-ray diffraction analysis. The aqueous stability of the compounds was studied by means of UV/Vis spectroscopy and the impact of the attached ligand scaffolds on the oxidation potentials (MoIV to MoV ) was studied by cyclic voltammetry. Utilizing polyvinylpyrrolidone (PVP) as a solubilizing agent, adequate aqueous solubility for biological tests was obtained. Anticancer activity tests and preliminary mode of action studies have been performed in vitro and in vivo.

1,4-Disubstituted 1,2,3-Triazoles as Amide Bond Surrogates for the Stabilisation of Linear Peptides with Biological Activity.

Peptides represent an important class of biologically active molecules with high potential for the development of diagnostic and therapeutic agents due to their structural diversity, favourable pharmacokinetic properties, and synthetic availability. However, the widespread use of peptides and conjugates thereof in clinical applications can be hampered by their low stability in vivo due to rapid degradation by endogenous proteases. A promising approach to circumvent this potential limitation includes the substitution of metabolically labile amide bonds in the peptide backbone by stable isosteric amide bond mimetics. In this review, we focus on the incorporation of 1,4-disubstituted 1,2,3-triazoles as amide bond surrogates in linear peptides with the aim to increase their stability without impacting their biological function(s). We highlight the properties of this heterocycle as a trans-amide bond surrogate and summarise approaches for the synthesis of triazole-containing peptidomimetics via the Cu(I)-catalysed azide-alkyne cycloaddition (CuAAC). The impacts of the incorporation of triazoles in the backbone of diverse peptides on their biological properties such as, e.g., blood serum stability and affinity as well as selectivity towards their respective molecular target(s) are discussed.

Fine-Tuning the Activation Mode of an 1,3-Indandione-Based Ruthenium(II)-Cymene Half-Sandwich Complex by Variation of Its Leaving Group.

Fine-tuning of the properties of a recently reported 1,3-indandione-based organoruthenium complex is attempted to optimize the stability under physiological conditions. Previous work has shown its capacity of inhibiting topoisomerase IIα; however, fast aquation leads to undesired reactions and ligand cleavage in the blood stream before the tumor tissue is reached. Exchange of the chlorido ligand for six different N-donor ligands resulted in new analogs that were stable at pH 7.4 and 8.5. Only a lowered pH level, as encountered in the extracellular space of the tumor tissue, was capable of aquating the complexes. The 50% inhibitory concentration (IC50) values in three human cancer cell lines differed only slightly, and their dependence on the utilized leaving group was smaller than what would be expected from their differences in cellular accumulation, but in accordance with the very minor variation revealed in measurements of the complexes' lipophilicity.

Solvent Bar Micro-Extraction of Heavy Metals from Natural Water Samples Using 3-Hydroxy-2-Naphthoate-Based Ionic Liquids.

Developments in the liquid micro-extraction of trace metals from aqueous phases have proven to be limited when extended from pure water to more complex and demanding matrices such as sea water or wastewater treatment effluents. To establish a system that works under such matrices, we successfully tested three task-specific ionic liquids, namely trihexyltetradecyl- phosphonium-, methyltrioctylphosphonium- and methyltrioctylammonium 3-hydroxy-2-naphthoate in two-phase solvent bar micro-extraction (SBME) experiments. We describe the influence of pH, organic additives, time, stirring rate and volume of ionic liquid for multi-elemental micro-extraction of Cu, Ag, Cd and Pb from various synthetic and natural aqueous feed solutions. Highest extraction for all metals was achieved at pH 8.0. Minimal leaching of the ionic liquids into the aqueous phase was demonstrated, with values < 30 mg L-1 DOC in all cases. Sample salinities of up to 60 g L-1 NaCl had a positive effect on the extraction of Cd, possibly due to an efficient extraction mechanism of the present chlorido complexes. In metal-spiked natural feed solutions, the selected SBME setups showed unchanged stability under all conditions tested. We could efficiently (≥85%) extract Cu and Ag from drinking water and achieved high efficacies for Ag and Cd from natural sea water and hypersaline water, respectively. The method presented here proves to be a useful tool for an efficient SBME of heavy metals from natural waters without the need to pretreat or modify the sample.

Introducing the 4-Phenyl-1,2,3-Triazole Moiety as a Versatile Scaffold for the Development of Cytotoxic Ruthenium(II) and Osmium(II) Arene Cyclometalates.

Herein we report the synthesis, anticancer potency in vitro, biomolecule interaction, and preliminary mode of action studies of a series of cyclometalated 1,2,3-triazole-derived ruthenium(II) (2a-e) and osmium(II) (3a-e) organometallics of the general form [(η6-p-cym)RuCl(κ2-C^N-L)] with varying substituents in postion 1 of the 1,2,3-triazole moiety. These cyclometalates were characterized by standard analytical methods and their structures unambiguously assigned by single crystal X-ray crystallography. The anticancer activity of these novel compounds was tested in the human tumor cell lines A549 (non-small cell lung cancer), SW480 (colon adenocarcinoma), and CH1/PA-1 (ovarian teratocarcinoma), and preliminary structure-activity relationships were derived from the obtained data sets. Various representatives exhibit promising antineoplastic effects with IC50 values down to the low micromolar range. The compounds readily formed stable DMSO adducts after aquation in DMSO-containing solution, but employing DMSO as solubilizer in cytotoxicity assays had no pronounced effect on the cytotoxicity, compared to analogous experiments with DMF for most compounds. We isolated and characterized selected DMSO adducts as triflate salts and found that they show activities in the same range as the parent chlorido metalacycles in MTT assays with the use of DMSO. Osmium(II) cyclometalates exhibited higher antiproliferative activities than their ruthenium(II) counterparts. The IC50 values within each metal series decreased with increasing lipophilicity, which was attributed to higher cellular accumulation. Investigations on their mode of action revealed that the prepared organometallics were unable to inhibit topoisomerase IIα. Still, the most cytotoxic representatives 2b and 3b showed pronounced effects on cell cycle distribution.

Thiomaltol-Based Organometallic Complexes with 1-Methylimidazole as Leaving Group: Synthesis, Stability, and Biological Behavior.

Thiomaltol, a potential S,O-coordinating molecule, has been utilized for the complexation of four different organometallic fragments, yielding the desired RuII , OsII , RhIII , and IrIII complexes having a "piano-stool" configuration. In addition to the synthesis of these compounds with a chlorido leaving group, the analogous 1-methylimidazole derivatives have been prepared, giving rise to thiomaltol-based organometallics with enhanced stability under physiological conditions. The organometallic compounds have been characterized by NMR spectroscopy, elemental analysis, and X-ray diffraction analysis. Their behavior in aqueous solution and their interactions with certain amino acids have been studied by ESI mass spectrometry. Their pH-dependent stability has been investigated by 1 H NMR in aqueous solution, and their cytotoxicity against three different cancer cell lines has been investigated. Furthermore, their capacity as topoisomerase IIα inhibitors as well as their effect on the cell cycle distribution and reactive oxygen species (ROS) generation have been elucidated.

Photoreduction of Terrigenous Fe-Humic Substances Leads to Bioavailable Iron in Oceans.

Humic substances (HS) are important iron chelators responsible for the transport of iron from freshwater systems to the open sea, where iron is essential for marine organisms. Evidence suggests that iron complexed to HS comprises the bulk of the iron ligand pool in near-coastal waters and shelf seas. River-derived HS have been investigated to study their transport to, and dwell in oceanic waters. A library of iron model compounds and river-derived Fe-HS samples were probed in a combined X-ray absorption spectroscopy (XAS) and valence-to-core X-ray emission spectroscopy (VtC-XES) study at the Fe K-edge. The analyses performed revealed that iron complexation in HS samples is only dependent on oxygen-containing HS functional groups, such as carboxyl and phenol. The photoreduction mechanism of Fe(III) -HS in oceanic conditions into bioavailable aquatic Fe(II) forms, highlights the importance of river-derived HS as an iron source for marine organisms. Consequently, such mechanisms are a vital component of the upper-ocean iron biogeochemistry cycle.

Amide-to-Triazole Switch in Somatostatin-14-Based Radioligands: Impact on Receptor Affinity and In Vivo Stability.

The use of metabolically stabilized, radiolabeled somatostatin (SST) analogs ([68Ga]Ga/[177Lu]Lu-DOTA-TATE/TOC/NOC) is well established in nuclear medicine. Despite the pivotal role of these radioligands in the diagnosis and therapy of neuroendocrine tumors (NETs), their inability to interact with all five somatostatin receptors (SST1-5R) limits their clinical potential. [111In]In-AT2S is a radiolabeled DOTA-conjugate derived from the parent peptide SST-14 that exhibits high binding affinity to all SSTR subtypes, but its poor metabolic stability represents a serious disadvantage for clinical use. In order to address this issue, we have replaced strategic trans-amide bonds of [111In]In-AT2S with metabolically stable 1,4-disubstituted 1,2,3-triazole bioisosteres. From the five cyclic triazolo-peptidomimetics investigated, only [111In]In-XG1 combined a preserved nanomolar affinity for the SST1,2,3,5R subtypes in vitro and an improved stability in vivo (up to 17% of intact peptide 5 min postinjection (pi) versus 6% for [111In]In-AT2S). The involvement of neprilysin (NEP) in the metabolism of [111In]In-XG1 was confirmed by coadministration of Entresto®, a registered antihypertensive drug, in vivo releasing the selective and potent NEP-inhibitor sacubitrilat. A pilot SPECT/CT imaging study conducted in mice bearing hSST2R-positive xenografts failed to visualize the xenografts due to the pronounced kidney uptake (>200% injected activity (IA)/g at 4 h pi), likely the result of the formation of cationic metabolites. To corroborate the imaging data, the tumors and the kidneys were excised and analyzed with a γ-counter. Even if receptor-specific tumor uptake for [111In]In-XG1 could be confirmed (1.61% IA/g), further optimization is required to improve its pharmacokinetic properties for radiotracer development.

Investigating the anticancer potential of 4-phenylthiazole derived Ru(II) and Os(II) metalacycles.

In this contribution we report the synthesis, characterization and in vitro anticancer activity of novel cyclometalated 4-phenylthiazole-derived ruthenium(II) (2a-e) and osmium(II) (3a-e) complexes. Formation and sufficient purity of the complexes were unambigiously confirmed by 1H-, 13C- and 2D-NMR techniques, X-ray diffractometry, HRMS and elemental analysis. The binding preferences of these cyclometalates to selected amino acids and to DNA models including G-quadruplex structures were analyzed. Additionally, their stability and behaviour in aqueous solutions was determined by UV-Vis spectroscopy. Their cellular accumulation, their ability of inducing apoptosis, as well as their interference in the cell cycle were studied in SW480 colon cancer cells. The anticancer potencies were investigated in three human cancer cell lines and revealed IC50 values in the low micromolar range, in contrast to the biologically inactive ligands.

Identification of the structural determinants for anticancer activity of a ruthenium arene peptide conjugate.

Organometallic Ru(arene)-peptide bioconjugates with potent in vitro anticancer activity are rare. We have prepared a conjugate of a Ru(arene) complex with the neuropeptide [Leu(5)]-enkephalin. [Chlorido(η(6)-p-cymene)(5-oxo-κO-2-{(4-[(N-tyrosinyl-glycinyl-glycinyl-phenylalanyl-leucinyl-NH2)propanamido]-1H-1,2,3-triazol-1-yl)methyl}-4H-pyronato-κO)ruthenium(II)] (8) shows antiproliferative activity in human ovarian carcinoma cells with an IC50 value as low as 13 µM, whereas the peptide or the Ru moiety alone are hardly cytotoxic. The conjugation strategy for linking the Ru(cym) (cym=η(6)-p-cymene) moiety to the peptide involved N-terminal modification of an alkyne-[Leu(5)]-enkephalin with a 2-(azidomethyl)-5-hydroxy-4H-pyran-4-one linker, using Cu(I)-catalyzed alkyne-azide cycloaddition (CuAAC), and subsequent metallation with the Ru(cym) moiety. The ruthenium-bioconjugate was characterized by high resolution top-down electrospray ionization mass spectrometry (ESI-MS) with regard to peptide sequence, linker modification and metallation site. Notably, complete sequence coverage was obtained and the Ru(cym) moiety was confirmed to be coordinated to the pyronato linker. The ruthenium-bioconjugate was analyzed with respect to cytotoxicity-determining constituents, and through the bioconjugate models [{2-(azidomethyl)-5-oxo-κO-4H-pyronato-κO}chloride (η(6)-p-cymene)ruthenium(II)] (5) and [chlorido(η(6)-p-cymene){5-oxo-κO-2-([(4-(phenoxymethyl)-1H-1,2,3-triazol-1-yl]methyl)-4H-pyronato-κO}ruthenium(II)] (6) the Ru(cym) fragment with a triazole-carrying pyronato ligand was identified as the minimal unit required to achieve in vitro anticancer activity.

Extraction of rare earth elements from aqueous solutions using the ionic liquid trihexyltetradecylphosphonium 3-hydroxy-2-naphthoate.

The task-specific ionic liquid trihexyltetradecylphosphonium 3-hydroxy-2-naphthoate has been described as a suitable extraction agent for numerous metals from aqueous phases, while additionally providing reduced leaching into the used matrices. Here, we investigate the extraction properties of this extractant towards rare earth elements. Of these, La, Ce, Nd, Ho und Lu were chosen as a representative mix of light and heavy elements. Single- as well as double-element extractions were carried out under varying conditions regarding pH, temperature and extraction time. The highest extraction efficacies and minimalized precipitation of the respective metals were recorded at a pH of 2.5. Satisfactory extraction efficacies (>80%) were achieved already after 6 hours for the elements Ce, Nd and Lu in single-element extraction experiments at room temperature. Increased temperatures improved the extraction efficacy for Nd from 36% at 20 °C to 80% at 30 °C after only 2 hours. Surprisingly, this effect was not observed for Ce in single-element experiments. In double-element feed solutions containing both Ce and Nd, however, the time-dependant extraction efficacy of Ce mirrored that of Nd. The pH in the aqueous extraction matrix changed during the extraction, showing a positive correlation with the extraction efficacy and revealing the extraction mechanism to be via anion exchange. The leaching was in good agreement with literature values, showed a positive correlation with extraction efficacies, and ranged for all extractions between 0.8 and 1.2%. Remarkably, increasing the temperature from 20 °C to 30 °C had no significant influence on leaching.

Half-Sandwich Rhodium Complexes with Releasable N-Donor Monodentate Ligands: Solution Chemical Properties and the Possibility for Acidosis Activation.

Cancer chemotherapeutics usually have serious side effects. Targeting the special properties of cancer and activation of the anticancer drug in the tumor microenvironment in situ may decrease the intensity of the side effects and improve the efficacy of therapy. In this study, half-sandwich Rh complexes are introduced, which may be activated at the acidic, extracellular pH of the tumor tissue. The synthesis and aqueous stability of mixed-ligand complexes with a general formula of [Rh(η5-Cp*)(N,N/O)(N)]2+/+ are reported, where (N,N/O) indicates bidentate 8-quinolate, ethylenediamine and 1,10-phenanthroline and (N) represents the releasable monodentate ligand with a nitrogen donor atom. UV-visible spectrophotometry, 1H NMR, and pH-potentiometry were used to determine the protonation constants of the monodentate ligands, the proton dissociation constants of the coordinated water molecules in the aqua complexes, and the formation constants of the mixed-ligand complexes. The obtained data were compared to those of the analogous Ru(η6-p-cymene) complexes. The developed mixed-ligand complexes were tested in drug-sensitive and resistant colon cancer cell lines (Colo205 and Colo320, respectively) and in four bacterial strains (Gram-positive and Gram-negative, drug-sensitive, and resistant) at different pH values (5-8). The mixed-ligand complexes with 1-methylimidazole displayed sufficient stability at pH 7.4, and their activation was found in cancer cells with decreasing pH; moreover, the mixed-ligand complexes demonstrated antimicrobial activity in Gram-positive and Gram-negative bacteria, including the resistant MRSA strain. This study proved the viability of incorporating releasable monodentate ligands into mixed-ligand half-sandwich complexes, which is supported by the biological assays.

Synthesis and Preclinical Evaluation of Radiolabeled [103Ru]BOLD-100.

The first-in-class ruthenium-based chemotherapeutic agent BOLD-100 (formerly IT-139, NKP-1339, KP1339) is currently the subject of clinical evaluation for the treatment of gastric, pancreatic, colorectal and bile duct cancer. A radiolabeled version of the compound could present a helpful diagnostic tool. Thus, this study investigated the pharmacokinetics of BOLD-100 in more detail to facilitate the stratification of patients for the therapy. The synthesis of [103Ru]BOLD-100, radiolabeled with carrier added (c.a.) ruthenium-103, was established and the product was characterized by HPLC and UV/Vis spectroscopy. In order to compare the radiolabeled and non-radioactive versions of BOLD-100, both complexes were fully evaluated in vitro and in vivo. The cytotoxicity of the compounds was determined in two colon carcinoma cell lines (HCT116 and CT26) and biodistribution studies were performed in Balb/c mice bearing CT26 allografts over a time period of 72 h post injection (p.i.). We report herein preclinical cytotoxicity and pharmacokinetic data for BOLD-100, which were found to be identical to those of its radiolabeled analog [103Ru]BOLD-100.

Anticancer Tungstenocenes with a Diverse Set of (O,O-), (O,S-) and (O,N-) Chelates-A Detailed Biological Study Using an Improved Evaluation via 3D Spheroid Models.

The synthesis, characterization and biological activity of tungstenocenes with varying biologically active (O,O-), (S,O-) and (N,O-) chelates are described. Complexes were characterized by 1H and 13C NMR, elemental analysis, ESI-mass spectrometry, FT-IR spectroscopy and X-ray diffraction analysis. The aqueous stability was studied by UV/Vis spectroscopy and the WIV to WV process by cyclic voltammetry. The cytotoxicity was determined by the MTT assay in A549, CH1/PA-1 and SW480 cancer cells as well as in IMR-90 human fibroblasts. Extensive biological evaluation was performed in three other human cancer cell lines (HCT116, HT29 and MCF-7) in monolayer and multicellular tumor spheroid cultures to better understand the mode of action. Lead compounds showed promising in vitro anticancer activity in all cancer cell lines. Further studies yielded important insights into apoptosis induction, ROS generation, different patterns in metal distribution (detected by LA-ICP-TOF-MS), changes in KI67 (proliferation marker) expression and DNA interactions. The results based on qualitative and quantitative research designs show that complexes containing (S,O-) chelates are more active than their (O,O-) and (N,O-) counterparts. The most striking results in spheroid models are the high antiproliferative capacity and the different distribution pattern of two complexes differing only in a W-S or W-O bond.

Organometallic ruthenium and osmium compounds of pyridin-2- and -4-ones as potential anticancer agents.

Organometallic Ru(II) compounds are among the most widely studied anticancer agents. Functionalizing metal centers with biomolecule-derived ligands has been shown to be a promising strategy to improve the antiproliferative activity of metal-based chemotherapeutics. Herein, the synthesis of a series of novel 3-hydroxypyridin-2-one-derived ligands and their M(II)(η(6)-p-cymene) half-sandwich complexes (M = Ru, Os) is described. The compounds were characterized by 1D- and 2D-NMR spectroscopy, and elemental analysis.

Systematic Study on the Cytotoxic Potency of Commonly Used Dimeric Metal Precursors in Human Cancer Cell Lines.

The cytotoxicities of seven dimeric metal species of the general formula [M(arene)Cl2 ]2 , commonly used as precursors for complex synthesis and deemed biologically inactive, are investigated in seven commonly employed human cancer cell lines. Four of these complexes featured a ruthenium(II) core, where p-cymene, toluene, benzene and indane were used as arenes. Furthermore, the osmium(II) p-cymene dimer, as well as the Cp* dimers of rhodium(III) and its heavier analogue iridium(III) were included in this work (Cp*=1,2,3,4,5-pentamethylcyclopentadienide). While the cytotoxic potencies of the ruthenium(II) and osmium(II) dimers are very low (or not even detectable at applicable concentrations), surprising activity, especially in cells from ovarian malignancies (with one or two-digit micromolar IC50 values), have been found for the rhodium(III) and iridium(III) representatives. This publication is aimed at all researchers using synthetic procedures based on functionalization of these dimeric starting materials to rationalize changes in biological properties, especially cytotoxicity in cancer cells.