Thiourea-Derived Chelating Ligands and Their Organometallic Compounds: Investigations into Their Anticancer Activity

Chemotherapeutic Activities of New η6-p-Cymene Ruthenium(II) and Osmium(II) Complexes with Chelating SS and Tridentate SNS Ligands

A series of new chelating bidentate (SS) alkylimidazole-2-thione-Ru(II)/Os(II) complexes (3ai, 3aii, 3aiii, 3bii/4aiii, 4bi, 4bii), and the tridentate (SNS) pyridine-2,6-diylimidazole-2-thione-Ru(II)/Os(II) complexes (5bi, 5civ/6bi, 6ci, 6civ) in the forms [MII(cym)(L)Cl]PF6 and [MII(cym)(L)]PF6 (M = Ru or Os, cym = η6-p-cymene, and L = heterocyclic derivatives of thiourea) respectively, were successfully synthesized. Spectroscopic and analytical methods were used to characterize the complexes and their ligands. Solid-state single-crystal X-ray diffraction analyses revealed a "piano-stool" geometry around the Ru(II) or Os(II) centers in the respective complexes. The complexes were investigated for in vitro chemotherapeutic activities against human cervical carcinoma (HeLa) and the non-cancerous cell line (Hek293) using the MTT assay. The compounds 3aii, 5civ, 5bi, 4aiii, 6ci, 6civ, and the reference drug, 5-fluorouracil were found to be selective toward the tumor cells; the compounds 3ai, 3aiii, 3bii, 4bi, 4bii, and 6bi, which were found not to be selective between normal and tumor cell lines. The IC50 value of the tridentate half-sandwich complex 5bi (86 ± 9 μM) showed comparable anti-proliferative activity with the referenced commercial anti-cancer drug, 5-fluorouracil (87 ± 15 μM). The pincer (SNS) osmium complexes 6ci (36 ± 10 μM) and 6civ (40 ± 4 μM) were twice as effective as the reference drug 5-fluorouracil at the respective dose concentrations. However, the analogous pincer (SNS) ruthenium complex 5civ was ineffective and did not show anti-proliferative activity, even at a higher concentration of 147 ± 1 μM. These findings imply that the higher stability of the chelating (SS) and the pincer (SNS) ligand architectures in the complexes improves the biological (anti-proliferative) activity of the complexes by reducing the chance of ligand dissociation under physiological conditions. In general, the pincer (SNS) osmium complexes were found to be more cytotoxic than their ruthenium analogues, suggesting that the anti-proliferative activity of the imidazole-2-thione-Ru/Os complexes depends on the ligand's spatial coordination, the nature of the metal center, and the charge of the metal complex ions.

Triazolyl- vs Pyridyl-Functionalized N-Heterocyclic Carbene Complexes: Impact of the Pendant N-Donor Ligand on Intramolecular C-C Bond Formation

Organometallic Rh(Cp*) (Cp* = η⁵-pentamethylcyclopentadienyl) complexes with monodentate N-heterocyclic carbene (NHC) ligands bearing a pendant anthracenyl substituent have been shown to undergo intramolecular C-C coupling reactions. Herein, two bidentate NHC ligands substituted with pyridyl or triazolyl donor groups were prepared along with the corresponding M^II/III (M = Ru^II, Os^II, Rh^III, Ir^III) complexes. While the Rh(Cp*) complex featuring an NHC-triazole bidentate ligand underwent the equivalent reaction as the monodentate Rh(NHC) complex, i.e., it formed a polydentate ligand, the pyridyl-pendant derivative was unequivocally shown to be unreactive. This contrasting behavior was further investigated by density functional theory (DFT) calculations that highlighted significant differences between the two types of Rh(III) complexes with pendant pyridyl or triazolyl N-coordinating groups. Modeling of the reaction pathways suggests that the initial formation of a dicationic Rh(III) species is unfavorable and that the internal ligand transformation proceeds first by dissociation of the coordinated N atom of the pendant group from the Rh center. After the formation of a neutral η⁴-fulvene ligand via combined proton/single electron transfer, a cycloaddition occurs between the exo-ene bond of fulvene and the 9' and 10' positions on the pendant anthracenyl group. The resulting experimental UV-visible spectrum recorded in methanol of the polydentate triazolyl-based Rh species revealed the loss of the vibronic coupling typically associated with an anthracenyl functional group. Moreover, TD-DFT modeling indicates the presence of an equilibrium process whereby the N-coordination of the pendant triazolyl group to the Rh^III center appears to be highly labile. Charge decomposition analysis (CDA) of the DFT-modeled species with the dissociated triazolyl group revealed a pseudo-η³-allylic interaction between the π-type MOs of the transformed anthracenyl group and the Rh^III center; thus, the singly attached chelating ligand is classified as having rare nonadenticity.

Impact of Coordination Mode and Ferrocene Functionalization on the Anticancer Activity of N-Heterocyclic Carbene Half-Sandwich Complexes

The substitution of phenyl rings in established drugs with ferrocenyl moieties has been reported to yield compounds with improved biological activity and alternative modes of action, often involving the formation of reactive oxygen species (ROS). Translating this concept to N-heterocyclic carbene (NHC) complexes, we report here organometallics with a piano-stool structure that feature di- or tridentate ligand systems. The ligands impacted the cytotoxic activity of the NHC complexes, but the coordination modes seemed to have a limited influence, which may be related to the propensity of forming the same species in solution. In general, the stability of the complexes in an aqueous environment and their reactivity to selected biomolecules were largely dominated by the nature of the metal center. While the complexes promoted the formation of ROS, the levels did not correlate with their cytotoxic activity. However, the introduction of ferrocenyl moieties had a significant impact on the antiproliferative potency of the complexes and, in particular, some of the ferrocenyl-functionalized compounds yielded IC₅₀ values in the low μM range.

Anthracenyl Functionalization of Half-Sandwich Carbene Complexes: In Vitro Anticancer Activity and Reactions with Biomolecules

N-Heterocyclic carbene (NHC) ligands are widely investigated in medicinal inorganic chemistry. Here, we report the preparation and characterization of a series of half-sandwich [M(L)(NHC)Cl₂] (M = Ru, Os, Rh, Ir; L = cym/Cp*) complexes with a N-flanking anthracenyl moiety attached to imidazole- and benzimidazole-derived NHC ligands. The anticancer activity of the complexes was investigated in cell culture studies where, in comparison to a Rh derivative with an all-carbon-donor-atom-based ligand (5a), they were found to be cytotoxic with IC₅₀ values in the low micromolar range. The Ru derivative 1a was chosen as a representative for stability studies as well as for biomolecule interaction experiments. It underwent partial chlorido/aqua ligand exchange in DMSO-d₆/D₂O to rapidly form an equilibrium in aqueous media. The reactions of 1a with biomolecules proceeded quickly and resulted in the formation of adducts with amino acids, DNA, and protein. Hen egg white lysozyme crystals were soaked with 1a, and the crystallographic analysis revealed an interaction with an l-aspartic acid residue (Asp119), resulting in the cleavage of the p-cymene ligand but the retention of the NHC moiety. Cell morphology studies for the Rh analog 3a suggested that the cytotoxicity is exerted via mechanisms different from that of cisplatin.

Anticancer Half-Sandwich Rhodium(III) Complexes

Platinum-based anticancer drugs are most likely the most successful group of bioinorganic compounds. Their apparent disadvantages have led to the development of anticancer compounds of other noble metals, resulting in several ruthenium-based drugs which have entered clinical trials on oncological patients. Besides ruthenium, numerous rhodium complexes have been recently reported as highly potent antiproliferative agents against various human cancer cells, making them potential alternatives to Pt- and Ru-based metallodrugs. In this review, half-sandwich Rh(III) complexes are overviewed. Many representatives show higher in vitro potency than and different mechanisms of action (MoA) from the conventional anticancer metallodrugs (cisplatin in most cases) or clinically studied Ru drug candidates. Furthermore, some of the reviewed Rh(III) arenyl complexes are also anticancer in vivo. Pioneer anticancer organorhodium compounds as well as the recent advances in the field are discussed properly, and adequate attention is paid to their anticancer activity, solution behaviour and various processes connected with their MoA. In summary, this work summarizes the types of compounds and the most important biological results obtained in the field of anticancer half-sandwich Rh complexes.

Impact of the Metal Center and Leaving Group on the Anticancer Activity of Organometallic Complexes of Pyridine-2-carbothioamide

Ru^II(cym)Cl (cym = η⁶-p-cymene) complexes of pyridinecarbothioamides have shown potential for development as orally active anticancer metallodrugs, underlined by their high selectivity towards plectin as the molecular target. In order to investigate the impact of the metal center on the anticancer activity and their physicochemical properties, the Os(cym), Rh- and Ir(Cp*) (Cp* = pentamethylcyclopentadienyl) analogues of the most promising and orally active compound plecstatin 2 were prepared and characterized by spectroscopic techniques and X-ray diffraction analysis. Dissolution in aqueous medium results in quick ligand exchange reactions; however, over time no further changes in the ¹H NMR spectra were observed. The Rh- and Ir(Cp*) complexes were investigated for their reactions with amino acids, and while they reacted with Cys, no reaction with His was observed. Studies on the in vitro anticancer activity identified the Ru derivatives as the most potent, independent of their halido leaving group, while the Rh derivative was more active than the Ir analogue. This demonstrates that the metal center has a significant impact on the anticancer activity of the compound class.