Due to the increasing impact of cancer on worldwide mortality, more and more attention is being devoted to the investigation of novel anticancer strategies. Among these, chemotherapy plays a key role in fighting cancer. This explains the increasing engagement of both pharmaceutical industry and academia towards the discovery of new chemotherapeutic agents. Over the recent years, metal-based drugs have attracted much attention due to their atypical physico-chemical properties compared to organic molecules. After the approval of cisplatin as a chemotherapeutic agent in 1978, several types of metal-based drugs have been explored. Among them, Ru-based anticancer drug candidates have become a central subject in this research field. However, most of the Ru-based compounds investigated over the last two decades express their cytotoxicity with a mechanism of action involving, among others, a ligand-exchange mechanism. In this Review, we give a complete overview of a specific class of antiproliferative ruthenium complexes, namely coordinatively saturated and substitutionally inert Ru(II) polypyridyl complexes. This implies that the cytotoxicity observed comes from the entire complex and not from a ligand-exchange. In this Review, we are presenting monomeric and dimeric Ru(II) polypyridyl complexes, which have been found to be toxic to cancer cells. More specifically, the monomeric Ru(II) polypyridyl complexes are analysed considering their direct interaction or not with DNA as cause of cell death, while dimeric Ru(II) polypyridyl complexes, are classified according to their biological targets. Very importantly, the cellular targets of these complexes are discussed in detail. Indeed, several targets were identified and different mechanisms of action were suggested.

Chemotherapy remains one of the dominant treatments to cure cancer. However, due to the many inherent drawbacks, there is a surge for new chemotherapeutic drugs. More specifically, the discovery of new drug candidates able to overcome severe side effects, the occurrence of resistance and the inefficacy toward metastatic tumours is highly desirable. In this work, we designed a new chemotherapeutic drug candidate against cancer, namely [Ru(DIP)2(sq)]PF6 (Ru-sq) (DIP = 4,7-diphenyl-1,10-phenanthroline; sq = semiquinonate ligand). The aim was to combine the great potential expressed by Ru(II) polypyridyl complexes and the singular redox and biological properties associated to the catecholate moiety. Several pieces of experimental evidence (e.g., X-ray crystallography, electron paramagnetic resonance, electrochemistry) demonstrate that the semiquinonate is the preferred oxidation state of the dioxo ligand in this complex. The biological activity of Ru-sq was then scrutinised in vitro and in vivo, and the results highlight the tremendous potential of this complex as a chemotherapeutic agent against cancer. Ru-sq was notably found have a much higher cytotoxic activity than cisplatin on several cell lines (i.e. in the nanomolar range), and, contrary to cisplatin, to have mitochondrial disfunction as one of its modes of action. The multicellular targets of Ru-sq could potentially be the key to overcome one of the main drawbacks of cisplatin i.e. the occurrence of resistance. Moreover, Ru-sq exhibited impressing activity on Multi Cellular Tumour Spheroids (MCTS) model, leading to a growth inhibition of the tumour even 13 days after treatment (20 μM). Very importantly, using two different in vivo models, it could be demonstrated that this compound is extremely well-tolerated by mice and has a very promising activity, curing, in some cases, tumour-bearing mice.<br>

Due to the great potential expressed by an anticancer drug candidate previously reported by our group, namely, Ru-sq ([Ru(DIP)2(sq)](PF6) (DIP: 4,7-diphenyl-1,10phenanthroline, sq: semiquinonate ligand), we describe in this work a structure-activity relationship (SAR) that involves a broader range of derivatives resulting from the coordination of different catecholate-type dioxo ligands to the same Ru(DIP)2 core. More in detail, we chose catechols carrying either electron-donating or electronwithdrawing groups EDG or EWG and investigated the physico-chemical and biological properties of their complexes. Several pieces of experimental evidences demonstrated that the coordination of catechols bearing EDGs led to deep red positively charged complexes 1-4 in which the preferred oxidation state of the dioxo ligand is the uninegatively charged semiquinonate. Complexes 5 and 6, on the other hand, are blue/violet neutral complexes, which carry an EWG substituted dinegatively charged catecholate ligand. The biological investigation of complexes 1-6 led to the conclusion that the difference in their physico-chemical properties has a strong impact on their biological activity. Thus, complexes 1-4 expressed much higher cytotoxicities than complexes 5 and 6. Complex 1 constitutes the most promising compound of the series and was selected for a more in-depth biological investigation. Apart from its remarkably high cytotoxicity (IC50 = 0.07-0.7 µM in different cancerous cell lines) complex 1 was taken up by HeLa cells very efficiently by a passive transportation mechanism. Moreover, its moderate accumulation in several cellular compartments (i.e. nucleus, lysosomes, mitochondria and cytoplasm) is extremely advantageous in the search of a potential drug with multiple modes of action. Further DNA metalation and metabolic studies pointed to the direct interaction of complex 1 with DNA and to the severe impairment of the mitochondrial function. Multiple targets, together with its 4 outstanding cytotoxicity, make complex 1 a valuable candidate in the field of chemotherapy research. Noteworthy, a preliminary biodistribution study on healthy mice demonstrated the suitability of complex 1 for further in vivo studies.

The global spread of the severe acute respiratory syndrome coronavirus 2 (SARS‐CoV‐2) has called for an urgent need for dedicated antiviral therapeutics. Metal complexes are commonly underrepresented in compound libraries that are used for screening in drug discovery campaigns, however, there is growing evidence for their role in medicinal chemistry. Based on previous results, we have selected more than 100 structurally diverse metal complexes for profiling as inhibitors of two relevant SARS‐CoV‐2 replication mechanisms, namely the interaction of the spike (S) protein with the ACE2 receptor and the papain‐like protease PL pro . In addition to many well‐established types of mononuclear experimental metallodrugs, the pool of compounds tested was extended to approved metal‐based therapeutics such as silver sulfadiazine and thiomersal, as well as polyoxometalates (POMs). Among the mononuclear metal complexes, only a small number of active inhibitors of the S/ACE2 interaction was identified, with titanocene dichloride as the only strong inhibitor. However, among the gold and silver containing complexes many turned out to be very potent inhibitors of PL pro activity. Highly promising activity against both targets was noted for many POMs. Selected complexes were evaluated in antiviral SARS‐CoV‐2 assays confirming activity for gold complexes with N‐heterocyclic carbene (NHC) or dithiocarbamato ligands, a silver NHC complex, titanocene dichloride as well as a POM compound. These studies might provide starting points for the design of metal‐based SARS‐CoV‐2 antiviral agents.

Cancer is one of the main causes of death worldwide. Chemotherapy, despite its severe side effects, is to date one of the leading strategies against cancer. Metal‐based drugs present several potential advantages when compared to organic compounds and they have gained trust from the scientific community after the approval on the market of the drug cisplatin. Recently, we reported the ruthenium complex ([Ru(DIP)2(sq)](PF6) (where DIP is 4,7‐diphenyl‐1,10‐phenantroline and sq is semiquinonate) with a remarkable potential as chemotherapeutic agent against cancer, both in vitro and in vivo. In this work, we analyse a structurally similar compound, namely [Ru(DIP)2(mal)](PF6), carrying the flavour‐enhancing agent approved by the FDA, maltol (mal). To possess an FDA approved ligand is crucial for a complex, whose mechanism of action might include ligand exchange. Herein, we describe the synthesis and characterisation of [Ru(DIP)2(mal)](PF6), its stability in solutions and under conditions that resemble the physiological ones, and its in‐depth biological investigation. Cytotoxicity tests on different cell lines in 2D model and on HeLa MultiCellular Tumour Spheroids (MCTS) demonstrated that our compound has higher activity than cisplatin, inspiring further tests. [Ru(DIP)2(mal)](PF6) was efficiently internalised by HeLa cells through a passive transport mechanism and severely affected the mitochondrial metabolism.

Four novel monocationic Ru(II) polypyridyl complexes have been synthesized with the general formula [Ru(DIP)2flv]X, where DIP is 4,7-diphenyl-1,10-phenanthroline, flv stands for the flavonoid ligand (5-hydroxyflavone in [Ru(DIP) 2 (5-OHF)](PF 6), genistein in [Ru(DIP)2(gen)](PF6), chrysin in [Ru(DIP)2(chr)](OTf), and morin in [Ru(DIP)2(mor)](OTf)) and X is the counterion, PF6, and OTf ̄ (triflate, CF3SO3̄), respectively. Following the chemical characterisation of the complexes by 1 H and 13 C-NMR, mass spectrometry and elemental analysis, their cytotoxicity was tested against several cancer cell lines. The most promising complex, [Ru(DIP) 2 (gen)](PF 6), was further investigated for its biological activity. Metabolic studies revealed that this complex severely impaired mitochondrial respiration and glycolysis processes, contrary to its precursor, Ru(DIP)2Cl2, which showed a prominent effect only on the mitochondrial respiration. In addition, its preferential accumulation in MDA-MB-435S cells (a human melanoma cell line previously described as mammary gland/breast; derived from metastatic site: pleural effusion), that are used for the study of metastasis, explained the better activity in this cell line compared to MCF-7 (human, ductal carcinoma).

Chemotherapy remains one of the dominant treatments to cure cancer. However, due to the many inherent drawbacks, there is a surge for new chemotherapeutic drugs. Many classes of compounds have been investigated over the years in order to discover new targets and synergistic mechanisms of action including multicellular targets. In this work, we designed a new chemotherapeutic drug candidate against cancer, namely [Ru(DIP)2(sq)]PF6 (Ru-sq) (DIP = 4,7-diphenyl-1,10-phenanthroline; sq = semiquinonate ligand). The aim was to combine the great potential expressed by Ru(II) polypyridyl complexes and the singular redox and biological properties associated to the catecholate moiety. Experimental evidences (e.g. X-ray crystallography, electron paramagnetic resonance, electrochemistry) demonstrate that the semiquinonate is the preferred oxidation state of the dioxo ligand in this complex. The biological activity of Ru-sq was then scrutinised in vitro and in vivo, and the results highlight the auspicious potential of this complex as a chemotherapeutic agent against cancer.

Poor aqueous solubility is one of the recurrent drawbacks of many compounds in medicinal chemistry. To overcome this limitation, the dilution of drug candidates from stock solutions of an organic solvent is common practice. However, the precise characterisation of these compounds in aqueous solutions is often neglected, leading to some uncertainties regarding the nature of the actual active species. In this communication, we demonstrate that two ruthenium complexes previously reported by our group for their chemotherapeutic potential against cancer, namely [Ru(DIP)2(sq)](PF6) and [Ru(DIP)2(3‐methoxysq)](PF6), where DIP is 4,7‐diphenyl‐1,10‐phenanthroline, sq=semiquinonate and 3‐methoxysq=3‐methoxysemiquinonate, form colloids in water‐DMSO (1 % v/v) mixtures that are invisible to the naked eyes. [Ru(DIP)2(3‐methoxysq)](PF6) was found to form a highly stable and monodispersed colloid with nanoaggregates of ∼25 nm. In contrast, [Ru(DIP)2(sq)](PF6) was found to form large reticulates of mostly spherical aggregates which size was found to increase over time. The difference in size and shape distribution of drug candidates is of tremendous significance as the study of their biological activity might be severely affected. Overall, we strongly believe that these observations should be taken into account by the scientific community working on the development of metal‐based drugs with poor water solubility.