Small Structural Differences between Two Ferrocenyl Diphenols Determine Large Discrepancies of Reactivity and Biological Effects

Thioredoxin Reductase and Organometallic Complexes: A Pivotal System to Tackle Multidrug Resistant Tumors?

Cancers classified as multidrug-resistant (MDR) are a family of diseases with poor prognosis despite access to increasingly sophisticated treatments. Several mechanisms explain these resistances involving both tumor cells and their microenvironment. It is now recognized that a multi-targeting approach offers a promising strategy to treat these MDR tumors. Inhibition of thioredoxin reductase (TrxR), a key enzyme in maintaining redox balance in cells, is a well-identified target for this approach. Auranofin was the first inorganic gold complex to be described as a powerful inhibitor of TrxR. In this review, we will first recall the main results obtained with this metallodrug. Then, we will focus on organometallic complexes reported as TrxR inhibitors. These include gold(I), gold(III) complexes and metallocifens, i.e., organometallic complexes of Fe and Os derived from tamoxifen. In these families of complexes, similarities and differences in the molecular mechanisms of TrxR inhibition will be highlighted. Finally, the possible relationship between TrxR inhibition and cytotoxicity will be discussed and put into perspective with their mode of action.

Unravelling the Role of Uncommon Hydrogen Bonds in Cyclodextrin Ferrociphenol Supramolecular Complexes: A Computational Modelling and Experimental Study

We sought to determine the cyclodextrins (CDs) best suited to solubilize a patented succinimido-ferrocidiphenol (SuccFerr), a compound from the ferrociphenol family having powerful anticancer activity but low water solubility. Phase solubility experiments and computational modelling were carried out on various CDs. For the latter, several CD-SuccFerr complexes were built starting from combinations of one or two CD(s) where the methylation of CD oxygen atoms was systematically changed to end up with a database of ca. 13 k models. Modelling and phase solubility experiments seem to indicate the predominance of supramolecular assemblies of SuccFerr with two CDs and the superiority of randomly methylated β-cyclodextrins (RAMEβCDs). In addition, modelling shows that there are several competing combinations of inserted moieties of SuccFerr. Furthermore, the models show that ferrocene can contribute to high stabilization by making atypical hydrogen bonds between Fe and the hydroxyl groups of CDs (single bond with one OH or clamp with two OH of the same glucose unit).

Succinimido–Ferrocidiphenol Complexed with Cyclodextrins Inhibits Glioblastoma Tumor Growth In Vitro and In Vivo without Noticeable Adverse Toxicity

SuccFerr (N-[4-ferrocenyl,5-5-bis (4-hydroxyphenyl)-pent-4-enyl]-succinimide) has remarkable antiproliferative effects in vitro, attributed to the formation of a stabilized quinone methide. The present article reports in vivo results for a possible preclinical study. SuccFerr is lipophilic and insoluble in water, so the development of a formulation to obviate this inconvenience was necessary. This was achieved by complexation with randomly methylated cyclodextrins (RAMEßCDs). This supramolecular water-soluble system allowed the in vivo experiments below to proceed. Application of SuccFerr on the glioblastoma cancer cell line U87 indicates that it affects the cellular cycle by inducing a blockade at G0/G1 phase, linked to apoptosis, and another one at the S phase, associated with senescence. Using healthy Fischer rats, we show that both intravenous and subcutaneous SuccFerr: RAMEßCD administration at 5 mg/kg lacks toxic effects on several organs. To reach lethality, doses higher than 200 mg/kg need to be administered. These results prompted us to perform an ectopic in vivo study at 1 mg/kg i.v. ferrocidiphenol SuccFerr using F98 cells xenografted in rats. Halting of cancer progression was observed after six days of injection, associated with an immunological defense response linked to the active principle. These results demonstrate that the properties of the selected ferrocidiphenol SuccFerr transfer successfully to in vivo conditions, leading to interesting therapeutic perspectives based on this chemistry.

α-Hydroxylactams as Efficient Entries to Diversely Functionalized Ferrociphenols: Synthesis and Antiproliferative Activity Studies

The [ferrocene-ene-phenol] motif has been identified as the pharmacophore responsible for the anticancer activity of the family of ferrocene-based molecules coined ferrocifens, owing to its unique redox properties. The addition of imide entities to the historical ferrociphenol scaffold tremendously enhanced the cytotoxic activity of a large panel of cancer cell cultures and preliminary studies showed that the reduction of one of the carbonyl groups of the imide groups to the corresponding α-hydroxylactams only slightly affected the antiproliferative activity. As a continuation to these studies, we took advantage of the facile conversion of α-hydroxylactams to highly electrophilic N-acyliminium ions to graft various substituents to the imide motif of phthalimido ferrocidiphenol. Cell viability studies showed that the newly synthesized compounds showed diverse cytotoxic activities on two breast cancer cell lines, while only one compound was significantly less active on the non-tumorigenic cell line hTERT-RPE1.

Heterogeneity of Response to Iron-Based Metallodrugs in Glioblastoma Is Associated with Differences in Chemical Structures and Driven by FAS Expression Dynamics and Transcriptomic Subtypes

Glioblastoma (GBM) is the most frequent and deadliest primary brain cancer in adults, justifying the search for new treatments. Some members of the iron-based ferrocifen family have demonstrated a high cytotoxic effect on various cancer cell lines via innovative mechanisms of action. Here, we evaluated the antiproliferative activity by wst-1 assay of six ferrocifens in 15 molecularly diverse GBM patient-derived cell lines (PDCLs). In five out of six compounds, the half maximal inhibitory concentration (IC₅₀) values varied significantly (10 nM < IC₅₀ < 29.8 µM) while the remaining one (the tamoxifen-like complex) was highly cytotoxic against all PDCLs (mean IC₅₀ = 1.28 µM). The pattern of response was comparable for the four ferrocifens bearing at least one phenol group and differed widely from those of the tamoxifen-like complex and the complex with no phenol group. An RNA sequencing differential analysis showed that response to the diphenol ferrocifen relied on the activation of the Death Receptor signaling pathway and the modulation of FAS expression. Response to this complex was greater in PDCLs from the Mesenchymal or Proneural transcriptomic subtypes compared to the ones from the Classical subtype. These results provide new information on the mechanisms of action of ferrocifens and highlight a broader diversity of behavior than previously suspected among members of this family. They also support the case for a molecular-based personalized approach to future use of ferrocifens in the treatment of GBM.

Thioredoxin reductase as a pharmacological target

Thioredoxin reductases (TrxRs) belong to the pyridine nucleotide disulfide oxidoreductase family enzymes that reduce thioredoxin (Trx). The couple TrxR and Trx is one of the major antioxidant systems that control the redox homeostasis in cells. The thioredoxin system, comprised of TrxR, Trx and NADPH, exerts its activities via a disulfide-dithiol exchange reaction. Inhibition of TrxR is an important clinical goal in all conditions in which the redox state is perturbed. The present review focuses on the most critical aspects of the cellular functions of TrxRs and their inhibition mechanisms by metal ions or chemicals, through direct targeting of TrxRs or their substrates or protein interactors. To update the involvement of overactivation/dysfunction of TrxRs in various pathological conditions, human diseases associated with TrxRs genes were critically summarized by publicly available genome-wide association study (GWAS) catalogs and literature. The pieces of evidence presented here justify why TrxR is recognized as one of the most critical clinical targets and the growing current interest in developing molecules capable of interfering with the functions of TrxR enzymes.

The photodynamic and intrinsic effects of Azure B on mitochondrial bioenergetics and the consequences of its intrinsic effects on hepatic energy metabolism

BACKGROUND

The present study aimed to characterize the intrinsic and photodynamic effects of azure B (AB) on mitochondrial bioenergetics, as well as the consequences of its intrinsic effects on hepatic energy metabolism.

METHODS

Two experimental systems were utilized: (a) isolated rat liver mitochondria and (b) isolated perfused rat liver.

RESULTS

AB interacted with mitochondria regardless of photostimulation, but its binding degree was reduced by mitochondrial energization. Under photostimulation, AB caused lipid peroxidation and protein carbonylation and decreased the content of reduced glutathione (GSH) in mitochondria. AB impaired mitochondrial bioenergetics in at least three distinct ways: (1) uncoupling of oxidative phosphorylation; (2) photoinactivation of complexes I and II; and (3) photoinactivation of the F_oF₁-ATP synthase complex. Without photostimulation, AB also demonstrated mitochondrial toxicity, which was characterized by the induction of lipid peroxidation, loss of inner mitochondrial membrane integrity, and uncoupling of oxidative phosphorylation. The perfused rat liver experiments showed that mitochondria were one of the major targets of AB, even in intact cells. AB inhibited gluconeogenesis and ureagenesis, two biosynthetic pathways strictly dependent on intramitochondrially generated ATP. Contrariwise, AB stimulated glycogenolysis and glycolysis, which are required compensatory pathways for the inhibited oxidative phosphorylation. Similarly, AB reduced the cellular ATP content and the ATP/ADP and ATP/AMP ratios.

CONCLUSIONS

Although the properties and severe photodynamic effects of AB on rat liver mitochondria might suggest its usefulness in PDT treatment of liver tumors, this possibility should be considered with precaution given the toxic intrinsic effects of AB on mitochondrial bioenergetics and energy-linked hepatic metabolism.

The photosensitiser azure A disrupts mitochondrial bioenergetics through intrinsic and photodynamic effects

Azure A (AA) is a cationic molecule of the class of phenothiazines that has been applied in vitro as a photosensitising agent in photodynamic antimicrobial chemotherapy. It is a di-demethylated analogue of methylene blue (MB), which has been demonstrated to be intrinsically and photodynamically highly active on mitochondrial bioenergetics. However, as far as we know, there are no studies about the photodynamic effects of AA on mammalian mitochondria. Therefore, this investigation aimed to characterise the intrinsic and photodynamic acute effects of AA (0.540 μM) on isolated rat liver mitochondria, isolated hepatocytes, and isolated perfused rat liver. The effects of AA were assessed by evaluating several parameters of mitochondrial bioenergetics, oxidative stress, cell viability, and hepatic energy metabolism. The photodynamic effects of AA were assessed under simulated hypoxic conditions, a suitable way for mimicking the microenvironment of hypoxic solid tumour cells. AA interacted with the mitochondria and, upon photostimulation (10 min of light exposure), produced toxic amounts of reactive oxygen species (ROS), which damaged the organelle, as demonstrated by the high levels of lipid peroxidation and protein carbonylation. The photostimulated AA also depleted the GSH pool, which could compromise the mitochondrial antioxidant defence. Bioenergetically, AA photoinactivated the complexes I, II, and IV of the mitochondrial respiratory chain and the F1FO-ATP synthase complex, sharply inhibiting the oxidative phosphorylation. Upon photostimulation (10 min of light exposure), AA reduced the efficiency of mitochondrial energy transduction and oxidatively damaged lipids in isolated hepatocytes but did not decrease the viability of cells. Despite the useful photobiological properties, AA presented noticeable dark toxicity on mitochondrial bioenergetics, functioning predominantly as an uncoupler of oxidative phosphorylation. This harmful effect of AA was evidenced in isolated hepatocytes, in which AA diminished the cellular ATP content. In this case, the cells exhibited signs of cell viability reduction in the presence of high AA concentrations, but only after a long time of incubation (at least 90 min). The impairments on mitochondrial bioenergetics were also clearly manifested in intact perfused rat liver, in which AA diminished the cellular ATP content and stimulated the oxygen uptake. Consequently, gluconeogenesis and ureogenesis were strongly inhibited, whereas glycogenolysis and glycolysis were stimulated. AA also promoted the release of cytosolic and mitochondrial enzymes into the perfusate concomitantly with inhibition of oxygen consumption. In general, the intrinsic and photodynamic effects of AA were similar to those of MB, but AA caused some distinct effects such as the photoinactivation of the complex IV of the mitochondrial respiratory chain and a diminution of the ATP levels in the liver. It is evident that AA has the potential to be used in mitochondria-targeted photodynamic therapy, even under low oxygen concentrations. However, the fact that AA directly disrupts mitochondrial bioenergetics and affects several hepatic pathways that are linked to ATP metabolism, along with its ability to perturb cellular membranes and its little potential to reduce cell viability, could result in significant adverse effects especially in long-term treatments.

Ferrocifen Loaded Lipid Nanocapsules: A Promising Anticancer Medication against Multidrug Resistant Tumors

Resistance of cancer cells to current chemotherapeutic drugs has obliged the scientific community to seek innovative compounds. Ferrocifens, lipophilic organometallic compounds composed of a tamoxifen scaffold covalently bound to a ferrocene moiety, have shown very interesting antiproliferative, cytotoxic and immunologic effects. The formation of ferrocenyl quinone methide plays a crucial role in the multifaceted activity of ferrocifens. Lipid nanocapsules (LNCs), meanwhile, are nanoparticles obtained by a free organic solvent process. LNCs consist of an oily core surrounded by amphiphilic surfactants and are perfectly adapted to encapsulate these hydrophobic compounds. The different in vitro and in vivo experiments performed with this ferrocifen-loaded nanocarrier have revealed promising results in several multidrug-resistant cancer cell lines such as glioblastoma, breast cancer and metastatic melanoma, alone or in combination with other therapies. This review provides an exhaustive summary of the use of ferrocifen-loaded LNCs as a promising nanomedicine, outlining the ferrocifen mechanisms of action on cancer cells, the nanocarrier formulation process and the in vivo results obtained over the last two decades.

Recent advances in iron-complexes as drug candidates for cancer therapy: reactivity, mechanism of action and metabolites

In this perspective, we discuss iron-complexes as drug candidates that are promising alternatives to conventional platinum-based chemotherapies owing to their broad range of reactivities and to the targeting of different biological systems. Breakthroughs in the comprehension of iron complexes' structure-activity relationship contributed to the clarification of their metabolization pathways, sub-cellular localization and influence on iron homeostasis, while enlightening the primary molecular targets of theses likely multi-target metallodrugs. Both the antiproliferative activity and elevated safety index observed among the family of iron complexes showed encouraging results as per their therapeutic potential and selectivity also with the aim of reducing chemotherapy side-effects, and facilitated more pre-clinical investigations. The purpose of this perspective is to summarize the recent advances that contributed in unveiling the intricate relationships between the structural modifications on iron-complexes and their reactivity, cellular trafficking and global mechanisms of action to broaden their use as anticancer drugs and advance to clinical evaluation.

Enantioselective Synthesis of Planar Chiral Ferrocifens that Show Chiral Discrimination in Antiproliferative Activity on Breast Cancer Cells

The design and first enantioselective synthesis of a series of chiral ferrocifens and ferrociphenols was realised by enantioselective palladium-catalysed intramolecular direct C-H bond activation followed by McMurry coupling. Biological evaluation revealed moderate anticancer activities on breast cancer cells and evidence of chiral discrimination between enantiomers. Treatment of the novel ferrocifens with Ag₂ O revealed that these systems are unable to form a neutral quinone methide, yet still demonstrate marked antiproliferative properties against both the hormone-dependent MCF-7 and hormone-independent MDA-MB-231 cell lines. This bioactivity arises from two mechanisms: Fenton-type chemistry and the anti-estrogenic activity associated with the tamoxifen-like structure.

Ferrocene-containing hybrids as potential anticancer agents: Current developments, mechanisms of action and structure-activity relationships

Cancer is one of the most fatal threatens to human health throughout the world. The major challenges in the control and eradication of cancers are the continuous emergency of drug-resistant cancer and the low specificity of anticancer agents, creating an urgent need to develop novel anticancer agents. Organometallic compounds especially ferrocene derivatives possess remarkable structural and mechanistic diversity, inherent stability towards air, heat and light, low toxicity, low cost, reversible redox, ligand exchange, and catalytic properties, making them promising drug candidates for cancer therapy. Ferrocifen, a ferrocene-phenol hybrid, has demonstrated promising anticancer properties on drug-resistant cancers. Currently, Ferrocifen is in pre-clinical trial against cancers. Obviously, ferrocene moiety is a useful template for the development of novel anticancer agents. This review will provide an overview of ferrocene-containing hybrids with potential application in the treatment of cancers covering articles published between 2010 and 2020. The mechanisms of action, the critical aspects of design and structure-activity relationships are also discussed.