Emerging Metallopharmaceuticals for the Treatment of Cancer

Cobalt(III)-Macrocyclic Scaffolds with Anti-Cancer Stem Cell Activity

Cobalt(III) compounds with tetradentate ligands have been widely employed to deliver cytotoxic and imaging agents into cells. A large body of work has focused on using cobalt(III)-cyclam scaffolds for this purpose. Here, we investigate the cytotoxic properties of cobalt(III) complexes containing 14-membered macrocycles related to cyclam. A breast cancer stem cell (CSC) in vitro model was used to gauge efficacy. Specifically, [Co(1,4,7,11-tetraazacyclotetradecane)Cl2]+ (1) and [Co(1-oxa-4,8,12-triazacyclotetradecane)Cl2]+ (2) were synthesised and characterised, and their breast CSC activity was determined. The cobalt(III) complexes 1 and 2 displayed micromolar potency towards bulk breast cancer cells and breast CSCs grown in monolayers. Notably, 1 and 2 displayed selective potency towards breast CSCs over bulk breast cancer cells (up to 4.5-fold), which was similar to salinomycin (an established breast CSC-selective agent). The cobalt(III) complexes 1 and 2 were also able to inhibit mammosphere formation at low micromolar doses (with respect to size and number). The mammopshere inhibitory effect of 2 was similar to that of salinomycin. Our studies show that cobalt(III) complexes with 1,4,7,11-tetraazacyclotetradecane and 1-oxa-4,8,12-triazacyclotetradecane macrocycles could be useful starting points for the development of new cobalt-based delivery systems that can transport cytotoxic and imaging agents into breast CSCs.

Novel Re(I) Complexes as Potential Selective Theranostic Agents in Cancer Cells and In Vivo in Caenorhabditis elegans Tumoral Strains

A series of rhenium(I) complexes of the type fac-[Re(CO)3(N^N)L]0/+, Re1-Re9, was synthesized, where N^N = benzimidazole-derived bidentate ligand with an ester functionality and L = chloride or pyridine-type ligand. The new compounds demonstrated potent activity toward ovarian A2780 cancer cells. The most active complexes, Re7-Re9, incorporating 4-NMe2py, exhibited remarkable activity in 3D HeLa spheroids. The emission in the red region of Re9, which contains an electron-deficient benzothiazole moiety, allowed its operability as a bioimaging tool for in vitro and in vivo visualization. Re9 effectivity was tested in two different C. elegans tumoral strains, JK1466 and MT2124, to broaden the oncogenic pathways studied. The results showed that Re9 was able to reduce the tumor growth in both strains by increasing the ROS production inside the cells. Moreover, the selectivity of the compound toward cancerous cells was remarkable as it did not affect neither the development nor the progeny of the nematodes.

Synthesis and evaluation of WK-X-34 derivatives as P-glycoprotein (P-gp/ABCB1) inhibitors for reversing multidrug resistance

The emergence of multidrug resistance (MDR) in malignant tumors is one of the leading threats encountered currently by many chemotherapeutic agents. A proposed strategy to overcome MDR is to disable the efflux function of P-glycoprotein (P-gp/ABCB1), a critical member of the ABC transporter family that significantly increases the efflux of various anticancer drugs from tumor cells. In this study, structural modification of a third-generation P-gp inhibitor WK-X-34 based on bioisosteric and fragment-growing strategies led to the discovery of the adamantane derivative PID-9, which exhibited the best MDR reversal activity (IC50 = 0.1338 μM, RF = 78.6) in this series, exceeding those of the reported P-gp inhibitors verapamil and WK-X-34. In addition, compared with WK-X-34, PID-9 showed decreased toxicity to cells. Furthermore, the mechanism studies revealed that the reversal activity of adamantane derivatives PID-5, PID-7, and PID-9 stemmed from the inhibition of P-gp efflux. These results indicated that compound PID-9 is the most effective P-gp inhibitor among them with low toxicity and high MDR reversal activity, which provided a fundamental structural reference for further discovery of novel, effective, and non-toxic P-gp inhibitors.

The Bulk Breast Cancer Cell and Breast Cancer Stem Cell Activity of Binuclear Copper(II)-Phenanthroline Complexes

Mononuclear copper(II)-phenanthroline complexes have been widely investigated as anticancer agents whereas multinuclear copper(II)-phenanthroline complexes are underexplored. Here the synthesis and characterisation of two new binuclear copper(II)-phenanthroline complexes 1 and 2 is reported, comprising of 2,9-dimethyl-1,10-phenanthroline or 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline, terminal chloride ligands, and bridging chloride or hydroxide ligands. The binuclear copper(II) complex containing 2,9-dimethyl-1,10-phenanthroline 1 displays nanomolar toxicity towards bulk breast cancer cells and breast cancer stem cells (CSCs) grown in monolayers, >50-fold greater than cisplatin (an anticancer metallodrug) and salinomycin (a gold-standard anti-CSC agent). Spectacularly, 1 exhibits >100-fold greater potency toward three-dimensionally cultured mammospheres than cisplatin and salinomycin. Mechanistic studies show that 1 evokes breast CSC apoptosis by elevating intracellular reactive oxygen species levels and damaging genomic DNA (possibly by an oxidative mechanism). To the best of our knowledge, this is the first study to probe the anti-breast CSC properties of binuclear copper(II)-phenanthroline complexes.

Payload Release Profile and Anti-Cancer Stem Cell Properties of Compositionally Different Polymeric Nanoparticles Containing a Copper(II) Complex

Cancer stem cells (CSCs) are linked to tumour relapse and metastasis, the main reason for cancer-related deaths. The application of polymeric nanoparticles as drug delivery systems to target CSCs is relatively unexplored. Here, we report the encapsulation of a CSC-potent copper(II) complex 1 by two compositionally different methoxy poly(ethylene glycol)-b-poly(D,L-lactic-co-glycolic) acid (PEG–PLGA) copolymers. Specifically, we used PEG–PLGA (5000:10,000 Da, 1:1 LA:GA) and PEG–PLGA (5000:10,000 Da, 4:1 LA:GA) polymers to prepare spherical nanoparticle formulations 1:1 NP15 and 4:1 NP15, respectively, both with a 15% feed of 1. The two formulations show distinct biophysical and in vitro properties. For example, (i) 4:1 NP15 displays a slower payload release profile than 1:1 NP15 in physiologically relevant solutions, (ii) 4:1 NP15 exhibits statistically greater potency towards breast CSCs than bulk breast cancer cells grown in monolayers, whereas 1:1 NP15 is equally potent towards breast CSCs and bulk breast cancer cells, and (iii) 4:1 NP15 shows significantly greater potency towards three-dimensionally cultured mammospheres than 1:1 NP15. This study shows that the release profile and anti-breast CSC properties of PEG–PLGA nanoparticle formulations (containing 1) can be perturbed (and possibly controlled) by modifying the proportion of glycolic acid within the PLGA component.

Towards Selective Delivery of a Ruthenium(II) Polypyridyl Complex-Containing Bombesin Conjugate into Cancer Cells

An increasing number of novel Ru(II) polypyridyl complexes have been successfully applied as photosensitizers (PSs) for photodynamic therapy (PDT). Despite recent advances in optimized PSs with refined photophysical properties, the lack of tumoral selectivity is often a major hurdle for their clinical development. Here, classical maleimide and versatile NHS-activated acrylamide strategies were employed to site-selectively conjugate a promising Ru(II) polypyridyl complex to the N-terminally Cys-modified Bombesin (BBN) targeting unit. Surprisingly, the decreased cell uptake of these novel Ru-BBN conjugates in cancer cells did not hamper the high phototoxic activity of the Ru-containing bioconjugates and even decreased the toxicity of the constructs in the absence of light irradiation. Overall, although deceiving in terms of selectivity, our new bioconjugates could still be useful for advanced cancer treatment due to their nontoxicity in the dark.

Targeting chemotherapy-resistant tumour sub-populations using inorganic chemistry: Anti-cancer stem cell metal complexes

Cancer stem cells (CSC) are a sub-population of tumours linked to metastasis and relapse. Current chemotherapeutic drug options are ineffective against CSCs at their administered doses. New families of cytotoxic agents, and new, highly specific ways of delivering them to CSCs, are needed to provide durable clinical outcomes. Inorganic compounds have recently emerged as a promising class of anti-CSC agents with clinically relevant potencies. In this short review, we present the very latest efforts (post-2020) on the development of anti-CSC metal complexes. The activities of the metal complexes in monolayer and three-dimensional CSC cultures and animal models is documented. The mechanism of action of the metal complexes with respect to their chemical structures is also highlighted.

Antioxidant and Anticancer Potential of the New Cu(II) Complexes Bearing Imine-Phenolate Ligands with Pendant Amine N-Donor Groups

Cu(II) complexes bearing NNO-donor Schiff base ligands (2a, b) have been synthesized and characterized. The single crystal X-ray analysis of the 2a complex revealed that a mononuclear and a dinuclear complex co-crystallize in the solid state. The electronic structures of the complexes are optimized by Density Functional Theory (DFT) calculations. The monomeric nature of 2a and 2b species is maintained in solution. Antioxidant activities of the ligands (1a, b) and Cu(II) complexes (2a, b) were determined by in vitro assays such as 1,1-diphenyl-2-picrylhydrazyl free radicals (DPPH^.) and 2,2'-azino-bis (3-ethylbenzothiazoline-6-sulfonic acid) radicals (ABTS⁺). Our results demonstrated that 2a showed better antioxidant activity. MTT assays were performed to assess the toxicity of ligands and Cu(II) complexes in V79 cells. The antiproliferative activity of compounds was tested against two human tumor cell lines: MCF-7 (breast adenocarcinoma) and SW620 (colorectal carcinoma) and on MRC-5 (normal lung fibroblast). All compounds showed high cytotoxicity in the all-cell lines but showed no selectivity for tumor cell lines. Antiproliferative activity by clonogenic assay 2b showed a more significant inhibitory effect on the MCF-7 cell lines than on MRC-5. DNA damage for the 2b compound at 10 µM concentration was about three times higher in MCF-7 cells than in MRC-5 cells.

Isolation, Detection and Analysis of Circulating Tumour Cells: A Nanotechnological Bioscope

Cancer is one of the dreaded diseases to which a sizeable proportion of the population succumbs every year. Despite the tremendous growth of the health sector, spanning diagnostics to treatment, early diagnosis is still in its infancy. In this regard, circulating tumour cells (CTCs) have of late grabbed the attention of researchers in the detection of metastasis and there has been a huge surge in the surrounding research activities. Acting as a biomarker, CTCs prove beneficial in a variety of aspects. Nanomaterial-based strategies have been devised to have a tremendous impact on the early and rapid examination of tumor cells. This review provides a panoramic overview of the different nanotechnological methodologies employed along with the pharmaceutical purview of cancer. Initiating from fundamentals, the recent nanotechnological developments toward the detection, isolation, and analysis of CTCs are comprehensively delineated. The review also includes state-of-the-art implementations of nanotechnological advances in the enumeration of CTCs, along with future challenges and recommendations thereof.

Antimicrobial activity and cytotoxicity of transition metal carboxylates derived from agaric acid

Carboxylato-type transition metal complexes with agaric acid, a bioactive natural compound derived from citric acid, were prepared, and tested in vitro for their antimicrobial activity and cytotoxicity. The products as well as agaric acid itself are amphiphilic compounds containing a hydrophilic head (citric acid moiety) and a hydrophobic tail (non-polar alkyl chain). The putative composition of the carboxylates was assigned on grounds of elemental analysis, infrared (IR) and high-resolution mass spectra (HR-MS), as well as in analogy with known complexes containing the citrate moiety. The metal carboxylates showed interesting activity in several microbial strains, especially against S. aureus (vanadium complex; MIC = 0.05 mg/ml). They were also tested for their cytotoxic activity in hepatocytes, the highest activity having been found in the copper(II) and manganese(II) complexes. Further research based on these preliminary results is needed in order to evaluate the influence of parameters like stability of the metal complexes in solution on the bioactivity of the complexes.

A review: Pharmacological aspects of metal based 1,2,4-triazole derived Schiff bases

Clinical reports have highlighted the radical increase of antibiotic resistance. As a result, multidrug resistance has emerged as a serious threat to human health. Many organic compounds commonly used as drugs in the past, no longer have pure organic mode of action rather need bio-transformation or more activation. Bulk of research has shown that they need trace amount of metal ions incorporated within the chemistry of bioactive molecules for enhancement of their potentiality to fight aggressively against resistance. The deficiency of some metal ions can also be responsible for many diseases like growth retardation, pernicious anemia and heart diseases in infants. To overcome these problems, there is a need to introduce novel strategies which have new mechanism of action along with significant spectrum of biological activity, enhanced safety and efficacy. Bioinorganic compounds have played imperative role in developing the new strategy in the form of "Metal Based Drugs". In current years there have been momentous rise of interest in the application of metal based Schiff base compounds to treat various diseases which are difficult to be treated with conventional methodologies. The unique properties of metal chelates acting as an intermediate between conventional organic and inorganic compounds provided innovative opportunities in the field of pharmaceutical chemistry. In this review, we have exclusively focused on the search of metal based 1,2,4-triazole derived Schiff base compounds (synthesized, reported and reviewed in the past ten years) that possess various biological activities such as antifungal, antibacterial, antioxidant, antidiabetic, anthelmintic, anticancer, antiproliferative, cytotoxic and DNA-intercalation activity.

A dithiacyclam-coordinated silver(i) polymer with anti-cancer stem cell activity

A cancer stem cell (CSC) active, solution stable, silver(i) polymeric complex bearing a dithiacyclam ligand is reported. The complex displays similar potency towards CSCs to salinomycin in monolayer and three-dimensional cultures. Mechanistic studies suggest CSC death results from cytosol entry, an increase in intracellular reactive oxygen species, and caspase-dependent apoptosis.

A photoactivated Ir(iii) complex targets cancer stem cells and induces secretion of damage-associated molecular patterns in melanoma cells characteristic of immunogenic cell death

The new iridium complex selectively targets cancer stem cells and has potential to induce immunogenic cell death in cancer cells.