Recent Development of Transition Metal Complexes as Chemotherapeutic Hypoxia Activated Prodrug (HAP)

Hypoxia is a state characterized by low concentration of Oxygen. Hypoxic state is often found in the central region of solid tumors. Hypoxia is associated with abnormal neovascularization resulted in poor blood flow in tissues and increased proliferation of tumor cells, imbalance between O2 supply and O2 consumption in tumor cells, high concentration of proton and strong reducibility. And, these abnormalities enhance the survival potency of the hypoxic tumours and increase the resistance towards chemotherapy and radiotherapy. One of the approach for treating hypoxic region of tumour is to use reducing environment of hypoxic tumours for reducing a molecule (hypoxia activated prodrug, HAP) and as a result the active drug will be released in hypoxic region in a controlled manner from the prodrug and kill the hypoxic tumour. Co(III) and Pt(IV) complexes with monodentate active drug molecule in the axial position can be reduced to Co(II) and Pt(II) moieties and as a result, the axial ligands (active drug) could come out from the metal center and could show its anticancer activity. In this review we have highlighted the research articles where transition metal-based complexes are used as chemotherapeutic hypoxia activated prodrug molecules which are reported in last 5 years.

Polypyridyl-based Co(III) complexes of vitamin B6 Schiff base for photoactivated antibacterial therapy

Herein, five novel polypyridyl-based Co(III) complexes of Schiff bases, viz., [Co(dpa)(L1)]Cl (1), [Co(dpa)(L2)]Cl (2), [Co(L3)(L2)]Cl (3), [Co(L3)(L1)]Cl (4), and [Co(L4)(L1)]Cl (5), where dpa (dipicolylamine) = bis(2-pyridylmethyl)amine; H2L1 = (E)-2-((2-hydroxybenzylidene)amino)phenol; H2L2 = (E)-5-(hydroxymethyl)-4-(((2-hydroxyphenyl)imino)methyl)-2-methylpyridin-3-ol; L3 = 4'-phenyl-2,2':6',2''-terpyridine (ph-tpy); and L4 = 4'-ferrocenyl-2,2':6',2''-terpyridine (Fc-tpy), were synthesized and characterized. Complexes 1, 3, and 4 were structurally characterized by single-crystal XRD, indicating an octahedral CoIIIN4O2 coordination core. The absorption bands of these complexes were observed in the visible range with a λmax at ∼430-485 nm. Complex 5 displayed an extra absorption band near 545 nm because of a ferrocene moiety. These absorptions in the visible region reflect the potential of the complexes to act as visible-light antimicrobial photodynamic therapy (aPDT) agents. All of these complexes showed reactive oxygen species (ROS)-mediated antibacterial effects against S. aureus (Gram-positive) and E. coli (Gram-negative bacteria) upon low-energy visible light (0.5 J cm-2, 400-700 nm) exposure. Additionally, 1-5 did not show any toxicity toward A549 (Human Lung adenocarcinoma) cells, reflecting their selective bacteria-killing abilities.

Cobalt-mediated oxidative DNA damage and its prevention by polyphenol antioxidants

Although cobalt is a required nutrient, it is toxic due to its ability to generate reactive oxygen species (ROS) and damage DNA. ROS generation by Co²⁺ often has been compared to that of Fe²⁺ or Cu⁺, disregarding the reduction potential differences among these metal ions. In plasmid DNA damage studies, a maximum of 15% DNA damage is observed with Co²⁺/H₂O₂ treatment (up to 50 μM and 400 μM, respectively) significantly lower than the 90% damage observed for Fe²⁺/H₂O₂ or Cu⁺/H₂O₂ treatment. However, when ascorbate is added to the Co²⁺/H₂O₂ system, a synergistic effect results in 90% DNA damage. DNA damage by Fe²⁺/H₂O₂ can be prevented by polyphenol antioxidants, but polyphenols both prevent and promote DNA damage by Cu⁺/H₂O₂. When tested for cobalt-mediated DNA damage affects, eight of ten polyphenols (epicatechin gallate, epigallocatechin gallate, propyl gallate, gallic acid, methyl-3,4,5-trihydroxybenzoate, methyl-4,5-dihydroxybenzoate, protocatechuic acid, and epicatechin) prevent cobalt-mediated DNA damage with IC₅₀ values of 1.3 to 27 μM and two (epigallocatechin and vanillic acid) prevent little to no DNA damage. EPR studies demonstrate cobalt-mediated formation of ^•OH, O₂^•-, and ^•OOH, but not ¹O₂ in the presence of H₂O₂ and ascorbate. Epigallocatechin gallate and methyl-4,5-dihydroxybenzoate significantly reduce ROS generated by Co²⁺/H₂O₂/ascorbate, consistent with their prevention of cobalt-mediated DNA damage. Thus, while cobalt, iron, and copper are all d-block metal ions, cobalt ROS generation and its prevention is significantly different from that of iron and copper.

Luminescent and Photofunctional Transition Metal Complexes: From Molecular Design to Diagnostic and Therapeutic Applications

There has been emerging interest in the exploitation of the photophysical and photochemical properties of transition metal complexes for diagnostic and therapeutic applications. In this Perspective, we highlight the major recent advances in the development of luminescent and photofunctional transition metal complexes, in particular, those of rhenium(I), ruthenium(II), osmium(II), iridium(III), and platinum(II), as bioimaging reagents and phototherapeutic agents, with a focus on the molecular design strategies that harness and modulate the interesting photophysical and photochemical behavior of the complexes. We also discuss the current challenges and future outlook of transition metal complexes for both fundamental research and clinical applications.

Phototherapeutic anticancer strategies with first-row transition metal complexes: a critical review

Photodynamic therapy (PDT) and photoactivated chemotherapy (PACT) are therapeutic techniques based on a photosensitizer (PS) and light. These techniques allow the spatial and temporal control of the activation of drugs with light. Transition metal complexes are attractive compounds as photoactivatable prodrugs since their excited states can be appropriately designed by subtle modifications of the ligands, the metal centre, or the oxidation state. However, most metal-based PSs contain heavy metals such as Ru, Os, Ir, Pt or Au, which are expensive and non-earth-abundant, contrary to first-row transition metals. In this context, the exploration of the photochemical properties of complexes based on first-row transition metals appears to be extremely promising. This did encourage several groups to develop promising PSs based on these metals. This review presents up-to-date state-of-the-art information on first-row-transition metal complexes, from titanium to zinc in regard to their application as PSs for phototherapeutic applications.

Photodynamic Therapy of Inorganic Complexes for the Treatment of Cancer†

Photodynamic therapy (PDT) is a medicinal tool that uses a photosensitizer and a light source to treat several conditions, including cancer. PDT uses reactive oxygen species such as cytotoxic singlet oxygen (¹ O₂ ) to induce cell death in cancer cells. Chemotherapy has historically utilized the cytotoxic effects of many metals, especially transition metal complexes. However, chemotherapy is a systemic treatment so all cells in a patient's body are exposed to the same cytotoxic effects. Transition metal complexes have also shown high cytotoxicity as PDT agents. PDT is a potential localized method for treating several cancer types by using inorganic complexes as photosensitizing agents. This review covers several in vitro and in vivo studies, as well as clinical trials that reported on the anticancer properties of inorganic pharmaceuticals used in PDT against different types of cancer.

Mixed-Ligand Cobalt(III) Complexes of a Naturally Occurring Coumarin and Phenanthroline Bases as Mitochondria-Targeted Dual-Purpose Photochemotherapeutics

The bioessential nature of cobalt and the rich photochemistry of its coordination complexes can be exploited to develop potential next-generation photochemotherapeutics. A series of six novel mixed-ligand cobalt(III) complexes of the formulation [Co(B)₂(L)]ClO₄ (1-6), where B is an N,N-donor phenanthroline base, namely, 1,10-phenanthroline (phen in 1 and 4), dipyrido[3,2-d:2',3'-f]quinoxaline (dpq in 2 and 5), and dipyrido[3,2-a:2',3'-c]phenazine (dppz in 3 and 6), and L is an O,O-donor dianionic ligand derived from catechol (1,2-dihydroxybenzene, cat^2-, in 1-3) or esculetin (6,7-dihydoxycoumarin, esc^2-, in 4-6), have been prepared and characterized, and their light-triggered cytotoxicity has been studied in cancer cells. The single-crystal X-ray diffraction structures of complexes 1 (as PF₆^- salt, 1a) and 2 show distorted octahedral geometries around the cobalt(III) center formed by the set of N₄O₂ donor atoms. The low-spin and 1:1 electrolytic complexes 1-6 display a d-d transition around 700 nm. Complexes 4-6 with a coordinated esc^2- ligand additionally display a π → π* intraligand transition centered at 403 nm. Complexes 4-6 possessing a naturally occurring and photoactive esc^2- ligand show high visible-light-triggered cytotoxicity against HeLa and MCF-7 cancer cells, yielding remarkably low micromolar IC₅₀ values while being much less toxic under dark conditions. Control complexes 1-3 possessing the photoinactive cat^2- ligand show significantly less cytotoxicity either in the presence of light or in the dark. The complex-induced cell death is apoptotic in nature caused by the formation of reactive oxygen species via a type 1 photoredox pathway. Fluorescence microscopy of HeLa cells treated with complex 6 reveals mitochondrial localization of the complex. A significant decrease in the dark toxicity of free esculetin and dppz base is observed upon coordination to cobalt(III). Complexes bind to calf-thymus DNA with significant affinity, but 6 binds with the greatest affinity. Complex 6 efficiently photocleaves supercoiled DNA to its nicked circular form when irradiated with visible light via a photoredox type 1 pathway involving hydroxyl radicals (HO^•). Thus, complex 6 showing remarkable visible-light-triggered cytotoxicity but negligible toxicity in the dark is a good candidate for cancer photochemotherapy applications.

Water soluble ionic Co(II), Cu(II) and Zn(II) diimine-glycinate complexes targeted to tRNA: structural description, in vitro comparative binding, cleavage and cytotoxic studies towards chemoresistant prostate cancer cells

Four new water soluble Co(ii), Cu(ii) and Zn(ii) ionic metal complexes (1-4) [Cu(diimine)(H2O)2(glycinate)]+[glycinate]-, [Co(diimine)(H2O)4]+[glycinate]- and [Zn(diimine) (H2O)4]+[glycinate]-, where diimine = 2,2'-bipyridine (1-3) and 1,10-phenanthroline (4) were synthesized and thoroughly characterized by spectroscopic and single X-ray crystallographic studies. Complex 1 possesses a triclinic crystal system with a penta-coordinated geometry whereas complexes 2-4 crystallized in an isostructural monoclinic system having distorted octahedral geometry. Density functional theory (DFT) studies for complexes 1-4 were performed to correlate their geometrical parameters and to calculate the energy of frontier molecular orbitals. The corroborative results of spectroscopic and voltammetric studies with ct-DNA and tRNA revealed that the complexes bind noncovalently via an electrostatic mode of binding with specificity for tRNA as compared to ct-DNA. Gel electrophoresis experiments revealed that all the complexes unwind the plasmid pBR322 DNA at low micromolar concentrations (2-9 μM) following an oxidative mechanism for Cu(ii) and Co(ii) complexes (1, 2 and 4) whereas the Zn(ii) complex (3) mediates DNA cleavage by the hydrolytic pathway. The tRNA cleavage showed concentration and time dependent activity of the complexes to promote RNA hydrolysis. Furthermore, the BSA binding ability of complexes 1-4 was monitored, which revealed that the complexes could quench the intrinsic fluorescence in a static manner. Complexes 1-4 were found to be non-toxic towards normal prostate epithelial cells, PNT2, but were potent against chemoresistant metastatic prostate cancer cells, Du145, with GI50 values ranging from 12.75-37 μM. Complexes 1 and 2 also showed cytotoxic activity against cancer stem cells having GI50 values of 14.70 and 14.90 μM, respectively. Molecular docking studies were performed with DNA and tRNA which further validated the spectroscopic analysis demonstrating the higher binding affinity of the complexes towards tRNA.

Near-IR light-induced photorelease of nitric oxide (NO) on ruthenium nitrosyl complexes: formation, reactivity, and biological effects

Polypyridyl backbone nitrosyl complexes of ruthenium with the molecular framework [RuII(antpy)(bpy)NO+/˙]n+ [4](PF6)3 (n = 3), [4](PF6)2 (n = 2), where antpy = 4'-(anthracene-9-yl)-2,2':6',2''-terpyridine and bpy = 2,2'-bipyridine, were synthesized via a stepwise synthetic route from the chloro precursor [RuII(antpy)(bpy)(Cl)](PF6) [1](PF6) and [RuII(antpy)(bpy)(CH3CN)](PF6)2 [2](PF6)2 and [RuII(antpy)(bpy)(NO2)](PF6) [3](PF6). After column chromatographic purification, all the synthesized complexes were fully characterized using different spectroscopic and analytical techniques including mass spectroscopy, 1H NMR, FT-IR and UV-vis spectrophotometry. The Ru-NO stretching frequency of [4](PF6)3 was observed at 1941 cm-1, which suggests moderately strong Ru-NO bonding. A massive shift in the νNO frequency occurred at Δν = 329 cm-1 (solid) upon reducing [4](PF6)3 to [4](PF6)2. To understand the molecular integrity of the complexes, the structure of [3](PF6) was successfully determined by X-ray crystallography. The redox properties of [4](PF6)3 were thoroughly investigated together with the other precursor complexes. The rate constants for the first-order photo-release of NO from [4](PF6)3 and [4](PF6)2 were determined to be 8.01 × 10-3 min-1 (t1/2 ∼ 86 min) and 3.27 × 10-2 min-1 (t1/2 ∼ 21 min), respectively, when exposed to a 200 W Xenon light. Additionally, the photo-cleavage of Ru-NO occurred within ∼2 h when [4](PF6)3 was irradiated with an IR light source (>700 nm) at room temperature. The first-order rate constant of 9.4 × 10-3 min-1 (t1/2 ∼ 73 min) shows the efficacy of the system and its capability to release NO in the photo-therapeutic window. The released NO triggered by light was trapped by reduced myoglobin, a biologically relevant target protein. The one-electron reduction of [4](PF6)3 to [4](PF6)2 was systematically carried out chemically (hydrazine hydrate), electrochemically and biologically. In the biological reduction, it was found that the reduction is much slower with double-stranded DNA compared to a single-stranded oligonucleotide (CAAGGCCAACCGCGAGAAGATGAC). Moreover, [4](PF6)3 exhibited significant photo-toxicity to the VCaP prostate cancer cell line upon irradiation with a visible light source (IC50 ∼ 8.97 μM).

Synthesis, characterization, and cytotoxic properties of mono- and di-nuclear cobalt(ii)-polypyridyl complexes

We report the synthesis and characterisation of mono- and di-nuclear cobalt(ii) complexes (1–3) containing L1, a polypyridyl ligand with pyrazole moieties.

The Potent Inhibitory Effect of a Naproxen-Appended Cobalt(III)-Cyclam Complex on Cancer Stem Cells

We report the potency against cancer stem cells (CSCs) of a new cobalt(III)-cyclam complex (1) that bears the nonsteroidal anti-inflammatory drug, naproxen. The complex displays selective potency for breast CSC-enriched HMLER-shEcad cells over breast CSC-depleted HMLER cells. Additionally, it inhibited the formation of three-dimensional tumour-like mammospheres, and reduced their viability to a greater extent than clinically used breast cancer drugs (vinorelbine, cisplatin and paclitaxel). The anti-mammosphere potency of 1 was enhanced under hypoxia-mimicking conditions. Detailed mechanistic studies revealed that DNA damage and cyclooxygenase-2 (COX-2) inhibition contribute to the cytotoxic mechanism of 1. To the best of our knowledge, 1 is the first cobalt-containing compound to show selective potency for CSCs over bulk cancer cells.

Enhanced third-order nonlinear optical properties determined in thin films using the Z-scan technique: bis(μ-4,4′-oxydibenzoato)bis[(4′-phenyl-2,2′:6′,2′′-terpyridine)cobalt(II)]

π-Conjugated organic materials exhibit high and tunable nonlinear optical (NLO) properties, and fast response times. 4′-Phenyl-2,2′:6′,2′′-terpyridine (PTP) is an important N-heterocyclic ligand involving π-conjugated systems, however, studies concerning the third-order NLO properties of terpyridine transition metal complexes are limited. The title binuclear terpyridine CoIIcomplex, bis(μ-4,4′-oxydibenzoato)-κ3O,O′:O′′;κ3O′′:O,O′-bis[(4′-phenyl-2,2′:6′,2′′-terpyridine-κ3N,N′,N′′)cobalt(II)], [Co2(C14H8O5)2(C21H15N3)2], (1), has been synthesized under hydrothermal conditions. In the crystal structure, each CoIIcation is surrounded by three N atoms of a PTP ligand and three O atoms, two from a bidentate and one from a symmetry-related monodentate 4,4′-oxydibenzoate (ODA2−) ligand, completing a distorted octahedral coordination geometry. Neighbouring [Co(PTP)]2+units are bridged by ODA2−ligands to form a ring-like structure. The third-order nonlinear optical (NLO) properties of (1) and PTP were determined in thin films using the Z-scan technique. The title compound shows a strong third-order NLO saturable absorption (SA), while PTP exhibits a third-order NLO reverse saturable absorption (RSA). The absorptive coefficient β of (1) is −37.3 × 10−7 m W−1, which is larger than that (8.96 × 10−7 m W−1) of PTP. The third-order NLO susceptibility χ(3)values are calculated as 6.01 × 10−8 e.s.u. for (1) and 1.44 × 10−8 e.s.u. for PTP.

A hybrid terpyridine-based bis(diphenylphosphino)amine ligand, terpy-C6H4N(PPh2)2: synthesis, coordination chemistry and photoluminescence studies

This paper describes the synthesis, metal complexes and photophysical studies of a novel hybrid ligand containing terpyridine and aminobis(phosphine) moieties.

Redox induced colour changes between red-violet and blue in hetero-metal complexes of the type [CoII(4′-ferrocenyl-2,2′;6′2′′-terpyridine)2]X2 (X = counter anion)

Cobalt complexes incorporating ferrocene–terpyridine ligands show a reversible color change in response to redox reactions involving the cobalt centers.

Metal-mediated DNA damage and cell death: mechanisms, detection methods, and cellular consequences

Metal ions cause various types of DNA damage by multiple mechanisms, and this damage is a primary cause of cell death and disease.

Enhancing the photocytotoxic potential of curcumin on terpyridyl lanthanide(III) complex formation

Lanthanide(III) complexes [Ln(R-tpy)(cur)(NO3)2] (Ln = La(III) in 1, 2; Gd(III) in 5, 6) and [Ln(R-tpy)(scur)(NO3)2] (Ln = La(III) in 3, 4; Gd(III) in 7, 8), where R-tpy is 4′-phenyl-2,2′:6′,2′′-terpyridine (ph-tpy in 1, 3, 5, 7), 4′-(1-pyrenyl)-2,2′:6′,2′′-terpyridine (py-tpy in 2, 4, 6, 8), Hcur is curcumin (in 1, 2, 5, 6) and Hscur is diglucosylcurcumin (in 3, 4, 7, 8), were prepared and their DNA photocleavage activity and photocytotoxicity studied. Complexes [La(ph-tpy)(cur)(NO3)2] (1) and [Gd(ph-tpy)(cur)(NO3)2] (5) were structurally characterized. The complexes in aqueous-DMF showed an absorption band near 430 nm and an emission band near 515 nm when excited at 420 nm. The complexes are moderate binders to calf-thymus DNA. They cleave plasmid supercoiled DNA to its nicked circular form in UV-A (365 nm) and visible light (454 nm) via (1)O2 and ˙OH pathways. The complexes are remarkably photocytotoxic in HeLa cells in visible light (λ = 400–700 nm) and are non-toxic in the dark. FACScan analysis of the HeLa cells treated with 2 and 4 showed cell death via an apoptotic pathway. Nuclear localization of 1–4 is evidenced from confocal imaging on HeLa cells. The hydrolytic instability of curcumin gets significantly reduced upon binding to the lanthanide ions while retaining its photocytotoxic potential.

Cobalt derivatives as promising therapeutic agents

Inorganic complexes are versatile platforms for the development of potent and selective pharmaceutical agents. Cobalt possesses a diverse array of properties that can be manipulated to yield promising drug candidates. Investigations into the mechanism of cobalt therapeutic agents can provide valuable insight into the physicochemical properties that can be harnessed for drug development. This review presents examples of bioactive cobalt complexes with special attention to their mechanisms of action. Specifically, cobalt complexes that elicit biological effects through protein inhibition, modification of drug activity, and bioreductive activation are discussed. Insights gained from these examples reveal features of cobalt that can be rationally tuned to produce therapeutics with high specificity and improved efficacy for the biomolecule or pathway of interest.