A luminescent ruthenium(II) complex for light-triggered drug release and live cell imaging

Making soluble Dy2 single-molecule magnets red emissive through their functionalization by ruthenium-cyanido luminophores

We present an efficient strategy for obtaining red-emissive molecular nanomagnets by exploring a heterometallic approach. We report the {[DyIII(4-pyridone)5]2[RuII(CN)2(phen)2]2}·(CF3SO3)6·2MeOH (phen = 1,10-phenanthroline) compound composed of exchange-coupled {DyIII2} single-molecule magnets functionalized by Ru(II)-cyanido units. The latter makes the resulting {DyIII2RuII2}6+ cations a unique example of a soluble lanthanide SMM exhibiting red charge-transfer photoluminescence in the solution and solid state, well enhanced when compared with the cyanido precursor.

Dual Photoreactive Ternary Ruthenium(II) Terpyridyl Complexes: A Comparative Study on Visible-Light-Induced Single-Step Dissociation of Bidentate Ligands and Generation of Singlet Oxygen

The versatile and tunable ligand-exchange dynamics in ruthenium(II)-polypyridyl complexes imposed by the modulation of the steric and electronic effects of the coordinated ligands provide an unlimited scope for developing phototherapeutic agents. The photorelease of a bidentate ligand from the Ru-center is better suited for potent Ru(II)-based photocytotoxic agents with two available labile sites for cross-linking with biological targets augmented with possible phototriggered 1O2 generation. Herein, we introduced a phenyl-terpyridine (ptpy) ligand in the octahedral Ru(II) core of [Ru(ptpy)(L-L)Cl]+ to induce structural distortion for the possible photorelease of electronically distinct bidentate ligands (L-L). For a systematic study, we designed four Ru(II) polypyridyl complexes: [Ru(ptpy)(L-L)Cl](PF6), ([1]-[4]), where L-L = 1,2-bis(phenylthio)ethane (SPH) [1], N,N,N',N'-tetramethylethylenediamine (TMEN) [2], N1,N2-diphenylethane-1,2-diimine (BPEDI) [3], and bis[2-(diphenylphosphino)phenyl]ether (DPE-Phos) [4]. The detailed photochemical studies suggest a single-step dissociation of L-L from the bis-thioether (SPH) complex [1] and diamine (TMEN) complex [2], while no photosubstitution was observed for [3] and [4]. Complex [1] and [2] demonstrated a dual role, involving both photosubstitution and 1O2 generation, while [3] and [4] solely exhibited poor to moderate 1O2 production. The interplay of excited states leading to these behaviors was rationalized from the lifetimes of the 3MLCT excited states by using transient absorption spectroscopy, suggesting intricate relaxation dynamics and 1O2 generation upon excitation. Therefore, the photolabile complexes [1] and [2] could potentially act as dual photoreactive agents via the phototriggered release of L-L (PACT) and/or 1O2-mediated PDT mechanisms, while [4] primarily can be utilized as a PDT agent.

Minimal Functionalization of Ruthenium Compounds with Enhanced Photoreactivity against Hard-to-Treat Cancer Cells and Resistant Bacteria

Metallocompounds have emerged as promising new anticancer agents, which can also exhibit properties to be used in photodynamic therapy. Here, we prepared two ruthenium-based compounds with a 2,2'-bipyridine ligand conjugated to an anthracenyl moiety. These compounds coded GRBA and GRPA contain 2,2'-bipyridine or 1,10-phenathroline as auxiliary ligands, respectively, which provide quite a distinct behavior. Notably, compound GRPA exhibited remarkably high photoproduction of singlet oxygen even in water (ϕΔ = 0.96), almost twice that of GRBA (ϕΔ = 0.52). On the other hand, this latter produced twice more superoxide and hydroxyl radical species than GRPA, which may be due to the modulation of their excited state. Interestingly, GRPA exhibited a modest binding to DNA (Kb = 4.51 × 104), while GRBA did not show a measurable interaction only noticed by circular dichroism measurements. Studies with bacteria showed a great antimicrobial effect, including a synergistic effect in combination with commercial antibiotics. Besides that, GRBA showed very low or no cytotoxicity against four mammalian cells, including a hard-to-treat MDA-MB-231, triple-negative human breast cancer. Potent activities were measured for GRBA upon blue light irradiation, where IC50 of 43 and 13 nmol L-1 were seen against hard-to-treat triple-negative human breast cancer (MDA-MB-231) and ovarian cancer cells (A2780), respectively. These promising results are an interesting case of a simple modification with expressive enhancement of biological activity that deserves further biological studies.

Anticancer Activity of Metallodrugs and Metallizing Host Defense Peptides-Current Developments in Structure-Activity Relationship

This article provides an overview of the development, structure and activity of various metal complexes with anti-cancer activity. Chemical researchers continue to work on the development and synthesis of new molecules that could act as anti-tumor drugs to achieve more favorable therapies. It is therefore important to have information about the various chemotherapeutic substances and their mode of action. This review focuses on metallodrugs that contain a metal as a key structural fragment, with cisplatin paving the way for their chemotherapeutic application. The text also looks at ruthenium complexes, including the therapeutic applications of phosphorescent ruthenium(II) complexes, emphasizing their dual role in therapy and diagnostics. In addition, the antitumor activities of titanium and gold derivatives, their side effects, and ongoing research to improve their efficacy and reduce adverse effects are discussed. Metallization of host defense peptides (HDPs) with various metal ions is also highlighted as a strategy that significantly enhances their anticancer activity by broadening their mechanisms of action.

Ruthenium(II) Polypyridyl-Based Photocages for an Anticancer Phytochemical Diallyl Sulfide: Comparative Dark and Photoreactivity Studies of Caged and Precursor Uncaged Complexes

The spatiotemporal control over the drug's action offered by ruthenium(II) polypyridyl complexes by the selective activation of the prodrug inside the tumor has beaconed toward much-desired selectivity issues in cancer chemotherapy. The photocaging of anticancer bioactive ligands attached synergistically with cytotoxic Ru(II) polypyridyl cores and selective release thereof in cancer cells are a promising modality for more effective drug action. Diallyl sulfide (DAS) naturally found in garlic has anticancer, antioxidant, and anti-inflammatory activities. Herein, we designed two Ru(II) polypyridyl complexes to cage DAS having a thioether-based donor site. For in-depth photocaging studies, we compared the reactivity of the DAS-caged compounds with the uncaged Ru(II)-complexes with the general formula [Ru(ttp)(NN)(L)]+/2+. Here, in the first series, ttp = p-tolyl terpyridine, NN = phen (1,10-phenanthroline), and L = Cl- (1-Cl) and H2O (1-H2O), while for the second series, NN = dpq (pyrazino[2,3-f][1,10]phenanthroline), and L = Cl- (2-Cl) and H2O (2-H2O). The reaction of DAS with 1-H2O and 2-H2O yielded the caged complexes [Ru(ttp)(NN)(DAS)](PF6)2, i.e., 1-DAS and 2-DAS, respectively. The complexes were structurally characterized by X-ray crystallography, and the solution-state characterization was done by 1H NMR and ESI-MS studies. Photoinduced release of DAS from the Ru(II) core was monitored by 1H NMR and UV-vis spectroscopy. When irradiated with a 470 nm blue LED in DMSO, the photosubstitution quantum yields (Φ) of 0.035 and 0.057 were observed for 1-DAS and 2-DAS, respectively. Intriguing solution-state speciation and kinetic behaviors of the uncaged and caged Ru(II)-complexes emerged from 1H NMR studies in the dark, and they are depicted in this work. The caged 1-DAS and 2-DAS complexes remained mostly structurally intact for a reasonably long period in DMSO. The uncaged 1-Cl and 2-Cl complexes, although did not undergo substitution in only DMSO but in the 10% DMSO/H2O mixture, completely converted to the corresponding DMSO-adduct within 16 h. Toward gaining insights into the reactivity with the biological targets, we observed that 1-Cl upon hydrolysis formed an adduct with 5'-GMP, while a small amount of GSSG-adduct was observed when 1-Cl was reacted with GSH in H2O at 323 K. 1-Cl after hydrolysis reacted with l-methionine, although the rate was slightly slower compared with that with DMSO, suggesting varying reaction kinetics with different sulfur-based linkages. Although 1-H2O reacted with sulfoxide and thioether ligands at room temperature, the rate was much faster at higher temperatures obviously, and thiol-based systems needed higher thermal energy for conjugation. Overall, these studies provide insight for thoughtful design of new generation Ru(II) polypyridyl complexes for caging suitable bioactive organic molecules.

Resurgence and Repurposing of Antifungal Azoles by Transition Metal Coordination for Drug Discovery

Coordination compounds featuring one or more antifungal azole (AA) ligands constitute an interesting family of candidate molecules, given their medicinal polyvalence and the viability of drug complexation as a strategy to improve and repurpose available medications. This review reports the work performed in the field of coordination derivatives of AAs synthesized for medical purposes by discussing the corresponding publications and emphasizing the most promising compounds discovered so far. The resulting overview highlights the efficiency of AAs and their metallic species, as well as the potential still lying in this research area.

Ligand Rigidity Steers the Selectivity and Efficiency of the Photosubstitution Reaction of Strained Ruthenium Polypyridyl Complexes

While photosubstitution reactions in metal complexes are usually thought of as dissociative processes poorly dependent on the environment, they are, in fact, very sensitive to solvent effects. Therefore, it is crucial to explicitly consider solvent molecules in theoretical models of these reactions. Here, we experimentally and computationally investigated the selectivity of the photosubstitution of diimine chelates in a series of sterically strained ruthenium(II) polypyridyl complexes in water and acetonitrile. The complexes differ essentially by the rigidity of the chelates, which strongly influenced the observed selectivity of the photosubstitution. As the ratio between the different photoproducts was also influenced by the solvent, we developed a full density functional theory modeling of the reaction mechanism that included explicit solvent molecules. Three reaction pathways leading to photodissociation were identified on the triplet hypersurface, each characterized by either one or two energy barriers. Photodissociation in water was promoted by a proton transfer in the triplet state, which was facilitated by the dissociated pyridine ring acting as a pendent base. We show that the temperature variation of the photosubstitution quantum yield is an excellent tool to compare theory with experiments. An unusual phenomenon was observed for one of the compounds in acetonitrile, for which an increase in temperature led to a surprising decrease in the photosubstitution reaction rate. We interpret this experimental observation based on complete mapping of the triplet hypersurface of this complex, revealing thermal deactivation to the singlet ground state through intersystem crossing.

An Overview of the Potential Medicinal and Pharmaceutical Properties of Ru(II)/(III) Complexes

This review examines the existing knowledge about Ru(II)/(III) ion complexes with a potential application in medicine or pharmacy, which may offer greater potential in cancer chemotherapy than Pt(II) complexes, which are known to cause many side effects. Hence, much attention has been paid to research on cancer cell lines and clinical trials have been undertaken on ruthenium complexes. In addition to their antitumor activity, ruthenium complexes are under evaluation for other diseases, such as type 2 diabetes, Alzheimer's disease and HIV. Attempts are also being made to evaluate ruthenium complexes as potential photosensitizers with polypyridine ligands for use in cancer chemotherapy. The review also briefly examines theoretical approaches to studying the interactions of Ru(II)/Ru(III) complexes with biological receptors, which can facilitate the rational design of ruthenium-based drugs.

Upconversion Nanoparticle-Based Cell Membrane-Coated cRGD Peptide Bioorthogonally Labeled Nanoplatform for Glioblastoma Treatment

Glioblastoma is hard to be eradicated partly because of the obstructive blood-brain barrier (BBB) and the dynamic autophagy activities of glioblastoma. Here, hydroxychloroquine (HDX)-loaded yolk-shell upconversion nanoparticle (UCNP)@Zn0.5Cd0.5S nanoparticle coating with the cyclic Arg-Gly-Asp (cRGD)-grafted glioblastoma cell membrane for near-infrared (NIR)-triggered treatment of glioblastoma is prepared for the first time. UCNPs@Zn0.5Cd0.5S (abbreviated as YSN, yolk-shell nanoparticle) under NIR radiation will generate reactive oxygen species for imposing cytotoxicity. HDX, the only available autophagy inhibitor in clinical studies, can enhance cytotoxicity by preventing damaged organelles from being recycled. The cRGD-decorated cell membrane allowed the HDX-loaded nanoparticles to efficiently bypass the BBB and specifically target glioblastoma cells. Exceptional treatment efficacy of the NIR-triggered chemotherapy and photodynamic therapy was achieved in U87 cells and in the mouse glioblastoma model as well. Our results provided proof-of-concept evidence that HDX@YSN@CCM@cRGD could overcome the delivery barriers and achieve targeted treatment of glioblastoma.

Photosubstitution in a trisheteroleptic ruthenium complex inhibits conjunctival melanoma growth in a zebrafish orthotopic xenograft model

In vivo data are rare but essential for establishing the clinical potential of ruthenium-based photoactivated chemotherapy (PACT) compounds, a new family of phototherapeutic drugs that are activated via ligand photosubstitution. Here a novel trisheteroleptic ruthenium complex [Ru(dpp)(bpy)(mtmp)](PF₆)₂ ([2](PF₆)₂, dpp = 4,7-diphenyl-1,10-phenanthroline, bpy = 2,2′-bipyridine, mtmp = 2-methylthiomethylpyridine) was synthesized and its light-activated anticancer properties were validated in cancer cell monolayers, 3D tumor spheroids, and in embryonic zebrafish cancer models. Upon green light irradiation, the non-toxic mtmp ligand is selectively cleaved off, thereby releasing a phototoxic ruthenium-based photoproduct capable notably of binding to nuclear DNA and triggering DNA damage and apoptosis within 24–48 h. In vitro, fifteen minutes of green light irradiation (21 mW cm⁻², 19 J cm⁻², 520 nm) were sufficient to generate high phototherapeutic indexes (PI) for this compound in a range of cancer cell lines including lung (A549), prostate (PC3Pro4), conjunctival melanoma (CRMM1, CRMM2, CM2005.1) and uveal melanoma (OMM1, OMM2.5, Mel270) cancer cell lines. The therapeutic potential of [2](PF₆)₂ was further evaluated in zebrafish embryo ectopic (PC3Pro4) or orthotopic (CRMM1, CRMM2) tumour models. The ectopic model consisted of red fluorescent PC3Pro4-mCherry cells injected intravenously (IV) into zebrafish, that formed perivascular metastatic lesions at the posterior ventral end of caudal hematopoietic tissue (CHT). By contrast, in the orthotopic model, CRMM1- and CRMM2-mCherry cells were injected behind the eye where they developed primary lesions. The maximally-tolerated dose (MTD) of [2](PF₆)₂ was first determined for three different modes of compound administration: (i) incubating the fish in prodrug-containing water (WA); (ii) injecting the prodrug intravenously (IV) into the fish; or (iii) injecting the prodrug retro-orbitally (RO) into the fish. To test the anticancer efficiency of [2](PF₆)₂, the embryos were treated 24 h after engraftment at the MTD. Optimally, four consecutive PACT treatments were performed on engrafted embryos using 60 min drug-to-light intervals and 90 min green light irradiation (21 mW cm⁻², 114 J cm⁻², 520 nm). Most importantly, this PACT protocol was not toxic to the zebrafish. In the ectopic prostate tumour models, where [2](PF₆)₂ showed the highest photoindex in vitro (PI > 31), the PACT treatment did not significantly diminish the growth of primary lesions, while in both conjunctival melanoma orthotopic tumour models, where [2](PF₆)₂ showed more modest photoindexes (PI ∼ 9), retro-orbitally administered PACT treatment significantly inhibited growth of the engrafted tumors. Overall, this study represents the first demonstration in zebrafish cancer models of the clinical potential of ruthenium-based PACT, here against conjunctival melanoma.

A new tris-heteroleptic photoactivated chemotherapy ruthenium complex induces apoptosis upon green light activation in a zebrafish orthothopic conjunctival melanoma xenograft model.

Fine-Feature Modifications to Strained Ruthenium Complexes Radically Alter Their Hypoxic Anticancer Activity†

In an earlier study of π-expansive ruthenium complexes for photodynamic and photochemo-therapies, it was shown that a pair of structural isomers differing only in the connection point of a naphthalene residue exhibited vastly different biological activity. These isomers are further explored in this paper through the activity of their functionalized derivatives. In normoxia, the inactive 2-NIP isomer (5) can be made as photocytotoxic as the active 1-NIP isomer (1) by functionalizing with methyl or methoxy groups, while methoxy variants of the 1-NIP isomer became inactive. In all cases, the singlet oxygen sensitization quantum yield was below 1%. Hypoxic photocytotoxicity was attenuated, with only three of the series showing any activity, notwithstanding the photodissociative ligands. The results here are consistent with the earlier findings in that seemingly minor structural modifications on the non-strained ligand can dramatically modulate the normoxic and hypoxic activity of these strained compounds and that these changes appear to exert a greater influence on photocytotoxicity than singlet oxygen sensitization or rates of photosubstitution in cell-free conditions would suggest.

The Development of Ru(II)-Based Photoactivated Chemotherapy Agents

Photoactivated chemotherapy (PACT) is a novel cancer treatment method that has drawn increasing attention due to its high selectivity and low side effects by spatio-temporal control of irradiation. Compared with photodynamic therapy (PDT), oxygen-independent PACT is more suitable for treating hypoxic tumors. By finely tuning ligand structures and coordination configurations, many Ru(II) complexes can undergo photoinduced ligand dissociation, and the resulting Ru(II) aqua species and/or free ligands may have anticancer activity, showing their potential as PACT agents. In this mini-review, we summarized the progress in Ru(II)-based PACT agents, as well as challenges that researchers in this field still face.

Photosensitive Ru(II) Complexes as Inhibitors of the Major Human Drug Metabolizing Enzyme CYP3A4

We report the synthesis and photochemical and biological characterization of the first selective and potent metal-based inhibitors of cytochrome P450 3A4 (CYP3A4), the major human drug metabolizing enzyme. Five Ru(II)-based derivatives were prepared from two analogs of the CYP3A4 inhibitor ritonavir, 4 and 6: [Ru(tpy)(L)(6)]Cl₂ (tpy = 2,2':6',2″-terpyridine) with L = 6,6'-dimethyl-2,2'-bipyridine (Me₂bpy; 8), dimethylbenzo[i]dipyrido[3,2-a:2',3'-c]phenazine (Me₂dppn; 10) and 3,6-dimethyl-10,15-diphenylbenzo[i]dipyrido[3,2-a:2',3'-c]phenazine (Me₂Ph₂dppn; 11), [Ru(tpy)(Me₂bpy)(4)]Cl₂ (7) and [Ru(tpy)(Me₂dppn)(4)]Cl₂ (9). Photochemical release of 4 or 6 from 7-11 was demonstrated, and the spectrophotometric evaluation of 7 showed that it behaves similarly to free 4 (type II heme ligation) after irradiation with visible light but not in the dark. Unexpectedly, the intact Ru(II) complexes 7 and 8 were found to inhibit CYP3A4 potently and specifically through direct binding to the active site without heme ligation. Caged inhibitors 9-11 showed dual action properties by combining photoactivated dissociation of 4 or 6 with efficient ¹O₂ production. In prostate adenocarcinoma DU-145 cells, compound 9 had the best synergistic effect with vinblastine, the anticancer drug primarily metabolized by CYP3A4 in vivo. Thus, our study establishes a new paradigm in CYP inhibition using metalated complexes and suggests possible utilization of photoactive CYP3A4 inhibitory compounds in clinical applications, such as enhancement of therapeutic efficacy of anticancer drugs.

Photorelease of a metal-binding pharmacophore from a Ru(II) polypyridine complex

The adoption of compounds that target metalloenzymes comprises a relatively low (<5%) percentage of all FDA approved therapeutics. Metalloenzyme inhibitors typically coordinate to the active site metal ions and therefore contain ligands with charged or highly polar functional groups. While these groups may generate highly water-soluble compounds, this functionalization can also limit their pharmacological properties. To overcome this drawback, drug candidates can be formulated as prodrugs. While a variety of protecting groups have been developed, increasing efforts have been devoted towards the use of caging groups that can be removed upon exposure to light to provide spatial and temporal control over the treatment. Among these, the application of Ru(ii) polypyridine complexes is receiving increased attention based on their attractive biological and photophysical properties. Herein, a conjugate consisting of a metalloenzyme inhibitor and a Ru(ii) polypyridine complex as a photo-cage is presented. The conjugate was designed using density functional theory calculations and docking studies. The conjugate is stable in an aqueous solution, but irradiation of the complex with 450 nm light releases the inhibitor within several minutes. As a model system, the biochemical properties were investigated against the endonucleolytic active site of the influenza virus. While showing no inhibition in the dark in an in vitro assay, the conjugate generated inhibition upon light exposure at 450 nm, demonstrating the ability to liberate the metalloenzyme inhibitor. The presented inhibitor-Ru(ii) polypyridine conjugate is an example of computationally-guided drug design for light-activated drug release and may help reveal new avenues for the prodrugging of metalloenzyme inhibitors.

Effect of the electronic structure on the robustness of ruthenium(ii) bis-phenanthroline compounds for photodissociation of the co-ligand: synthesis, structural characterization, and density functional theory study

Photodissociation of co-ligand in cis-[Ru(phen)₂(L)₂](PF₆)₂ (phen = 1,10-phenanthroline, L = isoquinoline 1; phthalazine 2), upon blue light irradiation was investigated via both experimental and DFT studies.

Luminescent naphthalimide-tagged ruthenium(ii)–arene complexes: cellular imaging, photocytotoxicity and transferrin binding

Two luminescent ruthenium(ii)–arene complexes containing a naphthalimide tagged morpholine moiety were studied for their biomaging, transferrin-binding and phototherapeutic activity.

Visible-to-NIR-Light Activated Release: From Small Molecules to Nanomaterials

Photoactivatable (alternatively, photoremovable, photoreleasable, or photocleavable) protecting groups (PPGs), also known as caged or photocaged compounds, are used to enable non-invasive spatiotemporal photochemical control over the release of species of interest. Recent years have seen the development of PPGs activatable by biologically and chemically benign visible and near-infrared (NIR) light. These long-wavelength-absorbing moieties expand the applicability of this powerful method and its accessibility to non-specialist users. This review comprehensively covers organic and transition metal-containing photoactivatable compounds (complexes) that absorb in the visible- and NIR-range to release various leaving groups and gasotransmitters (carbon monoxide, nitric oxide, and hydrogen sulfide). The text also covers visible- and NIR-light-induced photosensitized release using molecular sensitizers, quantum dots, and upconversion and second-harmonic nanoparticles, as well as release via photodynamic (photooxygenation by singlet oxygen) and photothermal effects. Release from photoactivatable polymers, micelles, vesicles, and photoswitches, along with the related emerging field of photopharmacology, is discussed at the end of the review.

Ruthenium Complexes as Anticancer Agents: A Brief History and Perspectives

Platinum (Pt)-based anticancer drugs such as cisplatin have been used to treat various cancers. However, they have some limitations including poor selectivity and toxicity towards normal cells and increasing chemoresistance. Therefore, there is a need for novel metallo-anticancers, which has not been met for decades. Since the initial introduction of ruthenium (Ru) polypyridyl complex, a number of attempts at structural evolution have been conducted to improve efficacy. Among them, half-sandwich Ru-arene complexes have been the most prominent as an anticancer platform. Such complexes have clearly shown superior anticancer profiles such as increased selectivity toward cancer cells and ameliorating toxicity against normal cells compared to existing Pt-based anticancers. Currently, several Ru complexes are under human clinical trials. For improvement in selectivity and toxicity associated with chemotherapy, Ru complexes as photodynamic therapy (PDT), and photoactivated chemotherapy (PACT), which can selectively activate prodrug moieties in a specific region, have also been investigated. With all these studies on these interesting entities, new metallo-anticancer drugs to at least partially replace existing Pt-based anticancers are anticipated. This review covers a brief description of Ru-based anticancer complexes and perspectives.

796 nm Activation of a Photocleavable Ruthenium(II) Complex Conjugated to an Upconverting Nanoparticle through Two Phosphonate Groups

The biological application of photoactivatable ruthenium anticancer prodrugs is limited by the need to use poorly penetrating high-energy visible light for their activation. Upconverting nanoparticles (UCNPs), which produce high-energy light under near-infrared (NIR) excitation, can solve this issue, provided that they form stable, water (H2O)-dispersible nanoconjugates with the prodrug and that there is efficient energy transfer from the UCNP to the ruthenium complex. Herein, we report on the synthesis and photochemistry of the ruthenium(II) polypyridyl complex [Ru(bpy)2(3H)](PF6)2 ([1](PF6)2), where bpy = 2,2-bipyridine and 3H is a photocleavable bis(thioether) ligand modified with two phosphonate moieties. This ligand was coordinated to the ruthenium center through its thioether groups and could be dissociated under blue-light irradiation. Complex [1](PF6)2 was bound to the surface of NaYF4:Yb3+,Tm3+@NaYF4:Nd3+@NaYF4 core-shell-shell (CSS-)UCNPs through its bis(phosphonate) group, thereby creating a H2O-dispersible, thermally stable nanoconjugate (CSS-UCNP@[1]). Conjugation to the nanoparticle surface was found to be most efficient in neutral to slightly basic conditions, resulting in up to 2.4 × 103 RuII ions per UCNP. The incorporation of a neodymium-doped shell layer allowed for the generation of blue light using low-energy, deep-penetrating light (796 nm). This wavelength prevents the undesired heating seen with conventional UCNPs activated at 980 nm. Irradiation of CSS-UCNP@[1] with NIR light led to activation of the ruthenium complex [1](PF6)2. Although only one of the two thioether groups was dissociated under irradiation at 50 W·cm-2, we provide the first demonstration of the photoactivation of a ruthenium thioether complex using 796 nm irradiation of a H2O-dispersible nanoconjugate.

Fluconazole analogues with metal-binding motifs impact metal-dependent processes and demonstrate antifungal activity in Candida albicans

Azole antifungals are an important class of antifungal drugs due to their low cost, ability to be administered orally, and broad-spectrum activity. However, their widespread and long-term use have given rise to adaptation mechanisms that render these compounds less effective against common fungal pathogens, including Candida albicans. New antifungals are desperately needed as drug-resistant strains become more prevalent. We recently showed that copper supplementation potentiates the activity of the azole antifungal fluconazole against the opportunistic fungal pathogen C. albicans. Here, we report eight new azole analogues derived from fluconazole in which one triazole group has been replaced with a metal-binding group, a strategy designed to enhance potentiation of azole antifungal activity by copper. The bioactivity of all eight compounds was tested and compared to that of fluconazole. Three of the analogues showed activity against C. albicans and two had lower levels of trailing growth. One compound, Flu-TSCZ, was found to impact the levels, speciation, and bioavailability of cellular metals.

Metal complexes as a promising source for new antibiotics

There is a dire need for new antimicrobial compounds to combat the growing threat of widespread antibiotic resistance. With a currently very scarce drug pipeline, consisting mostly of derivatives of known antibiotics, new classes of antibiotics are urgently required. Metal complexes are currently in clinical development for the treatment of cancer, malaria and neurodegenerative diseases. However, only little attention has been paid to their application as potential antimicrobial compounds. We report the evaluation of 906 metal-containing compounds that have been screened by the Community for Open Antimicrobial Drug Discovery (CO-ADD) for antimicrobial activity. Metal-bearing compounds display a significantly higher hit-rate (9.9%) when compared to the purely organic molecules (0.87%) in the CO-ADD database. Out of 906 compounds, 88 show activity against at least one of the tested strains, including fungi, while not displaying any cytotoxicity against mammalian cell lines or haemolytic properties. Herein, we highlight the structures of the 30 compounds with activity against Gram-positive and/or Gram-negative bacteria containing Mn, Co, Zn, Ru, Ag, Eu, Ir and Pt, with activities down to the nanomolar range against methicillin resistant S. aureus (MRSA). 23 of these complexes have not been reported for their antimicrobial properties before. This work reveals the vast diversity that metal-containing compounds can bring to antimicrobial research. It is important to raise awareness of these types of compounds for the design of truly novel antibiotics with potential for combatting antimicrobial resistance.

Transition Metal-Based Prodrugs for Anticancer Drug Delivery

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Transition metal complexes, of which the platinum(II) complex cisplatin is an example, have been used in medicine to treat cancer for more than 40 years. Although many successes have been achieved, there are problems associated with the use of these drugs, such as side effects and drug resistance. Converting them into prodrugs, to make them more inert, so that they can travel to the tumour site unchanged and release the drug in its active form only there, is a strategy which is the subject of much research nowadays. The new prodrugs may be activated and release the cytotoxic agent by differences in oxygen concentration or in pH, by the action of overexpressed enzymes, by differences in metabolic rates, etc., which characteristically distinguish cancer cells from normal ones, or even by the input of radiation, which can be visible light. Converting a metal complex into a prodrug may also be used to improve its pharmacological properties. In some cases, the metal complex is a carrier which transports the active drug as a ligand. Some platinum prodrugs have reached clinical trials. So far platinum, ruthenium and cobalt have been the most studied metals. This review presents the recent developments in this area, including the types of complexes used, the mechanisms of drug action and in some cases the techniques applied to monitor drug delivery to cells.

Toward Optimal Ru(II) Photocages: Balancing Photochemistry, Stability, and Biocompatibility Through Fine Tuning of Steric, Electronic, and Physiochemical Features

Ru(II) complex photocages are used in a variety of biological applications, but the thermal stability, photosubstitution quantum yield, and biological compatibility of the most commonly used Ru(II) systems remain unoptimized. Here, multiple compounds used in photocaging applications were analyzed and found to have several unsatisfactory characteristics. To address these deficiencies, three new scaffolds were designed to improve key properties through modulation of a combination of electronic, steric, and physiochemical features. One of these new systems, containing the 2,2'-biquinoline-4,4'-dicarboxylic acid (2,2'-bicinchoninic acid) ligand, fulfills several of the requirements for an optimal photocage. Another complex, containing the 2-benzothiazol-2-yl-quinoline ligand, provides a scaffold for the creation of "dual action" agents.

Photo-Uncaging of a Microtubule-Targeted Rigidin Analogue in Hypoxic Cancer Cells and in a Xenograft Mouse Model

Marine alkaloid rigidins are cytotoxic compounds known to kill cancer cells at nanomolar concentrations by targeting the microtubule network. Here, a rigidin analogue containing a thioether group was "caged" by coordination of its thioether group to a photosensitive ruthenium complex. In the dark, the coordinated ruthenium fragment prevented the rigidin analogue from inhibiting tubulin polymerization and reduced its toxicity in 2D cancer cell line monolayers, 3D lung cancer tumor spheroids (A549), and a lung cancer tumor xenograft (A549) in nude mice. Photochemical activation of the prodrug upon green light irradiation led to the photosubstitution of the thioether ligand by water, thereby releasing the free rigidin analogue capable of inhibiting the polymerization of tubulin. In cancer cells, such photorelease was accompanied by a drastic reduction of cell growth, not only when the cells were grown in normoxia (21% O₂) but also remarkably in hypoxic conditions (1% O₂). In vivo, low toxicity was observed at a dose of 1 mg·kg^-1 when the compound was injected intraperitoneally, and light activation of the compound in the tumor led to 30% tumor volume reduction, which represents the first demonstration of the safety and efficacy of ruthenium-based photoactivated chemotherapy compounds in a tumor xenograft.

Partially Solvated Dinuclear Ruthenium Compounds Bridged by Quinoxaline-Functionalized Ligands as Ru(II) Photocage Architectures for Low-Energy Light Absorption

Ruthenium compounds with coordinated photolabile molecules that can be selectively released by irradiation with a visible light source are finding increasing applications in photoactivated chemotherapy (PCT) as photocages. Earlier photocages based on mononuclear Ru(II) compounds lack absorption in the therapeutic window (λ > 600 nm). In previous work, we synthesized the first partially solvated tppz bridged (tppz= 2,3,5,6-tetrakis(pyridin-2-yl)pyrazine) dinuclear Ru(II) complex capable of photoinduced ligand exchange at both metal centers. To further explore the effect of the bridging ligand on Ru(II) photocage design, we used quinoxaline-functionalized bridging ligand platforms to prepare [{Ru^II(NCCH₃)₄}₂(μ-BL)](PF₆)₄[BL = dpq, 2,3-di(pyridin-2-yl)quinoxaline (1); BL = dpb, 2,3-di(pyridin-2-yl)benzo[g]quinoxaline (2)]. The compounds are capable of absorbing green light with tails extending beyond 650 nm which can be exploited for applications as PCT agents. Experimental results were additionally verified by DFT calculations. The use of two Ru(II) centers equipped with quinoxaline-based bridging ligands is a promising design strategy for the synthesis of a new family of dinuclear Ru(II) photocage prototypes with the ability to absorb low-energy visible light.

A mitochondria-targeting dinuclear Ir-Ru complex as a synergistic photoactivated chemotherapy and photodynamic therapy agent against cisplatin-resistant tumour cells

A mitochondria-targeting hetero-binuclear Ir(iii)-Ru(ii) complex was developed as a photoactivated chemotherapy (PACT) and photodynamic therapy (PDT) bifunctional agent to achieve a synergistic effective therapeutic outcome for the therapy of cisplatin-resistant tumour cells.

NIR-Light-Driven Generation of Reactive Oxygen Species Using Ru(II)-Decorated Lipid-Encapsulated Upconverting Nanoparticles

The biological application of ruthenium anticancer prodrugs for photodynamic therapy (PDT) and photoactivated chemotherapy (PACT) is restricted by the need to use poorly penetrating high-energy photons for their activation, i.e., typically blue or green light. Upconverting nanoparticles (UCNPs), which produce high-energy light under near-infrared (NIR) excitation, may solve this issue, provided that the coupling between the UCNP surface and the Ru prodrug is optimized to produce stable nanoconjugates with efficient energy transfer from the UCNP to the ruthenium complex. Herein, we report on the synthesis and photochemistry of the two structurally related ruthenium(II) polypyridyl complexes [Ru(bpy)₂(5)](PF₆)₂ ([1](PF₆)₂) and [Ru(bpy)₂(6)](PF₆)₂ ([2](PF₆)₂), where bpy = 2,2-bipyridine, 5 is 5,6-bis(dodecyloxy)-2,9-dimethyl-1,10-phenanthroline, and 6 is 5,6-bis(dodecyloxy)-1,10-phenanthroline. [1](PF₆)₂ is photolabile as a result of the steric strain induced by ligand 5, but the irradiation of [1](PF₆)₂ in solution leads to the nonselective and slow photosubstitution of one of its three ligands, making it a poor PACT compound. On the other hand, [2](PF₆)₂ is an efficient and photostable PDT photosensitizer. The water-dispersible, negatively charged nanoconjugate UCNP@lipid/[2] was prepared by the encapsulation of 44 nm diameter NaYF₄:Yb³⁺,Tm³⁺ UCNPs in a mixture of 1,2-dioleoyl-sn-glycero-3-phosphate and 1,2-dioleoyl-sn-glycero-3-phosphocholine phospholipids, cholesterol, and the amphiphilic complex [2](PF₆)₂. A nonradiative energy transfer efficiency of 12% between the Tm³⁺ ions in the UCNP and the Ru²⁺ acceptor [2]²⁺ was found using time-resolved emission spectroscopy. Under irradiation with NIR light (969 nm), UCNP@lipid/[2] was found to produce reactive oxygen species (ROS), as judged by the oxidation of the nonspecific ROS probe 2',7'-dichlorodihydrofluorescein (DCFH^2-). Determination of the type of ROS produced was precluded by the negative surface charge of the nanoconjugate, which resulted in the electrostatic repulsion of the more specific but also negatively charged ¹O₂ probe tetrasodium 9,10-anthracenediyl-bis(methylene)dimalonate (Na₄(ADMBMA)).

Diastereoselective Synthesis and Two-Step Photocleavage of Ruthenium Polypyridyl Complexes Bearing a Bis(thioether) Ligand

Thioethers are good ligands for photoactivatable ruthenium(II) polypyridyl complexes, as they form thermally stable complexes that are prone to ligand photosubstitution. Here, we introduce a novel symmetric chelating bis(thioether) ligand scaffold, based on 1,3-bis(methylthio)-2-propanol (4) and report the synthesis and stereochemical characterization of the series of novel ruthenium(II) polypyridyl complexes [Ru(bpy)₂(L)](PF₆)₂ ([1]–[3](PF₆)₂), where L is ligand 4, its methyl ether, 1,3-bis(methylthio)-2-methoxypropane (5), or its carboxymethyl ether, 1,3-bis(methylthio)-2-(carboxymethoxy)propane (6). Coordination of ligands 4–6 to the bis(bipyridine)ruthenium center gives rise to 16 possible isomers, consisting of 8 possible Λ diastereoisomers and their Δ enantiomers. We found that the synthesis of [1]–[3](PF₆)₂ is diastereoselective, yielding a racemic mixture of the Λ-(S)-eq-(S)-ax-OH_eq-[Ru]²⁺ and Δ-(R)-ax-(R)-eq-OH_eq-[Ru]²⁺ isomers. Upon irradiation with blue light in water, [1]–[3](PF₆)₂ selectively substitute their bis(thioether) ligands for water molecules in a two-step photoreaction, ultimately producing [Ru(bpy)₂(H₂O)₂]²⁺ as the photoproduct. The relatively stable photochemical intermediate was identified as cis-[Ru(bpy)₂(κ¹-L)(H₂O)]²⁺ by mass spectrometry. Global fitting of the time evolution of the UV–vis absorption spectra of [1]–[3](PF₆)₂ was employed to derive the photosubstitution quantum yields (Φ₄₄₃) for each of the two photochemical reaction steps separately, revealing very high quantum yields of 0.16–0.25 for the first step and lower values (0.0055–0.0093) for the second step of the photoreaction. The selective and efficient photochemical reaction makes the photocleavable bis(thioether) ligand scaffold reported here a promising candidate for use in e.g. ruthenium-based photo-activated chemotherapy.

Thioethers are excellent photocleavable ligands for ruthenium(II) polypyridyl complexes but may lead to the formation of several stereoisomers when they are present in bidentate ligands. Here, a chelating bis(thioether) ligand was found to coordinate to Ru(II) diastereoselectively, in spite of the four chiral centers of the resulting complex. Photosubstitution of this bis(thioether) ligand in water occurs via a selective, two-step process that involves a relatively stable mono(aqua) intermediate.

Copper potentiates azole antifungal activity in a way that does not involve complex formation

To survive, fungal pathogens must acquire nutrient metals that are restricted by the host while also tolerating mechanisms of metal toxicity that are induced by the host. Given this dual vulnerability, we hypothesized that a pathogen's access to and control of essential yet potentially dangerous metal ions would affect fungal tolerance to antifungal drug stress. Here, we show that Candida albicans becomes sensitized to both Cu limitation and Cu elevation during exposure in liquid culture to the antifungal drug fluconazole, a widely prescribed antifungal agent. Spectroscopic data confirm that while fluconazole forms a complex with Cu(ii) in water, interactions of fluconazole with neither Cu(ii) nor Cu(i) are observed in the cell culture media used for the cellular assays. This result is further supported by growth assays in deletion strains that lack Cu import machinery. Overall, we establish that increases in Cu levels by as little as 40 nM over basal levels in the growth medium reduce tolerance of C. albicans to fluconazole in a way that does not require formation of a Cu-fluconazole complex. Rather, our data point to a more complex relationship between drug stress and Cu availability that gives rise to metal-mediated outcomes of drug treatment.

Light-activated ruthenium (II)-bicalutamide prodrugs for prostate cancer

Targeted delivery of clinically approved anticancer drug to tumor sites is an effective way to achieve enhanced drug efficacy as well as reduced side effects and toxicity. Here bicalutamide is caged by the Ru(II) center through the nitrile group, and three photoactive Ru(II) complexes were designed and synthesized. Docking study showed that the ruthenium(II) fragments can effectively block the binding of complexes 1-3 with AR (androgen receptor) owing to the large steric structures, thus bicalutamide in complexes 1-3 could not interact with AR-LBD (ligand binding domain). Once irradiation with blue light (465nm), complexes 1-3 can release bicalutamide and anticancer Ru(II) fragments, which possesses dual-action of AR binding and DNA interaction simultaneously. In vitro cytotoxicity study on these complexes further confirmed that complexes 1-3 exhibited considerable cytotoxicity upon irradiation with blue light. Significantly, complex 3 could be activated at 660nm, which greatly increases the scope of complex 3 to treat deeper within tissue. Theoretical calculations showed that the lowest singlet excitation energy of complex 3 is lower than those of complexes 1-2, which explains the experimental results well. Moreover, the ³MC (metal centered) states of these complexes are more stable than their ³MLCT (metal to ligand charge transfer) states, indicating that the photoactive processes of these complexes are likely to result in ligand dissociation.

Synthesis, Characterisation and In Vitro Permeation, Dissolution and Cytotoxic Evaluation of Ruthenium(II)-Liganded Sulpiride and Amino Alcohol

Sulpiride (SPR) is a selective antagonist of central dopamine receptors but has limited clinical use due to its poor pharmacokinetics. The aim of this study was to investigate how metal ligation to SPR may improve its solubility, intestinal permeability and prolong its half-life. The synthesis and characterisation of ternary metal complexes [Ru(p -cymene)(L)(SPR)]PF6 (L1 = (R)-(+)-2-amino-3-phenyl-1-propanol, L2 = ethanolamine, L3 = (S)-(+)-2-amino-1-propanol, L4 = 3-amino-1-propanol, L5 = (S)-(+)-2-pyrrolidinemethanol) are described in this work. The stability constant of the [Ru(p -cymene)(SPR)] complex was determined using Job's method. The obtained value revealed higher stability of the metal complex in the physiological pH than in an acidic environment such as the stomach. The ternary metal complexes were characterised by elemental analysis, Fourier transform infrared spectroscopy (FT-IR), 1H and 13C nuclear magnetic resonance (NMR), differential scanning calorimetry (DSC), thermal analyses, Ultraviolet-Visible (UV-Vis). Solubility studies showed higher aqueous solubility for complexed SPR than the free drug. Dissolution profiles of SPR from the metal complexes exhibited slower dissolution rate of the drug. Permeation studies through the pig's intestine revealed enhanced membrane permeation of the complexed drug. In vitro methyl thiazolyl tetrazolium (MTT) assay showed no noticeable toxic effects of the ternary metal complexes on Caco-2 cell line.

Stimuli-Responsive Therapeutic Metallodrugs

The success of platinum-based anticancer agents has motivated the exploration of novel metal-based drugs for several decades, whereas problems such as drug-resistance and systemic toxicity hampered their clinical applications and efficacy. Stimuli-responsiveness of some metal complexes offers a good opportunity for designing site-specific prodrugs to maximize the therapeutic efficacy and minimize the side effect of metallodrugs. This review presents a comprehensive and up-to-date overview on the therapeutic stimuli-responsive metallodrugs that have appeared in the past two decades, where stimuli such as redox, pH, enzyme, light, temperature, and so forth were involved. The compounds are classified into three major categories based on the nature of stimuli, that is, endo-stimuli-responsive metallodrugs, exo-stimuli-responsive metallodrugs, and dual-stimuli-responsive metallodrugs. Representative examples of each type are discussed in terms of structure, response mechanism, and potential medical applications. In the end, future opportunities and challenges in this field are tentatively proposed. With diverse metal complexes being introduced, the foci of this review are pointed to platinum and ruthenium complexes.

Ruthenium-containing P450 inhibitors for dual enzyme inhibition and DNA damage

Cytochrome P450s are key players in drug metabolism, and overexpression in tumors is associated with significant resistance to many medicinal agents. Consequently, inhibition of P450s could serve as a strategy to restore drug efficacy. However, the widespread expression of P450s throughout the human body and the critical roles they play in various biosynthetic pathways motivates the development of P450 inhibitors capable of controlled local administration. Ruthenium complexes containing P450 inhibitors as ligands were synthesized in order to develop pro-drugs that can be triggered to release the inhibitors in a spatially and temporally controlled fashion. Upon light activation the compounds release ligands that directly bind and inhibit P450 enzymes, while the ruthenium center is able to directly damage DNA.

Ru(II) Polypyridyl Complexes Derived from Tetradentate Ancillary Ligands for Effective Photocaging

Metal complexes have many proven applications in the caging and photochemical release of biologically active compounds. Photocaging groups derived from Ru(II) traditionally have been composed of ancillary ligands that are planar and bi- or tridentate, such as 2,2'-bipyridine (bpy), 2,2':6',2″-terpyridine (tpy), and 1,10-phenanthroline (phen). Complexes bearing ancillary ligands with denticities higher than three represent a new class of Ru(II)-based photocaging groups that are grossly underdeveloped. Because high-denticity ancillary ligands provide the ability to increase the structural rigidity and control the stereochemistry, our groups initiated a research program to explore the applications of such ligands in Ru(II)-based photocaging. Ru(TPA), bearing the tetradentate ancillary ligand tris(2-pyridylmethyl)amine (TPA), has been successfully utilized to effectively cage nitriles and aromatic heterocycles. Nitriles and aromatic heterocycles caged by the Ru(TPA) group show excellent stability in aqueous solutions in the dark, and the complexes can selectively release the caged molecules upon irradiation with light. Ru(TPA) is applicable as a photochemical agent to offer precise spatiotemporal control over biological activity without undesired toxicity. In addition, Ru(II) polypyridyl complexes with desired photochemical properties can be synthesized and identified by solid-phase synthesis, and the resulting complexes show properties to similar to those of complexes obtained by solution-phase synthesis. Density functional theory (DFT) calculations reveal that orbital mixing between the π* orbitals of the ancillary ligand and the Ru-N dσ* orbital is essential for ligand photodissociation in these complexes. Furthermore, the introduction of steric bulk enhances the photoliability of the caged molecules, validating that steric effects can largely influence the quantum efficiency of photoinduced ligand exchange in Ru(II) polypyridyl complexes. Recently, two new photocaging groups, Ru(cyTPA) and Ru(1-isocyTPQA), have been designed and synthesized for caging of nitriles and aromatic heterocycles, and these complexes exhibit unique photochemical properties distinct from those derived from Ru(TPA). Notably, the unusually greater quantum efficiency for the ligand exchange in [Ru(1-isocyTPQA)(MeCN)₂](PF₆)₂, Φ₄₀₀ = 0.033(3), uncovers a trans-type effect in the triplet metal-to-ligand charge transfer (³MLCT) state that enhances photoinduced ligand exchange in a new manner. DFT calculations and ultrafast transient spectroscopy reveal that the lowest-energy triplet state in [Ru(1-isocyTPQA)(MeCN)₂](PF₆)₂ is a highly mixed ³MLCT/³ππ* excited state rather than a triplet metal-centered ligand-field (³LF) excited state; the latter is generally accepted for ligand photodissociation. In addition, Mulliken spin density calculations indicate that a majority of the spin density in [Ru(1-isocyTPQA)(MeCN)₂](PF₆)₂ is localized on the isoquinoline arm, which is opposite to the cis MeCN, rather than on the ruthenium center. This significantly weakens the Ru-N6 ( cis MeCN) bond, which then promotes the ligand photodissociation. This newly discovered effect gives a clearer perception of the interplay between the ³MLCT and ³LF excited states of Ru(II) polypyridyl complexes, which may be useful in the design and applications of ruthenium complexes in the areas of photoactivated drug delivery and photosensitizers.

Photoresponsive Drug/Gene Delivery Systems

Light as an external stimulus can be precisely manipulated in terms of irradiation time, site, wavelength, and density. As such, photoresponsive drug/gene delivery systems have been increasingly pursued and utilized for the spatiotemporal control of drug/gene delivery to enhance their therapeutic efficacy and safety. In this review, we summarized the recent research progress on photoresponsive drug/gene delivery, and two major categories of delivery systems were discussed. The first category is the direct responsive systems that experience photoreactions on the vehicle or drug themselves, and different materials as well as chemical structures responsive to UV, visible, and NIR light are summarized. The second category is the indirect responsive systems that require a light-generated mediator signal, such as heat, ROS, hypoxia, and gas molecules, to cascadingly trigger the structural transformation. The future outlook and challenges are also discussed at the end.

Photoactivation of imatinib-antibody conjugate using low-energy visible light from Ru(ii)-polypyridyl cages

Ru(ii)-polypyridyl cages with sterically bulky bidentate ligands provide efficient photochemical release of the anticancer drug imatinib using low energy visible light, imparting spatiotemporal control over drug bioavailability. The light-activated drug release is maintained when the Ru(ii) cage is covalently coupled to an antibody, which is expected to localize selectively on the tumor.

Illuminating cytochrome P450 binding: Ru(ii)-caged inhibitors of CYP17A1

New Ru(ii)-caged abiraterone complexes were synthesized that exhibit strong absorption in the visible region and release the steroidal CYP17A1 inhibitor abiraterone upon exposure to low energy visible light in buffer and prostate cancer cells. Photoinduced release results in abiraterone binding to its CYP17A1 target in an inhibitory mode.

To cage or to be caged? The cytotoxic species in ruthenium-based photoactivated chemotherapy is not always the metal

In metal-based photoactivated chemotherapy (PACT), two photoproducts are generated by light-triggered photosubstitution of a metal-bound ligand: the free ligand itself and an aquated metal complex. By analogy with cisplatin, the aquated metal complex is usually presented as the biologically active species, as it can typically bind to DNA. In this work, we show that this qualitative assumption is not necessarily valid by comparing the biological activity, log P, and cellular uptake of three ruthenium-based PACT complexes: [Ru(bpy)₂(dmbpy)]²⁺, [Ru(bpy)₂(mtmp)]²⁺, and [Ru(Ph₂phen)₂(mtmp)]²⁺. For the first complex, the photoreleased dmbpy ligand is responsible for the observed phototoxicity, whereas the second complex is not phototoxic, and for the third complex it is the ruthenium bis-aqua photoproduct that is the sole cytotoxic species.

Ru(ii) polypyridyl complexes as photocages for bioactive compounds containing nitriles and aromatic heterocycles

Photocaging allows for precise spatiotemporal control over the release of biologically active compounds with light. Most photocaged molecules employ organic photolabile protecting groups; however, biologically active compounds often contain functionalities such as nitriles and aromatic heterocycles that cannot be caged with organic groups. Despite their prevalence, only a few studies have reported successful caging of nitriles and aromatic heterocycles. Recently, Ru(ii)-based photocaging has emerged as a powerful method for the release of bioactive molecules containing these functional groups, in many cases providing high levels of spatial and temporal control over biological activity. This Feature Article discusses recent developments in applying Ru(ii)-based photocaging towards biological problems. Our groups designed and synthesized Ru(ii)-based platforms for the photoinduced delivery of cysteine protease and cytochrome P450 inhibitors in order to achieve selective control over enzyme inhibition. We also reported Ru(ii) photocaging groups derived from higher-denticity ancillary ligands that possess photophysical and photochemical properties distinct from more traditional Ru(ii)-based caging groups. In addition, for the first time, we are able to rapidly synthesize and screen Ru(ii) polypyridyl complexes that elicit desired properties by solid-phase synthesis. Finally, our work also defined steric and orbital mixing effects that are important factors in controlling photoinduced ligand exchange.

Platinum complexes as light promoted anticancer agents: a redefined strategy for controlled activation

Site-specific delivery and amenable activation of prodrugs are indispensible criteria for designing novel anticancer agents. Platinum based drugs vanguard the chemotherapeutic regimes and over the years significant attention has been paid to achieve more efficacious drugs with fewer adverse effects. The switch from platinum(ii) drugs to the inert platinum(iv) analogues proved advantageous but the new prodrugs still suffered from unspecific cytotoxic actions. Thus the photoactivation of an inert platinum prodrug specifically within neoplastic cells provided the desired spatio-temporal control over drug activation by means of illumination, thereby limiting the cytotoxic events to only at the targeted tumors. This article collates research on platinum complexes which exhibit potential light mediated anticancer effects and provides insights into the underlying mechanisms of activation. Fine tuning of the coordination sphere results in dramatic alteration of the redox and spectral properties of both ground and excited states and the cellular properties of the molecules. This concise article highlights the various light promoted strategies employed to attain a controlled release of active platinum(ii) and/or reactive oxygen species such as photoreduction, photocaging, photodissociation and photosensitization. Such dual action photoactive metal complexes with improved aqueous solubility and versatility are promising candidates for combination therapy which is likely to be the future of anticancer research.

Caging the uncageable: using metal complex release for photochemical control over irreversible inhibition

Photochemical control over irreversible inhibition was shown using Ru(ii)-caged inhibitors of cathepsin L. Levels of control were dependent on where the Ru(ii) complex was attached to the organic inhibitor, reaching >10 : 1 with optimal placement. A new strategy for photoreleasing Ru(ii) fragments from inhibitor-enzyme conjugates is also reported.