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

Photodynamic therapy photosensitizers and photoactivated chemotherapeutics exhibit distinct bioenergetic profiles to impact ATP metabolism

Energy is essential for all life, and mammalian cells generate and store energy in the form of ATP by mitochondrial (oxidative phosphorylation) and non-mitochondrial (glycolysis) metabolism. These processes can now be evaluated by extracellular flux analysis (EFA), which has proven to be an indispensable tool in cell biology, providing previously inaccessible information regarding the bioenergetic landscape of cell lines, complex tissues, and in vivo models. Recently, EFA demonstrated its utility as a screening tool in drug development, both by providing insights into small molecule-organelle interactions, and by revealing the peripheral and potentially undesired off-target effects small molecules have within cells. Surprisingly, technologies to quantify cellular bioenergetics have not been systematically applied in phototherapy development, leaving open several questions about how the mechanism of action of a compound can impact essential cellular functions. Here, we utilized the Seahorse analyzer to address this question for photosensitizers (PSs) for photodynamic therapy (PDT) and contrast these systems to molecules that photo-release a ligand and thus act as photocages or photoactivated chemotherapeutics (PACT), intending to understand the influence these two classes of compounds have on cellular bioenergetics. EFA results show that acute treatment of A549 lung adenocarcinoma cells with PDT agents induces a quiescent bioenergetic response as a result of mitochondrial respiration shutdown. The loss of oxidative phosphorylation is followed by disruption of glycolysis, which occurs after an initial increase in glycolytic respiration is unable to compensate for the interruption of the electron transport chain (ETC). In contrast, the PACT agents tested had little impact on cellular respiration, and the minor inhibition of these metabolic processes was not related to the mechanism of action, as reflected by a lack of correlation with photoejection efficiency. Notably, a system capable of both generating 1O2 and photo-releasing a ligand exhibited the dominant profile of a PDT agent and induced the quiescent bioenergetic state, indicating potential implications on cellular bioenergetics for so-called dual-action agents. These findings are presented with the aim to provide the necessary groundwork for expanding the application and utility of EFA to phototherapeutics and to highlight the role of metabolic alterations in PDT.

A serendipitous crossed aldol reaction in the ligand periphery of a Ru(II) polypyridyl complex in silica bed: prospects for delivering anticancer agents for photoactivated chemotherapy

The localized drug action in tumors to overcome the side effects of chemotherapy has become an impetus for the development of photoactivated chemotherapy (PACT). As potential PACT agents, ruthenium(II) polypyridyl complexes have emerged as efficient photocages for anticancer agents. Bioactive molecules possessing functional groups such as nitrile, thioether, pyridine, imidazole, etc. are often directly attached to the primary coordination sphere of Ru(II) polypyridyl complexes for this purpose. Herein, we propose an alternative design strategy to attach potential anticancer agents lacking these functional groups with Ru(II) polypyridyl complexes through a pyridyl linker moiety. The proposition is, however, a thoughtful extrapolation of a serendipitous crossed aldol reaction that took place between the Ru(II)-coordinated 4-Pyridinecarboxaldehyde (4-PyCHO) and acetone, discovered while the Ru(II)-complex [Ru(ttp)(dppz)(4-PyCHO)]2+ {[1]} [ttp = p-tolyl terpyridine, dppz = dipyrido[3,2-a:2',3'-c]phenazine, 4-PyCHO = 4-Pyridinecarboxaldehyde] was being purified by silica gel column chromatography with acetone/water/saturated aqueous KNO3 solution as the eluent. The resultant pure aldol product [Ru(ttp)(dppz)(4-PyCHAc)]2+ {[1-Ac]} [4-PyCHAc = aldol modified 4-Pyridinecarboxaldehyde, i.e., 4-hydroxy-4-(pyridin-4-yl)butan-2-one)], was unambiguously characterized by a variety of spectroscopic techniques and X-ray crystallography. Furthermore, a 1H NMR study after 470 nm light irradiation and subsequent ESI-MS analysis revealed that 4-PyCHO could be photo-released from [1-Ac] as its in situ generated aldol adduct 4-PyCHAc. Therefore, this finding serves as a proof-of-concept that provides a simpler alternative design strategy for appending cancer-selective agents having carbonyl groups with α-hydrogens to ruthenium(II) polypyridyl complexes and their photorelease for selective and targeted anticancer chemotherapy.

Sonoinduced Tumor Therapy and Metastasis Inhibition by a Ruthenium Complex with Dual Action: Superoxide Anion Sensitization and Ligand Fracture

Photoresponsive ruthenium(II) complexes have recently emerged as a promising tool for synergistic photodynamic therapy and chemotherapy in oncology, as well as for antimicrobial applications. However, the limited penetration power of photons prevents the treatment of deep-seated lesions. In this study, we introduce a sonoresponsive ruthenium complex capable of generating superoxide anion (O2•-) via type I process and initiating a ligand fracture process upon ultrasound triggering. Attaching hydroxyflavone (HF) as an "electron reservoir" to the octahedral-polypyridyl-ruthenium complex resulted in decreased highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) energy gaps and triplet-state metal to ligand charge transfer (3MLCT) state energy (0.89 eV). This modification enhanced the generation of O2•- under therapeutic ultrasound irradiation at a frequency of 1 MHz. The produced O2•- rapidly induced an intramolecular cascade reaction and HF ligand fracture. As a proof-of-concept, we engineered the Ru complex into a metallopolymer platform (PolyRuHF), which could be activated by low-power ultrasound (1.5 W cm-2, 1.0 MHz, 50% duty cycle) within a centimeter range of tissue. This activation led to O2•- generation and the release of cytotoxic ruthenium complexes. Consequently, PolyRuHF induced cellular apoptosis and ferroptosis by causing mitochondrial dysfunction and excessive toxic lipid peroxidation. Furthermore, PolyRuHF effectively inhibited subcutaneous and orthotopic breast tumors and prevented lung metastasis by downregulating metastasis-related proteins in mice. This study introduces the first sonoresponsive ruthenium complex for sonodynamic therapy/sonoactivated chemotherapy, offering new avenues for deep tumor treatment.

Prediction and Rationalization of Different Photochemical Behaviors of mer- and fac-Isomers of [Ru(pyridyltriazole)3]2

Facial and meridional isomerism of metal complexes is known to result in fundamental differences in photophysical properties. One may also envisage differences in their photochemical reactivity and therefore predict different outcomes of their light-triggered transformations. The fac- and mer-isomers of the complex [Ru(pytz)3]2+ (fac-1 & mer-1, pytz = 1-benzyl-4-(pyrid-2-yl)-1,2,3-triazole) were separated and isolated. mer-1 undergoes a predicted pytz photodechelation process in acetonitrile to yield trans-[Ru(κ2-pytz)2(κ1-pytz)(NCMe)]2+ (2) whereas unfavorable interligand steric interactions are predicted to, and indeed do prevent comparable photoreactivity for fac-1. Reversible photoisomerization of fac-1 and mer-1 is also observed, however. The differences in photochemical reactivity of the two isomers can be rationalized based on structural programming of the preferential accessibility of particular 3MC excited states due to differences in their interligand steric interactions. Here we present an initial predictive thought experiment, subsequent experimental verification, and computational rationalization of the differences in photochemical reactivity of these two isomeric complexes.

Photochemistry of Ru(II) Triazole Complexes with 6-Membered Chelate Ligands: Detection and Reactivity of Ligand-Loss Intermediates

Photochemical ligand release from metal complexes may be exploited in the development of novel photoactivated chemotherapy agents for the treatment of cancer and other diseases. Highly intriguing photochemical behavior is reported for two ruthenium(II) complexes bearing conformationally flexible 1,2,3-triazole-based ligands incorporating a methylene spacer to form 6-membered chelate rings. [Ru(bpy)2(pictz)]2+ (1) and [Ru(bpy)2(btzm)]2+ (2) (bpy = 2,2'-bipyridyl; pictz = 1-(picolyl)-4-phenyl-1,2,3-triazole; btzm = bis(4-phenyl-1,2,3-triazol-4-yl)methane) exhibit coordination by the triazole ring through the less basic N2 atom as a consequence of chelation and readily undergo photochemical release of the pictz and btzm ligands (ϕ = 0.079 and 0.091, respectively) in acetonitrile solution to form cis-[Ru(bpy)2(NCMe)2]2+ (3) in both cases. Ligand-loss intermediates of the form [Ru(bpy)2(κ1-pictz or κ1-btzm)(NCCD3)]2+ are detected by 1H NMR spectroscopy and mass spectrometry. Photolysis of 1 yields three ligand-loss intermediates with monodentate pictz ligands, two of which form through simple decoordination of either the pyridine or triazole donor with subsequent solvent coordination (4-tz(N2) and 4-py, respectively). The third intermediate, shown to be able to form photochemically directly from 1, arises through linkage isomerism in which the monodentate pictz ligand is coordinated by the triazole N3 atom (4-tz(N3)) with a comparable ligand-loss intermediate with an N3-bound κ1-btzm ligand also observed for 2.

RuBi-Ruxolitinib: A Photoreleasable Antitumor JAK Inhibitor

We describe the synthesis and biological testing of ruthenium-bipyridine ruxolitinib (RuBiRuxo), a photoreleasable form of ruxolitinib, a JAK inhibitor used as an antitumoral agent in cutaneous T-cell lymphomas (CTCL). This novel caged compound is synthesized efficiently, is stable in aqueous solution at room temperature, and is photoreleased rapidly by visible light. Irradiation of RuBiRuxo reduces cell proliferation and induces apoptosis in a light- and time-dependent manner in a CTCL cell line. This effect is specific and is mediated by a decreased phosphorylation of STAT proteins. Our results demonstrate the potential of ruthenium-based photocompounds and light-based therapeutic approaches for the potential treatment of cutaneous lymphomas and other pathologies.

Photopharmacology of Protease Inhibitors: Current Status and Perspectives

Proteases are involved in many essential biological processes. Dysregulation of their activity underlies a wide variety of human diseases. Photopharmacology, as applied on various classes of proteins, has the potential to assist protease research by enabling spatiotemporal control of protease activity. Moreover, it may be used to decrease side-effects of protease-targeting drugs. In this review, we discuss the current status of the chemical design of photoactivatable proteases inhibitors and their biological application. Additionally, we give insight into future possibilities for further development of this field of research.

Photoactivatable Ruthenium Complexes Containing Minimal Straining Benzothiazolyl-1,2,3-triazole Chelators for Cancer Treatment

Ruthenium(II) complexes containing diimine ligands have contributed to the development of agents for photoactivated chemotherapy. Several approaches have been used to obtain photolabile Ru(II) complexes. The two most explored have been the use of monodentate ligands and the incorporation of steric effects between the bidentate ligands and the Ru(II). However, the introduction of electronic effects in the ligands has been less explored. Herein, we report a systematic experimental, theoretical, and photocytotoxicity study of a novel series of Ru(II) complexes Ru1-Ru5 of general formula [Ru(phen)2(N∧N')]2+, where N∧N' are different minimal strained ligands based on the 1-aryl-4-benzothiazolyl-1,2,3-triazole (BTAT) scaffold, being CH3 (Ru1), F (Ru2), CF3 (Ru3), NO2 (Ru4), and N(CH3)2 (Ru5) substituents in the R4 of the phenyl ring. The complexes are stable in solution in the dark, but upon irradiation in water with blue light (λex = 465 nm, 4 mW/cm2) photoejection of the ligand BTAT was observed by HPLC-MS spectrometry and UV-vis spectroscopy, with t1/2 ranging from 4.5 to 14.15 min depending of the electronic properties of the corresponding BTAT, being Ru4 the less photolabile (the one containing the more electron withdrawing substituent, NO2). The properties of the ground state singlet and excited state triplet of Ru1-Ru5 have been explored using density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations. A mechanism for the photoejection of the BTAT ligand from the Ru complexes, in H2O, is proposed. Phototoxicity studies in A375 and HeLa human cancer cell lines showed that the new Ru BTAT complexes were strongly phototoxic. An enhancement of the emission intensity of HeLa cells treated with Ru5 was observed in response to increasing doses of light due to the photoejection of the BTAT ligand. These studies suggest that BTAT could serve as a photocleavable protecting group for the cytotoxic bis-aqua ruthenium warhead [Ru(phen)2(OH2)2]2+.

The Impact of Inorganic Systems and Photoactive Metal Compounds on Cytochrome P450 Enzymes and Metabolism: From Induction to Inhibition

While cytochrome P450 (CYP; P450) enzymes are commonly associated with the metabolism of organic xenobiotics and drugs or the biosynthesis of organic signaling molecules, they are also impacted by a variety of inorganic species. Metallic nanoparticles, clusters, ions, and complexes can alter CYP expression, modify enzyme interactions with reductase partners, and serve as direct inhibitors. This commonly overlooked topic is reviewed here, with an emphasis on understanding the structural and physiochemical basis for these interactions. Intriguingly, while both organometallic and coordination compounds can act as potent CYP inhibitors, there is little evidence for the metabolism of inorganic compounds by CYPs, suggesting a potential alternative approach to evading issues associated with rapid modification and elimination of medically useful compounds.

Ruthenium Pyrazole Complexes: A Family of Highly Active Metallodrugs for Photoactivated Chemotherapy

Ruthenium complexes bearing bis pyrazole (pzH) ligands, cis-[Ru(bpy)2(R-pzH)2]2+ (bpy = 2,2'-bipyridine, R = -H, -Cl), were examined as photoactivated anticancer prodrugs. A dicationic pyrazole complex deprotonated to give monocationic pyrazole-pyrazolate complexes, cis-[Ru(bpy)2(R-pz-)(R-pzH)]+, in an aqueous solution with pKa values of 9.5 and 7.2 for R = H and R = Cl, respectively. Upon deprotonation, relative quantum yields of photosubstitution decreased while lipophilicity of the complexes increased according to the measurements of water-octanol coefficients. The ruthenium complex with 4-chloropyrazole ligands displayed high cytotoxicity upon light irradiation (IC50 = 0.060 ± 0.016 μM) toward lung cancer cells, which was 7 times higher than that in the dark (IC50 = 0.44 ± 0.07 μM). Additional experiments for the ruthenium R-pyrazole complexes indicated that (1) selective photodissociation of the 4-chloropyrazole ligand occurs from cis-[Ru(bpy)2(4-Clpz-)(4-ClpzH)]+, (2) photoinduced ligand dissociation is dominant rather than photoinduced generation of singlet oxygen (1O2), and (3) induction of cell death occurs via the intrinsic pathway of apoptosis.

Red-Light Activation of a Microtubule Polymerization Inhibitor via Amide Functionalization of the Ruthenium Photocage

Photoactivated chemotherapy (PACT) is a promising cancer treatment modality that kills cancer cells via photochemical uncaging of a cytotoxic drug. Most ruthenium-based photocages used for PACT are activated with blue or green light, which penetrates sub-optimally into tumor tissues. Here, we report amide functionalization as a tool to fine-tune the toxicity and excited states of a terpyridine-based ruthenium photocage. Due to conjugation of the amide group with the terpyridine π system in the excited state, the absorption of red light (630 nm) increased 8-fold, and the photosubstitution rate rose 5-fold. In vitro, red light activation triggered inhibition of tubulin polymerization, which led to apoptotic cell death both in normoxic (21 % O2 ) and hypoxic (1 % O2 ) cancer cells. In vivo, red light irradiation of tumor-bearing mice demonstrated significant tumor volume reduction (45 %) with improved biosafety, thereby demonstrating the clinical potential of this compound.

Differences in Photophysical Properties and Photochemistry of Ru(II)-Terpyridine Complexes of CH3CN and Pyridine

A series of 22 Ru(II) complexes of the type [Ru(tpy)(L)(L')]n+, where tpy is the tridentate ligand 2,2';6,2″-terpyridine, L represents bidentate ligands with varying electron-donating ability, and L' is acetonitrile (1a-11a) or pyridine (1b-11b), were investigated. The dissociation of acetonitrile occurs from the 3MLCT state in 1a-11a, such that it does not require the population of a 3LF state. Electrochemistry and spectroscopic data demonstrate that the ground states of these series do not differ significantly. Franck-Condon line-shape analysis of the 77 K emission data shows no significant differences between the emitting 3MLCT states in both series. Arrhenius analysis of the temperature dependence of 3MLCT lifetimes shows that the energy barrier (Ea) to thermally populating a 3LF state from a lower energy 3MLCT state is significantly higher in the pyridine than in the CH3CN series, consistent with the photostability of complexes 1b-11b, which do not undergo pyridine photodissociation under our experimental conditions. Importantly, these results demonstrate that ligand photodissociation of pyridine in 1b-11b does not take place directly from the 3MLCT state, as is the case for 1a-11a. These findings have potential impact on the rational design of complexes for a number of applications, including photochemotherapy, dye-sensitized solar cells, and photocatalysis.

Ru(II) Phenanthroline-Based Oligothienyl Complexes as Phototherapy Agents

Ru(II) polypyridyl complexes have gained widespread attention as photosensitizers for photodynamic therapy (PDT). Herein, we systematically investigate a series of the type [Ru(phen)2(IP-nT)]2+, featuring 1,10-phenanthroline (phen) coligands and imidazo[4,5-f][1,10]phenanthroline ligands tethered to n = 0-4 thiophene rings (IP-nT). The complexes were characterized and investigated for their electrochemical, spectroscopic, and (photo)biological properties. The electrochemical oxidation of the nT unit shifted by -350 mV as n = 1 → 4 (+920 mV for Ru-1T, +570 mV for Ru-4T); nT reductions were observed in complexes Ru-3T (-2530 mV) and Ru-4T (-2300 mV). Singlet oxygen quantum yields ranged from 0.53 to 0.88, with Ru-3T and Ru-4T being equally efficient (∼0.88). Time-resolved absorption spectra of Ru-0T-1T were dominated by metal-to-ligand charge-transfer (3MLCT) states (τTA = 0.40-0.85 μs), but long-lived intraligand charge-transfer (3ILCT) states were observed in Ru-2T-4T (τTA = 25-148 μs). The 3ILCT energies of Ru-3T and Ru-4T were computed to be 1.6 and 1.4 eV, respectively. The phototherapeutic efficacy against melanoma cells (SK-MEL-28) under broad-band visible light (400-700 nm) increases as n = 0 → 4: Ru-0T was inactive up to 300 μM, Ru-1T-2T were moderately active (EC50 ∼ 600 nM, PI = 200), and Ru-3T (EC50 = 57 nM, PI > 1100) and Ru-4T (EC50 = 740 pM, PI = 114,000) were the most phototoxic. The activity diminishes with longer wavelengths of light and is completely suppressed for all complexes except Ru-3T and Ru-4T in hypoxia. Ru-4T is the more potent and robust PS in 1% O2 over seven biological replicates (avg EC50 = 1.3 μM, avg PI = 985). Ru-3T exhibited hypoxic activity in five of seven replicates, underscoring the need for biological replicates in compound evaluation. Singlet oxygen sensitization is likely responsible for phototoxic effects of the compounds in normoxia, but the presence of redox-active excited states may facilitate additional photoactive pathways for complexes with three or more thienyl groups. The 3ILCT state with its extended lifetime (30-40× longer than the 3MLCT state for Ru-3T and Ru-4T) implicates its predominant role in photocytotoxicity.

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.

Ruthenium-Based Photoactivated Chemotherapy

Ruthenium(II) polypyridyl complexes form a vast family of molecules characterized by their finely tuned photochemical and photophysical properties. Their ability to undergo excited-state deactivation via photosubstitution reactions makes them quite unique in inorganic photochemistry. As a consequence, they have been used, in general, for building dynamic molecular systems responsive to light but, more particularly, in the field of oncology, as prodrugs for a new cancer treatment modality called photoactivated chemotherapy (PACT). Indeed, the ability of a coordination bond to be selectively broken under visible light irradiation offers fascinating perspectives in oncology: it is possible to make poorly toxic agents in the dark that become activated toward cancer cell killing by simple visible light irradiation of the compound inside a tumor. In this Perspective, we review the most important concepts behind the PACT idea, the relationship between ruthenium compounds used for PACT and those used for a related phototherapeutic approach called photodynamic therapy (PDT), and we discuss important questions about real-life applications of PACT in the clinic. We conclude this Perspective with important challenges in the field and an outlook.

Tricolor visible wavelength-selective photodegradable hydrogel biomaterials

Photodynamic hydrogel biomaterials have demonstrated great potential for user-triggered therapeutic release, patterned organoid development, and four-dimensional control over advanced cell fates in vitro. Current photosensitive materials are constrained by their reliance on high-energy ultraviolet light (<400 nm) that offers poor tissue penetrance and limits access to the broader visible spectrum. Here, we report a family of three photolabile material crosslinkers that respond rapidly and with unique tricolor wavelength-selectivity to low-energy visible light (400-617 nm). We show that when mixed with multifunctional poly(ethylene glycol) macromolecular precursors, ruthenium polypyridyl- and ortho-nitrobenzyl (oNB)-based crosslinkers yield cytocompatible biomaterials that can undergo spatiotemporally patterned, uniform bulk softening, and multiplexed degradation several centimeters deep through complex tissue. We demonstrate that encapsulated living cells within these photoresponsive gels show high viability and can be successfully recovered from the hydrogels following photodegradation. Moving forward, we anticipate that these advanced material platforms will enable new studies in 3D mechanobiology, controlled drug delivery, and next-generation tissue engineering applications.

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.

Ruthenium(II) Polypyridyl Complexes and Metronidazole Derivatives: A Powerful Combination in the Design of Photoresponsive Antibacterial Agents Effective under Hypoxic Conditions

Ruthenium(II) polypyridyl complexes (RPCs) are gaining momentum in photoactivated chemotherapy (PACT), thanks to the possibility of overcoming the classical reliance on molecular oxygen of photodynamic therapy while preserving the selective drug activation by using light. However, notwithstanding the intriguing perspectives, the translation of such an approach in the development of new antimicrobials has been only barely considered. Herein, MTZH-1 and MTZH-2, two novel analogues of metronidazole (MTZ), a mainstay drug in the treatment of anaerobic bacterial infections, were designed and inserted in the strained ruthenium complexes [Ru(tpy)(dmp)(MTZ-1)]PF₆ (Ru2) and [Ru(tpy)(dmp)(MTZ-2)]PF₆ (Ru3) (tpy = terpyridine, dmp = 2,9-dimethyl-1,10-phenanthroline) (Chart 1). Analogously to the parental compound [Ru(tpy)(dmp)(5NIM)]PF₆ (Ru1) (5-nitroimidazolate), the Ru(II)-imidazolate coordination of MTZ derivatives resulted in promising Ru(II) photocages, capable to easily unleash the bioactive ligands upon light irradiation and increase the antibacterial activity against Bacillus subtilis, which was chosen as a model of Gram-positive bacteria. The photoreleased 5-nitroimidazole-based ligands led to remarkable phototoxicities under hypoxic conditions (<1% O₂), with the lead compound Ru3 that exhibited the highest potency across the series, being comparable to the one of the clinical drug MTZ. Besides, the chemical architectures of MTZ derivatives made their interaction with NimAunfavorable, being NimA a model of reductases responsible for bacterial resistance against 5-nitroimidazole-based antibiotics, thus hinting at their possible use to combat antimicrobial resistance. This work may therefore provide fundamental knowledge in the design of novel photoresponsive tools to be used in the fight against infectious diseases. For the first time, the effectiveness of the "photorelease antimicrobial therapy" under therapeutically relevant hypoxic conditions was demonstrated.

Ruthenium Complexes with Protic Ligands: Influence of the Position of OH Groups and π Expansion on Luminescence and Photocytotoxicity

Protic ruthenium complexes using the dihydroxybipyridine (dhbp) ligand combined with a spectator ligand (N,N = bpy, phen, dop, Bphen) have been studied for their potential activity vs. cancer cells and their photophysical luminescent properties. These complexes vary in the extent of π expansion and the use of proximal (6,6'-dhbp) or distal (4,4'-dhbp) hydroxy groups. Eight complexes are studied herein as the acidic (OH bearing) form, [(N,N)₂Ru(n,n'-dhbp)]Cl₂, or as the doubly deprotonated (O^- bearing) form. Thus, the presence of these two protonation states gives 16 complexes that have been isolated and studied. Complex 7_A, [(dop)₂Ru(4,4'-dhbp)]Cl₂, has been recently synthesized and characterized spectroscopically and by X-ray crystallography. The deprotonated forms of three complexes are also reported herein for the first time. The other complexes studied have been synthesized previously. Three complexes are light-activated and exhibit photocytotoxicity. The log(D_o/w) values of the complexes are used herein to correlate photocytotoxicity with improved cellular uptake. For Ru complexes 1-4 bearing the 6,6'-dhbp ligand, photoluminescence studies (all in deaerated acetonitrile) have revealed that steric strain leads to photodissociation which tends to reduce photoluminescent lifetimes and quantum yields in both protonation states. For Ru complexes 5-8 bearing the 4,4'-dhbp ligand, the deprotonated Ru complexes (5_B-8_B) have low photoluminescent lifetimes and quantum yields due to quenching that is proposed to involve the ³LLCT excited state and charge transfer from the [O₂-bpy]^2- ligand to the N,N spectator ligand. The protonated OH bearing 4,4'-dhbp Ru complexes (5_A-8_A) have long luminescence lifetimes which increase with increasing π expansion on the N,N spectator ligand. The Bphen complex, 8_A, has the longest lifetime of the series at 3.45 μs and a photoluminescence quantum yield of 18.7%. This Ru complex also exhibits the best photocytotoxicity of the series. A long luminescence lifetime is correlated with greater singlet oxygen quantum yields because the triplet excited state is presumably long-lived enough to interact with ³O₂ to yield ¹O₂.

Synthesis and photobiological evaluation of Ru(II) complexes with expanded chelate polypyridyl ligands

Photoreactive Ru(II) complexes capable of ejecting ligands have been used extensively for photocaging applications and for the creation of "photocisplatin" reagents. The incorporation of distortion into the structure of the coordination complex lowers the energy of dissociative excited states, increasing the yield of the photosubstitution reaction. While steric clash between ligands induced by adding substituents at the coordinating face of the ligand has been extensively utilized, a lesser known, more subtle approach is to distort the coordination sphere by altering the chelate ring size. Here a systematic study was performed to alter metal-ligand bond lengths, angles, and to cause intraligand distortion by introducing a "linker" atom or group between two pyridine rings. The synthesis, photochemistry, and photobiology of five Ru(II) complexes containing CH₂, NH, O, and S-linked dipyridine ligands was investigated. All systems where stable in the dark, and three of the five were photochemically active in buffer. While a clear periodic trend was not observed, this study lays the foundation for the creation of photoactive systems utilizing an alternative type of distortion to facilitate photosubstitution reactions.

Not All 3MC States Are the Same: The Role of 3MCcis States in the Photochemical N∧N Ligand Release from [Ru(bpy)2(N∧N)]2+ Complexes

Ruthenium(II) complexes feature prominently in the development of agents for photoactivated chemotherapy; however, the excited-state mechanisms by which photochemical ligand release operates remain unclear. We report here a systematic experimental and computational study of a series of complexes [Ru(bpy)₂(N^∧N)]²⁺ (bpy = 2,2'-bipyridyl; N^∧N = bpy (1), 6-methyl-2,2'-bipyridyl (2), 6,6'-dimethyl-2,2'-bipyridyl (3), 1-benzyl-4-(pyrid-2-yl)-1,2,3-triazole (4), 1-benzyl-4-(6-methylpyrid-2-yl)-1,2,3-triazole (5), 1,1'-dibenzyl-4,4'-bi-1,2,3-triazolyl (6)), in which we probe the contribution to the promotion of photochemical N^∧N ligand release of the introduction of sterically encumbering methyl substituents and the electronic effect of replacement of pyridine by 1,2,3-triazole donors in the N^∧N ligand. Complexes 2 to 6 all release the ligand N^∧N on irradiation in acetonitrile solution to yield cis-[Ru(bpy)₂(NCMe)₂]²⁺, with resultant photorelease quantum yields that at first seem counter-intuitive and span a broad range. The data show that incorporation of a single sterically encumbering methyl substituent on the N^∧N ligand (2 and 5) leads to a significantly enhanced rate of triplet metal-to-ligand charge-transfer (³MLCT) state deactivation but with little promotion of photoreactivity, whereas replacement of pyridine by triazole donors (4 and 6) leads to a similar rate of ³MLCT deactivation but with much greater photochemical reactivity. The data reported here, discussed in conjunction with previously reported data on related complexes, suggest that monomethylation in 2 and 5 sterically inhibits the formation of a ³MC_cis state but promotes the population of ³MC_trans states which rapidly deactivate ³MLCT states and are prone to mediating ground-state recovery. On the other hand, increased photochemical reactivity in 4 and 6 seems to stem from the accessibility of ³MC_cis states. The data provide important insights into the excited-state mechanism of photochemical ligand release by Ru(II) tris-bidentate complexes.

Ruthenium-Locked Helical Chirality: A Barrier of Inversion and Formation of an Asymmetric Macrocycle

Upon coordination to metal centers, tetradentate ligands based on the 6,6′-bis(2″-aminopyridyl)-2,2′-bipyridine (bapbpy) structure form helical chiral complexes due to the steric clash between the terminal pyridines of the ligand. For octahedral ruthenium(II) complexes, the two additional axial ligands bound to the metal center, when different, generate diastereotopic aromatic protons that can be distinguished by NMR. Based on these geometrical features, the inversion barrier of helical [Ru^II(L)(RR′SO)Cl]⁺ complexes, where L is a sterically hindered bapbpy derivative and RR′SO is a chiral or achiral sulfoxide ligand, was studied by variable-temperature ¹H NMR. The coalescence energies for the inversion of the helical chirality of [Ru(bapbpy)(DMSO)(Cl)]Cl and [Ru(bapbpy)(MTSO)(Cl)]Cl (where MTSO is (R)-methyl p-tolylsulfoxide) were found to be 43 and 44 kJ/mol, respectively. By contrast, in [Ru(biqbpy)(DMSO)(Cl)]Cl (biqbpy = 6,6′-bis(aminoquinolyl)-2,2′-bipyridine), increased strain caused by the larger terminal quinoline groups resulted in a coalescence temperature higher than 376 K, which pointed to an absence of helical chirality inversion at room temperature. Further increasing the steric strain by introducing methoxy groups ortho to the nitrogen atoms of the terminal pyridyl groups in bapbpy resulted in the serendipitous discovery of a ring-closing reaction that took place upon trying to make [Ru(OMe-bapbpy)(DMSO)Cl]⁺ (OMe-bapbpy = 6,6′-bis(6-methoxy-aminopyridyl)-2,2′-bipyridine). This reaction generated, in excellent yields, a chiral complex [Ru(L″)(DMSO)Cl]Cl, where L″ is an asymmetric tetrapyridyl macrocycle. This unexpected transformation appears to be specific to ruthenium(II) as macrocyclization did not occur upon coordination of the same ligand to palladium(II) or rhodium(III).

Ruthenium complexes based on the bapbpy ligand form helical chiral complexes due to the steric clash between their terminal pyridyl groups. The coalescence energy for the inversion of this helical chirality was 43 kJ/mol according to variable temperature NMR. Increasing the steric strain by replacing terminal pyridyl groups with quinolyl groups blocked helical interconversion, while introducing ortho-methoxy groups resulted in an unexpected ring-closing reaction, forming a dissymmetric macrocycle bound to ruthenium in excellent yields.

Ru(II) photocages enable precise control over enzyme activity with red light

The cytochrome P450 family of enzymes (CYPs) are important targets for medicinal chemistry. Recently, CYP1B1 has emerged as a key player in chemotherapy resistance in the treatment of cancer. This enzyme is overexpressed in a variety of tumors, and is correlated with poor treatment outcomes; thus, it is desirable to develop CYP1B1 inhibitors to restore chemotherapy efficacy. However, possible off-target effects, such as inhibition of liver CYPs responsible for first pass metabolism, make selective inhibition a high priority to avoid possible drug-drug interactions and toxicity. Here we describe the creation of light-triggered CYP1B1 inhibitors as "prodrugs", and achieve >6000-fold improvement in potency upon activation with low energy (660 nm) light. These systems provide a selectivity index of 4,000-100,000 over other off-target CYPs. One key to the design was the development of coordinating CYP1B1 inhibitors, which suppress enzyme activity at pM concentrations in live cells. The metal binding group enforces inhibitor orientation in the active site by anchoring to the iron. The second essential component was the biologically compatible Ru(II) scaffold that cages the inhibitors before photochemical release. These Ru(II) photocages are anticipated to provide similar selectivity and control for any coordinating CYP inhibitors.

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.

A supramolecular photosensitizer derived from an Arene-Ru(II) complex self-assembly for NIR activated photodynamic and photothermal therapy

Effective photosensitizers are of particular importance for the widespread clinical utilization of phototherapy. However, conventional photosensitizers are usually plagued by short-wavelength absorption, inadequate photostability, low reactive oxygen species (ROS) quantum yields, and aggregation-caused ROS quenching. Here, we report a near-infrared (NIR)-supramolecular photosensitizer (RuDA) via self-assembly of an organometallic Ru(II)-arene complex in aqueous solution. RuDA can generate singlet oxygen (¹O₂) only in aggregate state, showing distinct aggregation-induced ¹O₂ generation behavior due to the greatly increased singlet-triplet intersystem crossing process. Upon 808 nm laser irradiation, RuDA with excellent photostability displays efficient ¹O₂ and heat generation in a ¹O₂ quantum yield of 16.4% (FDA-approved indocyanine green: Φ_Δ = 0.2%) together with high photothermal conversion efficiency of 24.2% (commercial gold nanorods: 21.0%, gold nanoshells: 13.0%). In addition, RuDA-NPs with good biocompatibility can be preferably accumulated at tumor sites, inducing significant tumor regression with a 95.2% tumor volume reduction in vivo during photodynamic therapy. This aggregation enhanced photodynamic therapy provides a strategy for the design of photosensitizers with promising photophysical and photochemical characteristics.

Intraligand Excited States Turn a Ruthenium Oligothiophene Complex into a Light-Triggered Ubertoxin with Anticancer Effects in Extreme Hypoxia

Ru(II) complexes that undergo photosubstitution reactions from triplet metal-centered (3MC) excited states are of interest in photochemotherapy (PCT) due to their potential to produce cytotoxic effects in hypoxia. Dual-action systems that incorporate this stoichiometric mode to complement the oxygen-dependent photosensitization pathways that define photodynamic therapy (PDT) are poised to maintain antitumor activity regardless of the oxygenation status. Herein, we examine the way in which these two pathways influence photocytotoxicity in normoxia and in hypoxia using the [Ru(dmp)2(IP-nT)]2+ series (where dmp = 2,9-dimethyl-1,10-phenanthroline and IP-nT = imidazo[4,5-f][1,10]phenanthroline tethered to n = 0-4 thiophene rings) to switch the dominant excited state from the metal-based 3MC state in the case of Ru-phen-Ru-1T to the ligand-based 3ILCT state for Ru-3T and Ru-4T. Ru-phen-Ru-1T, having dominant 3MC states and the largest photosubstitution quantum yields, are inactive in both normoxia and hypoxia. Ru-3T and Ru-4T, with dominant 3IL/3ILCT states and long triplet lifetimes (τTA = 20-25 μs), have the poorest photosubstitution quantum yields, yet are extremely active. In the best instances, Ru-4T exhibit attomolar phototoxicity toward SKMEL28 cells in normoxia and picomolar in hypoxia, with phototherapeutic index values in normoxia of 105-1012 and 103-106 in hypoxia. While maximizing excited-state deactivation through photodissociative 3MC states did not result in bonafide dual-action PDT/PCT agents, the study has produced the most potent photosensitizer we know of to date. The extraordinary photosensitizing capacity of Ru-3T and Ru-4T may stem from a combination of very efficient 1O2 production and possibly complementary type I pathways via 3ILCT excited states.

Factors that influence singlet oxygen formation vs. ligand substitution for light-activated ruthenium anticancer compounds

This review focuses on light-activated ruthenium anticancer compounds and the factors that influence which pathway is favored. Photodynamic therapy (PDT) is favored by π expansion and the presence of low-lying triplet excited states (e.g. ³MLCT, ³IL). Photoactivated chemotherapy (PACT) refers to light-driven ligand dissociation to give a toxic metal complex or a toxic ligand upon photo substitution. This process is driven by steric bulk near the metal center and weak metal-ligand bonds to create a low-energy ³MC state with antibonding character. With protic dihydroxybipyridine ligands, ligand charge can play a key role in these processes, with a more electron-rich deprotonated ligand favoring PDT and an electron-poor protonated ligand favoring PACT in several cases.

Photodecaging of a Mitochondria-Localized Iridium(III) Endoperoxide Complex for Two-Photon Photoactivated Therapy under Hypoxia

Despite the clinical success of photodynamic therapy (PDT), the application of this medical technique is intrinsically limited by the low oxygen concentrations found in cancer tumors, hampering the production of therapeutically necessary singlet oxygen (¹O₂). To overcome this limitation, we report on a novel mitochondria-localized iridium(III) endoperoxide prodrug (2-O-IrAn), which, upon two-photon irradiation in NIR, synergistically releases a highly cytotoxic iridium(III) complex (2-IrAn), singlet oxygen, and an alkoxy radical. 2-O-IrAn was found to be highly (photo-)toxic in hypoxic tumor cells and multicellular tumor spheroids (MCTS) in the nanomolar range. To provide cancer selectivity and improve the pharmacological properties of 2-O-IrAn, it was encapsulated into a biotin-functionalized polymer. The generated nanoparticles were found to nearly fully eradicate the tumor inside a mouse model within a single treatment. This study presents, to the best of our knowledge, the first example of an iridium(III)-based endoperoxide prodrug for synergistic photodynamic therapy/photoactivated chemotherapy, opening up new avenues for the treatment of hypoxic tumors.

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.

Anticancer Agent with Inexplicable Potency in Extreme Hypoxia: Characterizing a Light-Triggered Ruthenium Ubertoxin

Tumor hypoxia renders treatments ineffective that are directly (e.g., radiotherapy and photodynamic therapy) or indirectly (e.g., chemotherapy) dependent on tumor oxygenation. This study introduces a ruthenium compound as a light-responsive anticancer agent that is water-soluble, has minimal dark cytotoxicity, is active at concentrations as low as 170 pM in ∼18.5% O₂ normoxia and near 10 nM in 1% O₂ hypoxia, and exhibits phototherapeutic indices as large as >500,000 in normoxia and >5,800 in 1% O₂ hypoxia using broadband visible and monochromatic blue light treatments. These are the largest values reported to date for any compound class. We highlight the response in four different cell lines to improve rigor and reproducibility in the identification of promising clinical candidates.

Nitroimidazole-Based Ruthenium(II) Complexes: Playing with Structural Parameters to Design Photostable and Light-Responsive Antibacterial Agents

5-Nitroimidazole (5NIMH), chosen as a molecular model of nitroimidazole derivatives, which represent a broad-spectrum class of antimicrobials, was incorporated into the ruthenium complexes [Ru(tpy)(phen)(5NIM)]PF₆ (1) and [Ru(tpy)(dmp)(5NIM)]PF₆ (2) (tpy = terpyridine, phen = phenanthroline, dmp = 2,9-dimethyl-1,10-phenanthroline). Besides the uncommon metal coordination of 5-nitroimidazole in its imidazolate form (5NIM), the different architectures of the spectator ligands (phen and dmp) were exploited to tune the "mode of action" of the resulting complexes, passing from a photostable compound where the redox properties of 5NIMH are preserved (1) to one suitable for the nitroimidazole phototriggered release (2) and whose antibacterial activity against B. subtilis, chosen as cellular model, is effectively improved upon light exposure. This study may provide a fundamental knowledge on the use of Ru(II)-polypyridyl complexes to incorporate and/or photorelease biologically relevant nitroimidazole derivatives in the design of a novel class of antimicrobials.

Using photocaging for fast time-resolved structural biology studies

Careful selection of photocaging approaches is critical to achieve fast and well synchronized reaction initiation and perform successful time-resolved structural biology experiments. This review summarizes the best characterized and most relevant photocaging groups previously described in the literature. It also provides a walkthrough of the essential factors to consider in designing a suitable photocaged molecule to address specific biological questions, focusing on photocaging groups with well characterized spectroscopic properties. The relationships between decay rates (k in s-1), quantum yields (ϕ) and molar extinction coefficients (ϵmax in M-1 cm-1) are highlighted for different groups. The effects of the nature of the photocaged group on these properties is also discussed. Four main photocaging scaffolds are presented in detail, o-nitrobenzyls, p-hydroxyphenyls, coumarinyls and nitrodibenzofuranyls, along with three examples of the use of this technology. Furthermore, a subset of specialty photocages are highlighted: photoacids, molecular photoswitches and metal-containing photocages. These extend the range of photocaging approaches by, for example, controlling pH or generating conformationally locked molecules.

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.

Ruthenium-based PACT agents based on bisquinoline chelates: synthesis, photochemistry, and cytotoxicity

The known ruthenium complex [Ru(tpy)(bpy)(Hmte)](PF6)2 ([1](PF6)2, where tpy = 2,2':6',2″-terpyridine, bpy = 2,2'-bipyridine, Hmte = 2-(methylthio)ethanol) is photosubstitutionally active but non-toxic to cancer cells even upon light irradiation. In this work, the two analogs complexes [Ru(tpy)(NN)(Hmte)](PF6)2, where NN = 3,3'-biisoquinoline (i-biq, [2](PF6)2) and di(isoquinolin-3-yl)amine (i-Hdiqa, [3](PF6)2), were synthesized and their photochemistry and phototoxicity evaluated to assess their suitability as photoactivated chemotherapy (PACT) agents. The increase of the aromatic surface of [2](PF6)2 and [3](PF6)2, compared to [1](PF6)2, leads to higher lipophilicity and higher cellular uptake for the former complexes. Such improved uptake is directly correlated to the cytotoxicity of these compounds in the dark: while [2](PF6)2 and [3](PF6)2 showed low EC50 values in human cancer cells, [1](PF6)2 is not cytotoxic due to poor cellular uptake. While stable in the dark, all complexes substituted the protecting thioether ligand upon light irradiation (520 nm), with the highest photosubstitution quantum yield found for [3](PF6)2 (Φ[3] = 0.070). Compounds [2](PF6)2 and [3](PF6)2 were found both more cytotoxic after light activation than in the dark, with a photo index of 4. Considering the very low singlet oxygen quantum yields of these compounds, and the lack of cytotoxicity of the photoreleased Hmte thioether ligand, it can be concluded that the toxicity observed after light activation is due to the photoreleased aqua complexes [Ru(tpy)(NN)(OH2)]2+, and thus that [2](PF6)2 and [3](PF6)2 are promising PACT candidates.

Modification of amyloid-beta peptide aggregation via photoactivation of strained Ru(ii) polypyridyl complexes

Alzheimer's disease (AD) is a chronic neurodegenerative disorder characterized by progressive and irreversible damage to the brain. One of the hallmarks of the disease is the presence of both soluble and insoluble aggregates of the amyloid beta (Aβ) peptide in the brain, and these aggregates are considered central to disease progression. Thus, the development of small molecules capable of modulating Aβ peptide aggregation may provide critical insight into the pathophysiology of AD. In this work we investigate how photoactivation of three distorted Ru(ii) polypyridyl complexes (Ru1-3) alters the aggregation profile of the Aβ peptide. Photoactivation of Ru1-3 results in the loss of a 6,6'-dimethyl-2,2'-bipyridyl (6,6'-dmb) ligand, affording cis-exchangeable coordination sites for binding to the Aβ peptide. Both Ru1 and Ru2 contain an extended planar imidazo[4,5-f][1,10]phenanthroline ligand, as compared to a 2,2'-bipyridine ligand for Ru3, and we show that the presence of the phenanthroline ligand promotes covalent binding to Aβ peptide His residues, and in addition, leads to a pronounced effect on peptide aggregation immediately after photoactivation. Interestingly, all three complexes resulted in a similar aggregate size distribution at 24 h, forming insoluble amorphous aggregates as compared to significant fibril formation for peptide alone. Photoactivation of Ru1-3 in the presence of pre-formed Aβ1-42 fibrils results in a change to amorphous aggregate morphology, with Ru1 and Ru2 forming large amorphous aggregates immediately after activation. Our results show that photoactivation of Ru1-3 in the presence of either monomeric or fibrillar Aβ1-42 results in the formation of large amorphous aggregates as a common endpoint, with Ru complexes incorporating the extended phenanthroline ligand accelerating this process and thereby limiting the formation of oligomeric species in the initial stages of the aggregation process that are reported to show considerable toxicity.

Metal complexes as optical probes for DNA sensing and imaging

Transition and lanthanide metal complexes have rich photophysical properties that can be used for cellular imaging, biosensing and phototherapy. One of the applications of such luminescent compounds is the detection and visualisation of nucleic acids. In this brief review, we survey the recent literature on the use of luminescent metal complexes (including Re^I, Ru^II, Os^II, Ir^III, Pt^II, Eu^III and Tb^III) as DNA optical probes, including examples of compounds that bind selectively to non-duplex DNA topologies such as quadruplex, i-motif and DNA mismatches. We discuss the applications of metal-based luminescent complexes in cellular imaging, including time-resolved microscopy and super-resolution techniques. Their applications in biosensing and phototherapy are briefly mentioned in the relevant sections.

Taking phototherapeutics from concept to clinical launch

More than four decades have passed since the first example of a light-activated (caged) compound was described. In the intervening years, a large number of light-responsive derivatives have been reported, several of which have found utility under a variety of in vitro conditions using cells and tissues. Light-triggered bioactivity furnishes spatial and temporal control, and offers the possibility of precision dosing and orthogonal communication with different biomolecules. These inherent attributes of light have been advocated as advantageous for the delivery and/or activation of drugs at diseased sites for a variety of indications. However, the tissue penetrance of light is profoundly wavelength-dependent. Only recently have phototherapeutics that are photoresponsive in the optical window of tissue (600-900 nm) been described. This Review highlights these recent discoveries, along with their limitations and clinical opportunities. In addition, we describe preliminary in vivo studies of prospective phototherapeutics, with an emphasis on the path that remains to be navigated in order to translate light-activated drugs into clinically useful therapeutics. Finally, the unique attributes of phototherapeutics is highlighted by discussing several potential disease applications.

Ru(II)-diimine complexes and cytochrome P450 working hand-in-hand

With a growing interest in utilizing visible light to drive biocatalytic processes, several light-harvesting units and approaches have been employed to harness the synthetic potential of heme monooxygenases and carry out selective oxyfunctionalization of a wide range of substrates. While the fields of cytochrome P450 and Ru(II) photochemistry have separately been prolific, it is not until the turn of the 21st century that they converged. Non-covalent and subsequently covalently attached Ru(II) complexes were used to promote rapid intramolecular electron transfer in bacterial P450 enzymes. Photocatalytic activity with Ru(II)-modified P450 enzymes was achieved under reductive conditions with a judicious choice of a sacrificial electron donor. The initial concept of Ru(II)-modified P450 enzymes was further improved using protein engineering, photosensitizer functionalization and was successfully applied to other P450 enzymes. In this review, we wish to present the recent contributions from our group and others in utilizing Ru(II) complexes coupled with P450 enzymes in the broad context of photobiocatalysis, protein assemblies and chemoenzymatic reactions. The merging of chemical catalysts with the synthetic potential of P450 enzymes has led to the development of several chemoenzymatic approaches. Moreover, strained Ru(II) compounds have been shown to selectively inhibit P450 enzymes by releasing aromatic heterocycle containing molecules upon visible light excitation taking advantage of the rapid ligand loss feature in those complexes.

Bis[pyrrolyl Ru(ii)] triads: a new class of photosensitizers for metal-organic photodynamic therapy

A new family of ten dinuclear Ru(ii) complexes based on the bis[pyrrolyl Ru(ii)] triad scaffold, where two Ru(bpy)2 centers are separated by a variety of organic linkers, was prepared to evaluate the influence of the organic chromophore on the spectroscopic and in vitro photodynamic therapy (PDT) properties of the compounds. The bis[pyrrolyl Ru(ii)] triads absorbed strongly throughout the visible region, with several members having molar extinction coefficients (ε) ≥ 104 at 600-620 nm and longer. Phosphorescence quantum yields (Φ p) were generally less than 0.1% and in some cases undetectable. The singlet oxygen quantum yields (Φ Δ) ranged from 5% to 77% and generally correlated with their photocytotoxicities toward human leukemia (HL-60) cells regardless of the wavelength of light used. Dark cytotoxicities varied ten-fold, with EC50 values in the range of 10-100 μM and phototherapeutic indices (PIs) as large as 5400 and 260 with broadband visible (28 J cm-2, 7.8 mW cm-2) and 625 nm red (100 J cm-2, 42 mW cm-2) light, respectively. The bis[pyrrolyl Ru(ii)] triad with a pyrenyl linker (5h) was especially potent, with an EC50 value of 1 nM and PI > 27 000 with visible light and subnanomolar activity with 625 nm light (100 J cm-2, 28 mW cm-2). The lead compound 5h was also tested in a tumor spheroid assay using the HL60 cell line and exhibited greater photocytotoxicity in this more resistant model (EC50 = 60 nM and PI > 1200 with 625 nm light) despite a lower dark cytotoxicity. The in vitro PDT effects of 5h extended to bacteria, where submicromolar EC50 values and PIs >300 against S. mutans and S. aureus were obtained with visible light. This activity was attenuated with 625 nm red light, but PIs were still near 50. The ligand-localized 3ππ* state contributed by the pyrenyl linker of 5h likely plays a key role in its phototoxic effects toward cancer cells and bacteria.

Breaking the barrier: an osmium photosensitizer with unprecedented hypoxic phototoxicity for real world photodynamic therapy

Hypoxia presents a two-fold challenge in the treatment of cancer, as low oxygen conditions induce biological changes that make malignant tissues simultaneously more aggressive and less susceptible to standard chemotherapy. This paper reports the first metal-based photosensitizer that approaches the ideal properties for a phototherapy agent. The Os(phen)2-based scaffold was combined with a series of IP-nT ligands, where phen = 1,10-phenanthroline and IP-nT = imidazo[4,5-f][1,10]phenanthroline tethered to n = 0-4 thiophene rings. Os-4T (n = 4) emerged as the most promising complex in the series, with picomolar activity and a phototherapeutic index (PI) exceeding 106 in normoxia. The photosensitizer exhibited an unprecedented PI > 90 (EC50 = 0.651 μM) in hypoxia (1% O2) with visible and green light, and a PI > 70 with red light. Os-4T was also active with 733 nm near-infrared light (EC50 = 0.803 μM, PI = 77) under normoxia. Both computation and spectroscopic studies confirmed a switch in the nature of the lowest-lying triplet excited state from triplet metal-to-ligand charge transfer (3MLCT) to intraligand charge transfer (3ILCT) at n = 3, with a lower energy and longer lifetime for n = 4. All compounds in the series were relatively nontoxic in the dark but became increasingly phototoxic with additional thiophenes. These normoxic and hypoxic activities are the largest reported to date, demonstrating the utility of osmium for phototherapy applications. Moreover, Os-4T had a maximum tolerated dose (MTD) in mice that was >200 mg kg-1, which positions this photosensitizer as an excellent candidate for in vivo applications.

Avobenzone incorporation in a diverse range of Ru(II) scaffolds produces potent potential antineoplastic agents

Four structurally distinct classes of polypyridyl ruthenium complexes containing avobenzone exhibited low micromolar and submicromolar potencies in cancer cells, and were up to 273-fold more active than the parent ligand. Visible light irradiation enhanced the cytotoxicity of some complexes, making them promising candidates for combined chemo-photodynamic therapy.