Effects of Methyl Substitution in Ruthenium Tris(2-pyridylmethyl)amine Photocaging Groups for Nitriles

Ru(II) Oligothienyl Complexes with Fluorinated Ligands: Photophysical, Electrochemical, and Photobiological Properties

A series of Ru(II) complexes incorporating two 4,4'-bis(trifluoromethyl)-2,2'-bipyridine (4,4'-btfmb) coligands and thienyl-appended imidazo[4,5-f][1,10]phenanthroline (IP-nT) ligands was characterized and assessed for phototherapy effects toward cancer cells. The [Ru(4,4'-btfmb)2(IP-nT)]2+ scaffold has greater overall redox activity compared to Ru(II) polypyridyl complexes such as [Ru(bpy)3]2+. Ru-1T-Ru-4T have additional oxidations due to the nT group and additional reductions due to the 4,4'-btfmb ligands. Ru-2T-Ru-4T also exhibit nT-based reductions. Ru-4T exhibits two oxidations and eight reductions within the potential window of -3 to +1.5 V. The lowest-lying triplets (T1) for Ru-0T-2T are metal-to-ligand charge-transfer (3MLCT) excited states with lifetimes around 1 μs, whereas T1 for Ru-3T-4T is longer-lived (∼20-24 μs) and of significant intraligand charge-transfer (3ILCT) character. Phototoxicity toward melanoma cells (SK-MEL-28) increases with n, with Ru-4T having a visible EC50 value as low as 9 nM and PI as large as 12,000. Ru-3T and Ru-4T retain some of this activity in hypoxia, where Ru-4T has a visible EC50 as low as 35 nM and PI as high as 2900. Activity over six biological replicates is consistent and within an order of magnitude. These results demonstrate the importance of lowest-lying 3ILCT states for phototoxicity and maintaining activity in hypoxia.

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.

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.

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 (τ_TA = 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.

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.

Singlet Oxygen Formation vs Photodissociation for Light-Responsive Protic Ruthenium Anticancer Compounds: The Oxygenated Substituent Determines Which Pathway Dominates

Ruthenium complexes bearing protic diimine ligands are cytotoxic to certain cancer cells upon irradiation with blue light. Previously reported complexes of the type [(N,N)₂Ru(6,6'-dhbp)]Cl₂ with 6,6'-dhbp = 6,6'-dihydroxybipyridine and N,N = 2,2'-bipyridine (bipy) (1_A), 1,10-phenanthroline (phen) (2_A), and 2,3-dihydro-[1,4]dioxino[2,3-f][1,10]phenanthroline (dop) (3_A) show EC₅₀ values as low as 4 μM (for 3_A) vs breast cancer cells upon blue light irradiation ( Inorg. Chem. 2017, 56, 7519). Herein, subscript A denotes the acidic form of the complex bearing OH groups, and B denotes the basic form bearing O^- groups. This photocytotoxicity was originally attributed to photodissociation, but recent results suggest that singlet oxygen formation is a more plausible cause of photocytotoxicity. In particular, bulky methoxy substituents enhance photodissociation but these complexes are nontoxic ( Dalton Trans 2018, 47, 15685). Cellular studies are presented herein that show the formation of reactive oxygen species (ROS) and apoptosis indicators upon treatment of cells with complex 3_A and blue light. Singlet oxygen sensor green (SOSG) shows the formation of ¹O₂ in cell culture for cells treated with 3_A and blue light. At physiological pH, complexes 1_A-3_A are deprotonated to form 1_B-3_B in situ. Quantum yields for ¹O₂ (ϕ_Δ) are 0.87 and 0.48 for 2_B and 3_B, respectively, and these are an order of magnitude higher than the quantum yields for 2_A and 3_A. The values for ϕ_Δ show an increase with 6,6'-dhbp derived substituents as follows: OMe < OH < O^-. TD-DFT studies show that the presence of a low lying triplet metal-centered (³MC) state favors photodissociation and disfavors ¹O₂ formation for 2_A and 3_A (OH groups). However, upon deprotonation (O^- groups), the ³MLCT state is accessible and can readily lead to ¹O₂ formation, but the dissociative ³MC state is energetically inaccessible. The changes to the energy of the ³MLCT state upon deprotonation have been confirmed by steady state luminescence experiments on 1_A-3_A and their basic analogs, 1_B-3_B. This energy landscape favors ¹O₂ formation for 2_B and 3_B and leads to enhanced toxicity for these complexes under physiological conditions. The ability to convert readily from OH to O^- groups allowed us to investigate an electronic change that is not accompanied by steric changes in this fundamental study.

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.

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.

Ru(ii) complexes with diazine ligands: electronic modulation of the coordinating group is key to the design of "dual action" photoactivated agents

Coordination complexes can be used to photocage biologically active ligands, providing control over the location, time, and dose of a delivered drug. Dual action agents can be created if both the ligand released and the ligand-deficient metal center effect biological processes. Ruthenium(ii) complexes coordinated to pyridyl ligands generally are only capable of releasing one ligand in H2O, wasting equivalents of drug molecules, and producing a Ru(ii) center that is not cytotoxic. In contrast, Ru(ii) polypyridyl complexes containing diazine ligands eject both monodentate ligands, with the quantum yield (φPS) of the second phase varying as a function of ligand pKa and the pH of the medium. This effect is general, as it is effective with different Ru(ii) structures, and demonstrates that diazine-based drugs are the preferred choice for the development of light-activated dual action Ru(ii) agents.

Synthesis and Avidin Binding of Ruthenium Complexes Functionalized with a Light-Cleavable Free Biotin Moiety

In this work the synthesis, photochemistry, and streptavidin interaction of new [Ru(tpy)(bpy)(SRR')](PF6)2 complexes where the R' group contains a free biotin ligand, are described. Two different ligands SRR' were investigated: An asymmetric ligand 1 where the Ru-bound thioether is a N-acetylmethionine moiety linked to the free biotin fragment via a triethylene glycol spacer and a symmetrical ligand 2 containing two identical biotin moieties. The coordination of these two ligands to the precursor [Ru(tpy)(bpy)Cl]Cl was studied in water at 80 °C. In such conditions the coordination of the asymmetric ligand 1 occurred under thermodynamic control. After the reaction, a mononuclear and a binuclear complex were isolated. In the mononuclear complex, the ratio of methionine- {[6](PF6)2} vs. biotin-bound {[7](PF6)2} regioisomer was 5.3 and the free biotin fragment of [6](PF6)2 allowed to purify it from its isomer [7](PF6)2 at small scales using avidin affinity chromatography. Coordination of the symmetrical ligand 2 afforded [Ru(tpy)(bpy)(2)](PF6)2 {[8](PF6)2} in synthetically useful scales (100 mg), good yield (82 %), and without traces of the binuclear impurity. In this complex, one of the biotin remains free whereas the second one is coordinated to ruthenium. Photochemical release of ligand 2 from [8](PF6)2 occurred upon blue light irradiation (465 nm) with a photosubstitution quantum yield of 0.011 that was independent of the binding of streptavidin to the free biotin ligand.

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.

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.

Sterically demanding methoxy and methyl groups in ruthenium complexes lead to enhanced quantum yields for blue light triggered photodissociation

Ruthenium complexes exhibit enhanced photodissociation quantum yields due to bulky, weak donor ligands, illustrating the impact of electronics and sterics.

Red Light Activation of Ru(II) Polypyridyl Prodrugs via Triplet-Triplet Annihilation Upconversion: Feasibility in Air and through Meat

Triplet-triplet annihilation upconversion (TTA-UC) is a promising photophysical tool to shift the activation wavelength of photopharmacological compounds to the red or near-infrared wavelength domain, in which light penetrates human tissue optimally. However, TTA-UC is sensitive to dioxygen, which quenches the triplet states needed for upconversion. Here, we demonstrate not only that the sensitivity of TTA-UC liposomes to dioxygen can be circumvented by adding antioxidants, but also that this strategy is compatible with the activation of ruthenium-based chemotherapeutic compounds. First, red-to-blue upconverting liposomes were functionalized with a blue-light sensitive, membrane-anchored ruthenium polypyridyl complex, and put in solution in presence of a cocktail of antioxidants composed of ascorbic acid and glutathione. Upon red light irradiation with a medical grade 630 nm PDT laser, enough blue light was produced by TTA-UC liposomes under air to efficiently trigger full activation of the Ru-based prodrug. Then, the blue light generated by TTA-UC liposomes under red light irradiation (630 nm, 0.57 W/cm²) through different thicknesses of pork or chicken meat was measured, showing that TTA-UC still occurred even beyond 10 mm of biological tissue. Overall, the rate of activation of the ruthenium compound in TTA-UC liposomes using either blue or red light (1.6 W/cm²) through 7 mm of pork fillet were found comparable, but the blue light caused significant tissue damage, whereas red light did not. Finally, full activation of the ruthenium prodrug in TTA-UC liposomes was obtained under red light irradiation through 7 mm of pork fillet, thereby underlining the in vivo applicability of the activation-by-upconversion strategy.