Cyclometalated Ruthenium(II) Complexes Derived from α-Oligothiophenes as Highly Selective Cytotoxic or Photocytotoxic Agents

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.

Chiral Iridium-Based TLD-1433 Analogues: Exploration of Enantiomer-Dependent Behavior in Photodynamic Cancer Therapy

Metallodrug-based photodynamic therapy (PDT) agents have demonstrated significant superiority against cancers, while their different chirality-induced biological activities remain largely unexplored. In this work, we successfully developed a pair of enantiopure mononuclear Ir(III)-based TLD-1433 analogues, Δ-Ir-3T and Λ-Ir-3T, and their enantiomer-dependent anticancer behaviors were investigated. Photophysical measurements revealed that they display high photostability and chemical stability, strong absorption at 400 nm with high molar extinction coefficients (ε = 5.03 × 104 M-1 cm-1), and good 1O2 relative quantum yields (ΦΔ ≈ 47%). Δ- and Λ-Ir-3T showed potent efficacy against MCF-7 cancer cells, with a photocytotoxicity index of ≤44 238. This impressive result, to the best of our knowledge, represents the highest value among reported mononuclear Ir(III)-based PDT agents. Remarkably, Λ-Ir-3T tended to be more potent than Δ-Ir-3T when tested against SK-MEL-28, HepG2, and LO2 cells, with consistent results across multiple test repetitions.

Green Light-Triggered Photocatalytic Anticancer Activity of Terpyridine-Based Ru(II) Photocatalysts

The relentless increase in drug resistance of platinum-based chemotherapeutics has opened the scope for other new cancer therapies with novel mechanisms of action (MoA). Recently, photocatalytic cancer therapy, an intrusive catalytic treatment, is receiving significant interest due to its multitargeting cell death mechanism with high selectivity. Here, we report the synthesis and characterization of three photoresponsive Ru(II) complexes, viz., [Ru(ph-tpy)(bpy)Cl]PF6 (Ru1), [Ru(ph-tpy)(phen)Cl]PF6 (Ru2), and [Ru(ph-tpy)(aip)Cl]PF6 (Ru3), where, ph-tpy = 4'-phenyl-2,2':6',2″-terpyridine, bpy = 2,2'-bipyridine, phen = 1,10-phenanthroline, and aip = 2-(anthracen-9-yl)-1H-imidazo[4,5-f][1,10] phenanthroline, showing photocatalytic anticancer activity. The X-ray crystal structures of Ru1 and Ru2 revealed a distorted octahedral geometry with a RuN5Cl core. The complexes showed an intense absorption band in the 440-600 nm range corresponding to the metal-to-ligand charge transfer (MLCT) that was further used to achieve the green light-induced photocatalytic anticancer effect. The mitochondria-targeting photostable complex Ru3 induced phototoxicity with IC50 and PI values of ca. 0.7 μM and 88, respectively, under white light irradiation and ca. 1.9 μM and 35 under green light irradiation against HeLa cells. The complexes (Ru1-Ru3) showed negligible dark cytotoxicity toward normal splenocytes (IC50s > 50 μM). The cell death mechanistic study revealed that Ru3 induced ROS-mediated apoptosis in HeLa cells via mitochondrial depolarization under white or green light exposure. Interestingly, Ru3 also acted as a highly potent catalyst for NADH photo-oxidation under green light. This NADH photo-oxidation process also contributed to the photocytotoxicity of the complexes. Overall, Ru3 presented multitargeting synergistic type I and type II photochemotherapeutic effects.

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.

Increasing the π-Expansive Ligands in Ruthenium(II) Polypyridyl Complexes: Synthesis, Characterization, and Biological Evaluation for Photodynamic Therapy Applications

Lack of selectivity is one of the main issues with currently used chemotherapies, causing damage not only to altered cells but also to healthy cells. Over the last decades, photodynamic therapy (PDT) has increased as a promising therapeutic tool due to its potential to treat diseases like cancer or bacterial infections with a high spatiotemporal control. Ruthenium(II) polypyridyl compounds are gaining attention for their application as photosensitizers (PSs) since they are generally nontoxic in dark conditions, while they show remarkable toxicity after light irradiation. In this work, four Ru(II) polypyridyl compounds with sterically expansive ligands were studied as PDT agents. The Ru(II) complexes were synthesized using an alternative route to those described in the literature, which resulted in an improvement of the synthesis yields. Solid-state structures of compounds [Ru(DIP)2phen]Cl2 and [Ru(dppz)2phen](PF6)2 have also been obtained. It is well-known that compound [Ru(dppz)(phen)2]Cl2 binds to DNA by intercalation. Therefore, we used [Ru(dppz)2phen]Cl2 as a model for DNA interaction studies, showing that it stabilized two different sequences of duplex DNA. Most of the synthesized Ru(II) derivatives showed very promising singlet oxygen quantum yields, together with noteworthy photocytotoxic properties against two different cancer cell lines, with IC50 in the micro- or even nanomolar range (0.06-7 μM). Confocal microscopy studies showed that [Ru(DIP)2phen]Cl2 and [Ru(DIP)2TAP]Cl2 accumulate preferentially in mitochondria, while no mitochondrial internalization was observed for the other compounds. Although [Ru(dppn)2phen](PF6)2 did not accumulate in mitochondria, it interestingly triggered an impairment in mitochondrial respiration after light irradiation. Among others, [Ru(dppn)2phen](PF6)2 stands out for its very good IC50 values, correlated with a very high singlet oxygen quantum yield and mitochondrial respiration disruption.

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

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

Cyclometalated Ru(II)-NHC complexes with phenanthroline ligands induce apoptosis mediated by mitochondria and endoplasmic reticulum stress in cancer cells

The exploration of ruthenium complexes as anticancer drugs has been the focus of intense investigation. In this study, we synthesized and characterized four C,N-cyclometalated ruthenium(II) complexes (Ru1-Ru4) coordinated with pyridine-functionalized N-heterocyclic carbene (NHC) and auxiliary ligands (e.g., acetonitrile, 1,10-phenanthroline, 3,4,7,8-tetramethyl-1,10-phenanthroline, and 4,7-diphenyl-1,10-phenanthroline). X-ray diffraction analysis showed that all of the four cycloruthenated complexes are hexa-coordinated in a typical octahedral geometry. In vitro cytotoxic studies revealed that cyclometalated Ru-NHC complexes Ru3 and Ru4 had stronger anticancer activity than their corresponding Ru-NHC precursor Ru1 and the clinically used cisplatin. For HeLa cells, Ru3 and Ru4 exhibited potent cytotoxicity with the IC₅₀ value of 4.31 ± 0.42 μM and 3.14 ± 0.23 μM, respectively, which was approximately three times lower than that of cisplatin. More interestingly, Ru3 and Ru4 not only effectively inhibited the proliferation of HeLa cells, but also exhibited potential anti-migration activity. In the scratch wound healing assay, Ru3 and Ru4 treatment significantly reduced the wound healing rate of HUVEC cells. Mechanistic studies showed that Ru3 and Ru4 caused a dual action mode of mitochondrial membrane depolarization and endoplasmic reticulum stress and finally induced apoptosis of HeLa cells.

Novel Ru(II)-bipyridine/phenanthroline-lapachol complexes as potential anti-cancer agents

For the first time, we herein report on the syntheses of two new Ru(II)/bipyridine/phenanthroline complexes containing lapachol as ligand: complex (1), [Ru (bipy)₂(Lap)]PF₆ and complex (2), [Ru(Lap)(phen)₂]PF₆, where bipy = 2,2'-bipyridine and ph en = 1,10-phenanthroline; Lap = lapachol (2-hydroxy-3-(3-methylbut-2-en-1- yl)naphthalene-1,4-dione). The complexes were synthesized and characterized by elemental analyses, molar conductivity, mass spectrometry, ultraviolet-visible and infrared spectroscopies, nuclear magnetic resonance (¹H, ¹³C), and single crystal X-ray diffraction, for complex (2). In addition, in vitro cytotoxicity was tested against six cancer cells: A549 (lung carcinoma); DU-145 (human prostate carcinoma); HepG2 (human hepatocellular carcinoma), PC-3 (human prostate adenocarcinoma); MDA-MB-231 (human breast adenocarcinoma); Caco-2 (human colorectal adenocarcinoma), and against two non-cancer cells, FGH (human gingival normal fibroblasts) and PNT-2 (prostate epithelial cells). Complex (1) was slightly more toxic and selective than complex (2) for all cell lines, except against the A549 cells, where (2) was more potent than complex (1). The complexes induced an increase in the reactive oxygen species, and the co-treatment with N-acetyl-L-cysteine remarkably suppressed the ROS generation and prevented the reduction of cell viability, suggesting that the cytotoxicity of the complexes is related to the ROS-mediated pathway. Further studies indicated that the complexes may bind to DNA via minor groove interaction. Our studies also revealed that free Lap induces gene mutations in Salmonella Typhimurium, nevertheless, the complexes demonstrated the absence of genotoxicity by the Ames test. The present study provides a relevant contribution to understanding the anti-cancer potential and genetic toxicological events of new ruthenium complexes containing the lapachol molecule as a ligand.

Bichromophoric ruthenium(II) bis-terpyridine-BODIPY based photosensitizers for cellular imaging and photodynamic therapy

Two multichromophoric homoleptic ruthenium(II) complexes [Ru(tpy-BODIPY)₂]Cl₂ (complexes 1 and 2, tpy = 4-phenyl-2,2:6,2-terpyridine, BODIPY = boron-dipyrromethene) were prepared, characterized and their phototherapeutic activity and bioimaging properties were studied. The complexes having structural similarity differ only by a phenylethynyl linker, and its overall influence on their physicochemical and photobiological behavior was evaluated. The terpyridine-BODIPY ligand L¹ was structurally characterized by X-ray crystallography. The complexes showed intense absorption near 500 nm (ε: ∼1.5 × 10⁵ M^-1 cm^-1 in DMSO), have a high singlet oxygen quantum yield (Φ_Δ: ∼0.6 in DMSO), and displayed low photobleaching thus making them suitable for PDT applications. The complexes showed high DNA binding affinity and induced DNA damage on light activation via multiple types of ROS production. Confocal laser scanning microscopy experiments revealed their incorporation in the cancer cells and complex 1 predominantly accumulated in lysosomes. The complexes displayed a significant PDT effect in cancerous cells with visible light activation with a high photocytotoxicity index (PI) value in HeLa cells. Both type-I and type-II photosensitization processes were involved in the PDT effect. The photodynamic action of complex 2 initiated cellular apoptosis. Finally, their diagnostic potential was evaluated against clinically relevant 3D multicellular tumor spheroids (MCTs).

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.

Metalloimmunotherapy with Rhodium and Ruthenium Complexes: Targeting Tumor-Associated Macrophages

Tumor associated macrophages (TAMs) suppress the cancer immune response and are a key target for immunotherapy. The effects of ruthenium and rhodium complexes on TAMs have not been well characterized. To address this gap in the field, a panel of 22 dirhodium and ruthenium complexes were screened against three subtypes of macrophages, triple-negative breast cancer and normal breast tissue cells. Experiments were carried out in 2D and biomimetic 3D co-culture experiments with and without irradiation with blue light. Leads were identified with cell-type-specific toxicity toward macrophage subtypes, cancer cells, or both. Experiments with 3D spheroids revealed complexes that sensitized the tumor models to the chemotherapeutic doxorubicin. Cell surface exposure of calreticulin, a known facilitator of immunogenic cell death (ICD), was increased upon treatment, along with a concomitant reduction in the M2-subtype classifier arginase. Our findings lay a strong foundation for the future development of ruthenium- and rhodium-based chemotherapies targeting TAMs.

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.

Unlocking the Potential of Ru(II) Dual-action Compounds with the Power of the Heavy-atom Effect

We report the synthesis, photochemical and biological characterization of two new Ru(II) photoactivated complexes based on [Ru(tpy)(Me₂ bpy)(L)]²⁺ (tpy = 2,2':6',2''-terpyridine, Me₂ bpy = 6,6'-dimethyl-2,2'-bipyridine), where L = pyridyl-BODIPY (pyBOD). Two pyBOD ligands were prepared bearing flanking hydrogen or iodine atoms. Ru(II)-bound BODIPY dyes show a red-shift of absorption maxima relative to the free dyes and undergo photodissociation of BODIPY ligands with green light irradiation. Addition of iodine into the BODIPY ligand facilitates intersystem crossing, which leads to efficient singlet oxygen production in the free dye, but also enhances quantum yield of release of the BODIPY ligand from Ru(II). This represents the first report of a strategy to enhance photodissociation quantum yields through the heavy-atom effect in Ru(II) complexes. Furthermore, Ru(II)-bound BODIPY dyes display fluorescence turn-on once released, with a lead analog showing nanomolar EC₅₀ values against triple negative breast cancer cells, >100-fold phototherapeutic indexes under green light irradiation, and higher selectivity toward cancer cells as compared to normal cells than the corresponding free BODIPY photosensitizer. Conventional Ru(II) photoactivated complexes require nonbiorthogonal blue light for activation and rarely show submicromolar potency to achieve cell death. Our study represents an avenue for the improved photochemistry and potency of future Ru(II) complexes.

BODIPY-Ruthenium(II) Bis-Terpyridine Complexes for Cellular Imaging and Type-I/-II Photodynamic Therapy

A series of multichromophoric ruthenium(II) complexes with the formulation [Ru(tpy-BODIPY)(tpy-R)]Cl₂ (1-4), having a heteroleptic Ru(II)-bis-tpy (tpy = 4'-phenyl-2,2':6',2″-terpyridine) moiety covalently linked to a boron-dipyrromethene (BODIPY) pendant, have been prepared and characterized and their application as a phototherapeutic and photodetection agent in cancer therapy has been explored. Ligand L¹ with a terpyridine-BODIPY moiety and complex 1 as its PF₆ salt (1a) have been structurally characterized by a single-crystal X-ray diffraction study. Complex 1a has a distorted-octahedral RuN₆ core with a Ru(II)-bis-terpyridine unit that is covalently linked to one photoactive BODIPY unit. The complexes exhibit strong absorbance near 502 nm (ε ≈ (3.7-7.8) × 10⁴ M^-1 cm^-1) and high singlet oxygen sensitization ability, giving singlet oxygen quantum yield (Φ_Δ) values ranging from 0.57 to 0.75 in DMSO. An emission-based study using complex 4 and Singlet Oxygen Sensor Green (SOSG) displays the formation of singlet oxygen inside the cells and also in the buffer medium upon light irradiation. DNA (pUC19) photocleavage experiments using ROS scavengers/stabilizers reveal photoinduced generation of singlet oxygen by a type-II process and of the superoxide anion radical by a type-I process. Complex 4 having a pendant biotin moiety as a cancer cell targeting group shows high photocytotoxicity with a remarkable phototherapeutic index (PI) value of >1400 in HeLa cancer cells with a low light dose activation (400-700 nm, 2.2 J cm^-2). The complexes display reduced activity in noncancerous HPL1D cells. The emission property of the complexes is used for cellular imaging, thus making them suitable as next-generation theranostic PDT agents.

Evaluating the mechanisms of action and subcellular localization of ruthenium(II)-based photosensitizers

The identification of ruthenium(II) polypyridyl complexes as photosensitizers in photodynamic therapy (PDT) for the treatment of cancer is progressing rapidly. Due to their favorable photophysical and photochemical properties, Ru(II)-based photosensitizers have absorption in the visible spectrum, can be irradiated via one- and two-photon excitation within the PDT window, and yield potent oxygen-dependent and/or oxygen-independent photobiological activities. Herein, we present a current overview of the mechanisms of action and subcellular localization of Ru(II)-based photosensitizers in the treatment of cancer. These photosensitizers are highlighted from a medicinal chemistry and chemical biology perspective. However, although this field is burgeoning, challenges and limitations remain in the photosensitization strategies and clinical translation.

String-Attached Oligothiophene Substituents Determine the Fate of Excited States in Ruthenium Complexes for Photodynamic Therapy

We explore the photophysical properties of a family of Ru(II) complexes, Ru-ip-nT, designed as photosensitizers (PSs) for photodynamic therapy (PDT). The complexes incorporate a 1H-imidazo[4,5-f][1,10]-phenanthroline (ip) ligand appended to one or more thiophene rings. One of the complexes studied herein, Ru-ip-3T (known as TLD1433), is currently in phase II human clinical trials for treating bladder cancer by PDT. The potent photocytotoxicity of Ru-ip-3T is attributed to a long-lived intraligand charge-transfer triplet state. The accessibility of this state changes upon varying the length (n) of the oligothiophene substituent. In this paper, we highlight the impact of n on the ultrafast photoinduced dynamics in Ru-ip-nT, leading to the formation of the function-determining long-lived state. Femtosecond time-resolved transient absorption combined with resonance Raman data was used to map the excited-state relaxation processes from the Franck-Condon point of absorption to the formation of the lowest-energy triplet excited state, which is a triplet metal-to-ligand charge-transfer excited state for Ru-ip-0T-1T and an oligothienyl-localized triplet intraligand charge-transfer excited state for Ru-ip-2T-4T. We establish the structure-activity relationships with regard to changes in the excited-state dynamics as a function of thiophene chain length, which alters the photophysics of the complexes and presumably impacts the photocytotoxicity of these PSs.

Strategies to promote permeation and vectorization, and reduce cytotoxicity of metal complex luminophores for bioimaging and intracellular sensing

Transition metal luminophores are emerging as important tools for intracellular imaging and sensing. Their putative suitability for such applications has long been recognised but poor membrane permeability and cytotoxicity were significant barriers that impeded early progress. In recent years, numerous effective routes to overcoming these issues have been reported, inspired in part, by advances and insights from the pharmaceutical and drug delivery domains. In particular, the conjugation of biomolecules but also other less natural synthetic species, from a repertoire of functional motifs have granted membrane permeability and cellular targeting. Such motifs can also reduce cytotoxicity of transition metal complexes and offer a valuable avenue to circumvent such problems leading to promising metal complex candidates for application in bioimaging, sensing and diagnostics. The advances in metal complex probes permeability/targeting are timely, as, in parallel, over the past two decades significant technological advances in luminescence imaging have occurred. In particular, super-resolution imaging is enormously powerful but makes substantial demands of its imaging contrast agents and metal complex luminophores frequently possess the photophysical characteristics to meet these demands. Here, we review some of the key vectors that have been conjugated to transition metal complex luminophores to promote their use in intra-cellular imaging applications. We evaluate some of the most effective strategies in terms of membrane permeability, intracellular targeting and what impact these approaches have on toxicity and phototoxicity which are important considerations in a luminescent contrast or sensing agent.

Photoinduced DNA Cleavage and Photocytotoxic of Phenanthroline-Based Ligand Ruthenium Compounds

The photophysical and biological properties of two new phenanthroline-based ligand ruthenium complexes were investigated in detail. Their DNA interaction modes were determined to be the intercalation mode using spectra titration and viscosity measurements. Under irradiation, obvious photo-reduced DNA cleavages were observed in the two complexes via singlet oxygen generation. Furthermore, complex 2 showed higher DNA affinity, photocleavage activity, and singlet oxygen quantum yields than complex 1. The two complexes showed no toxicity towards tumor cells (HeLa, A549, and A375) in the dark. However, obvious photocytotoxicities were observed in the two complexes. Complex 2 exhibited large PIs (phototherapeutic indices) (ca. 400) towards HeLa cells. The study suggests that these complexes may act as DNA intercalators, DNA photocleavers, and photocytotoxic agents.

Rational design of type I photosensitizers based on Ru(ii) complexes for effective photodynamic therapy under hypoxia

Photodynamic therapy (PDT) has been widely used in conjunction with molecular oxygen to cause cancer cell death. Hypoxia, the inherent property in solid tumors, is the obstacle during the process of PDT. It is urgent to develop PDT photosensitizers independent of the oxygen concentration. Herein, triphenylamine-modified Ru(ii) complexes have been used as photosensitizers to produce superoxide anions (O2-˙) and hydroxyl radicals (˙OH) through a type I photochemical process. Ru(ii) complexes with triphenylamine can provide a possibility to drive the reactive oxygen species production through low oxidation potential and good light-harvesting abilities. The investigation on light-mediated radical production showed that Ru4 could produce abundant ˙OH and O2-˙ compared to Ru1-Ru3 under hypoxic environments owing to the strong absorption. These radicals exhibit potent toxicity, which can damage the neighbouring biomolecules and cause the apoptosis of cancer cells. The PDT effect was evaluated in vitro under hypoxia, suggesting that Ru4 could maintain excellent performance in inducing a sharp decrease in the activity of cancer cells.

It Takes Three to Tango - the length of the oligothiophene determines the nature of the long-lived excited state and the resulting photocytotoxicity of a Ru(II) photodrug

TLD1433 is the first Ru(II) complex to be tested as a photodynamic therapy agent in a clinical trial. In this contribution we study TLD1433 in the context of structurally-related Ru(II)-imidozo[4,5-f][1,10]phenanthroline (ip) complexes appended with thiophene rings to decipher the unique photophysical properties which are associated with increasing oligothiophene chain length. Substitution of the ip ligand with ter- or quaterthiophene changes the nature of the long-lived triplet state from metal-to-ligand charge-transfer to 3ππ* character. The addition of the third thiophene thus presents a critical juncture which not only determines the photophysics of the complex but most importantly its capacity for 1O2 generation and hence the potential of the complex to be used as a photocytotoxic agent.

ENTRY FOR THE TABLE OF CONTENTS

A low-lying triplet intraligand state (3IL) determines the properties of the long-lived excited states in a series of Ru(II) complexes. The 3IL state can be accessed by increasing the length of an oligothiophene chain. The 3IL state is extremely efficient at generating 1O2 and thus enhances the potency of the complexes as PDT agents.

In Vitro and In Vivo Effect of Palladacycles: Targeting A2780 Ovarian Carcinoma Cells and Modulation of Angiogenesis

Palladacycles are versatile organometallic compounds that show potential for therapeutic use. Here are described the synthesis and characterization of mono- and dinuclear palladacycles bearing diphosphines. Their biological effect was investigated in A2780, an ovarian-derived cancer line, and in normal dermal fibroblasts. The compounds displayed selective cytotoxicity toward the A2780 cell line. Compound 3 decreased the cell viability through cell cycle retention in G0/G1, triggered apoptosis through the intrinsic pathway, and induced autophagy in A2780 cells. Compound 9 also induced cell cycle retention, apoptosis, and cellular detachment. Notably, compound 9 induced the production of intracellular reactive oxygen species (ROS). Our work demonstrated that compound 3 enters A2780 cells via active transport, which requires energy, while compound 9 enters A2780 cells mostly passively. The potential effect of palladacycles in angiogenesis was investigated for the first time in an in vivo chorioallantoic membrane model, showing that while compound 3 displayed an antiangiogenic effect crucial to fighting cancer progression, compound 9 promoted angiogenesis. These results show that palladacycles may be used in different clinical applications where pro- or antiangiogenic effects may be desirable.

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.

NIR-Absorbing RuII Complexes Containing α-Oligothiophenes for Applications in Photodynamic Therapy

The design of near-infrared (NIR)-active photosensitizers (PSs) for light-based cancer treatments such as photodynamic therapy (PDT) has been a challenge. While several NIR-RuII scaffolds have been reported, this approach has not been proven in cells. This is the first report of NIR-RuII PSs that are phototoxic to cancer cells, including highly pigmented B16F10 melanoma cells. The PS family incorporated a bis(1,8-naphthyridine)-based ligand (tpbn), a bidentate thiophene-based ligand (nT; n=0-4), and a monodentate 4-picoline ligand (4-pic). All compounds absorbed light >800 nm with maxima near 730 nm. Transient absorption (TA) measurements indicated that n=4 thiophene rings (4T) positioned the PDT-active triplet intraligand charge transfer (3 ILCT) excited state in energetic proximity to the lowest-lying triplet metal-to-ligand charge transfer (3 MLCT). 4T had low-micromolar phototoxicity with PIvis and PI733nm values as large as 90 and 12, respectively. Spectroscopic studies suggested that the longer-lived (τTA =3-6 μs) 3 ILCT state was accessible from the 3 MLCT state, but energetically uphill in the overall photophysics. The study highlights that phototoxic effects can be achieved with NIR-absorbing RuII PSs as long as the reactive 3 ILCT states are energetically accessible from the low-energy 3 MLCT states. It also demonstrates that tissue-penetrating NIR light can be used to activate the PSs in highly pigmented cells where melanin attenuates shorter wavelengths of light.

Os(II) Oligothienyl Complexes as a Hypoxia-Active Photosensitizer Class for Photodynamic Therapy

Hypoxia presents a challenge to anticancer therapy, reducing the efficacy of many available treatments. Photodynamic therapy is particularly susceptible to hypoxia, given that its mechanism relies on oxygen. Herein, we introduce two new osmium-based polypyridyl photosensitizers that are active in hypoxia. The lead compounds emerged from a systematic study of two Os(II) polypyridyl families derived from 2,2'-bipyridine (bpy) or 4,4'-dimethyl-2,2'-bipyridine (dmb) as coligands combined with imidazo[4,5-f][1,10]phenanthroline ligands tethered to n = 0-4 thiophenes (IP-nT). The compounds were characterized and investigated for their spectroscopic and (photo)biological activities. The two hypoxia-active Os(II) photosensitizers had n = 4 thiophenes, with the bpy analogue 1-4T being the most potent. In normoxia, 1-4T had low nanomolar activity (half-maximal effective concentration (EC50) = 1-13 nM) with phototherapeutic indices (PI) ranging from 5500 to 55 000 with red and visible light, respectively. A sub-micromolar potency was maintained even in hypoxia (1% O2), with light EC50 and PI values of 732-812 nM and 68-76, respectively -currently among the largest PIs for hypoxic photoactivity. This high degree of activity coincided with a low-energy, long-lived (0.98-3.6 μs) mixed-character intraligand charge-transfer (3ILCT)/ligand-to-ligand charge-transfer (3LLCT) state only accessible in quaterthiophene complexes 1-4T and 2-4T. The coligand identity strongly influenced the photophysical and photobiological results in this study, whereby the bpy coligand led to longer lifetimes (3.6 μs) and more potent photo-cytotoxicity relative to those of dmb. The unactivated compounds were relatively nontoxic both in vitro and in vivo. The maximum tolerated dose for 1-4T and 2-4T in mice was greater than or equal to 200 mg kg-1, an excellent starting point for future in vivo validation.

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.

Role of Photoactive Phytocompounds in Photodynamic Therapy of Cancer

Cancer is one of the greatest life-threatening diseases conventionally treated using chemo- and radio-therapy. Photodynamic therapy (PDT) is a promising approach to eradicate different types of cancers. PDT requires the administration of photosensitisers (PSs) and photoactivation using a specific wavelength of light in the presence of molecular oxygen. This photoactivation exerts an anticancer effect via apoptosis, necrosis, and autophagy of cancer cells. Recently, various natural compounds that exhibit photosensitising potentials have been identified. Photoactive substances derived from medicinal plants have been found to be safe in comparison with synthetic compounds. Many articles have focused on PDT mechanisms and types of PSs, but limited attention has been paid to the phototoxic activities of phytocompounds. The reduced toxicity and side effects of natural compounds inspire the researchers to identify and use plant extracts or phytocompounds as a potent natural PS candidate for PDT. This review focusses on the importance of common photoactive groups (furanocoumarins, polyacetylenes, thiophenes, curcumins, alkaloids, and anthraquinones), their phototoxic effects, anticancer activity and use as a potent PS for an effective PDT outcome in the treatment of various cancers.

The Self-Assembly of a Cyclometalated Palladium Photosensitizer into Protein-Stabilized Nanorods Triggers Drug Uptake In Vitro and In Vivo

Enhanced passive diffusion is usually considered to be the primary cause of the enhanced cellular uptake of cyclometalated drugs because cyclometalation lowers the charge of a metal complex and increases its lipophilicity. However, in this work, monocationic cyclometalated palladium complexes [1]OAc (N^N^C^N) and [2]OAc (N^N^N^C) were found to self-assemble, in aqueous solutions, into soluble supramolecular nanorods, while their tetrapyridyl bicationic analogue [3](OAc)₂ (N^N^N^N) dissolved as isolated molecules. These nanorods formed via metallophilic Pd···Pd interaction and π–π stacking and were stabilized in the cell medium by serum proteins, in the absence of which the nanorods precipitated. In cell cultures, these protein-stabilized self-assembled nanorods were responsible for the improved cellular uptake of the cyclometalated compounds, which took place via endocytosis (i.e., an active uptake pathway). In addition to triggering self-assembly, cyclometalation in [1]OAc also led to dramatically enhanced photodynamic properties under blue light irradiation. These combined penetration and photodynamic properties were observed in multicellular tumor spheroids and in a mice tumor xenograft, demonstrating that protein-stabilized nanoaggregation of cyclometalated drugs such as [1]OAc also allows efficient cellular uptake in 3D tumor models. Overall, serum proteins appear to be a major element in drug design because they strongly influence the size and bioavailability of supramolecular drug aggregates and hence their efficacy in vitro and in vivo.

Light activation of cyclometalated ruthenium complexes drives towards caspase 3 dependent apoptosis in gastric cancer cells

Polypyridyl ruthenium complexes have been intensively investigated for their remarkable antiproliferative properties and some are currently being tested in clinical trials. Here, we investigated the impact of illumination on the biological properties of a series of new cyclometalated ruthenium compounds with increased π-conjugation. We determined that various of these complexes display a bivalent biological activity as they are highly cytotoxic by themselves in absence of light while their cytotoxicity can significantly be elevated towards an IC₅₀ in the nanomolar range upon illumination. In particular, we showed that these complexes are particularly active (IC₅₀ < 1 μM) on two gastric cancer cell lines (AGS, KATO III) that are resistant towards cisplatin (IC₅₀ > 25 μM). As expected, light activation leads to increased production of singlet oxygen species in vitro and accumulation of reactive oxygen species in vivo. Importantly, we established that light exposure shifts the mode of action of the complexes towards activation of a caspase 3-dependent apoptosis that correlates with increased DNA damage. Altogether, this study characterizes novel ruthenium complexes with dual activity that can be tuned towards different mode of action in order to bypass cancer cell resistance mechanisms.

Dual-Action Ru(II) Complexes with Bulky π-Expansive Ligands: Phototoxicity without DNA Intercalation

We report the synthesis and photochemical and biological characterization of Ru(II) complexes containing π-expansive ligands derived from dimethylbenzo[i]dipyrido[3,2-a:2',3'-c]phenazine (Me₂dppn) adorned with flanking aryl substituents. Late-stage Suzuki couplings produced Me₂dppn ligands substituted at the 10 and 15 positions with phenyl (5), 2,4-dimethylphenyl (6), and 2,4-dimethoxyphenyl (7) groups. Complexes of the general formula [Ru(tpy)(L)(py)](PF₆)₂ (8-10), where L = 4-7, were characterized and shown to have dual photochemotherapeutic (PCT) and photodynamic therapy (PDT) behavior. Quantum yields for photodissociation of monodentate pyridines from 8-10 were about 3 times higher than that of parent complex [Ru(tpy)(Me₂dppn)(py)](PF₆)₂ (1), whereas quantum yields for singlet oxygen (¹O₂) production were ∼10% lower than that of 1. Transient absorption spectroscopy indicates that 8-10 possess long excited state lifetimes (τ = 46-50 μs), consistent with efficient ¹O₂ production through population and subsequent decay of ligand-centered ³ππ* excited states. Complexes 8-10 displayed greater lipophilicity relative to 1 and association to DNA but do not intercalate between the duplex base pairs. Complexes 1 and 8-10 showed photoactivated toxicity in breast and prostate cancer cell lines with phototherapeutic indexes, PIs, as high as >56, where the majority of cell death was achieved 4 h after treatment with Ru(II) complexes and light. Flow cytometric data and rescue experiments were consistent with necrotic cell death mediated by the production of reactive oxygen species, especially ¹O₂. Collectively, this study confirms that DNA intercalation by Ru(II) complexes with π-expansive ligands is not required to achieve photoactivated cell death.

Synthesis and Characterization of Ru(II) Complexes with π-Expansive Imidazophen Ligands for the Photokilling of Human Melanoma Cells

Ru(II) complexes were synthesized with π-expanding (phenyl, fluorenyl, phenanthrenyl, naphthalen-1-yl, naphthalene-2-yl, anthryl and pyrenyl groups) attached at a 1H-imidazo[4,5-f][1,10]phenanthroline ligand and 4,4'-dimethyl-2,2'-bipyridine (4,4'-dmb) coligands. These Ru(II) complexes were characterized by 1D and 2D NMR, and mass spectroscopy, and studied for visible light and dark toxicity to human malignant melanoma SK-MEL-28 cells. In the SK-MEL-28 cells, the Ru(II) complexes are highly phototoxic (EC₅₀  = 0.2-0.5 µm) and have low dark toxicity (EC₅₀  = 58-230 µm). The highest phototherapeutic index (PI) of the series was found with the Ru(II) complex bearing the 2-(pyren-1-yl)-1H-imidazo[4,5-f][1,10]phenanthroline ligand. This high PI is in part attributed to the π-rich character added by the pyrenyl group, and a possible low-lying and longer-lived ³ IL state due to equilibration with the ³ MLCT state. While this pyrenyl Ru(II) complex possessed a relatively high quantum yield for singlet oxygen formation (Φ_∆  = 0.84), contributions from type-I processes (oxygen radicals and radical ions) are competitive with the type-II (¹ O₂ ) process based on effects of added sodium azide and solvent deuteration.

Nanoencapsulation of a ruthenium(ii) complex with triazolopyrimidine in liposomes as a tool for improving its anticancer activity against melanoma cell lines

Two types of ruthenium(ii) complexes containing 1,2,4-triazolo[1,5-a]pyrimidines of the general formulas [RuCl₂(dmso)₃(L)] ((1)-(3)) and [RuCl₂(dmso)₂(L)₂] ((4)-(6)), where L represents 1,2,4-triazolo[1,5-a]pyrimidine (tp for (1)), 5,7-dimethyl-1,2,4-triazolo[1,5-a]pyrimidine (dmtp for (2)), 7-isobutyl-5-methyl-1,2,4-trizolo[1,5-a]pyrimidine (ibmtp for (3)), 5,7-diethyl-1,2,4-triazolo[1,5-a]pyrimidine (detp for (4)), 5,7-ditertbutyl-1,2,4-triazolo[1,5-a]pyrimidine (dbtp for (5)) and 5,7-diphenyl-1,2,4-triazolo[1,5-a]pyrimidine (dptp for (6)), have been synthesized and characterized by elemental analysis, infrared, multinuclear magnetic resonance spectroscopic techniques (¹H, ¹³C, and ¹⁵N), and X-ray (for (3), (4), and (5)). All these complexes have been thoroughly screened for their in vitro cytotoxicity against melanoma cell lines A375 and Hs294T, indicating cis,cis,cis-[RuCl₂(dbtp)₂(dmso)₂] (5) as the most active representative, in addition to being non-toxic to normal human fibroblasts (NHDF) and not inducing hemolysis of human erythrocytes. In order to develop an intravenous formulation for (5), liposomes composed of soybean phosphatidylcholine (SPC), cholesterol (Chol) and 1,2-distearoyl-sn-glycero-3-phosphoethanolamine-N-[amino(polyethylene glycol)-2000] (DSPE-PEG₂₀₀₀) were prepared and subsequently characterized. (5)-Loaded liposomes, with spherical morphology, assessed by transmission electron microscope (TEM), exhibited satisfactory encapsulation efficiency and stability. In in vitro experiments, PEG-modified (5)-loaded liposomes were more effective (10-fold) than free (5) for growth inhibition of both human melanoma cell lines. Furthermore, such an approach resulted in the reduction of cancer cell viability that was even 10-fold greater than that observed for free cisplatin.

Strained, Photoejecting Ru(II) Complexes that are Cytotoxic Under Hypoxic Conditions

A series of strained Ru(II) complexes were studied for potential anticancer activity in hypoxic tissues. The complexes were constructed with methylated ligands that were photolabile and an imidizo[4,5-f][1,10]phenanthroline ligand that contained an appended aromatic group to potentially allow for contributions of ligand-centered excited states. A systematic variation of the size and energy of the aromatic group was performed using systems containing 1-4 fused rings, and the photochemical and photobiological behaviors of all complexes were assessed. The structure and nature of the aromatic group had a subtle impact on photochemistry, altering environmental sensitivity, and had a significant impact on cellular cytotoxicity and photobiology. Up to 5-fold differences in cytotoxicity were observed in the absence of light activation; this rose to 50-fold differences upon exposure to 453 nm light. Most significantly, one complex retained activity under conditions with 1% O₂ , which is used to induce hypoxic changes. This system exhibited a photocytotoxicity index (PI) of 15, which is in marked contrast to most other Ru(II) complexes, including those designed for O₂ -independent mechanisms of action.

Cellular uptake of protic ruthenium complexes is influenced by pH dependent passive diffusion and energy dependent efflux

The lipophilic vs. hydrophilic properties of three protic ruthenium compounds were studied as a function of pH. Specifically, we measured Log(D_o/w) values for [(N,N)₂Ru(6,6'-dhbp)]²⁺ complexes (where N,N = 2,2'-bipyridine (1_A), 1,10-phenanthroline (2_A), 2,3-dihydro-[1,4]dioxino[2,3-f][1,10]phenanthroline (3_A) and 6,6'-dhbp is the diprotic 6,6'-dihydroxy-2,2'-bipyridine ligand) from pH 4.0 to 8.0. This study allowed us to demonstrate that as the ligand is deprotonated at higher pH values the resulting neutral charge on the complex improves its lipophilic properties. Thus, improved uptake by passive diffusion is expected with protic ligands on Ru(II). Furthermore, cellular studies have demonstrated that passive diffusion is the dominant pathway for cellular uptake. However, metabolic inhibition has also shown that energy dependent efflux reduces the amount of the ruthenium complex (as measured by mean fluorescence intensity) in the cells. These compounds have been shown by fluorescence microscopy to accumulate in the nuclei of cancer cells (MCF7, MDA-MB-231, and HeLa). Taken together, this data shows that uptake is required for toxicity but uptake alone is not sufficient. The greatest light activated toxicity appears to occur in breast cancer cell lines with relatively moderate uptake (MCF7 and MDA-MB-231) rather than the cell line with the greatest uptake of complex 3_A (normal breast cell line MCF-10A).

Photophysical Properties and Photobiological Activities of Ruthenium(II) Complexes Bearing π-Expansive Cyclometalating Ligands with Thienyl Groups

A new family of cyclometalated ruthenium(II) complexes [Ru(N^N)₂(C^N)]⁺ derived from the π-extended benzo[h]imidazo[4,5-f]quinolone ligand appended with thienyl groups (n = 1-4, compounds 1-4) was prepared and its members were characterized for their chemical, photophysical, and photobiological properties. The lipophilicities of 1-4, determined as octanol-water partition coefficients (log P_o/w), were positive and increased with the number of thienyl units. The absorption and emission bands of the C^N compounds were red-shifted by up to 200 nm relative to the analogous Ru(II) diimine systems. All of the complexes exhibited dual emission with the intraligand fluorescence (¹IL, C^N-based) shifting to lower energies with increasing n and the metal-to-ligand charge transfer phosphorescence (³MLCT, N^N-based) remaining unchanged. Compounds 1-3 exhibited excited state absorption (ESA) profiles consistent with lowest-lying ³MLCT states when probed by nanosecond transient absorption (TA) spectroscopy with 532 nm excitation and had contributions from ¹IL(C^N) states with 355 nm excitation. These assignments were supported by the lifetimes observed (<10 ns for the ¹IL states and around 20 ns for the ³MLCT states) as well as a noticeable ESA for 3 with 355 nm excitation that did not occur with 532 nm excitation. Compound 4 was the only member of the family with two ³MLCT emissive lifetimes (15, 110 ns), and the TA spectra collected with both 355 and 532 nm excitation was assigned to the ³IL state, which was corroborated by its 4-6 μs lifetime. The ESA for 4 had a rise time of approximately 10 ns and an initial decay of 110 ns, which suggests a possible ³MLCT-³IL excited state equilibrium that results in delayed emission from the ³MLCT state. Compound 4 was nontoxic toward human skin melanoma cells (SKMEL28) in the dark (EC₅₀ = >300 μM); 1-3 were cytotoxic and yielded EC₅₀ values between 1 and 20 μM. The photocytotoxicites with visible light ranged from 87 nM with a phototherapeutic index (PI) of 13 for 1 to approximately 1 μM (PI = >267) for 4. With red light, EC₅₀ values varied from 270 nM (PI = 21) for 3 to 12 μM for 4 (PI = >25). The larger PIs for 4, especially with visible light, were attributed to the much lower dark cytotoxicity for this compound. Because the dark cytotoxicity contributes substantially to the observed photocytotoxicity for 1-3, it was not possible to assess whether the ³IL state of 4 led to a much more potent phototoxic mechanism in the absence of dark toxicity. There was no stark contrast in cellular uptake and accumulation by laser scanning confocal and differential interference contrast microscopy to explain the large differences in dark toxicities between 1-3 and 4. Nevertheless, the study highlights a new family of Ru(II) C^N complexes where π-conjugation beyond a certain point results in low dark cytotoxicity with high photocytotoxicity, opposing the notion that cyclometalated Ru(II) systems are too toxic to be phototherapeutic agents.

The photoactivity of natural products - An overlooked potential of phytomedicines?

BACKGROUND

Photoactivity, though known for centuries, is only recently shifting back into focus as a treatment option against cancer and microbial infections. The external factor light is the ingenious key-component of this therapy: Since light activates the drug locally, a high level of selectivity is reached and side effects are avoided. The first reported photoactive medicines were plant extracts. Synthetic entities (so-called photosensitizers PSs), however, paved the route towards the clinical approval of the so-called photodynamic therapy (PDT), and thus natural PSs took a backseat in the past.

HYPOTHESIS

Many isolated bioactive phytochemicals hold a hidden photoactive potential, which is overlooked due to the reduced common awareness of photoactivity.

METHODS

A systematic review of reported natural PSs and their supposed medicinal application was conducted by employing PubMed, Scifinder, and Web of Science. The identified photoactive natural products were compiled including information about their natural sources, their photoyield, and their pharmacological application. Furthermore, the common chemical scaffolds of natural PS are shown to enable the reader to recognize potentially overlooked natural PSs.

RESULTS

The literature review revealed over 100 natural PS, excluding porphyrins. The PSs were classified according to their scaffold. Thereby it was shown that some PS-scaffolds were analyzed in a detailed way, while other classes were only scarcely investigated, which leaves space for future discoveries. In addition, the literature revealed that many PSs are phytoalexins, thus the selection of the starting material significantly matters in order to find new PSs.

CONCLUSION

Photoactive principles are ubiquitous and can be found in various plant extracts. With the increasing availability of light-irradiation setups for the identification of photoactive natural products, we anticipate the discovery of many new natural PSs in the near future. With the accumulation of chemically diverse PSs, PDT itself might finally reach its clinical breakthrough as a promising alternative treatment against multi-resistant microbes and cancer types.

Predictive Strength of Photophysical Measurements for in Vitro Photobiological Activity in a Series of Ru(II) Polypyridyl Complexes Derived from π-Extended Ligands

This study investigates the correlation between photocytotoxicity and the prolonged excited-state lifetimes exhibited by certain Ru(II) polypyridyl photosensitizers comprised of π-expansive ligands. The eight metal complexes selected for this study differ markedly in their triplet state configurations and lifetimes. Human melanoma SKMEL28 and human leukemia HL60 cells were used as in vitro models to test photocytotoxicity induced by the compounds when activated by either broadband visible or monochromatic red light. The photocytotoxicities of the metal complexes investigated varied over 2 orders of magnitude and were positively correlated with their excited-state lifetimes. The complexes with the longest excited-state lifetimes, contributed by low-lying 3IL states, were the most phototoxic toward cancer cells under all conditions.

Mechanisms of action of Ru(ii) polypyridyl complexes in living cells upon light irradiation

The unique photophysical properties of Ru(ii) polypyridyl complexes make them very attractive candidates as photosensitisers in Photodynamic Therapy (PDT). However, to date, there are not many studies exploring in detail the mechanism(s) of action of such compounds in living systems upon light irradiation. This feature article provides an overview of the most in-depth biological studies on such compounds.

Pyrazole appended quinoline-BODIPY based arene ruthenium complexes: their anticancer activity and potential applications in cellular imaging

Synthesis of four arene ruthenium complexes [Ru(η⁶-C₆H₆)(L1)Cl]PF₆, (1), [Ru(η⁶-C₁₀H₁₄)(L1)Cl]PF₆ (2), [Ru(η⁶-C₆H₆)(L2)Cl]PF₆ (3) and [Ru(η⁶-C₁₀H₁₄)(L2)Cl]PF₆ (4) based on quinoline-BODIPY were described and their photocytotoxicity was evaluated.