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

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.

Modulating Excited State Properties and Ligand Ejection Kinetics in Ruthenium Polypyridyl Complexes Designed to Mimic Photochemotherapeutics

Ruthenium(II) polypyridyl complexes have gained significant interest as photochemotherapeutics (PCTs) due to their synthetic viability, strong light absorption, well understood excited state properties, and high phototoxicity indexes. Herein, we report the synthesis, characterization, electrochemical, spectrochemical, and preliminary cytotoxicity analyses of three series of ruthenium(II) polypyridyl complexes designed to mimic PCTs. The three series have the general structure of [Ru(bpy)2(N-N)]2+ (Series 1), [Ru(bpy)(dmb)(N-N)]2+ (Series 2), and [Ru(dmb)2(N-N)]2+ (Series 3, where N-N is a bidentate polypyridyl ligand, bpy = 2,2'-bipyridine, and dmb = 6,6'-dimethyl-2,2'-bipyridine). In the three series, the N-N ligand was systematically modified to incorporate increased conjugation and/or electronegative heteroatoms to increase dπ-π* backbonding, red-shifting the lowest energy metal-to-ligand charge transfer (MLCT) absorptions from λmax = 454 to λmax = 580 nm, nearing the therapeutic window for PCTs (600-1100 nm). In addition, steric bulk was systematically introduced through the series, distorting the Ru(II) octahedra, making the dissociative 3dd* state thermally accessible at room and body temperatures. This resulted in a 4 orders of magnitude increase in photoinduced ligand ejection kinetics, and demonstrates the ability to modulate both the MLCT* and dd* manifolds in the complexes, which is critical in PCT drug design. Preliminary cell viability assays suggest that the increased steric bulk to lower the 3dd* states may interfere with the cytotoxicity mechanism, limiting photoinitiated toxicity of the complexes. This work demonstrates the importance of understanding both the MLCT* and dd* manifolds and how they impact the ability of a complex to act as a PCT agent.

Exploiting Orthogonal C-C Cross-Coupling Reactions for Chemistry-on-the-Complex: Modular Assembly of 2,6-Di(quinolin-8-yl)pyridine Ruthenium(II) Photosensitizer Triads

In this work, we present a concise modular assembly strategy using one universal heteroleptic 2,6-di(quinolin-8-yl)pyridine-based ruthenium(II) complex as a starting building block. Extending the concept from established ligand modifications and subsequent complexation (classical route), the later appearing chemistry-on-the-complex methodology was used for late-stage syntheses, i.e., assembling discrete building blocks to molecular architectures (here: dyad and triads). We focused on Suzuki-Miyaura and Sonogashira cross-couplings as two of the best-known C-C bond forming reactions. Both were performed on one building block complex bearing a bromine and TIPS-protected alkyne for functional group interconversion (bromine to TMS-protected alkyne, a benzyl azide, or a boronic acid pinacol ester moiety with ≥95% isolated yield and simple purification) as well as building block assemblies using both a triarylamine-based donor and a naphthalene diimide-based acceptor in up to 86% isolated yield. Additionally, the developed purification via automated flash chromatography is simple compared to tedious manual chromatography for ruthenium(II)-based substrates in the classical route. Based on the preliminary characterization by steady-state spectroscopy, the observed emission quenching in the triad (55%) serves as an entry to rationally optimize the modular units via chemistry-on-the-complex to elucidate energy and electron transfer.

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-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.

Novel Ru(II) complexes with multiple anticancer photoreactivity: ligand exchange, photoredox catalysis, reactive oxygen generation and endoperoxide formation

The hypoxic microenvironment and drug resistance of cancer cells have become a huge threat for clinical anticancer therapy. Anticancer phototherapy providing spatial and temporal control over drug activation may conquer this problem. Herein, we report a novel photoactivated Ru(II) complex (Ru2) with multiple activities including photochemotherapy, photodynamic and photocatalytic therapy, and endoperoxide formation. Upon white light irradiation, Ru2 can dissociate the coordinating ligands and form endoperoxides, produce diverse reactive oxygen species and catalytically oxidize cellular coenzymes. As a result, Ru2 shows promising antiproliferation activity toward cisplatin and 5-fluorouracil resistant tumor cell lines under normoxia and hypoxia. The multifunctional design strategy of metal-based anticancer drugs offers novel efficient therapeutics to combat drug-resistant cancer cells under hypoxia.

A near-infrared light-activatable Ru(ii)-coumarin photosensitizer active under hypoxic conditions

Photodynamic therapy (PDT) represents a promising approach for cancer treatment. However, the oxygen dependency of PDT to generate reactive oxygen species (ROS) hampers its therapeutic efficacy, especially against hypoxic solid tumors. In addition, some photosensitizers (PSs) have dark toxicity and are only activatable with short wavelengths such as blue or UV-light, which suffer from poor tissue penetration. Herein, we developed a novel hypoxia-active PS with operability in the near-infrared (NIR) region based on the conjugation of a cyclometalated Ru(ii) polypyridyl complex of the type [Ru(C^N)(N^N)₂] to a NIR-emitting COUPY dye. The novel Ru(ii)-coumarin conjugate exhibits water-solubility, dark stability in biological media and high photostability along with advantageous luminescent properties that facilitate both bioimaging and phototherapy. Spectroscopic and photobiological studies revealed that this conjugate efficiently generates singlet oxygen and superoxide radical anions, thereby achieving high photoactivity toward cancer cells upon highly-penetrating 740 nm light irradiation even under hypoxic environments (2% O₂). The induction of ROS-mediated cancer cell death upon low-energy wavelength irradiation along with the low dark toxicity exerted by this Ru(ii)-coumarin conjugate could circumvent tissue penetration issues while alleviating the hypoxia limitation of PDT. As such, this strategy could pave the way to the development of novel NIR- and hypoxia-active Ru(ii)-based theragnostic PSs fuelled by the conjugation of tunable, low molecular-weight COUPY fluorophores.

Recent Trends in the Development of Novel Metal-Based Antineoplastic Drugs

Since the accidental discovery of the anticancer properties of cisplatin more than half a century ago, significant efforts by the broad scientific community have been and are currently being invested into the search for metal complexes with antitumor activity. Coordination compounds of transition metals such as platinum (Pt), ruthenium (Ru) and gold (Au) have proven their effectiveness as diagnostic and/or antiproliferative agents. In recent years, experimental work on the potential applications of elements including lanthanum (La) and the post-transition metal gallium (Ga) in the field of oncology has been gaining traction. The authors of the present review article aim to help the reader "catch up" with some of the latest developments in the vast subject of coordination compounds in oncology. Herewith is offered a review of the published scientific literature on anticancer coordination compounds of Pt, Ru, Au, Ga and La that has been released over the past three years with the hope readers find the following article informative and helpful.

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.

Insights into enantioselective separations of ionic metal complexes by sub/supercritical fluid chromatography

Sub/supercritical fluid chromatography (SFC) is a green separation technique that has been used to separate a wide variety of compounds and is proven to be immensely useful for chiral separations. However, SFC is currently not thought to be applicable for ionic compounds due to their low solubility in CO2, even with additives and organic modifiers. Recently, a large amount of research has been centered on octahedral complexes of Ru(II) and Os(II) with bidentate polypyridyl ligands due to their ability to serve in cancer treatment and other biological activities. These compounds exist as the delta (Δ) and lambda (Λ) enantiomers. Previously, similar compounds have been enantiomerically separated using HPLC and capillary electrophoresis, but never with SFC. Cyclofructan-6 (CF6) derivatized with (R)-naphthyl ethyl (RN) groups has been proven to be an effective chiral stationary phase for these separations in HPLC. This column chemistry was expanded to SFC to provide the first chiral separation of a wide variety (23 complexes in total) of ionic octahedral polypyridyl complexes. Unexpected behavior for mixing methanol and acetonitrile as the organic modifier will be discussed, along with the effects of additives. Enantioselectivity on CF6-RN chemistry is shown to be dependent on the conjugation level and rigidity of the metal complexes. Mass transfer kinetic behavior is also shown, and high-efficiency baseline resolved rapid separations are shown for fast screening or quantitation of representative coordination complexes.

Balancing the interplay between ligand ejection and therapeutic window light absorption in ruthenium polypyridyl complexes

Ruthenium polypyridyl complexes have gained significant interest as photochemotherapies (PCTs) where their excited-state properties play a critical role in the photo-cytotoxicity mechanism and efficacy. Herein we report a systematic electrochemical, spectrochemical, and photophysical analysis of a series of ruthenium(II) polypyridyl complexes of the type [Ru(bpy)₂(N-N)]²⁺ (where bpy = 2,2'-bipyridine; N-N is a bidentate polypyridyl ligand) designed to mimic PCTs. In this series, the N-N ligand was modified through increased conjugation and/or incorporation of electronegative heteroatoms to shift the metal-to-ligand charge-transfer (MLCT) absorptions near the therapeutic window for PCTs (600-1100 nm) while incorporating steric bulk to trigger photoinduced ligand dissociation. The lowest energy MLCT absorptions were red-shifted from λ_max = 454 nm to 564 nm, with emission energies decreasing from λ_max = 620 nm to 850 nm. Photoinduced ligand ejection and temperature-dependent emission studies revealed an important interplay between red-shifting MLCT absorptions and accessing the dissociative ³dd* states, with energy barriers between the ³MLCT* and ³dd* states ranging from 850 cm^-1 to 2580 cm^-1 for the complexes measured. This work demonstrates the importance of understanding both the MLCT manifold and ³dd* state energy levels in the future design of ligands and complexes for PCT.

DNA Interaction, DNA Photocleavage, Photocytotoxicity In Vitro, and Molecular Docking of Naphthyl-Appended Ruthenium Complexes

With the development of metal-based drugs, Ru(II) compounds present potential applications of PDT (photodynamic therapy) and anticancer reagents. We herein synthesized two naphthyl-appended ruthenium complexes by the combination of the ligand with naphthyl and bipyridyl. The DNA affinities, photocleavage abilities, and photocytotoxicity were studied by various spectral methods, viscosity measurement, theoretical computation method, gel electrophoresis, and MTT method. Two complexes exhibited strong interaction with calf thymus DNA by intercalation. Production of singlet oxygen (1O2) led to obvious DNA photocleavage activities of two complexes under 365 nm light. Furthermore, two complexes displayed obvious photocytotoxicity and low dark cytotoxicity towards Hela, A549, and A375 cells.

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.

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.

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.

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.

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.

Synthesis, characterisation and biological activity of the ruthenium(II) complexes of the N4-tetradentate (N4-TL), 1,6-di(2'-pyridyl)-2,5-dibenzyl-2,5-diazahexane (picenBz2)

A series of complexes of the type rac-cis-β-[Ru(N₄-T_L)(N₂-bidentates)]²⁺ (where N₄-T_L = 1,6-di(2'-pyridyl)-2,5-dibenzyl-2,5-diazahexane (picenBz₂, N₄-T_L-2) and N₂-bidentates = 1,10-phenanthroline (phen, Ru-2), dipyrido[3,2-d:2',3'-f]quinoxaline (dpq, Ru-3), 7,8-dimethyl-dipyrido[3,2-a:2',3'-c] phenazine (dppzMe_2,Ru-4), 2-phenyl-1H-imidazo[4,5-f][1,10]phenanthroline (phenpyrBz, Ru-5), 2-(p-tolyl)-1H-imidazo[4,5-f][1,10]phenanthroline (phenpyrBzMe, Ru-6), 2-(4-nitrophenyl)-1H-imidazo[4,5-f][1,10]phenanthroline (phenpyrBzNO_2,Ru-7), were synthesised and characterised and X-ray crystallography of Ru-5 obtained. The in vitro cytotoxicity assays revealed that Ru-6 was 5, 2 and 19-fold more potent than oxaliplatin, cisplatin, and carboplatin, respectively displaying an average GI₅₀ value of ≈ 0.76 μM against a panel of 11 cancer cell lines.