Light-Activated Carbon Monoxide Prodrugs Based on Bipyridyl Dicarbonyl Ruthenium(II) Complexes

Insights into the cytotoxicity of photoactivatable Ru(II) carbonyl complexes towards human liver carcinoma cells

Metal carbonyl complexes have recently been extensively studied as carbon monoxide releasing molecules (CORMs) with the potential to positively affect the biological targets by transferring trace amounts of CO. CORMs have advantageous anti-inflammatory, anti-coagulative, anti-apoptotic, and anti-proliferative properties. In this contribution, three photoactivatable ruthenium(II) carbon monoxide releasing molecules of the general formula [RuCl2(CO)2LR] (LR = (2-phenyliminomethyl)quinoline derivative), R = OH, OCH3, and Cl) were synthesized and structurally characterized. Their aerated solutions in DMSO or DMSO-water mixture were stable under the dark conditions. A clear isosbestic point was observed upon the illumination of the complexes at 468 nm due to the release of CO. The position of the isosbestic point in the two media differs, indicating the presence of two distinct forms of the CO depleted species (iCORM). Under the dark conditions, the three complexes exhibited no cytotoxicity against human epithelial-like hepatocellular carcinoma (HepG2) cells. After being exposed to light, the complex, decorated with Cl, demonstrated weak potency. The acquired photocytotoxicity is most likely caused by iCORM, particularly as the cytotoxicity varies according to the diluent used to produce it. Under the dark conditions, the images of fluorescence microscopy showed that the complexes were visible outside the cells. However, the complexes were clearly able to enter the cytoplasmic membrane and maintain their autofluorescence when they were illuminated while the malignant cells were present.

Visible-light-induced CO-releasing properties and cytotoxicity of a Ru(II) carbonyl complex containing 2-(pyridin-2-yl)-quinoxaline

The photo-induced CO-releasing properties of the dark-stable complex [RuCl2(CO)2L] (L = 2-(pyridin-2-yl)quinoxaline) were investigated under 468 nm light exposure in the presence and absence of biomolecules such as histidine, calf thymus DNA and hen egg white lysozyme. The CO release kinetics were consistent regardless of the presence of these biomolecules, suggesting that they did not influence the CO release mechanism. The quinoxaline ligand demonstrated exceptional cytotoxicity against human acute monocytic leukemia cells (THP-1), with evidence of potential DNA damage ascertained by comet assay, while it remained non-toxic to normal kidney epithelial cells derived from African green monkey (Vero) cell lines. In contrast, upon light activation, the Ru(II) complex showed no toxicity against THP-1 cells but was detrimental to Vero cells. In human colorectal carcinoma (HCT-116) cells, the ligand and the Ru(II) complex produced ROS under light and dark conditions. However, HCT-116 cells retained their ability to consume oxygen and produce ATP following CO treatment, suggesting that the ROS levels were insufficient to cause significant cellular damage. Morphological features of apoptosis, including apoptotic bodies, chromatin condensation, cell shrinkage, and membrane leakage, were observed in the presence of both the ligand and its complex, irrespective of light exposure.

Insights into the photoactivatable CO releasing properties of dicarbonyl Ru(II) complex with 8-amino quinoline ligand: Experimental and theoretical studies

Reaction between the polymeric [RuCl2(CO)2]n and the N,N-bidentate ligand, 8-amino-quinoline (Quin), in methanol, afforded the photoactivated CO releasing molecule with the formula of trans-(Cl,Cl)-[RuCl2(CO)2Quin]. In the presence of biomolecules or in solvents with varying polarity and coordinating abilities, the solvatochromic characteristics and dark stability were investigated. A new board band emerged in the visible spectrum during the illumination, and its position varies according to the type of solvent used, indicating the role of the solvent in controlling the nature of the CO-depleted species. Spectral methods were used in combination with density functional theory simulations to get insight into the local minimum structure and the electronic properties of the Ru(II) complex. The results of the myoglobin assay showed that within the first two hours of illumination, one of the two CO molecules was released. The cytotoxic properties of the Ru(II)-based complex were investigated against normal mice bone marrow stromal cells and malignant human acute monocytic leukaemia cells.

Metal-based carbon monoxide releasing molecules with promising cytotoxic properties

Carbon monoxide, the "silent killer" gas, is increasingly recognised as an important signalling molecule in human physiology, which has beneficial biological properties. A particular way of achieving controlled CO administration is based on the use of biocompatible molecules that only release CO when triggered by internal or external factors. These approaches include the development of pharmacologically effective prodrugs known as CO releasing molecules (CORMs), which can supply biological systems with CO in well-regulated doses. An overview of transition metal-based CORMs with cytotoxic properties is here reported. The mechanisms at the basis of the biological activities of these molecules and their potential therapeutical applications with respect to their stability and CO releasing properties have been discussed. The activation of metal-based CORMs is determined by the type of metal and by the nature and features of the auxiliary ligands, which affect the metal core electronic density and therefore the prodrug resistance towards oxidation and CO release ability. A major role in regulating the cytotoxic properties of these CORMs is played by CO and/or CO-depleted species. However, several mysteries concerning the cytotoxicity of CORMs remain as intriguing questions for scientists.

Switching on Cytotoxicity of Water-Soluble Diiron Organometallics by UV Irradiation

The diiron compounds [Fe₂Cp₂(CO)₂(μ-CO)(μ-CSEt)]CF₃SO₃, [1]CF₃SO₃, K[Fe₂Cp₂(CO)₃(CNCH₂CO₂)], K[2], [Fe₂Cp₂(CO)₂(μ-CO)(μ-CNMe₂)]NO₃, [3]NO₃, [Fe₂Cp₂(CO)₂(PTA){μ-CNMe(Xyl)}]CF₃SO₃, [4]CF₃SO₃, and [Fe₂Cp₂(CO)(μ-CO){μ-η:¹η³-C(4-C₆H₄CO₂H)CHCNMe₂}]CF₃SO₃, [5]CF₃SO₃, containing a bridging carbyne, isocyanoacetate, or vinyliminium ligand, were investigated for their photoinduced cytotoxicity. Specifically, the novel water-soluble compounds K[2], [3]NO₃, and [4]CF₃SO₃ were synthesized and characterized by elemental analysis and IR and multinuclear NMR spectroscopy. Stereochemical aspects concerning [4]CF₃SO₃ were elucidated by ¹H NOESY NMR and single-crystal X-ray diffraction. Cell proliferation studies on human skin cancer (A431) and nontumoral embryonic kidney (HEK293) cells, with and without a 10-min exposure to low-power UV light (350 nm), highlighted the performance of the aminocarbyne [3]NO₃, nicknamed NIRAC (Nitrate-Iron-Aminocarbyne), which is substantially nontoxic in the dark but shows a marked photoinduced cytotoxicity. Spectroscopic (IR, UV-vis, NMR) measurements and the myoglobin assay indicated that the release of one carbon monoxide ligand represents the first step of the photoactivation process of NIRAC, followed by an extensive disassembly of the organometallic scaffold.

Intracellular Localization Studies of the Luminescent Analogue of an Anticancer Ruthenium Iminophosphorane with High Efficacy in a Triple-Negative Breast Cancer Mouse Model

The potential of ruthenium(II) compounds as an alternative to platinum-based clinical anticancer agents has been unveiled after extensive research for over 2 decades. As opposed to cisplatin, ruthenium(II) compounds have distinct mechanisms of action that do not rely solely on interactions with DNA. In a previous report from our group, we described the synthesis, characterization, and biological evaluation of a cationic, water-soluble, organometallic ruthenium(II) iminophosphorane (IM) complex of p-cymene, ([(η6-p-cymene)Ru{(Ph3P═N-CO-2N-C5H4)-κ-N,O}Cl]Cl (1 or Ru-IM), that was found to be highly cytotoxic against a panel of cell lines resistant to cisplatin, including triple-negative breast cancer (TNBC) MDA-MB-231, through canonical or caspase-dependent apoptosis. Studies on a MDA-MB-231 xenograft mice model (after 28 days of treatment) afforded an excellent tumor reduction of 56%, with almost negligible systemic toxicity, and a favored ruthenium tumor accumulation compared to other organs. 1 is known to only interact weakly with DNA, but its intracellular distribution and ultimate targets remain unknown. To gain insight on potential mechanisms for this highly efficacious ruthenium compound, we have developed two luminescent analogues containing the BOPIPY fluorophore (or a modification) in the IM scaffold with the general structure of [(η6-p-cymene)Ru{(BODIPY-Ph2P═N-CO-2-NC5H4)-κ-N,O}Cl]Cl {BODIPY-Ph2P = 8-[(4-diphenylphosphino)phenyl]-4,4-dimethyl-1,3,5,7-tetramethyl-2,6-diethyl-4-bora-3a,4a-diaza-s-indacene (3a) and 4,4-difluoro-8-[4-[[2-[4-(diphenylphosphino)benzamido]ethyl]carbamoyl]phenyl]-1,3,5,7-tetramethyl,2,6-diethyl-4-bora-3a,4a-diaza-s-indacene (3b)}. We report on the synthesis, characterization, lipophilicity, stability, luminescence properties, and cell viability studies in the TNBC cell line MDA-MB-231, nonmalignant breast cells (MCF10a), and lung fibroblasts (IMR-90) of the new compounds. The ruthenium derivative 3b was studied by fluorescence confocal microscopy. These studies point to a preferential accumulation of the compound in the endoplasmic reticulum, mitochondria, and lysosomes. Inductively coupled plasma optical emission spectrometry (ICP-OES) analysis also confirms a greater ruthenium accumulation in the cytoplasmic fraction, including endoplasmic reticulum and lysosomes, and a smaller percentage of accumulation in mitochondria and the nucleus. ICP-OES analysis of the parent compound 1 indicates that it accumulates preferentially in the mitochondria and cytoplasm. Subsequent experiments in 1-treated MDA-MB-231 cells demonstrate significant reactive oxygen species generation.

Photostable Red-Emitting Fluorescent Rhein-Magnesium(Ⅱ) Coordination Polymer Nanodot-Based Nanostructures With a Large Stokes Shift for Imaging Mitochondria in Cancer Cell

The mitochondria play a significant role in many cellular processes and are recognized as one of the most important therapeutic targets in cancer. Direct long-term imaging of the mitochondria is very crucial for treating cancer. However, the development of a red-emitting mitochondrial probe with a large Stokes shift and photostability remains highly challenging. Fluorescent metal complexes with superior physicochemical property have emerged as new fluorescent nanomaterials due to their increasing advantages in bioimaging. Herein, a luminescent pitaya-type nanostructure based on rhein-magnesium(II) (Rh-Mg) coordination polymer nanodots was used as a fluorescent nanoprobe to selectively image the mitochondria benefiting from the introduction of triphenylphosphine. The as-prepared Rh-Mg nanodot-based nanoprobe showed red emission peaking at 620 nm, a large Stokes shift (100 nm), and excellent photostability as compared with commercial mitochondrial probes. Due to these extraordinary features, this fluorescent nanoprobe was successfully used for mitochondrial targeting imaging of live cancer cell line Neuro-2a (mouse neuroblastoma) and BV2 microglial cells. Therefore, our results pave a new way for the design of fluorescent nanoprobes for imaging mitochondria in cancer cell.

Highly Efficient Ir(III)-Coumarin Photo-Redox Catalyst for Synergetic Multi-Mode Cancer Photo-Therapy

Four photo-catalysts of the general formula [Ir(CO6/ppy)₂ (L)]Cl where CO6=coumarin 6 (Ir1-Ir3), ppy=2-phenylpyridine (Ir4), L=4'-(3,5-di-tert-butylphenyl)-2,2' : 6',2''-terpyridine (Ir1), 4'-(3,5-bis(trifluoromethyl)phenyl)-2,2' : 6',2''-terpyridine (Ir2 and Ir4), and 4-([2,2' : 6',2''-terpyridin]-4'-yl)-N,N-dimethylaniline (Ir3) were synthesized and characterized. These photostable photo-catalysts (Ir1-Ir3) showed strong visible light absorption between 400-550 nm. Upon light irradiation (465 and 525 nm), Ir1-Ir3 generated singlet oxygen and induced rapidly photo-catalytic oxidation of cellular coenzymes NAD(P)H. Ir1-Ir3 showed time-dependent cellular uptake with excellent intracellular retention efficiency. Upon green light irradiation (525 nm), Ir2 provided a much higher photo-index (PI=793) than the clinically used photosensitizer, 5-aminolevulinicacid (5-ALA, PI>30) against HeLa cancer cells. The observed necro-apoptotic anticancer activity of Ir2 was due to the Ir2 triggered photo-induced intracellular redox imbalance (by NAD(P)H oxidation and ROS generation) and change in the mitochondrial membrane potential. Remarkably, Ir2 showed in vivo photo-induced catalytic anticancer activity in mouse models.

Light-activated cytotoxicity of dicarbonyl Ru(II) complexes with a benzimidazole coligand towards breast cancer

Reaction between [RuCl2(CO)2]n and 1H-benzimidazol-2-ylmethyl-(N-phenyl)amine ligands (LR) functionalized with various electron-donating and electron-withdrawing substituents on the phenyl ring (R = H, 4-CH3, 4-Cl, 4-COOCH3, and 3-COOCH3) afforded the dark-stable photoactivatable carbon monoxide prodrugs of the general formula [RuCl2(CO)2LR]. Release of the CO molecules from the Ru(II) compounds was examined by monitoring the electronic and IR spectra upon illumination at 365 nm. A noticeable decrease in the intensities of the two characteristic ν(CO) modes for Ru(CO)II2 species, and the growth of two new bands for the mono-carbonyl species and free CO, were the main features of the photolysis profiles. The cytotoxicity of the complexes towards breast cancer (MCF-7) cells was assessed with and without illumination at 365 nm. All the complexes except that with a 4-COOCH3 group (IC50 = 45.08 ± 3.5 μM) are nontoxic under dark conditions. Upon illumination, all the compounds acquired cytotoxicity in the following order: H > 4-COOCH3 > 4-CH3 > 4-Cl > 3-COOCH3. Investigation of the cytotoxicity of the CO-depleted fragments showed that the light-induced cytotoxicity can be attributed to the liberated CO and CO-depleted metal fragments, including the liberated benzimidazole ligands.

Bioorthogonal Reactions of Triarylphosphines and Related Analogues

Bioorthogonal phosphines were introduced in the context of the Staudinger ligation over 20 years ago. Since that time, phosphine probes have been used in myriad applications to tag azide-functionalized biomolecules. The Staudinger ligation also paved the way for the development of other phosphorus-based chemistries, many of which are widely employed in biological experiments. Several reviews have highlighted early achievements in the design and application of bioorthogonal phosphines. This review summarizes more recent advances in the field. We discuss innovations in classic Staudinger-like transformations that have enabled new biological pursuits. We also highlight relative newcomers to the bioorthogonal stage, including the cyclopropenone-phosphine ligation and the phospha-Michael reaction. The review concludes with chemoselective reactions involving phosphite and phosphonite ligations. For each transformation, we describe the overall mechanism and scope. We also showcase efforts to fine-tune the reagents for specific functions. We further describe recent applications of the chemistries in biological settings. Collectively, these examples underscore the versatility and breadth of bioorthogonal phosphine reagents.

Photofunctional transition metal complexes as cellular probes, bioimaging reagents and phototherapeutics

This critical review summarises the recent biological applications of transition metal complexes as cellular probes, bioimaging reagents and phototherapeutics.