Synthesis, Structures, and CO Release Capacity of a Family of Water-Soluble PhotoCORMs: Assessment of the Biocompatibility and Their Phototoxicity toward Human Breast Cancer Cells

Mitochondria-Accumulating Rhenium(I) Tricarbonyl Complexes Induce Cell Death via Irreversible Oxidative Stress and Glutathione Metabolism Disturbance

Mitochondria play a critical role in tumorigenesis. Targeting mitochondria and disturbing related events have been emerging as a promising way for chemotherapy. In this work, two binuclear rhenium(I) tricarbonyl complexes of the general formula [Re₂(CO)₆(dip)₂L](PF₆)₂ (dip = 4,7-diphenyl-1,10-phenanthroline; L = 4,4'-azopyridine (ReN) or 4,4'-dithiodipyridine (ReS)) were synthesized and characterized. ReN and ReS can react with glutathione (GSH). They exhibit good in vitro anticancer activity against cancer cell lines screened. Besides, they can target mitochondria, cause oxidative stress, and disturb GSH metabolism. Both ReN and ReS can induce necroptosis and caspase-dependent apoptosis simultaneously. We also demonstrate that ReN and ReS can inhibit tumor growth in nude mice bearing carcinoma xenografts. Our study shows the potential of Re(I) complexes as chemotherapeutic agents to kill cancer cells via a mitochondria-to-cellular redox strategy.

Tracking CO release in cells via the luminescence of donor molecules and/or their by-products

Carbon monoxide (CO) is a bioactive signalling molecule that is produced endogenously via the breakdown of heme. Beneficial health effects associated with the delivery of CO gas have spurred the development of CO-releasing molecules (CORMs) that can be used to provide specific amounts of the gas. In addition to their potential use as therapeutics, CORMs are needed to provide insight into the biological targets of CO. In this regard, light-activated CO-releasing molecules (photoCORMs), are valuable for examining the effects of localized CO release. Herein we examine luminescent CORMs and photoCORMs that have been reported for tracking CO delivery in cells. A variety of motifs are available that exhibit differing luminescence properties and cover a wide range of wavelengths. Trackable CO donors have been successfully applied to targeting CO delivery to mitochondria, thus demonstrating the feasibility of using such molecules in detailed investigations of the biological roles of CO.

Carbon monoxide sensitizes cisplatin-resistant ovarian cancer cell lines toward cisplatin via attenuation of levels of glutathione and nuclear metallothionein

Cisplatin resistance remains a major impediment to effective treatment of ovarian cancer. Despite initial platinum responsiveness, thiol-containing peptides and proteins, glutathione (GSH) and metallothionein (MT), bind and inactivate cisplatin in cancer cells. Indeed, high levels of GSH and MT in ovarian cancers impart cisplatin resistance and are predictive of poor prognosis. Cystathionine β-synthase (CBS), an enzyme involved in sulfur metabolism, is overexpressed in ovarian cancer tissues and is itself associated with cisplatin resistance. Treatment with exogenous carbon monoxide (CO), a known inhibitor of CBS, may mitigate cisplatin resistance in ovarian cancer cells by attenuation of GSH and MT levels. Using a photo-activated CO-releasing molecule (photoCORM), [Mn(CO)₃(phen)(PTA)]CF₃SO₃ (phen = 1,10-phenanthroline, PTA = 1,3,5-triza-7-phosphaadamantane) we assessed the ability of CO to sensitize established cisplatin-resistant ovarian cancer cell lines to cisplatin. Cisplatin-resistant cells, treated with both cisplatin and CO, exhibited significantly lower cell viability and increased poly (ADP-ribose) polymerase (PARP) cleavage versus those treated with cisplatin alone. These cisplatin-resistant cell lines overexpressed CBS and had increased steady state levels of GSH and expression of nuclear MT. Both CO treatment and lentiviral-mediated silencing of CBS attenuated GSH and nuclear MT expression in cisplatin resistant cells. We have demonstrated that CO, delivered from a photoCORM, sensitizes established cisplatin-resistant cell lines to cisplatin. Furthermore, we have presented strong evidence that the effects of CO in circumventing chemotherapeutic drug resistance is at least in part mediated by the inactivation of endogenous CBS.

Ultrafast Intersystem Crossing vs Internal Conversion in α-Diimine Transition Metal Complexes: Quantum Evidence

Whereas third row transition metal carbonyl α-diimine complexes display luminescent properties and possess low-lying triplet metal-to-ligand charge transfer (MLCT) states efficiently accessible by a spin-vibronic mechanism, first row analogues hold low-lying metal-centered (MC) excited states that could quench these properties. Upon visible irradiation, different functions are potentially stimulated, namely, luminescence, electron transfer, or photoinduced CO release, the branching ratio of which is governed by the energetics, the character, and the early time dynamics of the photoactive excited states. Simulations of ultrafast nonadiabatic quantum dynamics, including spin-vibronic effects, of [M(imidazole)(CO)₃(phenanthroline)]⁺ (M = Mn, Re) highlight the role of the metal atom. An ultrafast intersystem crossing process, driven by spin-orbit coupling, populates the low-lying triplet states of [Re(imidazole)(CO)₃(phen)]⁺ within the first tens of fs. In contrast, efficient internal conversion between the two lowest ¹MLCT states of [Mn(imidazole)(CO)₃(phen)]⁺ is mediated within 50 fs by vibronic coupling with upper MC and MLCT states.

Electrospray ionization study of tricarbonyl fac-[Re(CO)3 (PO)(X)]-type complexes: influence of ancillary co-ligands in the release of carbon monoxide

RATIONALE

fac-[Re(CO)₃ (PO)(X)]-type complexes (PO = chelated bidentate tertiary phosphine (1-), X = various neutral, mono-dentate ligands) represent a class of compounds that meets the synthetic criteria for the preparation of potential carbon monoxide (CO) release molecules (CORMs) for medicinal application. The aim of our investigation was to achieve qualitative information whether the nature of the ancillary X ligand might influence the release of CO.

METHODS

The release of CO has been investigated by means of product ion spectrometry of electrospray ionization (ESI)-generated [M + H]⁺ species, produced by multiple collisional experiments, using an ion trap mass spectrometer.

RESULTS

Tandem mass spectrometry applied to the protonated species [Re(CO)₃ (PO)(X) + H]⁺ of seven complexes (those including X = OH₂ (1), isonitrile (2, 3), imidazole (4), pyridine (5) and phosphine (6, 7)) shows initial loss of coordinated water (1) or pyridine (5), whereas the majority of investigated entries display initial, sequential release of CO groups. The energetics of CO release have been investigated by breakdown curves for selected collisionally activated decomposition processes involving CO, and compared with those involving X groups.

CONCLUSIONS

The nature of the co-ligand X drives the primary loss in the MSⁿ processes of [Re(CO)₃ (PO)(X) + H]⁺ compounds. When X = solvent, the energetics of these decompositions follow the trend H₂ O < MeOH < CO. In each case, loss of CO is a favored fragmentation route with associated energies following the trend: N-py ≤ P-phosphine < C-isonitrile. Overall, MSⁿ pathways indicate that [Re(PO)] (Re with chelated PO phosphine) constitutes the residual moiety. This behavior indicates that the presence of a functionalized phosphine is essential for a sequential, controlled release of CO.

Cationic Au(I) complexes with aryl-benzothiazoles and their antibacterial activity

Two cationic Au(I) complexes derived from aryl-benzothiazoles, namely [(PPh₃)Au(pbt)](OTf) (1) and [(PPh₃)Au(qbt)](OTf) (2) (where pbt = 2‑(pyridyl)benzothiazole and qbt = (quinolyl)benzothiazole, and OTf^- = trifluoromethanesulfonate anion), have been synthesized and structurally characterized by X-ray crystallography. Both complexes exhibit strong antibacterial effects against Gram-negative bacteria such as Acinetobacter baumannii and Pseudomonas Aeruginosa. Results of examination of the reactions of 1 and 2 indicate that these cationic Au(I) complexes rapidly cross the bacterial membrane and exert drug action by disrupting cellular function(s) through binding of cytosolic thiol-containing peptides (such as glutathione) and proteins to the highly reactive (PPh₃)Au⁺ intermediate formed upon in situ dissociation of pbt or qbt.

Water soluble derivatives of platinum carbonyl Chini clusters: synthesis, molecular structures and cytotoxicity of [Pt12(CO)20(PTA)4]2- and [Pt15(CO)25(PTA)5]2-

The reactions of [Pt3n(CO)6n]2- (n = 2-5) homoleptic Chini-type clusters with increasing amounts of 1,3,5-triaza-7-phosphaadamantane (PTA) result in the stepwise substitution of one terminal CO ligand per Pt3 triangular unit up to the formation of [Pt3n(CO)5n(PTA)n]2- (n = 2-5). Competition between the nonredox substitution with retention of the nuclearity and the redox fragmentation to afford lower nuclearity heteroleptic Chini-type clusters is observed as a function of the amount of PTA and the nuclearity of the starting cluster. Because of this, [Pt12(CO)20(PTA)4]2- and [Pt15(CO)25(PTA)5]2- are more conveniently obtained via the oxidation of [Pt9(CO)15(PTA)3]2-. All the new species were spectroscopically characterized, and the structures of [Pt12(CO)20(PTA)4]2- and [Pt15(CO)25(PTA)5]2- were determined by single-crystal X-ray diffraction. These clusters may be viewed as heteroleptic Chini-type clusters composed of stacks of four and five Pt3(μ-CO)3(CO)2(PTA) units, respectively. The solubility in water of [Pt12(CO)20(PTA)4]2- and [Pt15(CO)25(PTA)5]2- has been determined and their cytotoxicity towards human ovarian (A2780) cancer cells and their cisplatin-resistant strain (A2780cisR) has been evaluated.

Diiron Hexacarbonyl Complex Induces Site-Specific Release of Carbon Monoxide in Cancer Cells Triggered by Endogenous Glutathione

In this study, we have evaluated a water-soluble, nontarget reagent and a carrier-free diiron hexacarbonyl complex, [Fe₂{μ-SCH₂CH(OH)CH₂(OH)}₂(CO)₆] (TG-FeCORM), that can induce the site-specific release of carbon monoxide (CO) in cancer cells triggered by endogenous glutathione (GSH). The releasing rate of CO was dependent on the amount of endogenous GSH. Being the amount of endogenous GSH higher in cancer cells than in normal cells, the CO-releasing rate resulted faster in cancer cells. Moreover, the anti-inflammatory properties related to the intracellular CO release of TG-FeCORM were also confirmed in the living HeLa cells.

CO-Induced apoptotic death of colorectal cancer cells by a luminescent photoCORM grafted on biocompatible carboxymethyl chitosan

A photo-active luminescent rhenium carbonyl complex namely, [Re(CO)₃(phen)(pyAl)](CF₃SO₃) was grafted on a biocompatible carboxymethyl chitosan (CMC) matrix through Schiff base condensation reaction. The light-induced CO delivery from ReCMC has been shown to eradicate human colorectal adenocarcinoma cells (HT-29) very efficiently in a dose-dependent fashion. The onset of CO-induced apoptosis was realized by caspase-3,-7 detection aided by fluorescence confocal microscopy. ReCMC represents a unique example of a biocompatible and biodegradable antineoplastic agent that could find its use in cancer photopharmacology.

CO suppresses prostate cancer cell growth by directly targeting LKB1/AMPK/mTOR pathway in vitro and in vivo

BACKGROUND

CO is a freely diffusible gas that acts as a physiological mediator of many biological and cellular processes, which has been shown to possess anticancer effect in many kinds of cancers. However, the effect of CO on prostate cancer has not been demonstrated. Therefore, we analyzed the antitumor activities and related mechanisms of CO on prostate cancer in vitro and in vivo.

METHODS

Cell viability of LNCaP and PC-3 cells after CORM-2 treatment was measured by CCK-8 assay, whereas the ATP production were detected by ATP detection assay. The early apoptosis induced by CO was evaluated by flow cytometry, and the expression level of apoptosis-related molecules (Caspases 3, 8, 9 and cleaved-Caspases 3, 8, 9) was detected using Western blot. Matrigel in vitro invasion assay was used to evaluate the effect of CO on cell invasion. We then evaluated the impact of CO on the expression of several key regulators involved in the LKB1 signaling pathway. At last, xenograft tumor in nude mice was used to further investigate the antitumor effect of CO in vivo.

RESULTS

Our results showed that CO could significantly inhibit proliferation and invasion, and induce apoptosis in human prostate cancer cell lines. The expression of LKB1 could be up-regulated after CO treatment, and CO also could increase p-AMPK levels and decrease p-mTOR. Furthermore, LKB1 knockdown could weaken the effect of CO on prostate cancer cells. In vivo, CO treatment significantly suppressed tumor growth and induced apoptosis in xenografts tumor in nude mice.

CONCLUSIONS

CO possesses striking anticancer effect in human prostate cancer cells in vitro and in vivo, which is largely mediated by LKB1-AMPK-mTOR axis.

Photoactivated in Vitro Anticancer Activity of Rhenium(I) Tricarbonyl Complexes Bearing Water-Soluble Phosphines

Fifteen water-soluble rhenium compounds of the general formula [Re(CO)3(NN)(PR3)]+, where NN is a diimine ligand and PR3 is 1,3,5-triaza-7-phosphaadamantane (PTA), tris(hydroxymethyl)phosphine (THP), or 1,4-diacetyl-1,3,7-triaza-5-phosphabicylco[3.3.1]nonane (DAPTA), were synthesized and characterized by multinuclear NMR spectroscopy, IR spectroscopy, and X-ray crystallography. The complexes bearing the THP and DAPTA ligands exhibit triplet-based luminescence in air-equilibrated aqueous solutions with quantum yields ranging from 3.4 to 11.5%. Furthermore, the THP and DAPTA complexes undergo photosubstitution of a CO ligand upon irradiation with 365 nm light with quantum yields ranging from 1.1 to 5.5% and sensitize the formation of 1O2 with quantum yields as high as 70%. In contrast, all of the complexes bearing the PTA ligand are nonemissive and do not undergo photosubstitution upon irradiation with 365 nm light. These compounds were evaluated as photoactivated anticancer agents in human cervical (HeLa), ovarian (A2780), and cisplatin-resistant ovarian (A2780CP70) cancer cell lines. All of the complexes bearing THP and DAPTA exhibited a cytotoxic response upon irradiation with minimal toxicity in the absence of light. Notably, the complex with DAPTA and 1,10-phenanthroline gave rise to an IC50 value of 6 μM in HeLa cells upon irradiation, rendering it the most phototoxic compound in this library. The nature of the photoinduced cytotoxicity of this compound was explored in further detail. These data indicate that the phototoxic response may result from the release of both CO and the rhenium-containing photoproduct, as well as the production of 1O2.

Visible Light-Induced CO-Release Reactivity of a Series of ZnII–Flavonolate Complexes

A series of zinc–flavonolate complexes of the general formula [(L)Zn(R)]ClO4 (L = TPA (tris-2-(pyridylmethyl)amine)), 6-MeTPA (N,N-(6-methyl-2-pyridyl)methyl)bis(2-pyridylmethyl)amine)), 6-Me2TPA (N,N-bis(6-methyl-2-pyridyl)methyl)(2-pyridylmethyl) amine), BPQA (bis(2-pyridylmethyl)(2-quinolinemethyl)amine), and BQPA (bis(2-quinolinemethyl)(2-pyridylmethyl)amine), R = FLH (flavonol), 4-MeOFLH (4-methoxyflavonol), and 4-MeOFLTH (4-methoxyflavothione)) have been prepared and characterised by X-ray crystallography, elemental analysis, FT-IR, ESI-MS, 1H NMR, 13C NMR, UV-vis and fluorescence spectroscopy. All the complexes can be induced to release CO by visible light (λmax ranges from 414 to 503 nm). The maximum absorption wavelength of the complexes followed the order 4-MeOFLTH > 4-MeOFLH > FLH. Exposure of the complexes to visible light under aerobic conditions results in oxidative C–C bond cleavage and almost quantitative CO release. Cytotoxicity tests showed that the complexes had a low toxicity to HeLa cells in the concentration range of 1 to 50 μM. These advantages indicate that the series of complexes are likely to be applied to biological systems.

In Vitro Anticancer Activity and in Vivo Biodistribution of Rhenium(I) Tricarbonyl Aqua Complexes

Seven rhenium(I) complexes of the general formula fac-[Re(CO)3(NN)(OH2)]+ where NN = 2,2'-bipyridine (8), 4,4'-dimethyl-2,2'-bipyridine (9), 4,4'-dimethoxy-2,2'-bipyridine (10), dimethyl 2,2'-bipyridine-4,4'-dicarboxylate (11), 1,10-phenanthroline (12), 2,9-dimethyl-1,10-phenanthroline (13), or 4,7-diphenyl-1,10-phenanthroline (14), were synthesized and characterized by 1H NMR spectroscopy, IR spectroscopy, mass spectrometry, and X-ray crystallography. With the exception of 11, all complexes exhibited 50% growth inhibitory concentration (IC50) values that were less than 20 μM in HeLa cells, indicating that these compounds represent a new potential class of anticancer agents. Complexes 9, 10, and 13 were as effective in cisplatin-resistant cells as wild-type cells, signifying that they circumvent cisplatin resistance. The mechanism of action of the most potent complex, 13, was explored further by leveraging its intrinsic luminescence properties to determine its intracellular localization. These studies indicated that 13 induces cytoplasmic vacuolization that is lysosomal in nature. Additional in vitro assays indicated that 13 induces cell death without causing an increase in intracellular reactive oxygen species or depolarization of the mitochondrial membrane potential. Further studies revealed that the mode of cell death does not fall into one of the canonical categories such as apoptosis, necrosis, paraptosis, and autophagy, suggesting that a novel mode of action may be operative for this class of rhenium compounds. The in vivo biodistribution and metabolism of complex 13 and its 99mTc analogue 13* were also evaluated in naı̈ve mice. Complexes 13 and 13* exhibited comparable biodistribution profiles with both hepatic and renal excretion. High-performance liquid chromatography inductively coupled plasma mass-spectrometry (HPLC-ICP-MS) analysis of mouse blood plasma and urine postadministration showed considerable metabolic stability of 13, rendering this potent complex suitable for in vivo applications. These studies have shown the biological properties of this class of compounds and demonstrated their potential as promising theranostic anticancer agents that can circumvent cisplatin resistance.

Attenuation of Antioxidant Capacity in Human Breast Cancer Cells by Carbon Monoxide through Inhibition of Cystathionine β-Synthase Activity: Implications in Chemotherapeutic Drug Sensitivity

Drug resistance is a major impediment to effective treatment of breast cancer. Compared to normal cells, cancer cells have an increased antioxidant potential due to an increased ratio of reduced to oxidized glutathione (GSH/GSSG). This is known to confer therapeutic resistance. Here, we have identified a mechanism, unique to breast cancer cells, whereby cystathionine β-synthase (CBS) promotes elevated GSH/GSSG. Lentiviral silencing of CBS in human breast cancer cells attenuated GSH/GSSG, total GSH, nuclear factor erythroid 2-related factor 2 (Nrf2), and processes downstream of Nrf2 that promote GSH synthesis and regeneration of GSH from GSSG. Carbon monoxide (CO) reduced GSH/GSSG in three breast cancer cell lines by inhibiting CBS. Furthermore, CO sensitized breast cancer cells to doxorubicin. These results provide insight into mechanism(s) by which CBS increases the antioxidant potential and the ability for CO to inhibit CBS activity to alter redox homeostasis in breast cancer, increasing sensitivity to a chemotherapeutic.

Spin-Forbidden Excitation: A New Approach for Triggering Photopharmacological Processes with Low-Intensity NIR Light

Exposure to low-intensity radiation in the near-infrared (NIR) spectral region matching the optically transparent "phototherapeutic window" of biological tissues can be applied to directly populate spin-restricted excited states of light-responsive compounds. This unconventional and unprecedented approach is introduced herein as a new strategy to overcome some of the major unresolved problems observed in the rapidly emerging fields of photopharmacology and molecular photomedicine, where practical applications in living cells and organisms are still limited by undesired side reactions and insufficient light penetration. Water-soluble and biocompatible metal complexes with a significant degree of spin-orbit coupling were identified as target candidates for testing our new hypothesis. As a first example, a dark-stable manganese carbonyl complex acting as a visible-light-triggered CO-releasing molecule (Photo-CORM) is shown to be photoactivated by NIR radiation, although apparently no spectroscopically evident absorption bands are detectable in this low-energy region. This quite remarkable effect is ascribed to a strongly restricted, but obviously not completely forbidden optical population of the lowest triplet excited state manifold of the diamagnetic complex from the singlet ground state.

Eradication of HT-29 colorectal adenocarcinoma cells by controlled photorelease of CO from a CO-releasing polymer (photoCORP-1) triggered by visible light through an optical fiber-based device

The gaseous signaling molecule carbon monoxide (CO) has recently been recognized for its wide range of physiological activity as well as its antineoplastic properties. However, site-specific delivery of this noxious gas presents a major challenge in hospital settings. In this work, a visible light-sensitive CO-releasing molecule (photoCORM) derived from manganese(I) and 2-(quinolyl)benzothiazole (qbt) namely, [Mn(CO)₃(qbt)(4-vpy)](CF₃SO₃) (1), has been co-polymerized within a gas-permeable HEMA/EGDMA hydrogel. The resulting photoactive CO-releasing polymer (photoCORP-1) incorporates 1 such that neither the carbonyl complex nor its photoproduct(s) exits the polymer at any time. The material can be triggered to photorelease CO remotely by low-power broadband visible light (<1mWcm^-2) with the aid of fiber optics technology. The CO photorelease rates of photoCORP-1 (determined by spectrophotometry) can be modulated by both the concentration of 1 in the hydrogel and the intensity of the light. A CO-delivery device has been assembled to deliver CO to a suspension of human colorectal adenocarcinoma cells (HT-29) under the control of visible light and the extent of CO-induced apoptotic death of the cancer cells has been determined via Annexin V/Propidium iodide stain and flow cytometry. This photoactive CO-releasing polymer could find use in delivering controlled doses of CO to cellular targets such as malignant tissues in remote parts of the body.

An esterase-activated click and release approach to metal-free CO-prodrugs

Metal-free CO prodrugs with a biological trigger and tunable release rate are described herein for the first time.

Enhancement of the antiproliferative activity of [RuCp(PPh3)2(dmoPTA-1κP)]+via its coordination to one {CoCl2} unit: synthesis, crystal structure and properties of [RuCp(PPh3)2-μ-dmoPTA-1κP:2κ2N,N′-CoCl2](OTf)·0.25H2O

Antiproliferative activity up to 354-fold for WiDr-colon cells than cisplatin.