Excited-state dynamics of bis-dehydroxycurcumin carboxylic acid, a water-soluble derivative of the photosensitizer curcumin

Synthesis and Optical Properties of a Novel Hybrid Nanosystem Based on Covalently Modified nSiO2 Nanoparticles with a Curcuminoid Molecule

A new curcuminoid molecule (3) has been designed and synthesized, containing a central -(CH2)2-COOH chain at the α carbon of the keto-enol moiety in the structure. The carboxylic acid group is added to react with exposed amino groups on silica oxide nanoparticles (nSiO2), forming an amide bond to attach the curcuminoid moiety to the nSiO2 covalently. The Kaiser test quantifies the functionalization degree, yielding 222 μmol of curcuminoid per gram of nanoparticles. The synthesized hybrid nanosystem, nSiO2-NHCO-CCM, displays significant emission properties, with a maximum emission at 538 nm in dichloromethane, similar to curcuminoid 1 (without the central chain), which emits at 565 nm in the same solvent. Solvent-induced spectral effects on the absorption and emission bands of the new hybrid nanosystem are confirmed, similar to those observed for the free curcuminoid (1). The new nanosystem is evaluated in the presence of kerosene in water, showing an emission band at 525 nm as a detection response. The ability of nSiO2-NHCO-CCM to change its fluorescence when interacting with kerosene in water is notable, as it overcomes the limitation caused by the insolubility of free curcuminoid 1 in water, allowing for the exploitation of its properties when connected to the water-stable nanosystem for future detection studies.

Synthesis, Characterization, Fluorescence Properties, and DFT Modeling of Difluoroboron Biindolediketonates

We report a simple and efficient strategy to enhance the fluorescence of biocompatible biindole diketonates (bdks) in the visible spectrum through difluoroboronation (BF₂bdks complexes). Emission spectroscopy testifies an increase in the fluorescence quantum yields from a few percent to as much as >0.7. This massive increment is essentially independent of substitutions at the indole (-H, -Cl, and -OCH₃) and corresponds to a significant stabilization of the excited state with respect to non-radiative decay mechanisms: the non-radiative decay rates are reduced by as much as an order of magnitude, from 10⁹ s^-1 to 10⁸ s^-1, upon difluoroboronation. The stabilization of the excited state is large enough to enable sizeable ¹O₂ photosensitized production. Different time-dependent (TD) density functional theory (DFT) methods were assessed in their ability to model the electronic properties of the compounds, with TD-B3LYP-D3 providing the most accurate excitation energies. The calculations associate the first active optical transition in both the bdks and BF₂bdks electronic spectra to the S₀ → S₁ transition, corresponding to a shift in the electronic density from the indoles to the oxygens or the O-BF₂-O unit, respectively.

Asymmetric Phenyl Substitution: An Effective Strategy to Enhance the Photosensitizing Potential of Curcuminoids

Curcumin has been demonstrated to exhibit photosensitized bactericidal activity. However, the full exploitation of curcumin as a photo-pharmaceutical active principle is hindered by fast deactivation of the excited state through the transfer of the enol proton to the keto oxygen. Introducing an asymmetry in the molecular structure through acting on the phenyl substituents is expected to be a valuable strategy to impair this undesired de-excitation mechanism competing with the therapeutically relevant ones. In this study, two asymmetric curcumin analogs were synthesized and characterized as to their electronic-state transition spectroscopic properties. Fluorescence decay distributions were also reconstructed. Their analysis confirmed the substantial stabilization of the fluorescent state with respect to the parent compound. Nuclear magnetic resonance experiments were performed with the aim of determining the structural features of the keto-enol ring and the strength of the keto-enol hydrogen bond. Electronic structure calculations were also undertaken to elucidate the effects of substitution on the features of the keto-enol semi-aromatic system and the proneness to proton transfer. Finally, their singlet oxygen-generation efficiency was compared to that of curcumin through the 9,10-dimethylanthracene fluorescent assay.

Synthesis, Characterization and DNA-Binding Affinity of a New Zinc(II) Bis(5-methoxy-indol-3-yl)propane-1,3-dione Complex

The novel zinc(II) µ-oxo-bridged-dimeric complex [Zn2(µ-O)2(BMIP)2] (BMIP = 1,3-bis(5-methoxy-1-methyl-1H-indol-3-yl)propane-1,3-dione), 1, was synthetized and fully characterized. The spectral data indicate a zincoxane molecular structure, with the BMIP ligand coordinating in its neutral form via its oxygen atoms. Structural changes in 1 in dimethylsulfoxide (DMSO) were evidenced by means of spectroscopic techniques including infrared absorption and nuclear magnetic resonance, showing DMSO entrance in the coordination sphere of the metal ion. The resulting complex [Zn2(µ-O)2(BMIP)2(DMSO)], 2, readily reacts in the presence of N-methyl-imidazole (NMI), a liquid-phase nucleoside mimic, to form [Zn2(µ-O)2(BMIP)2(NMI)], 3, through DMSO displacement. The three complexes show high thermal stability, demonstrating that 1 has high affinity for hard nucleophiles. Finally, with the aim of probing the suitability of this system as model scaffold for new potential anticancer metallodrugs, the interactions of 1 with calf thymus DNA were investigated in vitro in pseudo-physiological environment through UV-Vis absorption and fluorescence emission spectroscopy, as well as time-resolved fluorescence studies. The latter analyses revealed that [Zn2(µ-O)2(BMIP)2(DMSO)] binds to DNA with high affinity upon DMSO displacement, opening new perspectives for the development of optimized drug substances.

Development of a Potential Gallium-68-Labelled Radiotracer Based on DOTA-Curcumin for Colon-Rectal Carcinoma: From Synthesis to In Vivo Studies

Colorectal cancer is the third most commonly occurring cancer in men and the second most commonly occurring cancer in women worldwide. We have recently reported that curcuminoid complexes labelled with gallium-68 have demonstrated preferential uptake in HT29 colorectal cancer and K562 lymphoma cell lines compared to normal human lymphocytes. In the present study, we report a new gallium-68-labelled curcumin derivative (⁶⁸Ga-DOTA-C21) and its initial validation as marker for early detection of colorectal cancer. The precursor and non-radioactive complexes were synthesized and deeply characterized by analytical methods then the curcuminoid was radiolabelled with gallium-68. The in vitro stability, cell uptake, internalization and efflux properties of the probe were studied in HT29 cells, and the in vivo targeting ability and biodistribution were investigated in mice bearing HT29 subcutaneous tumour model. ⁶⁸Ga-DOTA-C21 exhibits decent stability (57 ± 3% after 120 min of incubation) in physiological media and a curcumin-mediated cellular accumulation in colorectal cancer cell line (121 ± 4 KBq of radiotracer per mg of protein within 60 min of incubation). In HT29 tumour-bearing mice, the tumour uptake of ⁶⁸Ga-DOTA-C21 is 3.57 ± 0.3% of the injected dose per gram of tissue after 90 min post injection with a tumour to muscle ratio of 2.2 ± 0.2. High amount of activity (12.73 ± 1.9% ID/g) is recorded in blood and significant uptake of the radiotracer occurs in the intestine (13.56 ± 3.3% ID/g), lungs (8.42 ± 0.8% ID/g), liver (5.81 ± 0.5% ID/g) and heart (4.70 ± 0.4% ID/g). Further studies are needed to understand the mechanism of accumulation and clearance; however, ⁶⁸Ga-DOTA-C21 provides a productive base-structure to develop further radiotracers for imaging of colorectal cancer.

Solubilization of the chlorin TPCS2a in the presence of Pluronic® F127/Tween 80 mixtures

The efficacy of surfactant mixtures of Pluronic^® F127 and Tween 80 at overall concentration in the micromolar range and molar ratio 1:1, 1:10, and 10:1 in inhibiting aggregation of the photosensitizer meso-tetraphenyl chlorin disulphonate (TPCS_2a) was investigated in aqueous media at pH 2.9 by means of steady-state absorption and fluorescence emission spectroscopy as well as time-resolved fluorescence analysis. Corresponding experiments were performed at pH 7.4 in the absence of surfactants to determine the spectroscopic properties of a monomeric sample. Aggregation resulted in a red shift of the Soret absorption band and in substantial fluorescence quenching. The fluorescence lifetime of TPCS_2a was a particularly sensitive indicator of the aggregation state, as the monomer at pH 7.4 decayed with a ∼ 10 ns time constant, while aggregation resulted in subnanosecond decay. The critical micelle concentration (CMC) of the surfactant mixtures was determined spectrophotometrically in the presence of TPCS_2a. The ability of the surfactant mixtures to prevent aggregation at acidic pH was evaluated at overall surfactant concentration below and above CMC. Solubilization of TPCS_2a in Pluronic^® F127/Tween 80 mixtures prevented aggregation of the photosensitizer at overall surfactant concentrations much lower than those needed for both pure Pluronic^® F127 and pure Tween 80.

New curcumin-derived ligands and their affinity towards Ga3+, Fe3+ and Cu2+: spectroscopic studies on complex formation and stability in solution

Chemico-physical properties, metal chelating ability, antiproliferative activity and DNA binding of new curcuminoids with improved stability.

Inhibition effect of curcumin on UVB-induced secretion of pro-inflammatory cytokines from corneal limbus epithelial cells

AIM

To study the effects of curcumin on the secretion of interleukin (IL)-6 and IL-8 by corneal limbus epithelial cells.

METHODS

Human corneal limbus epithelial cells were isolated and cultured from donor eyes and irradiated by UVB at different dosages with or without curcumin. MTT test was used for studying the effects of UVB and curcumin on the cell viability. The role of mitogen-activated protein kinase (MAPK) and nuclear factor-kappa B (NF-κB) pathways on the UVB-induced secretion of IL-6 and IL-8 were tested by addition of their inhibitors to the culture with or without UVB-radiation. Levels of various signal pathways, IL-6 and IL-8 in the cells and in the conditioned culture medium were measured by ELISA analysis.

RESULTS

UVB at 20 mJ/cm² or less and curcumin at 20 µmol/L or less did not affect the cell viability of cultured limbus epithelial cells (P>0.05). UVB irradiation at 10 and 20 mJ/cm² induced a significant increase of secretion of IL-6 and IL-8 and upregulated NF-κB and phosphorylated MAPK pathways of cultured limbus epithelial cells (P<0.05). Various signal pathway inhibitors, including SP600125 (JNK inhibitor), SB203580 (p38 MAPK inhibitor) and BAY11-7082 (NF-κB inhibitor) significantly decreased the UVB-induced secretion of IL-6 and IL-8 secretion (P<0.05). Curcumin at 5-20 µmol/L significantly inhibited UVB-induced secretion of IL-6 and IL-8 by limbus epithelial cells in a dose-dependent manner; while curcumin alone did not affect the secretion of IL-6 and IL-8. The upregulation of NF-κB and MAPK pathways induced by UVB treatment was significantly inhibited by curcumin, suggesting that NF-κB and MAPK pathways are involved in the inhibitory effect of curcumin on UVB-induced production of IL-6 and IL-8.

CONCLUSION

Curcumin may be a promising agent to be explored for the prevention and treatment of pterygium.

Excited state dynamics of bis-dehydroxycurcumin tert-butyl ester, a diketo-shifted derivative of the photosensitizer curcumin

Bis-dehydroxycurcumin tert-butyl ester (K2T23) is a derivative of the natural spice curcumin. Curcumin is widely studied for its multiple therapeutic properties, including photosensitized cytotoxicity. However, the full exploitation of curcumin phototoxic potential is hindered by the extreme instability of its excited state, caused by very efficient non radiative decay by means of transfer of the enolic proton to the nearby keto oxygen. K2T23 is designed to exhibit a tautomeric equilibrium shifted toward the diketo conformers with respect to natural curcumin. This property should endow K2T23 with superior excited-state stability when excited in the UVB band, i.e., in correspondence of the diketo conformers absorption peaks, making this compound an interesting candidate for topical photodynamic therapy of, e.g., skin tumors or oral infections. In this work, the tautomeric equilibrium of K2T23 between the keto-enolic and diketo conformers is assessed in the ground state in several organic solvents by UV-visible absorption and by nuclear magnetic resonance. The same tautomeric equilibrium is also probed in the excited-state in the same environments by means of steady-state fluorescence and time-correlated single-photon counting measurements. These techniques are also exploited to elucidate the excited state dynamics and excited-state deactivation pathways of K2T23, which are compared to those determined for several other curcuminoids characterized in previous works of ours. The ability of K2T23 in photosensitizing the production of singlet oxygen is compared with that of curcumin.

In vitro study on potential pharmacological activity of curcumin analogues and their copper complexes

Curcumin and its derivatives have attracted great interest in the prevention and treatment of Alzheimer's disease, thanks both to the ability to hinder the formation of amyloid-beta (Aβ) aggregates and the ability to bind Cu (II) ion. In this article, we explore the ability of curcumin derivatives of K2T series to affect amyloid Aβ_1-40 aggregation. These derivatives were obtained by introducing the t-butyl ester group through a methylenic spacer on the central carbon atom of the β-diketo moiety of curcumin frame. The studied curcuminoids were demonstrated to inhibit Aβ_1-40 fibrillization at substoichiometric concentrations with IC₅₀ value near that of curcumin. In addition, the antioxidant properties and DNA interaction of their Cu(II) complexes is evaluated. The structure of Cu(II)-K2T31 complex is also proposed on the basis of DFT calculation.

SiO₂-CaO-P₂O₅ Bioactive Glasses: A Promising Curcuminoids Delivery System

In this paper, we report the study of the loading and the release of curcuminoids by bioactive glasses (BG) and mesoporous bioactive glasses (MBG). Through a detailed spectroscopic study, it was possible to determine the amount and the type of molecules released in water and in simulated body fluid (SBF). In particular, curcumin and K2T21 show a good ability to be released in di-keto and keto-enolic form, depending from the pH. However, after 24 h, the amount of pristine curcumin release is very low with a consequent increment of degradation products derived by curcuminoids. The presence of –OH groups on curcuminoids is a fundamental pre-requisite in order to obtain a high loading and release in polar solution such as water and SBF. The substrate on which we loaded the drugs does not seem to affect significantly the loading and the release of the drugs. The environment, instead, affects the release: for all the drugs, the release in SBF, buffered at pH of 7.4, is slightly worse than the release in water (basic pH values).

Elucidation of the relationships between H-bonding patterns and excited state dynamics in cyclovalone

Cyclovalone is a synthetic curcumin derivative in which the keto-enolic system is replaced by a cyclohexanone ring. This modification of the chemical structure might in principle result in an excited state that is more stable than that of curcumin, which in turn should produce an enhanced phototoxicity. Indeed, although curcumin exhibits photosensitized antibacterial activity, this compound is characterized by very fast excited-state dynamics which limit its efficacy as a photosensitizer. In previous works we showed that the main non-radiative decay pathway of keto-enolic curcuminoids is through excited-state transfer of the enolic proton to the keto-oxygen. Another effective deactivation pathway involves an intermolecular charge transfer mechanism occurring at the phenyl rings, made possible by intramolecular H-bonding between the methoxy and the hydroxyl substituent. In this paper we present UV-Vis and IR absorption spectra data with the aim of elucidating the intramolecular charge distribution of this compound and its solvation patterns in different environments, with particular focus on solute-solvent H-bonding features. Moreover, we discuss steady state and time-resolved fluorescence data that aim at characterizing the excited-state dynamics of cyclovalone, and we compare its decay photophysics to that of curcumin. Finally, because during the characterization procedures we found evidence of very fast photodegradation of cyclovalone, its photostability in four organic solvents was studied by HPLC and the corresponding relative degradation rates were calculated.

Curcumin derivatives as metal-chelating agents with potential multifunctional activity for pharmaceutical applications

Curcuminoids represent new perspectives for the development of novel therapeutics for Alzheimer's disease (AD), one probable mechanism of action is related to their metal complexing ability. In this work we examined the metal complexing ability of substituted curcuminoids to propose new chelating molecules with biological properties comparable with curcumin but with improved stability as new potential AD therapeutic agents. The K2T derivatives originate from the insertion of a -CH2COOC(CH3)3 group on the central atom of the diketonic moiety of curcumin. They retain the diketo-ketoenol tautomerism which is solvent dependent. In aqueous solution the prevalent form is the diketo one but the addition of metal ion (Ga(3+), Cu(2+)) causes the dissociation of the enolic proton creating chelate complexes and shifting the tautomeric equilibrium towards the keto-enol form. The formation of metal complexes is followed by both NMR and UV-vis spectroscopy. The density functional theory (DFT) calculations on K2T21 complexes with Ga(3+) and Cu(2+) are performed and compared with those on curcumin complexes. [Ga(K2T21)2(H2O)2](+) was found more stable than curcumin one. Good agreement is detected between calculated and experimental (1)H and (13)C NMR data. The calculated OH bond dissociation energy (BDE) and the OH proton dissociation enthalpy (PDE), allowed to predict the radical scavenging ability of the metal ion complexed with K2T21, while the calculated electronic affinity (EA) and ionization potential (IP) represent yardsticks of antioxidant properties. Eventually theoretical calculations suggest that the proton-transfer-associated superoxide-scavenging activity is enhanced after binding metal ions, and that Ga(3+) complexes display possible superoxide dismutase (SOD)-like activity.

Metal binding ability of curcumin derivatives: a theoretical vs. experimental approach

Theoretical calculations employing DFT at the B3LYP/6-311G++** level are used to investigate the tautomeric equilibrium in curcumin derivatives. The solvent effect is evaluated using the CPCM continuum solvation method. The results are compared with experimental data obtained from the X-ray crystal structure of K2A23 and UV-vis data. The KE tautomer is more stable in a vacuum and in the solid state, while in water the DK tautomer reaches a population of 90%. In agreement with spectroscopic data, theoretical calculations predict a slight prevalence of the DK form in non-aqueous solvent systems. The ability to chelate metal ions [Fe(3+), Ga(3+) and Cu(2+)] is then explored by means of (1)H, (13)C NMR and UV-Vis spectroscopy. From the calculation of the overall stability constants of metal complexes and (1)H NMR titrations with Ga(3+), it is clear that the more stable species has a 1 : 2 M/L molar ratio. The curcuminoid coordinates the metal ion through the keto-enol function in the dissociated form; in addition 2D (1)H (13)C NMR experiments suggest the involvement of carboxylic oxygen in metal coordination it was found in the solid state for the complex [Ga(K2A33)2]PF6. The rate of the complexation reaction is strongly influenced by the type of substituent on the aromatic ring of the curcuminoid (K2A33 ≈ K2A23 ≫ K2A21). In addition DPPH assay evidences how antioxidant ability of curcumin derivatives is mainly due to the presence of a phenolic group and metal coordination by a keto-enolic moiety does not affect it, especially for K2A21.

bis-Dehydroxy-Curcumin triggers mitochondrial-associated cell death in human colon cancer cells through ER-stress induced autophagy

BACKGROUND

The activation of autophagy has been extensively described as a pro-survival strategy, which helps to keep cells alive following deprivation of nutrients/growth factors and other stressful cellular conditions. In addition to cytoprotective effects, autophagy can accompany cell death. Autophagic vacuoles can be observed before or during cell death, but the role of autophagy in the death process is still controversial. A complex interplay between autophagy and apoptosis has come to light, taking into account that numerous genes, such as p53 and Bcl-2 family members, are shared between these two pathways.

METHODOLOGY/PRINCIPAL FINDINGS

In this study we showed a potent and irreversible cytotoxic activity of the stable Curcumin derivative bis-DeHydroxyCurcumin (bDHC) on human colon cancer cells, but not on human normal cells. Autophagy is elicited by bDHC before cell death as demonstrated by increased autophagosome formation -measured by electron microscopy, fluorescent LC3 puncta and LC3 lipidation- and autophagic flux -measured by interfering LC3-II turnover. The accumulation of poly-ubiquitinated proteins and ER-stress occurred upstream of autophagy induction and resulted in cell death. Cell cycle and Western blot analyses highlighted the activation of a mitochondrial-dependent apoptosis, which involves caspase 7, 8, 9 and Cytochrome C release. Using pharmacological inhibitions and RNAi experiments, we showed that ER-stress induced autophagy has a major role in triggering bDHC-cell death.

CONCLUSION/SIGNIFICANCE

Our findings describe the mechanism through which bDHC promotes tumor selective inhibition of proliferation, providing unequivocal evidence of the role of autophagy in contrasting the proliferation of colon cancer cells.