5,10,15,20-Tetrakis(N-Methyl-4-Pyridyl)-21H,23H-Porphine (TMPyP) as a Sensitizer for Singlet Oxygen Imaging in Cells: Characterizing the Irradiation-dependent Behavior of TMPyP in a Single Cell†

Photodynamic Activity of TMPyP4/TiO2 Complex under Blue Light in Human Melanoma Cells: Potential for Cancer-Selective Therapy

The combination of TiO₂ nanoparticles (NPs) and photosensitizers (PS) may offer significant advantages in photodynamic therapy (PDT) of melanoma, such as improved cell penetration, enhanced ROS production, and cancer selectivity. In this study, we aimed to investigate the photodynamic effect of 5,10,15,20-(Tetra-N-methyl-4-pyridyl)porphyrin tetratosylate (TMPyP4) complexes with TiO₂ NPs on human cutaneous melanoma cells by irradiation with 1 mW/cm² blue light. The porphyrin conjugation with the NPs was analyzed by absorption and FTIR spectroscopy. The morphological characterization of the complexes was performed by Scanning Electron Microscopy and Dynamic Light Scattering. The singlet oxygen generation was analyzed by phosphorescence at 1270 nm. Our predictions indicated that the non-irradiated investigated porphyrin has a low degree of toxicity. The photodynamic activity of the TMPyP4/TiO₂ complex was assessed on the human melanoma Mel-Juso cell line and non-tumor skin CCD-1070Sk cell line treated with various concentrations of the PS and subjected to dark conditions and visible light-irradiation. The tested complexes of TiO₂ NPs with TMPyP4 presented cytotoxicity only after activation by blue light (405 nm) mediated by the intracellular production of ROS in a dose-dependent manner. The photodynamic effect observed in this evaluation was higher in melanoma cells than the effect observed in the non-tumor cell line, demonstrating a promising potential for cancer-selectivity in PDT of melanoma.

Plasmonic Surface of Metallic Gold and Silver Nanoparticles Induced Fluorescence Quenching of Meso-Terakis (4-Sulfonatophenyl) Porphyrin (TPPS) and Theoretical-Experimental Comparable

Colloidal metallic nanoparticles have attracted a lot of interest in the last two decades owing to their simple synthesis and fascinating optical properties. In this manuscript, a study of the effect of both gold nanoparticles (Au NPs) and silver nanoparticles (Ag NPs) on the fluorescence emission (FE) of TPPS has been investigated utilizing steady-state fluorescence spectroscopy and UV–Vis spectrophotometry. From the observed electronic absorption spectra, there is no evidence of the ground state interaction between metallic Au NPs or Ag NPs with TPPS. On the other side, the FE spectra of TPPS have been quenched by both Ag and Au NPs. Via applying quenching calculations, Ag NPs showed only traditional static fluorescence quenching of TPPS with linear Stern–Volmer (SV) plots. On the contrary, quenching of TPPS emission by Au NPs shows composed models. One model is the sphere of action static quenching model that prevails at high quencher concentrations leading to non-linear SV plots with positive deviation. However, at low Au NPs concentrations, traditional dynamic quenching occurs with linear SV plots. The quantum calculations for TPPS structure have been obtained using Gaussian 09 software: in which the TPPS optimized molecular structure was achieved using DFT/B3LYP/6-311G (d) in a gaseous state. Also, the calculated electronic absorption spectra for the same molecule in water as a solvent are obtained using TD/M06/6-311G +  + (2d, 2p). Furthermore, the theoretical and experimental results comparable to UV–Vis spectra have been investigated.

The effect of two porphyrine photosensitizers TMPyP and ZnTPPS4 for application in photodynamic therapy of cancer cells in vitro

Photodynamic therapy (PDT) is one of the treatments for cancer. This therapy uses a combination of a photosensitizer (PS), light irradiation, and oxygen O₂, which is converted to cytotoxic ¹O₂ and other forms of reactive oxygen species (ROS), causing selective damage to the target tissue. In this work, we studied effect of two porphyrin photosensitizers TMPyP and ZnTPPS₄ at three different concentrations (0.25, 0.5, 5μM) after two irradiation doses (5 and 25 J/cm²). Photodynamic efect of TMPyP and ZnTPPS₄ were confirmed by a battery of in vitro tests including MTT, reactive oxygen species (ROS) production and mitochondrial membrane potential test (MMP). Morphological changes of the cells before and after treatment were imaged by atomic force microscopy (AFM). The most effective combination of irradiation dose and concentration for both PSs showed a concentration of 5 μM and a irradiation dose of 25 J/cm² in both cell cultures.

Developing [60]Fullerene Nanomaterials for Better Photodynamic Treatment of Non-Melanoma Skin Cancers

Skin cancer is the most common cancer in the U.S.A. and Europe. Its subtype, squamous skin carcinoma (SCC), if allowed to grow, has the potential to metastasize and can become deadly. Currently, carbon nanomaterials are being developed to treat cancer due to their attractive physicochemical and biological properties such as an enhanced permeability effect and their ability to produce reactive oxygen species. Here, we describe the synthesis of two water-soluble aminofullerenes (MonoaminoC₆₀ and HexakisaminoC₆₀), which were evaluated as novel [60]fullerene based photosentizers exhibiting anticancer properties. Moreover, the previously described neutral glycofullerene GF1 and its peracetylated lipophilic precursor MMS48 were compared with the aminofullerenes for their ability to generate reactive oxygen species and oxidize lipids. Remarkably, the generation of singlet oxygen and a superoxide radical by HexakisaminoC₆₀ was found to be markedly elevated in the presence of bovine serum albumin and NADH, respectively. Mechanistic studies of lipid peroxidation using cholesterol as a unique reporter molecule revealed that although all four fullerene nanomaterials primarily generated singlet oxygen, superoxide anion was also formed, which suggest a mixed mechanism of action (in which Type I and Type II photochemistry is involved). The [60]fullerene derivative HexakisaminoC₆₀ was also studied for its phototoxicity in squamous skin cancer cell line (A431) using the MTT test and propidium iodide staining.

A [60]fullerene nanoconjugate with gemcitabine: synthesis, biophysical properties and biological evaluation for treating pancreatic cancer

Background

The first-line chemotherapy drug that is used to treat pancreatic ductal adenocarcinoma is gemcitabine. Unfortunately, its effectiveness is hampered by its chemo-resistance, low vascularization and drug biodistribution limitations in the tumor microenvironment. Novel nanotherapeutics must be developed in order to improve the prognosis for patients with pancreatic cancer.

Results

We developed a synthetic methodology for obtaining a water-soluble nanoconjugate of a [60]fullerene-glycine derivative with the FDA-approved drug gemcitabine (nanoC60GEM). The proposed synthetic protocol enables a highly water-soluble [60]fullerene-glycine derivative (6) to be obtained, which was next successfully conjugated with gemcitabine using the EDCI/NHS carbodiimide protocol. The desired nanoconjugate was characterized using mass spectrometry and DLS, IR and XPS techniques. The photogeneration of singlet oxygen and the superoxide anion radical were studied by measuring 1O2 near-infrared luminescence at 1270 nm, followed by spin trapping of the DMPO adducts by EPR spectroscopy. The biological assays that were performed indicate that there is an inhibition of the cell cycle in the S phase and the induction of apoptosis by nanoC60GEM.

Conclusion

In this paper, we present a robust approach for synthesizing a highly water-soluble [60]fullerene nanoconjugate with gemcitabine. The performed biological assays on pancreatic cancer cell lines demonstrated cytotoxic effects of nanoC60GEM, which were enhanced by the generation of reactive oxygen species after blue LED irradiation of synthesized fullerene nanomaterial.

Sequence selective photoinduced electron transfer of a pyrene-porphyrin dyad to DNA

The binding modes of a pyrene-porphyrin dyad, (1-pyrenyl)-tris(N-methyl-p-pyridino)porphyrin (PyTMpyP), to various DNAs (calf thymus DNA (Ct-DNA), poly[d(G-C)2], and poly[d(A-T)2]) have been investigated using circular dichroism and linear dichroism measurements. Based on the polarization spectroscopic results, it can be shown that the pyrenyl and porphryin planes are skewed to a large extent for PyTMPyP in an aqueous environment and in the binding site of poly[d(G-C)2]. In this complex, a photoinduced electron transfer (PET) process between the pyrenyl and porphyrin moieties occurs. On the other hand, PET was not observed in the PyTMPyP-poly[d(A-T)2] complex, whereas the fluorescence intensity of TMPyP was enhanced. The molecular planes of the pyrene and porphyrin moieties are almost parallel in the poly[d(A-T)2] and Ct-DNA adducts. Moreover, the generation of 1O2 species occurs only for the PyTMPyP-Ct-DNA and PyTMPyP-poly[d(A-T)2] complexes. We discuss the photophysical properties of PyTMPyP which are attributed to the binding patterns and the sequence of DNA bases.

Microscopic time-resolved imaging of singlet oxygen by delayed fluorescence in living cells

Singlet oxygen is a highly reactive species which is involved in a number of processes, including photodynamic therapy of cancer. Its very weak near-infrared emission makes imaging of singlet oxygen in biological systems a long-term challenge. We address this challenge by introducing Singlet Oxygen Feedback Delayed Fluorescence (SOFDF) as a novel modality for semi-direct microscopic time-resolved wide-field imaging of singlet oxygen in biological systems. SOFDF has been investigated in individual fibroblast cells incubated with a well-known photosensitizer aluminium phthalocyanine tetrasulfonate. The SOFDF emission from the cells is several orders of magnitude stronger and much more readily detectable than the very weak near-infrared phosphorescence of singlet oxygen. Moreover, the analysis of SOFDF kinetics enables us to estimate the lifetimes of the involved excited states. Real-time SOFDF images with micrometer spatial resolution and submicrosecond temporal-resolution have been recorded. Interestingly, a steep decrease in the SOFDF intensity after the photodynamically induced release of a photosensitizer from lysosomes has been demonstrated. This effect could be potentially employed as a valuable diagnostic tool for monitoring and dosimetry in photodynamic therapy.

In Vitro Photodynamic Activity of N-Methylated and N-Oxidised Tripyridyl Porphyrins with Long Alkyl Chains and Their Inhibitory Activity in Sphingolipid Metabolism

A series of N-methylated and N-oxidised tripyridyl porphyrins were synthesised, characterised, and their PDT activity was studied with six cell lines. All the tested porphyrins with a long alkyl chain, except one, were more efficient for PDT than an N-methylated hydrophilic porphyrin and N-oxidised porphyrin without the long alkyl chain. Generally, N-methylated tripyridyl porphyrins were more active than those N-oxidised, but IC₅₀ values for phototoxicity of two N-oxides, named TOPyP3-C₁₇ H₃₃ O and TOPyP3-C₁₇ H₃₅ , were still in the nanomolar concentration range for most of the tested cell lines. However, TOPyP3-C₁₇ H₃₅ did not show phototoxicity on human foreskin fibroblast cells. Two methylated amphiphilic porphyrins, named TMPyP3-C₁₇ H₃₃ and TMPyP4-C₁₇ H_35, showed significant dark toxicity, whereas none of the oxidopyridyl porphyrins were toxic without light activation. The selected photosensitisers were shown to be apoptosis inducers, and had inhibitory effects on the clonogenic growth of HCT116 and HeLa cells. All three N-methylated amphiphilic porphyrins significantly reduced the migratory potential of HCT116 cells. Porphyrins TMPyP3-C₁₇ H₃₅ and TOPyP3-C₁₇ H₃₅ reduced the activity of acid ceramidase, whereas TOPyP3-C₁₇ H₃₃ O had a significant inhibitory effect on sphingosine kinase 1 activity in HeLa cells. Compounds with this dual activity were shown to be the most promising photosensitisers, with potential to treat invasive cancers.

Photodynamic inactivation of Escherichia coli - Correlation of singlet oxygen kinetics and phototoxicity

Photodynamic inactivation (PDI) of bacteria may play a major role in facing the challenge of the ever expanding antibiotic resistances. Here we report about the direct correlation of singlet oxygen luminescence kinetics and phototoxicity in E. coli cell suspension under PDI using the widely applied cationic photosensitizer TMPyP. Through direct access to the microenvironment, the time resolved investigation of singlet oxygen luminescence plays a key role in understanding the photosensitization mechanism and inactivation pathway. Using the homemade set-up for highly sensitive time resolved singlet oxygen luminescence detection, we show that the cationic TMPyP is localized predominantly outside the bacterial cells but in their immediate vicinity prior to photodynamic inactivation. Throughout following light exposure, a clear change in singlet oxygen kinetics indicates a redistribution of photosensitizer molecules to at least one additional microenvironment. We found the signal kinetics mirrored in cell viability measurements of equally treated samples from same overnight cultures conducted in parallel: A significant drop in cell viability of the illuminated samples and stationary viability of dark controls. Thus, for the system investigated in this work - a Gram-negative model bacteria and a well-known PS for its PDI - singlet oxygen kinetics correlates with phototoxicity. This finding suggests that it is well possible to evaluate PDI efficiency directly via time resolved singlet oxygen detection.

A Closer Look at Dark Toxicity of the Photosensitizer TMPyP in Bacteria

Photodynamic inactivation of bacteria (PIB) is based on photosensitizers which absorb light and generate reactive oxygen species (ROS), killing cells via oxidation. PIB is evaluated by comparing viability with and without irradiation, where reduction of viability in the presence of the photosensitizer without irradiation is considered as dark toxicity. This effect is controversially discussed for photosensitizers like TMPyP (5,10,15,20-Tetrakis(1-methyl-4-pyridinio)porphyrin tetra(p-toluensulfonate). TMPyP shows a high absorption coefficient for blue light and a high yield of ROS production, especially singlet oxygen. Escherichia coli and Bacillus atrophaeus were incubated with TMPyP and irradiated with different light sources at low radiant exposures (μW per cm²), reflecting laboratory conditions of dark toxicity evaluation. Inactivation of E. coli occurs for blue light, while no effect was detectable for wavelengths >450 nm. Being more susceptible toward PIB, growth of B. atrophaeus is even reduced for light with emission >450 nm. Decreasing the light intensities to nW per cm² for B. atrophaeus, application of TMPyP still caused bacterial killing. Toxic effects of TMPyP disappeared after addition of histidine, quenching residual ROS. Our experiments demonstrate that the evaluation of dark toxicity of a powerful photosensitizer like TMPyP requires low light intensities and if necessary additional application of substances quenching any residual ROS.

Delivery of fluorophores by calcium phosphate-coated nanoliposomes and interaction with Staphylococcus aureus biofilms

The delivery capacity and mechanical stability of calcium phosphate (CaP) coated 1,2-dioleoyl-sn-glycero-3-phosphate (DOPA) liposomes free and adsorbed on bacterial surface was investigated introducing either acridine orange (AO) or 5,10,15,20-Tetrakis(1-methyl-4-pyridinio)porphyrin (TMP) in the aqueous core of the liposomes. The obtained nanomaterials were thoroughly characterized by electron and optical microscopy and by fluorescence techniques. Distribution of the AO and TMP molecules between the aqueous liposomes core and the outer solution was demonstrated by the band shifts and broadening of the excitation-emission matrices and the modified Stern-Volmer model for fluorescence quenching. In aqueous suspensions, c.a. 40% of AO was released to the outer solution while only a small percentage of TMP was observed to reach the outer liposome surface. The nanoliposomes adhesion capacity and the leaking of fluorophore molecules to Staphylococcus aureus (S. aureus) biofilms were further evaluated. A close interaction between liposomes and S. aureus biofilm was evidenced by TEM and SEM imaging. Epifluorescence experiments demonstrated that CaP-coated liposomes have good biofilm staining capability after two hours incubation of the biofilms with the liposomes, thus supporting an important release of the fluorophores when in contact with the biofilm. Altogether, the obtained results strongly suggest that CaP-coated liposomes are capable of activating drug release when in presence of S. aureus biofilms and smears. The studies herein presented, indicate that CaP-coated liposomes are potential vehicles for the selective delivery of drugs to S. aureus biofilms, as is the case of the singlet oxygen photosensitizer TMP, a well known photodynamic antibacterial agent.

Real-time luminescence microspectroscopy monitoring of singlet oxygen in individual cells

A new setup for direct microspectroscopic monitoring of singlet oxygen ((1)O2) has been developed in our laboratory using a novel near-infrared sensitive InGaAs 2D-array detector. An imaging spectrograph has been inserted in front of the 2D-array detector, which allows us to acquire spectral images where one dimension is spatial and the other is spectral. The work presents a detailed examination of sensitivity and noise characteristics of the setup and its ability to detect (1)O2. The (1)O2 phosphorescence-based images and near-infrared luminescence spectral images recorded from single TMPyP-containing fibroblast cells reflecting spectral changes during irradiation are demonstrated. The introduction of spectral images addresses the issue of a potential spectral overlap of (1)O2 phosphorescence with near-infrared-extended luminescence of the photosensitizer and provides a powerful tool for distinguishing and separating them, which can be applied to any photosensitizer manifesting near-infrared luminescence.

The singlet-oxygen-sensitized delayed fluorescence in mammalian cells: a time-resolved microscopy approach

Microsecond kinetics of singlet-oxygen-sensitized delayed fluorescence (SOSDF) have been detected from individual living fibroblast cells as a proof-of-concept. These provide valuable information about excited state lifetimes and their changes during PDT-like treatment.

Modifying charge and hydrophilicity of simple Ru(II) polypyridyl complexes radically alters biological activities: old complexes, surprising new tricks

Compounds capable of light-triggered cytotoxicity are appealing potential therapeutics, because they can provide spatial and temporal control over cell killing to reduce side effects in cancer therapy. Two simple homoleptic Ru(II) polypyridyl complexes with almost-identical photophysical properties but radically different physiochemical properties were investigated as agents for photodynamic therapy (PDT). The two complexes were identical, except for the incorporation of six sulfonic acids into the ligands of one complex, resulting in a compound carrying an overall −4 charge. The negatively charged compound exhibited significant light-mediated cytotoxicity, and, importantly, the negative charges resulted in radical alterations of the biological activity, compared to the positively charged analogue, including complete abrogation of toxicity in the dark. The charges also altered the subcellular localization properties, mechanism of action, and even the mechanism of cell death. The incorporation of negative charged ligands provides a simple chemical approach to modify the biological properties of light-activated Ru(II) cytotoxic agents.

Two Ru(II) polypyridyl complexes with essentially identical photophysical properties but different charges and hydrophilicites were explored as potential agents for photodynamic therapy. Remarkably, the complex carrying a −4 overall charge exhibited excellent light-dependent cytotoxicity while remaining inactive in the dark. In contrast, the complex that was +2 charged caused significant cell death in the absence of the light trigger, likely because of its localization in the mitochondria and destruction of the membrane potential.

Photosensitizer-incorporated quadruplex DNA-gated nanovechicles for light-triggered, targeted dual drug delivery to cancer cells

A novel light-operated vehicle for targeted intracellular drug delivery is constructed using photosensitizer-incorporated G-quadruplex DNA-capped mesoporous silica nanoparticles. Upon light irradiation, the photosensitizer generates ROS, causing the DNA capping to be cleaved and allowing cargo to be released. Importantly, this platform makes it possible to develop a drug-carrier system for the synergistic combination of chemotherapy and PDT for cancer treatment with spatial/temporal control. Furthermore, the introducing of targeting ligands further improves tumor targeting efficiency. The excellent biocompatibility, cell-specific intracellular drug delivery, and cellular uptake properties set up the basis for future biomedical application that require in vivo controlled, targeted drug delivery.

Luminescence spectroscopy of singlet oxygen enables monitoring of oxygen consumption in biological systems consisting of fatty acids

The interaction of singlet oxygen ((1)O2) generated in a photosensitized process with well-known reference photosensitizers Perinaphthenone (PN) and TMPyP is investigated in a model system consisting of fatty acids and the respective exogenous photosensitizer (PS) in solution by direct detection of the luminescence photons of (1)O2 at 1270 nm. Such a model system is a first approach to mimic the complex environment of (1)O2 in a biological cell which consists mainly of water, proteins, sugars and lipids. Firstly, the important issue of oxygen consumption is evaluated which has to be considered during luminescence detection of (1)O2. It is known that the luminescence signal of (1)O2 is dependent on the oxygen concentration of the environment. Cellular components such as lipids represent oxygen consumers due to peroxidation of their unsaturated double bonds. Secondly, the experimental conditions for this model system regarding oxygen consumption are optimized to estimate the rates and rate constants of the coupled system. Thirdly, the triplet decay of the PS can provide more precise information about the actual oxygen concentration close to the PS and can be used, therefore, as a more precise method to determine the oxygen concentration in more complex systems such as a biological cell. The aim is to get a better understanding of photosensitized reactions of (1)O2 with cellular components to further improve methodologies, in particular at a cellular level using luminescence spectroscopy. In conclusion, luminescence detection might be a helpful tool to monitor precisely and promptly changes in oxygen concentration in a complex environment.

Toward a 3D cellular model for studying in vitro the outcome of photodynamic treatments: accounting for the effects of tissue complexity

Clinical therapies have traditionally been developed using two-dimensional (2D) cell culture systems, which fail to accurately capture tissue complexity. Therefore, three-dimensional (3D) cell cultures are more attractive platforms to integrate multiple cues that arise from the extracellular matrix and cells, closer to an in vivo scenario. Here we report the development of a 3D cellular model for the in vitro assessment of the outcome of oxygen- and drug-dependent therapies, exemplified by photodynamic therapy (PDT). Using a synthetic self-assembling peptide as a cellular scaffold (RAD16-I), we were able to recreate the in vivo limitation of oxygen and drug diffusion and its biological effect, which is the development of cellular resistance to therapy. For the first time, the production and decay of the cytotoxic species singlet oxygen could be observed in a 3D cell culture. Results revealed that the intrinsic mechanism of action is maintained in both systems and, hence, the dynamic mass transfer effects accounted for the major differences in efficacy between the 2D and 3D models. We propose that this methodological approach will help to improve the efficacy of future oxygen- and drug-dependent therapies such as PDT.

Preclinical photodynamic therapy research in Spain 3: Localization of photosensitizers and mechanisms of cell deathin vitro

Photodynamic therapy (PDT) is a subject of increasing biomedical research and represents a very promising therapeutic modality for palliative or even curative treatment of some superficial or endoscopically accessible tumors. In addition to the first photosensitizers (PSs) applied (hematoporphyrin-based drugs), second generation PSs with improved photophysical and photobiological properties are now studied using cell cultures, experimental tumors and clinical trials. On the other hand, there is a growing interest in the analysis of cell death mechanisms by apoptosis, which is especially relevant in oncology, because many anticancer drugs work, at least in part, by triggering apoptosis in neoplastic cells both in vitro and in vivo. The evaluation of cell death mechanisms is an important parameter to determine the efficacy and the potential toxicity of a treatment, allowing better adjustment of protocol. Using cell cultures, our research team has studied the mechanisms of cell damage and death implicated in the photodynamic processes, as well as the relationship between the cellular localization of the PS and the organelle damage during photosensitization. The results obtained in our laboratory provide a deeper understanding on the action mechanisms that lead to cell inactivation by PDT, and also allow selection of PSs with higher potential for clinical application than those currently in use.

Bisquinolinium compounds induce quadruplex-specific transcriptome changes in HeLa S3 cell lines

Background

Guanosine rich sequences capable of forming G-quadruplex (G4) motifs are enriched near the gene transcription start site (TSS) in the human genome. When probed at the single gene level, G-quadruplex motifs residing in promoter regions show substantial effects on gene transcription. Moreover, further changes in transcription levels are noticed when G4-motifs are targeted with G-quadruplex-specific small molecules.

Results

Global studies concerning general changes of the transcriptome via targeting promoter-based G-quadruplex motifs are very limited and have so far only been carried out with compounds displaying weak selectivity for quadruplex sequences. Here we utilize two G-quadruplex-specific bisquinolinium derivatives PhenDC3 and 360A and investigate their effects on the expression of the HeLa S3 transcriptome. Our results show wide-spread changes in the transcriptome with specificity for genes with G-quadruplex motifs near their transcription start sites (TSS). Using real-time PCR we further confirmed the specificity of PhenDC3 and 360A as potent molecules to target G-quadruplex-regulated genes.

Conclusions

Specific effects on quadruplex-containing genes have been observed utilizing whole-transcriptome analysis upon treatment of cultured cells with quadruplex-selective bisquinolinium compounds.

Irradiation- and sensitizer-dependent changes in the lifetime of intracellular singlet oxygen produced in a photosensitized process

Singlet oxygen, O(2)(a(1)Δ(g)), was produced upon pulsed-laser irradiation of an intracellular photosensitizer and detected by its 1275 nm O(2)(a(1)Δ(g)) → O(2)(X(3)Σ(g)(-)) phosphorescence in time-resolved experiments using (1) individual mammalian cells on the stage of a microscope and (2) suspensions of mammalian cells in a 1 cm cuvette. Data were recorded using hydrophilic and, independently, hydrophobic sensitizers. The microscope-based single cell results are consistent with a model in which the behavior of singlet oxygen reflects the environment in which it is produced; nevertheless, the data also indicate that a significant fraction of a given singlet oxygen population readily crosses barriers between phase-separated intracellular domains. The singlet oxygen phosphorescence signals reflect the effects of singlet-oxygen-mediated damage on cell components which, at the limit, mean that data were collected from dead cells and, in some cases, reflect contributions from both intracellular and extracellular populations of singlet oxygen. Despite the irradiation-induced changes in the environment to which singlet oxygen is exposed, the "inherent" intracellular lifetime of singlet oxygen does not appear to change appreciably as the cell progresses toward death. The results obtained from cell suspensions reflect key features that differentiate cell ensemble from single cell experiments (e.g., the ensemble experiment is more susceptible to the effects of sensitizer that has leaked out of the cell). Overall, the data clearly indicate that measuring the intracellular lifetime of singlet oxygen in a O(2)(a(1)Δ(g)) → O(2)(X(3)Σ(g)(-)) phosphorescence experiment is a challenging endeavor that involves working with a dynamic system that is perturbed during the measurement. The most important aspect of this study is that it establishes a useful framework through which future singlet oxygen data from cells can be interpreted.

Effects of environment on the photophysical characteristics of mesotetrakis methylpyridiniumyl porphyrin (TMPyP)

Porphyrins are an important class of organic molecules, with interesting linear and nonlinear optical properties given mainly by their extended π-conjugation structure. Their photophysical properties can be greatly affected by the surrounding environment, which can be used to tune its final properties. Here we report on an experimental study of the photophysical properties of meso-tetrakis (methylpyridiniumyl) porphyrin (TMPyP) in aqueous and in several organic solvents and its interaction with micelles formed from negatively charged sodium dodecylsulphate (SDS), positively charged cetyl trimethyl ammonium bromide (CTAB) and neutral TRITON X-100. By using the Z-scan technique, flash-photolysis and time-resolved fluorescence techniques, we were able to evaluate the excited state dynamics of the TMPyP, and observed that the tetrapyrrole ring plays important role due to hydrogen bonds formation between nitrogen atom and water, while the side groups determine the porphyrin localization in non-aqueous micelle part.

Single cell responses to spatially controlled photosensitized production of extracellular singlet oxygen

The response of individual HeLa cells to extracellularly produced singlet oxygen was examined. The spatial domain of singlet oxygen production was controlled using the combination of a membrane-impermeable Pd porphyrin-dendrimer, which served as a photosensitizer, and a focused laser, which served to localize the sensitized production of singlet oxygen. Cells in close proximity to the domain of singlet oxygen production showed morphological changes commonly associated with necrotic cell death. The elapsed postirradiation "waiting period" before necrosis became apparent depended on: (1) the distance between the cell membrane and the domain irradiated, (2) the incident laser fluence and, as such, the initial concentration of singlet oxygen produced and (3) the lifetime of singlet oxygen. The data imply that singlet oxygen plays a key role in this process of light-induced cell death. The approach of using extracellularly generated singlet oxygen to induce cell death can provide a solution to a problem that often limits mechanistic studies of intracellularly photosensitized cell death: it can be difficult to quantify the effective light dose, and hence singlet oxygen concentration, when using an intracellular photosensitizer.

Singlet oxygen in a cell: spatially dependent lifetimes and quenching rate constants

Singlet molecular oxygen, O(2)(a(1)Delta(g)), can be created in a single cell from ground-state oxygen, O(2)(X(3)Sigma(g)(-)), upon focused laser irradiation of an intracellular sensitizer. This cytotoxic species can subsequently be detected by its 1270 nm phosphorescence (a(1)Delta(g) --> X(3)Sigma(g)(-)) with subcellular spatial resolution. The singlet oxygen lifetime determines its diffusion distance and hence the intracellular volume element in which singlet-oxygen-initiated perturbation of the cell occurs. In this study, the time-resolved phosphorescence of singlet oxygen produced by the sensitizers chlorin (Chl) and 5,10,15,20-tetrakis(N-methyl-4-pyridyl)-21H,23H-porphine (TMPyP) was monitored. These molecules localize in different domains of a living cell. The data indicate that (i) the singlet oxygen lifetime and (ii) the rate constant for singlet oxygen quenching by added NaN(3) depend on whether Chl or TMPyP was the photosensitizer. These observations likely reflect differences in the chemical and physical constituency of a given subcellular domain (e.g., spatially dependent oxygen and NaN(3) diffusion coefficients), thereby providing evidence that singlet oxygen responds to the inherent heterogeneity of a cell. Thus, despite a relatively long intracellular lifetime, singlet oxygen does not diffuse a great distance from its site of production. This is a consequence of an apparent intracellular viscosity that is comparatively large.

Spatial and temporal dynamics of in vitro photodynamic cell killing: extracellular hydrogen peroxide mediates neighbouring cell death

Photodynamic killing of a cell population is generally considered to result from direct effects that occur in each cell. In some scenarios this may be an over-simplification and the potential for cell-cell signaling processes to contribute to the response of a population to photodynamic stress is addressed in this paper. Photodynamic killing of EMT6 cells in culture was studied in time and space using computerized time-lapse microscopy. The rate of cell killing was dependent on the fluence with both rapid and slower processes evident, the proportion of the former increasing with fluence. The spatial distribution of cell death was non-random and for the slow cell killing process was found to occur preferentially in the vicinity of dead or dying cells, suggesting a local signaling process. An inhibitory effect of extracellular catalase indicated the involvement of hydrogen peroxide in the spread of cell death and NADPH oxidase was determined as the principal source of hydrogen peroxide. This cell signaling pathway was observed for membrane-bound and mitochondrial photosensitizers but not for a nuclear photosensitizer. These secondary cell signalling pathways extend the oxidative damage to cells in space and time.

Photophysical properties and intracellular imaging of water-soluble porphyrin dimers for two-photon excited photodynamic therapy

We have investigated the photophysical properties and intracellular behaviour of a series of hydrophilic conjugated porphyrin dimers. All the dimers exhibit intense linear absorption at 650-800 nm and high singlet oxygen quantum yields (0.5-0.9 in methanol), as required for an efficient sensitiser for photodynamic therapy (PDT). They also exhibit fluorescence at 700-800 nm, with fluorescence quantum yields of up to 0.13 in methanol, and show extremely large two-photon absorption maxima of 8,000-17,000 GM in the near-IR. The dimers aggregate in aqueous solution, but aggregation is reduced by binding to bovine serum albumin (BSA), as manifested by an increase in fluorescence intensity and a sharpening in the emission bands. This process can be regarded as a model for the interaction with proteins under physiological conditions. Confocal fluorescence microscopy of live cells was used to monitor the rate of cellular uptake, intracellular localisation and photostability. Porphyrin dimers with positively charged substituents partition into cells more efficiently than the negatively charged dimers. The photostability of these dimers, in living cells, is significantly better than that of the clinical photosensitiser verteporfin. Analysis of the photophysical parameters and intracellular imaging data indicates that these dimers are promising candidates for one-photon and two-photon excited PDT.

Time dependence of singlet oxygen luminescence provides an indication of oxygen concentration during oxygen consumption

Singlet oxygen plays a major role in photodynamic inactivation of tumor cells or bacteria. Its efficacy depends critically on the oxygen concentration [O(2)], which can decrease in case oxygen is consumed caused by oxidative reactions. When detecting singlet oxygen directly by its luminescence at 1270 nm, the course of the luminescence signal is critically affected by [O(2)]. Thus, it should be feasible to monitor oxygen consumption during photo-oxidative processes. Singlet oxygen was generated by exciting a photosensitizer (TMPyP) in aqueous solution (H(2)O or D(2)O) of albumin. Chromatography shows that most of the TMPyP molecules are unbound, and therefore singlet oxygen molecules can diffuse in the solution. A sensor device for oxygen concentration revealed a rapid decrease of [O(2)] (oxygen depletion) in the solution during irradiation. The extent of oxygen depletion in aqueous albumin solution depends on the radiant exposure and the solvent. When detecting the luminescence signal of singlet oxygen, the shape of the luminescence signal significantly changed with irradiation time. Thus, local oxygen consumption could be monitored during photodynamic action by evaluating the course of singlet oxygen luminescence.

Singlet oxygen luminescence dosimetry (SOLD) for photodynamic therapy: current status, challenges and future prospects

As photodynamic therapy (PDT) continues to develop and find new clinical indications, robust individualized dosimetry is warranted to achieve effective treatments. We posit that the most direct PDT dosimetry is achieved by monitoring singlet oxygen (1O2), the major cytotoxic species generated photochemically during PDT. Its detection and quantification during PDT have been long-term goals for PDT dosimetry and the development of techniques for this, based on detection of its near-infrared luminescence emission (1270 nm), is at a noteworthy stage of development. We begin by discussing the theory behind singlet-oxygen luminescence dosimetry (SOLD) and the seminal contributions that have brought SOLD to its current status. Subsequently, technology developments that could potentially improve SOLD are discussed, together with future areas of research, as well as the potential limitations of this method. We conclude by examining the major thrusts for future SOLD applications: as a tool for quantitative photobiological studies, a point of reference to evaluate other PDT dosimetry techniques, the optimal means to evaluate new photosensitizers and delivery methods and, potentially, a direct and robust clinical dosimetry system.

Photodynamic molecular beacon as an activatable photosensitizer based on protease-controlled singlet oxygen quenching and activation

Molecular beacons are FRET-based target-activatable probes. They offer control of fluorescence emission in response to specific cancer targets, thus are useful tools for in vivo cancer imaging. Photodynamic therapy (PDT) is a cell-killing process by light activation of a photosensitizer (PS) in the presence of oxygen. The key cytotoxic agent is singlet oxygen ((1)O(2)). By combining these two principles (FRET and PDT), we have introduced a concept of photodynamic molecular beacons (PMB) for controlling the PS's ability to generate (1)O(2) and, ultimately, for controlling its PDT activity. The PMB comprises a disease-specific linker, a PS, and a (1)O(2) quencher, so that the PS's photoactivity is silenced until the linker interacts with a target molecule, such as a tumor-associated protease. Here, we report the full implementation of this concept by synthesizing a matrix metalloproteinase-7 (MMP7)-triggered PMB and achieving not only MMP7-triggered production of (1)O(2) in solution but also MMP7-mediated photodynamic cytotoxicity in cancer cells. Preliminary in vivo studies also reveal the MMP7-activated PDT efficacy of this PMB. This study validates the core principle of the PMB concept that selective PDT-induced cell death can be achieved by exerting precise control of the PS's ability to produce (1)O(2) by responding to specific cancer-associated biomarkers. Thus, PDT selectivity will no longer depend solely on how selectively the PS can be delivered to cancer cells. Rather, it will depend on how selective a biomarker is to cancer cells, and how selective the interaction of PMB is to this biomarker.

Control and Selectivity of Photosensitized Singlet Oxygen Production: Challenges in Complex Biological Systems

Singlet molecular oxygen is a reactive oxygen species that plays an important role in a number of biological processes, both as a signalling agent and as an intermediate involved in oxidative degradation reactions. Singlet oxygen is commonly generated by the so-called photosensitization process wherein a light-absorbing molecule, the sensitizer, transfers its energy of excitation to ground-state oxygen to make singlet oxygen. This process forms the basis of photodynamic therapy, for example, where light, a sensitizer, and oxygen are used to initiate cell death and ultimately destroy undesired tissue. Although the photosensitized production of singlet oxygen has been studied and used in biologically pertinent systems for years, the photoinitiated behaviour is often indiscriminate and difficult to control. In this Concept, we discuss new ideas and results in which spatial and temporal control of photosensitized singlet oxygen production can be implemented through the incorporation of the sensitizer into a conjugate system that selectively responds to certain triggers or stimuli.

Measuring the lifetime of singlet oxygen in a single cell: addressing the issue of cell viability

Singlet molecular oxygen, O(2)(a(1)Delta(g)), has been detected from single neurons and HeLa cells in time-resolved optical experiments by its 1270 nm phosphorescence (a(1)Delta(g)--> X(3)Sigma(-)(g)) upon irradiation of a photosensitizer incorporated into the cell. The cells were maintained in a buffered medium and their viability was assessed by live/dead assays. To facilitate the detection of singlet oxygen, intracellular H(2)O was replaced with D(2)O by an osmotic de- and rehydration process. The effect of this insult on the cells was likewise assessed. The data indicate that, in the complicated transition from a "live" to "dead" cell, the majority of our cells have the metabolic activity and morphology characteristic of a live cell. Quenching experiments demonstrate that the singlet oxygen lifetime in our cells is principally determined by interactions with intracellular water and not by interactions with other cell constituents. The data indicate that in a viable, metabolically-functioning, and H(2)O-containing cell, the lifetime of singlet oxygen is approximately 3 micros. This is consistent with our previous reports, and confirms that the singlet oxygen lifetime in a cell is much longer than hitherto believed. This implies that, in a cell, singlet oxygen is best characterized as a selective rather than reactive intermediate. This is important when considering roles played by singlet oxygen as a signaling agent and as a component in events that result in cell death. The data reported herein also demonstrate that spatially-resolved optical probes can be used to monitor selected events in the light-induced, singlet-oxygen-mediated death of a single cell.

A Europium(III) Complex as an Efficient Singlet Oxygen Luminescence Probe

A new europium(III) complex, [4'-(10-methyl-9-anthryl)-2,2':6',2"-terpyridine-6,6"-diyl]bis(methylenenitrilo) tetrakis(acetate)-Eu(3+), was designed and synthesized as a highly sensitive and selective time-gated luminescence probe for singlet oxygen ((1)O2). The new probe is highly water soluble with a large stability constant of approximately 10(21) and a wide pH available range (pH 3-10), and can specifically react with (1)O2 to form its endoperoxide (EP-MTTA-Eu(3+)) with a high reaction rate constant at 10(10) M(-1) s(-1), accompanied by the remarkable increases of luminescence quantum yield from 0.90% to 13.8% and lifetime from 0.80 to 1.29 ms, respectively. The wide applicability of the probe was demonstrated by detection of (1)O2 generated from a MoO(4)(2-)/H(2)O2 system, a photosensitization system of 5,10,15,20-tetrakis(1-methyl-4-pyridinio)porphyrin tetra(p-toluenesulfonate) (TMPyP), and a horseradish peroxidase catalyzed aerobic oxidation system of indole-3-acetic acid (IAA). In addition, it was found that the new probe could be easily transferred into living HeLa cells by incubation with TMPyP. A time-gated luminescence imaging technique that can fully eliminate the short-lived background fluorescence from TMPyP and cell components has been successfully developed for monitoring the time-dependent generation of (1)O2 in living cells.

Two-Photon Singlet Oxygen Microscopy: The Challenges of Working with Single Cells

A microscope is described in which singlet molecular oxygen, O2(a1deltag), is produced in a femtoliter focal volume via a nonlinear two-photon photosensitized process, and the 1270 nm phosphorescence from this population of O2(a1deltag) is detected in a photon counting experiment. Although two-photon excitation of a sensitizer is less efficient than excitation by a one-photon process, nonlinear excitation has several distinct advantages with respect to the spatial resolution accessible. Pertinent aspects of this two-photon O2(a1deltag) microscope were characterized using bulk solutions of photosensitizers. These data were compared to those obtained from a single biological cell upon linear one-photon excitation of a sensitizer incorporated in the cell. On the basis of the results obtained, we outline the challenges of using nonlinear optical techniques to create O2(aldeltag) at the single cell level and to then optically detect the O2(aldeltag) thus produced in a time-resolved experiment.

Optical detection of singlet oxygen from single cells

The lowest excited electronic state of molecular oxygen, singlet molecular oxygen, O(2)(a (1)Delta(g)), is a reactive species involved in many chemical and biological processes. To better understand the roles played by singlet oxygen in biological systems, particularly at the sub-cellular level, optical tools have been developed to create and directly detect this transient state in time- and spatially-resolved experiments from single cells. Data obtained indicate that, contrary to common perception, this reactive species can be quite long-lived in a cell and, as such, can diffuse over appreciable distances including across the cell membrane into the extracellular environment. On one hand, these results demonstrate that the behavior of singlet oxygen in an intact cell can be significantly different from that inferred from model bulk studies. More generally, these results provide a new perspective for mechanistic studies of intra- and inter-cellular signaling and events that ultimately lead to photo-induced cell death.