Intracellular Fate of the Photosensitizer Chlorin e4 with Different Carriers and Induced Metabolic Changes Studied by 1H NMR Spectroscopy

Simultaneous Application of Methylene Blue and Chlorin e6 Photosensitizers: Investigation on a Cell Culture

The application of photosensitizers for inhibition of oxidative phosphorylation in order to temporally decrease oxygen uptake by tumor cells in the course of photodynamic therapy (PDT) evokes growing interest. The aim of the study is to overcome tumor hypoxia for further photodynamic therapy with simultaneous use of type I photosensitizer methylene blue (MB) and type II photosensitizer chlorin e6.

Material and Methods

A photodynamic activity of MB and its combined use with chlorin e6 has been studied on the HeLa cell culture, their effect on cell metabolism in their co-accumulation and subsequent irradiation has also been assessed.

Results

MB generates reactive oxygen species in the cells in contrast to chlorin e6, which produces singlet oxygen. Besides, MB is converted to a colorless leucoform at low concentrations in the process of de-oxygenation. Incubation of cells with MB concurrently with chlorin e6 results in its greater fluorescence as compared to the incubation with MB only. MB concentration in the range of 1-10 mg/kg and the laser radiation dose of 60 J/cm2 do not cause cell death, probably, due to the MB transition to the photodynamically inactive leucoform. Cell death is observed after PDT in all samples with chlorin e6 and with MB at the 0-20 mg/kg concentration ranges and at 60 J/cm2 radiation dose. The phototoxicity of MB together with chlorin e6 is higher than that of chlorin e6 alone. The analysis of metabolic NADH cofactor lifetime after the incubation of the cells with MB and chlorin e6, and after PDT with them has revealed the presence of stress seen as an extension of NADH fluorescence cloud along the metabolic axis. After PDT with low concentrations of MB, the NADH fluorescent cloud on the phasor diagram shifts to the right towards short lifetimes (closer to anaerobic glycolysis along the NADH metabolic trajectory). The PDT with MB and chlorin e6 leads to the shift of the NADH fluorescence cloud on the phasor diagram to the left towards long lifetimes (closer to oxidative phosphorylation along the NADH metabolic trajectory). In this case, the cells die due to necrosis.

Conclusion

The co-accumulation of MB with chlorin e6 prevents MB reduction to a colorless leucoform, decreasing the oxygen uptake by the cells and making it possible to use simultaneously type I and II photodynamic reactions.

Encapsulation of photosensitizer worsen cell responses after photodynamic therapy protocol and polymer micelles act as biomodulators on their own

Photodynamic therapy (PDT) is a photochemical therapeutic modality used clinically for dermatological, ophthalmological and oncological applications. Pheo a was used as a model photosensitizer, either in its free form or encapsulated within poly(ethylene oxide)-block-poly(ε-caprolactone) (PEO-PCL) polymer micelles. Block copolymer micelles are water-soluble biocompatible nanocontainers with great potential for delivering hydrophobic drugs. Empty PEO-PCL micelles were also tested throughout the experiments. The goal was to conduct an in vitro investigation into human colorectal tumor HCT-116 cellular responses induced by free and encapsulated Pheo a in terms of cell architecture, plasma membrane exchanges, mitochondrial function, and metabolic disturbances. In a calibrated PDT protocol, encapsulation enhanced Pheo a penetration (flow cytometry, confocal microscopy) and cell death (Prestoblue assay), causing massive changes to cell morphology (SEM) and cytoskeleton organization (confocal), mitochondrial dysfunction and loss of integrity (TEM), rapid and massive ion fluxes across the plasma membrane (ICP-OES, ion chromatography), and metabolic alterations, including increased levels of amino acids and choline derivatives (1H NMR). The detailed investigation provides insights into the multifaceted effects of encapsulated Pheo-PDT, emphasizing the importance of considering both the photosensitizer and its delivery system in understanding therapeutic outcomes. The study also raises questions as to the broader impact of empty nanovectors per se, and encourages a more comprehensive exploration of their biological effects.

Fluorescence lifetime imaging and phasor analysis of intracellular porphyrinic photosensitizers applied with different polymeric formulations

The fluorescence lifetime of a porphyrinic photosensitizer (PS) is an important parameter to assess the aggregation state of the PS even in complex biological environments. Aggregation-induced quenching of the PS can significantly reduce the yield of singlet oxygen generation and thus its efficiency as a medical drug in photodynamic therapy (PDT) of diseased tissues. Hydrophobicity and the tendency to form aggregates pose challenges on the development of efficient PSs and often require carrier systems. A systematic study was performed to probe the impact of PS structure and encapsulation into polymeric carriers on the fluorescence lifetime in solution and in the intracellular environment. Five different porphyrinic PSs including chlorin e6 (Ce6) derivatives and tetrakis(m-hydroxyphenyl)-porphyrin and -chlorin were studied in free form and combined with polyvinylpyrrolidone (PVP) or micelles composed of triblock-copolymers or Cremophor. Following incubation of HeLa cells with these systems, fluorescence lifetime imaging combined with phasor analysis and image segmentation was applied to study the lifetime distribution in the intracellular surrounding. The data suggest that for free PSs, the structure-dependent cell uptake pathways determine their state and emission lifetimes. PS localization in the plasma membrane yielded mostly monomers with long fluorescence lifetimes whereas the endocytic pathway with subsequent lysosomal deposition adds a short-lived component for hydrophilic anionic PSs. Prolonged incubation times led to increasing contributions from short-lived components that derive from aggregates mainly localized in the cytoplasm. Encapsulation of PSs into polymeric carriers led to monomerization and mostly fluorescence emission decays with long fluorescence lifetimes in solution. However, the efficiency depended on the binding strength that was most pronounced for PVP. In the cellular environment, PVP was able to maintain monomeric long-lived species over prolonged incubation times. This was most pronounced for Ce6 derivatives with a logP value around 4.5. Micellar encapsulation led to faster release of the PSs resulting in multiple components with long and short fluorescence lifetimes. The hydrophilic hardly aggregating PS exhibited a mostly stable invariant lifetime distribution over time with both carriers. The presented data are expected to contribute to optimized PDT treatment protocols and improved PS-carrier design for preventing intracellular fluorescence quenching. In conclusion, amphiphilic and concurrent hydrophobic PSs with high membrane affinity as well as strong binding to the carrier have best prospects to maintain their photophysical properties in vivo and serve thus as efficient photodynamic diagnosis and PDT drugs.