Photophysical and photobiological properties of a sulfonated chlorin photosensitiser TPCS2a for photochemical internalisation (PCI)

This study investigated the photophysical and photobiological properties of a new amphiphilic chlorin photosensitiser, disulfonated tetraphenylchlorin (TPCS2a), for photochemical internalisation (PCI). The absorption and fluorescence spectra of TPCS2a were examined in a range of solvents together with fluorescence lifetime measurements. The fluorescence lifetime of TPCS2a was found to be 8.5 ns in methanol, whereas non-exponential decays were observed in distilled water due to sensitiser dimerisation. The singlet oxygen quantum yield of TPCS2a was determined as 0.62 in deuterated methanol by direct observation of singlet oxygen phosphorescence. In a human oral squamous carcinoma (HN5) cell line, intracellular co-localisation of TPCS2a and Alexa488-labelled saporin, a macromolecular toxin, was observed corresponding predominantly to a lysosomal distribution. Intracellular fluorescence redistribution of TPCS2a and Alexa488-saporin was observed after 405 nm irradiation. Using two-photon confocal microscopy at 840 nm, and fluorescence lifetime imaging (FLIM), the lifetime was measured as 6 ns in HN5 cells. PCI using TPCS2a was shown to be very effective, and a synergistic increase in saporin toxicity was achieved in HN5 cells where viability was significantly reduced after light exposure compared to saporin (25 nM) treatment alone. The results demonstrate the favourable photophysical and photobiological properties of TPCS2a for PCI, which induces the relocalisation of a macromolecular anti-cancer toxin inside cells and significantly enhances cell death.