Photodynamic modification of disulfonated aluminium phthalocyanine fluorescence in a macrophage cell line

Abstract

Disulfonated aluminium phthalocyanine (AlS₂Pc) is used experimentally as a photosensitiser for both photodynamic therapy (PDT) and photochemical internalisation (PCI). In this study we have focused on modifications in intracellular photosensitiser localisation and fluorescence intensity in macrophages during and after photoirradiation. Since macrophages are highly abundant in tumour tissue and readily accumulate AlS₂Pc both in vivo and in vitro, we investigated PDT-induced changes of AlS₂Pc fluorescence in the murine macrophage cell line J774A.1 using CCD fluorescence imaging microscopy. The distinct intracellular localization disappeared upon red laser irradiation and was replaced by a uniform distribution accompanied by a transient fluorescence intensity increase using higher AlS₂Pc concentrations, followed by photobleaching after further irradiation. A short period of irradiation was sufficient to induce the intracellular redistribution and intensity increase, which then continued in the dark without further laser irradiation. However in the absence of oxygen no fluorescence intensity increase or redistribution was observed. This finding favours the general assumption of photodynamic destruction of organelle membranes resulting in the observed redistribution of the phthalocyanine. No other long-lived fluorescent photoproducts were observed during irradiation. Under deoxygenated conditions slower photobleaching was observed, and photobleaching quantum yields were estimated under aerated and deoxygenated conditions. The participation of reactive oxygen intermediates (ROS) generated during irradiation was indicated by intracellular oxidation of 2′,7′-dichlorodihydrofluorescein to the fluorescent 2′,7′-dichlorofluorescein in macrophages. The oxygen dependence of these photomodification processes is relevant to the application of AlS₂Pc to photochemical internalisation which relies on photosensitiser redistribution in cells upon light exposure.