Photobleaching of photosensitizers applied for photodynamic therapy

It is well known that photosensitizers used for PDT are liable to photobleaching. This phenomenon should be taken into account when developing the appropriate tactics of treatment. The present paper deals with detailed investigation of photobleaching of two photosensitizers: ALA induced protoporphyrin IX (PPIX) and sulphonated aluminum phthalocyanines. The fluorescence decay curves during light irradiation in vivo and in vitro have been obtained and analyzed. The mathematical model taking into account both first and second order photobleaching as well as spatial inhomogeneity of light distribution in tissue is presented. The experimental data for fluorescence decay of ALA induced PPIX during light irradiation are fitted to this mode. As opposed to PPIX the photobleaching behavior for sulphonated aluminum phthalocyanines is rather complicated to be fitted by proposed mathematical mode. The fluorescence outburning and residual fluorescence for sulphonated aluminum phthalocyanines have been observe.d It has been shown that fluorescence maximum correlates with blood oxygen saturation decrease induced by PDT effect.

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