Photophysical behaviour and photodynamic activity of zinc phthalocyanines associated to liposomes

Abstract

Phthalocyanines are macrocyclic compounds that can be employed as photosensitizers in the treatment of various infections and diseases, as well as in photodynamic therapy. Nevertheless, a disadvantage for the clinical application of these compounds is their strong tendency to form oligomers (especially dimers), a phenomenon that reduces their efficiency as photosensitizers. In the present contribution, we have studied the photophysical and photochemical properties of ZnPc and ZnF₁₆Pc in an organic solvent (THF) and liposomal formulations (DMPC, DPPC and DSPC). Our results show that dye incorporation into liposomes decreases its aggregation degree, as revealed by absorption spectra, triplet quantum yield, and singlet oxygen quantum yield measurements. Additionally, we studied the photodynamic activity of both phthalocyanines in liposomal formulation on human cervical carcinoma (HeLa) cells. For ZnF₁₆Pc the photophysical behavior and phototoxicity in vitro correlate with the aggregation degree. The dimers are not photoactive and the photochemistry of ZnF₁₆Pc depends of the fraction present as monomer. On the other hand, ZnPc aggregation is minimal and its photophysical and photochemical properties are similar in the three liposomes studied. Nevertheless, its phototoxicity in vitro is liposome dependent.