Antimicrobial photodynamic therapy by means of porphycene photosensitizers.

Antimicrobial photodynamic therapy (APDT) is one of the promising tools for bacteria inactivation, which can be considered as an alternative to the common ways of treatment in the era of growing resistance to antibiotics. Presently applied phototherapeutic agents are often based on porphyrins. Porphycenes, isomers of porphyrin, exhibit even better spectral and photophysical properties regarding PDT and have therefore been proposed as photosensitizers in such applications as anticancer and antimicrobial PDT. We compare three different porphycenes in the study of photodestruction of commonly occurring bacteria: Enteroccocus faecalis, Staphylococcus aureus and Staphylococcus epidermidis. Special interest is drawn to the parent, unsubstituted porphycene, a compound which was not tested before in terms of its photosensitizing activity in the biological systems. The results show that two out of three investigated compounds, the parent porphycene and its quadruply substituted derivative, 2,7,12,17-tetrakis(β-methoxyethyl) exhibit very good ability of bacteria eradication and fulfill the criteria that are commonly required from the APDT photosensitizers. In contrast, 2,7,12,17-tetra-t-butylporphycene, of which the spectral and photophysical characteristics are very similar to those of the parent compound, is not photoactive. This is explained by its inability to penetrate into the bacteria cell. These results demonstrate extreme sensitivity of the photodestruction efficiency to minor structural variations in the photosensitizer.