Antimicrobial photodynamic therapy: photophysical and phototoxic activities of conjugates of apidaecin with different photosensitizers

Antimicrobial PhotoDynamic Therapy (APDT) represents a very promising strategy, particularly for the treatment of localized infectious diseases.[1] PDT involves the use of a non-toxic photosensitizer (PS) that forms reactive oxygen species (ROS) when irradiated with visible light of appropriate wavelength. ROS (e. g. 1O2) can cause irreversible damages to a variety of cellular components, resulting in cytotoxicity. Advantages of APDT over traditional antibiotics include a broad spectrum activity, also against antibiotic-resistant species and the lack of development of resistance mechanisms due to the multi-target process. Whereas Gram-positive bacteria can be efficiently killed by light after incubation with a number of PS, Gram-negative bacteria are less susceptible to photodynamic killing. Only cationic PS can bind efficiently to Gram-negative bacteria and induce their photoinactivation. Recently we have shown that the conjugation of a cationic antimicrobial peptide (apidaecin 1b ) to a neutral porphyrin afforded a new antibacterial agent with a broader spectrum activity than two individual components or a mixture of them.[2] Here we present the results obtained with two new PS-apidaecin conjugates in which either a porphycene (G), a structural porphyrin isomer, or a cationic porphyrin (Y) are covalently linked to the peptide N-terminal end. By a combination of spectroscopic and time resolved photophysical techniques, we have monitored the ability of conjugates to produce the main cytotoxic species (1O2) in different solvents and in E. coli (the prototypical Gram-negative bacterium), obtaining information on the possible localization of the PS in the cell. The photokilling activity of the conjugates was markedly concentration-dependent, and the most potent one, Y-apidaecin, caused complete killing of bacteria at 10 μM (E. coli) and 1.5 μM (S. aureus) concentrations. References 1. Hamblin, M. R.; Hasan, T. Photochem. Photobiol. Sci. 2004, 3, 436-450 2. Dosselli, R.; Gobbo, M.; Bolognini, E.; Campestrini, S.; Reddi E. ACS Med. Chem. Lett. 2010, 35-38.