PORPHYRIN-TYPE PHOTOSENSITISERS FOR KILLING BACTERIA AND CANCER CELLS WITH PHOTODYNAMIC THERAPY

The photosensitising properties displayed by many porphyrin-type molecules have been widely exploited for therapeutic purposes in the so called photodynamic therapy (PDT). In PDT, cells and tissues are first loaded with a photosensitizing drug which is then activated by illumination with suitable wavelengths of visible light to generate reactive oxygen species, mainly singlet oxygen, that kill cells through oxidative damages. Both prokaryotic and eukaryotic cells can be killed by PDT by selecting photosensitizers with physico-chemical properties favoring interaction/internalization with the particular cell type. We have been studying porphyrins and some derivatives to explore their potential usefulness for photodynamic therapy of tumours as well as for antimicrobial photodynamic therapy. In the last years we are exploring strategies that are able to improve efficiency and selectivity of PDT in killing cancer cells as well as bacteria. In the attempt of increasing the efficiency in killing bacteria, porphyrins have been conjugated to cationic antimicrobial peptides that are known to exhibit selectivity against bacterial cells. When compared to the corresponding free porphyrins, the conjugates showed very different outcomes, in terms of efficiency of bacteria photokilling, that were determined by the type of bacteria (Gram-positive or Gram-negative), the hydro/lipo-philicity and/or electric charge of the porphyrin that affect the strength of interaction of the conjugates with the bacterial cells. Our results showed that an anionic and hydrophobic porphyrin, devoid of photoactivity against the Gram-negative E. coli, acquired the ability to efficiently kill this bacterium when conjugated to the peptides. The effect of conjugation to cationic antimicrobial peptides of a tri-cationic porphyrin was less obvious very likely because, as other positively charged PS, this porphyrin exhibited by itself some photoactivity against both Gram-positive and –negative bacteria, which is preserved or even reduced by the conjugates [1]. In spite of the selectivity displayed by antimicrobial cationic peptides against bacterial cells, the porphyrin-peptide conjugates resulted very toxic by photoactivation also against eukaryotic cells. We found that normal human fibroblasts were completely killed by illumination with blue light after incubation with 5 and 3 µM of the free anionic or cation porphyrin respectively. The same effect was found with the porpyrin-peptides conjugates at concentrations 2-3 orders of magnitude lower than the free porphyrins. Therefore it appears that cationic antimicrobial peptides may represent useful carriers for improving internalization of drugs in mammalian cells, as already found for cell penetrating peptides. Based on this we are exploring the potential of the peptides to improve the delivery of PSs also to cancer cells. [1] Dosselli R., Tampieri C., Ruiz-Gonzalez R., De Munari S., Ragàs X., Sánchez-García D., Agut M., Nonell S., Reddi E., Gobbo M. J. .Med. Chem. 2013, 56, 1052.