Effect of the delivery system on the antimicrobial photosensitising activity of a cationic meso-substituted porphyrin

A monocationic meso-substituted porphyrin, namely 5-[4-(1-dodecanoylpyridinium)]-10,15,20- triphenyl-porphine (TDPyP), appears to be endowed with favourable properties to act as a photodynamic agent, based on the determination of its quantum yield for intersystem crossing to the lowest excited triplet state and singlet oxygen generation. The porphyrin appears to be fairly active in photosensitising the inactivation of two typical bacterial pathogens, such as the Gram- positive meticillin-resistant Staphylococcus Aureus (MRSA) and the Gram-negative Escherichia coli. This observation is in agreement with the well known photocydal effect of positively charged porphyrin derivatives against a variety of microbial pathogens owing to the fast ionic binding between the array of negatively charged groups at the external surface of the bacterial cells and the cationic moieties in the porphyrin molecule. In actual fact, the photosensitising activity of TDPyP against both MRSA and E. coli is markedly enhanced by its incorporation into lipid-type carriers with multiple positive charges, such as small unilamellar liposomes [1] or nanoaggregates of amphiphilic cyclodextrins (SC6NH2) . In particular, a 6 log decrease in the bacterial cells survival is achieved upon relatively short irradiation times in the presence of SC6NH2/TDPyP complexes with porphyrin concentrations as low as 1 or 5 μM. Effective population of the lowest triplet state for the system SC6NH2/TDPyP has been demonstrated by laser flash photolysis studies and unambiguous evidences for the singlet oxygen photosensitization have been obtained by direct infrared luminescence time-resolved experiments. Transmission electron microscopy studies show that the potentiating effect of the cyclodextrin carrier is a consequence of its ability to promote an efficient crossing of the very tightly organized three-dimensional architecture of the bacterial outer wall by the embedded porphyrin, so that a prompt interaction between the short- lived photogenerated oxygen derivatives and the nearby targets, whose integrity is critical for cell survival, can take place.