Development of an Iridium Complex as an Antimicrobial Photodynamic Therapy

Objectives: Photodynamic therapy (PDT) is a process where, upon illumination, a photsensitiser becomes toxic to either cells or bacteria often through the generation of reactive oxygen free radicals. Such technology has been used clinically for the treatment of periodontal disease but with unproven effectiveness. This may be due to the ability of periodontal pathogens such as Porphyromonas gingivalis to avoid antibacterial agents and host defences by invading host cells.
We hypothesise that by developing new photosensitisers which can localise to and kill bacteria within host cells that this may improve the efficiency of antibacterial PDT.
Methods: H357 oral epithelial cells were incubated with a novel mononuclear iridium (III) complex for 2 hours followed by illumination with a 405nm laser at 20mW/cm² for 3 min. Treated control samples were kept in the dark. Cell viability was then assessed using an MTT assay. Toxicity against P. gingivalis in suspension and 5 day biofilms was determined using a similar protocol followed by viable plating. Confocal structured illumination and electron microscopy was also used to visualise the localisation of the photosensitiser. Finally, an antibiotic protection assay was used to determine the effect of the PDT on intracellular organisms.
Results: Oral epithelial cells exhibited >90% viability following exposure to photosensitiser concentrations up to 1mM and laser light. PDT resulted in >99% killing of both planktonic and intracellular P.gingivalis (2nM) although biofilms were less sensitive (60% reduction). Microscopy indicated that the sensitiser passes through both host cell and bacterial membranes, the latter being damaged by the PDT.
Conclusions: We have shown that a novel iridium complex can be used as part of an antimicrobial PDT strategy to effectively target P. gingivalis including those present within host cells. Such information is of importance in the development of new clinical treatments for periodontal disease.