Novel approaches towards a detailed control of the mechanism and efficiency of photosensitized processes in vivo

The photoexcitation of an endogenous or exogenous biological chromophore can result in the re-emission of light as fluorescence or phosphorescence; this may be used for photodiagnostic applications or in the conversion of at least part of the absorbed radiation into thermal or chemical energy. The last two processes generate photothermal and photochemical sensitization respectively leading to the irreversible damage of cells and tissues; hence they can be used for phototherapeutic purposes. The efficacy and scope of photodiagnostic and phototherapeutic techniques are improving as we increase our knowledge about the influence of biological microenvironments on the mechanisms of photoprocesses and as new approaches are developed for controlling the biodistribution of photosensitizing agents in vivo and the depth/diffusion of incident light into tissues. In particular, even the localized and sublethal damage of cells can often be modulated to generate intracellular signalling in order to either induce a programmed cell death (apoptosis) or an acceleration of cell metabolism (photostimulation).