Mechanism of Photodynamic Cytotoxicity with Nanosecond Pulsed Light Excitation

Reaction Dynamics and Intracellular Kinetics of Photosensitizer

Abstract

Photodynamic therapy with pulsed light excitation has interesting characteristics; however, its photosensitization mechanism has not been fully elucidated. We investigated the correlation between oxygen consumption and photobleaching during photodynamic treatment with nanosecond pulsed light excitation and a secondgeneration, lysosomal sensitizer _{in vitro}, and obtained “fluorescence-oxygen diagram” at different pulse frequencies. The diagrams showed that initial oxygen-dependent photobleaching was switched to an oxygenindependent photobleaching, which was re-switched to an oxygen-dependent reaction at the higher frequencies of 10 and 30 Hz; such changes were not observed at 5 Hz. The complex behaviors revealed by the diagrams at 10 and 30 Hz were shown to be associated with the intracellular kinetics of the sensitizer, _i.e., initial localization in lysosomes and redistributionto the cytosol during the treatment. The pulse frequencydependent intracellular kinetics of the sensitizer also explained the higher cytotoxicity at 5 Hz than at 10 and 30 Hz.