Fluorescence photobleaching dynamics of meso tetra hydroxy phenyl chlorin (mTHPC)

The experimental results described elaborate a micro-spectroscopic technique to record the laser-induced fluorescence emission of the photodynamic therapy (PDT) photosensitizer meso tetra hydroxy phenyl chlorin (mTHPC – Foscan) from micron-scale locations within individual keratinocytes. The technique provided the ability to analyze mTHPC fluorescence dynamics in single fixed cells. The system has been utilized to monitor mTHPC fluorescence accumulation and mTHPC photobleaching dynamics during continuous laser exposure at different excitation powers. These parameters have been examined in normal keratinocytes (representing normal skin cells). The rate of photobleaching, when plotted as a function of light dose follows an inverse-dose-rate dependence. Specifically, the bleaching rate at the highest incident laser powers applied is limited by oxygen availability, which significantly increases the (1/ e ) bleaching dose. No photoproduct emission was evident across the spectral region investigated. These results are in qualitative agreement with previous studies which demonstrate that the photodynamic dose delivered during light exposure is critically dependent upon local fluence-rate and oxygen partial pressure.