Characterization of time-domain fluorescence properties of typical photosensitizers for photodynamic therapy

We report the investigation of fluorescence lifetime of delta-aminolevulinic acid (ALA) induced protoporphyrin IX (PpIX) and Photofrin® in vitro in MAT-LyLu (MLL) rat prostate adenocarcinoma cells. Photodyramic therapy (PDT) has been extensively investigated in the past decade as an effective treatment option for various types of invasive tumors. The efficacy of PDT treatment depends strongly on cell uptake and subsequent excitation of the photosensitizers. Characterization of fluorescence lifetime of these drugs provides the basis for further investigation of in vivo PDT dosage measurements using time-domain spectroscopy and imaging. Physiologically relevant concentrations of the photosensitizer solutions were prepared. A picosecond diode laser was used to excite the two drugs and the time-resolved fluorescence decay was recorded using a time-correlated single photon counting (TCSPC) system. MLL cells were incubated with the photosensitizers and were treated with light under well-oxygenated or hypoxic conditions. Fluorescence lifetime images of these cells were recorded by a confocal FLIM microscope. The measured fluorescence lifetimes of both photosensitizers are much longer than typical endogenous tissue fluorescence, which suggests time-domain methods are good candidates for in vivo PDT monitoring.