Confocal spectroscopy allows investigation of Photofrin bleaching kinetics during photodynamic therapy

We examine the bleaching kinetics of the photosensitizer Photofrin® using spectrally-resolved fluorescence measurements in optical sections of multicell tumor spheroids during photodynamic therapy. The results suggest that Photofrin photodegrades via two simultaneous bleaching mechanisms, and their relative importance is dependent on the initial Photofrin concentration.