A spectroscopic study of the photobleaching of protoporphyrin IX in solution

Photodynamic therapy (PDT) has developed into an important new clinical treatment for cancer during the past 30 years. The method is non-invasive and based on the photochemical activity of a photosensitising agent present in cells and tissues. In so-called ALA-PDT, protoporphyrin IX (Pp IX) is induced from aminolaevulinic acid (ALA) applied topically or systemically. It has been shown that Pp IX is photodegraded by a photo-oxidation process and that its photoproducts have a characteristic absorption band around 670 nm, as observed both in solution and in cells incubated with ALA. In this study, the involvement of oxygen in the photobleaching process was verified by studying the effect of oxygen depletion using the freeze-pump-thaw (FPT) method. A solution of Pp IX in dimethylformamide (DMF) was exposed to light in the wavelength region 600-700 nm (peak centred at 620 (+/-25) nm) both in the presence and in the absence of oxygen. The bleaching process was observed by absorbance and fluorescence measurements. Photobleaching was observed in the presence of oxygen, as verified by the build-up of a photoproduct absorbing at 670 nm. When the sample was deoxygenated with the FPT method, the photoproduct absorption peak at 670 nm was missing. These results confirm that the formation of photo-protopor-phyrin is a photo-oxidation process and that no photobleaching takes place in the absence of oxygen. When comparing our results to the studies carried out by N-2 bubbling, the N-2 bubbling seems to be insufficient to remove the oxygen completely from the solution.