PHOTODEGRADATION OF PROTOPORPHYRIN-DIMETHYLESTER IN SOLUTION AND IN ORGANIZED ENVIRONMENTS

The degradation of sensitizers used in photodynamic therapy (PDT) invo lves photooxidation either by molecular oxygen or by oxygen intermedia tes which leads to hydroxyaldehyde and formyl products or to ring open ing. Our investigations focused on the spectroscopic changes which pro toporphyrin-dimethylester (PP) exhibits upon irradiation. As the micro environment strongly influences the effects, we used an aprotic organi c solvent, L-alpha-phosphatidylcholine dioleoyl (DOPC) liposomes and i sogenic fibrosarcoma cells (SSKII) as carriers for PP. Hydroxyaldehyde product isomers develop a new absorption band centred around 670 nm a nd a new emission band at 676 nm. These characteristics can be used to discriminate them from formyl products and intact PP. In organic solv ents, the formation of the hydroxyaldehyde products dominates. In DOPC liposomes and cells, the hydroxyaldehyde yield drops and photooxidati on results in attack of the macrocycle. Time-resolved fluorescence spe ctroscopy of monomeric PP in an organic solvent gives a monoexponentia l decay time tau of 10.1) +/- 1.3 ns. Upon irradiation a second compon ent with a decay time of 4.9 +/- 0.6 ns, resulting from the hydroxy-al dehyde product, was detected. In liposomes and cells the monomeric dec ay time was significantly longer (15 ns) due to the altered microenvir onment. Additionally, we observed in liposomes and in cells a small co ntribution of a short component (1 ns) which is attributed to an aggre gated sensitizer species. In irradiated cells the aggregated fraction doubles, indicating a change in the microenvironment caused by the pho todynamic action of the sensitizer.