Characteristics of photosensitization of Pheophorbide a in liposomal media

Pheophorbide a (PPa), a decomposition product of chlorophyll a, is a photosensitizer. The photosensitization mechanisms (Type I and Type 11) of PPa in simple buffer solutions and in buffer solutions containing double-layered DPPC liposomes have been studied using techniques of ESR, spin-trapping, spin-counteraction and laser flash photolysis. The results showed that adding DPPC liposomes to the buffer solution caused an increase of efficiency of generating1O2 and PPa-by photoactivating PPa. The increase could be ascribed to the disaggregation of hydrophobic PPa caused by the addition of liposomes and the protective effect of liposomal media on the triplet state of PPa. It is concluded that the photosensitization of PPa in liposomal systems is different from that in simple aqueous solutions, and shows higher efficacy. The results will be useful to elucidating the mechanisms of photodynamic therapy of cancer. Project supported by the National Natural Science Foundation of China (Grant No. 39830090).