A spectroscopic study of photobleaching of Chlorin p6 in different environments

The photobleaching process of Chlorin p6 a photosensitizer used for photodynamic therapy was studied in neat phosphate buffer, phosphatidyl choline liposomes and in 10% fetal bovine serum. The full width half maxima (FWHM) of the soret band of Chlorin p6 increases in liposomes when compared to that in buffer and in 10% serum. This suggests that it may exist in aggregated form when bound to the lipid bilayer of the liposomes. The observed changes in the absorption spectra upon photoillumination are indicative of photomodification and/or photoproduct formation. A new nonfluorescent band was observed at 730 nm after photoirradiation. This band appears to arise due to aggregated species formed on photoirradiation. The rate of photobleaching of Chlorin p6 fluorescence in neat buffer and liposomes were comparable, but it was three times faster in the serum media. Those observations have been attributed to, negligible photobleaching rate of the aggregated species in liposomes and reduced diffusional motion of the drug when bound to serum proteins. Addition of histidine in serum medium reduces the photobleaching rate considerably while addition of mannitol has no effect on the rate. This indicates that the photobleaching of Chlorin p6 in these environments may proceed via a type II mechanism.