Subcellular localization patterns and their relationship to photodynamic activity of pyropheophorbide-a derivatives

To determine if subcellular localization is important to photodynamic thera py (PDT) efficacy, an in vitro fluorescence microscopy study was conducted with a congeneric series of pyropheophorbide-a derivatives in human pharyng eal squamous cell carcinoma (FaDu) cells and murine radiation-induced fibro sarcoma (:RIF) mutant cells. In the FaDu cells the octyl, decyl and dodecyl ether derivatives localized to the lysosomes at extracellular concentratio ns less than needed to produce a 50% cell kill (LD50). At extracellular con centrations equal or greater than the LD50 the compounds localized mainly t o mitochondria, The propyl, pentyl, hexyl and heptyl ether derivatives loca lized mainly to the mitochondria at all concentrations studied. This sugges ted that mitochondria are a sensitive PDT target for these derivatives. Sim ilar experiments were performed with two Photofrin(R)-PDT resistant RIF cel l lines, one of which was found to be resistant to hexyl ether derivative ( C6) mediated-PDT and the other sensitive to C6-PDT relative to the parent l ine, At extracellular concentrations of C6 below the LD,, of each cell line , the mutants exhibited lysosomal localization. At concentrations above the se values the patterns shifted to a mainly mitochondrial pattern. In these cell lines mitochondrial localization also correlated with PDT sensitivity, Localization to mitochondria or lysosomes appeared to be affected by the a ggregation state of the congeners, all of which are highly aggregated in aq ueous medium, Monomers apparently were the active fraction of these compoun ds because equalizing the extracellular monomer concentrations produced equ ivalent intracellular concentrations, photoxicity and localization patterns . Compounds that were mainly aggregates localized to the lysosomes where th ey were rendered less active. Mitochondria appear to be a sensitive target for pyropheophorbide-a-mediated photodamage, and the degree of aggregation seems to be a determinant of the localization site.