Subcellular Localization Patterns and Their Relationship to Photodynamic Activity of Pyropheophorbide‐ a Derivatives

To determine if subcellular localization is important to photodynamic therapy (PDT) efficacy, an in vitro fluorescence microscopy study was conducted with a congeneric series of pyropheophorbide‐ a derivatives in human pharyngeal squamous cell carcinoma (FaDu) cells and murine radiation‐induced fibrosarcoma (RIF) mutant cells. In the FaDu cells the octyl, decyl and dodecyl ether derivatives localized to the lysosomes at extracellular concentrations less than needed to produce a 50% cell kill (LD 50 ). At extracellular concentrations equal or greater than the LD 50 the compounds localized mainly to mitochondria. The propyl, pentyl, hexyl and heptyl ether derivatives localized mainly to the mitochondria at all concentrations studied. This suggested that mitochondria are a sensitive PDT target for these derivatives. Similar experiments were performed with two Photofrin®‐PDT resistant RIF cell 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 line. At extracellular concentrations of C6 below the LD 50 of each cell line, the mutants exhibited lysosomal localization. At concentrations above these 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 aggregation state of the congeners, all of which are highly aggregated in aqueous medium. Monomers apparently were the active fraction of these compounds because equalizing the extracellular monomer concentrations produced equivalent intracellular concentrations, photoxicity and localization patterns. Compounds that were mainly aggregates localized to the lysosomes where they were rendered less active. Mitochondria appear to be a sensitive target for pyro‐pheophorbide‐a‐mediated photodamage, and the degree of aggregation seems to be a determinant of the localization site.