The A isomer of fimaporfin, a synthetic light-activated compound, with potential photosensitizing activity. Upon administration, fimaporfin A incorporates into the cell's endosome and lysosome membranes. Subsequently, cytotoxic agents are administered and accumulate in endosomal and lysosomal compartments; upon local activation by light, fimaporfin A produces reactive oxygen species (ROS), such as singlet oxygen, damaging endo/lysosomal membranes and accumulated cytotoxic agents are released into the tumor cell cytosol. This photochemical internalization (PCI) method can enhance the efficacy and selectivity of cytotoxic agents.