ABSTRACT
Lipson and Baldes reported in 1960 (1) that a derivative of hematoporphyrin (HpD) was preferentially concentrated by malignant tissue to a greater degree than pure hematoporphyrin (2). This derivative serves as a cancer marker by producing a characteristic red fluorescence when excited by light of appropriate wavelength (3-10). Furthermore, it absorbs light and uses the photon energy in a series of chemical reactions that eventuate in the death of the cell. Cytotoxic agents are generated by the transfer of energy from the excited porphyrin (sensitizer) to oxygen. The major cytotoxic agent is believed to be singlet oxygen, an excited oxygen species (11,12). Superoxide and the hydroxyl radical are also produced but are believed to play only minor roles (13,14).
