HEMATOPORPHYRIN AS A PHOTOSENSITIZER OF TUMORS*

Abstract— The ability of hematoporphyrin (Hp) to act as a photosensitizer of cells in vitro or in vivo is a matter of dispute, while hematoporphyrin derivative (Hpd), a mixture of porphyrins including hematoporphyrin, has been consistently found to be an effective photosensitizer both in vitro and in vivo. Until recently the actual component of the Hpd mixture responsible for these effects had not been identified. We have found that those preparations of Hp which contain, as an impurity, a porphyrin similar to that found to be responsible for the tumor photosensitizing ability of Hpd, may be effective photosensitizers of tumors but are generally of low efficacy. This material accounts for the entire photosensitizing activity of both Hp and Hpd in the SMT‐F mammary carcinoma in DBA/2 HeHa mice.