INCREASED EFFICACY OF IN-VITRO PHOTOFRIN(R) PHOTOSENSITIZATION OF HUMAN ORAL SQUAMOUS-CELL CARCINOMA BY IRON AND ASCORBATE

Photofrin(R), a photosensitizer used in the photodynamic therapy of ca ncer, selectively localizes in cellular membranes, Upon exposure to vi sible light, Photofrin(R) produces singlet oxygen (O-1(2)) which react s with membrane polyunsaturated fatty acids forming lipid hydroperoxid es. Transition metals, such as Fe2+, catalyze the production of cytoto xic free radicals from lipid hydroperoxides. Ascorbate reduces ferric to ferrous iron, further augmenting lipid peroxidation. Therefore, to increase the efficacy of Photofrin(R) photosensitization, we added 20 mu M ferrous sulfate and 100 mu M ascorbic acid, in an aqueous layer o ver SCC-25 oral squamous cell carcinoma cells during in vitro illumina tion. In electron paramagnetic resonance spin trapping experiments, us ing POBN (alpha-(4-pyridyl-1-oxide)-N-tert-butylnitrone), we observed that the presence of this pro-oxidant combination greatly increases th e production of membrane-derived lipid free radicals. The effect was t ime dependent but only partially concentration dependent. Trypan blue dye exclusion demonstrated that this increase in lipid radical formati on correlated with cytotoxicity. These observations support the hypoth esis that Photofrin(R) photosensitization leads to lipid hydroperoxide formation, which increases the cell's susceptibility to iron-induced Fenton chemistry. The resulting free radical-mediated Lipid peroxidati on results in cell death. From these data we hypothesize that the effi cacy of photodynamic therapy of superficial cancer might be increased by the topical application of the pro-oxidant combination of iron and ascorbate. Furthermore, their use will probably allow lower doses of P hotofrin(R) without compromising antitumor effect. (C) 1997 Elsevier S cience S.A.