Photodynamic therapy (PDT) is rapidly becoming an accepted therapeutic modality for the treatment of some types of malignant tumors. An important feature of PDT is its absolute dependence on molecular oxygen during light irradiation.Hypoxic tumor cells, either pre-existing or photochemically depleted of their oxygen supply during light irradiation, are resistant to PDT treatment, and contribute to treatment failures. We hypothesize that tumor response to PDT can be improved by combining PDT with Hyper-oxygenation, which may simultaneously compensate for the oxygen depletion by PDT and increase oxygenation of the pre-existing hypoxic cells. The doubling time of mammary carcinoma tumors, implanted in either leg or flank of C3H mice, was evaluated after PDT treatment with/without addition of hyperoxygenation. By adding hyperoxygenation to a non- curative PDT dose, a further delay in the tumor regrowth was observed. For a sub-curative does PDT treatment, the addition of hyperoxygenation resulted in an increase in tumor cure. The results indicate that tumor response can be improved by combining PDT and hyperoxygenation.
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