Photodynamic therapy (PDT) has been used clinically for the treatment of malignant brain gliomas. However, the efficacy of this treatment to date has remained equivocal. This study focused on determining the sensitivity of 9L gliosarcoma in Fischer 344 rats to PDT with increasing doses of 632 nm light and making a comparison of the responses of normal and tumor tissue in the brain at these doses. This sensitivity was then correlated with the concentrations of Photofrin® present in these tissues at the time of treatment. Our study indicates that the level of Photofrin® in the tumor was 13 times that present in normal brain 48 h after injection. However, this selective localization of the photosensitizer was not reflected in a selective tissue response to PDT. There was minimal tumor response to a dose of$35\ {\rm J}\ {\rm cm}^{-2}$, which has been reported previously to cause necrosis to the normal brain. Increasing energy dose levels resulted in an increased tumor response to PDT; however, normal tissue remained more sensitive than tumor tissue at all energy dose levels examined. These data indicate that, although Photofrin® is retained to a significantly higher degree in the tumor than in the normal brain tissue, the normal brain is more sensitive than the tumor to PDT under the conditions outlined in this study.

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