The relative sensitivities of different cancer histologies in a single site to photodynamic therapy (PDT) are unknown and methods to predict PDT sensitivity have not been described. The in vitro response to PDT of six established human lung cancer cell lines and one normal lung fibroblast cell line was studied using the clonogenic assay. Dose-response curves were determined for cells incubated in 25 µg/ml of Photofrin II for 2 h, followed by exposure to 630-nm light to total energies of 0-3150 J/m2. None of the cell lines were sensitive to sensitizer alone or light alone. Differences in inherent PDT sensitivities as evaluated by survival curve parameters n, Do, and light dose to yield 1% survival were observed among the cell lines. No clear correlation was found when inherent PDT sensitivity was compared with sensitizer uptake; however, a general association was noted between PDT sensitivity and the plating efficiency of the cell line. These data illustrate that differences in inherent PDT exist for in vitro systems. Such differences may explain some of the failures seen in clinical PDT.

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