RESISTANCE TO PHOTODYNAMIC THERAPY IN RADIATION INDUCED FIBROSARCOMA‐1 and CHINESE HAMSTER OVARY‐MULTI‐DRUG RESISTANT CELLS in vitro

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Abstract
A degree of resistance to photodynamic therapy (PDT) has been induced in radiation‐induced fibrosarcoma‐1 (RIF‐1) tumor cells by repeated photodynamic treatment with Photofrin (4 or 18 h incubation) in vitro to the 0.1‐1% survival level, followed by regrowth from single surviving colonies. The resistance is shown as increased cell survival in the strain designated RIF‐8A, compared to the wild‐type RIF‐1 cells, when exposed to increasing Photofrin concentration for 18 h incubation and fixed light exposure. No difference was found between RIF‐1 and RIF‐8A in the uptake of Photofrin per unit cell volume at 18 h incubation. Resistance to PDT was also observed in Chinese hamster ovary‐multi‐drug resistant (CHO‐MDR) cells compared to the wild‐type CHO cells, possibly associated with decreased cellular concentration of Photofrin in the former. By contrast, the PDT‐resistant RIF‐8A cells did not show any cross‐resistance to Adriamycin, nor was there any significant drug concentration difference between RIF‐1 and RIF‐8A. These findings suggest that different mechanisms are responsible for PDT‐induced resistance and multi‐drug resistance.