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In this study, we investigated the synergisticeffect of l-buthionine sulfoximine (BSO) on the chlorin e6(Ce6)-based photodynamic therapy (PDT) of cancer cells. Among various cancer cells, HCT116 cells have highestintracellular L-glutathione (GSH) level and SNU478 cellsshowed the lowest GSH level. BSO alone showed negligibleintrinsic cytotoxicity against CCD986sk cells. SinceHCT116 and SNU478 cells showed the highest and thelowest intracellular GSH levels, respectively, those wereused to test synergistic effect on the Ce6-based PDT. In theabsence of light, BSO and Ce6 combination did not practicallyincrease reactive oxygen species (ROS) in either ofHCT116 or SNU478 cells, while light irradiation increasedROS level dose-dependently. 10 μM BSO treatment significantlydepleted total GSH level in cancer cells, i.e. total GSHlevel decreased to one-fourth of the control in HCT116 cells while it decreased to two-fifth of the control treatment atSNU478 cell. BSO showed synergistic effect on the ROSproduction in HCT116 cells while it has practically no benefitsin ROS production of SNU478 cells. No synergisticeffect was observed in viability of SNU478 cells becauseBSO itself was cytotoxic to SNU478 cells. However, BSOhad negligible cytotoxicity against HCT116 cells andshowed synergistic anticancer effect in combination withCe6-based PDT. Furthermore, the addition of glutathionereduced ethyl ester (GSH-OEt), recovered intracellular GSHlevel, and cell viability with reduced the intracellular ROSlevel. We suggest that synergistic effect of BSO in the Ce6-based PDT should be considered with intrinsic intracellularGSH level of cancer cells.