OBJECTIVE To investigate the in vitro lethal effect of photodynamic therapy (PDT) using the photosensitizer hematoporphyrin on the human pancreatic cancer cell line Panc-1, the major influencing factors and the mechanisms of treatment. METHODS Three factors-the time needed for photosensitizer and cell incubation, the photosensitizer concentration (PhoC) and the exposure dose (ExpD)-were examined with different levels of these factors. Optical density (OD) was used as a measure of CCK-8 in the experiment, and was converted to the rate of cell survival. The separate effect of each factor on the photodynamic action was studied, and the interactions were investigated. The effects of different incubation times and PhoC levels on the fluorescence intensity (FI) of the intracellular photosensitizer were determined,and the mechanisms of these factors leading to the therapeutic effects of PDT discussed.RESULTS An increase in the photosensitizer and cell incubation time, an increase of PhoC, and enhancement of the ExpD, produced a corresponding decrease in the rate of Panc-1 cell survival after PDT (P , 0.05). PDT achieved its maximum lethal effects 16 h after starting the incubation, with a PhoC of 10 mg/L and an ExpD of 20 J/cm2; at these levels a synergistic interaction between PhoC and the ExpD occurred, decreasing the cell survival rate (P , 0.05). Neither simple administration of photosensitizer without ExpD (0 J/cm2) or illumination in the absence of PhoC (0 mg/L) affected the rate of cell survival (P.0.05). With an increase of PhoC and lengthening of the incubation time, the FI of the intracellular photosensitizer accordingly increased (P.0.05), and attained its maximum value at a PhoC of 10 mg/L and 36 h after the incubation. With an increase of PhoC,the FI of the photosensitizer, hematoporphyrin, in the solution increased progressively at first and then decreased (fluorescence quenching).CONCLUSION PDT with the photosensitizer hematoporphyrin has clear lethal effects on the human pancreatic cancer cell line Panc-1,but the presence of a photosensitizer and laser irradiation by themselves do not have independent lethal effects. The three influencing factors-the time for photosensitizer and cell incubation,PhoC and ExpD-correlate positively with the PDT response,within certain limits. Beyond these limits, the PDT response does not significantly increase. The main mechanism of the PDT response lies in the effect of these factors on the level of the intracellular photosensitizer and the fluorescence quenching of the photosensitizer.A synergistic effect exists between PhoC and ExpD.