Abstract: Objective To investigate the in vitro and in vivo effects of 5-aminolevulinic acid (5-ALA)-mediated photodynamic therapy against oral squamous cell carcinoma (OSCC) and preliminarily explore the possible mechanisms.Methods SCC25 cells were divided into the control group (5-ALA of 0 mg/L) and the experimental group (5-ALA of 10,25,50,100 and 150 mg/L).The production ofprotoporphyrin Ⅸ (PpⅨ) induced by 5-ALA in SCC25 cells was detected using the flow cytometry.SCC25 ceils were divided into the control group (5-ALA of 0 mg/L),lazer alone group,5-ALA alone group (5-ALA of 100 mg/L) and the 5-ALA combined with laser irradiation group (5-ALA of 5,10,25,50 and 100 mg/L),the cytotoxicity of 5-ALA combined with laser irradiation (wave length 635 nm,power density 87 mW/cm2 and laser dose 10.4 J/cm2) was evaluated in SCC25 cells using the methyl thiazolyltetrazolium assay (incubation times of 4,8 and 12 h in each group) and the induction effect of combination treatment on the cell apoptosis was assessed by the flow cytometry (incubation time of 12 h in each group).The intracellular production of reactive oxygen species (ROS) triggered by 5-ALA combined with laser irradiation was determined using a fluorescence probe method (incubation time of 12 h in each group).A mouse OSCC xenograft model bearing SCC25 tumor was built,and the mice were divided into control group (saline),5-ALA group (5-ALA of 50 mg/kg)and 5-ALA combined with laser irradiation group (5-ALA of 10,25 and 50 mg/kg).Antitumor effect of 5-ALA combined with laser irradiation (wave length 635 nm,power density 158 mW/cm2 and laser dose 94.8 J/cm2) was further measured.Results 5-ALA induced the production of PpⅨ in SCC25 cells in a drug concentration (0-150 mg/L)-and incubation time (0-24 h)-dependent manner.When the 5-ALA concentration was 100 mg/L,the intracellular PpⅨ production was in a relatively stable state.Cell viability and apoptosis rate of 5,10,25,50,100 mg/L 5-ALA combined with laser irradiation are,respectively,(82.3±5.2)%,(3.13±0.38)%;(74.6±9.3)%,(5.38±0.55)%;(38.3±9.7)%,(17.97±2.72)%;(9.2±3.8)%,(24.47±3.37)%;(7.2±0.8)%,(43.01±5.96)%,which indicated that 5-ALA combined with laser irradiation notably inhibited the growth of SCC25 cells and also induced significant cell apoptosis compared with the control group [(96.3± 6.0)%,(0.35±0.13)%,P<0.05].After combination treatment (5-ALA of 5,10,25,50 and 100 mg/L combined with laser irradiation,the mean fluorescence intensity of dichlorofluorescein is (1.46±0.12)× 104,(2.16±0.30)× 104,(3.57±0.34)× 104,(81.70± 13.05)× 104,(113.00±7.35)× 104,respectively,a large amount of ROS was produced in SCC25 cells compared with the control group [(0.96± 0.15) × 104,P<0.05],which was in positive correlation with the intracellular PpⅨ content.5-ALA (concentration of 10,25 and 50 mg/kg) combined with laser irradiation greatly suppressed the tumor growth in SCC25 tumor-bearing mice compared to the control group (P<0.05).Conclusions 5-ALA-mediated photodynamic therapy can trigger the generation of intracellular ROS that has significant cytotoxicity and apoptosis induction effect,and thus inhibit the tumor growth both in vitro and in vivo.