Objective To increase the killing effect of aminolevulinic acid (ALA)-based photodynamic therapy (PDT) on a human skin squamous cell carcinoma cell line A431 by poly lactic-co-glycolic acid nanoparticles (PLGA NPs).Methods ALA-loaded PLGA NPs (ALA PLGA NPs) were prepared using a modified double emulsion solvent evaporation method,and characterized in terms of size,encapsulation efficiency,loading capacity and morphology.Transmission electron microscopy was carried out to observe the morphology of A431 cells after uptake of ALA PLGA NPs.To optimize incubation time,multi-mode microplate reader was used to describe the fluorescence kinetics of protoporphyrin Ⅸ generated by A431 cells during 24 hours of incubation with 0.1 mmol/L ALA,1 mmol/L ALA,2.7 g/L ALA PLGA NPs containing about 0.1 mmol/L ALA,and PLGA NPs without ALA separately.Some A431 cells were divided into 10 groups:control group receiving neither treatment nor irradiation,0.1 and 1 mmol/L ALA dark/PDT group incubated 0.1 and 1 mmol/L ALA respectively,ALA PLGA NP dark/PDT group incubated with ALA PLGA NPs of 2.7 g/L,PLGA NP dark/PDT group incubated with PLGA NPs of 2.7 g/L,simple irradiation group irradiated with a He-Ne laser (wavelength:635 nm,power density:8.6 mW/cm2,energy density:8 J/cm2) only.The dark groups were kept in darkness strictly by wrapping in aluminum foil,and PDT groups were irradiated using a He-Ne laser.After another 24 hours of culture following irradiation,methyl thiazolyl tetrazolium (MTF) assay was conducted to estimate the survival rate of cells.To study the effect of ALA PLGA NPs on cell apoptosis,some A431 cells were divided into three groups:control group receiving neither treatment nor irradiation,ALA-PDT group and ALA PLGA NP PDT group receiving PDT after incubation with ALA and ALA PLGA NPs respectively.Flow cytometry was performed to detect the apoptosis of A431 cells at 12 and 24 hours,separately,after the photodynamic therapy.Data were statistically analyzed using SPSS 13.0 software package by means of a t test.Results The prepared ALA PLGA NPs were spherical with a mean particle size of (65.6 ± 26) nm,encapsulation efficiency of (65.8 ± 7.2) ％,and drug loading capacity of (0.62 ± 0.27)％.ALA PLGA NPs could be uptaken by A431 cells and gathered in the cytoplasm.The PpIX fluorescence kinetic study showed that the fluorescence intensity increased with time within 24 hours in A431 cells incubated with ALA or ALA PLGA NPs.After 6 and 24 hours of incubation,the A431 cells incubated with 2.7 g/L ALA PLGA NPs showed a significant increase in the protoporphyrin Ⅸ fluorescence intensity compared with those incubated with 0.1 mmol/L ALA (both P ＜ 0.01).Further more,the survival rate of A431 cells was statistically lower in the ALA PLGA NP PDT group than in the 0.1 mmol/L ALA PDT group at 6 and 24 hours (t =35.685,5.262,respectively,both P ＜ 0.01).Elevated apoptosis rate was observed in the ALA PLGA NP PDT group compared with the ALA PDT group at 12 ((13.10 ± 0.50)％ vs.(4.90 ± 0.13)％,t =9.074,P＜ 0.01) and 24 ((30.17 ± 1.02)％ vs.(11.6 ± 0.59)％,t =9.095,P ＜ 0.01) hours.Conclusions ALA PLGA NPs can promote the formation of protoporphyrin Ⅸ,strengthen the killing effect of ALA-PDT on A431 cells in vitro,and enhance the apoptosis induced by ALA-PDT in tumor cells.