wellplates with approximately 8 000 cells per well and incubated in a humid 5 co 2 incubator at 37 u00b0 c after 18 24 h incubation the medium was removed and the cells were washed with 100 u03bcl of serum free medium cells were then treated with the pei solutions for 16 h the serum free media was removed and replaced with 100 u03bcl of fresh media with 20 u03bcl of the mts reagent solution the cells were incubated for 2 h at 37 u00b0 c in a 5 co 2 incubator cell viability was assessed by measuring the absorbance at 490 nm using a plate reader spectramax m5 and expressed as the percentage of viable cells treated pei normalized to viable control cells in the absence of pei according to the results shown in fig2 the ic50 of pei 25d was 30 u03bcl ml in the a549 cell lines while the ic50 of pei 800 d was never reached even at concentrations more than 69 times higher gt 2750 u03bcg ml the concentrations of pei 800d used in our study were much lower 48 84 u03bcg ml and thus are much lower than the ic50 indicating their lack of toxicity.