The Effect of Charge on Cellular Uptake and Phototoxicity of Polylysine Chlorin_e6 Conjugates

The effect of charge on the cellular uptake, localization and phototoxicity of conjugates between chlorin_e6 (c_e6) and poly-L-lysine was studied in vitro. These conjugates (average MW 35-55 kDa) were synthesized to have polycationic, polyanionic or neutral charges. Two human cell lines (A431 epidermoid carcinoma cells and EA.hy926 hybrid endothelial cells) were studied and the cellular uptake of c_e6 delivered by the conjugates of varying charge and free c_e6 was measured at conjugate c_e6 equivalent concentrations up to 0.4 μM. Uptake was time and concentration dependent and temperature dependent in the case of neutral and anionic conjugates. Relative uptake at 6 h for A431 cells was 73:15:4:1 and for EA.hy926 cells was 63:11:3:1 for cationic, anionic, neutral and free c_e6, respectively, but EA.hy926 cells took up 1.5-2 times as much c_e6 from all the conjugates as A431 cells. Localization as studied by fluorescence microscopy indicated that the cationic conjugate was in aggregates bound to the plasma membrane, while the other forms were internalized in organelles and membranes. Phototoxicity was measured by 3-(4,5-dimethylthiazol-2-yl)-diphenyltetrazolium bromide (MTT) assay after irradiation with 5-20 J cm^-2 of 666 nm light. In contrast to the uptake, the order of phototoxicity for both cell types per mole of c_e6 uptake per cell was neutral ≫ anionic > cationic > free c_e6. Polymeric c_e6 conjugates bearing positive, negative and neutral charges may have different tissue-localizing properties and could play a role in photodynamic therapy.