Degree of substitution of chlorin e6 on charged poly-L-lysine chains affects their cellular uptake, localization and phototoxicity towards macrophages and cancer cells.

Macromolecular photosensitizer conjugates are under investigation as improved delivery vehicles for dyes used in photodynamic therapy. We have previously described the use of conjugates between photosensitizers such as chlorine6 (ce6) and poly-L-lysine (pL) chains which are versatile molecular species because the size of the chain can be varied, and the overall charge can be altered from cationic through neutral to anionic. We now report on a series of pL-ce6 conjugates in their cationic (native), neutral (acetylated) and anionic (succinylated) forms, where the number of ce6 molecules attached to each chain was varied (pL: ce6 ratios, 1:4, 1:8, 1:12, and 1:16). The fluorescence emissions were measured in both saline and a disaggregating solvent. We studied two cell lines (an epithelial ovarian cancer, OVCAR-5 and a mouse macrophage, J774) and measured cellular uptake, subcellular localization (by confocal fluorescence microscopy) and phototoxicity. The cellular uptake of the conjugates with four substitution ratios all delivered at 2 μM ce6 equivalent concentration showed a maximum at 12 ce6 per chain for both cationic and anionic conjugates, but the uptake of the neutral conjugate was proportional to the substitution ratio. The macrophages took up several times more ce6 than the ovarian cancer cells. Confocal fluorescence micrographs showed more cellular fluorescence with the lower substitution ratios, and more lysosomal localization with the cationic conjugates. The phototoxicity was much higher for the neutral conjugates. For the cationic and neutral conjugates the 12 ce6 per chain was the most effective at killing cells, while for the anionic conjugate it was the 16 ce6 per chain. The anionic conjugate was better at killing OVCAR-5 cells, while the cationic was better for J774 cells, and the neutral was approximately the same. These data will help to optimize the parameters to be used in preparing polymeric-photosensitizer conjugates for photodynamic therapy.