pH-Responsive Self-assembled Polymer-Photosensitizer Conjugate for Activable Photodynamic Therapy

This paper reports synthesis, aqueous self-assembly and relevance in the pH-triggered activable photodynamic therapy of an amphiphilic polyurethane (P1S), functionalized with a heavy-atom free organic photosensitizer. Condensation polymerization between 1,4-diisocyanatobutane and two different dihydroxy monomers (one having pendant hydrophilic wedge and the other having 1,3-dihydroxypropan-2-one with a reactive carbonyl-group) in presence of measured amount of (S)-2-methylbutan-1-ol (chain-stopper) and DABCO catalyst produces the reactive pre-polymer P1. Hydrazide- functionalized thionated-naphthalenemonimide (NMIS), that acts as a photosensitizer, was reacted with the carbonyl-functionality of P1 to get the desired polymer-photosensitizer conjugate P1S in which the dye was attached to the polymer backbone by the acid-labile hydrazone linker. In water, P1S adopted intra-chain H-bonding stabilized folded structure, which further assembled to produce polymersome structure (Dh ~ 200 nm), in which the hydrophobic membrane consists of aggregated NMIS and trialkoxy-benzene chromophores, as evident from the UV/Vis, CD and small-angle X-ray diffraction studies. In the aggregated state, the NMIS loses its reactive oxygen species (ROS)-generation ability and remains in the dormant state. However, in acidic condition (pH 5.5), the photosensitizer is detached (presumably due to the cleavage of the hydrazone-linker) and regains its full ROS-generation activity under photoirradiation, as evidenced from the standard DCFH assay. To test the possibility of such pH-activable intra-cellular ROS generation, P1S was treated with HeLa cells as it is known that cancer cells are more acidic than normal cells. Indeed, photoirradiation induced intra-cellular ROS generation was evident by the DCFH assay, resulting in efficient cell killing.