Fabrication, characterization, thermal stability and nanoassemblies of novel pullulan-aspirin conjugates

Present study deals with homogeneous and one-pot synthesis of novel macromolecular prodrugs (MPDs) of aspirin onto naturally occurring hydrophilic biopolymer pullulan. Pullulan-aspirin conjugates were synthesized by using green carboxylic acid activating reagent 1,1′-carbonyldiimidazole (CDI). The aspirin was first reacted with CDI to prepare aspirin-imidazolide at RT for 24 h which in situ reacted with pre-dissolved pullulan and the reaction preceded further for 24 h at 80 °C under nitrogen. Degree of substitution (DS 0.32-0.40) of aspirin onto pullulan was calculated from 1H NMR spectroscopy. Spectroscopic techniques confirmed the high covalent drug loading and purity. Thermal analysis has revealed that new MPDs of aspirin are thermally more stable than pure aspirin. The activation energy, order and frequency factor of the degradation reactions were calculated using Broido, Friedman and Chang models. The amphiphilic pullulan-aspirin conjugates self-assembled in nanoparticles without further structural modifications at solvent interface in the range of 500-680 nm as examined by transmission electron microscopy. These novel pullulan-aspirin conjugates with masked COOH functional group could be potentially safe prodrugs for the stomach.