Degradation of poly(glycoamidoamine) DNA delivery vehicles: Polyamide hydrolysis at physiological conditions promotes DNA release

Poly(glycoamidoamine)s (PGAAs) are a group of efficient and degradable gene delivery vehicles that consist of three main functionalities: carbohydrate groups, secondary amines, and amide bonds. Herein, we have created nonhydroxylated models to these structures by polymerizing oxylate, succinate, or adipate groups with pentaethylenehexamine. The resulting polymers (named O4, S4, and A4, respectively) were created to understand how the absence of hydroxyl groups and changes in the amide bond spacing affect polymer degradation, plasmid DNA (pDNA) complexation, toxicity, and transfection efficiency in vitro. An additional model was also created that retains a galactaramide unit, but we have replaced the secondary amines with ethyleneoxide units (GO2) to understand the effects of the amine groups on polymer degradation. We have found that the secondary amines and hydroxyls are necessary to facilitate rapid degradation of these polymers, and analogues lacking hydroxyls or amines did not degrade over the time course of the study. Through electron-withdrawing and hydrogen bonding, the hydroxyls appear to activate the carbonyls of the amide bond to hydrolysis via an inductive electron withdrawing effect. Through titration experiments, PGAA degradation appears not to affect the polymer buffering capacity. Furthermore, we have found that PGAA degradation may enhance gene expression by releasing pDNA from polyplexes (polymer-pDNA complexes) and, thus, exposing it to undergo transcription and translation. The difference in the optimal pH that promotes degradation of the PGAAs and the hydroxylfree analogues may prove to be a useful means to achieve pH-regulated DNA release from polyplexes by specifically modulating the chemical structures.