Fabrication of biodegradable micelles with reduction-triggered release of 6-mercaptopurine profile based on disulfide-linked graft copolymer conjugate

This research is aimed to develop a biodegradable micelle delivery system with sheddable poly (ethylene glycol) shell to achieve the reduction-triggered intracellular sustained release of 6-mercaptopurine (6-MP) and decreased toxicity. Firstly, the amino-disulfide linked poly (ethylene glycol) monomethyl ether (mPEG-SS-NH₂) was synthesized by the amidation reaction between cystamine and active ester of mPEG and p-nitrophenyl chloroformate (p-NPC) (mPEG-NPC). And then, the five-member rings in poly (l-succinimide) (PSI) were successively opened by mPEG-SS-NH₂ and 2-(pyridyldithio)-ethylamine (PDA) to produce the graft copolymer of mPEG-SS-NH-graft-PAsp-PDA. To avoid the drug initial burst, 6-MP was covalently conjugated with mPEG-SS-NH-graft-PAsp-PDA by thoil-disulfide exchange reaction to give the resultant product mPEG-SS-NH-graft-PAsp-MP. The product was found to form spherical micelles in aqueous media because of its amphiphilic nature with average particle size of 160nm measured by dynamic light scattering (DLS). It was found that the mPEG-SS-NH-graft-PAsp-MP micelles, though stable in phosphate buffer solution (PBS), were prone to aggregation in the presence of dithiothreitol (DTT). The in vitro drug release studies revealed the release of 6-MP were distinct from the conventional micelles whose drugs loaded by physical encapsulation. Sustained release profile of 6-MP over 85h was found in the presence of DTT (40mM) simulating the intracellular condition while minimal drug release was observed within 24h at the level of DTT corresponding to extracellular environment. Remarkably, the cell viability results showed there was essential decrease of cytotoxicity to HL-60 cell line compared to free 6-MP.