Thermo- and pH-responsive copolymers based on PLGA-PEG-PLGA and poly(l-histidine): Synthesis and in vitro characterization of copolymer micelles

A series of novel thermo- and pH-responsive block copolymers of PHis-PLGA-PEG-PLGA-PHis composed of poly(ethylene glycol) (PEG), poly(d,l-lactide-co-glycolide) (PLGA) and poly(l-histidine) (PHis) were synthesized and used for the construction of stimuli-responsive copolymer micelles. The starting polymers of PLGA-PEG-PLGA and PHis were synthesized by ring-opening polymerization of dl-lactide and glycolide with PEG as an initiator and l-histidine N-carboxylanhydride with isopropylamine as an initiator, respectively. The final copolymer was obtained by the coupling reaction of PHis with PLGA-PEG-PLGA. The copolymer micelles were constructed to have an inner core consisting of two hydrophobic blocks (PLGA and deprotonated PHis) and an outer hydrophilic PEG shell. The temperature- and pH-induced structure changes of the micelles were characterized by an alteration in particle size, a decrease in pyrene florescence intensity, and a variation of 1H NMR spectra in D2O. It was speculated that the hydrophobic–hydrophilic transitions of PEG and PHis in response to temperature and pH variations accounted for the destabilization of micelles. In vitro release profiles, cell cytotoxicity and intracellular location studies further confirmed the temperature- and pH-responsive properties of the copolymer micelles. These results demonstrate the potential of the developed copolymers to be stimuli-responsive carriers for targeted delivery of anti-cancer drugs.