Facile construction of dual-bioresponsive biodegradable micelles with superior extracellular stability and activated intracellular drug release.

It is still a major challenge for targeted cancer chemotherapy to design stable biodegradable micellar drug delivery systems which show a rapid and complete intracellular drug release. Here, reversibly core-crosslinked pH-responsive biodegradable micelles were developed based on poly(ethylene glycol)-poly(2,4,6-trimethoxybenzylidene-pentaerythritol carbonate-co-pyridyl disulfide carbonate) [PEG-P(TMBPEC-co-PDSC)] copolymers and investigated for intracellular doxorubicin (DOX) release. PEG-P(TMBPEC-co-PDSC) copolymers formed micelles with a small size of 58.6nm were readily crosslinked by the addition of dithiothreitol (DTT). Notably, in vitro release studies showed that under physiological conditions only ca. 19.9% of DOX was released from the reversibly crosslinked micelles in 24h at a low micelle concentration of 40μg/mL. The release of DOX was accelerated at pH5.0 or in the presence of 10mM glutathione (GSH) at pH7.4, in which 64.2% and 44.1% of DOX was released, respectively, in 24h. The drug release was further boosted at pH5.0 and 10mM GSH, with 98.8% of DOX released in 12h. Moreover, DOX release was also facilitated by a 4h incubation at pH5.0 followed by incubation at pH7.4 with 10mM GSH. Confocal microscopy indicated that DOX was delivered and released into the nuclei of RAW 264.7 cells following a 12h incubation with DOX-loaded reversibly crosslinked micelles. MTT assays revealed that DOX-loaded reversibly crosslinked micelles had much higher antitumor activity than irreversibly crosslinked controls, with low IC50 values of 1.65 and 1.14μg/mL for HeLa and RAW 264.7 cells, respectively, following a 48h incubation. The blank crosslinked micelles had a low cytotoxicity of up to a concentration of 0.8mg/mL. These reversibly crosslinked pH-sensitive biodegradable micelles with superior extracellular stability but activated intracellular drug release provide a novel platform for tumor-targeting drug delivery.