Swollen Micelles for the Preparation of Gated, Squeezable, pH-Responsive Drug Carriers

Natural variations in pH levels of tissues in the body make it an attractive stimuli to trigger drug release from a delivery vehicle. A number of such carriers have been developed but achieving high drug loading combined with low leakage at physiological pH and tunable controlled release at the site of action is an ongoing challenge. Here we report a novel strategy for the synthesis of entirely hydrophilic stimuli-responsive nanocarriers with high passive loading efficiency of doxorubicin (DOX), which show good stability at pH 7 and rapid tunable drug release at intracellular pH. The particles (D_h = 120-150 nm), are prepared by cross-linking the core of swollen micelles of the triblock copolymer poly[poly(ethylene glycol) methyl ether methacrylate-b-N,N'-di(methylamino)ethyl methacrylate-b-tert-butyl methacrylate] (poly(PEGMEM A)-b- PDMAEMA-b-PtBMA)). After subsequent deprotection of the tert-butyl groups a hydrophilic poly(methacrylic acid) (PMAA) core is revealed. Due to the negative charge in the acidic core the particles absorb 100% of the DOX from solution at pH 7 at up to 50 wt % DOX/polymer, making them extremely simple to load. Unlike other systems, the DMAEMA "gating" shell ensures low drug leakage at pH 7, whereas physical shrinkage of the MAA core allows rapid release below pH 6. The particles deliver DOX with high efficiency to human pancreatic cancer AsPC-1 cell lines, even lowering the IC50 of DOX. As the particles are stable as a dry powder and can be loaded with any mixture of positively charged drugs without complex synthetic or purification steps, we propose they will find use in a range of delivery applications.