Novel pH-responsive nanovectors for controlled release of ionisable drugs

A family of novel 'smart' drug nanovectors based on 2-(methacryloyloxy)ethyl-3-chloro-4-hydroxybenzoate (MCH), a functional pH-responsive monomer, and poly(methoxy-ethylene glycol) methacrylate (PEGMA) block copolymers is presented. Modification of the monomers' relative ratios allowed facile switching from micellar (PEGMA₁₁-b-MCH₂₁) to polymersome-based (PEGMA₁₁-b-MCH₃₈) drug delivery nanocarriers. The ability of the latter to incorporate model anticancer drugs - tamoxifen, paclitaxel and doxorubicin hydrochloride - was investigated. High drug loading - up to 18 w/w% - was observed for tamoxifen, a hydrophobic drug which bears an amino group able to form ion pairs with MCH acidic functionalities. Non-loaded nanovectors were characterized (CAC, DLS and TEM), and were found to be very stable under physiological conditions (PBS pH 7.4, 37 °C), even in the presence of 10% plasma proteins for at least 48 h. Tamoxifen loaded nanocarriers showed slow drug release at pH 7.4 and faster release after exposure to weakly acidic environments, due to the loss of polymer/drug ionic interactions. Importantly, at pH 7.4 tamoxifen-loaded PEGMA₁₁-b-MCH₂₁ micelles were found to be less cytotoxic than free tamoxifen against MCF-7 cells, while under more acidic conditions, at pH 6.8, the opposite behaviour was observed, with a 10-fold increase in cytotoxicity for the micellar nanocarriers. The empty nanocarriers were found to be non-toxic in 48 h incubation time experiments. Pharmacokinetic studies proved the increased half-life and the slower clearance of tamoxifen after encapsulation in the micelles. These PEGMA-b-MCH based nanoassemblies could represent a novel promising delivery platform for low molecular weight ionisable drugs.