Zwitterionic polymeric micelles that undergo a pH-triggered positive charge for enhanced cellular uptake

Abstract

To achieve a good stealth property and to enhance the uptake by tumor cells, polymeric micelles containing a slightly negatively charged and zwitterionic corona at pH 7.4 (i.e., blood pH) and a positively charged surface at a slightly acidic pH (i.e., tumor extracellular pH) were prepared. The amphiphilic diblock copolymer which was used to prepare the polymeric micelles contains a hydrophilic block comprised of nonionic hydrophilic groups, negatively charged groups (carboxyl groups) and pH-triggered positive charge-generation groups (morpholino groups). The zeta potential of the micelles was found to increase as the pH decreased over the pH range covering the blood and tumor tissue pH ranges, and the corona of the micelles should contain zwitterionic groups. This could be due to that, in the pH range studied, more morpholino groups become protonated as the pH decreased, whereas the carboxyl groups were almost completely deprotonated to form carboxylate anions. Furthermore, by adjusting the molar ratio of morpholino to carboxyl groups, the zeta potential of the polymeric micelles was controlled to achieve a slightly negative value at pH 7.4. Thus, the combination of a slightly negatively charged surface and a zwitterionic corona suggests that these micelles would possess good stealth property. When the pH decreased from 7.4 to 6.8 or 6.5, the zeta potential value of the micelles became positive due to increased protonation of the morpholino groups. Correspondingly, the cellular uptake of the micelles by HeLa cells was enhanced.