pH-triggered degradable polymeric micelles for targeted anti-tumor drug delivery

2-(Octadecyloxy)-1,3-dioxan-5-amine (OD) with an acid degradable ortho ester group was synthesized, and conjugated to hyaluronic acid (HA) backbone to prepare pH-responsive and tumor-targeted hyaluronic acid-2-(octadecyloxy)-1,3-dioxan-5-amine (HOD) conjugates. ¹H NMR was used to confirm the structures of the OD and HOD. The studies of pH-responsive behavior showed that HOD micelles were stable under physiological conditions while they were degraded in the tumor acidic microenvironment. Doxorubicin (DOX)-loaded HOD micelles (DOX/HOD) with a narrow size distribution were prepared and characterized. The increased release of DOX from DOX/HOD micelles was presented at low pH condition. From in vitro cytotoxicity assays against MCF-7 cells, the blank micelles exhibited low cytotoxicity, but DOX/HOD micelles had the higher cytotoxicity than pH insensitive control and free DOX. Cellular uptake experiments and confocal images demonstrated that pH-sensitive DOX/HOD micelles could be internalized efficiently by CD44 receptor mediated endocytosis, and then DOX was rapidly released due to pH-induced degradable of OD to cell nucleus compared to the non-sensitive micelles. Furthermore, endocytosis inhibition studies presented that DOX/HOD micelles were internalized into cells mainly via caveolae-mediated routes. In vivo study of micelles in tumor-bearing mice indicates that HOD micelles were more effectively accumulated into the tumor tissue than HOA micelles. These results verify that the pH-sensitive HOD micellar system is a promising nanocarrier for enhanced internalization of antitumor drugs to cancer cells.