Targeted acid-labile conjugates of norcantharidin for cancer chemotherapy

Amides with beta-carboxylic acid groups are stable and negatively charged at physiological pH, but hydrolyze back to the corresponding amines and anhydrides once in an acidic environment. We thereby developed charge-reversal nanocarriers for drug delivery. In these systems, the anhydrides served only as groups to amidize the amines in cationic polymers and had no therapeutic use after being cleaved from the carrier in the cells. Herein, we utilized the characteristic anhydride structure of norcantharidin (NCTD) and the pH-dependent hydrolysis of its beta-carboxylic amides and developed novel acid-labile conjugates for the targeted delivery of NCTD. In this study, NCTD was used as both an anticancer drug and an acid-labile anhydride to react with cationic polymers including PEI and PLL to form beta-carboxylic amides. The obtained conjugates (PEI-NCTD and PLL-NCTD) had not only significantly improved NCTD solubility and high drug loading contents (as high as 72.3% and 56.8%, respectively, for PEI-NCTD and PLL-NCTD), but also favorable acid-labile capabilities. These conjugates were stable and negatively charged at neutral pH, but once in acidic environments (e. g. endo/lysosomes), they hydrolyzed and regenerated not only NCTD as an antitumor drug, but also the PEI or PLL, which could assist the endo/lysosomal escape and release of the drugs. Further, functionalizing the acid-labile conjugates with targeting moiety folic acid (FA) increased the cellular uptake of the conjugates into folate receptor-overexpressing tumor cells and thereby the in vitro cytotoxicity. These targeted acid-labile conjugates may help to reduce the side effects of NCTD and improve its clinical applications.