a novel cell - targeting , ph - sensitive polymeric carrier was employed in this study for delivery of the anticancer drug bortezomib ( btz ) to cancer cells . 
 our strategy is based on facile conjugation of btz to catechol - containing polymeric carriers that are designed to be taken up selectively by cancer cells through cell surface receptor - mediated mechanisms . 
 the polymer used as a building block in this study was poly(ethylene glycol ) , which was chosen for its ability to reduce nonspecific interactions with proteins and cells . 
 the catechol moiety was exploited for its ability to bind and release borate - containing therapeutics such as btz in a ph - dependent manner . in acidic environments , such as in cancer tissue or the subcellular endosome , btz dissociates from the polymer - bound catechol groups to liberate the free drug , which inhibits proteasome function . 
 a cancer - cell - targeting ligand , biotin , was presented on the polymer carriers to facilitate targeted entry of drug - loaded polymer carriers into cancer cells . 
 our study demonstrated that the cancer - targeting drug  polymer conjugates dramatically enhanced cellular uptake , proteasome inhibition , and cytotoxicity toward breast carcinoma cells in comparison with nontargeting drug  polymer conjugates . 
 the ph - sensitive catechol  boronate binding mechanism provides a chemoselective approach for controlling the release of btz in targeted cancer cells , establishing a concept that may be applied in the future toward other boronic acid - containing therapeutics to treat a broad range of diseases .