COMMUNICATION pubs.acs.org/JACS Catechol Polymers for pH-Responsive, Targeted Drug Delivery to Cancer Cells Jing Su, †,^ Feng Chen,#,r Vincent L. Cryns,||,^,#,r and Phillip B. Messersmith*,†,‡,§,||,^,# †Biomedical Engineering Department, ‡Materials Science and Engineering Department, §Chemical and Biological Engineering Department, and C)hemistry of Life Processes Institute, Northwestern University, Evanston, Illinois 60208, United States ^Institute for Bionanotechnology in Medicine, #Robert H. Lurie Comprehensive Cancer Center, and rCell Death Regulation Lab, Feinberg School of Medicine, Northwestern University, Chicago, Illinois 60611, United States bS Supporting Information ABSTRACT: A novel cell-targeting, pH-sensitive poly- meric 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 environ- ments, 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 drama- tically 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 chemo- selective 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. argeted drug delivery systems aim to improve the eﬃcacy Tand reduce the toxicity of potent therapeutics in treatments for a variety of diseases. For example, chemotherapy is one of the major systemic treatments for cancer, but it is linked to serious side eﬀects because of its toxicity to normal proliferating cells. Cancer-targeting delivery systems have been developed to alter the biodistribution of drugs, aiming to achieve drug accumulation in cancer tissue through the enhanced permeability and reten- tion (EPR) eﬀect and/or targeting of the cancer cell surface.1 Advanced delivery systems for anticancer drugs often utilize envi- ronmentally triggered mechanisms.2 4 For example, the extra- cellular pH in tumor tissue is slightly lower than that in normal tissue, and this has been exploited to accomplish pH-triggered drug release in tumor tissue.5 In addition, the signiﬁcantly increased acidity in subcellular compartments such as the endosome also oﬀers a complementary route to increases in the eﬃcacy of anticancer drug delivery via pH-initiated release of drugs from endocytosed drug carriers.5,6 Here we describe a novel polymer conjugate of the anticancer drug bortezomib (BTZ) that can be used for pH-sensitive delivery to speciﬁc cancer cells. BTZ is a dipeptide boronic acid analogue that inhibits cancer cell proteasome through direct binding between its boronic acid group and threonine residues in the active sites of several proteases.7,8 This drug is currently marketed as Velcade (Millenium Pharmaceuticals) and has been approved for multiple myeloma and mantle cell lymphoma treatment; however, BTZ is less active against many solid tumors and has signiﬁcant dose-limiting toxicity.9,10 In addition to the possibility that diﬀerent cancer cells may have distinct protea- some sensitivity to BTZ,11,12 the unfavorable pharmacokinetic properties of BTZ, including nonspeciﬁc binding to proteins and rapid hepatic clearance from blood, may also contribute to its limited eﬃcacy against solid tumors.13,14 This suggests that polymeric delivery vehicles capable of enhancing the pharmaco- kinetic properties of BTZ may improve its activity against solid tumors. Moreover, strategies to direct BTZ to cancer cells by speciﬁc cell surface recognition epitopes and/or trigger pH- regulated drug release may facilitate drug delivery and release in tumors, thereby potentially increasing the eﬃcacy and redu- cing the toxicity of BTZ. It has been previously reported that polyphenols can chemi- cally block the proteasome-inhibiting activity of BTZ through conjugation to the boronic acid active site and that the formed conjugates cannot permeate freely through the cell membrane.15 18 Our design takes advantage of the facile conjugation of BTZ to the 1,2-benzenediol (catechol) moiety to form a membrane- impermeable compound in order to reduce nonselective cellular uptake of the drug. An important characteristic of the boronic acid catechol conjugate is that it is formed through dynamic covalent chemistry that is reversible in a pH-sensitive manner:19 at neutral or alkaline pH, BTZ and catechol form a stable boronate ester, which deactivates the cytotoxicity of BTZ; in a low-pH environment, the BTZ catechol ester conjugate readily dissociates to release free BTZ and catechol groups. This mechanism has the potential to be exploited in two contexts: (1) extracellularly for localized drug release within the mildly acidic tumor interstitium and (2) intracellularly in the more Received: April 4, 2011 Published: July 13, 2011 r 2011 American Chemical Society 11850 dx.doi.org/10.1021/ja203077x | J. Am. Chem. Soc. 2011, 133, 11850–11853