Journal of the American Chemical Society cells, while both cell types were equally sensitive to free BTZ (Figure 5C). This result suggests that cancer cells are more susceptible to BPC BTZ toxicity than noncancer cells, a major potential therapeutic advantage of our targeted polymeric carrier platform. This work has demonstrated targeted delivery of an anticancer drug to cancer cells using catechol-presenting polymers. These polymeric drug carriers have a chemically deﬁned mechanism for drug loading and release through the pH-sensitive cate- chol boronic acid interaction. With the use of biotin to target cancer cells, the proteasome inhibitor bortezomib delivered by such polymeric carriers showed enhanced cytotoxicity against breast cancer cells compared to noncancer cells in vitro. Variation of the catechol polymer architecture and composition and the choice of boronic acid-containing therapeutic can potentially provide great tailorability of catechol polymer boronic acid com- plexes for a variety of drug delivery applications. For instance, alternative targeting ligands can be exploited to alter the tissue biodistribution, and modiﬁcations of the chemical structure of the catechol (or, more generally, diol groups) can be exploited to ﬁne-tune the pH sensitivity of the diol boronic acid complex for drug release in diﬀerent tissue environments.24,25 Since the boronic acid structure exisits in many potent therapeutics,26 28 this pH-sensitive strategy may provide a versatile, chemoselective approach for targeted drug delivery to diseased tissues resulting from a broad spectrum of disorders. ’ ASSOCIATED CONTENT bS Supporting Information. Synthesis of catechol-presenting polymers, preparation of BTZ polymer conjugates, NMR study, drug release measurement, and cytotoxicity assays. This material is available free of charge via the Internet at http://pubs. acs.org. ’ AUTHOR INFORMATION Corresponding Author philm@northwestern.edu ’ ACKNOWLEDGMENT This work was supported by NIH Grant DE014193, the NCI Center of Cancer Nanotechnology Excellence at Northwestern University, and the Breast Cancer Research Foundation. We are grateful to Dr. Jinsong Wu of the Electron Probe Instrumenta- tion Center at Northwestern University for help with cryo-TEM analysis. ’ REFERENCES (1) Kwon, G. S.; Kataoka, K. Adv. Drug Delivery Rev. 1995, 16, 295–309. (2) Duncan, R. Nat. Rev. Cancer 2006, 6, 688–701. (3) Rapoport, N. Prog. Polym. Sci. 2007, 32, 962–990. (4) MacEwan, S. R.; Callahan, D. J.; Chilkoti, A. Nanomedicine (London) 2010, 5, 793–806. (5) Lee, E. S.; Gao, Z.; Bae, Y. H. J. Controlled Release 2008, 132, 164–170. (6) Torchilin, V. 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