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RESEARCH PRODUCT

DNA Block Copolymer Micelles – A Combinatorial Tool for Cancer Nanotechnology

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Selective drug targeting of a specific organ or tissue is a challenging task. This holds especially true for chemotherapeutic cancer treatment because most of the available anticancer agents cannot distinguish between cancerous and healthy cells, leading to systemic toxicity and undesirable side effects. One effective approach to address this problem is the application of polymeric nanoparticles equipped with targeting units for tumor-specific delivery. For instance dendrimers, highly branched macromolecules, can be equipped with targeting units as well as with anticancer drugs because of their large number of surface functionalities. Amphiphilic block copolymers, which self-assemble in dilute aqueous solutions into three-dimensional spherical micelles with a hydrophilic corona and a hydrophobic core, are another attractive option. These nanosized objects, with a typical size of 10– 100 nm, are able to accommodate lipophilic drugs in their interior and alter their kinetics in vitro and in vivo. Different polymeric systems such as shell crosslinked nanoparticles (SCKs), poly(D,L-lactic-co-glycolic acid)-b-poly(ethylene glycol), poly(ethylene glycol-b-e-caprolactone) block copolymers and poly(N-isopropylacrylamide acrylic acid) microgels have also been successfully utilized in combination with targeting units. Folate receptors (FRs), which are highly expressed on the surface of various cancer cells, emerged as new targets for specific localization of chemotherapeutics incorporated into nanoparticle systems. The family of FRs currently consists of three known isoforms: FRa, FRb and FRc. FRa is expressed primarily in cancer cells such as ovarian, testicular, breast, colon, renal and malignant nasopharyngeal carcinomas. The process that mediates targeting of the folate-linked nanoparticle to the receptor and subsequent internalization is identical to that for the free folate. As reviewed by Reddy et al., folates, after binding to their receptors, are taken up by the cells via the receptor-mediated endocytic pathway. Recently, a new type of amphiphilic block copolymer has emerged that is composed of a hydrophobic synthetic polymer component and a biological segment consisting of an oligodeoxynucleotide (ODN) sequence. Micelles composed of these materials exhibit a corona of single-stranded (ss) DNA and have been utilized for the delivery of antisense ODNs, for the hybridization with DNA-coated gold nanoparticles and as programmable, three-dimensional (3D) scaffolds for DNA-templated organic reactions. Here, we introduce DNA block copolymer micelles as a highly modular system for chemotherapeutic drug delivery. ODN-modified targeting units were “clicked” into the micelle corona by hybridization, allowing perfect control of surface functionalities of the nanoparticle system. The interior of the micelles was loaded efficiently with a hydrophobic anticancer drug. Cell culture experiments revealed that cellular uptake strongly depends on the density of targeting units on the surface of the carriers. As a result, cancer cells were efficiently killed when targeting units and chemotherapeutic acted together within the DNA block copolymer drug delivery system (Fig. 1). Polypropylene oxide (PPO) was selected as the hydrophobic component of the DNA block copolymer to provide a polymer with proven biocompatibility toward different cell types when administered as a constituent component of amphiphilic block copolymer micelles. For the generation of the DNA-b-PPO copolymer, a phosphoramidite-functionalized PPO (Mn=6800 g mol ) was synthesized and attached to the 5’ terminus of the nucleic acid fragment (5’-CCTCGCTCTGCTAATCCTGTTA-3’, 22mer, Mw= 6700 g mol) via automated solid phase synthesis as reported previously. The resulting block copolymer was analyzed and purified by denaturing polyacrylamide gel electrophoresis (PAGE) and the molecular weight was confirmed by matrixassisted laser desorption ionization time-of-flight (MALDITOF) mass spectrometry (Supporting Information, SI). Dynamic Light Scattering (DLS) measurements of the DNA C O M M U N IC A IO N