Smart “Turn‐on” Magnetic Resonance Contrast Agents Based on Aptamer‐Functionalized Superparamagnetic Iron Oxide Nanoparticles

Aptamers are single-stranded DNA or RNA molecules that can bind a variety of chemical and biological molecules with high affinity and selectivity. They are isolated from a large random pool of DNA or RNA molecules by using a combinatorial-biology technique called systematic evolution of ligands by exponential enrichment (SELEX). 2] They are often comparable to antibodies in their selective and sensitive binding to a broad range of molecules. The major advantage of these molecules over antibodies lies in the relative ease with which they can be selected for any target analyte and their stability against biodegradation and denaturation. Due to these properties aptamers are good candidates for making chemical and biological sensors in many fields, such as medical diagnostics and environmental monitoring. Therefore, aptamers have been converted into fluorescent, colorimetric, and electrochemical sensors. While aptamer sensors have been wide ACHTUNGTRENNUNGly explored in vitro, their application in vivo, particularly in huACHTUNGTRENNUNGmans, remains a significant challenge because of the difficulty in light penetration through the skin and signal interference from cellular components. Magnetic resonance imaging (MRI) is a powerful method for noninvasive three-dimensional imaging of cells and human bodies. One active area of research in this rapidly advancing field is the development of novel MRI contrast agents, particularly smart agents that can produce a contrast in response to small molecules or biomolecular markers in cells or human bodies. Such smart MRI contrast agents could dramatically change the way we study cellular function and could help to achieve early diagnosis for diseases. Toward this goal, organic receptors and proteins have been coupled to MRI contrast agents, such as gadolinium and superparamagnetic iron oxide nanoparticles. These contrast agents result in a change of MRI contrast in the presence of molecules to which these organic receptors or proteins bind. To develop a general strategy for obtaining smart MRI agents for any molecule of choice, we report a method for the production of smart contrast agents by combining aptamer technology with superparamagnetic iron oxide nanoparticles—specifically a method based on adenosine DNA aptamer-functionalized superparamagnetic iron oxide nanoparticles—and show that MRI contrast can be dramatically enhanced in the presence of adenosine in human serum. In addition, this MRI enhancement was highly selective for adenosine as the presence of all other nucleobases did not result in enhancement effect. We chose superparamagnetic iron oxide nanoparticles as the contrast agent to be functionalized by aptamers since they are efficient at dephasing the spins of neighboring water protons; this leads to a change in the spin–spin relaxation time (T2). Weissleder and co-workers have shown that oligonucleotide-functionalized, cross-linked dextran-coated superparamagnetic iron oxide nanoparticles (CLIOs) can form clusters when linked with a complementary sequence; each nanoparticle can on average be functionalized with three oligonucleotides. The magnetic properties of dispersed nanoparticles are significantly different from those in clusters, and can be ACHTUNGTRENNUNGdetected by MRI. As compared to disperse nanoparticles, the clusters are more effective in decreasing the T2 relaxation time, and therefore give rise to a darker T2-weighted MR image. To expand the method to aptamer-based sensing of molecules other than nucleic acids, we combined CLIOs with adenosine aptamer (Scheme 1). The adenosine sensor consisted of CLIO aggregates that were prepared by using three components: CLIO functionalized with either 3’ or 5’ thiol-modified DNA (3’Adap–CLIO and 5’Adap–CLIO, respectively) and a linker DNA (linker–Adap) that could hybridize to both 3’and 5’Adap–CLIOs; this led to the formation of clusters. One segment of the linker was the sequence for the adenosine aptamer (Scheme 1, in green). Seven bases of this aptamer were involved in hybridization with 5’Adap–CLIO. In the presence of adenosine the aptamer undergoes structure switching in order to form the adenosine binding pocket ; 44,45] this results in disruption of base-pairing interactions with 5’Adap–CLIO. The five remaining base pairs between linker–Adap and 5’Adap– CLIO are not enough to hold the clusters together at room temperature; this leads to the disassembly of the clusters. The dispersed nanoparticles result in a higher T2 as compared to the clusters ; thus the adenosine-induced disassembly can be monitored as an increase in T2 values and enhancement in brightness of T2-weighted MR images. [a] D. Mazumdar, H. -K. Kim, Prof. Y. Lu Department of Chemistry University of Illinois at Urbana–Champaign 600 S. Mathews Avenue, Urbana, IL 61801 (USA) Fax: (+1) 217-333-2685 E-mail : yi-lu@uiuc.edu [b] M. V. Yigit, Prof. Y. Lu Center for Biophysics and Computational Biology University of Illinois at Urbana–Champaign 607 S. Mathews Avenue, Urbana, IL 61801 (USA) [c] J. H. Lee, Prof. Y. Lu Department of Materials Science and Engineering University of Illinois at Urbana–Champaign 1304 W. Green Street, Urbana, IL 61801 (USA) [d] M. V. Yigit, D. Mazumdar, J. H. Lee, Dr. B. Odintsov, Prof. Y. Lu Beckman Institute for Advanced Science and Technology University of Illinois at Urbana–Champaign 405 N. Mathews Avenue, Urbana, IL 61801 (USA) [] These authors contributed equally to this work. Supporting information for this article is available on the WWW under http://www.chembiochem. org or from the author.