Aptamer-functionalized, ultra-small, monodisperse silica nanoconjugates for targeted dual-modal imaging of lymph nodes with metastatic tumors.

Metastases are responsible for 90% of human cancer deaths. Most solid tumors metastasize through the circulation system, and the sentinel lymph node (LN) is typically the first site reached by the disseminating malignant cancer cells. The detection of LN metastases is therefore crucial for accurate tumor staging and therapeutic decision making. The current standard method for LN assessment is lymphography using a vital blue dye. However, this method is invasive, involving extended nodal dissection, and can give a false negative result if an LN is missed in surgery. A non-invasive LN imaging technique is urgently needed to improve the accuracy of tumor staging. Various techniques for sentinel LN imaging have been investigated, such as near-infrared (NIR) fluorescence imaging, computed tomography (CT), magnetic resonance imaging (MRI), positron emission tomography (PET), and ultrasound and photoacoustic imaging. However, each technique has its drawbacks, and none is sufficient to provide all the necessary information for LN assessment. PET is the most sensitive and specific technique for in vivo molecular imaging, but it suffers from low spatial resolution. In contrast, fluorescence imaging has high resolution and allows spatial visualization, which is helpful for intraoperative guidance; but its application is limited by poor tissue penetration. Therefore, combination of both PET and fluorescence imaging together potentially permit non-invasive assessment of LNs with high sensitivity and excellent spatial resolution. Silica nanoparticles (NPs) are widely used for biomedical imaging applications because of the good biocompatibility and optical transparency of silica. We recently developed a versatile, size-controlled, monodisperse, drug/dye silica nanoconjugate (NC) that allows for conjugation with a variety of functional moieties. The robust silane chemistry and the formulation strategy permit the construction of multifunctional NCs, such as multi-modal imaging probes for in vivo applications. It is generally accepted that the physicochemical properties of NPs, especially their size, play a vital role in the systemic and lymphatic biodistribution. Because the size of the silica NCs can be precisely controlled, they are ideal for investigating size effects on their trafficking behavior in the lymphatic system. The silica NCs are multifunctional and give excellent size control for the preparation of nanoparticulate probes with optimized properties for improved imaging of LN metastases. There have been many studies on the targeting of primary tumors, but very few attempts have been made to actively target metastatic tumors specifically. As an alternative to antibodies for cancer targeting, aptamers, single-stranded oligonucleotides that can bind to target molecules with high specificity and affinity, have attracted much attention because they are small, easy to synthesize, non-immunogenic, and can be modified to resist denaturation and biodegradation. The capability of aptamers to target primary tumors has been demonstrated in several studies in vivo. However, active targeting of lymphatic metastases using aptamers has not been reported. Herein, we report a convenient, one-pot synthesis of monodisperse, size-controlled silica NC probes for dualmodal LN imaging using PET and NIR fluorescence. Monodisperse 20 nm silica NCs accumulated in sentinel LNs more rapidly and to a greater extent than larger NCs (200 nm) and were superior for efficient LN imaging. To further enhance the targeting of LNs with metastatic tumors, we functionalized the 20 nm silica NCs with a 26-mer G-rich DNA aptamer (Apt) derived from AS1411, which has high binding affinity for nucleolin (NCL), a protein that is over-expressed in the cytoplasm and on the plasma membrane of some cancer cells, including breast cancer cells. The NCL-Apt-functionalized silica NCs showed markedly enhanced uptake in LNs with metastatic tumors in a murine breast tumor model, and [*] L. Tang, I. Chaudhury, Q. Yin, C. Yao, Prof. Dr. J. Cheng Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign 1304 W. Green Street, Urbana, IL, 61801 (USA) E-mail: [email protected]