Activatable aptamer probe for contrast-enhanced in vivo cancer imaging based on cell membrane protein-triggered conformation alteration

Abstract

Aptamers have emerged as promising molecular probes for in vivo cancer imaging, but the reported "always-on" aptamer probes remain problematic because of high background and limited contrast. To address this problem, we designed an activatable aptamer probe (AAP) targeting membrane proteins of living cancer cells and achieved contrast-enhanced cancer visualization inside mice. The AAP displayed a quenched fluorescence in its free state and underwent a conformational alteration upon binding to target cancer cells with an activated fluorescence. As proof of concept, in vitro analysis and in vivo imaging of CCRF-CEM cancer cells were performed by using the specific aptamer, sgc8, as a demonstration. It was confirmed that the AAP could be specifically activated by target cancer cells with a dramatic fluorescence enhancement and exhibit improved sensitivity for CCRF-CEM cell analysis with the cell number of 118 detected in 200 μl binding buffer. In vivo studies demonstrated that activated fluorescence signals were obviously achieved in the CCRF-CEM tumor sites in mice. Compared to always-on aptamer probes, the AAP could substantially minimize the background signal originating from nontarget tissues, thus resulting in significantly enhanced image contrast and shortened diagnosis time to 15 min. Furthermore, because of the specific affinity of sgc8 to target cancer cells, the AAP also showed desirable specificity in differentiating CCRF-CEM tumors from Ramos tumors and nontumor areas. The design concept can be widely adapted to other cancer cell-specific aptamer probes for in vivo molecular imaging of cancer.switchable aptamer probe | in vivo imaging | activatable fluorescent molecular imaging | cancer detection | cell surface protein

Use of a conformational switching aptamer for rapid and specific ex vivo identification of central nervous system lymphoma in a xenograft model

“…To circumvent this, we developed fluorescently coupled aptamers that are activated upon specific binding-induced conformational change and visualized through the principle of Förster Resonance Energy Transfer (FRET), which has been previously described [16][17][18]. This can effectively produce an agent that is optically silent until bound and "switches on" after binding to a specific target [16]. This decreases the noise due to nonspecific binding and eliminates the need for multiple time-consuming tissue preparation, staining, and rinsing procedures.…”

Aptamer-based molecular imaging

“…When applied to tumor mouse model, the activatable probe was rapidly taken up in mouse in 5 min and significantly enriched at tumor site as early as 15 min and retained for more than 2 h after probe injection. On the other hand, the free probes were essentially cleared from the nontumor sites at 1 h. Recognition of the activatable probe was specific to the CCRF-CEM tumor (Shi et al, 2011).…”

“…(from part of the Fig. 5A in Shi et al, 2011). (E) Imaging tenascin-C, a tumor marker, in a MDA-MB-435 breast tumor bearing nude mice by 2D-sctintigraphy using the probe 1 in noncoding RNA (ncRNA).…”

Nucleic Acid Fluorescent Probes for Biological Sensing

“…The probe could be specifically activated by target cancer cells with a dramatic fluorescence enhancement, which achieved contrast-enhanced cancer visualization inside mice. 128 DNA-scaffolded oligodeoxyriboside fluorophores (ODFs) have been developed by Kool and co-workers for cellular enzyme imaging. 7 In the ODF system, the DNA bases are replaced by several fluorophores that are directly attached to the backbone in close proximity.…”