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.