Conventional diagnostic imaging methods have had a profound impact on cancer treatment. Yet, improvements are needed in both sensitivity and specificity. Molecular imaging offers the possibility of obtaining in vivo target-specific information with high sensitivity and specificity. However, to achieve this goal, it will be necessary to develop new imaging probes that can achieve high tumor-to-background ratios (TBR). In order to design such probes, one of three strategies can be used: 1. maximize signal from the target 2. minimize signal from the background or 3. do both. Achieving any of these leads to improved TBR, which, in turn, improves sensitivity and specificity for detecting cancers. Optical imaging with fluorescent probes has the advantages of portabililty and real time capabilities without ionizing radiation and at modest cost. The limited depth of light penetration in tissue, however, necessitates that fluorescence imaging reserved for surfaces such as those encountered during endoscopic or surgical procedures. A unique feature of fluorescence is that optical probes can be designed so that they do not emit light, if they are quenched, but will generate light upon interacting with target molecules. Recent advances in activatable optical agents that emit signal only after enzymatic digestion, or acidification, represent an exciting strategy for improving TBRs by minimizing signal from the background. Several different photo-chemical mechanisms have been utilized including self-quenching, fluorescence resonance energy transfer, H-dimer formation and photo-induced electron transfer. This attribute of �turning on� only when interacting or binding to the target molecule, fits the definition of an �activatable� imaging probe. Therefore, activatable agents have low to no background signal prior to activation and generate signal only after being bound to the specific molecular target. These activatable fluorescence probes maximize the target signal while minimizing the background signal, thereby resulting in higher TBRs than conventional �always on� imaging agents.

Activatable fluorescent imaging probes for cancer detection and diagnosis

248th National Meeting (2014)

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