Optical and multimodal peptide-based probes for in vivo molecular imaging

Molecular imaging consists of non-invasive monitoring of spatial-temporal distribution of molecular or cellular processes, and may be used for early disease detection and real-time monitoring of therapeutic responses. Several strategies have been developed over the last two decades. Early attempts used monoclonal antibodies or antibody fragments and, although specific targeting was achieved, these probes was largely unsuccessful. In the quest for better agents, labeled peptides were then used. Peptides are easier to synthesize, less likely to be immunogenic, and have rapid blood clearance, which results in adequate target-to-background ratios in a short period of time. This review discusses state-of-the-art cancer imaging by means of labeled peptides, the radionuclide, optical and nanoplatform-based imaging techniques which can provide functional information of the disease and track biochemical processes in vivo. The advantages and disadvantages of each technique are discussed. Lastly, the emphasis of this paper is on the new multimodal probes which can overcome individual limitations and exploit the individual strengths of the latest molecular imaging techniques.