Targeted molecular imaging techniques have become indispensable tools in modern diagnostics because they provide accurate and specific diagnosis of disease information. Conventional nonspecific contrast agents suffer from low targeting efficiency; thus, the use of molecularly targeted imaging probes is needed depending on different imaging modalities. Although recent technologies have yielded various strategies for designing smart probes, utilization of peptide-based probes has been most successful. Phage display technology and combinatorial peptide chemistry have profoundly impacted the pool of available targeting peptides for the efficient and specific delivery of imaging labels. To date, selected peptides that target a variety of disease-related receptors and biomarkers are in place. These targeting peptides can be coupled with the appropriate imaging moieties or nanoplatforms on demand with the help of sophisticated bioconjugation or radiolabeling techniques. This review article examines the current trends in peptide-based imaging probes developed for in vivo applications. We discuss the advantage of and challenges in developing peptide-based probes and summarize current systems with respect to their unique design strategies and applications.
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