Accurate and rapid detection of tumors is of great importance for interrogating the molecular basis of cancer pathogenesis, preventing the onset of complications, and implementing a tailored therapeutic regimen. In this era of molecular medicine, molecular probes that respond to, or target molecular processes are indispensable. Although numerous imaging modalities have been developed for visualizing pathologic conditions, the high sensitivity and relatively innocuous low energy radiation of optical imaging method makes it attractive for molecular imaging. While many human diseases have been studied successfully by using intrinsic optical properties of normal and pathologic tissues, molecular imaging of the expression of aberrant genes, proteins, and other pathophysiologic processes would be enhanced by the use of highly specific exogenous molecular beacons. This review focuses on the development of receptor-specific molecular probes for optical imaging of tumors. Particularly, bioconjugates of probes that absorb and fluoresce in the near infrared wavelengths between 750 and 900 nm will be reviewed.

Key words: Optical imaging, Contrast agents, Molecular probes, Carbocyanine dyes, Cell surface receptors, Somatostatin, Bombesin, Folate, Epidermal growth factor, Neurotensin.