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Quantum dot800-poly(ethylene glycol)-c(Arg-Gly-Asp-d-Tyr-Lys).


Leung K1.


Molecular Imaging and Contrast Agent Database (MICAD) [Internet]. Bethesda (MD): National Center for Biotechnology Information (US); 2004-2013.
2011 Mar 21 [updated 2011 Jun 16].

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National Center for Biotechnology Information, NLM, NIH, Bethesda, MD


Optical fluorescence imaging is increasingly used to monitor biological functions of specific targets in small animals (1-3). However, the intrinsic fluorescence of biomolecules poses a problem when fluorophores that absorb visible light (350–700 nm) are used. Near-infrared (NIR) fluorescence (700–1,000 nm) detection avoids the background fluorescence interference of natural biomolecules, providing a high contrast between target and background tissues. NIR fluorophores have wider dynamic range and minimal background as a result of reduced scattering compared with visible fluorescence detection. They also have high sensitivity, resulting from low infrared background, and high extinction coefficients, which provide high quantum yields. The NIR region is also compatible with solid-state optical components, such as diode lasers and silicon detectors. NIR fluorescence imaging is becoming a noninvasive alternative to radionuclide imaging in small animals (4, 5). Fluorescent semiconductor quantum dots (QDs) are nanocrystals made of CdSe/CdTe-ZnS with radii of 1–10 nm (6-8). They can be tuned to emit signals in a range of wavelengths by changing their sizes and composition, thus providing broad excitation profiles and high absorption coefficients. They have narrow, symmetric emission spectra with long, excited-state lifetimes (20–50 ns) compared with those of fluorescent dyes (1–10 ns). They process good quantum yields of 40%–90% and possess high extinction coefficients. They are more photo-stable than conventional organic dyes. They can be coated and capped with hydrophilic materials for additional conjugation with biomolecules, such as peptides, antibodies, nucleic acids, and small organic compounds, which have been tested in vitro and in vivo (8-12). Although many cells have been labeled with QDs in vitro with little cytotoxicity, there are limited studies of long-term toxicity of QDs in small animals (13-21); little is known about the toxicity and the mechanisms of clearance and metabolism of QDs in humans. The performance of various nanoparticles for biomedical imaging has been reviewed (22, 23). Integrins are a family of heterodimeric, cell-surface glycoproteins that mediate diverse biological events involving cell–cell and cell–matrix interactions (24). Integrins comprise an α and a β subunit, and they are important for cell adhesion and signal transduction. The αvβ3 integrin is the most prominent receptor class, affecting tumor growth, tumor invasiveness, metastasis, tumor-induced angiogenesis, inflammation, osteoporosis, and rheumatoid arthritis (25-30). The αvβ3 integrin is strongly expressed on tumor cells and activated endothelial cells. In contrast, expression of αvβ3 integrin is weak on resting endothelial cells and on most normal tissues. The αvβ3 antagonists are being studied as anti-tumor and anti-angiogenic agents, and the agonists are being studied as angiogenic agents for coronary angiogenesis (29, 31, 32). A tripeptide sequence comprising Arg-Gly-Asp (RGD) has been identified as a recognition motif used by extracellular matrix proteins (vitronectin, fibrinogen, laminin, and collagen) to bind to a variety of integrins including αvβ3. Various radiolabeled cyclic RGD peptides have been introduced for tumor imaging and tumor angiogenesis (33). QDs that do not contain Cd (non-Cd QDs) have been prepared as an alternative to the more toxic Cd-based QDs. These newly developed non-Cd QDs exhibit improved blood half-life and minimal reticuloendothelial system accumulation with rapid renal clearance (34). These QDs easily extravasate leaky tumor blood vessels because of their enhanced permeability and retention effects. Gao et al. (35) coated non-Cd QD800 (InAs/InP/ZnSe, emission maximum at 800 nm) with di-stearoyl-poly(ethylene glycol)-phosphatidylethanolamine 2000 (DSPE-PEG2000) and c(RGDyK) to form QD800-PEG-RGD for NIR fluorescence imaging of αvβ3 integrin expression in the tumor vasculature in mice. QD800-PEG-RGD has been shown to have a high accumulation in tumors with little extravasation and a predominant liver accumulation.

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