Cypate-Gly-Arg-Asp-Ser-Pro-Lys

Integrins are a family of cell surface heterodimeric glycoproteins that mediate diverse biological events involving cell-cell and cell-matrix interactions (1). They consist of an α and a β subunit. They are important for cell adhesion and signal transduction. The αβ integrin is the most prominent receptor class affecting tumor growth, tumor invasiveness, metastasis, tumor-induced angiogenesis, inflammation, osteoporosis, and rheumatoid arthritis (2-7). The αβ integrin is strongly expressed on tumor cells and activated endothelial cells. In contrast, expression of αβ integrin is weak on resting endothelial cells and most normal tissues. The αβ antagonists are being studied as anti-tumor and anti-angiogenic agents (8, 9), and the agonists are being studied as angiogenic agents for coronary angiogenesis (10, 11). A tripeptide sequence consisting of Arg-Gly-Asp (RGD) is identified as a recognition motif used by extracellular matrix proteins (vitronectin, fibrinogen, laminin, and collagen) to bind to a variety of integrins including αβ. Various radiolabeled antagonists and peptides were introduced for imaging of tumors and tumor angiogenesis (12). Optical fluorescence imaging is increasingly used to obtain biological functions of specific targets (13-15). However, the intrinsic fluorescence of biomolecules poses a problem when visible light (350-700 nm) absorbing fluorophores are used. Near-infrared (NIR) fluorescence (700-1000 nm) detection avoids the background fluorescence interference of natural biomolecules, providing 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 complement to radionuclide imaging. Cypate is a reactive carbocyanine dye, which is derived from indocyanine green (ICG) (16). Cypate was previously conjugated to octreotate (Cyp-OC). Cyp-OC was not toxic to rats up to 10 μmol/kg (17). From the results of investigating a small library of RGD peptides for their binding activity to the αβ integrin, a linear hexapeptide, Gly-Arg-Asp-Ser-Pro-Lys (GRDSPK), lacking the RGD sequence was conjugated with Cypate as Cyp-GRD to study biodistribution of the tracer in tumor-bearing mice (18). Cypate is a NIR fluorescent dye with an absorbance maximum at 778 nm and an emission maximum at 805 nm with a high extinction coefficient of 224,000 (mol/L)−cm−. Cyp-GRD was found to have a high and long-lasting accumulation in αβ-positve A549 human non-small cell lung carcinomas in nude mice. The binding of Cyp-GRD to the integrin receptor was found to be specific both and .