Galactosyl serum albumin-rhodamine green

Optical fluorescence imaging is increasingly used to study biological functions of specific targets (1, 2). 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 non-invasive alternative to radionuclide imaging in small animals. On their cell surface, a variety of cancer cells express receptors (lectins) that bind to glycosylated proteins (3, 4). The β-D-galactose receptor binds and internalizes proteins that contain galactose sugar residues. Galactosyl serum albumin (GSA) was labeled with rhodamine green (RhodG) to study biodistribution of the tracer in tumor-bearing mice (5). RhodG is an optical fluorescence dye with an absorbance maximum at 502 nm and an emission maximum at 527 nm with a high extinction coefficient of 75,000 Mcm. GSA-RhodG was found to have a high accumulation in a variety of human ovarian adenocarcinomas in nude mice (5).