Galactosamine-serum albumin-rhodamineX

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 a 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. A variety of cancer cells express on their cell surface receptors (lectins) that bind to glycosylated proteins (3, 4). The β-D-galactose receptor binds and internalizes proteins that contain galactose sugar residues. Galactosamine-serum albumin (GmSA) was labeled with rhodamine green (RhodG) to study biodistribution of the tracer in tumor-bearing mice and was found to have a high accumulation in a variety of human ovarian adenocarcinomas in nude mice (5). A self-quenching GmSA complex was create by conjugating 20 rhodamineX (ROX) molecules to GmSA (GMSA-rhodamineX (GmSA-20ROX)) (6). ROX is an optical fluorescence dye with an absorbance maximum at 595 nm and an emission maximum at 610 nm. GmSA-20ROX was found to have a high accumulation in peritoneal micrometastases of human SHIN3 ovarian adenocarcinomas in nude mice (6).