LLP2A-biotin-streptavidin-Alexa Fluor 680

Optical fluorescence imaging is increasingly being used to observe 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. Integrins are a family of cell-surface heterodimeric glycoproteins that mediate diverse biological events involving cell–cell and cell–matrix interactions (3). They consist of an α and a β subunit, and they are important for cell adhesion and signal transduction. The αβ integrin plays an important role in hematopoiesis of lymphocytes; on the other hand, the αβ integrin affects tumor growth, tumor invasiveness, and metastasis (4). The αβ integrin is strongly expressed on lymphoid tumor cells (5). LLP2A was identified as a peptidomimetic ligand to bind to αβ integrin on human Jurkat T-lymphoid leukemia cells using “one-bead one-compound” combinatorial libraries (6). LLP2A was conjugated with Alexa Fluor 680 (Alexa680) biotin and streptavidin (SA) interaction to study biodistribution of the tracer in tumor-bearing mice. Alexa680 is a NIR fluorescent dye with an absorbance maximum at 684 nm and an emission maximum at 707 nm with a high extinction coefficient of 183,000 (mol/L)cm. LLP2A-biotin-SA-Alexa680 was found to have a high specific accumulation in αβ-positve lymphoid tumor cells in nude mice.