The non-invasive near-infrared (NIR) fluorescence imaging dye Indocyanine green (ICG) is approved by the United States Food and Drug administration (FDA) for ophthalmologic angiography to determine cardiac output and liver blood flow and function. This dye is also used in cancer patients for the detection of solid tumors, localization of lymphnodes, and for angiography during reconstructive surgery, visualization of retinal and choroidal vasculature, and photodynamic therapy1-3. In cancer diagnostics and therapeutics, ICG could be used as both an imaging dye and a hyperthermia agent.
ICG is a tricarbocyanine-type dye with NIR-absorbing properties (peak absorption around 800 nm) and emission maximum at 807 nm (10% v/vDMSO/PBS). Little absorption in the visible range accounts for the low autofluorescence, tissue absorbance, and scattering at NIR wavelengths (700-900 nm).
This click reaction is 3 orders of magnitude faster than commonly used Staudinger ligation and offers the opportunity to develop optimal conjugates. Cyanobenzothiazole (CBT) active groups provide an efficient and convenient way to site-selectively link ICG dyes to 1,2- or 1,3-aminothiols on various substrates (Antibodies, peptides, proteins, nucleic-acid, small molecule drugs etc.) without any additional activation. The labeling reaction with aminothiols is selective over reaction with simple thiols.
The CBT click chemistry can be used together with all other biocompatible click reactions (like azide, alkyne, triphenylphosphine, tetrazine etc.), as it is very selective.
In addition in ICG-CBT labeling procedure no side product is formed as here is no leaving group (unlike NHS esters).