Synthesis, fluorescence and biodistribution of a bone-targeted near-infrared conjugate

Enhanced imaging of early-stage bone abnormalities, such as primary tumors or metastases is highly required as the widely-used bone scan frequently lacks the desired sensitivity. Near IR (NIR) fluorescence imaging affords high contrast and enhanced sensitivity, as body tissue expresses minimal autofluorescence at NIR range (600–1200 nm). Indocyanine green (ICG), a biocompatible NIR dye, is widely used in the imaging of various organs, such as liver, heart and blood circulation. We report the preparation and in-vivo testing of a bone-targeting ICG derivative, in comparison to the parent molecule(s). Since ICG itself is chemically unreactive, and could not form conjugates, we prepared two novel ICG conjugatable derivatives. The overall ICG structure was maintained while only a replacement of one or two sulfonate groups with carboxylic acids resulted in new linkers for covalent binding to biomolecules. These derivatives were evaluated for their fluorescence and biodistribution in comparison to ICG and were found to be comparable. One of the novel ICG-derivatives was conjugated to a bone-targeting moiety and this new compound was found to bind to growing regions of the skeleton, and emit fluorescence for as long as two weeks in young mice.

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► A novel bone-targeted Near-IR fluorescent dye has been prepared and tested in vivo.
► The new compound combines indocyanine green fluorescent characteristics with the strong bone affinity of a bisphosphonate.
► Preparation of the novel dye required the synthesis of a novel indocyanine green derivative, which is described in full.
► Injection of the novel dye at minute quantities resulted in a two-week long fluorescent signal in young mice.