Pamidronate-IRDye78

Optical fluorescence imaging is increasingly used to obtain biological functions of specific targets and in small animals (1, 2). However, the intrinsic fluorescence of biomolecules poses a problem when visible light (350-700 nm) absorbing fluorophores are used because of high tissue absorption and scatter. Near-infrared (NIR) fluorescence (700-900 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 to 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 was also compatible with solid-state optical components such as diode lasers and silicon detectors. NIR fluorescence imaging is becoming a non-invasive alterative to radionuclide imaging and in small animals. IRDye78 is a heptamethine indocyanine-type NIR fluorophore with peak absorption at 771 nm, and peak excitation emission at 806 nm. It provides a quantum yield of 14.2%. It has a molecular weight of 1083 Da. IRDye78 is a highly charged IR-786 derivative, which localized to mitochondria at low concentrations and endoplasmic reticulum (ER) at high concentrations in vitro (3). IRDye78 was shown to be a useful perfusion agent in myocardium. IRDye78 -hydroxysuccinimide (NHS) ester can be conjugated to antibodies and low-molecular weight ligands with one or more free primary amines. Osteoblasts (mineralization) and osteoclasts (demineralization) are two importance cell types in development and integrity of vertebral skeleton (4, 5). Osteoblast-like cells are present in vascular tissues and play a role in arteriosclerosis (6). Microcalcifications are found in breast tissue (7, 8). Hydroxyapatite (HA) is a mineral product deposited in the bone and vascular tissue by the osteoblast. HA binds pyrophosphates and phosphonates with high affinity. Nitrogen-containing synthetic diphosphonates are inhibitors of farnesyl diphosphate synthase (FDPS) of osteoclast and are used for treatment of osteoporosis (bone resorption) (9). Inhibition of FDPS inhibits osteoclast bone-resorption activity and induces osteoclast apoptosis (10, 11). Diphosphonates bind to bone mineral with high affinity for their long duration of action. For example, [Tc]Methylene diphosphonate (MDP) was developed for bone scanning (12) and it is believed that it binds to HA. Pamidronate (Pam) is a diphosphonate derivative with a single primary amine for conjugation with IRDye78-NHS ester to form Pam78. Pam78 exhibits rapid and specific binding to HA and . Zaheer et al. (13) demonstrated NIR fluorescence detection of bone with Pam78 in nude mice. A simple and rapid rat model of focal calcification in breast cancer tumors was validated by Pam78 NIR imaging (14).