Cat K 680 FAST is a Cathepsin K (Cat K) activatable fluorescent agent that is optically silent upon injection and produces fluorescent signal after cleavage by disease related Cathepsin K. Cat K is highly expressed in osteoclasts, suggesting a specialized role in bone resorption.

Cat K is found extracellularly along the bone and cartilage and intracellularly in lysozomes of osteoclasts attached to the bone. In addition to expression by osteoclasts, Cat K is also expressed by a broad range of other cell types including ovarian cells, colonic tissue, alveolar and bronchial epithelial cells, synovial fibroblasts, breast cancer cells, macrophages.

Cat K 680 FAST fluorescent probe is a member of a family of activatable imaging agents comprising a novel architecture, termed F.A.S.T. (Fluorescent Activatable Sensor Technology), that confers an improved pharmacokinetic profile with earlier imaging time points. This architecture offers higher target specific signal with reduced background while also reducing the optimal imaging time after injection.