Table 2.

Ligands for optical labeling and imaging

Detection ligand	Advantages	Disadvantages
Whole antibody	Easiest to formulate. Many clinically approved antibodies available for labeling. Long history of optical- and radio-labeling of antibodies.	Long blood half-life decreases specificity of signal, especially before blood pool clearance of imaging agent
Engineered or chemically produced antibody fragments	These structures retain high binding affinity. Clearance times well suited for imaging.	Somewhat more complex to formulate compared to whole antibodies
Targeting peptides	High throughput screening methods can yield high-specificity agents with rapid clearance times	Somewhat more complex to formulate compared to whole antibodies
Small molecules	High specificity, rapid clearance. Additional targets including intracellular targets, available for imaging	Fluorochromes and their comparable size to small molecules may affect pharmacokinetics and biodistribution of the resulting labeled ligands