Ga-Labeled anti-EpCAM diabody against epithelial cell adhesion molecule

Epithelial cell adhesion molecules (EpCAM) are found on the cell surface of epithelial cells of many epithelial tissues such as the pancreas, jejunum, colon, kidney, salivary gland, and prostate (1). EpCAM is responsible for intracellular signaling and polarity, and it mediates cell differentiation, proliferation, migration, and adhesion (2, 3). It is a pan-epithelial differentiation antigen that is expressed on almost all carcinomas (4). Approximately 70% of human prostate cancers show high levels of EpCAM expression (5, 6). Single-chain variable fragments (scFvs) of antibodies with a molecular mass of 25 kDa are cleared very rapidly from the circulation, but they exhibit poor tumor retention because they have a lower affinity than the parent antibody (7). On the other hand, bivalent antibody fragments possess more ideal tumor-targeting characteristics, including rapid tissue penetration, high target retention, and rapid blood clearance. The diabody fragment (a dimer of scFvs; molecular mass, 55 kDa) has been evaluated for targeting in several tumor antigen systems and has demonstrated rapid tumor localization and high-contrast imaging (7, 8). In particular, human IgG anti-EpCAM (clone 42 (huIgG), ~150kDa) was isolated from a human naïve antibody library. The monomeric scFv (clone 42 with 18 amino acid linker), dimeric scFv (diabody), and trimeric scFv (tribody) were genetically constructed. These fragments retain excellent EpCAM-binding properties (dissociation constant () = 0.24–11.8 nM) and were developed as EpCAM imaging agents. They were conjugated with the bifunctional chelating agent '-bis[2-hydroxy-5-(carboxyethyl)benzyl]ethylenediamine-'-diacetic acid (HBED-CC) to enable labeling with Ga for imaging of EpCAM expression in solid tumors (9).