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125I-T84.66 scFv-human serum albumin.


Leung K.


Molecular Imaging and Contrast Agent Database (MICAD) [Internet]. Bethesda (MD): National Center for Biotechnology Information (US); 2004-2013.
2008 Jul 02 [updated 2008 Sep 02].


Carcinoembryonic antigen (CEA) was first identified from extracts of human colon adenocarcinoma (1) and fetal gut (2). It is a β-glycoprotein, and its predominant expression on the cell surface is increased in a variety of carcinomas and in certain inflammatory conditions such as inflammatory bowel disease (3, 4). CEA has a molecular weight of ~180 kDa, and it can be shed and detected in the serum (5). CEA expression is observed in patients with various carcinomas of the colon, lungs, thyroid, uterus, ovaries, pancreas, and medullary thyroid (6). Radiolabeled monoclonal antibodies (mAbs) have been developed for both the diagnosis and treatment of tumors (7, 8). Single-chain variable fragments (scFvs) of mAbs 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 (9). 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) and the minibody fragment (a dimer of scFvs-CH3 with a linker comprising 18 amino acids; molecular mass = 80 kDa) of a murine/human chimeric anti-CEA T84.66 antibody have been evaluated for targeting in several tumor antigen systems with rapid tumor localization and high-contrast imaging (9, 10). In particular, murine/human chimeric anti-CEA T84.66 diabody and minbody, which retain excellent CEA-binding properties (dissociation constant = 0.01–0.6 nM) (11-13), were developed as CEA imaging agents. However, the pharmacokinetics of radiometal labeled antibodies, with high liver and kidney uptake, are generally not ideal for imaging (14). Albumin is known to accumulate in tumors (15) and has a long blood half-life (16). Furthermore, albumin fusion proteins have been shown to prolong the in vivo circulation of small peptides and proteins (17). A T84.66 single-chain antibody (scFv) was joined with human serum albumin (HSA) using recombinant technology to form T84.66 scFv-albumin fusion protein (T84.66 scFv-HSA), which was radiolabeled with 125I for localization of CEA-positive tumors in mice with single-photon emission computed tomography (SPECT) (18).

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