Radioiodinated anti–TAG-72 CC49 (Fab’)2 antibody fragment

Excerpt

Radioiodinated anti–TAG-72 CC49 (Fab’)₂ antibody fragment (¹³¹I-CC49 (Fab’)₂), which is formed by the conjugation of ¹²⁵I/¹³¹I with an anti–tumor-associated glycoprotein 72 (TAG-72) (Fab’)₂ antibody fragment, has been developed for gamma imaging of cancers that express TAG-72 (1-4). ¹²⁵I has a physical half-life (t_½) of 60 days, and ¹³¹I has a t_½ of 8 days. Although both can be used for in vivo gamma imaging, ¹²⁵I has a gamma energy that is too low and ¹³¹I has a gamma energy that is too high. Another radioiodine, ¹²³I, is more ideal for single-photon emission tomography (SPECT) and planar gamma imaging.

The TAG-72 antigen was isolated from the LS-174T human colon cancer xenograft as a high molecular weight glycoprotein (molecular mass of 10⁶ Da) with mucin-like characteristics (5-8). It is expressed on a variety of human adenocarcinomas such as pancreatic, breast, colorectal, prostate, endometrial, and ovarian cancers. This antigen has also been shown to be shed into the serum of cancer patients (9). The murine monoclonal antibody B72.3 (MAb B72.3) against TAG-72 mucin was initially generated by immunization of mice with a membrane-enriched fraction of a human breast carcinoma (10). With the use of affinity-purified TAG-72 from LS-174T as an immunogen, CC49 and other anti–TAG-72 MAbs with higher affinity constants (K_a) have been produced and characterized (5, 6, 10, 11). CC49 MAb appears to react with a unique disaccharide sialyl-Tn (STn) epitope on TAG-72 (12, 13).

Radiolabeled MAbs have been developed for both the diagnosis and treatment of tumors (14). Radiolabeled B72.3 and CC49 exhibit excellent tumor localization capabilities with potential diagnostic and therapeutic applications in the clinical setting (15, 16). Because of their relatively large size, intact radiolabeled MAbs tend to have unfavorable imaging kinetics, poor tumor penetration, and high potential for human anti-mouse antibody response (11, 17-19). One possible approach to minimize these problems is reducing intact antibodies to smaller antibody fragments such as F(ab’)₂ and Fab’ (20). Another approach is the development of genetic engineering methods to obtain single-chain Fv constructs (scFv) and multivalent scFv constructs (11, 21, 22). The F(ab’)₂ and Fab’ fragments can generally be prepared by simple enzymatic cleavage. Pepsin digestion of the intact IgG removes the antibody constant region and produces the F(ab’)₂ fragment with bivalent antigen-binding capability and a molecular weight of 100,000 (14). Because of the smaller size, the F(ab’)₂ fragment has a faster blood clearance and a better tumor penetration than the intact IgG (2, 23). The removal of the Fc portion during the enzymatic cleavage also reduces nonspecific binding of F(ab’)₂ to Fc receptors. The in vitro and in vivo properties of radioiodinated CC49 (Fab’)₂ fragments have been studied and compared to those of the single-chain Fv constructs (1-4, 24).