**6.1 CEA**

174 12 Chapters on Nuclear Medicine

engineering have led to a number of alternative constructs for imaging and therapy. These

• **Fragment antigen binding (Fab):** The area around the antibody hinge is more susceptible to proteolysis than the tightly folded domains and thus this is the point at which cleavage usually takes place. Proteolysis above the disulphide bonds in the hinge region with, for example papain, results in Fab fragments which are monovalent for

• **Divalent fragment antigen binding (F(ab´)2):** If antibody proteolysis occurs below the hinge disulphide bonds, with enzymes such as pepsin, it results in a divalent F(ab´)2

• **Single chain fragment (scFv):** This antibody fragment consists only the variable heavy (VH) and the variable light (VL) chain of an antibody tethered together by a flexible linker, attaching the carboxyl terminus of the VL sequence to the amino terminus of the

• **Nanobody:** Nanobodies are small antibody fragments (15 kDa) derived from the naturally occurring heavy-chain-only antibodies (Friedman. et al., 2009). These fragments have also been referred to VHH as they consist of a variable domain (VH) of

When compared to intact IgGs, these antibody fragments have the advantages of faster biodistribution, rapid penetration into the tissues and improved tumor-to-normal organs ratio. These forms (e.g. F(ab')2, Fab or other molecular constructs) achieve maximum tumor accretion more quickly with improved tumor/blood ratio that make earlier visualization, possible (Colcher et al., 1998). The use of fragments of antibodies has also been advocated as a possible means to reduce the immunogenicity of rodent antibodies in human beings. Antibody fragments generated by proteolysis or by genetic engineering, have been tested both in vitro and in vivo. As monovalent binding entities, antibody fragments suffer from relatively low avidity binding. Hence, to increase their binding avidity, they have also been

• **Diabody:** "Diabody" is one of the engineered multivalent constructs. Diabodies can be produced to high levels in the form of stable dimmers (Wu & Yazaki, 2000; Hudson & Kortt, 1999; Lawrence et al., 1998). However, the stability of such dimeric species varies

• **Tribody:** In some cases, particularly with direct fusions of VH and VL, stable trimeric species are produced which have been termed "tribody" (Todorovskaet al., 2001). Some tribodies show improved avidity for antigens as expected from the increased number of

In recent years, there has been a significant improvement in the understanding of molecular events and critical pathways involved in breast cancer. The studies have confirmed the feasibility of using radiolabeled antibodies for imaging and therapy of primary and metastatic breast cancer and that a diverse array of molecules can serve as targets for localizing antibodies (Carlos et al., 2006). Theoretically, an ideal target for radionuclide detection and therapy of metastatic breast cancer would be tumor-specific, generously expressed on all the breast cancer cells, not expressed by normal tissues and not released into the blood circulation (Mohammadnejad et al., 2010). For antibody screening of breast

binding sites, although this is not always the case (Todorovskaet al., 2001).

alternatives characterized by smaller size include:

the heavy chain (H) of antibodies (Meel et al., 2010).

engineered into multivalent constructs include:

from one antibody to another.

**6. Target antigens** 

antigen binding (~55 kDa).

fragment (~110 kDa).

VH sequence (~25 kDa).

First described by Gold and Freedman in 1965, CEA was thought to be a specific marker for colon adenocarcinoma. However, subsequent studies demonstrated CEA expression in other human adenocarcinomas including the surface membrane of breast cancer cells (Jonathan et al., 2000). Expression of CEA has been reported in 10% to 95% of breast cancers (Hong et al., 2008, Denardo, 2005). Preliminary studies with 99mTc-labeled CEA antibody appeared to indicate a useful role for this agent in distinguishing between benign and malignant breast lesions in patients with indeterminate mammographic findings (Denardo, 2005). Therapy studies specifically in breast cancer have also been performed with T84.66 (Koppe et al., 2005). T84.66 is a well characterized murine IgG1 antibody with high specificity and affinity for a unique epitope of CEA molecules. A chimeric form of T84.66 (cT84.66) has been also used in clinical studies for the scintigraphic detection of breast cancer and in phase I/II therapy trials and scFv-based anti-CEA constructs are under study (Denardo, 2005). Another mAb that has been classified in this group is NP4. NP-4 belongs to the murine IgG1 subclass and is specific for CEA, reacting with a class III peptide epitope of CEA molecules (Richman & DeNardo, 2001).
