**5.5.1 Antibody derivatives**

The use of mAbs has presented challenges in radionuclide imaging. Because of their large size (molecular weight of ~150 kDa), mAbs penetrate slowly and have long residence time in the blood circulation (days-weeks) due to active recycling by the neonatal Fc receptor, leading to limited tumor-to-normal organs ratio in biodistribution and low contrast images for the detection of biomarkers/tumors (Jonathan et al., 2000). Advances in protein

Breast Cancer: Radioimmunoscintigraphy and Radioimmunotherapy 175

cancer, it is essential that the antibody is well characterized with little cross-reactivity to other antigens. The antibodies show cross-reactivity with, e.g. leucocytes, potentially yielding false-positive images. The majority of breast cancer targeting antibody studies have

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

MUC-1 mucins are large, complex glycoproteins that have a polypeptide core with multiple oligosaccharide side chains (Mukhopadhyay et al., 2011). The mature molecule is anchored within the cell surface by a characteristic transmembrane domain, but most of the mucins are expressed extracellular. This polar distribution is lost with neoplastic transformation and increased heterogenous MUC-1 synthesis is a common feature of breast cancer. This glycoprotein is aberrantly over expressed in adenocarcinomas including 80% of breast cancers (Salouti et al., 2011). Targeting, using antibodies directed against the MUC-1 antigen has been tested in patients with breast cancer using various antibodies. BrE-3 is an IgG1 antibody directed against the peptide epitope of the MUC-1 antigen that have been evaluated for RIT (Mohammadnejad et al, 2010). The studies of the application of BrE-3 mAbs in patients with breast cancer showed minimal cross reactivity with normal breast tissues (Howell et al., 1995; Blank et al., 1992). Because the majority of the patients rapidly showed HAMA response against the murine BrE-3 mAb, a humanized version of BrE-3 (hBrE-3) was developed (Kramer et al., 1993). Kramer and colleagues investigated the pharmacokinetics and biodistribution of 111In-MX-DTPA-labeled hBrE-3 in seven patients with metastatic breast cancer (Kramer et al., 1993). hBrE-3 was proved to have a lower immunogenicity compared to murine BrE-3 (only one patient developed a HAMA response) while tumor-targeting properties were preserved (Denardo, 2005). Monoclonal antibody 170H.82 (m170) is a murine IgG1 prepared using a synthetic asialo GM1 terminal disaccharide immunogen related to the Thomsen–Friedenreich disaccharide and selected by reactivity with MUC-1 expressing cancer cell membranes (Jonathan et al., 2000). The results of experimens showed that labeled m170 was effective for imaging of primary and metastatic breast cancer and was able to detect lesions as small as 1 cm in size using SPECT with an overall clinical accuracy of 92%. In particular, mAb 170H.82 has been studied in

used antibodies against MUC1, CEA, TAG-72 and L6 antigens (Carlos et al., 2006).

**6.1 CEA** 

**6.2 MUC-1** 

CEA molecules (Richman & DeNardo, 2001).

engineering have led to a number of alternative constructs for imaging and therapy. These alternatives characterized by smaller size include:


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 engineered into multivalent constructs include:

