**5.5 Antibodies**

172 12 Chapters on Nuclear Medicine

small-molecule radiopharmaceuticals include 125I-daunorubicin, 111In-folate, 123/131I metaiodo- benzyl-guanidine (MIBG), 125I-iododeoxyuridine and 111In-bleomycin (Jalilian et al., 2006, 2007)**.** Small molecules such as tyrosine kinase inhibitors (TKIs) are less specific than therapeutic monoclonal antibodies (mAbs) (Huang & Armstrong,2004) and some of them can inhibit multiple targets simultaneously including cell receptors or signal

The natural ability of oligonucleotides and the oligonucleotide mimetic peptide nucleic acids (PNAs) and phosphorodiamidate morpholinos (MORF) to anneal with RNA and DNA makes them the appealing vehicles to bring radionuclides in close proximity to the RNA/DNA. Both 125I and 111In have been used to radiolabel oligonucleotides and have been applied successfully to target over-expression of certain genes involved with cancer

Aptamers are synthetically based DNA or RNA oligonucleotides that are highly stable structures and are considered to have low immunogenicity. They are selected for their ability to bind to a target of interest (Perkins & Missailidis, 2007). Hicke et al. demonstrated that the aptamers cleared quickly from the blood, reaching maximum tumor uptake within 10 min, but then decreasing to approximately 2% by 3 h. However, the rapid clearance from the blood and tissues resulted in highly favorable tumor/blood and kidney ratios but there was considerable additional clearance of the 99mTc through the liver and intestines (Hicke et al., 2006). However, using aptamers as the radiopharmaceuticals needs further

Affibody molecules are 58 amino-acid, three-helix bundle affinity proteins and are derived from the β-domain of the five-domain Ig-binding region protein A from *Staphylococcus aureus (*Orlova et al., 2007). They represent highly specific binders, selected by phage display from a library generated by randomization of 13 amino acids in helix 1 and 2, which are responsible for the Fc-binding site. Recently, affibody molecules have been investigated for tumor targeting purposes both for targeted imaging and therapy. The first generated and used affibody molecule for radionuclide imaging was ZHER2 with a binding affinity of 50 nM to HER2 protein (Capalaet al., 2009). Affibody molecules represent a promising novel class of targeting molecules that can be used as relatively small, high-affinity, cancer-specific ligands and that are well suited for tumor molecular imaging and therapy, providing a possible new route for imaging of tumor-specific receptors (Orlova et al., 2006). Since these structures are derived from a *Staphylococcal* protein, the potential immunogenicity of these

During the past decade, proof of the principle that peptide receptors can be used successfully for in vivo targeting of human cancers, has been provided (Ferro-Flores et al., 2010). Peptides used for tumor targeting show some advantages over antibodies. Peptides are small and show rapid diffusion into the target tissues resulting in rapid pharmacokinetics (Ferro-Flores et al., 2010). Their fast blood clearance can lead to high tumor to background ratio shortly after administration of the radiopeptide. In addition, they

molecules may be a concern (Sharkey & Goldenberg, 2008).

transduction pathway proteins leading to a higher risk for toxicity (Xia et al., 2005).

**5.2 Oligonucleotides/ PNAs/ MORF** 

(Cornelissen & Vallis, 2010).

improvements.

**5.4 Peptides** 

**5.3 Affibody molecules** 

Antibodies are now increasingly recognized as important biological agents for the detection and treatment of cancer (Sharkey & Goldenberg, 2006). In the 1970s, polyclonal antibodies were already essential components of medical diagnosis as well as for therapy as antitoxins for the prevention of tetanus and other diseases (Goldsmith & Signore, 2010). The development of monoclonal antibody technology by Kohler and Milstein in the 1970s accelerated the exploitation of the chemo-specificity of antibodies for diagnostic and therapeutic purposes (Goldsmith & Signore, 2010). In the last two decades several different monoclonal antibodies have been approved by the Food and Drug Administration (FDA) for therapeutic purposes and some of these have also been radiolabeled for diagnostic and therapeutic purposes (Xiao et al., 2008). For imaging, it is highly desirable that targeting agents are rapidly excreted from the body. It is also essential that the targeting agent binds rapidly to its target, reducing the time between injection and imaging. The application of monoclonal antibodies for therapy and diagnosis is limited by generation of an immune response known as human anti-mouse antibody (HAMA) response (Salouti et al., 2011). One of the most successful approaches to overcome immunogenicity is "humanization" of rodent mAbs by genetic engineering (Waldmann&Morris, 2008). A simple approach to make an antibody to be more humanized is the replacement of the constant domains of the antibody with constant domains of a human antibody. The resulting chimeric antibody contains only the variable regions of murine origin and would therefore be expected to be less immunogenic in people. Many chimeric antibodies have been prepared and shown to retain the full antigen binding ability of the parent murine antibody as well as taking on the constant region effecter functions of the human antibody used (Waldmann&Morris, 2008)**.** Humanization of rodent antibodies can be taken further to produce fully humanized antibodies in the form of reshaped, engineered human antibodies, in which much of the variable domain sequences are also replaced by human antibody sequence. In these approaches, the antigen binding loops are derived from the rodent antibody and much of the supporting framework is humanized (Waldmann&Morris, 2008)**.** Targeting agents that have been approved for breast cancer include trastuzumab and pertuzumab directed against human epidermal growth factor receptor 2 (HER2) and bevacizumab, directed against vascular endothelial growth factor (VEGF) (Goldsmith & Signore, 2010). Several other targeting agents are currently under evaluation in preclinical and clinical trials (Carl & Roland, 2001).
