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[99mTc]Epidermal growth factor receptor–specific nanobody .

Authors

Chopra A1.

Source

Molecular Imaging and Contrast Agent Database (MICAD) [Internet]. Bethesda (MD): National Center for Biotechnology Information (US); 2004-2013.
2008 Apr 22 [updated 2008 May 13].

Author information

1
National Center for Biotechnology Information, NLM, NIH, Bethesda, MD 20894, Email: micad@ncbi.nlm.nih.gov

Excerpt

A neoplastic tumor is the result of an altered expression of the proteins that regulate the survival and proliferation of a cell. The molecular changes associated with the development of tumors such as changed expression (up- or downregulation) of certain receptors or metabolic regulatory components in the cell are often used for the identification, diagnosis, and treatment of cancers or other pathological conditions. The molecular changes are usually detected with monoclonal antibodies (mAbs) developed against specific targets, and early detection of the cancer may result in the development of a suitable treatment and favorable outcome for the patient. To this end, mAbs may be labeled with radionuclides for noninvasive in vivo detection of the target tissue to diagnose a specific condition; techniques such as positron emission tomography and single-photon emission computed tomography (SPECT) are known to be sensitive enough to follow tumor binding and to image tumors with these radiolabeled mAbs (1, 2). However, the use of labeled mAbs has limitations because these molecules are relatively large and do not penetrate solid tumors; in addition they stay in blood circulation for a long time and tend to accumulate in the liver (3, 4). Investigators are constantly searching and evaluating alternatives to the conventional mAbs that have similar applications, but have superior production and stability characteristics. One alternative to the conventional antibodies is the antibody variable fragment (Fv), genetically engineered by linking the variable-heavy (VH) and -light (VL) chain components of an antibody, which are also involved in antigen binding, through a flexible peptide linker to obtain a single-chain Fv (scFv) (5). The scFv has its limitations in that the peptide linkers are susceptible to enzymatic digestion under in vivo conditions, may show a reduced affinity toward the target, and tend to aggregate easily (6). Another alternative to the use of conventional antibodies is the use of a unique class of immunoglobulins found in the Camelidae species (camels and llamas) that consists of only the heavy chain component of the traditional mAb. These immunoglobulins contain a single variable domain that interacts with the antigen (7). These are the smallest, functional, antigen-binding fragments that can be produced from an immunoglobin, and the domains are known as VHH or a nanobody. Nanobodies are more stable than conventional mAbs and are inexpensively produced in a variety of expression systems such as yeast, filamentous fungi, or even plants (8-10). Also, nanobodies have reduced aggregation compared to the scFv fragments, and they have been shown to have low immunogenicity in mice because repeated administration in these animals did not evoke an anti-nanobody response (11). The epidermal growth factor receptor (EGFR) has a characteristic overexpression in a variety of cancers such as those of the breast, ovaries, lungs, head and neck, bladder, and colon (12). EGFR is a 170-kDa transmembrane protein that promotes cell proliferation on binding of the autocrine epidermal growth factor (EGF) or the transforming growth factor α (TGFα), and EGFR operates through a receptor-associated tyrosine kinase–mediated signal transduction pathway. The progression of some cancers is attributed either to both organ increased level and activation of EGFR by EGF and TGFα or to a constitutive activation of the tyrosine kinase pathway caused by the development of a mutated receptor kinase. In an effort to develop therapy against cancers that are attributed to an increased EGFR activity or a mutated receptor kinase, a variety of EGFR inhibitors have been evaluated that either compete with EGF and TGFα for receptor binding (e.g., mAbs that target the receptor) or are small molecules that inhibit activation of the receptor tyrosine kinase signaling pathway (13). EGFR is the target of more than 350 clinical trials approved by the United States Food and Drug Administration. Roovers et al. (14) generated anti-EGFR nanobodies that were labeled with meta-stable technetium (99mTc), and these nanobodies were used by Huang et al. to investigate molecular imaging (15) and by Gainkam et al. to study biodistribution and tumor imaging (16) in mice bearing xenograft tumors.

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