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125I-Single-chain antibody B1 targeting a pancreatic β-cell surface antigen.

Authors

Leung K1.

Source

Molecular Imaging and Contrast Agent Database (MICAD) [Internet]. Bethesda (MD): National Center for Biotechnology Information (US); 2004-2013.
2012 Feb 02 [updated 2012 May 17].

Author information

1
National for Biotechnology Information, NLM, NIH, Bethesda, MD

Excerpt

The β-cell mass (BCM) is a useful indicator of the function of the pancreas. In type I diabetes, β-cells are destroyed, leading to a dramatic decrease in BCM. In type II diabetes, there is a slow and continuous decrease in BCM because of peripheral insulin resistance and increased demand for insulin. Vesicular monoamine transporter (VMAT2) is present in brain monoaminergic neurons and is responsible for collecting neurotransmitters (e.g., dopamine, norepinephrine, and serotonin) from the cytoplasm and storing them in vesicles for synaptic release (1). VMAT2 is also highly expressed by β-cells in the endocrine pancreas but not in the exocrine pancreas. VMAT2 is expressed mainly on the β-cells in the islets of Langerhans and comprises 1%–2% of the pancreatic mass. VMAT2 is a potential target for noninvasive assessment of BCM and pancreatic function. VMAT2 has been studied noninvasively in vivo with positron emission tomography (PET) using [11C]dihydrotetrabenazine (2-hydroxy-3-isobutyl-9-[11C]methoxy-10-methoxy-1,2,3,4,6,7-hexahydro-11bH-benzo[a]quinolizine, also known as [11C]DTBZ) in the human brain and pancreas. However, a human clinical study showed that the high nonspecific binding in the exocrine pancreas may appear to overestimate BCM on pancreatic [11C]DTBZ PET scans (2). Glucagon-like peptide-1 receptor (GLP-1R) has been shown to be highly overexpressed in human insulinomas and gastrinomas and has been evaluated with various noninvasive imaging studies of β-cells (3-5). Other β-cell imaging probes have also been studied (6-8). Brogren et al. (9) prepared a rat monoclonal autoantibody (IC2) with lymphocytes isolated from the spleen and lymph nodes of a spontaneously diabetic BB rat. IC2 is a rat IgM kappa monoclonal antibody. The binding of IC2 to β-cells is sensitive to protease treatments. IC2 shows little binding to other rat tissues; however, IC2 exhibits binding to mouse, hamster, and human β-cells (8). Moore et al. (10) showed 111In-diethylenetriamine pentaacetic acid-IC2 (111In-DTPA-IC2) binding to β-cells in normal mice and reduced binding to β-cells in mice treated with streptozotocin (STZ) to induce BCM loss and diabetes. However, the pharmacokinetics of intact radiolabeled mAb, with high liver uptake and slow blood elimination, are generally not ideal for imaging (11). Smaller antibody fragments, such as single-chain antibody (SCA), Fab', or F(ab')2, have better imaging pharmacokinetics because they are rapidly excreted by the kidneys (12). SCA comprises a single polypeptide in which the VH (variable heavy chain) and VL (variable light chain) domains are attached to one another by a flexible linker. Using phage-display screening of human SCA library with a rat β-cell insulinoma INS-1 tumor cell line, Ueberberg et al. (13) identified SCA B1 to be β-cell-specific. 125I-SCA B1 has been evaluated as a potential imaging probe for β-cells.

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