|
|
| Fig. 1: Schematic of an antibody structure. (Source: Wikimedia Commons) |
Antibodies are large proteins produced by cells of the immune system to recognize, bind to, and coordinate the fight with pathogens such as bacteria, viruses, fungi, and parasites. Antibodies are made up of two heavy chains and two light chains of amino acid molecules coming together to form a Y shape (see Fig. 1). The two forks of the Y make up the variable region and determine what target, or antigen, the antibody binds to - whether thats a unique receptor on the surface of a bacteria, a cancer cell, or something else that scientists are interested in. The base of the Y is known as the constant region and is involved in binding with and communicating signals to immune cells in the area.
Antibodies can be engineered to have particular sequences and structures, allowing them to bind to particular targets. By fusing antibody-producing B-cells with tumor cells, biomedical researchers have created hybridomas that can mass-produce identical copies of these engineered antibodies as monoclonal antibodies. These monoclonal antibodies have a variety of important applications in studying, diagnosing, and treating disease.
One fascinating recent medical advance is the development of monoclonal antibodies that are fused with radioactive isotopes to form radiolabeled antibodies with unique applications in fields like cancer imaging. Two particularly important isotopes in medicine are I-123 and I-131. I-123 decays into Te-123 by electron capture and emits 28 keV and 149 keV γ rays. I-131 undergoes both β decay into Xe-131 and γ emission at 364 keV. [1]
When discussing nuclear medicine, three measures of half-life are often used: the physical half-life, which describes the time it takes for the amount of the isotope to decrease by half due to radioactive decay; the biological half-life, which describes the time it takes for the amount of isotope to decrease by half due to bodily excretion of the isotope; and the effective half-life, which combines the two measures. The physical, biological, and effective half-lives of I-131 outside of the thyroid compartment are 8 days, 12 days, and 8 days respectively. [2] In contrast, I-123 has a physical half-life of 13.2 hours - a far shorter decay time that allows it to be cleared more quickly from the body. [1]
Given their different decay patterns and behaviors, monoclonal antibodies fused with I-131 and I-123 have very different impacts on the body, which differentes their potential applications. γ radiation simply passes through cells relatively unimpeded, allowing them to be accurately detected by external instrumentation and used in an imaging context. On the other hand, β radiation can lyse thyroid cells, stun thyroid tissue, and cause high rates of mutation and cell damage. [3] Using an antibody fused with I-131 would expose internal tissues to β decay and potentially kill the cells, making it a potentially toxic imaging strategy but an effective therapeutic one that can be specifically delivered to cancer cells for elimination.
©Arjun Kumar. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] F. A. Mettler and M. J. Guiberteau, Essentials of Nuclear Medicine Imaging, 6th Ed. (Elsevier, 2012), Ch. 4.
[2] R. J. Amdur and E. L. Mazzaferri, Essentials of Thyroid Cancer Management (Springer US, 2005), p. 165.
[3] M. F. Villani et al., "Usefulness of Iodine-123 Whole-Body Scan in Planning Iodine-131 Treatment of the Differentiated Thyroid Carcinoma in Children and Adolescence," Nucl. Med. Commun. 39, 1121 (2018).
