Download Arsenic isotopes in biology As and As are important radioactive

Stable
isotope
75
As
Relative
atomic mass
74.921 595
Mole
fraction
1
Arsenic isotopes in biology
73
As and 76 As are important radioactive tracers used in environmental and biomedical studies to
quantify arsenic uptake [266]. 74 As has been used to investigate the biotransformation
(modification of a chemical compound by an organism) of arsenate by mammals. In one study,
rabbits were injected with 74 As-labeled arsenate. After a given amount of time, blood and blood
products were sampled and tested for the presence and quantity of labeled arsenate metabolites
[266]. Inhalation of dust or smoke containing 74 As is thought to be a causal agent of lung cancer.
In one study [267], the “absorption rate from the bronchial tree (a respiratory tract conducts air
into the lungs) was rapid for the first several days and then tapered off slowly. In three patients
an average of 45 per cent of the inhaled arsenic was eliminated in the urine in 10 days and ~0.5
per cent in the stools. The remainder must be assumed to have been deposited in the body,
exhaled, and/or eliminated in body secretions and excreta over a long period of time.” See Figure
1.
Fig. 1: Combined urine and fecal elimination of inhaled 74 As over a 10-day period. The ratio of
urine to fecal elimination was approximately 30 to 1 (modified from [267]).
Arsenic isotopes in medicine
72
As and 74 As are useful in molecular imaging because they are radioactive isotopes that emit
positrons that can be designed to bind to monoclonal antibodies (moAb), which accumulate in
tumors and then 72 As- or 74 As-labeled ligands will bind to the moAbs. Once the 72 As- or 74 Aslabeled ligand binds to the moAb, positron emission tomography (PET) can be used to
visualize the exact location of the tumor [265, 268]. A specific example of using radiolabeled
antibodies for better imaging of tumors is the combination of 74 As with bavituximab, which is an
antibody that binds strongly to unique lipids on the surface of tumors. When a thiol group is
introduced to bavituximab, arsenic is able to bind covalently, creating a simple and elegant
radio-label for targeting cancerous tumors [265, 268].
Glossary
atomic number (Z) – The number of protons in the nucleus of an atom.
covalently - share electron pairs between atoms. [return]
CT scan (X-ray computed tomography or X-ray CT, computerized axial tomography scan
or CAT scan) – a computerized tomography (CT) scan combines a series of X-ray images taken
from different angles and uses computer processing to create cross-sectional images, or slices, of
the bones, blood vessels and soft tissues inside your body [702].
electron – elementary particle of matter with a negative electric charge and a rest mass of about
9.109 × 10–31 kg.
element (chemical element) – a species of atoms; all atoms with the same number of protons in
the atomic nucleus. A pure chemical substance composed of atoms with the same number of
protons in the atomic nucleus [703].
gamma rays (gamma radiation) – a stream of high-energy electromagnetic radiation given off
by an atomic nucleus undergoing radioactive decay. The energies of gamma rays are higher
than those of X-rays; thus, gamma rays have greater penetrating power.
half-life (radioactive) – the time interval that it takes for the total number of atoms of any
radioactive isotope to decay and leave only one-half of the original number of atoms.
isotope – one of two or more species of atoms of a given element (having the same number of
protons in the nucleus) with different atomic masses (different number of neutrons in the
nucleus). The atom can either be a stable isotope or a radioactive isotope.
ligand – the atoms or groups joined to the central atom [703]. [return]
monoclonal antibodies (mAb or moAb) - identical immune cells that are all clones of a unique
parent cell, in contrast to polyclonal antibodies, which are made from several different immune
cells. [return]
neutron – an elementary particle with no net charge and a rest mass of about 1.675 × 10–27 kg,
slightly more than that of the proton. All atoms contain neutrons in their nucleus except for
protium (1H).
positron – the antimatter counterpart of the electron, with a mass identical to that of the electron
and an equal but opposite (positive) charge. [return]
positron emission tomography (PET) scan – an imaging technique that is used to observe
metabolic activity within the body. The system detects pairs of gamma rays emitted indirectly
by a radioactive isotope used as a tracer, which emits positrons and which is introduced into the
body on a biologically-active molecule. Three-dimensional images of the concentration of the
radioactive isotope within the body are then constructed by computer analysis. The imaging
often is performed with an X-ray CT scan in the same instrument. [return]
proton – an elementary particle having a rest mass of about 1.673 × 10–27 kg, slightly less than
that of a neutron, and a positive electric charge equal and opposite to that of the electron. The
number of protons in the nucleus of an atom is the atomic number.
radioactive decay – the process by which unstable (or radioactive) isotopes lose energy by
emitting alpha particles (helium nuclei), beta particles (positive or negative electrons), gamma
radiation, neutrons or protons to reach a final stable energy state.
radioactive isotope (radioisotope) – an atom for which radioactive decay has been
experimentally measured (also see half-life). [return]
radiolabeled – a mixture of an isotopically unmodified compound with one or more analogous
radioactive isotopically substituted compound(s). [return]
stable isotope – an atom for which no radioactive decay has ever been experimentally measured.
thiol – any of various compounds having the general formula RSH, which are analogous to
alcohols, but in which sulfur replaces the oxygen of the hydroxyl group and which have
disagreeable odors [718]. [return]
tracer - substance used for tracking purposes. [return]
X-rays – electromagnetic radiation with a wavelength ranging from 0.01 to 10 nanometers—
shorter than those of UV rays and typically longer than those of gamma rays.
References
265. M. Jennewein, Lewis, M.A., Zhao, D., et al. Clinical Cancer Research. 14, 1377 (2008).
10.1158/1078-0432.CCR-07-1516
266. J. De Kimpe, Cornelis, R., Mees, L., and Vanholder, R. FUNDAMENTAL AND
APPLIED TOXICOLOGY. 34 240 (1996).
267. M. S. M. R. H. Holland, and H. C. Lanz. Cancer Research. 19 (11), 1154 (1959).
268. M. Jennewein, Hermanne, A., Mason, R. P., Thorpe, P. E., and Rösch, F. Accelerators,
Spectrometers, Detectors and Associated Equipment. 569 (2), 512 (2006).
702. M. Clinic. Test and Procedures: CT scan. 2016 June 22.
http://www.mayoclinic.org/tests-procedures/ct-scan/basics/definition/prc-20014610
703. I. U. o. P. a. A. Chemistry. Compendium of Chemical Terminology, 2nd ed. (the "Gold
Book"). Blackwell Scientific Publications, Oxford (1997).
718. Merriam-Webster. thiol. Merriam-Webster. 2016 June 22. http://www.merriamwebster.com/dictionary/thiol