There are about 200 radioisotopes that are in current medical use. Radioisotopes can be employed as either radiation sources, for radiotherapy applications, or as radioactive tracers, which are commonly used as diagnostic agents.
Biomedical
applications of radiopharmaceuticals
There
are about 200 radioisotopes that are in current medical use. Radioisotopes can
be employed as either radiation sources, for radiotherapy applications, or as
radioactive tracers, which are commonly used as diag-nostic agents. They are
also used to determine the biodistribution of particu-lar compounds (in which
those radioisotopes are incorporated).
Use
of radioisotopes as tracers and diagnostic agents depends on the ease of
detection of the radiation emitted by the isotope and the ability of the
isotopic element to be incorporated into the molecule that is being traced
(such as during the biodistribution studies of new drug candi-dates). As
diagnostic agents, radioactive elements are typically adsorbed or incorporated
on a carrier. Thus, chemical identity, ability to use dur-ing synthesis, and
the form of the radioisotopes are important for their applications as tracers
and diagnostic agents. Diagnostic uses of radio-isotopes can be exemplified by
thyroid function studies using low dose 131I, erythrocyte tagging
for identification of type of anemia using 51Cr, and metabolic
studies using 14C. The 14C radioisotope detection in
breath can be used to detect the presence of ulcer-causing bacteria Helicobacter pylori. There is an increasing preference for the use of
nonradioactive methods of analyses,
wherever possible, due to the handling risks associated with radioactive
isotopes.
For
the use of radioisotopes as diagnostic agents and tracers, the dose of
radiation administered to the patients or normal healthy volunteers should be
as low as possible, while maintaining accuracy and sensitivity of analytical
detection. Thus, the radioisotopes for diagnostic use should ideally be
compounds with low half-life that exhibit rapid elimination kinetics and are
administered in low doses. Typically, radioisotopes that emit gamma rays are
used for diagnostic use, since gamma rays are the most penetrating; the
radiation does not stay in the body and is quickly received by the detector.
Specialized analytical methods are often devel-oped to analyze low
concentration of radioisotopes in plasma and tissue samples.
Technetium-99m
(99mTc)—a metastable nuclear isomer of technetium-99 (99Tc),
with a half-life of about 6 hours and biological elimination half-life of about
1 day, is the most common radioisotope used in medicine. 99Tc is sourced at the
hospitals from its more stable and easily transportable parent isotope,
molybdenum-99 (99Mo), with a half-life of about 66 hours, in lead
containers. The hospital extracts and uses the needed quantities of 99mTc,
as 99Mo degrades to 99mTc.
Use
of radioisotopes as radiation sources for radiotherapy aims to utilize the
tissue damage that results from radiation to, for example, reduce the amount of
cancerous tissue. Selection of radioisotopes as radiotherapy agents depends
mainly on the type and energy of radiation emitted by the isotope and its depth
of tissue penetration. Chemical identity and reactivity are of relatively less
importance for radiotherapy applications.
Radioisotopes
used for therapy can be applied to the target tissue either from an external
source or on administration to the patient as a drug.
1. External source application of radiation has the
advantages of dura-tion and amount of dose titration, with direct observation
of the target tissue, and of being able to remove the radiation source—and
terminate treatment—at any time. Radioisotopes used for external therapy are
exemplified by cobalt (60Co) and cesium (137Cs). They
have been used for the treatment of undesired lesions.
2. Internal application, or administration of the
radiotherapy agent to the patient, has a limitation that the source of
radiation cannot be removed once administered. Therefore, the amount of
radio-isotope administered to the patient must be carefully controlled.
Radioisotopes that have been used for internal therapy include gold (198Au),
iridium (192Ir), phosphorus (32P as sodium phosphate),
yttrium (90Y), iodine (131I as sodium iodide), and
palladium (103P).
·
Colloidal gold (198Au) suspensions have been used
in the cases of fluid accumulation in the abdomen (peritoneal cavity) or chest
(plural cavity), associated with malignant tumors. The colloidal suspension
diffuses throughout the fluid and, over time, tends to aggregate at the surface
of the cavity.
·
Nylon ribbons containing iridium (192Ir) seeds at
periodic inter-vals can be implanted into the interstitial cavity, such as
abdom-inal, for the treatment of tumors. These ribbons are surgically
removable.
·
Radiophosphorus (32P) can be injected parenterally
as a solution of a highly soluble sodium salt. The phosphorus tends to
accumu-late in rapidly proliferating cells and tissues. Accordingly, it has
been used for the treatment of polycythemia vera (too many red blood cells
produced by the bone marrow) and chronic granulo-cytic or myeloid leukemia (too
many blood cells produced by the bone marrow). At relatively high doses (1.5–5
mCi), 32P accu-mulates in the bone marrow and can suppress the
production of blood cells.
·
Yttrium (90Y) has a strong affinity for chelating
agents, which can be used for targeting carriers. Yttrium chelate with pentetic
acid or diethylenetriaminepentaacetic acid (DTPA) can be used for localization
to the lymphatics, and its chelate with ethylenediami-netetraacetic acid (EDTA)
can lead to localization in the bone.
·
Iodine (131I), used as a water-soluble salt,
sodium iodide, is per-haps the most commonly known radioisotope used for the
treat-ment of goiter, Graves’ disease, and thyroid cancer. Iodine is
selectively taken up by the thyroid gland in the neck. The uptake of
radioactive iodine can cause localized tissue destruction by radiation produced
within the gland. Targeted uptake of 131I by select tissues can be
achieved by incorporation into compounds such as metaiodobenzylguanidine
(mIBG). The 131I-labeled mIBG is selectively taken up by the adrenal medullary
tissues and can be used to treat carcinomas of or metastases from the adrenal
medullary glands.
·
Radioimmunotherapy is the targeting of radioisotopes to
spe-cific cells, tissues, and tumor types by covalent conjugation of a
radioisotope to monoclonal antibodies or their antigen-binding fragments. For
example, 131I can be conjugated to antibodies by using
N-hydroxysuccinimide (NHS) to produce radiolabeled antibodies. Copper isotope, 67Cu,
can be conjugated to anti-bodies by using the chelating agent
[6-p-nitrobenzyl]-l,4,8,ll-tetraazacyclotetradecane-N, N′, N″, N‴ tetraacetate
(TETA) for radioimmunotherapy.
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