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Technetium Radiochemistry

The most commonly used radioisotope of technetium is tech-netium-99m, which is produced by molybdenum-99 decay in a molybdenum-99/technetium-99m generator. Technetium-99m (t1/2 = 6 hours) decays by IT to technetium-99 (t1/2 = 2.1 X 105 years) with principal y-ray emissions of 140 keV (100%). The radiotracer is collected as pertechnetate ions (99mTcO4-) from the generator by elution from the alumina column with sterile normal saline (0.9% sodium chloride). Long-lived radioactive impurities such as molybdate ions (99MoO4) that coelute from the generator column with pertechnetate ions (99mTcO4-) theoretically may shorten the manufacturer's recommended expiration time of each elution since the Nuclear Regulatory Commission (NRC) and United States Pharmacopeia (USP) limits molybendum-99 breakthrough to no more than 0.15 ^Ci of 99Mo per 1 mCi of 99mTc in the preparation.5 In clinical practice, molybendum-99 breakthrough is a rare occurrence.

Technetium, is a transition state metal and is the only "artificial" element with a lower atomic number than uranium. No stable isotopes exist in nature. All known isotopes of technetium are radioactive, and as such, has hindered the study of technetium's chemistry. However, milligram quantities of Tc-99 (a weak emitter; t1/2 = 2.1 X 105 years) are now available for characterization of technetium complexes. Technetium can exist in eight oxidation states, from -1 to +7. The +7 (99mTcO4-) and +4 state (99mTcO2, or hydrolyzed-reduced technetium-99m) are the most stable forms.4

The chemistry of technetium is similar to that of rhenium and is dominated by forming compounds by bonding between the electron-deficient metal and electronegative groups, which are capable of donating electron pairs. Some examples of these electronegative groups include amines, carboxylic acids, hydroxyls, isonitriles, oximes, phosphates, phosphines, and sulfhydryls. Various oxidation states (e.g., +1, +3, +5, etc.) are prepared in the presence of the reducing agent and stabilized by complexation with available ligands.

Tc-99m radiopharmaceuticals are by far the most commonly used radiotracers in day-to-day diagnostic nuclear medicine practice. Almost all technetium radiopharmaceuti-cals are metal-electron donor complexes. Compounds that contain two or more electron donor groups and bind to a metal are called chelating agents. As the pertechnetate (99mTcO4-) ion, technetium will not form metal-donor complexes. However, the pertechnetate ion can be reduced by a stannous salt or other methods to an oxidation state that will complex with various monodentate, bidentate, or poly-dentate ligands. The most common reducing agent used in clinical practice is the stannous ion.

Technetium-99m radiopharmaceuticals are prepared by combining sodium pertechnetate (Na99mTcO4) with nonra-dioactive components in a sterile reaction vial. The primary chemical substances in the vial are the complexing agent (ligand) and a stannous salt-reducing agent (stannous chloride, stannous fluoride, or stannous tartrate). The oxidation state of technetium in various complexes as well as the actual chemical structure of many radiopharmaceuticals has yet to be characterized. Several reviews of technetium chemistry are available for further study.17-20

After preparation of the radiopharmaceutical, tests for ra-diochemical purity should be carried out to ensure that the radiotracer is in the correct radiochemical form. Analytical quality control methods include paper and thin-layer chro-matography, column chromatography, and solvent extraction. Likely, radiochemical impurities include unreacted sodium pertechnetate (Na99mTcO4), hydrolyzed-reduced technetium (99mTcO2) colloid, technetium-tin colloid, and possibly radiochemical complexes different from the one expected (e.g., 99mTc-monodentate rather than 99mTc-biden-tate ligand, etc.). The sterile vials containing the stannous salt and the ligand are lyophilized under a sterile inert gas atmosphere (i.e., nitrogen or argon). The ligand in the reaction vial determines the final chemical structure of the 99mTc-complex and the biological fate after intravenous injection of the radiopharmaceutical.

thyroid, Meckel's diverticulum (stomach tissue in the intestine), salivary glands, and to detect processes that disrupt the blood-brain barrier (i.e., tumors, abscesses, strokes).

Like iodide, pertechnetate is trapped by the thyroid. Pertechnetate, however, cannot be utilized to prepare thyroid hormone. The patient receives an intravenous injection of 5 to 10 mCi (185-370 MBq) of Tc-99m pertechnetate, and thyroid images are obtained 20 to 30 minutes after injection. The usual dose for other imaging procedures such as Meckel's diverticulum and salivary gland imaging is also 5 to 10 mCi (185-370 MBq), whereas 20 mCi (740 MBq) is used for blood-brain barrier imaging.

Sodium Pertechnetate (99mTc)

Technetium ("mTc) Bicisate (Neurolite). Technetium (99mTc) bicisate is a neutral, lipophilic complex that crosses the blood-brain barrier and is selectively retained in the brain.

The kit used to prepare the product contains two vials. Vial "A" contains the ligand, tin, and other components that must be protected from light, whereas vial "B" contains a buffer solution. The radiopharmaceutical is prepared by adding sodium pertechnetate (99mTc) to the shielded "B" vial containing the buffer. The vial A containing lyophilized bicisate, EDTA, mannitol, and stannous chloride is reconstituted with 3 mL of normal saline (0.9% NaCl). Within 30 seconds, 1 mL of the saline solution from vial A is transferred to vial B and allowed to incubate at room temperature for 30 minutes. The tagged product must be used within 6 hours of preparation. The structure of the technetium complex is [N,N'-ethylene-di-L-cysteinato(3-)]oxo [99mTc]tech-netium(V) diethyl ester.21

After intravenous injection of 20 mCi (740 MBq) of Tc-99m bicisate, about 5% of the injected dose is localized within the brain cells 5 minutes after injection. Ester hydrolysis traps the radiotracer within the brain. This radiotracer is indicated for the evaluation of stroke, but it is used offlabel to clinically evaluate various brain disorders, especially brain perfusion ("brain death").

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