Tcessential compounds

There are a significant number of different Tc complexes that have been employed as radiotracers.12,13 Compounds of Tc(V) are particularly prominent as they are readily prepared in water from TcO^ via SnCl2 reduction. Ceretec™, for example, is a Tc(V) complex of the hexamethylpropyleneami-neoxime (HMPAO) ligand, which is used to image cerebral blood flow (Figure 18.1a).14 The HMPAO ligand forms a neutral square pyramidal complex with the {TcO}3+ core, which is a prominent feature in Tc(V) chemistry.15 Neurolite™ (Figure 18.1b) is similarly used to image cerebral blood flow and consists of a chelate, 1-ethylcysteinate dimer (ECD), that is made up of two cysteine ethyl ester units which form a neutral complex with the {TcO}3+ core.16 The square pyramidal complex freely crosses the blood-brain barrier whereupon an ester group of one of the isomers of Tc-ECD is hydrolyzed by an esterase, which in turn prevents premature efflux of the imaging agent from the brain.

Figure 18.1 Tc(V) Chelate complexes r^

n'OX

Figure 18.1 Tc(V) Chelate complexes

The 99mTc complex of mercaptoacetyltriglycine (MAG3) (Figure 18.1c) is used as an agent to assess kidney function.17,18 Like Ceretec™ and Neurolite™, TcMAG3 forms a square pyramidal complex with Tc(V) with the basal plane consisting of the three nitrogen and one sulfur donor atoms. The carboxylic acid group, which can be used as a site to link targeting agents, does not coordinate to the metal center and is believed to help facilitate excretion via the kidneys.

One of the most important roles Tc radiopharmaceuticals play in diagnostic medicine is in helping to assess cardiac function. The structures of approved cardiac imaging agents vary significantly and involve coordination to different Tc cores. These include the technetium dioxo complex [TcO2{P(R)2CH2CH2P(R)2}2]+ (Myoview™),19 which has an approximately octahedral geometry about Tc (Figure 18.2a), and the Tc(III) complex, 99mTc-teboroxime (CardiotecTM), which contains boronic acid adducts of oxime type ligands. Tc-teboroxime is formally a seven-coordinate-complex containing a labile chloride as the seventh ligand.20

Technetium isonitrile complexes of the type [Tc(CNR)6]+ are also widely used to image heart function.21 The hexakis-((2-methoxy-2-methyl-1-propyl) isonitrile) complex of Tc(I) (Figure 18.2b, Tc-Sestamibi, Tc-MIBI or CardioliteTM)

Figure 18.2 (a) 99mTc-teboroxirne (Cardiotec™) (b) Tc-Sestamibi (Cardiolite™)

Figure 18.2 (a) 99mTc-teboroxirne (Cardiotec™) (b) Tc-Sestamibi (Cardiolite™)

is used for myocardial perfusion imaging studies and breast cancer imaging (under the name Miraluma™).22 99mTc-Sestamibi is indicated for detecting coronary artery disease by localizing myocardial ischemia and infarction and for evaluating myocardial function. MiralumaTM is used as a second-line screening method after mammography for patients who have had an abnormal mam-mogram.

The preparation of Tc-isonitrile complexes in water23 deserves special mention as it is recognized as a milestone in Tc chemistry. [Tc(CNR)6]+ complexes, unlike most organometallic compounds, can be prepared in high yield in aqueous solutions where the concentration of the metal is extremely low. Tc-Sestamibi is currently prepared using a kit which consists of, amongst other components, a tetrakis(isonitrile) Cu(I) complex as the source of the isonitrile ligands and stannous chloride as the reducing agent. The product can be prepared in high yield by heating the kit with 99mTcO4 in boiling water for 10min. Tc-Sestamibi was the first organometallic compound to be approved for routine clinical use as a diagnostic agent.

Another prevalent use of Tc is for the detection of cancer metastases to bone. Tc complexes of phosphonate ligands are routinely used to image increased calcium ion concentrations associated with stress to bone.24 The mechanism of localization is believed to be associated with the interaction of the ligand hydroxyl groups with free calcium ions. One of the most widely employed agents is based on the methylenediphosphonate (MDP) ligand. The exact structure of the product(s) produced when MDP is labeled with 99mTc is not known conclusively.25,26

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