**9. Chemistry of indium and oxidation state for labeling**

Indium belongs to aluminum which are naturally occurring transition metals. Its chemical and physical properties are enlisted in **Table 5**. Indium is a soft silvery white metal which is not found in free elemental form but found in the form of combined state such as halides InCl3, InBr3 InI3 and InF3, sulphide and oxide (In2O3). Indium exists in three oxidation state +3, +2 and +1 but indium in +3 oxidation state it appears more stable. Thirty-nine isotopes of indium have been reported but only three isotopes such as indium-111, indium-113 and indium-115 are commonly found. Indium-111 with half-life of 66.32 hours are used in radiopharmaceutical for imaging purpose


*Single-Photon Emission Computed Tomography (SPECT) Radiopharmaceuticals DOI: http://dx.doi.org/10.5772/intechopen.93449*


#### **Table 5.**

which is used to reduce the pertechnetate 99mTcO4 from +7 oxidation state to lower oxidation state +4 (TcO2.xH2O) is zinc with HCl. However, 20% of technetium reduces to technetium metal by this method. In technetium-99mhydroxyethylidene diphosphonate (HEDP) complex, it is observed that the oxidation state of technetium is changeable which is highly dependent upon the pH of the method which is used to synthesize the complex. In acidic medium, the oxidation state of technetium is +3; in alkaline medium, it is +5; and in neutral medium, it is +4 [11]. This means that a slight change in pH can change the oxidation state of technetium pointing to the fact that they may exist as a mixture of all oxidation states like +3, +5 and +4 in technetium-99m-

• Complex of technetium in +3 oxidation state (Tc+3). A number of technetium-

ethylenediamine tetraacetic acid (EDTA), DMSA (dimercaptosuccinic acid) and hepatobiliary iminodiacetic acid. However, the oxidation state of

• Complex of technetium in +1 oxidation state (Tc+1). This oxidation state is stabilized with the help of coordinate covalent bond with different types of ligands in aqueous medium. In this oxidation state, compounds are usually

Indium belongs to aluminum which are naturally occurring transition metals. Its chemical and physical properties are enlisted in **Table 5**. Indium is a soft silvery white metal which is not found in free elemental form but found in the form of combined state such as halides InCl3, InBr3 InI3 and InF3, sulphide and oxide (In2O3). Indium exists in three oxidation state +3, +2 and +1 but indium in +3 oxidation state it appears more stable. Thirty-nine isotopes of indium have been reported but only three isotopes such as indium-111, indium-113 and indium-115 are commonly found. Indium-111 with half-life of 66.32 hours are used in radiopharmaceutical for imaging purpose

> ,2s<sup>2</sup> ,2p<sup>6</sup> ,3s<sup>2</sup> ,3p<sup>6</sup>

,3d10,4s2

,4p<sup>6</sup>

,4d10,5s2

,5p<sup>1</sup>

technetium in the complex EDTA and DTPA become +4 in alkaline as well as in neutral medium. A variety of technetium complexes in which technetium exists in +3 oxidation state are used for myocardial scanning. These include complexes of technetium-99m with phosphine, arsine and BATOs (boronic

99m complexes exist with +3 oxidation state in acidic medium. These complexes include DTPA (diethylenetriamine pentaacetic acid,

hydroxyethylidene diphosphonate (HEDP) complex.

acid adduct of technetium dioxime comples).

**9. Chemistry of indium and oxidation state for labeling**

**Properties of indium Values** Atomic number 49 Atomic mass (amu) 114.818

Density gm/cm<sup>3</sup> (at 25°C) 7.31

Melting point in Kelvin 429.75 Boiling point in Kelvin 2353.15 Occurrence Solid state (naturally)

Electronegativity 1.78

Oxidation state +1,+2,+3, (+3 more stable)

Electronic configuration 1s<sup>2</sup>

**10**

stable in water and air.

*Medical Isotopes*

*Physical and chemical properties of indium.*

[12]. γ-radiation emitted by indium-111 have an energy of 247 keV and 172 keV and the percentage of γ-radiation emitted by indium-111 is 90.6% with minimal β-radiation emission that make the indium �111 a good imaging radiotracer.

These γ-emitting radionuclide labeled compounds can be utilized to identify the exact position and location of the infection in different parts and organs such as brain, arteries, joints, bones and tissues. In **Table 6**, a number of compounds bound


#### **Table 6.**

*General radiopharmaceuticals developed based on SPECT imaging.*

with γ-emitting radionuclides (indium-111 and technetium-99m) along with their sensitivity and imaging purpose are shown.

are similar in biological properties, except for the energy emitted from different isotopes of the same element which is used for labeling [14]. This method used for in vitro study. Examples of isotope exchange labeling reactions are labeling of the triiodothyronine (T3) with I-125, labeling of thyroxine with I-125, and labeling with

*Single-Photon Emission Computed Tomography (SPECT) Radiopharmaceuticals*

In this process of labeling, a molecule of known biological function is labeled with a radionuclide. This labeling occurs by forming covalent bond or co-ordinate covalent bond. The attaché radiotracer is unknown (foreign) to the molecule, and labeling does not occur due to the exchange of its isotope. In most of these types of compounds, chelation is the cause for bond formation. In such bonds, more than one atom donates a pair of electrons to the foreign acceptor atom that is mostly a transition metal. Majority of Tc-99m labeled compounds are developed by this

The biosynthesis method involves the growth of the microorganisms in a culture medium that contains the radiotracer. When microorganisms (bacteria) grow in such a medium, the radiotracer is introduced into the metabolites that are produced by the metabolic activity of the organism. This metabolite is then chemically separated. Example of such product is preparation of 57Co-B12 by using a bacterium *Streptomyces griseus*.

It is of limiting interest and cannot be preceded on large scale for labeling because it has low specific activity of the bounded molecule. The method involves generation of recoil ions or atoms as particles are emitted by the nucleus. These generated atoms or ions then form a bond with the targeted molecule. This high

Radioactive and very reactive daughter ions that are produced by nuclear decay

A chelating agent is a substance that has the ability to form multiple bonds with a single metal ion, thus acts as a multidendate ligand. Bi-functional chelating agent is that which has two are more separate covalent or coordinate covalent bonds with a ligand which is polydendate in nature. The labeling process using bi-functional chelating agent involves the bond formation at two sites: one bond is formed by the bi-functional chelating agent with macromolecule such as protein and antibody and

process are used in excitation labeling process. In β-decay and electron capture processes, there is a production of highly energetic charged particle ions which have the ability to label the compound of interest. When Kr-77 undergoes the decay process, it yields Br-77. These (Br-77) energetic ions are able to bind the compound of interest when exposed to it [16]. A number of proteins are labeled with I-123 when protein is exposed to Xe-123 which decays into energetic I-123 and label the

**12. Indirect method labeling using bi-functional chelating agent**

process such as binding of Tc-99m with DTPA, gluceptate, etc.

C-14, S-35 and H-3 labeled compounds [15].

*DOI: http://dx.doi.org/10.5772/intechopen.93449*

**11.2 Introduction of a foreign label**

**11.3 Biosynthesis**

**11.4 Recoil labeling**

**11.5 Excitation labeling**

**13**

energy of recoil atoms gives poor yield.

protein. Main disadvantage of this method is poor yield.
