**11.2 Introduction of a foreign label**

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

#### **11.3 Biosynthesis**

with γ-emitting radionuclides (indium-111 and technetium-99m) along with their

The radiolabeling of antibiotics, drugs, peptides, proteins and organic species with different radiotracer has increased reasonably from imaging point of view in medical, biochemical and other associated fields. In the field of medical imaging, compounds are labeled with two types of radionuclides: (a) compound labeled with those radionuclide that emitted the gamma radiation and have large number of application and especially used for in vivo imaging of a number of organs and (b) secondly, the compounds are labeled with radionuclide that emitted the *β*-radiation and have limited in vitro study and therapeutic treatment of the disease site. During the labeling process of a compound with a radiotracer, atoms or group of atoms of compound are replaced by different or similar atoms or group of atoms of the radiotracers [13]. In order to obtain, certain type of the labeling, the labeling process is carried out under constant conditions of temperature, pressure and incubation time. There are mainly six methods for labeling of the compound with

Isotopic exchange Labeling of the compounds with C-14, S-35, I-135 labeling of T3 and T4 and H-3.

> Tc-99m labeled radiopharmaceuticals Labeling of the hormones with I-125 Labeling of the cells with In-111 F-18 fluorodeoxyglucose

Labeling of the compounds with Br-77 from Kr-77 decay

In-111 DTPA albumin Tc-99m DTPA antibody

Co-57 cyanocobalamin Se-75 selenomethionine Excitation labeling Labeling of the compounds with I-223 from Xe-123 decay

Compounds label with H-3

Introduction of foreign label Labellng of the proteins with I-125.

Biosynthesis Labeling of the compounds with C-14

**11. Direct method labeling without bi-functional chelating agent**

In this method, some atoms from the compound which is to be labeled is replaced by isotope of the same atom of the element having different atomic mass (more or less) such as I-123, I-124, I-125, I-127, and I-131. the compound is labeled with isotope of the same element so the compound to be labeled and radiolabeled

or metal cheater. These are discussed below.

Recoil labeling Iodinated compounds

*Methods for labeling of the compound with radiotracers [13].*

**11.1 Isotopes exchange labeling**

**Table 7.**

**12**

In this type of labeling process, there is no need of bi-functional chelating agents

sensitivity and imaging purpose are shown.

**10. Methods of radiolabeling**

*Medical Isotopes*

radiotracer as shown in **Table 7**.

Labeling with bifunctional Chelating Agent

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*.

#### **11.4 Recoil labeling**

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 energy of recoil atoms gives poor yield.

#### **11.5 Excitation labeling**

Radioactive and very reactive daughter ions that are produced by nuclear decay 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 protein. Main disadvantage of this method is poor yield.
