**5.1 Somatostatin receptors**

Naturally, existing somatostatin (SST) complexes of peptide formation are of two types. One with 14 amino acids (SST14) and other with 28 amino parts and designated as SST28. In human beings, SST receptors have been recognized on cell surface of neuroendocrine region and on lymphocytes. Seglitide and somatuline are synthetically prepared somatostatin analogs with more stability than SST14 because the latter one join SSTR sub-types with equivalent association. To image growing SSTR cells, iodine labeled octreotide radioactive tracer pioneered the functioning of radiolabeled peptides [69] (**Figure 18**).

**111**

*Localization Mechanisms of Radiopharmaceuticals DOI: http://dx.doi.org/10.5772/intechopen.94099*

• **111In-DTPA-pentetreotide**was developed with specific amounts of 111In (10 μg) but showed short residing time as released from body circulation via kidneys (approximately 50% in 5 h). After that, **90Y-DOTA-octreotide** was formed with

• **99mTc-P829 peptide** exhibited peculiar binding ability with SSTR types showing the potential to image rapidly growing cells. The radioactive product is

A 28 amino acid neuroendocrine moderator with diverse variety of biological activity in various cells and tissues. Although, receptors of cell membrane are extensively present along gastrointestinal tract. However, few receptor cells found on adenocarci-

**123I-VIP**was formulated with extraordinary activity ratio of radionuclide of about 200 MBq/μg of product to be specifically localized in liver, lung, pancreatic

Steroid chemicals such as estrogen and progesterone have high binding affinity with intracellular receptors which are used to identify hormone dependent proliferating cells in case of breast cancer. Various types of hormonal analogs radiolabeled with PET imaging radiotracers**. 18F-fluoro-estradiol (FES),** with specificity for estrogen hormone, has revealed greater potential for analyzing metastatic tumor cells. **123I-methoxy-iodovinylestradiol** (radioactive ligand) was used to diagnose

Plasma lipoprotein transports cholesterol towards adrenal gland that act as the

substrate for synthesis of steroid hormone. **131I-iodomethyl-norcholeterol** 

the similar capacity to bind proliferating cells revealing SSTR type 2.

nomas, malignant tumors, neuroblastomas, and pancreatic cancerous cells.

FDA approved to image lung cancerous cells [71].

*Schematic representation of radiotheranostics using radiolabeled peptides [70].*

**5.2 Vasoactive intestinal peptide (VIP) receptor**

estrogen receptors in breast carcinogenic cells [73].

**5.4 Low density lipoprotein (LDL) receptors**

and gastrointestinal malignant cells [72].

**5.3 Steroid hormone receptor**

**Figure 18.**

*Localization Mechanisms of Radiopharmaceuticals DOI: http://dx.doi.org/10.5772/intechopen.94099*

**Figure 18.**

*Medical Isotopes*

**110**

antibodies.

**Figure 17.**

**5. Specific receptor binding**

**5.1 Somatostatin receptors**

radiolabeled peptides [69] (**Figure 18**).

Receptors are attachment sites for ligand molecules i.e. polypeptide hormones and neurotransmitters. In case of antigen–antibody complex formation, antigen expression on cell surface is considered a definite receptor site for antibody binding. Antigen fragments are present on the upper surface and within the cells or sometimes released into body fluids. Localization of definite receptor-binding radiotracers depends upon multiple factors such as blood clearance, affinity of the tracer & receptor and blood flow of the tumor tissue. Receptors are of various types with specificity of basic compound including peptides, steroid hormones, and

*Pathophysiological mechanisms of significant radiotracers used in investigation of malignant cells [67].*

Naturally, existing somatostatin (SST) complexes of peptide formation are of two types. One with 14 amino acids (SST14) and other with 28 amino parts and designated as SST28. In human beings, SST receptors have been recognized on cell surface of neuroendocrine region and on lymphocytes. Seglitide and somatuline are synthetically prepared somatostatin analogs with more stability than SST14 because the latter one join SSTR sub-types with equivalent association. To image growing SSTR cells, iodine labeled octreotide radioactive tracer pioneered the functioning of *Schematic representation of radiotheranostics using radiolabeled peptides [70].*

