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

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

**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 and gastrointestinal malignant cells [72].

#### **5.3 Steroid hormone receptor**

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 estrogen receptors in breast carcinogenic cells [73].
