**Abstract**

Scintigraphic techniques have opened a new era of developments in the localization of infectious and cancerous foci. Diseases area targeting mechanisms of radiopharmaceuticals encompasses visualization, characterization, and measurement of physiological and biological functioning at targeted sites in addition to measure the area and density of the disease. The accumulation of a radiopharmaceutical at specific organ is based upon numerous processes such as enzymatic interactions, receptor binding site, transport of chemical species and elimination of damaged cells from circulation by a normal metabolic process. PET and SPECT are developing scanning techniques that provides effective diagnostic tool to identify pathophysiology of diseased cells. In this chapter, we are exploring and explaining different mechanisms of radiopharmaceutical localization for imaging and therapeutic processes. The knowledge of these mechanisms will help to develop target based new radiopharmaceuticals using variety of medically used radioisotopes either for imaging or therapy of diseased cells.

**Keywords:** radiopharmaceuticals, SPECT imaging, antibiotics, infections, cancer

### **1. Introduction**

Dysfunction and disruption of healthy physiological and biochemical cycles lead to the formation of malignancies. As the treatment of many diseases involves biochemical reactions so it may provide a basis for diagnosis. Several radiopharmaceuticals widely available that use to visualize the functioning and structure of body cells, tissues, and organs. These radiopharmaceuticals formulated for treating several malignancies, pain palliation due to bony metastases, joint diseases, and many other similar conditions. Nuclear medicine basically, a medical specialty comprises a carrier molecule and radiotracer that image the regional biochemistry of the body. The biochemical nature of carrier molecule and radiotracer; effects on organ uptake, retention, transportation and biodistribution towards targeted area. So, it's essential to know about the biochemistry of radiopharmaceuticals for better understanding [1]. Nuclear pharmacists must understand how radiopharmaceuticals localize and initiate its work, aka action mechanism. This expertise was required to assess the substrate specific and non-specific nature of labeled drug, its pharmacokinetics and biodistribution because life matters. As the radiopharmaceuticals provide us an opportunity for timely diagnostics using blood flow,

multi-molecular cell localization, bio-energies, tissue metabolism, physiological functioning of the organ, intercellular and intracellular communicative pathways [2]. Different radiopharmaceuticals are used to image different organs based on the functioning of the organ. For example, the labeled iodine would be ideal for imaging thyroid malignancies, because inorganic iodine absorbed more in the thyroid. Similarly, radiolabeled phosphate widely used for the bone scan as it is observed that phosphate ions more accumulated in the bone. Hence one can use the same labeled atoms for organ imaging, which are more accumulated there.

A radiopharmaceuticals localization mechanism is specific to targeted organs depends on processes as varied as antigen–antibody reactions, physical particle trapping, receptor site binding, removal of deliberately damaged cells from circulation, and transportation of a chemical species across a cell membrane and into the cell via a normally operational metabolic cycle. Chemically, radiochemistry plays a crucial role in producing these compounds and in conducting quality assurance procedures to ensure purity [1, 3].

Some other factors also important for the selection and action of radiopharmaceuticals like for diagnosis gamma emitters were preferably choose (beta emitter in case of therapeutic), energy threshold 100–250 Kev, high T/NT ratio last but not least teff must be moderately long.

Furthermore, insoluble radiopharmaceuticals such as 99mTc-MAA and 99mTc-SC are used to represent the lungs and liver/spleen, diagnostic tests, respectively. Since it is well known that these two organs extract particles from the bloodstream, selection based entirely on particle size instead to chemical composition.

The mechanisms explained are not specific to radiopharmaceuticals, but these may be appropriate for some instances to explain the localization mechanisms of nuclear medicines. Radiopharmaceuticals are not limited to a mechanism but requires a combination of more than one mechanism. Lastly, a comprehensive overview of radiopharmaceuticals characteristics, their mode of action and detailed examples are given.
