Clinical Radiopharmacy

Radiopharmaceutical or Radiotracer Properties

1. Must have the basic characteristics of any compound for injection in human beings (non-toxic, non-pyrogenic, sterile, etc.).
2. If adequate for use in clinical diagnosis should ideally be emitting only gamma rays and  in the range of 100 - 200 KeV.
3. The specific activity (Activity/gr) must be high, without adding non-radioactive carrier in order to obtain a high count average in small volume and mass injection.
4. Should ideally be soluble in water and remain soluble when mixed with body fluids. Lipid solubility is a determining factor in the biological distribution of radiochemicals, as it allows its diffusion through the cell membrane, due to the composition of it (phospholipids). The greater the lipid solubility the higher the diffusion across the cell membrane and therefore the greater its location in the organ under study.
5. The burden of a compound is a major determinant of solubility. A greater burden implies greater solubility in water. Nonpolar molecules tend to be solubilized in organic solvents and lipids.
6. Must be stable both in vitro and in vivo. The labeled chemical species must be stable for at least the minimum time of the exam. Preferably the stability or useful life of the radiochemical, should be determined by the physical half-life of the radionuclide.
7. The biological distribution of a radiochemical is the feature that allows to establish its utility for functional studies or collection of static image. In-vivo stability can be affected by binding to plasma proteins or cells after being injected. Preferably it should remain as a free compound and attach to proteins or cells only when concentrating on the target organ (organ of interest.)
8. Each radiopharmaceutical or radiotracer has a characteristic biological half-life. Once achieved its objective, the radiopharmaceutical should disappear from the body through excretion pathways and physical decay. This leads us to define the effective half-life of the radiopharmaceutical dependending on the biological and radionuclide physical half-life and determined by:


            Tf x Tb
Tef = ----------------
            Tf + Tb

The rate of radioactivity from the organ of interest (or the body) is directly proportional to the radionuclide physical decay (Tf) and biological excretion (Tb). A radionuclide that has a long physical half-life can be considered a good radiopharmaceutical as long as the biological half-life is relatively short and vice versa. The effective half-life should be short, ideally equal to 1-1.5 times the time needed to complete the study.
9. The relationship activity in the target tissue activity  versus in the non-target tissue must be high. The main objective of the radiopharmaceutical is to reach and locate in the organ of interest, since the activity concentrated in areas not under study may interfere with obtaining a good image.
10. The blood clearance should be high because it is directly related to the speed of concentration in the target, which influences the time required to perform the test.
11. Must be of low cost and readily available in Nuclear Medicine Centers.

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