Pharmaceutical Manufacturing Handbook: Production and

The terms tracer, radiotracer , and radiodiagnostic agent , although long used as
equivalent to radiopharmaceutical, should be avoided. The preferred and correct
term is radiopharmaceutical , as the other names can be confusing or do not clearly
show the nature of these compounds as pharmaceuticals.
The composition of radiopharmaceuticals is not constant as it varies with time
as the radionuclide disintegrates. Very often, the half - life of the labeled molecule is
so short that it must be readily prepared just before its administration to the patient.
This implies in many cases the use of “ semimanufactured ” , such as radionuclide
generators, precursors, and cold kits that are also considered a medicinal product
according to directive 2001/83/EC.
1.3.1.3 Ideal Characteristics of Radiopharmaceuticals
Radiopharmaceuticals should have several specifi c characteristics that are a combination
of the properties of the radionuclide used as the label and of the fi nal radiopharmaceutical
molecule itself. The radiopharmaceutical should ideally be easily
produced (both the radionuclide and the unlabeled molecule) and readily available.
The half - life of the radionuclide should be adequate to the diagnostic or therapeutic
purpose for which it is designed. It has to be considered that radiopharmaceuticals
disappear from the organism by a combination of two different processes. The biological
half - life (showing the disappearance of a radiopharmaceutical from the body
due to biological processes such as metabolization, excretion, etc.) and the physical
half - life (due to the radioactive decay of the radionuclide). The combination of both
parameters gives the effective half - life :
T
e
TT p b
=
T + T
p b
INTRODUCTION 61
where T e is the effective half - life, T p the physical half - life, and T b the biological half -
life. Radiopharmaceuticals should have an effective half - life adequate to the use for
which they are intended. It should be short (hours) for diagnostic radiopharmaceuticals
(not longer than the time necessary to complete the study in question) and
longer for therapeutic radiopharmaceuticals (most often days) as the intended
effect should have a suffi cient duration.
The type of decay of the radiopharmaceutical should also be adequate for its
intended use. Diagnostic radiopharmaceuticals should decay by γ emission, electron
capture, or positron emission, and never emit α or even β particles. On the contrary,
therapeutic radiopharmaceuticals should decay by α or β emission because the
intended effect is in fact radiation damage to specifi c cells.
Regarding the energy emission of diagnostic radiopharmaceuticals, the fi nally
produced γ rays should be powerful enough to be detected from outside of the body
of the patient. The ideal energy for nuclear medicine equipment is around 150 keV.
γ rays should be monochromatic and photon abundance should be high to decrease
the imaging time.
1.3.1.4 Radioactive Decay
Radionuclides are unstable nuclei that are stabilized upon radioactive decay. More
than 2000 unstable nuclides have been described so far, most of them radioactive.