Radiopharmaceuticals

Radiopharmaceuticals are radioactive drugs. They are used in hospitals mainly for the diagnosis and, to a limited but growing extent, for the therapy of diseases.

Radiopharmaceuticals for diagnosis are labelled with a radioisotope (also termed radionuclide or radioactive atom) that decays with the emission of electromagnetic radiation, i.e. gamma- or roentgen (= X-) radiation (or positrons, as the annihilation of a positron with an electron generates two gamma rays of 511 keV almost immediately after the emission of the positron). Gamma- and roentgen-radiation is of the same nature as radio waves, television-waves and light. As electromagnetic radiation has a high penetrating power and is absorbed only to a limited extent by tissues, the gamma or X-rays emitted after the administration of a diagnostic radiopharmaceutical in the patient´s body can be detected outside the body using a gamma camera or PET camera (PET = positron emission tomography). With the aid of powerful computer programs, this information is converted into scintigraphic images showing the distribution of the radioactive compound in the patient´s body. If the radiopharmaceutical is taken up and handled by a pathological tissue or organ to a different extent than by healthy tissues, the scintigraphic image shows the localisation and status of a particular disease, such as a tumour, metastasis, or infection. The images can also allow the evaluation of, for example, the functional status of an organ, the density of receptors at a particular site, or the levels of metabolism in some tissues.


Dual-head gamma camera	PET-camera

Sequential scintigraphic images (1 min/image) of kidneys and urinary bladder after intravenous administration of a radiopharmaceutical that is rapidly excreted by the kidney. The rate of uptake of radioactivity (tracer agent) in the kidneys and the rate of transfer from the kidneys to the urinary bladder permits the evaluatio of the functional status of the kidneys. This is, for instance, very useful for evaluation of the function of a transplant kidney.

Radiopharmaceuticals for treatment of a disease (therapeutic radiopharmaceuticals) are mostly labelled with a radionuclide that decays with the emission of β^--particles. Such β^--particles are in fact electrons and have a short range in tissue, thereby delivering all their energy in a small area. In this way, hyperfunctioning or malignant cells are killed or their function impaired.

For diagnostic radiopharmaceuticals, the half-life of the radioisotope should be short to limit the radiation dose and thus limit the biological effects of the ionising radiation. A number of radioisotopes used in diagnostic radiopharmaceuticals are listed with their half-lives in Table 1, below:

Table 1. Principal radionuclides used in diagnostic radiopharmaceuticals

Radionuclide	Symbol	Half-life	Mode of decay	Energy of gamma(s) (keV)
Technetium-99m	^99mTc	6.01 h	isomeric transition	141
Iodine-123	¹²³I	13.27 h	electron capture	159
Indium-111	¹¹¹In	67.31 h	electron capture	171, 245
Gallium-67	⁶⁷Ga	78.27 h	electron capture	91, 93, 185, 209, 300
Thallium-201	²⁰¹Tl	72.91 h	electron capture	69-71 (X-rays), 80 (X)
Krypton-81m	^81mKr	13.1 s	isomeric transition	190
Carbon-11	¹¹C	20.38 min	positron emission	511 (annihilation)
Nitrogen-13	¹³N	9.96 min	positron emission	511 (annihilation)
Oxygen-15	¹⁵O	122.24 s	positron emission	511 (annihilation)
Fluorine-18	¹⁸F	109.77 min	positron emission	511 (annihilation)

In view of the short half-life of some of these radionuclides, the preparation of diagnostic radiopharmaceuticals is usually carried out in the hospital radiopharmacy shortly before administration to the patient. This is especially true in the case of radiopharmaceuticals labelled with very short-lived radionuclides (such as the PET radionuclides carbon-11, nitrogen-13, oxygen-15 and to a lesser extent fluorine-18 and the gamma emitters iodine-123 and technetium-99m). Such radiopharmaceuticals are prepared using the eluate of a radionuclide generator (e.g. ^99mTc-labelled radiopharmaceuticals), labelling kits and/or radioactive and non-radioactive starting materials. For more information about generators and details of their eluates, see Decay properties of generators


Isolation of granulocytes from patient's blood for subsequent labelling with technetium-99m

Radiopharmaceuticals consisting of radiolabelled autologous blood cells taken from patients (erythrocytes, granulocytes, thrombocytes) are also mainly prepared "in house". The withdrawal of blood from a patient, isolation of the intended type of cells, their labelling with technetium-99m or indium-111, the reinjection of the radiolabelled cells and the clinical examination (acquisition of images) should be performed within a limited time period and ideally, all these actions should be done during a single visit of the patient to the Nuclear Medicine department.
Even radiopharmaceuticals that contain radionuclides with a sufficiently long half-life to allow the product to be delivered in a ready-for-use form usually require the preparation of individual patient doses before administration, except when individual doses are delivered to the Nuclear Medicine department by a central radiopharmacy.
Thus, one of the special features of all these radioactive pharmaceutical products is the need for much more handling prior to patient administration than is the case for other pharmaceuticals.

Back to Introduction

Shortcuts