Immunoscintigraphy

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The immunoscintigraphy is an imaging technique of nuclear medicine . It is used for diagnostic purposes primarily in oncology , but also to investigate inflammatory reactions . In this case, be radioactively labeled monoclonal antibodies or fragments of antibodies the patient administered with the aid of a gamma camera shown imagewise. The progress of the disease, for example metastasis , can be determined from the scintigram obtained, and the therapeutic intervention can be planned based on this. Immunoscintigraphy is a special form of scintigraphy .

principle

Different types of antibodies or antibody fragments

Monoclonal antibodies and their fragments are able to bind highly specifically to certain proteins ( antigens ) on the cell membrane . These antigens are the target structure ( target ) for the antibodies ( ligand ) in pathological, especially degenerate cells ( cancer cells ). Antibodies and antigen interact according to the lock and key principle . The antibodies can be labeled with the help of radioisotopes . In the case of immunoscitigraphy, radioactive labeling is only used for diagnostic purposes. Only high-energy γ-emitters with high tissue penetration and a short half-life (maximum a few days) are used. In complete contrast, radioimmunotherapy essentially uses β emitters with a very short range in the tissue. The tracers obtained after labeling are generally considered to be immunoconjugates or, in particular, radioligands . If the radioligand is administered to a patient, it is called a radiopharmaceutical . The radioligands are usually administered intravenously. The labeled antibodies should circulate in the body for as long as possible and ideally only bind to the target structures (antigens) of the diseased cells. The antibodies subsequently accumulate there. Antibodies that are not bound continue to circulate in the body or are excreted again after a while.

Fragments of a monoclonal antibody, such as Fab , F (ab) 2 or scFv may contain as carrier ( carrier ) for a radionuclide used.

The γ-emitters with short half-life used in scintigraphy, such as technetium -99m (from a technetium generator ), gallium -67, indium -111 or iodine -123, are generally used as radionuclides . Polyvalent metals are often bound to the carrier protein via a strong complexing agent such as DOTA .

application

Immunoscintigraphy is mainly used in oncology for cancer diagnosis. Usually, immunoscintigraphy is used to detect a relapse after therapeutic measures. The procedure is limited to tumors or metastases with a diameter of over 10 mm. Smaller tumor foci are only detectable in exceptional cases. Immunoscintigraphy can also be used to check whether a patient is responding to a therapeutic antibody. If the diagnostic agent binds sufficiently to the cancer cells, a radioimmunoconjugate , for example, can be administered in a subsequent therapy . If the connection is inadequate, such a therapy would be pointless. The radiopharmaceutical , which consists of a monoclonal antibody and a radioisotope, is usually administered intravenously to the patient. After several hours, often 4 and 24, a scintigram is created. The recording time is usually between 30 and 90 minutes. Side effects are extremely rare. Incompatibilities in the form of allergic reactions due to the antibody (foreign protein) are possible. The radiation exposure corresponds to one or two computer tomographies.

In addition to oncological applications, immune inflammation scintigraphies are also performed , albeit much less frequently. With this procedure, foci of inflammation can be localized in the patient's body, for example in the case of fever of unknown origin (FUG).

example

Arcitumomab is a drug based on a fragment of a murine monoclonal antibody that is conjugated with the radioisotope 99m technetium. The immunoconjugate binds to the carcinoembryonic antigen , which is overexpressed in a large number of colorectal carcinomas . Metastases or recurrences can be detected using this diagnostic tool using SPECT .

Immune PET

A variant of immunoscintigraphy is immuno-PET (immune positron emission tomography). Instead of a gamma emitter, a positron emitter, for example 68 gallium, is used as the radionuclide. The emitted positron annihilates with an electron in the immediate vicinity of the radionuclide to form two high-energy gamma rays that fly apart at an angle of 180 ° and are detected. For the principle of PET, see positron emission tomography . Immuno-PET has some advantages over immuno-scintigraphy, such as a lower detection limit. However, the process is still not well established and is not widely used.

further reading

Individual evidence

  1. ^ A b German Society for Nuclear Medicine: Cancer Diseases - Oncology. Retrieved October 5, 2012.
  2. ^ V. Ivančević: Immunoscintigraphy with the monoclonal NCA-95 antigranulocyte antibody and the NCA-90 antigranulocyte antibody Fab 'fragment for the diagnosis of inflammation in problem indications and for bone marrow scintigraphy. Habilitation thesis, Humboldt University Berlin, 2001.
  3. ↑ J. Meller: The immune inflammatory scintigraphy with Tc-99m-MAK BW 250-183 in fever of unknown origin. Dissertation, University of Göttingen, 1996.
  4. ^ GA van Dongen et al: Immuno-PET: a navigator in monoclonal antibody development and applications. In: Oncologist 12, 2007, pp. 1379-1389, PMID 18165614 .
  5. ^ I. Verel et al: The promise of immuno-PET in radioimmunotherapy. In: J Nucl Med 46, 2005, pp. 164S-171S, PMID 15653665 .