With "tailor-made" radioisotopes, tumors should be combated more efficiently in the future. In a preclinical study, a European research group succeeded in demonstrating the effectiveness of radionuclides of the element terbium (chemical symbol: Tb) - a rare earth metal - for cancer therapy. In contrast to radioactive isotopes, which have been used for a long time and which mostly arise as "waste products" in commercial nuclear reactors, terbium obviously has ideal properties for medicine.

Radionuclide medicine treats cancer patients with the help of radioactive substances that are injected into the bloodstream. The radiation of these substances can destroy cancer cells and stop the growth of tumors in the body of the patient or even to regress tumors. Since the emitted radiation does not distinguish between healthy and diseased tissue, it is necessary to "pack" the radioactive isotope into a biological component that binds specifically to the cancer cells. In this way, the drug can concentrate on the diseased tissue and take up the fight against the tumor there.

Tailoring in the early phase

Many of the nuclear isotopes commonly used in nuclear medicine, such as iodine-131 or yttrium-90, often do not have ideal properties in terms of energy and the type and duration of radioactive radiation from a medical point of view. "It would be ideal to be able to select the most suitable radioisotopes at an early stage of the development of medicines," explains physicist Ulli Köster of the Institut Laue-Langevin (ILL) in Grenoble. Him it is together with colleagues of Paul Scherrer Institute in the Swiss city of Villingen, the Technical University of Munich and the European Research Center Cern near Geneva, succeeded in producing four tailor-made terbium isotopes that could possibly fulfill the medical requirements. The results of the first animal experiments were promising.

Nuclides for diagnosis and therapy

To produce the artificial radionuclides with the mass numbers 149, 152, 155 and 161, the scientists fired at films of tantalum and gadolinium with neutrons or high-energy protons. Although the terbium isotopes differ in the number of neutrons and in the nature of their radioactive decay, from a chemical and biological point of view, they are quite similar to each other. This is a great advantage for radionuclide medicine. Packaged in a molecular complex consisting essentially of the vitamin B9, the radioisotopes can dock on certain tumors and have their effect there.

This is different depending on the isotope: For example, the isotopes terbium-155 and 161 emit gamma radiation, which almost completely leaves the patient's body, so that these two radionuclides are particularly suitable for the diagnosis and localization of tumors. In contrast, terbium-152 emits positrons that rapidly degrade into gamma radiation. This isotope is therefore predestined for diagnosis with positron emission tomography. Terbium-149 and 161 have particular therapeutic benefits. The latter emits beta-rays, which penetrate a few millimeters to a few centimeters deep into the tissue and destroy cells there. The alpha particles emitted by terbium-149 penetrate only a few tenths of a millimeter into the tissue. Terbium-161 emits so-called beta particles in addition to so-called Auger electron from the atomic shell, which due to their lower energy only a few microns far, which is ideal for the fight against small tumors and metastases.

Promising pilot study

The researchers around Köster have examined the potential diagnostic and therapeutic benefit of the Terbiumquadrupletts in mice. In five out of eight experimental animals a regression of the tumors could be observed. However, Köster and his colleagues themselves concede that the number of laboratory animals was too small to be able to draw too great conclusions from these results. That new oncological procedures in pre-clinical trials provide promising results but disappoint in clinical trials is an all-too-familiar pattern in cancer medicine. In the journal "The Journal of Nuclear Medicine" (doi:10.2967 / jnumed.112.107540) published test series therefore has the character of a pilot study. It shows that the concept as such works - even if only for tumors that have suitable receptors for the vitamin complex in which the terbium nuclides are embedded.

Chance for better therapy

For radionuclide medicine, despite this limitation, the study could be an important step forward. Already there is a shortage of certain nuclides. Institutes like Cern or the ILL, which are mainly involved in the study of fundamental physical relationships, could jump in here, says Köster: "They can accelerate the development of promising new therapies by providing high-quality radioisotopes that are not yet commercially available . "