Europium as a label in time-resolved immunofluorometric assays

The orientational freedom of molecular probes. The orientation factor in intramolecular energy transfer.

The use of radiometal-labeled small complexes and biomolecules as diagnostic agents is a relatively new area of medical research. In the late 1940s and early 1950s, the use of nuclear technology for medical purposes began, with nuclear reactors, accelerators, and cyclotrons being applied to medical isotope production. In 1959, Brookhaven National Laboratory developed the 99Mo/99mTc generator,1 and in 1964 the first 99mTc radiotracers were developed at the University of Chicago.2 Research into 99mTc radiopharmaceuticals was the beginning of the study of coordination chemistry as it relates to diagnostic imaging. Although 99mTc is the most widely used radionuclide for diagnostic imaging,3-5 a myriad of other radiometals have been or are being investigated for their uses in nuclear medicine. This review will discuss the non-technetium-labeled radiometal-labeled agents used in gamma scintigraphy and positron emission tomography (PET).

Radiometal-Labeled Agents (Non-Technetium) for Diagnostic Imaging

Conjugation of antibodies with bifunctional chelating agents: Isothiocyanate and bromoacetamide reagents, methods of analysis, and subsequent addition of metal ions☆

The relaxation of excited molecular states is often followed by the emission of light. Molecular excited states are related to the orbital, vibrational, and rotational levels of the orbital electrons. In addition, the electron spin determines the singlet and triplet states. In the singlet state the spin of an electron is paired, whereas in the triplet state the spin is not paired. When a molecule is excited to an upper singlet energy level, such as S2 (Figure 1), it can rapidly (10−10 sec) go to the lowest excited state S1 without emission of light. From S1 the molecule may go to any of the rotational and vibrational levels of the ground electronic state S0. This happens either by fluorescence emission or by nonradiative internal processes, referred to as intersystem crossing. From T1, the molecule can return to S0 by nonradiative processes or by a radiative process called phosphorescence. Under suitable conditions S1 and T1 can also transfer their excitation energy to other molecules.

Time-Resolved Fluorescence of Lanthanide Probes and Applications in Biotechnology

Human serum albumin has been conjugated to 1-(p-bnezenediazonium)-(ethylenedinitrilo)tetraacetic acid, a powerful chelating agent, and radioactive 111indium ions have been added specifically to the chelating groups. The product, with a specific radioactivity of about 1 mCi/mg of protein, was employed as a radiotracer in scintillation scanning studies with human volunteers. Results show that 48 hr after injection, practically all of the label remains attached to albumin. This is confirmed by electrophoresis of serum proteins; 7 days after injection, 85% of the radioactivity in the serum is still in the albumin fraction. These observations agree with in vitro studies of the labeled albumin in human serum, where loss of the metal ion from the chelating group to the protein transferrin amounts to less than 3% after 1 week and less than 5% after 2 weeks. Measurements of the distribution of label in mice up to 23 days after injection suggest that metabolism of the labeled protein does not lead to binding of indium ions by transferrin. The binding of indium and other metal ions by transferrin has previously posed a major impediment to the use of metal chelates for in vivo diagnostic procedures. Demonstration of the kinetic inertness of the chelate in these experiments suggests the use of related chelates as physical probes of biological systems.

https://www.pnas.org/content/pnas/73/11/3803.full.pdf

Charles S.-H. Leung

Papers

Covalent attachment of metal chelates to proteins:the stability in vivo and in vitro of the conjugate of albumin with a chelate of 111indium

The attachment of metal-chelating groups to proteins: Tagging of albumin by diazonium coupling and use of the products as radiopharmaceuticals

Attachment of fluorescent metal chelates to macromolecules using "bifunctional" chelating agents.

Properties In Vivo of Chelate-Tagged Proteins and Polypeptides

'Bifunctional' chelating agents for the binding of metal ions to biological molecules: principles and practical aspects