Correlating Labeling Chemistry and in-vitro Test Results with the Biological Behavior of Radiolabeled Proteins

Is there life after technetium: what is the potential for developing new broad-based radionuclides?

The use of radionuclides for medical and for a multitude of other basic research applications has continued to grow at a very rapid pace. Procedures, based on their use as radiotracers for nuclear medicine imaging and for radiotherapy of cancer and other pathology, have become firmly established as important clinical modalities. It is estimated that on an annual basis in the United States alone, radionuclides are used medically in over 13 million imaging procedures, in over 100 million laboratory tests, and in an ever increasing number (> 100,000) for therapeutic administrations. One out of every four hospital patients undergoes a procedure that involves the use of radionuclides. Diagnostic imaging methods using planar/single-photon emission computed tomography and positron-emission tomography (PET) imaging, as well as the measurement of in vivo organ function, physiology, or biochemistry, have become indispensable tools in patient workup and management. More than 80% of all imaging studies (mostly anatomic) currently use technetium-99m (99mTc), because it has turned out to be the ideal isotope from various considerations. However, over the past few years, nuclear medicine has experienced a slow but steady evolution towards functional studies, quantitative PET imaging, and novel therapeutic approaches. New radionuclides are required for these applications, and their development has attracted considerable interest. This article reviews the current status and future prospects for the development of many new potential isotopes. Practical issues, such as the feasibility of large-scale production and wide-spread availability in a continuous reliable fashion, are addressed. To date, the data are not sufficient to answer the question as to whether any of these radionuclides (or their applications, for that matter) will eventually assume as broad-based a role as that of 99mTc. Nonetheless, there are a number of promising radionuclides that could assume an important place in the future practice of nuclear medicine.

Criteria for the selection of radionuclides for targeting nuclear antigens for cancer radioimmunotherapy

The potential of utilizing immunoconjugates to selectively deliver radionuclides for the destruction of tumors has stimulated much research activity. From dosimetric and other considerations, the choice of radiolabel is an important factor that needs to be optimized for maximum effectiveness of radioimmunotherapy (RIT). This paper reviews and assesses a number of present and future radionuclides that are particularly suitable for RIT based on the various physical, chemical, and biological considerations. Although intermediate to high-energy beta emitters (with and without gamma photons in their emission) possess a number of advantages for most RIT, the use of alpha, Auger, and short range conversion electron emitters could be attractive for targeting nuclear antigens when the radioimmunoconjugate is internalized into tumor cells. Factors relating to the production and availability of candidate radionuclides as well as their stable chemical attachment to monoclonal antibodies are discussed.

Progress in research on ligands, nuclides and techniques for labeling monoclonal antibodies

This paper reviews the current methods for radiolabelling monoclonal antibodies with particular emphasis on radiometals useful for radioimmunoscintigraphy. The discussion, however, is equally applicable to therapeutic radionuclides. The advantages and the pitfalls of the various techniques are critically evaluated. Both direct labeling methods, as well as indirect methods using the bifunctional chelating agent approach, are covered. Recent work on the development and synthesis of new and more specific chelating agents, including the approach of utilizing rigid polyaminocarboxylates, is described. Preliminary promising results with these newer generation chelating agents are presented.

Reduced technetium 99m labelled NCA-95/CEA-antibody uptake in liver due to gentle antibody reconstitution. Technical note

The influence of reconstituting a murine monoclonal IgG1 antibody kit with pertechnetate technetium 99m on antibody distribution in the liver, spleen and sternal bone marrow of patients was examined. The 99mTc-labelled antibody used is directed against non-specific cross-reacting antigen (NCA-95) and carcinoembryonic antigen (CEA) and has been successfully applied for imaging tissue inflammation and bone marrow scanning. Radioactivity uptake was determined in the liver, spleen, bone marrow and a precordial background region in a consecutive series of 25 patients, examined with an antibody preparation, routinely radiolabelled according to the manufacturer's recommendations and in 14 patients, in whom the antibody was reconstituted with special care, avoiding bubble formation and dropping of buffer into the antibody-containing vial. Gentle compared with routine antibody reconstitution caused a highly significant reduction of the antibody uptake in the liver, as determined by count densities, normalized to injected dose and acquisition time (13.2 +/- 5.5 vs 20.1 +/- 6.0 cpm per pixel, means +/- SD, P = 0.008). The liver to background ratio was reduced from 3.4 +/- 1.4 to 1.9 +/- 0.5 (P less than 0.001). Spleen, sternal bone marrow and precordial background count rates were not significantly affected. These results clearly demonstrate that gentle antibody reconstitution can decrease non-specific antibody uptake in the liver by 34% +/- 6.4% (means +/- SEM). Thus, scan quality is improved, and the potential deleterious camouflage of underlying structures is avoided.

The Effect of Human Serum on the Binding Activity of Radiolabelled Monoclonal Antibodies

Three murine monoclonal antibodies (MoAbs), MBrl and MOv2 of IgM isotype and MOv8 of IgG isotype, with restricted reactivity for breast or ovarian carcinomas, were labelled with 125I in the perspective of obtaining specific and stable radioimmunopharmaceutical reagents. The radiolabeled MoAbs were analyzed for their « in vitro » stability in human blood. They were incubated at 37 °C for various lengths of time in human or, as a control, in murine blood and their binding capacity was evaluated by solid-phase RIA and compared with that obtained after incubation with buffer. In human blood, serum and plasma, but not with other components such as erythrocytes, leukocytes, HSA and IgG, the MoAbs revealed a loss of binding reactivity which was marked and constant for the IgM MoAbs, and only occasional for the IgG MoAb. In murine serum the decrease was not so rapid. The same change in the binding capacity was observed when the MoAbs were labelled with 3H or 35S, excluding the involvement of dehalogenating mechanisms. In the perspective of using MoAbs for intracavity therapy the effect of ascitic or pleural fluids on their binding activity was also evaluated. The inhibition of the binding reactivity was not as evident and was not related to the MoAb isotype.