An approach for immunoradiometric assay with metallic radionuclides: gallium-67-deferoxamine-dialdehyde starch-IgG.

Radiogallium (Ga) labeling of an immunoglobulin-G-deferoxamine conjugate (DF-IgG) to a high-specific radioactivity was performed to allow the development of a radiometallic immunoradiometric assay (IRMA) system. To increase the specific radioactivity of Ga-DF-IgG, we used dialdehyde starch (DAS) as a multi-site spacer for the binding of DF to IgG. Six DF molecules bound to each IgG molecule after DAS conjugation. DF-DAS-IgG was then labeled with the previously reported 67Ga labeling solution, producing labeled IgG with a specific radioactivity of 11,766 MBq/mg IgG. Using this method, we labeled an anti-CA125 tumor-associated antigen monoclonal antibody (130-22), allowing the first application of 67Ga-DF-DAS-IgG to an IRMA system. With this system, a higher sensitivity could be obtained than with 125I IRMA. In addition, a very high correlation (r = 0.995) was obtained between serum CA125 levels as determined by 67Ga IRMA and 125I IRMA. Gallium-67-labeled antibodies with a high-specific radioactivity appear to hold promise for use in highly sensitive radioassay systems.