Technetium-99m-labeled antibodies. Stability compared to 125I labeled antibodies.

It is essential in any method for radiolabeling antibody with 99mTc that the labeling procedure is rapid and reliable, producing a highly stable 99mTc-antibody complex with minimal effect on the immunoreactivity of the antibody. In the present study, analysis of the stability and homogeneity of radiolabeled (99mTc and 125I) antibodies (HMFG1 and PR1A3) was carried out by fast protein liquid chromatography (FPLC) using superose-6 and S-200 columns, and by polyacrylamide gel electrophoresis (PAGE) followed by autoradiography. Superose 6 and S-200 gel filtration analysis showed the radiolabel (99mTc or 125I) eluting with a retention time identical to that of native antibody. No peaks of relative molecular size (Mr) corresponding to possible antibody fragments were seen in either the UV or the radioactive FPLC elution profiles. PAGE analysis of 99mTc labeled antibody, however, revealed the presence of a number of radiolabeled antibody fragments (Mr < IgG) that were not detected by the same analysis of 125I labeled antibody. The stability of the radiolabeled antibodies in serum in vitro was also studied. FPLC (superose-6) analysis after 45 h incubation in normal serum in vitro revealed 3.3% (HMFG1), and 20% (PR1A3) of the 99mTc on a molecule or aggregate with a Mr greater than that of IgG. There is also the appearance of small amounts of 99mTc-labeled material with a Mr < IgG in the later fractions (2.2% for HMFG1 and 4.9% for PR1A3). Similar results were obtained using radioiodinated antibody, although the small amount of low molecular size material detected as a single peak with a longer retention time than the 99mTc equivalent corresponds to free iodide.