Differences in tumour and normal tissue concentrations of iodine- and indium-labelled monoclonal antibody. I. The effect on image contrast in clinical studies.

The poor imaging characteristics of 131I have resulted in the use of alternative radionuclides for radiolabelling monoclonal antibodies. Clinical imaging studies have shown that, in addition to the more suitable energy of emission of 111In over 131I for gamma camera detection, the 111In-labelled antibody appears to clear from the bloodstream at a faster rate than that of 131I-labelled antibody, resulting in greater tumour-to-background image contrast. However, measurements of the activity in blood samples from patients demonstrate that both 131I- and 111In-labelled antibodies clear from the circulation at similar rates. This discrepancy is probably due to the different biological fates of the two radionuclides and warrants further scientific study.