Differences in tumour and normal tissue concentrations of iodine- and indium-labelled monoclonal antibody. II. Biodistribution studies in mice with human tumour xenografts.

The blood, tumour and whole-body levels and survivals of 131I- and 111In-labelled monoclonal antibody (791T/36) have been compared in mice with human tumour xenografts. The blood levels and survivals of 131I- and 111In-labelled antibody were similar when expressed as proportions of the injected doses. However, the whole-body survival of 111In following administration of 111In-labelled antibody was over twice as long as that of 131I after administration of 131I-labelled antibody, principally because free 131I was rapidly excreted but free 111In was retained, primarily in liver, spleen and kidneys. Consequently, when expressed in relation to the whole body, blood levels of 111In became progressively lower than those of 131I following administration of labelled antibodies. In mice with human tumour xenografts the proportion of the injected dose of 111In from 111In-labelled antibody deposited in tumour tissue was 4-5 times higher than that of 131I from 131I-labelled antibody. When compared with the whole-body levels of radiolabel the difference was less marked, although 111In accumulation in tumour was more rapid. The higher levels and longer retention of 111In in tumour produced tumour-to-blood ratios that were 7-8 times those achieved with 131I-labelled antibody.