Differences in tumour and normal tissue concentrations of iodine- and indium-labelled monoclonal antibody.

Abstract

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 ¹³¹In 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.