Influence of radiolabel on the in vivo processing of intact and fragmented anti-tumour monoclonal antibody.

The anti-colon carcinoma MoAb35 and its F(ab')2 fragment were labelled with 131I or 67Cu and their biodistributions compared in nude mice bearing human tumour xenografts. Two-fold higher levels of 67Cu were found in the xenografts than 131I, with no significant difference in whole-body distribution. For the F(ab')2 fragment tumour levels were not significantly different between 131I and 67Cu. A very high accumulation of 67Cu was found in the kidney (36.7% ID/g). Whether deiodination in the kidney could account for the difference in nuclide accumulation was checked by repeating the study with 131I, linked to the F(ab')2 by a method known to resist deiodination. The results showed a slight increase in tumour accumulation of 131I but kidney levels remained low at 3.1% ID/g 24 hours post-injection compared to 42.4% ID/g for 67Cu, suggesting that deiodination does not play a role in the observed low levels of 131I in the kidney. Ongoing studies on the distribution and processing of radioimmunoconjugates will hopefully assist in their application in clinical studies.