Comparison between 125IUdR and 51Cr as cell labels in investigations of tumor cell migration

YAC-1 tumor cells double-labeled with Na2[51Cr]O4 [51Cr] and [125I]iododeoxyuridine [125IUdR] were injected intravenously into Balb/c mice in order to investigate their migration and fate 0-4 h after the injection. Whereas the clearance of tumor cells from the lung tissue was similar as judged with both labels, the kinetics of isotope uptake in the liver were strikingly different. Thus, retention of 51Cr in the liver was very high compared to a much lower and only transient retention of 125I. A higher retention of non-tumor cell-associated 51Cr was also observed in most other organs, resulting in overestimation of the number of viable tumor cells in these organs. Moreover, a marked spontaneous release (greater than 10% after 12 h) makes 51Cr less suitable as a cell label than 125IUdR. On the other hand, we found that the release of 125I from dead cells in vivo depends at least partially on host factors such as macrophages. Consequently, caution must be exerted when tumor cell migration is investigated in animals treated with drugs that might affect the reticuloendothelial system. We conclude that 125IUdR is superior to 51Cr as a cell label for investigation of tumor cell migration in vivo, even though some doubt about the reliability of the number of tumor cells in liver and carcass, predicted by this radiolabel, still remains.