Reutilization of 125I-UdR during growth of a solid mammary carcinoma: implications for the 125I-UdR loss technique.

Reutilization of thymidine (TdR) and 5-iodo-2'-deoxyuridine (I-UdR) released by dying tumour cells was assayed in the syngeneic adenocarcinoma EO 771 by injecting heat killed, labelled tumour cells into tumours. 3H and 125I liberation from labelled breakdown products was measured in tumours of various sizes without or with separation of tumours into viable and necrotic portions. Internal reutilization of 3H-TdR was considerably greater than that of 125I-UdR. 125I-UdR released by dying tumour cells was reutilized at about 10%. There was no significant increase in 125I-UdR reutilization during tumour growth. It is concluded that measurements of radioactivity loss by the 125I-UdR technique can result in underestimating the real cell loss depending on the amount of internal reutilization by the tumours investigated. Compared with 3H-TdR 125I-UdR is the tracer of choice for long term studies of cell loss.