125IUdR loss as a measure of tumour cell loss: influence of reutilization and influx of labelled host cells.

Two potential artifacts, the reutilization of label released from the gut and the influx into solid tumours of labelled host cells, have been measured by occluding the blood flow to tumours during, and for 30 min after, injection of 125IUdR. Previous work has shown that occluded EMT6 and KHJJ tumours exhibit a substantial increase in label, to 30-40% of the total activity in non-occluded tumours within 4 days post-125IUdR injection. In B16 and RIF-1 tumours the influx of label is minimal. Almost all of this entry of 125IUdR results from the influx of labelled host cells. Significant influx was also demonstrated in Lewis lung and KHT tumours. Minimal changes in the extent of influx of labelled host cells were found following irradiation of EMT6 and RIF-1 tumours with 600 or 1400 rad. These doses also resulted in little or no additional loss of 125IUdR despite the fact that 60 to 95% of the tumour cells were killed. Thus, the lysis of the killed cells must proceed very slowly. The conditions necessary for the use of 125IUdR loss to assess cell killing, as opposed to cell lysis, are reviewed.