Biological Stability of 5-lodo-2′-deoxyuridine labelled with Iodine-125 after its Incorporation into the Deoxyribonucleic Acid of the Mouse

Abstract

HUGHES et al.¹ have shown that the thymidine analogue 5-iodo-2′-deoxyuridine (IDU) given to Swiss mice 5–10 weeks old is incorporated primarily into the DNA of rapidly proliferating tissues such as intestinal epithelium and bone marrow ; the amount of label retained by the mouse as a whole decreases rapidly with time. This is as expected, since as mice mature the cell populations of their tissues approach steady-state conditions and, therefore, only those cells with a short life-span are proliferating rapidly enough to incorporate significant amounts of IDU. However, if IDU is given to infant or embryonic mice, where the cell populations of most tissues are rapidly increasing, a different labelling pattern is seen. This is illustrated in Fig. 1, where the retention of iodine-125 activity by the mouse as a whole is plotted against the time elapsed since administration (intraperitoneal) of ¹²⁵IDU to: (a) a litter of eight mice five days old; (b) a litter of five mice fifty-five days old; (c) a litter of seven mice five-days old which had received 25 µmole of thymidine simultaneously with the IDU. The ordinate represents the average retention of iodine-125 by each individual mouse expressed as the percentage of the activity originally administered. The experimental procedures used to obtain these results have been described elsewhere¹.