Reutilization of 131I-5-Iodo-2′-Deoxyuridine as Compared to 3H-Thymidine in Mouse Duodenum and Mammary Tumor2

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Abstract
The relative utilization and reutilization of tritiated thymidine (3H-TDR) and 5-iodo-2′-deoxyuridine (131I-IUDR) were studied in the duodenum and mammary tumor of C3H mice. Both radiotracers were injected into the same tumor-bearing mice, and then the specific activity of the tissue DNA was measured at various times post injection. Relative reutilization was estimated by a comparison of the rates of loss (half-times) of the two radioisotopes. In the duodenum, the absolute reutilization was estimated by a comparison of half-time for the loss of each radioisotope with the generation time of the crypt cells. TDR was incorporated into the DNA of both tissues three times more extensively than IUDR. TDR was appreciably reutilized in the duodenum and probably in the tumors. Minimal IUDR reutilization was indicated in the duodenum, but there was no evidence of IUDR reutilization in the tumors. Exogenous TDR was administered in the drinking water in an attempt to modify reutilization of 3H-TDR and 131I-IUDR. Very large doses (20 μmole/ml) may have had some small effect on IUDR, but one must be concerned with secondary effects at this level, since 40 μmole/ml did produce clinical evidence of host toxicity. Thus these data show that IUDR is favored over TDR for experiments where reutilization may be a problem, but unfortunately, one cannot rule out the possibility of low levels of IUDR reutilization.

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