Iodine-131-iododeoxyuridine (
IUdR) uptake and retention was measured in two C6
glioma cell lines (C6m and C6a) with different growth characteristics. Animals with intracerebral (i.c.) C6a
tumors had a mean survival of 16 days, whereas only 1 of 20 animals with i.c. C6m
tumors died during an 8-wk period of observation. The growth of i.c. C6m
tumors could be described by the Gompertz equation;
tumor doubling time increased from 1.9 to 5.2 days between Days 8 and 16 after
tumor inoculation. Corresponding measurements of 131I-IUdR uptake and retention (24 hr after
IUdR administration) by i.c. C6m
tumors were also time-dependent and decreased from 0.075 to 0.027 to 0.011 %dose/g in 8-, 10- and 16-day-old
tumors, respectively. Iodine-131-IUdR uptake in "rapidly growing" i.c. C6a
tumors was substantially higher (0.30 %dose/g at 24 hr) than that in "slowly growing" i.c. C6m
tumors and corresponded with differences in the survival data. Subcutaneous C6a
tumors had comparable high uptake values (0.49 %dose/g at 24 hr), and 93% of total
tumor radioactivity was recovered in
DNA 24 hr after
IUdR administration. Clearance of radioactivity was rapid in nonproliferative tissues; more than 80% of plasma radioactivity was cleared in 24 hr.
Tumor-to-cortex radioactivity ratios ranged from 100/1 to 120/1 and 150/1 between 24, 48 and 96 hr after
IUdR injection respectively. A "washout strategy," which reduces tissue background activity and increases specificity for PET and SPECT imaging of
IUdR-
DNA incorporation, is possible with longer-lived
radioisotopes of
iodine.