IODODEOXYURIDINE UPTAKE AND RETENTION AS A MEASURE OF TUMOR-GROWTH

Iodine-131-iododeoxyuridine (IUdR) uptake and retention was measured i n two C6 glioma cell lines (C6m and C6a) with different growth charact eristics. Animals with intracerebral (i.c.) C6a tumors had a mean surv ival of 16 days, whereas only 1 of 20 animals with i.c. C6m tumors die d during an 8-wk period of observation. The growth of i.c. C6m tumors could be described by the Gompertz equation; tumor doubling time incre ased from 1.9 to 5.2 days between Days 8 and 16 after tumor inoculatio n. Corresponding measurements of I-131-IUdR uptake and retention (24 h r after IUdR administration) by i.c. C6m tumors were also time-depende nt and decreased from 0.075 to 0.027 to 0.011 %dose/g in 8-, 10- and 1 6-day-old tumors, respectively. Iodine-131-IUdR uptake in ''rapidly gr owing'' i.c. C6a tumors was substantially higher (0.30 %dose/g at 24 h r) than that in ''slowly growing'' i.c. C6m tumors and corresponded wi th differences in the survival data. Subcutaneous C6a tumors had compa rable high uptake values (0.49 %dose/g at 24 hr), and 93% of total tum or radioactivity was recovered in DNA 24 hr after IUdR administration. Clearance of radioactivity was rapid in nonproliferative tissues; mor e than 80% of plasma radioactivity was cleared in 24 hr. Tumor-to-cort ex radioactivity ratios ranged from 100/1 to 120/1 and 150/1 between 2 4, 48 and 96 hr after IUdR injection respectively. A ''washout strateg y,'' which reduces tissue background activity and increases specificit y for PET and SPECT imaging of IUdR-DNA incorporation, is possible wit h longer-lived radioisotopes of iodine.