Tissue distribution of 125I-labeled fetal thyroid hormone in nude mice xenografted with Ewing sarcoma

Biodistribution of fetal thyroid hormone (RT3) was studied in nude mice with Ewing's sarcoma xenografts. At 30 min and 1, 2, 4, 8, and 12 hr after injection, blood, tumor and normal organs were measured to determine the amount of radioactivity per gram of tissue. The amount of radioactivity in the blood of tumor-bearing mice decreased sharply from 7.64% of injected dose per gram of tissue (% ID/g) at 30 min after inoculation to 0.45%/ID/g at 12 hr. The % of injected dose per gram for 125I-labeled RT3 in the tumor reached 1.92 at 1 hr after injection and decreased to 0.22 at 12 hr. The radiolocalization indices for tumor to other organs at 12 hr ranged from 1.35 to 4.43. This study suggests increased localization of RT3 in the Ewing's sarcoma as compared to other tissues in the nude mouse.