IODIDE UPTAKE AND EXPERIMENTAL I-131 THERAPY IN TRANSPLANTED UNDIFFERENTIATED THYROID-CANCER CELLS EXPRESSING THE NA+ I- SYMPORTER GENE/

I-131 therapy is a widely accepted treatment for differentiated thyroi d cancers which can accumulate iodide. We evaluated the efficiency of I-131 therapy against tumors which are transfected with the Na+/I- sym porter (NIS) gene. We transfected the rat NIS cDNA expression vector i nto malignantly transformed rat thyroid cells (FRTL-Tc) which do not c oncentrate iodide. The resultant cell line (Tc-rNIS) accumulated I-125 60-fold in vitro. The FRTL-Tc cells formed solid tumors after injecti on of cells into subcutaneous tissues of Fischer 344 rats. Tumors form ed with Tc-rNIS cells accumulated up to 27.3% of total I-125 administe red, and concentrated I-125 11 to 27-fold in the tumors. Extracorporea l measurement of radioactivity in the tumors revealed that I-125 accum ulation peaked at 90 min, and decreased to half levels 6 h after the i njections. To investigate the effect of I-131 administration on the tu mor growth, we injected (NaI)-I-131 2 and 3 weeks after the transplant ation of the cells. The (NaI)-I-131 did not change the tumor volume si gnificantly in either the FRTL-Tc or the Tc-rNIS-induced tumors. The s hort (6 h) effective half life of I-131 in the tumors diminished the r adiation dose to the tumor cells. However, this approach may prove ben eficial in the treatment of radiosensitive cancers, and could be emplo yed diagnostically.