The metabolism of 4-iodantipyrine-125I (RIAP) in rat organism after intravenous injection

The purpose of this study was to investigate RIAP metabolism in rats after intravenous administration. The distribution, excretion and passage with milk to the newborn animals were studied. Electrophoresis of the urine of the rats treated with this radioisotope was done. A rapid accumulation of the radioisotope in the thyroid was observed (the T1/2 value was about 4 hours and the greatest accumulation of RIAP was after 15 hours). The biological time of RIAP elimination from the thyroid was 5 days. 125I concentration in other organs was below 1% of the injected dose. Excretion was mainly with urine in three phases. In the urine two 125I-containing fractions were found: a iodide fraction and an unindentified fraction. RIAP administration to lactating females caused passage of 10 to 20 percent of the administered dose to the newborns with milk. RIAP administration to pregnant rats raised this proportion by several percent. Thyroid blockade with potassium iodide had no effect on RIAP passage with milk to the newborns. These results suggest a rapid loss of iodine from RIAP after intravenous injection. (author)