The metabolism of 4-iodantipyrine--I125 (RIAP) in rat organism after intravenous injection.

Radioactive antipyrine labelled with I125 (RIAP) is used in radioisotope diagnostic methods for determination of the total water content of the organism, and cerebral and cardiac blood flow. 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 as well. A rapid accumulation of the radioisotope in the thyroid was observed (the T 1/2 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. I125 concentration in other organs was below 1% of the injected dose. Excretion was mainly with urine in three phases. In the urine of the rats two I125-containing fractions were found (a iodide fraction and an unidentified fraction). RIAP administration to lactating females caused passage of between 10 and 20% of the administered dose to pass to the newborns with milk. RIAP administration to pregnant rats raised this proportion by several per cent. 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.