Abstract: The aim of this study was to investigate the influence of ionic ¹²⁵I on the results of pharmacokinetics study for biologic products. The ionic ¹²⁵I bound to plasma protein was studied through measuring precipitation rates of plasma protein bound ionic ¹²⁵I after TCA precipitation in vitro and in vivo, with which precipitation rates of ¹²⁵I-RGD-Sak in plasma of SD rats at different time were compared. The results of the experiment show that ionic ¹²⁵I could be bound to plasma protein and be deposited by TCA[in vitro: (1.26±0.14)%; in vivo: (1.38±0.33)%]. The precipitation rates were independent of the reaction time(10 to 1440 min) and ionic ¹²⁵I activity(14500 to 120000 count/min). Also, ionic ¹²⁵I attached to the surface of precipitate contributed a lot to the precipitation rate, which could be eliminated after 2 to 3 times TCA precipitation. The influence of the ionic 125I should be calibrated in ¹²⁵I tracing method applied to pharmacokinetics study for biological products.

Key words: Ionic ¹²⁵I , Pharmacokinetics, Plasma protein binding, Drug concentration in blood