Validation of I-125-hCG as a marker for elimination of hCG and stability of I-125-hCG after in vivo injection in humans

We have recently introduced I-125-hCG as an elimination marker in patients with human chorionic gonadotrophin (hCG) producing testicular cancer. I-125 -hCG is a well-known reagent in clinical biochemistry and is used extensive ly in hCG assays. Previous studies have shown that the iodination process l eaves the hCG molecule mainly intact. The iodination, purification and stab ility of I-125-hCG tracer are described. The aim of the present study was t o determine whether or not I-125 is associated with hCG after the injection of I-125-hCG intravenously (i.v.) in humans. Three different methods were used. Following injection of I-125-hCG, the plasma disappearance of radioac tivity and hCG were followed for a period of 28 days in 13 normal subjects. Serum from a normal healthy male following injection of I-125-hCG was anal ysed using a double antibody direct binding radioimmunoassay specific for h olo-hCG and high performance liquid chromatography (HPLC). Following inject ion of I-125-hCG in eight normal healthy males and five normal healthy fema les, the disappearance of radioactivity and hCG showed identical paths in t he 28 days follow-up period. The bindable radioactive fraction of immunolog ically active hCG in serum of a normal healthy male following injection of I-125-hCG was between 57.0% and 72.1%, and was constant over time. HPLC sho wed similar elution pattern of serum from a normal healthy male injected i. v, with I-125-hCG and I-125-hCG. Using three different methods, we were abl e concurrently to demonstrate the association of I-125 with hCG in humans u p to 28 clays after injection of radiolabelled hCG i.v. Thus, information a bout the expected elimination of hCG can be obtained by following the elimi nation of activity in plasma after injection of I-125-hCG.