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

We have recently introduced 125I-hCG as an elimination marker in patients with human chorionic gonadotrophin (hCG) producing testicular cancer. 125I-hCG is a well-known reagent in clinical biochemistry and is used extensively in hCG assays. Previous studies have shown that the iodination process leaves the hCG molecule mainly intact. The iodination, purification and stability of 125I-hCG tracer are described. The aim of the present study was to determine whether or not 125I is associated with hCG after the injection of 125I-hCG intravenously (i.v.) in humans. Three different methods were used. Following injection of 125I-hCG, the plasma disappearance of radioactivity and hCG were followed for a period of 28 days in 13 normal subjects. Serum from a normal healthy male following injection of 125I-hCG was analysed using a double antibody direct binding radioimmunoassay specific for holo-hCG and high performance liquid chromatography (HPLC). Following injection of 125I-hCG in eight normal healthy males and five normal healthy females, the disappearance of radioactivity and hCG showed identical paths in the 28 days follow-up period. The bindable radioactive fraction of immunologically active hCG in serum of a normal healthy male following injection of 125I-hCG was between 57.0% and 72.1%, and was constant over time. HPLC showed similar elution pattern of serum from a normal healthy male injected i.v. with 125I-hCG and 125I-hCG. Using three different methods, we were able concurrently to demonstrate the association of 125I with hCG in humans up to 28 days after injection of radiolabelled hCG i.v. Thus, information about the expected elimination of hCG can be obtained by following the elimination of activity in plasma after injection of 125I-hCG.