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

We have recently introduced (125)I-hCG as an elimination marker in patients with human chorionic gonadotrophin (hCG) producing testicular cancer. (125)I-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 (125)I-hCG tracer are described. The aim of the present study was to determine whether or not (125)I is associated with hCG after the injection of (125)I-hCG intravenously (i.v.) in humans. Three different methods were used. Following injection of (125)I-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 (125)I-hCG was analysed using a double antibody direct binding radioimmunoassay specific for holo-hCG and high performance liquid chromatography (HPLC). Following injection of (125)I-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 (125)I-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 (125)I-hCG and (125)I-hCG. Using three different methods, we were able concurrently to demonstrate the association of (125)I 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 (125)I-hCG. © 1999 Cancer Research Campaign