Characterisation of antibodies to urinary gonadotrophin peptide

There is now general recognition that human chorionic gonadotropin (hCG), its beta subunit and its fragments are valuable diagnostic markers of trophoblastic and some non-trophoblastic malignancies. Urinary gonadotropin peptide (UGP) contains at least one epitope which cross-reacts with the beta-subunit of hCG. In order to assess the potential of UGP as a tumour marker in its own right, it was paramount that any measurements made could be regarded as specific for UGP and not cross-reactive with either hCG or human luteinising hormone (hLH). Four antibodies were tested, two polyclonal (AK12 and DR-Pool) and two monoclonal (2C2 and 6D3). Initial screening using radioiodinated (125I) UGP, hCG, hCG beta-subunit and hLH showed that the polyclonal antibodies bound to all four gonadotrophins, whilst the monoclonal antibodies bound only to the radioiodinated UGP. The antibodies were tested in both radioimmunoassay (RIA) and immunoradiometric assay (IRMA-sandwich assay) systems. The parameters measured were sensitivity and specificity for UGP. The polyclonal antibodies used in the RIA system produced a sensitive assay (0.2 ng/ml UGP) which was relatively specific; cross-reactions for the AK12 antibody (at 50% inhibition) were 5% for hCG, 11% for its beta-subunit, 0.4% for the alpha-subunit and 2.6% for hLH. The monoclonal antibodies performed optimally in the IRMA system. Immobilised 2C2 and radiolabelled (125I) AK12 produced a system that had a sensitivity of 0.4 ng/ml UGP and cross-reactivity (50% maximum binding) of 2% for the hCG beta-subunit and less than 1% for hCG, hLH and the hCG alpha-subunit.