Evaluation of RU-27987 as a ligand to determine the progesterone receptor

RU-27987, a synthetic progestin, which was recently developed by Roussel Uclaf, Paris, was tested for its validity as a ligand to determine the progesterone receptor in breast cancer. The results were compared to those obtained with R-5020 and ORG-2058, two ligands that are already in use worldwide. The intra- and interassay variation of receptor determination was similar for all 3 ligands. Receptor levels were analyzed with each of the 3 progestins in control cytosols and in 26 mammary carcinoma samples. A good correlation between receptor levels was found although the values of ORG-2058 were somewhat lower, but not significantly. This resulted in a lower proportion of receptor positive samples for ORG-2058 (11/26) compared to R-5020 (13/26) and RU-27987 (14/26). The affinity to the progesterone and to the glucocorticoid receptor, as well as the precision of the Scatchard plot analysis were comparable for the 3 ligands tested. Intra- and interassay variation of receptor determination were also similar. We therefore conclude, that RU-27987 is a suitable ligand to determine progesterone receptor in mammary carcinoma.