Construction of Immunoradiometric Assay for Circulating c‐erbB‐2 Protooncogene Product in Advanced Breast Cancer Patients

The human c‐erbB‐2 protooncogene product (erbB‐2 protein) is a 185 kilodalton glycoprotein closely related to epidermal growth factor receptor protein. In this study, we measured the concentration of circulating erbB‐2 protein in cancer patients by means of a new immunoradiometric assay (IRMA). Two monoclonal antibodies (MoAbs), SV2‐61γ and 6G10, recognize erbB‐2 protein but bind to separate epitopes. SV2‐61γ was used as an immunoadsorbent and 6G10 as an (125)I‐labeled probe. A serum was considered positive for erbB‐2 protein if the percent binding exceeded the mean of the normal group by more than 3 standard deviations. Eleven of 21 patients with advanced breast cancer and 1 of 15 with advanced gastric cancer were positive. Serum erbB‐2 protein levels correlated well with the therapy and the status of the patients with breast cancer. On the contrary, all patients with advanced colon, ovarian, or pancreatic cancers, showed levels below the cut‐off value. These results suggest that circulating erbB‐2 protein can be measured using the newly constructed IRMA. Since c‐erbB‐2 protooncogene amplification and overexpression are accepted as a good marker of aggressiveness, relapsing potency, and poor prognosis, this IRMA should be a promising tool with which to help manage breast cancer patients.