Prognostic significance of progesterone receptor levels in estrogen receptor-positive patients with metastatic breast cancer treated with tamoxifen: results of a prospective Southwest Oncology Group study.

PURPOSE: Southwest Oncology Group (SWOG) protocol 8228 is a prospective trial designed to investigate the prognostic significance of progesterone receptor (PgR) levels in estrogen receptor (ER)-positive breast cancer patients who were treated with tamoxifen. This study was undertaken because the value of PgR measurements in advanced breast cancer had been assessed previously only in studies that were small, retrospective, or included heterogeneously treated patients. METHODS: Receptor assays were performed only in the laboratories that met strict quality control guidelines. Of the 398 patients entered, 342 patients were eligible and assessable for the study end points of objective clinical response, time to treatment failure, and overall survival. RESULTS: Multivariate analysis shows that elevated PgR levels significantly and independently correlated with increased probability of response to tamoxifen, longer time to treatment failure, and longer overall survival. Overall response rate (defined as complete response CR, partial response PR, or stable disease SD for greater than 6 months) in this trial was 54%. Response rates to tamoxifen were 43%, 53%, and 61% in subsets of patients with less than 10, 10 to 99, and more than 100 fmol/mg PgR, respectively. Exploratory subset analysis using PgR and other prognostic variables identified ER-positive patient subsets with response rates to tamoxifen ranging from 24% (premenopausal patients) to 86% (postmenopausal patients with ER greater than 38 and PgR greater than 329 fmol/mg). No groups of ER-positive patients were identified who had such a low response rate as to absolutely preclude considering the use of tamoxifen. Multivariate analysis showed the independent, statistically significant predictors were: for response to tamoxifen, menopausal status, PgR, and ER; for time to treatment failure, menopausal status, disease-free interval (DFI), PgR, and ER; and for overall survival DFI, PgR, ER, site of disease, and history of adjuvant therapy. CONCLUSION: We conclude that knowledge of PgR levels together with other clinical information can improve the pretreatment assessment of ER-positive breast cancer patients with metastatic disease.