BACKGROUND.
Human breast cancer cells in vitro exhibit increased levels of progestin receptors (PgR) after brief exposure to physiologic concentrations of estrogens. Prior clinical studies have positively correlated the responsiveness of metastatic breast cancer to progestin therapy with the level of PgR in the tumor cells. METHODS. These observations were used as the scientific basis for a randomized clinical trial by the Eastern Cooperative Oncology Group (ECOG) to compare the effectiveness of megestrol acetate (MEG) alone in a daily dose of 160 mg with MEG alternated with premarin in a dose of 1.25 niglday on the first 3 days of a 14 day cycle (PREIMEG). From 1985 through 1989,266 eligible and fully evaluable patients were randomized to 1 of the treatment arms and accrued to this trial. All patients were postmenopausal with biochemical estrogen cytosol protein receptor (ER) positive (m 10 fm/mgI tumors. The treatment groups were balanced with respect to performance status, number of involved organ systems, and PgR levels. RESULTS. Forty-five of 135 (33%) (95% confidence interval (CI], 25-42%) patients receiving MEG experienced a partial (PR) or complete (CR) response. Thirty-one of 131 (23%) (95% CI, 17-32%) patients receiving PREIMEG achieved a PR or CR. Survival was not influenced by treatment selection. However, median time to pro-'t''