Estrogen and progesterone receptors: correlation of response rates, site and timing of receptor analysis.

156 patients with advanced breast cancer of known estrogen receptor (ER) and progesterone receptor (PgR) status treated by endocrine therapy were studied. Regarding values for ER and PgR greater than or equal to 5 fmole/mg cytosol protein as positive, patients were divided into 4 phenotypic subgroups: ER+PgR+ (43%), ER+PgR- (26%), ER-PgR+ (8%), and ER-PgR- (23%). In patients with tumor phenotype ER+PgR+, responses were seen in 20/30 (67%) assessable initial treatments when receptor assays were performed on tumor recurrence or on primary tumor immediately before endocrine therapy, and in only 11/32 (34%) assessable initial treatments when receptor analysis was performed on primary tumor and there was intervening local therapy before endocrine therapy was started for tumor recurrence (P less than 0.05). Responses to first endocrine therapy for each tumor phenotype were ER+PgR+ 50%, ER+PgR- 27%, ER-PgR+ 27%, and ER-PgR- 6%. Four of 16 (25%) patients with ER+PgR+ tumors responded to subsequent secondary endocrine therapy, but such responses were not observed in 20 patients with other tumor phenotypes. Duration of response was similar for each phenotype, but patients with ER-PgR- tumors had a significantly shorter survival from time of initial endocrine treatment than patients of any other phenotype. These results suggest that repeat steroid receptor assays on accessible tumor immediately before endocrine therapy may result in improved predictability.