The influence of estrogen receptor status on response rate to endocrine therapy in 85 patients with metastatic breast cancer was determined in a retrospective study. The specific purpose of this study was to assess the role of estrogen receptor determinations in the light of a host of clinical variables known or suspected to influence response rates to endocrine therapy. Thirty-four of 52 patients whose tumors contained significant amounts of estrogen receptor (>10 fmole/mg cytoplasmic protein) had objective responses to endocrine therapy while only 3 33 patients whose tumors did not possess estrogen receptor (< 10 fmole/mg cytoplasmic protein) responded (P < 0.0001). A quantitative relationship was found between the amount of estrogen receptor and response rate. The quantity of estrogen receptor was not associated with the duration of response. The predictive value of the estrogen receptor assay was not associated with the receptor dissociation constant. Prior treatment with endocrine or chemotherapy did not diminish the ability of estrogen receptor determinations to predict response to subsequent endocrine therapy. Response rate in estrogen receptor positive tumors was not affected by extent of disease, site of involvement with metastatic tumor, or prior therapy. We conclude that these negative prognostic factors are less important in predicting response to endocrine therapy than estrogen receptor values.
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