Interrelationship between Estradiol and Tamoxifen Responses for Clinical Breast Carcinoma Cells Cultured on Contact‐sensitive Plates

An in vitro assay system for predicting the estradiol (E(2)) sensitivity of clinical cancer cells was applied to 54 patients with breast carcinoma to compare the responses to E(2) and tamoxifen (TAM) with the estrogen receptor (ER) status. We found that 18 of the 35 cases in the ER‐positive group and 6 of the 19 cases in the ER‐negative group were stimulated by E(2). It is suggested that ER status alone can not predict the response of cultured cells to E(2) in clinical breast cancer. Cell growth of 11/35 (31%) of the ER‐positive cases and that of 8/19 (42%) of the ER‐negative cases was inhibited by E(2). Since the cases inhibited by E(2) could not be distinguished by ER status alone, an assay system based on a quantitative proliferative response was considered necessary. There were 20 (83%) cases of inhibition by TAM among the 24 stimulated by E(2). Only 18/35 (51%) of the ER‐positive group exhibited growth inhibition by TAM. In our (CSP) assay, 20 (83%) of the 24 cases stimulated by E(2) were inhibited by TAM, 10 (91%) of the 11 E(2)‐insensitive cases were insensitive to TAM and 13 (68%) of the 19 cases inhibited by E(2) were stimulated by TAM. In short, TAM response and E(2) response tended to be inversely related (43/54=80%,P<0.01). Furthermore, the E(2)‐response rate showed a good correlation with the TAM‐response rate (R(2)= 0.825). These results indicate the feasibility of predicting individual tumor responses to either E(2) or TAM by using CSPs.