Prospective observational trial of re-antiestrogen therapy after becoming resistant to ethinylestradiol treatment in hormone-dependent metastatic breast cancer.

Abstract

161 Background: After heavily treated estrogen deprivation therapy by aromatase inhibitors (AIs), an additional estrogen therapy may be useful in a certain condition. Furthermore, the estrogen therapy might sensitize a subsequent anti-estrogen therapy after the acquired resistance to estrogen. Methods: In the patients who obtained disease control (SD or more) by ethinylestradiol (EE2) therapy (1.5-3mg TID), the efficacy of anti-estrogen therapy using AI or fulvestrant was examined as a subsequent endocrine therapy. Primary endpoint was clinical benefit rate (CBR), and secondary endpoints were response rate (RR) and time to treatment failure (TTF). (Registration; UMIN000002831, prospective observational trial, sub-protocol analysis) Results: EE2 therapy was performed in 22 of postmenopausal breast cancer patients with ER-positive HER2-negative in primary or recurrent tumor, who had heavily pre-treated by endocrine therapies, including chemotherapies. At the median observation period of 13.6 months, RR of 41% (9/22) and CBR of 50% (11/22) were obtained (CR; 0, PR; 9, long SD; 2, SD; 4, PD; 3) and 4 patients withdrew this trial due to adverse events of early endocrine related symptoms and cerebral infarction. Although two patients were still ongoing, the median of duration of disease-control was 46 weeks (range; 23-62+). In thirteen patients who obtained disease control (SD or more) by EE2 therapy, fulvestrant for 6 patients or AI for 7 patients was used as a subsequent therapy. Fifty percent of fulvestrant group (3/6) and 43% of AI group (3/7) showed clinical benefit (PR or long SD). TTF of fulvestrant group was 15 weeks (median; range; 5-55+) and that of AI group was 19 weeks (median; range 5-33+). Furthermore, two patients who were responders to previous AI, got long SD to the subsequent EE2 re-treatment. Conclusions: EE2 therapy after resistance to estrogen deprivation therapy could be very useful, and a subsequent anti-estrogen therapy by AI or fulvestrant could be also effective in a certain condition. Thus, the changes of the endocrine environment by EE2 may induce the sensitization to hormone dependency in acquired hormone-resistant tumors. Clinical trial information: 000002831.