Subsequent endocrine therapy after resistance to ethinylestradiol treatment for the late-stage metastatic breast cancer: A retrospective cohort study.

Abstract

148 Background: Aromatase inhibitor (AI) is a most commonly used as the endocrine therapy in postmenopausal hormone-dependent breast cancer. Paradoxically, estrogen additive therapy using ethinylestradiol can be useful after long-term estrogen deprivation therapies with AI. Furthermore, there is a possibility of the beneficial effect of AI or fulvestrant as a subsequent endocrine therapy after EE2 failure. Methods: Ethinylestradiol (EE2; 3mg/day, TID) therapy was performed in 20 patients with metastatic breast cancer (median; 62 years-old, the mean observation time: 13.6 months.), who were heavily treated by sequential endocrine therapies (3rd or more line) including cytotoxic chemotherapies. We examined the efficacy of a subsequent endocrine therapy of AI or fulvestrant after becoming resistance to the EE2 therapy in the patients who got the disease control by the prior EE2 therapy. The primary endpoint was clinical benefit rate (CBR) and the secondary endpoint was time to treatment failure (TTF). (Registration number; UMIN 000002831, additional analysis). Results: Median TTF of EE2 treatment was 46 weeks (23-62+, 2 cases were ongoing). The response rate was 41% (9/22), the clinical benefit rate was 50% (11/22). The stable disease (< 6 months) was 18% (4/22) and another 3 cases were judged as progressive disease. Four cases withdrew due to nausea, fatigue and muscle-skeletal pain. In 20 cases progressed after disease control (SD or more) of EE2, a subsequent endocrine therapy, fulvestrant for 10 cases and AI for 10 cases, was performed. Fulvestrant group showed 50% (3 in 6) of CBR and 15 weeks (5-55+) of TTF, and AI-treated group showed 43% (3 in 7) of CBR and 19 weeks (5-33+) of TTF. Two in three cases, who became resistance to anti-estrogen treatment, got long SD by further EE2 re-therapy. Conclusions: Some cases showed clinical benefits of a subsequent therapy by AI or fulvestrant after becoming to EE2 failure. Taken together, sequential use of estrogen and anti-estrogen therapy (vice versa) could be one of the options as a salvage endocrine therapy for the patients with end-stage breast cancer.