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Estrogen receptor downregulators: new antihormonal therapy for advanced breast cancer
1University of Nottingham and Nottingham City Hospital, United Kingdom.
Background:
Fulvestrant is the first agent in the new class of estrogen receptor (ER) downregulators to be evaluated in clinical studies. The binding of ER downregulators to ERs inhibits the activation functions of the receptors (AF-1 and AF-2). ER downregulators also disrupt the dimerization and nuclear localization of ERs. In contrast to ER downregulators, when selective estrogen receptor modulators (SERMs) bind to ERs, only AF-2 is inhibited; AF-1 remains active. Therefore, with SERMs, AF-1 can still recruit coactivators, which results in a partially inactivated transcription and a reduced rate of tumor cell division. Consistent with its mechanism of action, animal models have shown that fulvestrant demonstrates a longer suppression of tumor growth than does tamoxifen, and it has antitumor activity in tamoxifen-resistant tumors. Two Phase III trials have been conducted comparing fulvestrant (250 mg IM monthly) with anastrozole (1 mg oral tablet daily) in postmenopausal patients with advanced breast cancer progressing after prior endocrine therapy. Objective response in fulvestrant-treated patients in the 2 studies ranged from 17.5% to 20.7% compared with 15.7% to 17.5% for anastrozole-treated patients, and the median duration of response ranged from 14.3 to 19.3 months in the fulvestrant group compared with 10.5 to 14.0 months in the anastrozole group. No statistically significant differences were demonstrated for any of the predefined end points. In both studies, fulvestrant was as well tolerated as anastrozole.
Objective:
This review describes the pharmacology, clinical activity, safety, dosing, and pharmacokinetic profile of fulvestrant.
Conclusions:
Based on evidence to date, fulvestrant will likely be an important agent in the treatment of tamoxifen-resistant metastatic breast cancer.
Insights
Fulvestrant, a novel estrogen receptor downregulator, shows comparable efficacy and tolerability to anastrozole in advanced breast cancer. It is a promising treatment for tamoxifen-resistant metastatic breast cancer.
Area of Science:
- Endocrinology
- Oncology
- Pharmacology
Background:
- Fulvestrant is the first estrogen receptor (ER) downregulator, inhibiting ER activation functions, dimerization, and nuclear localization.
- Unlike selective estrogen receptor modulators (SERMs), fulvestrant offers complete ER inhibition, leading to enhanced antitumor activity in animal models, including tamoxifen-resistant tumors.
Purpose of the Study:
- This review details the pharmacology, clinical activity, safety, dosing, and pharmacokinetics of fulvestrant.
- To evaluate fulvestrant's role in treating advanced breast cancer, particularly in cases resistant to prior endocrine therapy.
Main Methods:
- Two Phase III clinical trials compared fulvestrant (250 mg IM monthly) with anastrozole (1 mg oral daily) in postmenopausal patients with advanced breast cancer.
- Efficacy endpoints included objective response rates and duration of response. Safety and tolerability were also assessed.
Main Results:
- Objective response rates for fulvestrant ranged from 17.5% to 20.7%, compared to 15.7% to 17.5% for anastrozole.
- Median duration of response was longer with fulvestrant (14.3–19.3 months) versus anastrozole (10.5–14.0 months).
- No statistically significant differences were observed in predefined endpoints; both treatments were well tolerated.
Conclusions:
- Fulvestrant demonstrates comparable efficacy and tolerability to anastrozole in advanced breast cancer patients progressing after endocrine therapy.
- Fulvestrant is positioned as a significant therapeutic option for tamoxifen-resistant metastatic breast cancer.