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Fulvestrant Therapy for Breast Cancer Does Not Foster Cross-Resistance: Presented at IBCC

By Alison Palkhivala

BANFF, AB -- August 5, 2003 -- Fulvestrant (Faslodex), a new endocrine therapy for breast cancer with a unique mechanism of action, does not appear to foster cross-resistance with other endocrine therapies.

Working on behalf of the Faslodex Study Investigators, Steven E. Come, MD, Beth Israel Deaconess Medical Center, Boston, Mass, and colleagues reviewed 3 randomized, phase III clinical trials in which postmenopausal women with metastatic or locally advanced breast cancer received hormonal therapy. Results were presented here in a poster at the Second Annual Future of Breast Cancer: An International Breast Cancer Congress.

In 2 of the trials, 428 women were randomized to monthly intra-muscular injections of fulvestrant 250 mg and 423 received oral anastrozole 1 mg daily. In the other trial, 313 patients received monthly injections of fulvestrant 250 mg and 274 took oral tamoxifen 20 mg daily. Patients in the fulvestrant versus anastrozole studies had not received more than one prior hormonal therapy, and patients in the fulvestrant versus tamoxifen study had not received any endocrine or cytotoxic treatment for advanced disease.

During an open-label follow-up, 253 patients who had participated in 1 of the 3 trials were evaluated for response to subsequent endocrine therapies. Specifically, the investigators assessed the presence of clinical benefit from these subsequent therapies, defined as complete response, partial response, or stable disease at 24 weeks or more.

Among 54 patients who received clinical benefit from fulvestrant therapy following tamoxifen therapy in the fulvestrant versus anastrozole trials, 25 (46%) also derived clinical benefit from subsequent endocrine therapy. In addition, among 51 patients who did not derive clinical benefit from fulvestrant, 18 (35%) obtained clinical benefit from subsequent endocrine therapy.

With respect to the patients in the fulvestrant versus tamoxifen trial, among 47 who derived clinical benefit from fulvestrant, 27 (57%) also derived clinical benefit from subsequent endocrine therapy. Among 36 patients who did not derive clinical benefit from fulvestrant, 15 (42%) derived clinical benefit from subsequent endocrine therapy.

The authors concluded in their poster that "patients who achieve [clinical benefit] during treatment with fulvestrant may remain sensitive to further treatment with other endocrine agents, including the aromatase inhibitor tamoxifen. Patients who do not benefit from fulvestrant may achieve [clinical benefit] from further endocrine therapy."

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