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Type of Breast Cancer Determines Effectiveness of Different Anti-Estrogen Agents

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Among women with hormone-positive breast cancer, certain subtypes of cancers benefit differently from the anti-estrogen agents, letrozole and tamoxifen. These results were recently published in the Journal of Clinical Oncology.

Hormone-positive breast cancer refers to breast cancer that is stimulated to grow from the female hormone, estrogen. Therefore, a standard treatment component for hormone-positive breast cancer includes an anti-estrogen agent that works to block or inhibit estrogen from its stimulatory growth effects on cancer cells.

One common anti-estrogen used in the treatment is tamoxifen. A newer class of anti-estrogen agents, referred to aromatase inhibitors (AIs) are also used in the treatment of hormone-positive breast cancer.

Tamoxifen and AIs produce their anti-estrogen effects through different biologic pathways, and each has different side effects. More recently, AIs have become commonly used as initial anti-estrogen therapy; however, researchers continue to explore which agents produce optimal effects in different types of breast cancer.

Researchers recently evaluated data to compare the effectiveness of tamoxifen and the AI, letrozole, in different types of hormone-positive breast cancer.

The study included evaluation of data from 2,923 women who participated in the Breast International Group BIG 1-98 trial. Median follow-up was 8.1 years. Participants were divided into two main groups for the data evaluation: those with invasive ductal carcinoma (IDC), meaning the cancer started in the milk ducts, which are ducts that are responsible for carrying the milk from where it is produced to the nipples; and those with invasive lobular carcinoma (ILC), meaning the cancer started in the milk-producing lobules of the breast.

The researchers also evaluated data from two subtypes of breast cancer: luminal B (LB)-like, meaning the cancer cells are rapidly dividing; and luminal A (LA)-like, meaning the cancer cells are not dividing as rapidly.

Overall, letrozole provided superior cancer-free survival compared to tamoxifen among patients with ILC – in both the LB and LA-like subtypes.

Among patients with IDC cancers, letrozole provided superior cancer-free survival among patients with the LB-like subtype.

However, among patients with IDC cancers, letrozole and tamoxifen provided similar cancer-free survival among the LA-like subtype.

Overall, letrozole provided a greater improvement in cancer-free survival among ILC breast cancer types compared to IDC, regardless of the luminal type.

The researchers concluded that letrozole provides superior cancer-free survival than tamoxifen in hormone-positive IDC and ILC breast cancer, except for women diagnosed with IDC with the LA-like subtype. Among women with this type of breast cancer, letrozole is comparable to tamoxifen.

The researchers stated that “the magnitude of benefit of adjuvant letrozole is greater for patients diagnosed with lobular carcinoma versus ductal carcinoma.” These results will help guide healthcare providers to determine optimal treatment choices for individual patients, taking certain cancer characteristics into consideration.

Reference: Filho O, Giobbie-Hurder A, Mallon E, et al. Relative Effectiveness of Letrozole Compared With Tamoxifen for Patients With Lobular Carcinoma in the BIG 1-98 Trial. Journal of Clinical Oncology. 2015; 33 (25): 2772-2779.